ImagineNano Abstract Booklet Poster Contributions (part I) by Phantoms Foundation

Graphene Self Switching Diodes Feras Al-Dirini 1

1,2,3

, Stan Skafidas

1,2,3

, Ampalavanapillai Nirmalathas

1,3

Electrical and Electronic Engineering Department, University of Melbourne, Parkville, Australia 2 Centre for Neural Engineering, University of Melbourne, Parkville, Australia 3 Victorian Research Laboratory, National ICT Australia, Parkville, Australia ferasa@student.unimelb.edu.au

Abstract Self-Switching Diodes (SSD) are two terminal Nano-devices that rectify current based purely on a field effect, without the need for a third gate terminal to apply this field. They can be fabricated simply by etching two L-shaped trenches back-to-back giving rise to asymmetrical nanowires that have rectifying characteristics similar to those of conventional diodes [1]. SSDs can operate at high frequencies, reaching up to terahertz at room temperature [2], and have shown a great potential for high frequency detector applications [3]. The device's channel length and the charge carrier mobility in the material used to build the device have both proven to be the limiting factors that limit the device's detection frequency [4]; with a shorter channel and higher charge carrier mobility, a higher detection frequency can be achieved. Furthermore, as the channel becomes shorter and the charge carrier mobility becomes higher, the device begins to enter ballistic operation through which resonant detection may be achieved at very high frequencies in the terahertz range [5]. Another important advantage of SSDs is the simplicity of their fabrication; their simple geometry requires only two photolithography steps during fabrication, one to etch the trenches and the other to define the contacts. Also, since rectification in these devices is based purely on a field effect that does not depend on any junctions or barriers they do not require any doping during fabrication, further simplifying the process. Nevertheless, one requirement needs to be satisfied in order to achieve this simple fabrication process, and that is SSDs need to be built on substrates in which current conduction is confined in a two-dimensional plane. This had previously been achieved through the use of either compound semiconductor heterostructures [1] or Silicon-On-Insulator wafers [6]. With the above mentioned factors in mind, namely high charge carrier mobility, the ability for extreme downscaling and two-dimensional confinement of conduction, Graphene, which is a 2-dimentional material with ultrahigh charge carrier mobility, stands out to be a very strong candidate that may fulfill such needs of SSDs. In this work we present the use of Graphene to make Graphene Self-Switching Diodes (G-SSD). The GSSD device structure is shown in Figs. 1 and 2, and as can be seen from the figures G-SSDs are similar to SSDs in their architecture except that the channel in them is a nanoribbon rather than a nanowire. This nanoribbon can be either an armchair nanoribbon or a zigzag nanoribbon, where armchair and zigzag refer to the structure of the edges of the nanoribbon, but more importantly result in different electronic properties of the channel. Armchair nanoribbons can be either metallic or semiconducting, while zigzag nanoribbons are metallic, and this suggests that a G-SSD may behave differently from another G-SSD based on the type of nanoribbon that constitutes its channel. In order to study this effect, two G-SSDs, one with a zigzag nanoribbon (Fig. 1) and the other with a semiconducting armchair nanoribbon (Fig. 2), were investigated using atomistic quantum simulations, and the I-V Curves of both devices were calculated based on NEGF and the Extended Huckel Method. The results for both cases, a G-SSD with a zigzag nanoribbon and another with an armchair nanoribbon, are presented in Figs. 3 and 4 respectively. As can be seen from Fig. 3, for a device with a zigzag nanoribbon as the channel, a linear I-V curve is obtained, suggesting that the device does not operate well as a rectifier. This result is consistent with what has become settled in the literature, namely that zigzag nanoribbons behave close to metals, and since the device's channel is a zigzag nanoribbon it is expected that its conductance will not be affected significantly by a field effect and hence it will conduct equally in both directions, resulting in a straight line for the I-V curve. On the other hand, Fig. 4 shows a different behavior for the semiconducting armchair channel G-SSD. A non-linear IV curve, in which conduction is much higher in the forward direction when compared to the reverse direction, is obtained resulting in rectifying characteristics that resemble a functional rectifying G-SSD. These results confirm the ability of realizing SSDs with strong non-linear characteristics using Graphene, by ensuring that the channel in the device is an armchair nanoribbon, launching a new line of Graphene devices that exploit Graphene's unique properties, and paving the way towards achieving resonant Terahertz detection using Graphene Self-Switching Diodes in the near future.

References [1] [2] [3]

[4] [5] [6]

Song, A.M., Missous, M., Omling, P., Peaker, A.R., Samuelson, L. & Seifert, W. 2003, "Unidirectional electron flow in a nanometer-scale semiconductor channel: A self-switching device", Applied Physics Letters, vol. 83, no. 9, pp. 1881-1883. Balocco, C., Kasjoo, S.R., Lu, X.F., Zhang, L.Q., Alimi, Y., Winnerl, S. & Song, A.M. 2011, "Roomtemperature operation of a unipolar nanodiode at terahertz frequencies", Applied Physics Letters, vol. 98, no. 22. Balocco, C., Song, A.M., Åberg, M., Forchel, A., González, T., Mateos, J., Maximov, I., Missous, M., Rezazadeh, A.A., Saijets, J., Samuelson, L., Wallin, D., Williams, K., Worschech, L. & Xu, H.Q. 2005, "Microwave detection at 110 GHz by nanowires with broken symmetry", Nano Letters, vol. 5, no. 7, pp. 1423-1427. Iniguez-de-la-Torre, I., Mateos, J., Pardo, D., Song, A.M. & González, T. 2010, "Enhanced Terahertz detection in self-switching diodes", International Journal of Numerical Modelling: Electronic Networks, Devices and Fields, vol. 23, no. 4-5, pp. 301-314. Iñiguez-De-La-Torre, I., Rodilla, H., Mateos, J., Pardo, D., Song, M. & Gonzlez, T. 2009, "Terahertz tunable detection in self-switching diodes based on high mobility semiconductors: InGaAs, InAs and InSb", Journal of Physics: Conference Series, vol. 193. Farhi, G., Morris, D., Charlebois, S.A. & Raskin, J.-. 2011, "The impact of etched trenches geometry and dielectric material on the electrical behaviour of silicon-on-insulator self-switching diodes", Nanotechnology, vol. 22, no. 43.

Figures

Figure 1 – G-SSD with a zigzag edged channel

Figure 2 – G-SSD with an armchair edged channel and hydrogen passivated dangling bonds

Figure 3 - IV curve for the G-SSD with a zigzag channel

Figure 4 - IV curve for the G-SSD with an armchair channel

1 Conditions for Production of Defect Free Graphene on Catalyst Melt Surface in CVD – Synthesis N.I.Alekseyev, V.V.Luchinin

St.Petersburg Electrotechnical University “LETI”, ul. Professora Popova, 5, St.-Petersburg, Russia. NIAlekseyev@yandex.ru As of now the research for the graphene monolayer production ways via CVD- methods on a crystalline substrate has not been a success. In this regard, some interest starts to concentrate on graphene growth process on molten substrate which does not impose its intrinsic crystalline matrix to graphene. On the contrary, the metal atoms in a surface layer of the melt place themselves fit to a crystalline lattice, organized by the carbon atoms. The monolayer graphene growth was practically realized on the copper melt surface [1]. Methane CH4 was used in this process as a gaseous carbon carrier. We considered the nucleation of the ensemble of the islands on the copper catalyst surface, whose surface thin layer is supersaturated with carbon. The nucleation of the islands, their rapid growth and consequent supersaturation decrease were considered in self-consistent way within the Lifshitz-Slëzov’ homogeneous nucleation theory (explosive nucleation). At the same time, such calculation reveals the conditions under which hexagonal shape of the islands is of preserved. Subsequent calculation of the fast nucleation stage, when new islands are not formed whereas the old ones absorb carbon atoms in diffusion mode, allows the parameters region to be determined, within which both the dispersion and the shape deterioration of the islands are minimal. Plunged into this parameter region in the conditions of the absence of stiff crystal matrix the hexagonal islands can agglomerate to form defect free graphene. The calculation show that practically found combination of the parameters can only be implemented for melted copper. Their approximate correspondence with the experimental date enables this method to be improved. [1].D.Geng, B.Wu, Y.Guo, L.Huang, Y.Xue, J.Chen, G.Yu, L.Jiang, W.Hu, Y.Liu. Uniform hexagonal graphene flakes and films grown on liquid copper surface. www.pnas.org/cgi/doi/10.1073/pnas.1200339109

Production of graphene-like thin film from carbon black by wet chemical method M. Alfè*, V. Gargiulo*, R. Di Capua**, A. Ciajolo* * Istituto di Ricerche sulla Combustione (IRC)-CNR, Napoli, Italy ** SPIN-CNR UOS Napoli, via Cintia, I-80126, Napoli, Italy alfe@irc.cnr.it Abstract Graphene attractive chemical-physical properties are largely exploited for numerous applications including the fabrication of electronic and optoelectronic devices, energy-storage materials and mechanical resonators. Graphene can also be used as conductive sheet upon which nanometer scale devices may be patterned to create single electron or few electron transistors [1]. It is widely accepted that the bottleneck in the graphene-based technology is its production on a large scale [1]. Various approaches have been used to produce graphene or graphene-like materials including one-step graphite exfoliation, chemical vapor deposition (CVD) of methane gas, graphite stamping, graphite oxide reduction and carbon nanotube unzipping. Among these, the production of graphite oxide (GO) from graphite powder and its further reduction (through chemical, thermal, or ultraviolet-assisted reduction methods) to graphene-like material is a convenient and cheap way of graphene-like sheet fabrication.’ A new approach for producing graphene-like thin films from carbon black (CB) has been recently reported [2]. In the proposed method graphene-like layers were produced in mild conditions and in aqueous environments with high yields (55% mass yield) through a two-step CB oxidation and reduction. The graphene-like layers presented a good suspendibility in water and when they dry undergo to self-assembling resulting in water-insoluble thin film. The selected oxidative treatment provides the partial demolition of the CB microstructure and the functionalization at the edge of the basal planes of the graphitic layers (Figure 1, middle) and not on them. In this way, differently from GO, the oxidized CB preserves the original graphitic network useful for the conductivity and electronic properties. This approach for making graphene-like thin film is environmentally advantageous because all procedures are performed in aqueous media, and it is highly compatible to an industrial scale-up process. The selected CB (furnace carbon black, N110 type) is arranged in chain-like aggregates of spherical primary particles (15-20 nm) with a narrow size distribution (Figure 1, left). CB was oxidized according to the procedure reported by Kamegawa [3]. The CB powder was treated with hot nitric acid (67 wt.%, 100 °C) under stirring for 90 hours. The purified hydro philic product (HNP) was reduced with hydrazine hydrate (100 °C under reflux) for 24 h obtaining a HNPR (Figure 1, right). Additional information about the sample preparation were reported in Alfè et al. [1]. The dried HNPR was insoluble in water and in the most common organic solvents, both polar and non-polar (water, ethanol, NMP, dichloromethane, heptane, DMF). This was attributed to an increase of hydrophobicity of the material caused by a decrease in the polar functionalities on the surface and consequent intimate self-assembling interaction between the restored graphitic planes. Elemental analysis indicates that the hydrogen content tends to increase from 0.48 to 1.21 after the oxidation step, testifying the reduction of the dimension of the graphitic basal planes. This finding was also confirmed by UV-visible spectroscopy. The increase of the H/C ratio from 0.058 (raw CB) to 0.27 (HNP) also confirms this hypothesis. The oxidation results in the introduction of oxygenated functional groups (hydroxyl, carboxylic, carbonylic) as testified by the increase of the oxygen content from 0.6% (raw CB) to 44% (HNP). The increase of nitrogen from 0.04% (raw CB) to 1.12% (HNP) as a consequence of the introduction of nitro groups is also observed. The HNP reduction step introduces nitrogen functionalities likely in the form of hydrazones or similar structures, leading to an increase of nitrogen percentage from 1.12 (HNP) to 6.09 (HNPR). Is it noteworthy that the oxygenated functionalities are not completely removed upon the chemical reduction. However it represents an expected result, since it is well known that CB oxidized with nitric acid has not oxygen atoms on basal planes in the form of epoxides, but only on their edges [3]. The H/C ratio kept nearly constant (0.270.32) after the reduction step suggesting that the size of the graphitic units remains rather unchanged. The ATR-FTIR investigation demonstrates the introduction of different types of oxygenated functionalities that are completely absent in the raw CB. In both HNP and HNPR the presence of C=O is detected. The intensity of the C=O signal is significantly lower in the HNPR as a consequence of the removal of carboxylic-carbonylic functionalities upon the reductive treatment. The reductive treatment also introduce NH2 functionalities in the form of hydrazones. The presence of nitrogen atoms in both HNP and HNPR in the form of nitro groups is also detected.

Thermogravimetric analyses indicate that the oxygenated functionalities begins to decompose at 150 °C (up to 40% weight loss). The HNPR mass loss is lower (up to 20% weight loss) as a consequence of the lower number of oxygenated functionalities, partially removed upon chemical reduction. The bulk oxidation of both HNP and HNPR (590 °C) oc curs at a lower temperature with respect to the pristine CB (690 °C) consistently with the strong d egradation of the graphitic backbone upon the wet oxidative treatment. It is noteworthy that the oxidation of HNP and HNPR occurs at the same temperature, suggesting a similar size of the graphitic core. The HNPR is characterized by a good suspendibility in water (ζ-potential lower than -30 mV in a wide pH range). When a HNPR suspension dries on a plane surface the graphene-like layers undergo to selfassembling resulting in flat water-insoluble thin film that easily conform to any feature of that surface. AFM measurements provide information about the surface features of the HNPR film. Figure 2 shows the topographic images acquired on the HNPR deposited on a mica plate and dried at room TM temperature. The image, acquired in the True Non Contact mode of operation of a XE-100 Park system, is reported together with a line profile representative of the surface morphology. The surface of the HNPR film looks atomically flat over large areas. Raw CB and HPN did not reveal any electrical current up to an applied voltage of 10 V. On the contrary, on the HPN film shows an ohmic behavior of the I−V characteristic. The evaluated resistivity on a 20 nm thick film is 6 Ω m. For a convenient exploitation of the process and film thickness control it is fundamental to explore carefully each aspect of the film preparation. It has been found that the quality of the thin film is strongly dependent on the pH of the graphene-like layers suspension. Studies are underway to clarify this important point. References [1] K. S. Novoselov, V. I. Fal′ko, L. Colombo, P. R. Gellert, M. G. Schwab, K. Kim, Nature, 490 (2012),192–200. [2] M. Alfè, V. Gargiulo, R. Di Capua, F. Chiarella, J.N. Rouzaud, A. Vergara, A. Ciajolo A,. ACS Appl. Mater. Interfaces, 4 (2012) 4491−4498. [3] K. Kamegawa,.K. Nisiukubo, H. Yoshida, Carbon, 36 (1998) 433-441. Figures

Figure 1

Figure 2. Non-contact AFM topographic image on the HNPR, 5 µm x 5 µm; together with the height profiles, along the white line in the image.

A Q-DLTS and transport study of graphene quantum dots in insulated matrix I.V.Antonova, N.A.Nebogatikova, V.Ya. Prinz A.V.Rzhanov Institute of Semiconductor Physics SB RAS, 630090, Lavrent’ev Avenue 13, Novosibirsk, Russia antonova@isp.nsc.ru Deep level transient spectroscopy (DLTS) is a powerful method for study of a thermal activated emission of charge carriers from localized states in 2D films and multilayered structures. The widespread used conventional capacitance modification of the deep level transient spectroscopy technique (C-DLTS) was developed for uniform semiconductor structures and not for structures involving dielectric layers. In contrast, the Q-DLTS technique, which measures the charge (Q) rather than the capacitance, enables the characterization of traps (quantum-confinement levels) in complicated structures with insulated layers and quantum wells or quantum dots (QD) [1]. The latter is possible because the charge in measured structure shows considerable variations as traps in different layers of the structure undergo recharging while the capacitance is determined by the capacitance of dielectric layers. Q-DLTS in combination with temperature dependence of conductivity in the temperature range 80-300 K were used to characterize a 2D system with few-layer graphene QDs (thickness ~1 nm) imbedded in an insulating matrix of fluorinated graphene. It was found that a few-minute treatment of graphene or few-layer graphene (FLG) in an HF aqueous solution led to strong changes in the structural and electrical properties of graphene samples involving a 11 step-like increase in their resistivity (up to 10 Ohm/□) [2]. Fluorination of FLG is suggested to occur during treatment of samples in HF aqueous solution. The process first proceeds at grain boundaries, and the strong increase in resistivity can be attributed to the formation of an insulating network blocking the conductivity in the graphene layer. The observed transformation of the network sample surface morphology into a nanoswell relief during longer HF treatments due to a self-organized corrugated process suggests the presence of graphene conductive islands (quantum dots) in the insulating matrix of functionalized graphene after the step-like increase of structure resistivity. The quantum dot size can be estimated as a half of nanoswell period. In this case QD sizes have to be equal to ~50-70 nm or smaller. The possibility of graphene quantum dot formation in a fluorinated graphene matrix was demonstrated in the theoretical study by Ribas et al. [3]. Formation of graphene quantum dots was also predicted for a corrugated graphene surface due to huge changes of the strain at nanometer scale [4]. Q-DLTS spectra exhibited one or two peaks in the temperature range 200 - 300 K (Fig.1a) with thermal activation energies E1 = 0.26 – 0.31 eV and E2 = 0.14 – 0.16 eV extracted from the Arrhenius plots (Fig.1b) (here, the energy values are given with statistical range of different measurements). The QDLTS measurements were performed by varying the time window m while keeping the temperature unchanged. Here, the time window is m = (t2 - t1)/ln(t2/t1), where t1 and t2 are the times at which the QDLTS signal (due to the relaxation of the dielectric-trapped charge Q = Q(t2) – Q(t1)) was measured at the end of the filling pulse. Thus, the Q-DLTS measurements allowed us to extract one more important parameter, the time constant of carrier emission (electrons in our case) from observed levels. The carrier emission time for both levels was found to range from 0.5 to 35 ms in the temperature interval 200 – 300 K (see Fig.1a). Reference samples treated in HF aqueous solution for shorter times (before the increase of resistivity) displayed no peaks in their Q-DLTS spectra. Current-voltage characteristics for different temperatures are shown in Fig. 2a. The considerable hysteresis was observed in the current values measured during different directions of voltage sweep. The voltages at which the current step was observed were almost identical (~7 - 8 V) in the temperature range 150 – 300 K. Arrhenius plots of the current values in the saturation region are shown in Fig. 2b. The activation energies extracted from those plots are E1* = 0.33 – 0.37 eV and E2* = 0.10 – 0.14 eV. Interestingly, roughly identical values of activation energies were obtained from Q-GLTS measurements and the temperature dependence of conductivity. It can be estimated from [5] that band gap 0.10 eV corresponds to QD size ~40 nm. This size of QD is good correlated with the expected QD size determined from period of the corrugated surface. Interpretation of these activation energies and comparison of transport properties of investigated graphene layers with Q-DLTS results are discussed in the report. Acknowledgements This study was supported by the Russian Foundation for Basic Research (Grants Nos. 11-02-00722 and 12-02-01275) and the Ministry of Science and Education of the Russian Federation, project N 8028 References

[1]. I.V. Antonova, E.P. Neustroev, S.A. Smagulova, V.A. Jedrzejewski, I. Balberg, J. Appl. Phys., 106 (2009) 064306; I.V.Antonova, D.V.Nikolaev, O.V. Naumova, V.P. Popov, Electrochem&Solid State Lett. 7 (2004) F21. [2]. N. A. Nebogatikova, I. V. Antonova, V. A. Volodin, V. Ya. Prinz, Physica E, in press. [3]. M. A. Ribas, A. K. Singh, P. B. Sorokin, B. I. Yakobson, Nano. Res., 4 (2011) 143. [4]. R. M. Taziev, V. Ya. Prinz, Nanotechnology, 22 (2011) 305705. [5]. J.GĂźttinger, F.Molitor, C.Stampfer, S.Schnez, A.Jacobsen, S.DrĂśscher, T.Ihn, K.Ensslin, Rep.Prog.Phys., 75 (2012) 126502. Figures -2 -3

270 K

-4

200 K

-5

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Q, C

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-1

step 5 K Uo = 3 V U=6V

T ), (s K )

-1

ln(

10 0

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-7 -8

log( m, s)

-1

10 /T, K

Fig.1. (a) Q-DLTS spectra for a sample of thickness ~1 nm treated in HF aqueous solution for 7 min. (b) Arrhenius plots extracted from repeated Q-DLTS measurements made on the same sample. 0

300 K

280 K

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I, A

Current, A

5 Voltage, V

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Fig.2. (a) Current-voltage characteristics measured with two directions of voltage sweep at different temperatures (b) Saturated current as a function of the reverse temperature.

Second-Harmonic Generation: a powerful technique for the Visualization of Graphene a

Maarten Vanbel , Stefaan Vandendriessche , Thierry Verbiest Rik Paesen , Marcel Ameloot , c, c a,d a, c Alexander Klekachev Cedric Huyghebaert , Stefan De Gendt , Inge Asselberghs a

Department of Chemistry, Celestijnenlaan 200D, B3001 Leuven, Belgium; BIOMED, University c Hasselt and transnational University Limburg, Agoralaan Building C, 3590 Diepenbeek, Belgium; Imec, d Kapeldreef 75, 3001 Leuven, Belgium; Department of Chemistry, Celestijnenlaan 200F, B3001 Leuven, Belgium maarten.vanbel@chem.kuleuven.be inge.asselberghs@imec.be Introduction Although the first theoretical report on graphene was already published in the 1947 by Wallace [1] the research was boosted by the first observation of an isolated graphene flake by Novoselov and Geim in 2004 [2-3]. Initially, many fascinating physical properties were discovered, such as the extremely high electron and hole mobilities, its intrinsic mechanical strength, the optical transparency and good thermal conductivity, to name a few. Also due to the further development of techniques to synthetize large graphene sheets, nowadays, graphene is a well investigated material in a variety of fields among which are FETs, large scale electronics, flexible electronics, sensors, photonics, energy storage, spintronics ... However, in this work we will further explore the potential of second-harmonic generation (SHG) [4-5-6] imaging as an alternative technique to visualize and characterize graphene structures. The intrinsic surface sensitivity can play an important role in order to characterize the interfaces between stacked layers deposited onto graphene. At this stage, we will benchmark the pristine graphene properties probed by SHG and link them with the widely applied Raman spectroscopy. Additionally, two-photon fluorescence (2PF) images, created by a non-equilibrium electron-hole plasma induced by the ultrashort laserpulses, are recorded [7]. Finally, it is our goal to compare these results with the results obtained from synthetic obtained graphene samples focussing on CVD graphene samples grown on Cu. Experimental techniques The exfoliated graphene samples [3] were prepared via standard micromechanical cleavage from graphite and are deposited onto 170 Âľm thick microscope cover slips. Prior to graphene deposition these glass slides are modified with location markers using standard photolithography. The graphene flakes were firstly screened by optical microscopy. Both the number of layers and the quality of the pristine sample is confirmed by Raman spectroscopy. [8]. A sheet of single layer CVD graphene is transferred from a Cu foil, as obtained from Prof B.J. Cho, KAIST, Korea, grown using their standard techniques as reported in [9-10]. The SHG and 2PF data were recorded by a commercial Zeiss multiphoton microscope (Zeiss LSM 510 META). The fundamental wavelength was 810 nm and the images were recorded with a 10x objective. Results and discussion The optical images of the probed samples are represented in figure 1a and 2a, of respectively exfoliated and CVD graphene. By Raman spectroscopy, the number of layers is determined, and in case of the CVD graphene samples the quality of the graphene confirmed. However, in case of the CVD sample we do not have the exact optical image available, but an image of a similar sample is used. Both the SHG and 2PF data are recorded in a region of a uniform sample domain. The inset of the figures shows the images of the SHG (Fig. 1(b) and 2(b)) and 2PF (Fig. 1(c)-2(c)) response. In case of exfoliated graphene, the gradient of intensity overlaps well with the optical image taken, and the signal intensity can additively be linked to the number of graphene layers. In case of the CVD sample area, uniform images are recorded for both SHG and 2PF. The little bright dot in the image can by due to the presence of polymer residuals from the transfer process. Moreover, due to the very high laser powers used, the pristine nature of the graphene flakes cannot be maintained for a long time. However, a clear difference is noted in the sustainability of the graphene flakes towards the illumination time, in which the CVD samples clearly survive longer the imaging process. Conclusions In this work we report on the combined SHG and 2PF imaging of the graphene samples, and compare with the results obtained via optical and Raman spectroscopy. We compare the properties of both

exfoliated and CVD graphene, and observe a distinct difference in the laser sustainability for the two graphene types.

Figure 1: 1(a) optical image of the probed graphene flake, the square demarcates the probed area. 1(b) SHG image 1(c) 2PF image of the same area.

Figure 2: 2(a) optical image of a similar transferred CVD sheet, the line indicates a similar uniform area that is probed. 2(b) SHG image 2(c) 2PF image of the same area as the SHG image is taken. Acknowledgements Authors would like to acknowledge Prof B.J. Cho (KAIST, Korea) for the proving us with CVD on Cu wafers. Thanks to Frederik Drieskens for assistance with sample preparation and characterization. References [1] Wallace, P. R., Physical Review 71, (1947) 622â&#x20AC;&#x201C;634. [2] K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, Proceedings of the National Academy of Sciences of the United States of America, (The National Academy of Sciences of the USA, Washington Dc (2005) 10451â&#x20AC;&#x201C;10453. [3] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science, 306, (2004) 5696. [4] J. Litwin, J. Sipe, and H. M. van Driel, Phys. Rev. B 31 (1985) 5543. [5] A.V. Klekachev, I. Asselberghs, C. Huyghebaert, M. Vanbel, M.A. Van der Veen, A. L. Stesmans, M.H. Heyns, S. De Gendt, T. Verbiest, I.Proc. SPIE 8474, Optical Processes in Organic Materials and Nanostructures, 847405 (2012). [6] T. Verbiest, K. Clays, and V. Rodriguez, Second-order Nonlinear Optical Characterization Techniques (CRC Press, 2009) [7] W.-T. Liu, S. W. Wu, P. J. Schuck, M. Salmeron, Y. R. Shen, and F. Wang, Phys Rev B 82, (2010) 081408(R). [8] A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, Physical Review Letters, 97(18) 187401 (2006). [9] J. K. Park, S. M. Song, J. H. Mun , B.J. Cho, Nano Letters, 11(12), 5383-5386 (2011). [10] T. Yoon, W. C. Shin, T. Y. Kim J.H. Mun, T.S. Kim, B.J. Cho, Nano Letters, 12(3), 1448-1452 (2012).

Behavior of different types of graphene nanoribbons with water molecules 1

I.G. Ayala , R. Pis-Diez and N.A. Cordero

Physics Department, University of Burgos, Spain CeQuinor, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Argentina isagomez@ubu.es

Abstract Graphene is the ideal candidate for the manufacture of next generation electronic devices because of its very high carrier mobility and quantum-hall effect. However, the absence of band-gap limits its usage for some purposes, like digital switching, where a high value of the on-off current ratio is an essential requirement. Fortunately, this limitation can be overcome by inducing quantum confinement and edge effects as in the case of narrow width graphene nanoribbons (GNRs) with controllable widths and smooth edges [1]. Therefore producing high quality GNRs at low cost on a large scale and in a reproducible manner is a very interesting target nowadays. Moreover, another issue of concern is that the synthesis of graphene by conventional methods involves the use of toxic chemicals and these methods usually result in the generation of hazardous waste and poisonous gases [2]. Therefore, there is a need to develop green methods to produce graphene by following environmentally friendly approaches. One process that involves such a material as water can be a good candidate for this aim. We have used a Tight-Binding code (DFTB+) to analyze the interaction of water molecules with the edges of bilayer graphene nanoribbons changing the water concentration and the molecules orientation, the nanoribbon width and the kind of edge, zig-zag or armchair. Besides, we have studied the difference between using pure carbon nanoribbons and nanoribbons with their dangling bonds passivated with hydrogen atoms.

References [1] T.S. Li, Y.C. Huang, M.F. Lin &S.C. Chang, Philosophical Magazine, Conductance of bilayer grapheme nanoribbons with different widths (2010) 3177. [2] K. Samba Sivudu and Yashwant Mahajan, Centre for Knowledge Management of Nanoscience and Technology (CKMNT), Mass production of high quality graphene: An analysis of worldwide patents (2012) http://www.nanowerk.com/ Figures

High coverage Width 3

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22,1 Å

Oral Spin-Strain Phase Diagram of Defective Graphene E.J.G. Santos1, S.Riikonen2, D. SĂĄnchez-Portal1, and A. Ayuela1 1 Centro de FĂsica de Materiales CFM-MPC, CSIC-UPV/EHU, and Donostia International Physics Center (DIPC), San SebastiĂĄn, Spain 2 Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, Finland swxayfea@sw.ehu.es

Graphene can sustain elastic deformations as large as 20 %, and it is typically under a strain of several percent when deposited on surfaces [1]. However, there are not many experimental conditions under which layers of graphene remain strictly planar. Strong rippling of this kind can be induced by adsorbates [2] and defects [3] such as that recently discovered for OH impurities [4]. Understanding the interaction between defect-induced deformations and ripples is one of the main challenges now faced in the study of the electronic structure of graphene. By theoretical calculations, it has been shown that substitutional doping [5] and the presence of defects [6] in graphene, and in graphitic materials in general, produce magnetism that is of interest in the potential use of these materials in spintronics. In experiments, the irradiation [6] and ion bombardment [7] of carbon-based materials create vacancies which indeed are linked with magnetic signals. The vacancies [8] and edges [9] present in graphene layers have been the focus of detailed theoretical studies. Although there have been both studies of the changes in the electronic structure induced by rippling and studies of defects in graphene, to our knowledge there have been no studies of the magnetism of rippled graphene. Such a study is of particular interest when considered alongside the effect of strain on the structural and electronic properties of graphene.

Using calculations on defective graphene from first principles, we herein consider the dependence of the properties of the monovacancy of graphene under isotropic strain, with a particular focus on spin moments [10]. At zero strain, the vacancy shows a spin moment of 1.5 Bohr magnetons that increases to 2 Bohr magnetons when the graphene is in tension. The changes are more dramatic under compression, in that the vacancy becomes non-magnetic when graphene is compressed more than 2 %. This transition is linked to changes in the atomic structure that occurs around vacancies, and is associated with the formation of ripples. For compressions slightly greater than 3 %, this rippling leads to the formation of a heavily reconstructed vacancy structure that consists of two deformed hexagons and pentagons. Our results suggest that any defect-induced magnetism that occurs in graphene can be controlled by applying a strain, or some other mechanical deformations.

Oral References [1] M. Huang et al, Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 7304. [2] H. C. Schniepp et al., ACS Nano 2008, 2, 2577. [3] U. Bangert et al., Phys. Stat. Sol. A, 2009, 206, 1117. [4] R. C. Thompson-Flagg, M. J. B. Moura and M. Marder, Eur. Phys. Lett. 2009, 85, 46002; D.W. Boukhalov and M. I. Katsnelson, J. Am. Chem Soc. 2008, 130, 10697; C. Gรณmez-Navarro, J. et al Nanoletters 2010, 10, 1144. [5] Y.H. Lee, S.G. Kim, and D. Tomรกnek, Phys. Rev. Lett. 1997, 78, 2393; E. J. G. Santos, A. Ayuela, et al Phys. Rev. B, 2008, 78, 195420; E. J. G. Santos, D. Sanchez-Portal, and A. Ayuela, Phys. Rev. B 2010, 81, 125433; E. J. G. Santos, A. Ayuela and D. Sรกnchez-Portal, New J. Phys. 2010, 12, 053012. [6] P.O. Lehtinen, A.S. Foster, A. Ayuela, A. Krasheninnikov, K. Nordlund, and R.M. Nieminen, Phys. Rev. Lett. 2003, 91, 017202; Y. Ma, P. O. Lehtinen, A. S. Foster, and R. M. Nieminen, New J. Phys. 2004, 6, 68. [7] P. Esquinazi et al, Phys. Rev. Lett. 2003, 91, 227201; H. Ohldag et al, Phys. Rev. Lett. 2007, 98, 187204; A. V. Krasheninnikov and F. Banhart, Nat. Mater. 2007, 6, 723; C. Gomez-Navarro et al , Nature Materials 2005, 4, 534. [7] M. M. Ugeda et al, Phys. Rev. Lett. 2010, 104, 096804. [8] V. M. Pereira et al, Phys. Rev. Lett. 2006, 96, 036801; O. V. Yazyev, Phys. Rev. Lett. 1998, 101, 037203. [9] M. Fujita et al, Phys. Soc. Jap. 1996, 65, 1920; T. Enoki, Y. Kobayashi, and K. I. Fukui, Int. Rev. Phys. Chem. 2007, 26, 609. [10] E. J. G. Santos, S. Riikonen, D. Sรกnchez-Portal, and A. Ayuela, J. Phys. Chem. C, 2012, 116, 7602.

Figures

Figure caption: (A) Graphene with vacancies under an isotropic compression of 1.2%. As an example we use the 10x10 unit cell (highlighted). The inset shows the geometry of the vacancies and the atomic labels. The local bending at the vacancies is described by the angle theta between the pentagon and the plane defined by three C atoms around the vacancy, i.e. two equivalent atoms labeled 1 and a third atom labeled 2. Note the rippling of graphene sheets with vacancies at the saddle points. (B) A different vacancy structure for a compression slightly under 3%. It has two distorted hexagons and pentagons, while the central C atom shows strong sp_3 hybridization.

The Interface between Polyaromatic Hydrocarbons (PAH’s) and Graphene: Insight from Molecular Simulations. Elias Gebremedhn1, Liping Chen1, Andrea Minoia1, Andrea Schlierf2, Vincenzo Palermo2, David Beljonne1 1

Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc, 20, B-7000 Mons, Belgium. ISOF – National Research Council, Area della Ricerca di Bologna, Via P. Gobetti 101 - 40129 Bologna, Bologna, Italy.

EliasGebremedhn.AZENE@umons.ac.be

Among other graphene production methods, liquid phase exfoliation of graphene, in particular, is pointed out interesting [1,2], due to its ease, scalability and versatility for a wide range of applications, such as inkjet printing [3,4], aerogels [5,6], conductive thin films [7,8], and solar cells [9,10]. This method involves the chemical wet dispersion of graphite followed by ultrasonication in water [11,12] or organic solvents [12,13]. Solvents which minimize the interfacial tension with graphene flakes are the most suitable ones for high yield and stability of the graphene suspension [11]. For water, the “natural solvent”, with too high surface tension (γ ~ 72 mN/m [14]), surfactants [1,2,11] can be used to stabilize the suspension. Some study [2], for example, showed the aqueous exfoliation of graphene, using amphiphilic molecules, with concentrations up to 1.5 mg/mL. The usual amphiphils used consist of Polyaromatic hydrocarbon (PAH) cores with functionalized hydrophilic side chains such as alkyl carboxylates [1] or oligoethers [2]. It is believed that the hydrophobic aromatic cores interact with both sides of the graphene surface via non-covalent interaction, while the hydrophilic side chains interact with the solvent medium [1], hence, leading to the stable dispersion of the graphene sheets in the aqueous solution. Understanding the PAH-graphene interface and the exact mechanism of the exfoliation process is very helpful for designing surfactants with enhanced effects. In this work, the interface between graphene and pyrene compounds with systematic pattern of functionalization was studied, theoretically, using force field methods. We have compared the relative conformation, in ethanol and vacuo, of pyrene compounds with long-chain polar side-groups on graphene surface. We show that in vacuo, the functional side groups interact with the surface to form a mixture of conformations, while in ethanol they are fully desorbed from the surface and hang in the solvent medium. We have also studied the adsorption mechanism of pyrene-based pH-indicator dyes (pyrene sulfonates), on the surface of graphene, in aqueous medium. When the pyrene core is highly functionalized with sulfonate, local minima were observed on the adsorption free energy profiles, which correspond to a metastable conformation in which the molecules are in an edge-on interaction with the surface. In addition, the more the number of functional groups, the more the pyrene core is engulfed and becomes less accessible. This reduces the attractive dispersive interaction of the core with the surface and the hydrophobicity of the molecule, resulting in lower adsorption free energy. But we show that this situation is also dependent on the pattern of functionalization; in the case when two corners of the core bear sulfonates, higer dipole moment and adsorption free energy were found than for the case only one corner is functionalized. Our results demonstrate the role of polar groups vs. aromatic cores in suspension stabilization and how the extent and pattern of functionalization affects the affinity of polyaromatic hydrocarbons (PAH’s) to wards graphene.

References [1] A. Xiaohong, S. Trevor, S. Rakesh, W. Christopher, M. L. Kim, W. Morris, K. N. Saroj, T. Saikat, K. Swastik, Nano Lett. 10 (2010) 4295. [2] L. Dong-Woo, K. Taehoon, L. Myongsoo, Chem. Commun. 47 (2011) 8259. [3] L. Huang, Y. Huang, J. Liang, X. Wan, Y. Chen, Nano Res. 4 (2011) 675. [4] K.-Y. Shin, J.-Y. Hong, J. Jang, Chem. Commun. 47 (2011) 8527. [5] M. A. Worsley, P. J. Pauzauskie, S. O. Kucheyev, J. M. Zaug, A. V. Hamza, J. H., Jr. Satcher, T. F. Baumann, Acta Mater. 57 (2009) 5131. [6] M. A. Worsley, P. J. Pauzauskie, T. Y. Olson, J. Biener, J. H., Jr. Satcher, T. F. Baumann, J. Am. Chem. Soc. 132 (2010) 14067. [7] G. Eda, M. Chhowalla, Nano Lett. 9 (2009) 814. [8] D. Li, M. B. Mueller, S. Gilje, R. B. Kaner, G. G.Wallace, Nat. Nanotechnol. 3 (2008) 101. [9] Q. Liu, Z. Liu, X. Zhong, L. Yang, N. Zhang, G. Pan, S. Yin, Y. Chen, J. Wei, Adv. Funct. Mater. 19 (2009) 894. [10] Z. Liu, D. He, Y. Wang, H. Wu, J. Wang, H. Wang, Sol. Energy Mater. Sol. Cells. 94 (2010) 2148. [11] M. Lotya, Y. Hernandez, PJ. King, RJ. Smith, V. Nicolosi, L. S. Karlsson, F. M. Blighe, S. De, Z. Wang, I. T. McGovern, G. S. Duesberg, J. N. Coleman. J. Am. Chem. Soc. 131 (2009) 3611. [12] Y. Hernandez, et al., Nature Nanotech. 3 (2008) 563. [13] T. Hasan, F. Terrisi, Z. Sun, D. Popa, V. Nicolsi, G. Privitera, F. Bonaccorso, A. C. Ferrari, Phys. Status Solidi. B. 247 (2010) 2953. [14] J. Israelachvili, Intermolecular and surface force. Boston: Academic Press third edition (2011).

Figures

PBA

PSSA

PS1

PS2

PS4

PS3

Figure 1: Structures of studied molecules: Pyrene-1-butyric acid (PBA), Pyrene-1-propylsulfonic acid (PPSA), 1-pyrenesulfonate (PS1), 6,8-dihydroxy-1,3-pyrenedisulfonate (PS2), 8-hydroxypyrene-1,3,6-trisulfonate (PS3) and pyrene-1,3,6,8-tetrasulfonate (PS4).

PBA

PS4 PS3 PS1 PS2

PSSA

Figure 2: Snapshots of simulated structures of PSA and PSSA on graphene surface in ethanol.

Figure 3: Simulated potential of mean force (PMF) curves of the adsorption process of PSx ions on graphene and snapshots of typical conformations of PS4 as it approachs to the surface of graphene, corresponding to points on the PMF curve-as indicated by letters.

Improved Nanoscale Electrical Characterization with Graphene-Coated Atomic Force Microscope Tips 1

1,2

A. Bayerl , M. Lanza , T. Gao , M. Porti , M. Nafria , G. Y. Jing , Y. F. Zhang , 3 2 Z. F. Liu , and H. L. Duan 1

Universitat Autònoma de Barcelona, Edifici Q - Campus UAB, Bellaterra, Spain Dept. of Mechanics and Aerospace Engineering, College of Engineering, Beijing, China 3 Academy for Advanced Interdisciplinary Studies,Beijing, China 4 Physics Department, Northwest University, Xi’an, China Email: Albin.Bayerl@uab.es

Abstract Conductive atomic force microscope (CAFM) has been shown to be a powerful technique to characterize the electrical properties of both conductive and thin insulating materials at areas as small 2 as 10-100nm [1–3]. One of the main challenges associated with this technique is the poor reliability of the conductive tips. Traditionally, metal-varnished silicon tips are commonly utilized in these kinds of applications [4]. However, due to the low stability of the metallic varnish, these tips can wear out very fast when measuring high currents and/or because of intense tip–sample frictions. In this work, we will show how the intrinsic properties of a commercially available CAFM tip can be modified if it is coated with a sheet of CVD-grown graphene. A graphene single layer (GSL) sheet grown on copper foils by CVD [5] was transferred onto different PtIr varnished CAFM tips. This prototype tip has been used to perform nanoscale electrical characterization of both conductive and thin insulating materials. The performance of graphene coated tips has been first assessed from the analysis of HfO 2/SiO2/Si stacks. I-V curves at different positions have been measured with as-received (Figure 1a) and graphene-coated (Figure 1b) tips. As it can be observed, the I–V curves measured with the as-received tips show a decrease of the measured current with the number of recorded I-V curves. Since this sample is very homogeneous [6], Figure 1a shows that the as-received tip loses its conductivity by 8 2 wearing out due to very high driving current densities (J ≈ 10 A/cm when it reaches the current compliance) [7]. On the other hand, the graphene-coated tip supports several measurements (Figure 1b), which entirely reached the current compliance level approximately at the same onset voltage, without appreciable wearing. In a second measurement series, the resistance of graphene-coated tips to the high frictions is analyzed from current maps. Figure 2 shows AFM topography and current maps obtained with an asreceived PtIr-coated tip (a-d) and a graphene-coated tip (e-h) on a as-grown GSL on Cu stacks. The current maps are recorded by biasing the samples at –0.1V (e. g., substrate injection of electrons) and keeping them under the high vacuum ambient conditions. From all the topographic images (a, b, e, f) the typical stepped copper surface (covered by GSL) can be observed [8]. On the other hand, the current maps (c, d, g, h) clearly overlap with the topographic images, showing current everywhere except at the step edges, probably due to the loss of contact between the tip and the sample at those locations. Comparing the images obtained with the two kinds of tips, one can easily see that as-received 2 tips allow scanning an area of 6μm (6 scans of 1μm × 1μm) before the tip loses its conduction (using a contact force of 5nN). The tip wearing can be observed from the lowering of the current scale in Figure 2d. However, the maps collected using graphene-coated tips suggest that these tips preserve their 2 intrinsic properties, since no loss of conductivity is observed even after scanning an area of 903µm and even using contact forces up to 50nN. Therefore, it is successfully demonstrated that conductive tips for the CAFM, fabricated by coating commercially available metal-varnished tips with a sheet of GSL following already established standard transfer process steps, show ultra-high performance. Graphene-coated tips are much more resistant to both high currents and frictions than commercially available metal-varnished CAFM tips, leading to much longer lifetimes and preventing false imaging due to tip–sample interaction. In total, seven different graphene-coated tips have been fabricated and characterized using the procedures here described, and successful results for all of them were observed. The novel devices can be interesting not only for reducing tip replacement costs, but also for those applications that require high stability and low tip–sample interaction.

References [1] J. Petry, W. Vandervorst, X. Blasco, Microelectronic Engineering, vol. 72 (2004), pp. 174-179. [2] H. J. Uppal, I. Z. Mitrovic, S. Hall, B. Hamilton, V. Markevich, A. R. Peaker, J. Vac. Sci. Tech. B, vol. 27 (2009), pp. 443-447. [3] A. Paskaleva, V. Yanev, M. Rommel, M. Lemberger, A. J. Bauer, „Improved insight in charge trapping of high-k YrO2/SiO2 stacks by use of tunneling atomic force microscopy“, J. Appl. Phys., vol. 84 (2008), pp. 024108-024108-7. [4] W. Frammelsberger, G. Benstetter, J. Kiely, R. Stamp, Appl. Surf. Sci., vol. 253 (2007), pp. 3615-3626. [5] Y. Zhang, T. Gao, Y. Gao, S. Xie, Q. Ji, K. Yan, H. Peng, Z. Liu, ACS Nano, vol. 5 (2011), pp. 4014-4022. [6] L. Aguilera, W. Polspoel, A. Volodin, C. Van Haesendonck, M. Porti, W. Vandervorst, M. Nafria, X. Aymerich, Proc. of Int. Reliab. Phys. Symp. 2009, pp. 657-658. [7] X. Blasco, M. Nafria, X. Aymerich, Rev. Sci. Instrum., vol. 76 (2005), pp. 016105 - 016105-3. [8] G. H. Han , F. Günes , J. J. Bae , E. S. Kim , S. J. Chae , H. J. Shin , J. Y. Choi , D. Pribat , Y. H. Lee , Nano Lett., vol. 11 (2011), pp. 4144–4148. Figures

Current Current

Topography Topography

Figure 1. I–V curves measured on the bare surface of an HfO2/SiO2 stack using a) a PtIr coated tip, and st rd th th th th b) a graphene-coated tip. Each graph shows the 1 , 3 , 5 , 10 , 15 , and 20 I–V curve measured. All I–V curves were collected at fresh different locations by applying a Ramped Voltage to the tip (sample substrate grounded).

Figure 2 . Topographic (a,b,e,f) and current (c,d,g,h) maps measured on the surface of GSL/Cu stacks recorded with as-received (a–d) and graphene-coated tips (e–h). All scans are 1µm x 1µm, and the images of each column have been collected simultaneously. The images clearly show that as-delivered tips experiment a substantial conductivity loss after 6 scans, while in contrast the graphene-coated tips keep measuring current even after have been used to scan an area 150 times larger.

Chemical reduced graphene in polymer fibers - a novel route to mobile charge storage devices? Wilhelm Steinmann, Markus Beckers, Benjamin Weise, Thomas Gries Institut für Textiltechnik, Otto-Blumenthal-Straße 1, 52074 Aachen, Germany wilhelm.steinmann@ita.rwth-aachen.de I.

Introduction

Since graphene was fabricated successfully for the first time by Geim and Novoselov, the research interests on graphen have increased enormously. The physical properties of this two-dimensional carbon variation offer multiple, potential applications in future. Due to the graphen’s high mobility and charge carrier density, graphen is qualified as potenzial material for charge storage devices and supercapacitors. At Institut für Textiltechnik (ITA), RWTH Aachen University, Germany, novel grapheme polymer supercapacitors are in the focus of research. Chemically reduced grapheme is spinned in polymer fibers at temperatures above 200 °C by using an extruder. Afterwards, the fibers can be continue processed which offers an application as fiber battery in clothing, for example. In the following abstract, the chemical fabrication of grapheme is described as well as the spinning of polymer fibers. Additionally, an overview of possible future applications is given.

II.

Fabrication of graphene

The chemical reduction of graphite oxid is a simple und cheap possiblity for fabrication of graphene. Graphite oxide is given in a suspension with destillied water and is then sonicated. As a consequence, the graphite oxide layers get exfoliated to graphene oxide. The suspension is then heated to a temperature range between 50 °C and 90 °C. High concentrated sodium or potassium hydroxide solution is given to the suspension which enables a reduction of the functional groups in graphene oxide. After that, the suspension is cooled down to room temperature and centrifugalized in order to separate the graphene from the suspension. The resulting graphene gets washed with destilled water and finally freeze-dried.

III.

Spinning into polymer fibers

By using a DSM mini extruder, the graphene layers are spinned into a polymer at temperatures above 200 °C. The forced control spinning guarantees a consistent allocation of the graphene into the polymer. Polypropylene is a promising polymer for fabrication of mobile charge devices, because the molecular structure is very linear and thus a high degree of crystallinity is reached. The high linearity enables a homogenous allocation of graphene in the polypropylene fiber. Furthermore, polypropylene is available at industrial scales and not dangerous to human and environment. Polypropylene offers a high mechanical stability which makes it applicable for applications with highest requirements.

IV.

Applications and summary

The research at Institut für Textiltechnik (ITA) pursues the goal to make polymers and polymer composites applicable for different applications at industrial standards. Beside the development of fiber reinforced plastics and other applications, the development of charge storage devices came into the focus of research at ITA. Graphene polymer composites offer themselves for aftertreatments, with the

consequence that an application as textile battery in clothing is a realistic scenario. The polymer fibers exhibit a low cross-section and a high flexibility which enables a simple bunching of the fibers. Thus, cheap and high performance charge storage devices in form of polymer fiber clusters for industrial and research applications. In summary, it can be stated that research activites on application of graphene-polymer fiber composites offers an enormous potential which has to be utilized. The versatile applications could solve existing problems in energy storage. Thus, the electrical power supply of the human population could be granted independent from weather conditions like wind or sun. The costs of energy storage systems could be reduced enormously. On the other side, textile batteries in clothing would offer many applications, for example mobile monitoring systems of bodily functions. Perilous complications like apoplectic strokes or cardiac infarction could be treated faster because the arrival time of an emergency physician would drop if the monitory system is connected to a rescue centre. These examples only show a little part of potential application fields, because a complete listing of the applications would blast this abstract. The intention is to show the enormous potential of research activities on graphene charge storage devices. It is worthwile to intensify and support this research.

References [1] Fan X et al. Advanced Materials, 2008, 20, 4490-4493. doi:10.1002

A Facile CVD Synthesis of BN Doped Graphene Using Boric Acid and Nitrogen Gas George Bepete,a Damien Voiryb, Manish Chhowalla,b Zivayi Chiguvarea and Neil J. Covillea a

DST/NRF Centre of Excellence in Strong Materials and Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, WITS 2050, Johannesburg, South Africa

Materials Science and Engineering, Rutgers University, 607 Taylor Road, Piscataway, New Jersey 08854, United States george.bepete@students.wits.ac.za

Abstract Chemical doping of graphene has been shown to be an effective way of permanently modulating the electronic properties in single layer graphene. Several theoretical studies have shown that simultaneous incorporation of B and N in graphene is thermodynamically possible yielding small planar hexagonal boron nitride (h-BN) domains in graphene and most importantly that this process achieves band gap modulation in graphene.1 The band gap opening in graphene, due to doping with BN, has been attributed to the breaking of localized symmetry. Here we demonstrate a facile, safe process for the chemical vapour deposition synthesis of BN doped graphene on copper using methane, boric acid powder and nitrogen gas, revealing that the B and the N are incorporated within the graphene structure forming a B-N-C system. We provide a simple reproducible experimental method to synthesize large area BN doped graphene which can be used to make high quality BN doped graphene for possible applications in nanoelectronic devices. Optical microscopy confirmed that continuous films were grown and XPS confirmed that both B- and N- can be substituted into the graphene structure in the form of BN to give a B-N-C system. A novel structure for the BN doped graphene is proposed. References [1] Fan, X.; Shen, Z.; Liu, A.Q.; Kuo, J. Nanoscale, 4 (2012), 4, 2157-2165. Figures

Figure 1. Graphene and BN doped graphene films grown on Cu using boric acid and nitrogen gas. (a) Optical images of pristine graphene transferred onto SiO2/Si substrate. (b) Optical image of BN doped

graphene transferred onto a SiO2/Si substrate. (c) Low-magnification TEM image of BN doped graphene on a plain Cu TEM grid showing a continuous graphene film. (d) High-magnification TEM image of BN doped graphene on a plain Cu grid showing small particles of Cu and Fe remaining after the etching process. (e) AFM image of BN doped graphene showing wrinkled graphene, and (f) AFM section analysis of BN graphene showing an average film thickness of 1.25 nm.

(a)

N1s

Intensity (a.u)

399.8 eV

404

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191.0 eV

188.7 eV 186.7 eV

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C1 s

(c)

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284.0 eV

285.1 eV

286.6 eV 289.1 eV

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286

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Binding energy (eV)

Figure 2. High resolution XPS spectras measured on BN doped graphene showing (a) the N1s core level revealing only a single peak at 399.8 eV, (b) B1s core level with three peaks at ca. 191.0, 188.7 and 186.7 eV of BN, BC3 and B4C respectively and (c) the C1s core level, obtained from BN doped graphene

Transfer printing of graphene structures onto gold contacts for transistors and photo detectors 1

Iris Bergmair , Barbara Einwögerer , Wolfgang Hackl , Thomas Fromherz , Maria Losurdo , Giovanni 3 4 5 5 6 1 Bruno , Nalin Rupesinghe , Christoph Giesen , Michael Heuken , Thomas Müller , Michael Mühlberger 1

PROFACTOR GmbH, Functional Surfaces&Nanostructures, Im Stadtgut A2, 4407 Steyr Gleink, Austria Institute of Semiconductor and Solid State Physics, Johannes Kepler University of Linz, Altenbergerstr. 69, 4040 Linz, Austria 3 Institute of Inorganic Methodologies and of Plasmas-CNR, Via Orabona 4, 70126 Bari, Italy 4 Aixtron Ltd, Buckingway Business Park, Anderson Road, Swavesey Cambridge CB24 4FQ, United Kingdom 5 Aixtron SE, Kaiserstr. 98, 52134 Herzogenrath, Aachen, Germany 6 Institute of Photonics, Vienna University of Technology, Gußhausstraße 25-29, 1040 Vienna, Austria

iris.bergmair@profactor.at Abstract (Arial 10) We present an innovative technique to realize patterned graphene on gold contacts for the realization of transistors and photo detectors. In previous work we described how to realize micro and nanostructures on graphene using UVNanoimprint Lithography (NIL) [1]. NIL is a powerful technique to realize micro and nanostructures on large area within seconds by using a nano patterned stamp [2]. Nevertheless the structuring of already transferred graphene to other substrates has high challenges like the insufficient adhesion of graphene and defects which occurred during transfer. Further for applications like transistors and photo detectors gold contacts have to be realized on graphene. For photolithography this is a simple task realizing µm sized structures, but going down to nanostructured Au contacts it might become challenging. Of course e-beam writing can be used to define nanopatterned Au contats on graphene this method is not scalable for mass production due to the long writing time on large area. Therefore we have chosen NIL to demonstrate the scalability for mass production of our process. One drawback is the fact that NIL creates a residual layer which has to be etched away before deposition of Au. Doing this process on graphene does not work since etching residual layer also removes the graphene and no contact to Au can be formed. Therefore we used a transfer print process for the gold contacts. We first fabricated the Au contacts on a substrate and transfer it afterwards into an Ormocere® (working like a glue) to another substrate. This method was described in ref. [3] for nanostructured gratings. The gold structures are imprinted in liquid Ormocomp and hardened with UV-light. Afterwards the Au sticks to the Ormocomp and can be peeled off from the substrate (Figure 1). As a next step graphene is patterned on CVD graphene grown samples on Cu from Aixtron (Figure 2). Due to the fact that the graphene adheres quite well to the Cu a patterning of the graphene is quite easy in comparison to the trials of patterning already transferred graphene on SiO 2. Here the adhesion is often insufficient and routes have to be found to increase the graphene adhesion to the substrate before a structuring can be successful. In our case the patterns are created on the Cu substrate by photolithography (Figure 3, 4) and transferred afterwards. The transfer method is done using a thermal tape. This tape is pressed on the structured graphene on the Cu (Figure 5,6) and the Cu is removed by a succeeding wet chemical etching step. After etching of the Cu the structured graphene remains on the tape and is pressed onto a SiO2 wafer with 90 nm (Figure 7). In Figure 8 such graphene patterns on SiO2 are shown. Some residues of the transfer tape can be found on the graphene, but can be avoided in the future by using and testing different tapes. The aim is to combine the transfer process of Au structures in Ormocomp and transferring structured graphene on top for fabrication of graphene transistors and photo detectors. The contact of graphene to gold has to be examined and some annealing/cleaning methods applied. Raman measurements, optical as well as transport measurements will be performed. The authors acknowledge funding from the NILgraphene project within the NILaustria Cluster and the European Community’s 7th Framework Programme under grant agreement no. 314578 MEM4WIN (www.mem4win.org). References [1] I. Bergmair et al. Nanotechnology 23, 335301 (2012) [2] Chou, S.Y. et al., Appl. Phys. Lett. 67 (21) 3114 (1995) [3] I. Bergmair et al. Nanotechnology, 22, 325301 (2011)

(a) v

(b)

(c)

Figure 1: Shows the schematic transfer process for Au contacts into Ormocomp a) Au structures on Si substrate fabricated by NIL, imprinting of Au structures into Ormocomp and UV curing of material c) Peeling off Au structures d) shows a micrograph image of one Au contact fabricated by NIL.

(d)

(e)

50 µm (f)

Figure 3: Microstructures patterend on quarter of 4” AIX graphene/Cu wafer

Figure 4: Microscope image of microstructures on 4” AIX graphene/Cu wafer

(g)

(h)

Figure 2: Shows the schematic structuring process of graphene on Cu and transfer to thermal release tape. d) photolithography on graphene on Cu wafer, e) etching of graphene, f) removing photoresist, g) adhesion of thermal release tape on graphene h) etching of Cu

Figure 7: i) j) schematic drawing of structured graphene transferring on i) adhesion of graphene on tape onto 90 nm SiO2 j) thermal release of tape

10 µm Figure 5: AFM topography image of structured graphene on Cu wafer

200 µm

10 µm Figure 6: AFM phase image of structured graphene on Cu wafer

10 µm

Figure 8: microscope image of transferred patterned graphene dots

Contribution (Poster)

Growth parameters and functionalization of epitaxial graphene on silicon carbide 3

G. Bidan , M. Baraket , B. Kumar , F. Duclairoir , L. Dubois , D. Rouchon , M. Paillet , J.-R. Hutzinger , 4 4 2 4 1 2 A. Tiberj , A. Zahab , G. Lapertot , J.-L. Sauvajol , P. Maldivi , F. Lefloch 1

Laboratoire de Chimie Inorganique et Biologique (UMR-E 3 CEA -UJF), CEA/INAC ; Service de 3 Physique, Statistique, Magnétisme et Supraconductivité, CEA/INAC ; INAC/DIR, CEA Grenoble, 38054, France 4 Laboratoire Charles Coulomb, UM2, Montpellier, 34095, France 5 CEA-Leti-MINATEC, Campus/DTSI/SCMC, Grenoble, 38402, France gerard.bidan@cea.fr

Current trends indicate that graphene researches for applications related to electronics are split into two main routes. One route concerns mainly “organic electronics” in the broad acceptation, from transparent electrodes for photovoltaics, displays and touch screen to cheaper flexible electronics which could be introduced in the large dairy retailing markets. These technologies involve large areas of graphene sheets grown on various metal by chemical vapour deposition. On the other hand, epitaxial graphene grows on SiC can be produced as wafer-scale high quality material compatible with microelectronics technology [1]. However, at the moment the booming of this material is notably hampered by the low homogeneity of the number of layers; and the electronic roadmap mainly forecasts the use of such graphene for high-frequency logic applications [2]. Another critical step for application in electronic devices will be to open a gap. The optimization of SiC graphene growth and functionalization linked to its fundamental understanding are therefore on-going topics. The studies presented in this poster are related to the optimization of growth of graphene on the Si face of 6H-SiC [3]. A comparative study of graphene growth under vacuum or in inert media at atmospheric pressure will be discussed. The surface reconstruction occurring during a specific step of annealing in presence of hydrogen at high temperature (>1500°C) along with a more controlled growth under Ar allowed to obtain better quality graphene in terms of homogeneity, surface roughness and domain size on large areas. A combined Raman-optical system study along with an AFM analysis revealed a sample with 2/3 of monolayer and 1/3 of bilayer or more with domains in the micrometer scales (Fig. 1a). The device fabrication (Fig. 1b) and the chemical functionalization of the graphene substrate with compounds from the porphyrin families will be presented. The preliminary results on the chemical, structural and electrical properties of bare and modified graphene will be discussed.

References: [1] M. Ruan, Y. Hu, Z. Guo, R. Dong, J. Palmer, J. Hankinson, C. Berger, and W. A. de Heer, MRS Bull., 37 (2012) 1138. [2] K.S. Novoselov, V. I. Fal'ko, L. Colombo, P. R. Gellert, M. G. Schwab, K. Kim, Nature, 490, (2012) 192. [3] K. V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G. L. Kellogg, L. Ley, J. L. McChesney, T. Ohta, S. A. Reshanov, J. Röhrl, E. Rotenberg, A. K. Schmid, D. Waldmann, H. B. Weber, T. Seyller, Nat. Mater., 8, (2009) 203.

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Figure 1: a) AFM image of SiC graphene grown on Si face (annealed under H2, growth at high T°C for 30 min under Ar); b) OM image of the graphene device geometry.

Coupled MD/NEGF simulation of transport properties under straining of a graphene bridge Anders Blom, Søren Smidstrup, Kurt Stokbro QuantumWise /S, Lersø Parkallé 107, 2100 Copenhagen, Denmark anders.blom@quantumwise.com Abstract Recent years have seen a rapid progress in the application of graphene in flexible displays, which are supposed to be able to be subjected to a large mechanical deformation without affecting the fundamental operation of the device. The challenge is, however, that the active components – the transistors or light-emitting diodes – are an integrated part of the flexible substrate itself. It is therefore crucial to understand how these electronic components behave under non-ideal conditions, such as strain, temperature, or other external influence. In this work we have performed a molecular dynamics (MD) simulation of how the electronic transport properties of a graphene-like bridge between two capped carbon nanotubes (Fig. 1) change as the system is adiabatically stretched in the transport direction. The nanotubes are pulled apart slowly enough (0.1 Å/ps) that the structure has time to equilibrate adiabatically in the MD routine, which uses a Verlet integration with time step 1 fs, initialized with a room-temperature Maxwell–Boltzmann distribution of the ion velocities. The entire simulation is run for 40 ps, at which point the graphene bridge begins to break away from the nanotubes. The calculation is made possible by integrating two separate codes – and also two separate computational approaches – into a single simulation framework. The MD part is performed using a classical Tersoff potential [1] as implemented in Tremolo-X [2], whereas the electronic transport properties are evaluated using a non-equilibrium Green’s function (NEGF) approach [3], in which the electronic structure part is computed with a semi-empirical Slater–Koster method [4], as implemented in our software package Atomistix ToolKit (ATK) [5]. The entire computation is driven by a single Python script, where the NEGF calculation is inserted as a “hook” into the MD simulation that is called after a specified number of MD steps. The results show that the electronic properties are remarkably stable when the system is strained, up until the point where the graphene bridge begins to tear apart from the nanotubes. The change in the linear response current at 0.5 V bias, as the strain increases, is comparable to the variations caused by pure temperature fluctuations (without any strain applied). At the same time, those variations in the current are almost 10% (standard deviation), at an average current of 3.7 A. The presented methodology is generic and can easily be applied to any type of device structure, based on graphene or other materials, and it is possible to freely mix and match the methods used for the force evaluation in the MD part and the electronic structure calculation for the NEGF, choosing between DFT, extended Hückel, Slater-Koster tight binding as desired.

References [1] J. Tersoff, Phys. Rev. B 37, 6991 (1988). [2] Tremolo-X, Fraunhofer Institute for Algorithms and Scientific Computing SCAI (www.tremolo-x.com). [3] M. Brandbyge, J.-L. Mozos, P. Ordejón, J. Taylor, and K. Stokbro, Phys. Rev. B 65, 165401 (2002). [4] K. Stokbro, D. E. Petersen, S. Smidstrup, A. Blom, M. Ipsen and K. Kaasbjerg, Phys. Rev. B 82, 075420 (2010). [5] Atomistix ToolKit, QuantumWise A/S (www.quantumwise.com).

Figures

Figure 1: The initial (t=0 sec) device structure used for the coupled MD/NEGF simulation, consisting of two capped (5,5) carbon nanotubes joined by a 2 atom wide zigzag graphene nanoribbon. The structure shown in the figure is the result of a combined force/stress optimization which minimized the forces to 0.05 eV/Ă&#x2026; and the stress to 0.0005 3 eV/Ă&#x2026; , starting with the graphene bridge being flat.

Figure 2: Transmission spectrum, in units of the quantum conductance, evaluated at 0.1 ps interval during the MD run. Essentially all transmission spectra are identical, except for the precise position of the Fermi level (dotted). This causes a large (and growing with time) fluctuation in the conductance (the transmission at the Fermi level), but when the spectrum is integrated to obtain the linear response current, the variation more or less disappears (Fig. 3).

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Figure 3: Running average (window=20 samples) of the linear response current at 0.5 V source-drain bias as a function of the simulation time for (a) a pure MD simulation without straining the sample (time step 1 fs), and (b) the case where strain is adiabatically applied.

ppt-Level Detection of CO, NH3 and H2 Gases

Behiye BOYARBAY KANTAR *, Hidayet ÇETİN **, Enes YAYAN ** and Enise AYYILDIZ * * Faculty of Science, Department of Physics, Erciyes University, 38039 Kayseri, Turkey **Faculty of Arts and Sciences, Department of Physics, Bozok University, 66100 Yozgat, Turkey bboyarbay@erciyes.edu.tr Abstract Graphene is widely regarded as one of the most promising materials for sensor applications [1-6]. In the study, carbon monoxide (CO), ammonia (NH3) and hydrogen (H2) gas sensing behaviors of graphene field effect sensor have been investigated. The measurements are made under 3.0 x 10-5 mbar vacuum environment at room temperature. We demonstrate that a simple two terminal graphene field effect gas sensors can detect gas molecules at extremely low concentrations with dedection limits (DLs) as low as parts per trillion (ppt) at room temperature. DLs have been measured as 7.2 ppt, 2.2 ppt and 10 ppt for CO, NH3 and H2 gases, respectively.

References [1] F. Schedin, A.K. Geim, S.V. Morozov , E.W. Hill, P. Blake, M.I. Katsnelson and K.S. Novoselov, Nat. Mater., 6 (2007), 652-655. [2] J.T, Robinson, F.K. Perkins, E.S. Snow, Z.Q. Wei and P.E. Shehan, Nano Lett., 8 (2008), 31373140. [3] J. D. Fowler, M. J. Allen, V.C. Tung, Y. Yang, R.B. Kaner, B. H. Weiller, ACS Nano, 3 (2009) , 301– 306. [4] G. Lu, L. E. Ocola, J. Chen, Appl. Phys. Lett., 94 (2009), 083111 (3 pp) [5] G. Lu, L. E. Ocola, J. Chen, Nanotechnology, 20 (2009), 445502 (9 pp) [6] V. Dua, S. P. Surwade, S. Ammu, S.R. Agnihotra, S. Jain, K.E Roberts, S. Park, R.S. Ruoff, S.K. Manohar, Angew. Chem., Int. Ed. 2010, 12 (2010), 2154– 2157.

Acknowledgement This work was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK), Grant No: TBAG-108T930 and Erciyes University Research Funds, Grant No: FBD-10-3031

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Optical analysis of grain size in graphene on copper revealed by wet chemical oxidation A. Mameli, G.V. Bianco, M.M. Giangregorio, P. Capezzuto, M. Losurdo and G. Bruno* CNR-IMIP, Institute of Inorganic Methodology and Plasmas, Dept of Chemistry, Univ. Campus, Via Orabona 4, 70125 Bari, Italy *giovanni.bruno@ba.imip.cnr.it

Abstract Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows the production of large area graphene as required for its application as transparent conductive layer substituting ITO. CVD-graphene presents a polycrystalline structure that strongly influences both mechanical and electrical properties. Nowadays, it is well consolidated, among different users of graphene, that analyzing graphene grains after growth, is important for quality-control. In fact, graphene is a well ordered material and contains internal boundaries, commonly known as "grain boundaries". When graphene is grown, the carbon atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain impact others and forms interfaces where the atomic orientations differ. It has been established that the transport properties of the graphene improve as the grain size increases. Therefore, the growth conditions must be carefully controlled to obtain large grain size. Specifically, a strong correlation has been reported between grain sizes and sheet resistance in CVD1 graphene . Improving the quality of CVD-graphene requires the optimization of nucleation and growth rates 1,2 in the synthesis process, in order to provide an increase in grain average size . Therefore, diagnostic methodologies for the accurate monitoring of grain sizes and distribution directly on the growing substrate is needed. In this work we present an effective methodology for analyzing grain size and distribution in 3,4 graphene as grown on copper by optical microscopy. In contrast to transmission electron microscopy and 3,5 scanning tunneling microscopy , this optical technique is not expensive and time-consuming and, above all, effective for the diagnostic on large scale. We exploit a wet chemistry approach for the selective oxidation of the metal substrate through graphene grains making their boundaries visible by optical microscopy. Specifically, we use the Fenton's reaction as a source of hydroxyl radicals to functionalize graphene grain boundaries. This diagnostic approach has been optimized by using Raman spectroscopy to probe and confirm chemical and structural changes in graphene and the Cu substrate upon the wet treatment. We also demonstrate the feasibility of this methodology for highlighting defect sites in graphene and, hence, for probing the retention of its structural quality upon post-growth processing such as transferring on other substrate. The authors acknowledge funding from the European Community's 7th Framework Programme under grant agreement no. 314578 MEM4WIN (www.mem4win.org).

References [1] Duong, D.L. et al., Nature, 490 (2012) 235â&#x20AC;&#x201C;239 [2] Losurdo M. et al., Phys. Chem. Chem. Phys., 13 (2011) 20836-20843 [3] Qingkai Y. et al., Nature Materials, 10 (2011) pp. 443-449 [4] Rasool, H.I. et al., Nano Lett., 11 (2011) 251-256 [5] Gao L., Nano Lett., 10 (2010) 3512â&#x20AC;&#x201C;3516

Fig. 1 - Optical images of CVD-graphene on copper before and after oxidation. The grain boundaries are clearly visible after treatment, it can be seen also the growth of graphene grain across the copper grain boundaries.

Systematic Raman analysis of graphene films grown on Cu foils by ethanol Chemical Vapor Deposition: effect of synthesis temperature G.Faggio1, N.Lisi2, S. Santangelo3, R.Giorgi2, Th.Dikonimos2, F. Buonocore2, V. Morandi4, L. Ortolani4, G. Messina1 Dept. of Information Engineering, Infrastructure and Sustainable Energy, University “Mediterranea” of Reggio Calabria, Via Graziella Loc. Feo di Vito, 89122 Reggio Calabria, Italy 2 ENEA, Materials Technology Unit, Surface Technology Laboratory Casaccia Research Centre, Roma, Italy 3 Dept. of Civil Engineering, Energy, Environment and Materials, University “Mediterranea” of Reggio Calabria, Via Graziella Loc. Feo di Vito, 89122 Reggio Calabria, Italy 4 Nat. Res. Council - Inst. of Microelectronics & Microsystems, Via Gobetti 101, 40129 Bologna, Italy 1

gfaggio@unirc.it Abstract Catalytic Chemical Vapor Deposition (C-CVD) is one of the most promising methods developed for large scale synthesis of graphene [1, 2]. Typically, graphene C-CVD takes place by exposure of a transition metal substrate to a hydrocarbon gas. Inspired by the carbon nanotube growth using alcohol catalytic CVD [3-5], recently there have been a few attempts of graphene CVD growth with ethanol and other alcohol precursors. One advantage of using ethanol lies in its harmless, low cost, and easy-handling nature. In addition, other advantages could rise from a different growth kinetic due to its weakly oxidising nature [6]. As known [7, 8], Raman spectroscopy is a non-destructive and powerful technique for evaluating the structural properties of graphene providing useful information on the defects (D-band) and in-plane vibration of sp2-carbon (G-band). The second order 2D Raman band has been used as a simple and efficient way to identify the number of graphene sheets. In this work, we report on a systematic Raman analysis of graphene films grown by CVD on thin copper foil substrates using ethanol as precursor gas, with different dilution in hydrogen or argon. Aim of the work is optimizing the growth of CVD graphene films in terms of quality and number of graphene layers. For this purpose, we investigated the effect of synthesis temperature (860-1070°C) and hydrogen flow (0 or 100 sccm) by coupling Raman spectroscopy with Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) to study morphology and spatial homogeneity of the films previously transferred to SiO 2/Si substrates. Figure 1 shows Raman spectra of graphene films obtained at different hydrogen flow (ΦH2) (0 sccm in Fig. 1a, and 100 sccm in Fig. 1b) by varying reaction temperatures in the range 860–1070 °C, for a growth duration of 10 min. Multilayer graphene films are obtained only for growth temperature greater than 930°C. Regardless of ΦH2 value, the D-band intensity reduces with increasing synthesis temperature, indicating that high quality graphene films can be obtained at high temperatures. A further reduction of defect density is achieved by adding hydrogen into growth mixture (Fig. 1b). Figure 2 shows a typical SEM image of CVD graphene film grown at high temperature. References [1] X. Li, W. Cai, Jinho An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, R.S. Ruoff, Science, 324 (2009) 1312. [2] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S. Dresselhaus and J. Kong, Nano. Lett., 9 (2009) 30 [3] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett., 360 (2002) 229. [4] L.X. Zheng, M.J. O’Connell, S.K. Doorn, X.Z. Liao, Y.H. Zhao, E.A. Akhadov, M.A. Hoffbauer, B.J. Roop, Q.X. Jia, R.C. Dye, D.E. Peterson, S.M. Huang, J. Liu and Y.T. Zhu, Nature Materials, 3 (2004) 673. [5] H.E. Unalan, M. Chhowalla, Nanotechnology, 16 (2005) 2153. [6] Y. Guojun, G. Jinlong, W. Sen, Z. Dezhang, H. Suixia, Z. Zhiyuan, Carbon, 44 (2006) 1218. [7] A.C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth and A. K. Geim, Phys. Rev. Lett., 97 (2006) 187401. [8] L.M. Malard, M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, Phys. Reports, 473 (2009) 51.

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Figure2 SEM micrograph of a sample grown at high temperature by ethanol CVD

Graphene substrate selection for the optimization of biosensor performance G. Burwell† , S. Teixeira† , P.R.Kidambi‡ , S. Hofmann‡ , A. Castaing† , O.J Guy† February 11, 2013 †College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, United Kingdom ‡Department of Engineering, University of Cambridge, Cambridge CB3 0FA, United Kingdom g.burwell.436734@swansea.ac.uk

Abstract The use of graphene in healthcare biosensing applications will be a disruptive technology in the near future [1]. Graphene sensors greatly outperform existing diagnostic analytical techniques - such as enzyme-linked immunobsorbant assays (ELISAs) in terms of sensitivity and sample throughput time. Advances such as these are crucial in the early detection of disease biomarkers, which greatly enhances the like of successful treatment and patient life expectancy [2]. Diﬀerent methods of producing graphene are currently being investigated, such as chemical vapor deposition (CVD) methods on transition metal substrates [3][4], epitaxial growth on silicon carbide (SiC) by annealing at high temperature [5], and chemical exfoliation techniques [6], to name a few. These production routes all oﬀer both advantages and disadvantages when compared to one another. In this work, we fabricate sensor devices from graphene produced by mechanical exfoliation, CVD on Cu, epitaxial growth on 4H-SiC(0001), 4H-SiC(000-1), and chemical exfoliation. We compare the morphology, defect density, and chemical purity of these graphene devices using scanning electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy, respectively. The surface modification is performed by firstly terminating the graphene with -OH groups using a Fenton reaction [7], then reacted with 3-Aminopropyl-triethoxysilane (APTES) in order to obtain an amine-terminated surface [8]. The surface amine groups are then used to link the graphene to a monoclonal antibody. In order to be able to react with the surface amine groups, the carboxylic acids on the antibody are activated. However, in order to prevent the antibody from cross-linking, the majority of amine groups on the antibody are blocked using Di-tert-butyl dicarbonate. The antibody can now be reacted with the amine on the surface. The groups blocking the amines on the antibody are subsequently removed by mild acidic treatment. Raman spectroscopy, Fourier-transform IR spectroscopy, and fluorescence microscopy are used to monitor the chemical modification and attachment of the monoclonal antibody to the graphene device. Electron-beam induced defects are used to control the defect density of graphene, which are monitored using Raman spectroscopy. Defects cause a significant change in the chemical behavior of the graphene surface, creating local energy minima and maxima on the surface [9], which leads to the inhomogenous attachment of the covalently attached species. In this work, we demonstrate the sensitivity of graphene biosensor devices fabricated on a number of substrates, with the aim of producing a generic, adaptable biosensor platform that can be used with any monoclonal antibodies. We also consider other practical issues such as cost and processability of the biosensor devices.

References [1] M. Pumera, A. Ambrosi, A. Bonanni, E. L. K. Chng, and H. L. Poh, “Graphene for electrochemical sensing and biosensing,” Trac-Trends In Analytical Chemistry, vol. 29, pp. 954–965, Oct. 2010. [2] M. Pepe, R. Etzioni, Z. Feng, J. Potter, M. Thompson, M. Thornquist, M. Winget, and Y. Yasui, “Phases of biomarker development for early detection of cancer,” Journal of the National Cancer Institute, vol. 93, no. 14, pp. 1054–1061, 2001. [3] L. De Arco, Y. Zhang, A. Kumar, and C. Zhou, “Synthesis, transfer, and devices of single-and few-layer graphene by chemical vapor deposition,” Nanotechnology, IEEE Transactions on, vol. 8, no. 2, pp. 135–138, 2009. [4] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. Dresselhaus, and J. Kong, “Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition,” Nano Letters, vol. 9, no. 1, pp. 30–35, 2008.

Figure 1: Raman spectra of CVD grown graphene with low (Top, LHS) and high (Top, RHS) defect densities, SiC epitaxial graphene (Bottom, LHS), Schematic of chemical attachment (Bottom, RHS) [5] P. Sutter, “Epitaxial graphene how silicon leaves the scene,” Nature Materials, vol. 8, pp. 171–172, Mar. 2009. [6] S. Park and R. Ruoﬀ, “Chemical methods for the production of graphenes,” Nature nanotechnology, vol. 4, no. 4, pp. 217–224, 2009. [7] R. Bradley, K. Cassity, R. Andrews, M. Meier, S. Osbeck, A. Andreu, C. Johnston, and A. Crossley, “Surface studies of hydroxylated multi-wall carbon nanotubes,” Applied Surface Science, 2012. [8] J. Kathi and K. Rhee, “Surface modification of multi-walled carbon nanotubes using 3-aminopropyltriethoxysilane,” Journal of Materials Science, vol. 43, no. 1, pp. 33–37, 2008. [9] D. Boukhvalov and M. Katsnelson, “Chemical functionalization of graphene with defects,” Nano letters, vol. 8, no. 12, pp. 4373–4379, 2008.

Enhancement of the thermal conductivity of films by the use of stable graphene dispersions B. Iraola, L. Bilbao, I. Obieta, M. A. Mendizabal, I. Bustero TECNALIA, Mikeletegui, 2, San Sebastián, Spain Izaskun.bustero@tecnalia.com Abstract Graphene is an extraordinary good heat conductor, measurements revealed that graphene's near room temperature thermal conductivity is in the range from 3500-5300 W/mK [1]. The superb thermal conduction property of graphene is beneficial for electronic applications and establishes graphene as an excellent material for thermal management [2]. Thermal Interface Materials (TIMs) are typically made of polymer or silicone matrixes filled with thermally conductive particles. The biggest challenge of polymerbased TIMs is to achieve a low thermal resistance. The effective thermal resistance of a TIM can be decreased by reducing the bond-line thickness (BLT), reducing the contact resistance and increasing its thermal conductivity. This study has been focused on the improvement of the thermal conductivity of polymer and sol-gel coatings deposited on Al substrates by the addition of graphene. The method selected for fabricating the conductive films on the Al requires the preparation of stable graphene dispersions because the graphene platelets tend to form agglomerates due to their strong van der Waals interactions. Stable graphene dispersions in aqueous media were prepared with the aid of dodecylbenzene sulphonic acid sodium (SDBS) by adjusting the ratio of surfactant, the ultrasonication time and the initial graphene concentration. Dispersions of XGnP-M-25 graphene nanoplatelets from XG Science with initial concentrations from 0,1, 1 and 10mg/l were sonicated (tip and bath) in the presence of different ratios of SDBS during different time. The resulting dispersion was then centrifuged at 5000rpm during 4 hours to remove any aggregates, and the concentration of the supernatant was measured through absorbance measurements at a wavelength of 660 nm using the reported extinction coefficient for graphene -1 -1 dispersions in surfactant/water solutions  1390 (L g m ) [3]. Dispersed SDBS has negligible effects at this wavelength value. All absorbance measurements were measured against a blank of the appropriate SDBS/solvent mixture. The highest final concentration of 0,13mg/mL was achieved at an initial concentration of 10mg/ml and a surfactant ratio 1:1. It was also noted that tip sonication is more efficient as compared to bath sonication. Different concentrations of the stable graphene suspension were introduced in polymer and sol-gel solution to increase their thermal conductivity. The graphene solutions have been deposited by immersion techniques at different extraction speeds in order to control the film thickness. The thickness of the film at high extraction speed was around 20m, measured by scaning electron microscopy (SEM). The thermal conductivity was measured at room temperature by a Hot Disk sensor TPS 2500 S. Significant differences in the thermal conductivity values were observed depending on the percentage of graphene incorporated in the coating. Enhancement up to 46% with 1%w of graphene was measured reaching the percolation threshold. These preliminary results are very promising for the development of new thermally conductive coatings based on graphene that will allow reducing the amount of additive when comparing with currently used conventional.

References [1] Balandin A.A. et al., Nano Lett. 8, (2008). 902 [2]Ghosh, S. et al.. Appl. Phys. Lett. 92, (2008) 151911 [3] Lotya M, Hernandez Y, King PJ, Smith RJ, Nicolosi V, Karlsson LS, et al. J Am Chem Soc 131(10) (2009) 3611–20. [4] Balandin A.A. et al., Nano Letters 12, (2008) 861

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Figure 1. Comparison of final graphene concentration for different dispersion conditions: initial graphene concentrations, initial concentration:surfactant ratio, tip versus bath sonication, sonication time.

Nanopatterned graphene using spherical block copolymer lithography Alberto Cagliani, David Mackenzie, Lisa K. Tschammer and Peter Bøggild DTU-Nanotech, Ørsteds Plads Bygning 345ø, Kgs. Lyngby 2800, Denmark Alberto.Cagliani@nanotech.dtu.dk

Abstract The high carrier mobility at room temperature and the possibility of an easy integration with silicon technology makes graphene an attractive candidate for pushing forwards the boundaries of electronics [1]. Unfortunately the absence of a band gap makes it difficult to fabricate FET transistor with a high onoff ratio [1]. Many techniques have been developed to overcome this problem by engineering the graphene properties at the nanoscale. The possibility of creating semiconducting nanoribbons has been extensively investigated, demonstrating that graphene nanoribbons (GNR) with a 2-20 nm width have a band gap useful for FET applications [2][3]. The disadvantage of such narrow structures is a limited maximal current. A possible solution is to fabricate a large number of nanoribbons working in parallel to increase the total current [4]. Pedersen and coworkers [5], suggested graphene anti-dot lattice as an alternative route to bandgap engineering, which was flowingly demonstrated in the form of a graphene nanomesh [6][7]. In this case cylindrical block copolymer lithography has been used, since this approach provides sufficient large areas of nanopatterned graphene with critical dimensions comparable to state-of-the-art GNRs. However, cylindrical block copolymers present several drawbacks. 2 First off, achieving a good uniformity on a large scale is highly challenging (so far limited to 1 cm ) and the alignment of the cylinders depends strongly on the surface pre-treatment, which is generally a nontrivial procedure. Here we present a method to achieve nanopatterning of graphene with spherical block copolymer in a simpler, more robust and ultimately scalable way, at the cost of reduced ordering of the patterns. Moreover, we show a strong response to small concentrations of NO2. The process sequence used to pattern micro-cleaved and CVD graphene is depicted in Figure 1. A 48 nm thick layer of PS-b-PMMA block copolymer is spin cast on top, without any pretreatment of the surface. After annealing, the PMMA spheres segregate at the top of the thin film, forming a hexagonal lattice. The local hexagonal order was found to be robust within a large range of temperatures and annealing times. Moreover, the morphology of the block copolymer after annealing is uniform across an entire 4” inch wafer, with short range hexagonal order everywhere. In order to obtain an etch mask for the SiOx layer the block copolymer has to be etched (see Figure1 b-c-d). First, the PMMA spheres are removed using a standard UV and acetic acid step, but, in contrast to cylindrical block copolymers, this step is not sufficient to open the nanomask until the SiOx layer and extra plasma etching step is needed [6]. For this reason an O2/Ar plasma is used to vertically etch the remaining polystyrene layer until the SiOx layer is reached. As shown in Figure 2 the SiOx layer is not reached for all the polystyrene nanopits at the same etching time. This is most likely due to variation in the diameter of the PMMA spheres. Here we tune the etching time to maximize the number of fully open nanopits, avoiding at the same time overetching of the nanomask. When the nanomask is formed, the SiOx layer and graphene are etched using a CHF3/CF4 plasma and an oxygen plasma. As shown in Figure 3 the process time for the SiOx etching determines the morphology of the nanopatterned graphene, ranging from few sparse nanoholes to a densely nanoporous, nearly discontinuous graphene layer. The removal percentage for four different etching times is also shown. The size distribution of the holes is much larger compared to typical results with cyclindrical block copolymers and the local order is destroyed for the longest etch 2 time. Thanks to the uniformity of the process samples up to few cm of CVD nanopattered graphene have been obtained. The response of nanopatterned CVD graphene to small concentrations of NO2 has been studied by transferring nanopatterned CVD graphene on electrodes and monitoring the change of the resistance for different concentrations. Figure 4 shows a clear improvement compared to unpatterned CVG graphene. This behavior is attributed to the increased number of reactive sites along the edges due to the nanopatterning. References [1] Geim, a K. Science, 324 (2009), 1530–4. [2] Ponomarenko, L. a, et al., Science, 320 (2008), 356–8. [3] Li, X., Wang, X., Zhang, L., Lee, S., & Dai, H. Science 319 (2008), 1229–32. [4] Liu, G., et al. ACS nano, 6(8) (2012), 6786–92. [5] Pedersen, T., et al. Physical Review Letters, 100 (2008), 1-4. [6] Bai, J., Zhong, X., Jiang, S., Huang, Y., & Duan, X. Nature nanotechnology, 5(3) (2010), 190-4. [7] Liang, X., et al. Nano letters, 10(7) (2010), 2454-60.

Figures (a)

(c)

Figure 1. (a) Graphene is microcleaved on 90 nm SiO2. (b) Graphene is covered with 15 nm of e-beam oxide. The spherical PS-b-PMMA is spin cast on top and annealed. The crosssectional view shows the inner geometry of the block copolymer. (c) After an exposure to UV light the PMMA sheperes are removed in acetic acid. (c) An oxygen plasma is used to etch the polystyrene nanomask up to the point where the nanopits are all open. (d) A fluorine based plasma is used to etch the SiOx layer. An oxygen plasma removes the residual block copolymer and patterns the graphene. (e) A 5% HF dip removes the remaning SiOx.

(b)

(d)

Figure 2. (a) SEM micrograph of a spherical block copolymer PS-b-PMMA 48 nm thick layer (inset). The PMMA spheres are the black dots. The local hexagonal order is highlighted in red. The image has been post-processed to enhance the contrast. Large micrograph shows a top view of the polystyrene nanomask after 30 seconds of O2/Ar etching. (b) After 78 seconds ecthing approximately 50% of the pits are open. (c) Afetr 80 seconds etching 90% of the pits are clear. (d) After 82 seconds etching 95% of the pits reach the SiOx layer and some of the holes are merged. Scale bars are 200 nm (100 nm for inset)

Figure 4. Response of nanopatterned CVD graphene to NO2. The graph shows a clear improvement in the sensitivity due to the nanopatterning. All samples have been annealed at 230 degrees to remove contaminants.

Figure 3. SEM micrographs of microcleaved graphene after nanopatterning. The scale bars are 200 nm. (a) After a 26 seconds etching of the SiOx protective mask. (b) After a 32 seconds etching. (c) Percentage of removed CVD and microcleaved graphene per 3 different etching times.

Selective removal of graphene by means of micro- and nanosecond laser irradiation. N.Campos1, A.Perez-Mas2, P.Alvarez2, D.Gómez1, C. Vázquez3, A. Hadarig3, S.Ver Hoeye3, A.L. Elías4, 4 3 3 3 3 2 M. Terrones , M. Fernández , R. Camblor , G. Hotopan , F. Las Heras , and R. Menendez 1

ITMA Materials Technology, C/ Calafates 11, 33417, Avilés, Spain Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain 3 Area of Signal Theory and Communications, Department of Electrical Engineering, University of a Oviedo, Campus de Viesques, Edificio Polivalente s/n, mod. 8, 1 p, E-33203, Gijón, Spain 4 Department of Physics, 104 Davey Lab., The Pennsylvania State University, University Park, PA 16802, USA 2

n.campos@itma.es Abstract Selective removal of graphene by laser irradiation allows the complex patterning of graphene films with linewidth resolution in the range of tens of micrometers. Thereby, it may enable the exploitation of its exotic electronic properties in geometry-dependent applications, and makes technologically feasible its integration in microdevices. To the best of our knowledge, up to the date only femtosecond lasers have been used to address the challenge of facile graphene patterning (see for example [1], [2]). In this work, the use of nano and microsecond laser is inquired, because their lower cost of processing could facilitate the integration of graphene in devices at industrial scale. Graphene films have been synthesized on copper foil by means of a chemical vapor deposition process using methane as a precursor, as detailed in [3], and afterwards transferred onto silicon oxide substrates by means of the well known PMMA-based transfer process. With comparative purpose, two different laser sources with pulse widths of microsecond (diode, 1064nm) and nanosecond (Nd:YAG 532nm) were used to scribe pairs of parallel lines on the samples at a separation distance of 70µm. Series of preliminary tests have been carried out and, on its basis, different conditions have been selected to pattern three samples with each laser. All the samples were inspected by means of optical microscopy (as in figure 1), and Raman spectroscopy was used to prove the complete removal of graphene in the laser grooves, as well as to determine the grade of graphene damage in areas close to the edges. Figure 2 shows an example of the Raman characterization carried out, Although the borders of the lines scribed with nanosecond laser were better defined, it can be concluded from Raman measurements that interstitial graphene was more damaged than in the case of microsecond laser scribed samples. References [1]

G. Kalita, L. Qi, Y. Namba, K. Wakita and M. Umeno, Mat. Let. 65 (2011) 1569.

[2]

W. Zhang, L. Li, Z.B. Wang, A.A. Pena, D.J. Whitehead, M.L. Zhong, Z. Lin, H.W. Zhu, App. Phys. A, 109 (2012) 291.

[3] R. Lv, Q. Li, A.R. Botello-Méndez, T. Hayashi, B. Wang, A. Berkdemir, Q. Hao, A.L. Elías, R. CruzSilva, H.R. Gutiérrez, Y. A. Kim, H. Muramatsu, J. Zhu, M. Endo, H. Terrones, J. Charlier, M. Pan, M. Terrones, Sci. Rep. 2 (2012) 586

Figures

Fig. 1: Optical microscopy images showing examples of the different grooves obtained when varying the conditions of scribing (current, frequency and scan speed).

Fig. 2: Optical micrographs of the laser-scribed lines (nanosecond and microsecond) on Si/SiO2/graphene samples showing the Raman spectra of the different sections.

Acknowledgements This work has been supported by the Spanish Ministry of Economy and Competitiveness through the th TECNIGRAF project (IPT-2011-0951-390000) and by the European Comission within the 7 Framework Programme through the INSIDDE project (Project number: 600849).

Twisted bilayer graphene: phonon dispersion of microscopic rainbows 2

Jessica Campos-Delgado , Luiz Gustavo Cançado , Gerardo Algara-Siller , Ado Jorio , Carlos A. 4 3 1 Achete , Ute Kaiser , Jean-Pierre Raskin 1

ICTEAM, Université catholique de Louvain, 1348 Louvain-la-Neuve, Belgium Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 30123-970, Brazil 3 Ulm University, Group of electron microscopy for materials science, Ulm, 89081, Germany 4 Divisão de Metrologia de Materiais, INMETRO, Xerém, RJ, 25250-020, Brazil

jessica.campos@uclouvain.be Abstract In this work we study naturally produced bilayer graphene presenting different rotational angles. The production of the samples is carried out by low pressure CVD of methane using copper film as catalyst. The grown graphene layer is mostly a monolayer film with distributed bilayer and multi-layer graphene patches formed at the nucleation centers. The rotational angle between layers has proven to influence the electrical properties of bi-layer graphene [1,2,3] and generates superlattice structures known as Moiré patterns [4]. We demonstrate that different colorations appear visually for certain misorientation angles of bilayer graphene when graphene is transferred on an optimal SiO2 thickness of 100 nm stacked on top of a Si substrate, as exemplified in Figure 1A. In particular, Raman spectroscopy constitutes a powerful tool to probe bilayer graphene since the differences in orientation are sensed through the intensity of the G band [5,6] and the positions of the R bands [4]. We have studied these spectra of colorations combining Raman spectroscopy and high resolution transmission electron microscopy. Our investigations reveal that angles in the range of 9°-11° can be attributed to blue colorations, yellow colorations appear for rotational angles between 11° and 13° and finally pink-reddish colorations are present for angles of 13° up to 15° [7]. Moreover, we demonstrate that these superlattices provide θ-dependent q wavevectors that activate phonons in the interior of the Brillouin zone. We show this superlattice-induced Raman scattering can be used to probe the phonon dispersion in twisted bi-layer graphene [8]. We have found features in the -1

100-900 cm region (see Figure 1B) that are successfully attributed to phonon branches of graphene (ZA, TA, LA, ZO phonons) and to layer breathing vibrations (ZO’ phonons).

References [1] J.M.B. Lopes dos Santos, N.M.R. Peres, A.H. Castro Neto. Graphene bilayer with a twist: electronic structure. Phys. Rev. Lett. 99, 256802, 2007. [2] G. Li, A. Luican, J.M.B. Lopes dos Santos, A.H. Castro Neto, A. Reina, J. Kong, E.Y. Andrei. Observation of Van Hove singularities in twisted graphene layers. Nature Physics 6, 109-113, 2010. [3] A. Luican, G. Li, A. Reina, J. Kong, R.R. Nair, K.S. Novoselov, A.K. Geim, E.Y. Andrei. Single-layer behavior and its breakdown in twisted graphene layers. Phys. Rev. Lett. 106, 126802, 2011. [4] V. Carozo, C.M. Almedia, E.H.M. Ferreira, L.G. Cançado, C.A. Achete, A. Jorio. Raman signature of graphene superlattices. Nano Lett. 11, 4527-4534, 2011.

[5] R. Havener, H. Zhuang, L. Brown, R. G. Hennig, J. Park. Angle-resolved Raman imaging of interlayer rotations and interactions in twisted bilayer graphene. Nano Lett. 12, 3162-3167, 2012. [6] K. Kim, S. Coh, L.Z. Tan, W. Regan, J.M. Yuk, E. Chatterjee, M.F. Crommie, M. L. Cohen, S.G. Louie, A. Zettl. Raman spectroscopy study of rotated double-layer graphene: misorientation-angle dependence of electronic structure. Phys. Rev. Lett. 108, 246103, 2012. [7] J. Campos-Delgado, G. Algara-Siller, U. Kaiser, J.-P. Raskin. Twisted bi-layer graphene: microscopic rainbows (Submitted in 2013). [8] J. Campos-Delgado, L.G. Canรงado, C.A. Achete, A. Jorio, J.-P. Raskin. Raman-scattering study of the phonon dispersion in twisted bi-layer graphene. arXiv:1301.3795 [cond-mat.mes-hall].

Figures

Figure 1. A) Optical micrograph of bilayer graphene transferred to Si/SiO2 substrate (100 nm silicon dioxide thickness). Pink-, red-, yellow- and blue- colored areas can be identified. A color scale is included as guide to the colors pictured. B) Raman spectroscopy of bilayer graphene, new families of features (highlighted in yellow) can be identified. Curve coloring corresponds to E laser used: blue shades correspond to spectra recorded with Elaser=2.54 eV, green shades Elaser=2.41 eV and red shades Elaser=1.96 eV. Exception for the bottom (gray) spectrum (Elaser=2.41 eV), which corresponds to Si substrate reference signal.

Theoretical Study of the Effects of Electron Density Distribution in Frontier Orbitals with OpenShell Graphene Fragments Wei-Chih Chen, Ito Chao* Institute of Chemistry, Academia Sinica, No.128, Section 2, Academia Road, Taipei 11529, Taiwan Email: ichao@gate.sinica.edu.tw Abstract Open-shell fragments of graphene have attracted increased attention in recent years because of their unique spin carrier property.[1] Through the experimental efforts, these neutral Ď&#x20AC;-radicals could be stabilized by steric protection or delocalized Ď&#x20AC;-conjugated system, such as the phenalenyl-based molecules.[2] Here, we use zero sum rule[3] to extend the molecular size and investigate their electronic properties. The molecules studied in this work can be categorized into two kinds of open-shell structures: the ones with even electron density distribution in the frontier orbitals and the ones with uneven distribution. These molecules afford small reorganization energies in our computational results, especially in the even distribution case (smaller than 19 and 34 meV for hole and electron transfer, respectively). These ultra-low values can be interpreted with the strong nonbonding character in the frontier orbitals, so that the bond length alteration is small during charge transfer.[4],[5] Considering the overlap of bilayer open-shell graphene fragments, the even distribution molecules exhibit larger electronic coupling and binding energy than the uneven. Therefore, we propose that open-shell graphene fragments with uniform electron density distribution in the frontier orbitals would be better charge transfer materials than the ones with uneven electron density distribution. References [1] Y. Morita, S. Suzuki, K. Sato and T. Takui, Nat. Chem., 3 (2011), 197. [2] S. Nishida, K. Kariyazono, A. Yamanaka, K. Fukui, K. Sata, T. Takui, K. Nakasuji, and Y. Morita, Chem. Asian J., 6 (2011), 1188. [3] E. V. Anslyn and D. A. Dougherty, Modern Physical Organic Chemistry (University Science Books, Sausalito, 2006). [4] M. Y. Kuo, H. Y. Chen, and I. Chao, Chem. Eur. J., 13 (2007), 4750. [5] Y. C. Chang and I. Chao, J. Phys. Chem. Lett., 1 (2010), 116. Figures

Figure 1. Example of a molecule with (a) even or (b) uneven electron density distribution in the frontier orbitals.

Twisting Bilayer Graphene Superlattices Po-Wen Chiu1*, Chun-Chieh Lu,1 Yung-Chang Lin,1 Zheng Liu,2 Chao-Hui Yeh,1 Kazu Suenaga,2 1 2

Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan

*E-mail: pwchiu@ee.nthu.edu.tw

ABSTRACT Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of lowdimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decomposition of SiC; (ii) chemical vapor deposition (CVD) on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layer one atop the other, later of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycrystalline structures. The present study investigates bilayer graphene grown by ambient pressure CVD on polycrystalline Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes are shown to result from the twist, with angle determined by transmission electron microscopy. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with welldefined geometry.

50 m

2 m

Figure 1 C. C. Lu et al.

Properties of graphene-like materials derived from fuel-rich flames of diverse hydrocarbons Carmela Russo, Michela AlfĂ¨, Barbara Apicella, Fernando Stanzione, Antonio Tregrossi, Anna Ciajolo Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, IRC-CNR Piazzale tecchio 80, 80125 Naples, Italy ciajolo@irc.cnr.it Abstract Soot constitutes the carbon particulate matter, in form of particles and aggregates, that can be produced in combustion by a step-wise mechanism starting from organic precursors, mainly hydrocarbons, and passing through polycyclic aromatic hydrocarbons (PAH) formed in high-temperature partially oxidative (fuel-rich) conditions. Combustion-formed PAH from low- to high-mass are assumed as the fundamental molecules to which graphene is extrapolated upon the decrease to molecular size, whereas soot structure is mainly based on two-dimensional graphene layers grouped, cross-linked and/or layered each other in a turbostratic way [1]. The properties of PAH and soot as topology and morphology, size, 2 3 sp /sp ratio and hydrogen content strongly depend on the burning conditions [1-3]. Thus, in line of principle the bottom-up process leading through PAH intermediates to soot with different graphenic character can be controlled by a suitable change of the combustion conditions. In this work soot has been synthesized in the well-controlled combustion conditions of premixed laminar fuel-rich flames and its properties have been modulated changing the residence time into the flame and the precursors. Premixed flames burning methane (M), ethylene (E) and benzene (B) have been used to produce soot in a temperature range (Tmax [1700-1770K]) appropriate for obtaining high soot yields [2]. Hydrocarbon gas provides both carbon species necessary for materials growth and the high temperature to its decomposition, thus no external sources of energy are required. Indeed, a premixed flame in partially oxidative condition represents a carbon-rich chemically reactive environment able to generate graphenic nanostructures during short residence times in a continuous single-step inexpensive process. The maximum soot yield spans from a minimum value of 1% to about 3.5% in dependence on the fuel. In particular, methane (M) exhibits the lowest sooting tendency, whereas benzene (B) shows the highest one [2]. Soot has been deposited on a quartz plate inserted horizontally at selected positions of the flame for the minimum time required both to collect a sufficient material for the further characterization and to limit the thermal degradation of the deposited sample at flame temperature [3]. Soot, after dichloromethane extraction of low MW aromatic species [3], has been characterized by UVVisible spectroscopy and Raman spectroscopy with an excitation wavelength of 514nm and the correlations between the Tauc band gap, Eg, and the I(D)/I(G) ratio, reported in Figure 1, have been obtained. Size exclusion chromatography performed on soot sampled has shown that this material is mainly composed by carbon aggregates with a band gap ranging from 0.4 to 0.1 with a minor contribution (<20%) of less organized carbon structures with a bang gap around 1 [3]. From HR-TEM analysis it has been observed that the graphene layers, grouped in number of 2-3, have a layer length much below 1 nm [1, 2]. For this layer size it has been shown that I(D)/I(G) increases as the graphite crystallite size increases [4]. Thus, in this work the band gap decrease accompanied by the increase the I(D)/I(G) ratio, testifies an increase in the order extent at higher residence times in the flame, and particularly in the benzene flame. Overall, the well-controlled combustion conditions of a premixed flame have shown to be suitable to produce different carbon particulate matters burning different fuels in a narrow temperature range. A wide range of band gaps, i.e. electronic properties, can be easily obtained, showing the great flexibility of the flame reactor to produce carbonaceous materials with tailored properties by selecting the appropriate combustion conditions. References [1] AlfĂ¨ M.,Apicella B., Barbella R., Rouzaud J.-N., Tregrossi A., Ciajolo A., Proceedings of the Combustion Institute, 32 (2009) 697. [2] Russo C., AlfĂ¨ M., Rouzaud J.-N., Stanzione F., Tregrossi A., Ciajolo A., Proceedings of the Combustion Institute, 34 (2013) 1885. [3] Russo C, Stanzione F., Ciajolo A., Tregrossi A., Proceeding of Combustion Institute, 34 (2013) 3661. [4] Ferrari, A. C., Robertson, J. Phys. Rev. B 61 (2000) 14095.

Figures

0.45 0.4 0.35

Eg, eV

0.3 0.25 0.2 0.15 0.1

0.05 0 0.6

0.7

0.8

0.9

I(D)/I(G) Figure 1 Correlation between Tauc band gap, Eg, and I(D)/I(G) ratio of soot for methane (M), ethylene (E) and benzene (B) flames.

The Properties of Mono-, Double- and Triplelayer CVD Graphene Transferred by Electrochemical Delamination 1,2

1,3

Tymoteusz Ciuk , Iwona Pasternak , Aleksandra Krajewska , Jan Sobieski , and Wlodek Strupinski

1) Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland, 2) Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 3) Institute of Optoelectronics, Military University of Technology, Gen. S. Kaliskiego 2, 00-908 Warsaw, Poland 4) Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland tymoteusz.ciuk@gmail.com Abstract The realization of a low-cost and large-scale application of graphene has become possible with the advancement of its CVD growth on copper foil. It is, however, absolutely vital that graphene is transferred reliably and time-effectively if it is to be widely exploited. In this work we investigate the properties of a CVD graphene grown on copper foil as a monolayer and consecutively transferred onto a high-resistivity Si/SiO2 substrate to form a mono-, double- and triple layer. The transfer method is based on the electrochemical delamination [1] performed at a high pace of 1mm per second. We illustrate the influence of different electrolyte compositions on the quality and the properties of graphene films. We relate the obtained physical and electrical characteristics to graphene transferred by copper etching. The characterization of delaminated graphene includes Raman spectroscopy and SEM imaging. Further assessment of graphene properties is performed with a classical DC Hall method and a contactless technique that utilize a single post dielectric resonator operating at microwave frequencies [2,3].

References [1]Yu Wang, Yi Zheng, et al., ACS Nano, 5 (2011) 9927â&#x20AC;&#x201C;9933. [2]J. Krupka, W. Strupinski, Appl. Phys. Letters, 96 (2010) 082101. [3] J. Krupka, W. Strupinski, J. Nanosci. Nanotechnol., 11 (2011) 3358â&#x20AC;&#x201C;3362.

Rapid time-controlled emission of single Dirac fermions from graphene quantum dots 1,2

M. R. Connolly , K. L. Chiu , S. Giblin , M. Kataoka , J. Fletcher , C. Chua , J. P. Griffiths , G. A. C. 2 3 2 1 Jones , V. I. Falâ&#x20AC;&#x2122;ko , C. G. Smith , T. J. B. M. Janssen 1

National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, CB3 0HE, UK 3 Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK

mrc61@cam.ac.uk

Electrical transport measurements are traditionally used to uncover the properties of a conductor by driving randomly generated electron wavepackets through a device and extracting information about scattering, quantum interference, and localization from the time-averaged current density. Control over the trajectory and emission time of individual wavepackets provides a more direct way to explore and manipulate the quantum nature of single electronic excitations [1]. Given that low energy electron wavepackets in graphene have the same linear energy dispersion relation as massless Dirac fermions, time-controlled release in graphene also allows fundamental questions about quantum measurement to be explored in a relativistic setting. Multiple emitters, for instance, could spatially separate and control the collision of pairs of Dirac fermions to assess how the valley or layer degree of freedom affects particle indistinguishability.

Here we describe graphene single electron emitters which work by adiabatically transfering single charges between two series coupled quantum dots [Fig. 1(a) and Ref. [2]]. The application of phaseshifted radiofrequency signals to all-graphene side-gates results in a single charge being transferred between the dots each cycle, and generates a net current equal to the fundamental electronic charge times the drive frequency [3]. We discuss the role played by the geometry of the quantum dots on pumping, the experimental conditions required to achieve good pump performance, and the conformity of the experimental data to theoretical models based on pumping through metallic islands [4]. We also explore the quantization accuracy and robustness of the pumped current as a function of drive frequency. Quantized pumping is observed up to GHz frequencies, an order of magnitude higher than previously achieved using conventional metallic or semiconductor adiabatic pumps. References [1] E. Bocquillon, et al., Phys. Rev. Lett. 108, 196803 (2012) [2] M. R. Connolly, et al., arXiv:1207.6597 (2012) [3] Pothier, H., et al., Europhysics Letters, 17(3) 249-254 (1992) [4] Winkler, N., et al., Phys. Rev. B 79, 235309 (2009)

Figures

(a)

(b)

Fig. 1 (a) Schematic showing the process of transferring a single charge per cycle through a graphene double quantum dot: (1) The plunger gate attracts an electron on to the left quantum dot , changing the number of charges on the dots from (N1, N2) to (N1+1, N2), (2) the right plunger gate increases, shifting the electron on to the right quantum dot [(N1+1, N2) to (N1, N2 +1)], (3) the plunger gates return to their original values, and the electron is pumped out to the drain contact. Charges are thus pumped when the loop makes an excursion enclosing a triple point. (b) Plot showing the relation between the triple points in the DC zero-bias current and the plateau in the pumped current obtained by rapidly modulating the gate voltages at a frequency of 12 MHz. The same cycle of gate voltages generates current with opposite polarity when the rotation is centred on different triple points because electrons are pumped in the opposite direction [1].

CVD synthesis of graphene from acetylene catalyzed by a reduced CuO thin film deposited on SiO2 substrates 1

1,2

A. Cortés , C. Celedón , H. Häberle . 1

Universidad Técnica Federico Santa María, Departamento de Física, Avenida España 1680, Valparaíso-Chile, 2 Instituto Balseiro (U. N. de Cuyo and CNEA), Avenida Bustillo 9500, Bariloche-Argentina, andrea.cortes@usm.cl

Abstract Chemical vapor deposition (CVD) synthesis is a technique widely used to grow graphene on metallic substrates such as Copper (Cu) and Nickel (Ni). Copper, unlike nickel, allows better control of the growth of graphene due to the low bulk carbon solubility. Copper has been extensively researched in the last decade as a substrate for graphene (mono- and poly-crystalline), in the form of foil or thin film grown in High Vacuum (HV) or Ultra High Vacuum (UHV) conditions [1-3]. Direct deposition of graphene on dielectric surfaces is of great interest given their potential use in sensor fabrication and electronic or optical applications [4-8]. In this study direct growth of graphene or few-layer graphene (FLG), without any transfer process onto a silicon dioxide (SiO2) substrate by CVD was used. An intermediate thin film of cupric oxide (CuO), pre-deposited by DC sputtering was applied as catalysts. Unlike e-beam evaporation or resistive evaporation, DC sputtering is fast and easy to implement at industrial level. Figure 1 represents a schematic picture of the surface after growth is completed, revealing the possibility of finding a complex structure at the surface interface CuO was used as a catalyst precursor and acetylene as the carbon source for growth using chemical vapor deposition. By raising the temperature the CuO thin layer was partially reduced to Cu in a H2 environment. The oxide is reduced preferentially on the surface, generating a metallic layer on top of the oxide. During graphene growth, oxygen from the metal oxide may diffuse to the surface thus changing the catalytic properties of the metallic layer. After graphene growth, the reduced Cu layer is almost completely evaporated from underneath the graphene layer. When the process is completed, graphene lies mostly in contact with the SiO2 dielectric layer. Raman spectroscopy (λ= 633 nm) was used to characterize and confirm the presence of a single and/or few-layers of graphene. This procedure has the advantage of not requiring any post processing to transfer the thin film onto a dielectric substrate or the use of ultra-high vacuum during synthesis. The growth and deposition of graphene on SiO2 by CVD, has been carried out without any additional chemical or transfer procedure. References [1] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang et al. Science, 324 (2009) 1312-1314. [2] M. P. Levendorf, C. S. Ruiz-Vargas, S. Garg, J. Park. Nano Lett., 9 (2009) 4479-4483. [3] Y. Zhang, Z. Li, P. Kim, L. Zhang, C. Zhou. ACS Nano, 6 (2012) 126-132. [4] M. H. Rümmeli, A. Bachmatiuk, A. Scott, F. Börrnert, J. H. Warner, V. Hoffman et al. ACS Nano, 4 (2010) 4206-4210. [5] J. Hofrichter, B. N. Szafranek, M. Otto, T. J. Echtermeyer, M. Baus, A. Majerus et al. Nano Lett., 10 (2010) 36-42. [6] J. Chen, Y. Wen, Y. Guo, B. Wu, L. Huang, Y. Xue et al. J. Am. Chem. Soc.133 (2011) 1754817551. [7] Y. H. Lee, J. H. Lee. Apply Phys. Lett., 96 (2010) 083101. [8] A. Ismach, C. Druzgalski, S. Penwell, A. Schwartzberg, M. Zheng, A. Javey et al. Nature, 10 (2010) 1542 -1548.

Figures

Figure 1. The schematic image of a graphene sheet grown on SiO2 partially covered by Cu. Magenta, brown and the black transparent surface represent respectively SiO2, the Cu agglomerates and the graphene sheets.

Figure 2. Graphene Raman shift on SiO2, recorded on different sections of the surface The first spectrum is consistent with a bilayer graphene. The last two spectra corresponded to single layer graphene.

Exfoliation of graphene oxide using ionic liquids: experimental and molecular modelling approach B Coto, M Blanco, N Uranga, M Mahrova, J Barriga, A Marcaide IK4-TEKNIKER, I帽aki Goenaga 5, 20600 Eibar, Spain borja.coto@tekniker.es Abstract 2

Graphene, a one-atom-thick planar sheet of sp hybridized carbon, has received much attention due to its outstanding properties such as large specific surface area, high electrical and thermal conductivity, excellent chemical stability and mechanical stiffness. Graphite, which is cheap and readily available, consists of stacked graphene sheets. Therefore, one of the most convenient methods for the mass production of graphene sheets is the exfoliation of graphite in the liquid phase. Recently, many attempts to produce graphene sheets in large quantities via chemical reduction of exfoliated graphite oxide (GO) have been reported. During the oxidation process of graphite, the unique electronic properties of graphene dramatically degrade. The electrical conductivity of the graphene oxide sheets can be partially restored by the reduction step; however, this results in their irreversible agglomeration. Therefore, different strategies to disperse graphene sheets before or during reduction step have been used, including stabilization by various polymeric dispersants or surfactants and covalent/non-covalent functionalization [1]. In this context, ionic liquid (ILs) can be used for functionalization of graphene. They can adsorb on the graphene surface through the noncovalent interactions of anion and/or cation with graphene. ILs present several advantages such as enhanced ionic conductivity, thermal stability and excellent mechanical properties. The graphene modified with ILs are endowed with improved conductivity, excellent hydrophilicity and positive charged [2]. The repulsion between the resultant cation-charged GO sheets, the charge transfer between the ions and graphene and the high solubility of the grafted IL contribute to the exfoliation of graphite into graphene sheets and to prepare long-term stable graphene dispersions using ILs [3]. In this work, several ILs with different chemical structures have been synthesized and employed for the modification of graphite and graphite oxide in order to show the possible exfoliation of graphene layers in both materials. The graphite oxide employed for the study has been prepared by the Hummers method. The average interlayer spacing between the exfoliated graphene layers in graphite and graphite oxide has been measured by X-ray diffraction (XRD) (figure 1). Molecular dynamics simulations were also used to study the influence of ILs in the interlayer spacing (figure 2).

References [1] M Tunckol, J Durand, P Serp, Carbon, 50 (2012) 4303-4334. [2] R Marcilla, M S谩nchez-Paniagua, B L贸pez-Ruiz, E L贸pez-Cabarcos, E Ochoteco, H Grande, D Mecerreyes, Journal of Polymer Science Part A: Polymer Chemistry, 44 (2006) 3958-3965. [3] MH Ghatee, F Moosavi, Journal of Physical Chemistry C, 115 (2011) 5626-5636. Figures

(a) Figure 1. XRD pattern of: a) graphite and b) graphite oxide.

Figure 2. Molecular model of graphite oxide with adsorbed ionic liquid.

(b)

Role of zero-energy modes in low energy quantum transport through disordered graphene 1

2,3

Alessandro Cresti , Frank Ortmann , Thibaud Louvet , Dinh Van Tuan and Stephan Roche

IMEP-LAHC (UMR 5130), Grenoble INP, Minatec, 3 Parvis Louis Néel, F-38016 Grenoble, France 2 CIN2 (ICN-CSIC) and Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spain 3 Ecole Normale Supérieure de Lyon, 46, Allée d’Italie, 69007 Lyon France crestial@minatec.inpg.fr

The interplay between disorder and symmetry in driving quantum transport at the Dirac point of graphene is a highly debated subject of fundamental interest, whose investigation represents a theoretical and experimental challenge. Of particular importance is the case of resonant scatterers, i.e. short-range defects giving rise to resonant states, typically at low energy. The theoretical and numerical studies on this subject have mainly focused on the case of pseudovacancy disorder, for which the orbitals corresponding to the missing atoms are simply removed from the tight-binding Hamiltonian of the system. This type of disorder represent a wide class of defects for 2 3 which the hybridization of the carbon atoms passes from sp to sp , as for example in the case of hydrogen adsorbates. We consider pseudo-vacancies homogeneously distributed over the two sublattices of graphene (compensated case) or only over a single lattice (uncompensated case). The main results of the literature predict that: (i) uncompensated vacancies induce very localized zeroenergy states and reduce the density-of-states around the Dirac point thus opening a gap [1]; (ii) compensated vacancies at exactly the Dirac point give rise to a supermetallic state that makes the 2 minimum conductivity rise above 4e /h when increasing the defect density [2]; (iii) for uncompensated vacancies the conductivity increase is rapidly suppressed even for very low defect densities [2]. In this contribution [3], we get to the heart of the issue by simulating electronic transport through graphene in the presence of compensated and uncompensated vacancies at low energies around the Dirac point. To this aim, we consider two different and complimentary system configurations: 2D graphene (investigated by the Kubo-Greenwood approach) and graphene strips with high aspect ratio in between two highly doped contacts (for which we made use of the Green’s function technique), see Fig.1. In the case of uncompensated vacancies (Fig.2), both configurations show the opening of a band gap that increases as the square root of the defect density and leads to conductivity suppression. Very close to the Dirac point, a conductivity peak unveils the presence of very localized zero-energy states, whose contribution to transport is however almost irrelevant, as we will explain. Compensated vacancies globally preserve the sublattice symmetry, thus entailing a completely different conductivity behavior. For 2D graphene, the semiclassical conductivity at the Dirac point shows a 2 marked peak above 4e /h, which is consequence of the important contribution of the induced zeroenergy modes, see Fig.3(a). However, the effect of Anderson localization is evident from the quantum conductivity, which turns out to be progressively reduced for long quantum coherence times. In the case of the graphene strip, where transport is dominated by tunneling through the undoped region, the conductivity shows a peak close to the Dirac point, see Fig.3(b). The peak height (averaged over several disorder realizations) is found to be a universal function of the vacancy density times the square 2 of the strip length. For very low values of this parameter, the peak fluctuates around 4e /h and can be occasionally higher for specific disordered realizations. This is a signature of the competing effects of the low-energy increase of the density-of-states due to the zero-energy modes and the scattering due to the disordered distribution of the defects. When increasing the density, the conductivity peak decreases logarithmically, thus evidencing the progressively dominant role of scattering. Our results provide a broad numerical perspective on this interesting problem and clarify the interplay between sublattice symmetry and transport at the Dirac point, thus giving a possible explanation of the observed Anderson localization in ultraclean samples [4]. References [1] V.M. Pereira et al., Phys. Rev. Lett., 96 (2006) 036801. [2] P.M. Ostrovsky et al., Phys: Rev. Lett., 105 (2010) 266803. [3] A. Cresti et al., submitted. [4] L.A. Ponomarenko et al., Nat. Phys., 7 (2011) 958.

Figures

(a)

(b)

(a)

Figure 1: (a) Two-dimensional graphene with two pseudovacancies indicated by the letter V. (b) Strip configuration composed of a graphene strip with high aspect ratio (width W much larger than length L) between two highly doped contacts. Transport occurs by electron tunnelling through the undoped region.

(b)

Figure 2: (a) Semiclassical conductivity as a function of energy in 2D graphene with uncompensated vacancy density n=0.8%. (b) Quantum conductivity as a function of energy for a strip with W=150 nm, different lengths L and uncompensated vacancies with density n=0.1%.

(a)

(b)

Figure 3: (a) Semiclassical (ď łSC) and quantum (ď ł) conductivity for 2D graphene with density of compensated vacancies n=0.8%. Note that the peak at the Dirac point is progressively suppressed due to localization effects. (b) Inset: Average conductivity (over 20 disorder realizations) as a function of the energy for a strip with width W=150 nm, length L=5 nm and compensated vacancies with density up to 2 n=2%. Note the peak at E=0. Main frame: Height of the conductivity peak as a function of nxL . The 2 conductivity fluctuates around the pristine minimum conductivity as long as of nxL <10, then it decreases.

High resolution magneto-Raman microscopy of Graphene at low temperatures and high magnetic fields 1

C. Dal Savio , C. Faugeras , P. Kossacki , M. Potemski and K. Karrai 1

attocube systems AG, Königinstr. 11a RGB, D-80539 München, Germany 2 LNCMI-Grenoble, CNRS-UJF-UPS-INSA, France claudio.dalsavio@attocube.com

Abstract The electronic properties of Graphene are very much reminiscent of high quality layered semiconductors devices. Such two dimensional layered systems do exhibit striking electrical and optical properties when submitted to strong magnetic fields and cryogenic temperatures. Very pure graphene 2 can be obtained in form of tiny flakes of few µm by exfoliating the surface of high quality natural single crystal of graphite. The operation of exfoliation leaves behind high quality flakes of graphene on the surface of the natural graphite substrate. The flake are however so small that they require the use of local probes techniques to detect them and measure their properties. We have designed and built a high resolution confocal microscope capable of measuring spectroscopic optical Raman properties of surfaces at low temperature and high magnetic fields. We have based our optical microscopes instruments not only on standard liquid helium bath magnet cryostat but also on an ultra-low vibration closed-cycle magnet cryostat system, and this in order to gain total independence from the need of liquid cryogens enhancing this way enormously the ease of use of the system as well as the logistics usually involved with liquid based cryostat. In this presentation we report on sub-micro magneto-Raman scattering experiments performed on the surface of a freshly exfoliated single crystal of natural graphite [1]. Graphene flakes left on graphite are expected to be of very high electronic quality but are not easy to spot since they show no contrast in standard optical microscopy. In this work we image natural graphene flakes using high spatial resolution confocal Raman scattering microscopy in high magnetic fields (0-9 Tesla) at 7 K. Graphene flakes on graphite are revealed in the presence of a strong magnetic field, as first imaged in [1], when the E2g phonon energy coincides with the electron-hole separation between the valence and conduction Landau levels (-N,+M) of the Dirac cone. Resonant hybridization of the E2g phonon and the Dirac fermion magnetoexciton is a specific signature of graphene flakes [2-3] and display very rich Raman scattering spectra varying strongly as a function of magnetic field [1]. In the figures below the magnetic field evolution of Raman spectra are taken in region where the hybridization between the E2g phonon and the (-2,+1) and (-1,+2) magneto-exciton takes place. We map the Raman scattering over 7x7 µm with 600 nm spatial resolution on three different scattering bands namely i) centered, ii) blue-shifted and iii) red-shifted from the E2g phonon peak and this at 4.3 T (lower three images) and 5.3 T (upper three images). These two magnetic fields are chosen to be just bellow and just above the resonant conditions for hybridization. As expected at 4.3 T the graphene flake appears bright in the blue shifted image (lower right), it appears bright in the red shifted image at 5.3 T while it is darker in the Raman scattering mapping centered on the E2g (both center images). The upper right and lower left image have been shown here for completion with a much enhanced contrast.

References [1] C. Faugeras et al., M. Amado, P. Kossacki, M. Orlita, M. Kühne, A.A.L. Nicolet, Yu. I. Latyshev, and M. Potemski, Phys. Rev. Lett. 107, 036807 (2011). [2] C. Faugeras et al. Phys. Rev. Lett. 103, 186803 (2009). [3] J. Yan et al. Phys. Rev. Lett. 105, 227401 (2010)

Figures

6.3 T 6.1 5.9 5.7 5.3 T

5.5 5.3 5.1 4.9 4.7 4.5 4.3 4.1

4.3 T

3.9 3.7 3.5 T 1540 1560 1580 1600 1620 Raman shift

E2g peak

Red shifted (8 cm-1 )

Blue shifted (8 cm-1 )

(cm-1)

Figure 1: Left: series of Raman spectra of the E2G phonon peak evolving as a function of the magnetic field. The spectra are shifted vertically for clarity. The spectra shows a contribution in the Raman peak that clearly displays an avoided crossing behavior centered at about 4.7 T. Right: series of images of the graphene flakes a two different fields (5.3 and 4.3T) and three different Raman bands (see text for details)

Raman shift (cm-1)

1586

1584

1582

1581 -10

-8

-6

-4

-2

Magnetic field (Tesla) Figure 2: Evolution of the center of mass of the Raman peak of the E2G band as a function of the magnetic field between -9T and +9T. The oscillations are a clear signature of the graphene flake. The same type of measurement done away from the graphen shows no shifts within the spectral resolution.

Spin transport in graphene and the role of ferromagnetic tunnel contacts André Dankert, Johan Bergsten, M. Venkata Kamalakar, Saroj P. Dash Chalmers University of Technology, Department of Microtechnology and Nanoscience-MC2, SE-41296, Göteborg, Sweden andre.dankert@chalmers.se Abstract Spintronics is a vision of using the spin of the electrons instead of its charge to perform both 1 information storage and processing in a single device . Several schemes considering the spin of 2 3 electrons have been proposed, for example Spin-Field Effect Transistor and Spin Logic devices . Performance of such spintronic devices depend on the spin relaxation time and spin diffusion length in the materials, defined by spin relaxation mechanisms. For instance the strong interest in graphene based spintronic devices stems from their long spin coherence lengths, because of the absence of 4 hyperfine interactions and weak spin-orbit coupling . However, the full potential of graphene spintronics 5,6 has not been explored, and also much of the physics revealed are not yet understood . We will present pure spin transport and precession measurements in Graphene devices at room temperature, with and without tunnel barriers. The aim is to understand the basic spin injection, transport and relaxation in Graphene, and the effect of ferromagnetic tunnel contacts. Graphene-tunnel barrier-ferromagnet nano devices with multiple contacts were prepared by standard electron beam lithography and lift off technique. The spin transport and spin precession measurements are performed in non-local spin-valve and Hanle geometries respectively. We will discuss in details the measured spinsignal and the spin life time on different devices.

Non-local Voltage (V)

B

Graphene Cobalt Tunnel Barrier

1.0

0.5

0.0 -0.14

-0.07

0.00

0.07

0.14

B (Tesla)

Figure 1. Non-local Hanle spin precession measurement in a graphene device at room temperature.

References [1] D. Awschalom, et al., Nature Phys. 3, 153 (2007). [2] S. Datta, et al., Appl. Phys. Lett. 56, 665 (1990). [3] H. Dery, et al., H. Dery, et al., Nature 447, 573 (2007); IEEE Trans. Elec. Dev 59, 259 (2012). [4] N. Tombros et al., Nature 448, 571 (2007). [5] M. Guimaraes et al., Nano Lett. 12, 3512 (2012). [6] P. J. Zomer et al. Phys. Rev. B 86, 161416(R) (2012).

Large-area micro-ellipsometry mapping of thickness and electronic properties of epitaxial graphene on bulk 3C-SiC 1

V. Darakchieva, A. Boosalis, A. Zakharov, T. Hofmann, M. Schubert, T. Iakimov, R. Vasiliauskas, 1 and R. Yakimova 1

Department of Physics, Chemistry and Biology, Linköping University, Linköping SE 581 83, Sweden; 2 Department of Electrical Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, USA 3 Lund University, Maxlab, S-22100 Lund, Sweden vanya@ifm.liu.se

Abstract Epitaxial graphene (EG) grown by sublimation epitaxy on SiC holds great promise for large-scale production of next generation fast electronic devices. Despite significant progress and intense research efforts in the field, state-of-the-art EG shows mobility parameters that are still orders of magnitude lower than those of exfoliated graphene. Understanding the physical origin of the substantially different transport properties of epitaxial and free-standing graphene remains one of the major issues, and prevents further technological advances. The key point is to identify and control how the substrate affects graphene uniformity, thickness and carrier mobility. EG on the hexagonal polytypes of SiC has been extensively studied. On the other hand, cubic 3C-SiC substrates have not been explored, mostly due to the fact that they are not commercially available. However, 3C-SiC can offer advantages over the other poltytypes in terms of isotropic growth surfaces and device performance. Mapping nonuniformities in EG on a large-scale and possible interrelation to its electronic and transport properties will be beneficial for both studying substrate effects and device production. However, the simultaneous mapping of these properties presents a significant challenge due to the domain structure of EG and the fact that characterization techniques with different footprints need to be employed. In this work, we report large-area micro-ellipsometry mapping of thickness and electronic properties of EG grown on thick bulk-like 3C-SiC(111) layers. We explore both Si and C polarities of the substrate and discuss in detail the determining factors of EG thickness uniformity in relation to its electronic and transport properties. The EG layers were grown by high temperature sublimation in Ar2 atmosphere [1] under optimized conditions on the Si- and C-face of home-grown 3C-SiC(111). The thick (few hundreds of micrometers) 3C-SiC layers were grown by sublimation epitaxy on 6H-SiC (0001) [2]. Spectroscopic ellipsometry mapping (SE) from 1.25 eV up to 5.45 eV was performed with an M2000 rotating compensator ellipsometer from J. A. Woollam Co. on a circular area of the samples with a diameter of 2 0.5 cm and with a microspot of 30x30µm . Details about the experimental and modeling procedures can be found in Refs. 3-4. SE thickness maps [Figs 1(a) and (c)] demonstrate that 1 monolayer (ML) graphene can be achieved on Si- and C-polarities of the 3C-SiC(111) substrates in good agreement with low-energy electron microscopy and Raman scattering spectroscopy. Large domains with 2 homogeneous ML coverage with size of ~2x2 mm are grown at the Si-face 3C-SiC [Fig. 1(a)]. In this case, few thick graphite-like islands are formed and their nucleation sites can be correlated with an increased interface roughness of the substrate. On the C-polar 3C-SiC the formation of the thick graphite islands is suppressed [Fig. 1(c)]. However, the areas of EG with homogeneous thickness have considerably smaller size and are randomly distributed indicating a different formation mechanism than for EG on the Si-face of 3C-SiC. Our results indicate that the polarity of the 3C-SiC substrates critically affects the formation mechanism and growth kinetics of EG, which will be discussed in relation to substrate defects and surface status. Furthermore, the maps of the free-charge carrier scattering time show that higher mobility can be achieved in the homogeneous areas of 1 ML EG, while the thicker graphite islands show that the carrier mobility drastically decreases [Figs. 1(b) and (d)]. Finally, correlation between the number of MLs and the energy of the critical point (CP) associated with an exciton enhanced Van Hove singularity at ~4.5 eV may be established for both substrate polarities (Fig.2). The CP energy positions and dielectric function shape will be further discussed in view of strain and the interaction of EG with the substrate. The reported results can be used in future works on the application of optical micro-spectroscopy techniques to study electronic properties and monitor graphene thickness homogeneity.

References

[1] R. Vasiliauskas, M. Marinova, M. Syv¨aj¨arvi, R. Liljedahl, G. Zoulis, J. Lorenzzi, G. Ferro, S. Juillaguet, J. Camassel, E. K. Polychroniadis, and R. Yakimova, J. Cryst. Growth 324, 7 (2011). [2] . C. Virojanadara, M. Syvajarvi, R. Yakimova, et al, Phys. Rev. B 78, 245403 (2008). [3] A. Boosalis, T. Hofmann, V. Darakchieva, R. Yakimova, T. Tiwald, M. Schubert, and D. Sekora, Mat. Res. Soc. Symp. Proc. 1407, aa20-43 (2012). [4] A. Boosalis, T. Hofmann, V. Darakchieva, R. Yakimova and M. Schubert, Appl. Phys. Lett. 101, 011912 (2012).

Figures

Figure 1 Spectroscopic ellipsometry maps of the thickness of epitaxial graphene grown on the Si-face (a) and C-face (c) of 3C-SiC(111), and the respective free-charge carrier scattering times (b) –Si face, and (d) – C-face.

Figure 2 Critical point transition energy as a function of graphene thickness for epitaxial layers on Si-face (squares) and C-face (circles) of 3C-SiC (111).

Epitaxial growth of vertical III-V semiconductor nanowires on graphene Dasa L. Dheeraj1, Mazid A. Munshi1, Vidar T. Fauske2, Dong-Chul Kim1, Antonius T. J. van Helvoort2, BjĂ¸rn-Ove Fimland1 and Helge Weman1 1

Department of Electronics and Telecommunications, 2 Department of Physics, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway Email id: dheeraj.dasa@ntnu.no Semiconductor nanowires (NWs) have today advanced to a level beyond thin films with respect to design freedom, including structuring of both material composition and crystal phase with high spatial precision, making them promising for a number of electronic and optoelectronic device applications1. Graphene on the other hand, a zero-bandgap semiconductor, has some unique and complementary properties to conventional semiconductors that is believed to revolutionize future devices2. Apart from being an excellent electrical and thermal conductor, graphene is also transparent and flexible and thus has the potential to become an ideal electrode material for especially optoelectronic devices3, 4. Thereby, if these complementary materials classes can be combined, various unique hybrid devices can be realized. In this work, we show that by utilizing the reduced contact area of NWs, epitaxial growth of semiconductors on graphene can be achieved.5 Highly uniform vertical GaAs NWs were grown both on graphite and few-layer graphene using molecular beam epitaxy. Scanning electron microscopy and cross-sectional transmission electron microscopy studies revealed the epitaxial relationship of the NWs with the graphitic substrates in spite of a lattice mismatch of 6.3%. In addition, we present a generic atomic model which describes the epitaxial growth configurations of the semiconductor atoms on graphene and should in principle be applicable to all conventional semiconductor materials. Finally, a prototype of a single GaAs nanowire photodetector was also fabricated which demonstrates a high-quality material essential for successful optoelectronic device applications. References 1. Yang, P.; Yan, R.; Fardy, M. Nano Letters, 10 (2010) 1529. 2. Geim, A. K.; Novoselov, K. S. Nature Mater. 6 (2007) 183. 3. Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Nature Photon. 4 (2010) 611. 4. Chung, K.; Lee, C.-H.; Yi, G.-C. Science 330 (2010) 655. 5. Munshi, A. M.; Dheeraj, D. L.; Fauske, V. T.; Kim, D.-C.; van Helvoort, A. T. J.; Fimland, B.-O.; Weman, H. Nano Letters 12 (2012) 4570.

Figures

Figure 1. (a) Schematic drawing of self-catalyzed GaAs nanowires on a graphitic surface. (b) SEM image of nanowires grown on graphite by a two-temperature growth technique. In the inset a tilted-view image of one of the nanowires shows a uniform hexagonal cross-section. The scale bars are 200 nm in the main figure, and 100 nm in the inset [5].

Figure 2. TEM images of a representative GaAs nanowire grown on few-layer epitaxial graphene synthesized on a 6H-SiC(0001) substrate. (a) Cross-sectional bright-field TEM image of the selected vertical GaAs nanowire. The bottom part of the nanowire has a mixture of ZB and WZ segments with twins and stacking faults, whereas the rest of the nanowire (above the two red arrows) is nearly defect-free ZB. (b) Cross-sectional high-resolution TEM image showing the interface region of the graphene layers and the vertical GaAs nanowire marked with a red box in (a). Inset shows a magnified high-resolution TEM image of the nanowire/graphene/SiC interface area from the center part of (b), where the lattice fringes of the GaAs nanowire and the (0002) graphene layers separated by ~3.4 Ă&#x2026; can be seen. (c,d) Fast Fourier transforms from the highresolution TEM image in (b), from the nanowire/graphene/SiC and graphene/SiC interface regions, respectively [5].

Spin Transport and Spin Precession in Graphene AndrĂŠ Dankert, Johan Bergsten, M. Venkata Kamalakar, Saroj P. Dash Chalmers University of Technology, Department of Microtechnology and Nanoscience-MC2, SE-41296, GĂśteborg, Sweden saroj.dash@chalmers.se Abstract Spin degree of freedom of electrons is one of the alternative state variables under consideration for processing information, beyond the charge based CMOS technology. The potential of spintronics based research lies in the possibilities for a new generation of computers that can be non-volatile, faster, smaller, and capable of simultaneous data storage and processing - all with reduced energy consumption [1]. The strong interest in Graphene based spintronic devices stems from their proposed long spin coherence lengths (100 Âľm), because of the absence of hyperfine interactions and weak spinorbit coupling. A novel concept called all spin logic uses spin in ferromagnets to store information and communicate between them using a spin current [2]. All spin logic is particularly powerful in that it combines various spin related phenomena such as spin-injection, spin-transport and spin-detection with that of magnetization dynamics. Pure spin transport in Graphene and multilayer graphene on SiO2 substrate has been demonstrated at room temperature [3, 4], and more recently with high mobility Graphene devices [5-7]. The spin life time is found to be around few 100 pico seconds on all these measurements. Recently, a large spin-valve signal in two-terminal geometry at low temperature has also been demonstrated, using high resistive tunnel contacts [8]. In spite of all these advances, very little progress has been made in basic understanding of spin injection, transport and detection process in Graphene and its nanostructures. The observed spin life time differ by orders of magnitude than theoretical prediction. Furthermore, the basic functions such as large magnetoresistance in two-terminal spin-valve geometry at room temperature have not been demonstrated, and the spin manipulation has not yet been realized. Here we present pure spin transport and precession measurements in multilayer Graphene (around 10 layers) at room temperature. Graphene-ferromagnet nano devices with multiple contacts were prepared on SiO2 substrate by standard electron beam lithography and lift off technique. Ferromagnetic contacts on graphene with and without a tunnel barrier are tested to achieve high current densities and also larger spin signal, for realization of spin torque switching in all spin logic device [2]. The spin transport and spin precession measurements are performed in non-local spin-valve and Hanle geometries respectively [3]. We also compare the spin life time obtained from nonlocal measurements with the spin precession measurements in three-terminal Hanle geometry [9]. To perform the spin-transport measurements we use a non-local geometry where the charge current path is separated from the voltage contacts to exclude spurious signals such as AMR and stray Hall voltage. Figure 1a shows a typical non-local spin valve signal at room temperature by sweeping the in-plane magnetic field. Different switching fields of injector and detector ferromagnetic electrodes allow us to achieve a parallel or antiparallel configuration by a field sweep. To extract the spin relaxation time, we perform Hanle precession measurements and fit the data with the solutions for the Bloch equations in the diffusive regime. The Hanle precession measurements are done by measuring the non-local resistance as a function of an applied perpendicular magnetic field (Figure 1b). The spin life time were found to be around 100 pico seconds at room temperature. We also compare the spin life time measured in three-terminal Hanle geometry [9], in which the same tunnel interface is used for injection and detection of spin accumulation in Graphene. To be noted, very similar spin life times were extracted from both the non-local and three-terminal Hanle measurements. Nonlocal measurements were also performed at higher perpendicular magnetic fields up to 2T, where the rotation and saturation of ferromagnet occurs in the perpendicular direction. As shown in figure 1c, we observe anisotropy in spin relaxation in multilayer graphene between the parallel and perpendicular injected spin direction [10]. The perpendicularly injected spin at 2T magnetic field shows a decrease of signal amplitude compared to spins parallel to the layer at zero magnetic field. Furthermore, we will discuss the detailed investigation of spin-valve and Hanle measurements at different temperatures and bias voltages.

References [1] D. Awschalom, et al., Nature Phys. 3, 153 (2007). [2] H. Dery, et al., IEEE Trans. Elec. Dev 59, 259 (2012). [3] N. Tombros et al., Nature 448, 571 (2007). [4] T. Maassen, et al., Phys. Rev. B 83, 115410 (2011). [5] M. Guimaraes et al., Nano Lett. 12, 3512 (2012). [6] P. J. Zomer, S. P. Dash, et al. Appl. Phys. Lett. 99, 232104 (2011). [7] P. J. Zomer et al. Phys. Rev. B 86, 161416(R) (2012). [8] B. Dlubak et al., Nature Physics 8, 557(2012). [9] S. P. Dash et al. Nature 462, 491(2009). [10] N. Tombros et al., Phys. Rev. Lett. 101, 046601 (2008).

(a)

(b)

(c)

Figure 1. Nonlocal spin transport and spin precession measurements in multilayer Graphene devices. (a) Nonlocal spin valve measurement as a function of the in-plane magnetic field at room temperature, (b) Nonlocal Hanle measurement as a function of perpendicular magnetic field at room temperature, (c) Nonlocal measurement with higher perpendicular magnetic field, showing anisotropy in spin signal.

Enhancement of the DC electrical conductivity of Graphite Nanoplatelets through the control of the process parameters G. De Bellis, A. Tamburrano, A.G. D’Aloia, F. Marra, L. Paliotta, A. Bregnocchi, M.S. Sarto CNIS, Research Center for Nanotechnology applied to Engineering of Sapienza University, via Eudossiana 18, 00184 Rome, Italy giovanni.debellis@uniroma1.it Abstract Ever since its first obtainment as a single layer in 2004 through mechanical exfoliation [1], graphene and its related applications have received an ever growing attention, thanks to the peculiar properties of this material. Graphene mass production has recently become a crucial issue, in view of possible applications in various fields, such as flexible electronics [2], supercapacitors [3] and nanocomposites [4]. Among the different synthesis routes available, chemical exfoliation of graphene oxide (GO) and thermal exfoliation of graphite intercalation compounds (GIC) seem to be the most promising candidates for large yield production [5]. The work presented here is part of a larger study, whose main goal is to maximize the exfoliation yield of Thermally Expanded Graphite Oxide (TEGO) obtained starting from GIC (preferred over the liquid exfoliation route of GO, because of the simultaneous reduction of the material), and to tune the properties of the resulting graphite nanoplatelets (GNP) through the proper control of the synthesis parameters. GNPs are synthesized through thermal exfoliation of commercially available GIC, as reported elsewhere in detail [6]. The resulting TEGO is then dispersed in a suitable solvent mixture and the suspension is tip sonicated using an ultrasonic processor, thus obtaining GNPs. The sonicated GNP suspension is then subjected to vacuum filtration in order to obtain GNP thick films, having an average thickness in the range of 100-200 μm (depending on the sonication cycle and solvent used), suitable for electrical properties investigation. Several process parameters are believed to be critical for the resulting physical and structural characteristics of GNPs. In our previous works we already carried out a systematic study of the influence of several parameters on the DC properties of GNP thick films. The investigated parameters include: i) GIC exfoliation temperature and process duration; ii) TEGO sonication duty cycle; iii) Temperature of the dispersion under sonication; iv) The solvent used for GNP dispersion. In particular the solvents investigated include acetone, N,N-Dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP) and mixtures thereof. The previously investigated exfoliation temperature range goes from 750°C to 1250°C (an almost two fold increase of the DC electrical conductivity was found, as the GIC expansion temperature was raised from 750 to 1250°C [7]). The pulsed sonication cycle was preferred all throughout our research, in order to avoid overheating of the suspensions, and various cycles (differing on the ON and OFF duration of the process) have been considered. Finally the influence of the proper control of the suspension temperature during the sonication process has been addressed. The study presented here aims at investigating the effect on the final DC electrical properties of the fabricated GNP films of several parameters, such as GIC exfoliation temperature, post-synthesis annealing temperature and type of solvent (DMF- or NMP-acetone mixtures). In particular the investigated GIC exfoliation temperatures are 1150°C, 1250°C and 1350°C setting the process duration at 5s. The TEGO sonication cycle was carried out using two different solvent mixtures, namely acetone:DMF and acetone:NMP, both at 9:1 volume ratio. Finally the effect on the DC conductivity of the GNP films of two subsequent annealing steps, carried out at 250°C and 350°C, was investigated. After proper conditioning, as-produced GNP thick films, without metal coating, are characterized through electron microscopy, using a Zeiss Auriga FESEM. The obtained micrographs show a layered structure (Fig1(a)), demonstrating the high porosity of such films. It should be noted that only TEGOs collected from the top of the expansion crucible were utilized to produce the GNP films. In fact SEM analysis demonstrated that TEGOs collected from the bottom of the crucible are characterized by a blistered surface, due to entrapment of residual intercalating species in the gas phase (due to the poor heat exchange), as shown in Fig. 1(b) and confirmed by EDX (not shown). Fig 1 (c) and (d) demonstrate the absence of such blistering for GIC exfoliated at 1150°C and 1250°C respectively, whose corresponding TEGO have been collected from top of the crucible. On the other hand TEGO obtained at 1350°C 5s expansion, show the presence of pits (Fig.1(e)). The DC sheet resistance of the GNP film is measured using the four-point probe method at room temperature, before and after each annealing step. The measurement is repeated by positioning the test-probes in six different configurations over the sample. Fig.2(a) shows the measured sheet resistances of the films fabricated in the different conditions (GIC exfoliation temperature and TEGO

sonication medium) and after each annealing cycles. The measured electrical conductivity of the produced films after the second annealing step is shown in Fig.2(b): the obtained values are in good agreement with data recently published in literature for few layer GNP-films [8]. Finally, the estimated porosity of the films shown in Fig.2(c) demonstrates the crucial effect of the solvent on the morphological characteristics of GNPs, as well as on TEGO exfoliation degree Such effect will be confirmed by statistical investigations, based on results of SEM and AFM analyses, on the lateral size and thickness of GNPs produced under the different conditions, as previously described. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos et al. Science, 306 (2004) 666. [2] Eda, G., Fanchini, G. & Chhowalla, M.. Nature Nanotechnol. 3, (2008) 270–274 [3] J.J. Yoo, K.Balakrishnan, J. Huang, V. Meunier, B. G. Sumpter, A. Srivastava, M. Conway, A. L. Mohana Reddy, J. Yu, R. Vajtai, and P.M. Ajayan. Nano Lett.,11 (4), (2011) 1423–1427 [4] X. Huang,Xi. Qi, F. Boeyab and H. Zhang. Chem. Soc. Rev. 41, (2012) 666-686 [5] K. S. Novoselov, V. I. Falko, L. Colombo, P. R. Gellert, M. G. Schwab & K. Kim. Nature 490, (2012), 192. [6] G De Bellis, A Tamburrano, A Dinescu, ML Santarelli, and MS Sarto. Carbon 49 (2011):4291–4300 [7] G. De Bellis, F. Ruggeri, A. Broggi, A. Tamburrano, M. L. Santarelli, and M. S. Sarto, “Effect of the synthesis parameters on the dc resistance of graphite nanoplatelets thick films”, GraphITA, L’Aquila, Italy, (2011). [8] S. Wang,P.K. Ang, Z. Wang, A.L.L. Tang,J.T.L. Thong, and KP Loh, Nano Lett.10, (2010), 92-98. Figures

(b)

(c)

(a) (d)

(e)

Fig.1 – SEM images of GNP thick film (a), and TEGO exfoliated at different temperatures and collected from the bottom (b) and top (c-e) of the crucible.

(a) (b) (c) Fig. 2 – Measured sheet resistance (a), electrical conductivity (b) and porosity (c) of produced GNP thick films.

Solution-based graphene for high-performance flexible electronics 1

1,5

Cédric Sire, Joël Azevedo, Stéphane Campidelli, Sylvie Lepilliet, Jung-Woo T. Seo, Mark C. Hersam,3 Sébastien Sorgues,4 Christophe Colbeau-Justin,4 Jean-Jacques Benattar,5 Gilles Dambrine,2 Henri Happy,2 Vincent Derycke1 1

CEA Saclay, IRAMIS, Service de Physique de l’Etat Condensé, Laboratoire d’Electronique Moléculaire, F-91191 Gif sur Yvette, France 2 Institut d’Electronique, de Microélectronique et de Nanotechnologie, UMR-CNRS 8520, BP 60069, Avenue Poincaré, F-59652 Villeneuve d’Ascq, France 3 Department of Materials Science and Engineering and Department of Chemistry, Northwestern University, Evanston 60208-3108, Illinois, USA 4 Univ Paris-Sud and CNRS, Laboratoire de Chimie Physique, UMR 8000, F-91405 Orsay, France 5 CEA Saclay, IRAMIS, Service de Physique de l’Etat Condensé, GMOB, F-91191 Gif sur Yvette, France vincent.derycke@cea.fr

Abstract The potential of graphene transistors for radio-frequency (RF) electronics was recently demonstrated by several groups using exfoliated, SiC-based and CVD-based graphene. The most recent studies reached de-embedded current gain cut-off frequencies (f T) in the 100-300 GHz range with room for improvement at both the material and device levels. In parallel, graphene is being explored for large scale electronics on flexible substrates via CVD growth on metal foils associated with transfer methods. This progress is notably driven by the perspective of replacing ITO as the material of choice for the transparent electrodes required in applications such as touch screens, flat panel displays or organic photovoltaic cells. However, the combination of these two properties, namely high speed and flexibility, remains an open challenge. In particular for the viable development of fast and flexible electronic applications in the areas of portable / wearable communicating devices with low power consumption, this combination should be achieved with a source of material adapted to large-scale exploitation. Progress in the field notably implies to (i) build and characterize RF flexible devices, and (ii) to develop low-cost methods for the controlled production of large scale and high quality 2D materials thin films. Concerning the first point, printed electronics based on organic materials is a well-established field. Organic materials are particularly well suited for flexible circuits due to their mechanical resiliency. Yet, their low charge mobility limits their ultimate operating frequency. While several examples of organic devices and circuits operating in the kHz to MHz range have been demonstrated, these approaches fall well short of the GHz range. Graphene transistors on flexible substrates were realized by several groups, but up to now, their high frequency performances were not evaluated. We recently demonstrated [1] that solution-based single-layer graphene ideally combines the required properties and presents important advantages over alternative graphene sources. For this study, we employed solution-based, predominantly single-layer graphene flakes isolated via density gradient ultracentrifugation [2]. The devices (see Figure 1) operate at low bias (VDS<0.7 V), achieve current gain cut-off frequencies f T as high as 2.2 GHz before de-embedding (8.7 GHz after de-embedding), power gain cut-off frequency fMAX of 550 MHz and have a constant transconductance in the GHz range [1]. RF measurements directly performed on bent samples show the remarkable mechanical stability of these devices and demonstrate the advantages of solution-based graphene FETs over organic materials for analog RF electronics. Concerning the second challenge, the CEA developed an original method for the controlled formation of highly-order thin films of nano-objects (including nanowires, carbon nanotubes and graphene oxide flake) based on the transfer of surfactant-stabilized water films [3,4]. We modified and optimized this method [5] to allow the homogeneous and large-scale assembly of very large graphene oxide flakes onto all type of substrates including organic flexible ones. The film thickness can be very precisely controlled from individual flakes to multi-layers (see Figure 2). The films show remarkably low roughness and the flakes are almost totally wrinkles-free. After reduction, the reduced graphene oxide (rGO) films reach transparencies and conductivities compatible with their integration into prototype photovoltaic cells. We notably used Time Resolved Microwave Conductivity (TRMC) and graphene/silicon solar cells to study the mechanism of charge separation at the carbon/silicon interface. References [1] C. Sire, F. Ardiaca, S. Lepilliet, J-W. T. Seo, M.C. Hersam, G. Dambrine, H. Happy, V. Derycke, Nano Lett. 12 (2012) 1184. [2] Green, A. A.; Hersam, M. C. Nano Lett. 9 (2009) 4031-4036.

[3] Guolei Tang, Xinfeng Zhang, Shihe Yang, Vincent Derycke, Jean-Jacques Benattar, Small 6 (2010), 1488. [4] J. Azevedo, C. Costa-Coquelard, P. Jegou, T. Yu and J.-J. Benattar, Journal of Physical Chemistry C 115 (2011) 14678. [5] J. Azevedo, S. Campidelli, D. He, S. Sorgues, C. Colbeau-Justin, J-J. Benattar, V. Derycke, Waferscale, ultra-smooth and thickness-controlled graphene oxide films based on very large flakes, submitted. Figures

30 25

After Joule annealing

SL-Graphene

H21 , U (dB)

15 10 5 0 -5

fMAX = 550 MHz

-10 fT (ext) = 2.2 GHz

-15 -20 -25

fT (int) = 8.7 GHz

0.1

frequency (GHz)

Figure 1 : Solution-based single-layer graphene flakes assemble by dielectrophoresis (DEP) on a polyimide substrate and integrated into high-frequency top-gated FET device structures. Evolution of the current gain H21 (before and after de-embedding) and of the power gain U of such flexible FET after Joule annealing. The de-embedded cut-off frequency f T is 8.7 GHz.

Figure 2 : (left) AFM image of a sub-monolayer assembly of large and wrinkle-free graphene oxide flakes. (right) Two-inch quartz wafer covered with an ultra-smooth reduced graphene oxide multi-layer film forming a conductive and transparent electrode.

AFM-Raman and Tip Enhanced Raman studies of graphene P. Dorozhkin, A. Shelaev, A. Shchokin, E. Kuznetsov, S. Timofeev, V. Bykov NT-MDT Co., Build. 100, Zelenograd Moscow, 124482 Russia, dorozhkin@ntmdt.com

We demonstrate design and applications in graphene research of an Atomic Force Microscope integrated with Confocal Raman/Fluorescence/Rayleigh microscopy and Tip Enhanced Raman Scattering microscopy. Different graphene samples are investigated by different types of AFM and spectroscopy techniques providing comprehensive information about the sample. We study in details how the thickness (number of monolayers) in graphene affects its physical properties: surface potential (work function), local friction, elastic modulus, capacitance, conductivity, charge distribution, Raman and Rayleigh light scattering etc. Results for graphene flakes are qualitatively compared to those for carbon nanotubes of different diameters. We show how electrostatic charging of graphene flakes can be effectively measured and modified by AFM cantilever. Raman C-peak of graphene is studied by using high transmission volume bragg grating filters. Studies are performed both in ambient air conditions and in controlled atmosphere and humidity.

a), b). AFM – Raman configuration: schematics (a) and white light image (b); Raman laser is tightly (400 nm spot diameter) focused onto the very end of a “nose”- shaped AFM cantilever using 100x objective; Graphene layer is positioned below the cantilever and under the laser spot; while scanning the sample, AFM and Raman data is obtained simultaneously. c) – g). Various AFM images characterizing different physical properties of the sample - Topography (c), Electrostatic Force (d), Force Modulation (elastic properties) (e), Kelvin Probe (f), Lateral Force (g). h) – j). Confocal optical images – Rayleigh light (h), Raman 2D band mass center (i), Raman G band intensity (j). The ultimate goal of integrating AFM with Raman/fluorescence spectroscopy is to break diffraction limit and to bring spatial resolution of optical methods down to resolution of AFM (a few nm). We also present results of Tip Enhanced Raman Spectroscopy (TERS) or “nano-Raman” mapping of graphene on Si substrate. We demonstrate near field Raman enhancement effect due to resonant interaction of light with localized surface plasmon at the apex of a metal AFM probe. Actual plasmonic and near field nature of the Raman enhancement is proven by a number of ways: dependence of the enhancement on the excitation wavelength and polarization, enhancement versus tip-sample distance curves, observation of selective enhancement of Raman signal from thin surface layer etc. Finally, the ultimate performance of TERS is demonstrated by measuring Raman 2D maps with subwavelength lateral resolution – determined not by the wavelength of light, but by the localization area of the surface plasmon electromagnetic field.

Excitons and terahertz transitions in narrow gap carbon nanotubes and graphene nanoribbons 1

C. A. Downing , R. R. Hartmann , I. A. Shelykh and M. E. Portnoi

School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK Science Institute, University of Iceland, Dunhagi 3, IS-107, Reykjavik, Iceland C.A.Downing@exeter.ac.uk

Abstract We calculate the exciton binding energy in narrow band gap single-walled carbon nanotubes with and graphene nanoribbons, accounting for the quasi-relativistic dispersion of electrons and holes. Exact analytical solutions of the quantum relativistic two-body problem are obtained for several limiting cases. We show that the binding energy scales with the band gap, and conclude on the basis of the data available for semiconductor nanotubes that there is no transition to an excitonic insulator in quasimetallic nanotubes and that their proposed THz applications [1] are feasible. Depending on the presence of a metallic gate and the carrier density, exciton can be either described by a short-range electron-hole interaction potential [2] or by an unscreened potential, similar to that considered by Loudon in the 1950s [3, 4]. Our analysis shows that the Loudon potential is a good fit for the quasi-one-dimensional Coulomb potential, obtained by averaging the three-dimensional Coulomb potential with the envelope functions (Fig. 1). We report exact analytic solutions for the quasi-relativistic Loudon problem for an exciton with a zero total momentum along the nanoribbon or nanotube axis. The complex four-component structure of the electron-hole relative motion wavefunction which is obtained when two graphene sublattices and two types of particles are taken into account, results in a counterintuitive dip in the shape of the particle density distribution within the exciton, shown in Fig 2. The vanishing exciton binding energy with decreasing the energy gap removes for narrow gap graphene-based nanostructures the undesirable effect of strongly-bound dark excitons, which is known to suppress optical emission in semiconducting nanotubes. However, the Coulomb interaction remains very important as it smears the van Hove singularity in the one-dimensional density of states [5]. We report the resulting shape of the terahertz emission from narrow gap carbon nanotubes and nanoribbons with the Coulomb effect taken into account, for both the long-range and short-range interaction models [6].

References [1] M. E. Portnoi, O. V. Kibis, and M. Rosenau da Costa, Superlattices Microstruct. 43, 399 (2008). [2] R. R. Hartmann, I. A. Shelykh, and M. E. Portnoi, Phys. Rev. B 84, 035437 (2011). [3] R. Loudon, Am. J. Phys. 27, 649 (1959). [4] R. J. Elliott and R. Loudon, J. Phys. Chem. Solids 8, 382 (1959). [5] H. Haug and S. W. Koch, Quantum Theory of the Optical and Electronic Properties of Semiconductors (World Scientific, Singapore, 2004). [6] C. A. Downing and M. E. Portnoi, (in preparation, 2013).

Figures

Fig 1. A plot of the dimensionless quasi-one dimensional Coulomb potential as a function of scaled relative coordinate x/R, with the regularized potential of Loudon denoted by a solid (red) line and the averaged potential given by the dotted (blue) line.

Fig 2. The density of a 1s-exciton for a (10,10) carbon nanotube with a magnetic field induced gap of 10 meV (2.5 THz) corresponding to a magnetic field of 15 T along the nanotube axis. The density represents the probability of finding the electron and hole comprising the exciton at the indicated relative separation. Red and blue colours correspond to the highest and lowest values of density, respectively.

Structural, electronic and transport properties of quasi-1D BNC heterostructures. Simon M.-M. Dubois, Xavier Declerck, Jean-Christophe Charlier, Mike C. Payne Cavendish Laboratory, University of Cambridge, 19, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom smmfnd2@cam.ac.uk Two dimensional hexagonal BN (h-BN), an isomorph of graphene with a lattice mismatch of only 1.7%, is a wide gap insulator as its bulk counterpart [1]. Advances in the synthesis of hybrid BNC heterostructures offer new opportunities to engineer the electronic properties of low-dimensional systems. Recently, it has been shown that the introduction of h-BN nanodomains into graphene enables to induce a tunable band gap in the honeycomb lattice [2]. Besides, lateral junctions between electrically conductive graphene and insulating h-BN provide new ways to embed electrically isolated elements within single atomic layers [3]. Not only the two-dimensional BNC heterostructures hold promises for new applications but also the corresponding quasi-1D nanoribbons. In this work, we detail the impact of the edges on the stability and electronic structures of graphene and h-BN nanoribbons and we explore the electronic and transport properties of various prototypical quasi-1D BNC systems. Our results, obtained by means of firstprinciples calculations, emphasize the potential of those systems for applications in future electronic and spintronic devices. References [1] X. Blase, A. Rubio, S.G. Louie, and M.L. Cohen, Phys. Rev. B 51 (1995) 6868. [2] L. Ci, L. Song, C. Jin, et. al., Nature Materials 9 (2010) 430. [3] M.P. Levendorf, C.-J. Kim, L. Brown, et. al., Nature 488 (2012) 627.

Title:

Superconductivity in Two-Dimensional Crystals

Authors: M. S. El Bana 1, 2 and S. J. Bending1 Affiliations: 1

Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK

Department of Physics, Ain Shams University, Cairo, Egypt.

E-mail: meb26@bath.ac.uk

Abstract Since the first isolation of graphene in 2004 interest in superconductivity and the superconducting proximity effect in monolayer or few-layer crystals has grown rapidly [1]. Here we describe investigations of the superconducting transition in few molecular layer dichalcogenide flakes. Our lithographicallydefined 4-terminal devices have been realised by micromechanical cleavage from a 2H-NbSe 2 single crystal onto Si/SiO2 substrates followed by the deposition of Cr/Au contacts (c.f., the atomic force microscope image in Figure 1). Atomic force microscopy and Raman spectroscopy have been used to characterise the quality and number of molecular layers present in our flakes [2]. While very thin NbSe2 flakes do not appear to conduct, slightly thicker flakes are superconducting with an onset T c that is only slightly depressed from the bulk value (7.2K). Figure 2 plots the 4-point resistance of one of our devices from 0-300K. The resistance typically shows a small, sharp high temperature transition followed by one or more broader transitions which end in a wide tail to zero resistance at low temperatures (c.f., inset of Fig. 2). The temperature of the highest transition drops slowly as the flake thickness decreases in agreement with earlier works [3,4]. Estimates of the 300K resistivity for our flakes are several times higher than that found in bulk single crystals, and we speculate that these multiple resistive transitions are related to disorder in the layer stacking rather than lateral inhomogeneity as was proposed by Frindt [3]. The behaviour of several flakes has been characterised as a function of temperature, applied field and backgate voltage. We find that the resistance and transition temperatures depend weakly on the gate voltage, with both conductivity and Tc decreasing as the electron concentration is increased. The application of a small perpendicular magnetic field rapidly suppresses the highest temperature T c and for H>0.2T the transition broadens into a single featureless curve. Our results will be analysed in terms of available theories for these phenomena. References: [1] Novoselov K. S., et al., PNAS 102, (2005), 10451. [2] Dattatray J. Late, et al., Advanced Functional Materials 22, (2012), 1894-1905. [3] Frindt, R. F., Physical Review Letters 28, (1972), 299. [4] Neal E. Staley, et al., Physical Review B 80, (2009), 184505.

Figure 1:

3.8Âľm

Figure 2:

Full Title Development of nanostructured materials for solar cells and other applications. Presenting Author: Eduardo Elizalde, Co-Authors. Sergio Pinilla, Arancha G贸mez, Teresa Campo, Carmen Morant Organization, Address, City, Country Departamento de F铆sica Aplicada, Universidad Aut贸noma de Madrid, Cantoblanco, 28049 Madrid, Spain Contact@E-mail: eduardo.elizalde@uam.es Abstract (Arial 10)

The main goal of this research has been the creation of a nanostructured silicon surface capable of a high reduction of the reflectance, using an inexpensive and scalable method.

Vertically aligned silicon nanowire (Si NW) arrays have been fabricated using an electroless etching (EE) method. This method involves etching of silicon wafers in a silver nitrate and hydrofluoric acid based solution. We have studied different parameters of this growth and its effects over the reflectivity. The Si NWs were characterized by SEM and XRD. Si NWs have been directly synthesized by the use of a nitrate and hydrofluoric acid based solution. This method gives a constant rate of growth for each temperature being quite sensitive for this parameter. During the attack of the solution, homogeneously dispersed silver ions come into contact with the substrate surface. Then galvanic reactions take place simultaneously, and as a result, silicon is lost into the solution as SiF6, leaving pits on the wafer surface. The preferential deposition of the silver ions into these pits leads to the SiNWs formations. Conclusion: 1) Si NWs can be synthesized by electroless etching method. The temperature and etching time are critical factors that determine the length of the Si NW and the reflectance of the attacked wafer. 2) Vertically aligned Si NWs only grow on the polished face of the wafer. 3) The growth rate is determined by the temperature of the solution. Figures

The SEM images show a periodic ordination of vertically aligned silicon nanowires. After an attack of 50 minutes at room temperature, we get an average length of 2.5microns.

CVD Grown Monolayer Graphene Doping By Adsorbtion of V-VII Group Elements 1

D. Erts , J.

Andzane1, J. Kosmaca1, G. Kunakova1, S. Kubatkin2, A. Lartsev2, J. D. Holmes3 1

Institute of Chemical Physics, University of Latvia, Riga, Latvia 2 Chalmers University of Technology, Gothenburg, Sweden 3 Department of Chemistry, University College Cork, Cork, Ireland Donats.Erts@lu.lv

Graphene is very highly promising candidate for future electronics due to its twodimensionality and an extremely high charge carrier mobility [1]. The application of graphene in future electronic devices requires control of the character and density of its charge carriers. Control of the type and density of charge carriers can be realized both by the electrostatic gate and the chemical doping methods. While electrostatic gating has been successfully demonstrated [2], it does require a DC bias source, while the development of useful chemical doping methods, working â&#x20AC;&#x2122;off-lineâ&#x20AC;&#x2122; is a real challenge. Different molecules such as NO2, H2O, NH3 has been used for p-type doping of graphene [3]. However, NO2, H2O, and NH3 are very reactive chemicals and cannot be used in an electronic material. An alternative is presented by the heavier elements, which are less reactive, and can induce the spin-orbit interaction in graphene. It is expected that bismuth as well as antimony could be able to extract electrons out of the graphene sheet. Recently, the Dirac point shift towards the Fermi level and increase of the line width of the bands for bismuth doped epitaxial graphene were predicted theoretically and shown experimentally [4]. Antimony, which is located in group V of the periodic table just above bismuth, is expected to give similar results. Abstract

Here we present the CVD grown monolayer graphene doping by adsorption methods. V-VII group elements as antimony, sulfur, iodine were used as dopants. Doping methods include both full-surface modification and local surface modification. For the local graphene surface doping antimony sulfide nanowires and nanowire clusters are used. Graphene samples are prepared for doping and further research using electronic lithography and in-situ nanomanipulation techniques. Experiments are carried out both under high vacuum and ambient conditions. References [1] Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva,I.V.; Firsov, A. A., Science, 306 (2004), 666. [2] Oostinga, J. B.; Heersche, H. B.; Liu, X.; Morpurgo, A. F.; Vandersypen, L. M. K. Nature Mater., 7 (2008) 151. [3] Wehling, T. O.; Novoselov, K. S.; Morozov, S. V.; Vdovin, E. E.; Katsnelson, M. I.; Geim, A. K.; Lichtenstein, A. I. Nano Lett., 8 (2008) 173. [4] Gierz, I; Riedl, C; Starke, U; Ast, C. R.; Kern, K. Nano Lett., 8 (2008) 4603.

Graphene functionalization : A XPS layer dependent study 1

A. Felten , A. Eckmann , C. Casiraghi 1

3,4

, J.-J. Pireaux

Research Center in Physics of Matter and Radiation (PMR), University of Namur, Namur, Belgium 2 School of Physics and Astronomy, University of Manchester, UK 3 Physics department, Free University Berlin, Germany 4 School of Chemistry and Photon Science Institute, Manchester University, UK alexandre.felten@fundp.ac.be

Graphene based devices fabrication is currently impeded by its metallic behavior and its lack of reactivity making it hard to create graphene based transistors or sensors. In order to overcome this problem, various approaches to tune and control the electronic properties of graphene have been considered. In this work, we investigate the controlled modification of graphene using mild oxygen (O2) plasma treatment [1]. The treatments were performed on monolayer and bilayer graphene and graphite. The comparison between these three samples allows us to study the difference of reactivity when increasing the number of layers. X-ray photoelectron spectroscopy (XPS) using a micro beam probe is then performed on pristine and plasma modified graphene and graphite. This technique is able to give chemical bonding information on single exfoliated graphene flakes. We studied the carbon 1s peak evolution with different plasma parameters such as treatment time, power or sample position inside the chamber. The functionalized graphene layers present a shoulder at high binding energy (286-289 eV) which corresponds to carbon-oxygen bonds such as hydroxyl, carbonyl and carboxyl. We show that changing plasma parameters allows tuning of the functionalization. In addition, the amount and type of functional groups are found to vary with the number of graphene layers.

[1] A. Felten et al., Small, 2012, early view available online.

Removal of Selenium Ions from Aqueous Media by Magnetic Graphene Oxide You Fu, Jingyi Wang, Qingxia Liu*, Hongbo Zeng* Department of Chemical Engineering, University of Alberta, T6G 2V4, Edmonton, Canada yf6@ualberta.ca; qingxia2@ualberta.ca; hongbo.zeng@ualberta.ca Abstract Selenium is an essential nutrient element for life at trace concentrations, but extremely toxic at higher [1] concentrations. Selenium is widely distributed in sulfide minerals and coal ashes. During mining and mineral processing, selenium ions can be released into water and soils, and then further transferred to the plants, animals and life cycles. A guideline value of less than 40 ppb selenium in drinking-water is [2] given by the World Health Organization (WHO). Excessive selenium can be normally accumulated through mine drainage water, industrial wastewater and agricultural drainage water. The removals of selenium from these water systems are crucial to reduce its environmental impact and enhance the 20 2sustainable development of natural resources. Among all the valence states (i.e., Se , Se , SeO3 , and 2SeO4 ), Se (IV) (selenite) and Se (VI) (selenate) are more mobile and toxic. [3]

Among the different methods developed for the removal of selenium (such as ion exchange, air flotation, chemical precipitation, emulsion liquid membranes, nanofiltration, biological reduction, reverse osmosis, solvent extraction and adsorption), the adsorption method is considered to be more efficient and economic due to its fast removal rate and minimum pretreatment of samples. However, conventional adsorbents, such as silica gel, clay, activated carbon, molecular sieve, ferrihydrite, ferric oxyhydroxide/peat/resins, and activated aluminium oxide, only show limited removal capacity for Se (IV) [3] and perform poorly in removing Se (VI). In this work, a water-dispersible magnetic graphene oxide (MGO) was synthesized using a two-step [4] method: (1) preparation of graphene oxide (GO) by a modified Hummers method , and (2) in situ growth of magnetic iron oxide nanoparticles. The chemical and surface properties of GO and MGO were characterized by various techniques such as X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Atomic Force Microscope (AFM), X-ray Photoelectron Spectroscopy analysis (XPS), Transmission Electron Microscopy (TEM), Scanning Electron Microscope (SEM), Quantum Design 9T-PPMS magnetometer and Thermo Gravimetric Analysis (TGA). The XRD patterns manifested that the magnetic particles were mainly Fe3O4 and ď §-Fe2O3 particles. FTIR and XPS spectra explained the bonding mode of Fe with O. AFM image illustrated the thickness of monolayer GO. Together, TEM (Figure 1.) and SEM images showed the uniform distribution of magnetic particles on monolayer GO sheets. Thermo stability was proved according TGA results. Magnetic hysteresis loop showed about equal to zero value for Mr (remanence) and Hc (coercivity), indicating MGO is superparamagnetic. MGO was applied to remove selenium ions (both Se (IV) and Se (VI)) in industrial wastewater. The systematic adsorption tests showed high removal ratio of ~99% for Se (IV) and ~85% for Se () in wastewater within 10 seconds. Adsorption capacity (Q (mg/g)) under different initial (a) Se (IV) and (b) Se (VI) concentrations were well fitted both Freundlich and Langmuir Models (Figure 2.), indicating a 6 large potential saturated adsorption capacity (64.10 mg/g for Se (IV) and 1.44*10 mg/g for Se (VI)). The [5] synthesis and adsorption mechanisms of the MGO were proposed. Additionally, the MGO could be separated effectively under an external magnetic field and reused after recycling. It is evident from this work that MGO is a novel and very promising adsorbent for selenium removal, and our results provide new insights into the development of novel graphene-based nanomaterials for water treatment and [6] environmental applications. References [1] L. Schomburg, U. Schweizer, J. KĂśhrle, Cellular and molecular life sciences 61 (2004) 1988. [2] I. Brouwer, A. De Bruin, O. B. Dirks, J. Hautvast, The Lancet 331(1988) 223. [3] L. Twidwell, "The Removal of Arsenic, Selenium and Metals from Aqueous Solution by Iron Precipitation and Reduction Techniques", presented at TMS2011 Annual Meeting, San Diego, CA, (2011). [4] S. M. Paek, E. J. Yoo, I. Honma, Nano letters 9 (2008) 72. [5] Y. Fu, J. Wang, Q. Liu, H. Zeng. Advanced Functional Materials. (submitted) [6] Y. Fu, J. Wang, Q. Liu, H. Zeng. [US Patent]. (File No. 69147-58)

Figures

Figure 1. TEM images of (a) GO, (b) MGO and (c) HRTEM image of selected area of magnetic particles, insert: FFT image of selected area electron diffraction pattern of magnetic particles; (d) EDS spectrum indicates the presence of elements O, C, and Fe; (e) EELS spectrum of iron oxide nanoparticles in MGO with an atomic percentage of Fe (17.77 %) and O (82.23 %).

Figure 2. Fitting curves using Freundlich and Langmuir Models under different initial (a) Se (IV) and (b) Se (VI) concentrations.

Imaginenano-Graphene 2013 Enabling the Era of Carbon Electronics: Turning Diamond into Graphene Arnaldo Galbiati Solaris Photonics, London, U.K. admin@solaris-photonics.com Abstract The potential uses of carbon in electronics has grown rapidly in the last 10 years as the breadth of its potential applications has begun to be appreciated fully. We are now able to design electronic devices based on the allotropes of carbon materials, including diamond, diamond-like carbon and graphene. While graphene is an electrically conductive 2-dimensional sheet of sp2 bonded carbon atoms, in diamond the carbon atoms bond in an sp3tetrahedral structure and the wide band gap (5.5 eV) semiconductor diamond is formed. The combination of extreme electronic and thermal properties found in synthetic diamond produced by chemical vapor deposition (CVD) make it an ideal semiconductor material. Experimental studies have demonstrated charge-carrier mobilities of >3000 cm2V–1s–1, thermal conductivities >2000 Wm–1K–1, and has a breakdown field strength in excess of 10 MVcm–1. Diamond is the perfect semiconductor material wherever high frequencies, high powers, high temperatures or high voltages are required. We will present a novel technology that allows the fabrication of graphene directly on diamond substrates thus enabling the creation of novel graphene on diamond electronics devices enabling the new era of carbon electronics

References [1] A. Galbiati, “Contacts on Diamond”, US Patent US8119253, (2012). Figures: [2] Balmer et al., “Diamond as an Electronic Material”, Materials Today, Issue 1-2, (2008), p.25

Two Point-Contact Method for the Electrical Characterization of Graphene-On-Insulator Samples 1

Cristina Fernandez , Noel Rodriguez , Carlos Márquez , Blanca Biel , Francisco Gamiz 1

Nanoelectronics Laboratory, CITIC-UGR, Dept. of Electronics, University of Granada, Spain noel@ugr.es, fgamiz@ugr.es

Abstract Graphene is a promising candidate as a material for future electronics [1]. However, a long way has still to be run. For example, new characterization tools to study and monitor its electrical properties has to be developed. These new tools should target its key electronic applications (i.e. Graphene-On-Insulator for device fabrication). In this work, we introduce a new method for the electrical characterization of graphene layers based on a two point-contact configuration, Figure 1.(a). Two tungsten needles acting as source and drain electrodes, are applied on the insulator surface (SiO2) covered by the graphene layer. For the calibration of the method, the pressure of the needles on the surface should be adjusted. Figure 1.(b) shows the drain current (VD=1V, VS=0V, VB=floating) as a function of the probe pressure. A good saturation is achieved above 50gr for a tip radius of 25µm. This initial calibration is essential to guarantee reproducible measurements. Figure 2.(a) shows the current between the two needles, for a given bias point, as a function of the needle inter-distance, d. As observed, the resistance remains relatively independent of d showing a metallic behavior. Considering a form factor for the current flow of W/d=0.7 [2] and assuming an 12 -2 electron density of 10 cm [3], the extracted value for the surface conductivity is 0.75µS which turns 2 into a mobility of 4700 cm /Vs. This value is lower than expected if only the optical phonon scattering due to the SiO2 substrate would be considered, and may reflect the impact of the needle to graphene contacts. In Figure 2.(b) the dependence of the driven current with the substrate bias (vertical electric field) is minimal (< 5%) as expected from a zero-band-gap material. Capacitance measurements were performed with an Agilent 4294A impedance analyzer and a series resistance model. For these measurements, the surface needles were short-circuited acting as low electrodes (LC, LP) and the gate was connected to the high (HC, HL) electrode [4]. The resulting capacitance curves as a function of the frequency of the AC excitation signal are shown in Figure 3.(a). Different configurations of the surface needles (single needle, two-needle with different inter-distances) have been considered. For a given sample, the maximum value of the capacitance is independent of the needle distribution confirming the reliability of the technique. It is worth noting that, for defective-free graphene layer, the capacitance obtained corresponds to that of an ideal plate capacitor (C=SGraph εSiO2/tBOX=45nF). The different cut-off frequencies must be further investigated and may be attributed to limited carrier spreading effects. Finally, Figure 3.(b) shows the capacitance curves obtained when the high and low potentials are exchanged. The results are identical demonstrating no impact of the silicon substrate.

Acknowledgements This work has been partially funded by Spanish Government through project TEC-2011-28660 and Junta de Andalucia under project TIC-2010-6209. Thanks are due to Graphenea and AMO GmbH for supplying graphene samples.

References [1] A. K. Geim and K. S. Novoselov, Nature Materials, 6 (2007) 183. [2] K. Komiya, N. Bresson, S. Sato, S. Cristoloveanu and Y. Omura, IEEE Transactions On Electron Devices, 52 (2005) 406. [3] Chen, J. H. et al., Nature Nanotechnology, 3 (2008), 206. [4] D. K. Schroder, John Wiley and Sons, Third Ed. (2006).

Figures

Figure 1: (a) Schematic of the two Point-Contact technique for electrical characterization of Graphene. (b) Drain current as a function of the needle pressure for a given bias point in a square shaped graphene on insulator layer satisfying d << L. The current remains saturated above 50 gr. d=1.59mm.

Figure 2: (a) Drain current at a given bias point as a function of the distance between the needles, substrate electrode is floating. (b) Drain current at a given bias point as a function of the substrate bias 18 -3 from -5V to 5V. TBOX=90nm, ND-substrate=10 cm .

Figure 3: Source-Drain to substrate capacitance as a function of the excitation signal frequency from 40Hz to 60MHz. (a) Capacitance curves measured using one and two needles and considering several distances between them, d. (b) Capacitance curves measured exchanging the high and low potentials in the experimental setup.

Study of EUV induced defects on few-layer graphene 1

1,3

A. Gao , P.J. Rizo , E. Zoethout , L. Scaccabarozzi , C.J. Lee , V. Banine , F. Bijkerk 1. FOM-Dutch Institute for Fundamental Energy Research, Edisonbaan 14,3439 MN Nieuwegein, the Netherlands. 2. ASML, De Run 6501, 5504DR Veldhoven, the Netherlands. 3. MESA+ Institute for Nanotechnology, PO Box 217, University of Twente, 7500 AE, Enschede, the Netherlands. a.gao@differ.nl Abstract 1-3 Defects in graphene greatly affect its properties . Radiation induced-defects may reduce the long-term survivability of graphene-based nano-devices. Here, we expose few-layer graphene to extreme ultraviolet (EUV, 13.5nm) radiation and show there is a power-dependent increase in defect density. We also show that exposure to EUV radiation in an H2 background increases graphene's dosage sensitivity. This may be due to reactions caused by the EUV induced hydrogen plasma. The nature of the defects 3 was studied with X-ray photoelectron spectroscopy (XPS), which showed that the sp bonded carbon and oxide fractions increase with exposure. The experimental results are important for understanding the defect-creating mechanisms upon photon interaction as well as designing graphene-based components for EUV lithography systems. 2

Graphene samples grown on 25x25mm Ni/Si substrate by CVD were obtained from Graphene Laboratories, Inc. Each sample had 1 to 7 layers, with a spatial average of 4. The samples were exposed to EUV, and for comparison purposes, hydrogen radicals, under conditions summarized in table 1. Raman spectroscopy and XPS were used to study the defects in graphene. Fig. 1 shows the Raman spectra of the five samples. The spectrum for the sample exposed to EUV in a hydrogen background (SEUV+H2) has the highest D peak intensity. The spectra for the samples exposed to atomic hydrogen (SH) and EUV irradiation (SEUV) show slightly lower D peak intensities. The pristine sample (Sref), and the one exposed to molecular hydrogen (SH2) have the lowest D peak intensities. An increased D peak intensity indicates increased defect density. Fig. 2 and Fig. 3 show the D/G ratio dependence on the EUV power for SEUV and SEUV+H2. In Fig. 2, the D/G ratio maps of the SEUV and SEUV+H2 show clear differences between exposed and unexposed areas. The two maps also coincide with the EUV intensity profiles, as shown in Fig.2. The relationship between D/G ratio and EUV power is plotted in Fig. 3. For SEUV +H2, the D/G ratio first grows as the EUV power increases, then saturates. It seems, however, for SEUV, that the D/G ratio shows a linear dependence on EUV power. After Raman spectroscopy, the samples were examined by XPS. Fig. 4 shows the curve fitting results. 2 3 There are four components for the graphene samples: carbide, sp bonds, sp bonds and –COH. Their 2 concentrations are shown in table 2. For the SH, SEUV and SEUV +H2, the sp concentration decreases 3 2 3 while the sp concentration increases, indicating a sp phase to sp phase transformation. In the case of SEUV and SEUV +H2, the increase of –COH concentration suggests that graphene may be reacting with 3 background water. However, for SEUV+H2, oxidation is more prevalent than sp formation, despite what might be expected to be a reducing environment. We speculate that the added H2, together with EUV radiation, created more OH radicals from the background water. The Raman and XPS results reported here show that there are defects induced on graphene after EUV irradiation, which are reflected by an increase of D peak intensity. EUV irradiation, in the absence of hydrogen, introduces defects, both through oxidation with the residual water background, and, more 2 directly, by breaking sp bonds. Bonding-breaking defects are caused by EUV photons, energetic electrons and ions. In a hydrogen atmosphere, oxidation is still significant, due to the formation of OH groups by hydrogen and water plasma, generated during EUV irradiation. References [1] Florian Banhart, Jani Kotakoski, and Arkady V. Krasheninnikov, ACS Nano, 2011 5 (1), 26-41. [2] J. Hicks, R. Arora, E. Kenyon, P. Chakraborty, H. Tinkey, J. Hankinson, C. Berger, W. de Heer, E. Conrad, and J. Cressler, Applied Physics Letters 2011, 99, 232102-232102. [3] S. Y. Zhou, Ç. Ö. Girit, A. Scholl, C. J. Jozwiak, D. A. Siegel, P. Yu, J. T. Robinson, F. Wang, A. Zettl,and A. Lanzara, PHYSICAL REVIEW B 2009,80, 121409.

Figures

EUV(hr) H2(mbar) Background(mbar)

Sref 0 0 -7 6x10

Table 1 Exposure conditions SH2(molecular) SH(atomic) NA NA -2 -2 5x10 5x10 * -7 -7 6x10 6x10

SEUV 8 0 -7 6x10

SEUV+H2 8 -2 5x10 -7 6x10

*Atomic hydrogen was generated by injecting 5x10-2 mbar molecular hydrogen into the chamber and flowing the gas over a tungsten filament held at 2000oC.

Sref SH(atomic) SEUV SEUV+H2

Table 2 Atomic concentration of the four components for different samples 2 3 carbide(%) sp (%) sp (%) 5.7 79.3 9.0 3.6 73.2 16.3(↑7.3) 5.5 69.1 16.1(↑7.1) 4.2 67.7 14.7(↑5.8)

-COH(%) 5.0 6.9(↑0.9) 9.2(↑3.3) 13.3(↑7.4)

* Arrows indicate the change in concentration relative to the reference sample.

D peak range

Figure 1. Comparison of Raman spectra for the five samples

EUV intensity profile for SEUV

SEUV

SEUV+H2

Figure 2. D/G ratio map profile versus EUV intensity profile

SEUV+H2

EUV Intensity profile for SEUV+H2

SEUV

Figure 3. D/G ratio versus EUV power

Figure 4. XPS curve fitting results for SEUV+H2.

Growth of Single-Layer Graphene on Single Crystal Pt(111) Substrate 2

Jianhua Gao , Keisuke Sagisaka , and Daisuke Fujita

1,2

International Center for Young Scientists (ICYS) 2

Nano Characterization Unit

National Institute for Materials Science (NIMS), 1-2-1 Sengen Tsukuba, Ibaraki 305-0047, Japan GAO.Jian-Hua@nims.go.jp Abstract Epitaxial growth of graphene on transition metal substrates have been evidenced as an effective way for graphene synthesis. Furthermore, graphene growth on Pt(111) substrate is attractive because of the weak bonding interaction with the substrate

[1-3]

, which will affect the structure and electronic properties

of the graphene. In our work we successfully fabricated single-layer graphene on metal substrates by surface segregation and chemical vapor deposition, respectively. The graphene were investigated by spatially resolved scanning Auger microscope, atomic force microscopy and scanning tunneling microscopy, as shown in Fig. 1 (a) and (b). By surface segregation technique, we can produce largearea, single-layer graphene islands more than microns from carbon-doped Pt(111) substrate. By chemical vapor deposition technique, we can produce uniform single-layer graphene islands with several microns up to wafer-scale single-layer graphene, which will be an ideal system to study the electronic properties and interaction between graphene and the substrate. References [1] P. Sutter, J. T. Sadowski, and E. Sutter, Phys. Rev. B 80, 245411 (2009). [2] N. Levy, S. A. Burke, K. L. Meaker, M. Panlasigui, A. Zettl, F. Guinea, A. H. Castro Neto, M. F. Crommie, Science 329, 544 (2010). [3] M. M. Ugeda,D. Fernández-Torre, I. Brihuega, P. Pou, A. J. Martínez-Galera, Rubén Pérez, and J. M. GómezRodríguez, Phys. Rev. Lett. 107, 116803 (2011). [4] L. B.Gao, W. C. Ren, H. L. Xu, L. Jin, Z. X. Wang, T. Ma, L. P. Ma, Z. Y. Zhang, Q. Fu, L. M. Peng, X. H. Bao, H. M. Cheng, Nat. Commun. 3, 699 (2012).

Figures

Fig. 1 Scanning Auger map CKLL image of monolayer graphene/Pt(111) by surface segregation (a) and chemical vapor deposition (b) technique. The inset are atomic force microscopy and scanning tunneling microscopy image.

Physical model and parameters for graphane bipolar junction transistors

Behnaz Gharekhanlou, Sina Khorasani and Reza Sarvari Sharif University of Science and Technology, Azadi Ave., Tehran, Iran gharekhanlou@ee.sharif.ir Abstract The celebrated monolayer graphene, as a gapless semimetal, may be hydrogenated to obtain a semiconductor with an energy gap, named graphane. The planar arrangement and unique physical and electronic transport features of graphene, combined with the possibility of doping graphane, allows patterning and creation of p and n regions, in such a way that two-dimensional p-n rectifying junctions become practicable. Our recent analyses reveal that ideal I-V characteristics for such junctions may be expected [1]. Based on the theoretical model of planar graphane diodes developed in our earlier works, here we construct a physical model to predict the behaviour of bipolar junction transistors based on graphane. We derive the small-signal equivalent model and estimate the performance of the device. References [1] B. Gharekhanlou and S. Khorasani, IEEE Trans. Electr. Dev., vol. 57 (2009), pp. 209-214.. [2] A. K. Geim and K. S. Novoselov, vol. 6, no. 3 (2007), pp. 183-191. [3] N. Jung, N. Kim, S. Jockusch, N.J. Turro, and P. Kim, L.Brus, Nano Lett., vol. 9 (2009), p. 4133. [4] I. Gierz, C. Riedl, U. Starke, C.R. Ast, and K. Kern, Nano Lett., vol. 8 (2008), p. 4603. [5] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K.Saha, U.V. Waghmare, K.S. Novoselov, H.R. Krishnamurthy, A. K. Geim, A.C. Ferrari, and A.K. Sood, Nat. Nanotech., vol. 3 (2008), p. 210. [6] X. Wang, X. Li, L. Zhang, Y. Yoon, P. K. Weber,H. Wang, J. Guo, H. Dai, Science, vol. 324 (2009), p. 768. [7] F. Cervantes-Sodi, G. Cs´anyi, S. Piscanec, A. C. Ferrari, Phys. Rev. B, vol. 77 (2008), p. 165427. [8] J. O. Sofo, A. S. Chaudhari, and G. D. Barber, Phys. Rev. B, vol. 75, no. 15 (2007.), 153401. [9] B. Gharekhanlou, M. Alavi, and S. Khorasani, Semicond. Sci. Technol., vol. 23, no. 7 (2008), p.075026. [10] B. Gharekhanlou and S. Khorasani, “An overview of tight-binding method for two-dimensional carbon structures,” in Graphene: Properties, Synthesis and Application, edited by Z. Xu, New York: Nova Science, Cha. 1, (2011). [11] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hil1, P. Blake, M.I. Katsnelson, and K.S. Novoselov, Nature Mater, vol. 6 (2007), pp. 652–655. [12] D.C. Elias, R.R. Nair, T.M.G. Mohiuddin, S.V. Morozov, P. Blake, M.P. Halsall, A.C. Ferrari, D.W. Boukhvalov, M.I. Katsnelson, A.K. Geim, and K.S. Novoselov, Science, vol. 323, no. 5914 (2009), pp. 610-613. [13] J.H. Chen, C. Jang, M.S. Fuhrer, and E.D. Williams, and M. Ishigami, Nature Physics, vol. 4 (2008), pp. 377–381. [14] T.O. Wehling, E. Sasioglu, C. Friedrich, A. I. Lichtenstein, M. I. Katsnelson, and S. Blügel, Phys. Rev. Lett., vol. 106 (2011), p.236805.

[15] G. Savini, A.C. Ferrari, and F. Giustino, Phys Rev Lett., vol. 105 (2010), p. 037002.

Fig. 1. Depletion charge distribution in the n-p-n transistor. 0.15 0.1

B-C junction

E-B junction

0.05

Potential(V)

0 -0.05 -0.1 -0.15 -0.2 -0.25 -0.3 Emitter -0.35 -4

Collector

Base

-2

2 Distance(nm)

Fig. 2. Electric field distribution of the n-p-n transistor. 6

x 10

8 7

rπ(MΩ.m)

6 5 4 3 2 1 0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

VBE(V)

Fig. 3. Voltage dependence of the input resistance. 100 90

Cutoff Frequency(MHz)

80 70 60 50 40 30 20 10 0 -0.05

0.05

0.1

0.15 JC(μA/cm)

0.2

0.25

0.3

Fig. 4. Voltage dependence of the diffusion capacitance.

0.35

Doping Graphene with Organic Molecules – A Theoretical Study Angelos Giannakopoulos, Liping Chen, David Beljonne Chemistry of Novel Materials, Avenue Maistriau 23, Mons, Belgium angelos@giannakopoulos.biz Abstract Graphene has conquered the field of Cutting Edge Technology as the ultimate next generation material. However, in order to be used widely in applications, one should be able to tune its electronic properties (i.e. work function). This may be achieved by deposition of electron acceptor or donor molecules on the surface of Graphene. In this work, we investigate the interaction between Graphene and an organic molecule, hexaazatriphenyl hexacarbonitrile (HATCN). HATCN is a strongly electron deficient molecule widely used in organic LEDs for hole injection [1,2]. By means of first principle computational techniques, we study the evolution in the work function of Graphene due to the adsorption of HATCN, as a function of the relative orientation and density of the doping molecules. Our modeling work points to a change from a lying-down to a standing-up configuration as the coverage increases, which is also observed in layer growth experiment over gold(111) and silver(111) surfaces [3,4]. The preferential standing-up configuration is confirmed by simulated X-ray Photoelectron Spectra (XPS) and Near Edge X-ray Absorption Fine Structure Spectra (NEXAFS) in excellent agreement with experimental data. References [1] Liang-Sheng Liao et al.; Adv. Mater. 20 (2008) 324–329 [2] L. S. Liao and K. P. Klubek; Applied Physics Letters 92 (2008) 223311 [3] P. Frank et al.; Chemical Physics Letters 473 (2009) 321–325 [4] P. Frank et al.; J. Phys. Chem. C 114 (2010) 6650–6657

Graphene antidot lattice waveguides Jesper Goor Pedersen, Tue Gunst, Troels Markussen and Thomas Garm Pedersen DTU Nanotech - Department of Micro- and Nanotechnology, Technical University of Denmark, DTU Building 345 East, DK-2800 Kongens Lyngby, Denmark jeped@nanotech.dtu.dk Abstract In recent years, the concept of graphene antidot lattices (GALs) – periodic perforations of graphene – has proven an efficient way of tuning the properties of graphene [1]. In particular, experimental indications of a transport gap in graphene have recently been demonstrated [2]. While bulk GALs are interesting in their own right, there is a rich opportunity for developing graphene-based devices based on combinations of GALs and pristine graphene. We have recently theoretically proposed one such device, namely GAL waveguides [3]. Here, a region of pristine graphene is sandwiched between regions of GALs, as illustrated in Fig. 1. As the low-energy properties of GALs are quite well-described by the addition of a mass term to the Dirac equation of graphene, the surrounding GALs allow tight confinement of carriers within the central, pristine graphene region. This is in contrast to the case of electrostatic confinement, which in graphene is hindered by Klein tunneling. By including the effect of the GALs via a position-dependent mass term in the Dirac equation we arrive at analytical expressions for the dispersion relation and eigenstate spinors of the localized states. We find excellent agreement between the analytical results and more exact tight-binding simulations, as shown in Fig. 2. The results reveal tightly confined states, with eigenstates resembling those of graphene nanoribbons. Contrary to nanoribbons, however, GAL waveguides do not have sharply defined edges. We find that overall GAL waveguides resemble graphene nanoribbons but without the intricacies related to edges. In particular, GAL waveguides are always semiconducting. Using recursive Green’s functions techniques we calculate the transmission through a GAL waveguide attached to semi-infinite graphene leads and compare to the transmission through a graphene nanoribbon of comparable dimensions. We find that generally transmission is higher through the GAL waveguides. Including disorder in the shape of carbon atoms randomly removed at the edges of the antidots, we find that the waveguides are quite robust to disorder. Relying on the close analogies with photonic crystal waveguides we expect negligible backscattering through bends in the waveguide. We find that the transmission through a kink in the waveguide, shown in Fig. 3, is nearly identical to the transmission through a straight waveguide of similar length, despite the fact that the waveguide in Fig. 3 switched between zigzag and armchair orientations. With the benefit of the surrounding GAL in terms of mechanical stability and the possibility of carrying away Joule heating from the device, we believe that GAL waveguides may offer an attractive way of realizing electronic wires in graphene-based electronics.

References [1] T. G. Pedersen, C. Flindt, J. Pedersen, N. A. Mortensen, A.-P. Jauho, and K. Pedersen, Physical Review Letters, 100 (2008), 136804. [2] A. J. M. Giesbers, E. C. Peters, M. Burghard, and K. Kern, Physical Review B, 86 (2012), 045445 [3] J. G. Pedersen, T. Gunst, T. Markussen and T. G. Pedersen, Physical Review B, 86 (2012) 245410.

Figures Fig. 1. (upper panel) Schematic illustration of GAL waveguides; the band gap of the surrounding GAL regions confine carriers to the central waveguiding region of pristine graphene. (lower panel) One example of a GAL waveguide. Black dots indicate carbon atoms, while the red (blue) circles illustrate the probability distribution of the lowest eigenstate at the G point of the waveguide. The size of the circle shows the absolute square of the probability density while color indicates sign. Note the strong confinement.

Fig. 2. (left) Dispersion relation of a GAL waveguide. Thick blue lines indicate results from the TB calculation. The solid red lines are results from a Dirac equation, where the GALs are included via a position-dependent mass term. The dashed, red line shows the analytic result obtained in the infinite mass limit. For comparison, the bulk graphene dispersion relation is shown with black, dashed lines. The shaded grey region indicates the projected bands of the surrounding GAL, which define the range below which localized states are expected. (right) Corresponding DOS of the TB model. Note the clear van Hove singularities characteristic of onedimensionality.

Fig. 3. (left) Waveguide bend, with a transition between zigzag and armchair directions. The arrows show the bond current, illustrating the tight confinement to the waveguide and negligible backscattering through the bend. (right) Conductance of the waveguide â&#x20AC;&#x2DC;kinkâ&#x20AC;&#x2122; shown to the left, compared to a structure where the waveguide runs straight through.

Electrically conductive pressure sensitive adhesives containing graphene oxide 1 2 2 1 Karolina Górka , Małgorzata Wojtoniszak , Ewa Mijowska , Zbigniew Czech 1

West Pomeranian University of Technology in Szczecin, Institute of Chemical Organic Technology, ul. Pułaskiego 10, 70-322, Szczecin, Poland 2 West Pomeranian University of Technology in Szczecin, Institute of Chemical and Environment Engineering, ul. Pułaskiego 10, 70-322, Szczecin, Poland gorka.karolina@o2.pl

Abstract Adhesion substances found the wide application in the field of electronics and electrical engineering. Due to the unique structure, electric, thermal and mechanical properties, grapheme oxide hold great promise for potential applications including electrically conductive adhesives [1-3]. Electrically conductive pressure sensitive adhesives (PSA) containing graphene oxide are environmentally friendly alternative to so far applied PbSn solder which are being withdrawn of the electronic assembly as an effect of applicable Directive (Restriction of Hazardous Substances - RoHS) making impossible applying lead, cadmium, mercury and hexavalent chromium in electronic components [4-5]. In this study, receiving and properties of electrically conductive pressure sensitive adhesives containing graphene oxide will be described. Detailed description of the graphene oxide was carried out with UV-Vis spectroscopy, FT-IR spectroscopy, Raman spectroscopy and confocal laser scanning microscopy. Conducted series of experiments pointed up that acrylic pressure-sensitive adhesives basis was synthesised with good initial performances, excellent adhesion, good adhesion and high tack. The electrical conductivity is incorporated into acrylic adhesive polymer after adding electrically conductive additive [6], graphene oxide. Functional electrically conductive phase is compound from nanostructures of coal which provides the significant resistance reduction of adhesive, increasing in addition the conductance [2,4,7]. After an addition of graphene oxide electrical conductivity of the PSA layers was examined and the most important physicochemical and mechanical properties of pressure-sensitive adhesives such as peel adhesion, shear strength and tack were determined by standard A.F.E.R.A. (Association des Fabricants Europeens de Rubans Auto-Adhesifes) procedures. Methods of receiving electrically conductive pressure sensitive adhesives containing graphene oxide weren't described so far in literature and are not commercially available. This type of conductivity PSA including grapheme oxide are promising new composite materials which can be applied in many areas of electronic and electrical engineering industrial for the shielding of the electromagnetic and radio interference, earthing and carrying static charges, for fixing elements of electronics, for the connecting an electric wires replacing soldering and of many other applications.

References [1] Yanwu Zhu, Shanthi Murali, Weiwei Cai, Xuesong Li, Ji Won Suk, Jeffrey R. Potts, Rodney S. Ruoff, Advanced materials, 35 (2010) 3906-3924. [2] James J. Licari, Dale W. Swansow, Adhesives Technology for Electronic Applications, Materials, Processing, Reliability (2nd Edition), Elsevier, 2011. [3] Yi (Grace) Li, Daniel Lu, C.P. Wong, Electrical Conductive Adhesives with Nanotechnologies, Springer, 2009. [4] Marcin Słoma, Małgorzata Jakubowska, Ryszard Jezior, Materiały elektroniczne, 35 (2007) 65-83. [5] Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment.

[6] Zbigniew Czech, Robert Pełech, Agnieszka Kowalczyk, Arkadiusz Kowalski, Rafał J. Wróbel, Electrically conductive acrylic pressure-sensitive adhesives containing carbon black, Polish Journal of Chemical Technology, 4 (2011) 77-81. [7] K. Gilleo, Area Array Packaking Handbook: Manufacturing and Assembly, McGraw-Hill HandBooks, New York, 2011. Figures

Figure 1. Transmission electron microscopy (a) and atomic force microscopy (b) images and height profile (down panel of image b) of grapheme oxide.

Adiabatic quantum pumping in graphene with magnetic barriers E. Grichuk, E. Manykin National Research Center “Kurchatov Institute”, 123182, Kurchatov Sq. 1, Moscow, Russia evgeny.sg@gmail.com Most research of electronic transport in graphene is focused on stationary problems. A variety of new effects emerges when one considers non-stationary ones. An interesting phenomenon is a quantum pump effect, in which a periodic modulation of parameters of a quantum system produces a finite dc current through it even in the absence of an external bias [1]. Quantum pumping in graphene has recently attracted increasing attention of researchers [2-6]. The unusual electronic spectrum of graphene was demonstrated to have a significant impact on the effect. The potential use of a quantum pump effect in graphene-based spintronics was also discussed. Motivated by possible applications of a quantum pump effect in graphene valleytronics we extend previous studies of quantum pumping in graphene by taking into consideration the valley degree of freedom of electrons. The system that we examine is a standard two terminal quantum pump device that is formed by a wide graphene strip with two electric barriers (produced by top metallic gates) whose heights can be periodically modulated in time and one stationary magnetic barrier (see Fig.1). The model employs the low-energy Dirac approximation and incorporates the possible existence of a finite band gap in graphene spectrum. By using a δ-function approximation for a magnetic barrier profile, analytical expressions for bilinear total and valley pumping responses are derived within the scattering matrix approach [7]. These results are compared to numerical ones for a double δ-function, a square and a triple square magnetic barriers. We find that a finite magnetic field breaks the perfect Klein tunneling so that all propagating modes become sensitive to pumping. At the same time a magnetic barrier decreases the overall efficiency of a quantum pump. The joint use of a magnetic barrier and band gap engineering in graphene breaks the valley symmetry and gives a way to generate valley-polarized currents in graphene-based quantum pumps (see Fig.2). The parameters of a device can be adjusted such that a pure valley current is produced. A δ-function and a square magnetic barrier profiles, which are employed in our model are widely used for studying the electron transport through magnetic barriers and can be viewed as simplified approximations to those created experimentally by ferromagnetic strips. More realistic smooth profiles can be analyzed in the same framework. The considered pump effect might be found useful in the field of graphene valleytronics, e. g., as a source of valley-polarized and pure valley currents. Experimentally they could be detected using, for example, the valley Hall effect [8]. We gratefully acknowledge financial support from the Russian Fund for Basic Research, project No. 1002-00399 and from the Ministry of Education and Science of the Russian Federation, project No. 8364. References [1] P. W. Brouwer, Phys. Rev. B, 58 (1998) R10135. [2] R. Zhu, H. Chen, Appl. Phys. Lett., 95 (2009) 122111. [3] E. Prada, P. San-Jose, H. Schomerus, Phys. Rev. B, 80 (2009) 245414. [4] E. Grichuk, E. Manykin, EPL, 92 (2010) 47010. [5] R.P. Tiwari, M. Blaauboer, Appl. Phys. Lett., 97 (2010) 243112. [6] Q. Zhang, J.F. Liu, Z. Lin, K.S. Chan, J. Appl. Phys., 112 (2012) 073701. [7] E. Grichuk, E. Manykin, submitted. [8] D. Xiao, W. Yao, Q. Niu, Phys. Rev. Lett., 99 (2007) 236809.

Figures

Figure 1: (a) A schematic structure of a proposed graphene device. The device is formed by a wide graphene ribbon with two square electric barriers and (b) a single δ-function magnetic barrier or (c) a square magnetic barrier or (d) a double δ-function magnetic barrier or (e) a triple square barrier.

Figure 2: Contour plot of (a, c) the total and (b, d) the valley pumping responses for (a, b) the double δfunction magnetic barrier and (c, d) the triple square barrier in graphene with gapped spectrum as a function of the transverse canonical momentum q and the Fermi energy E. (e) The total (Qc) and the valley (Qv) pumped charges for a wide ribbon as a function of the Fermi energy E. Parameters: B=0.1 T; L=81.1 nm; EL=6.6 meV; gap ∆=4EL. Em=EL is the minimum value of the Fermi energy, above which the single (δ-function or square) magnetic barrier has a finite transparency.

Micro-Raman analysis of the influence of hydrogen intercalation on the epitaxial graphene grown on 4H-SiC(0001) substrate 1,2

K.Grodecki , W.Strupinski , A.Wysmolek , R.StÄ&#x2122;pniewski , J.M.Baranowski

1,2

1.Institute of Electronic Materials Technology, Warsaw, Poland 2.Faculty of Physics, University of Warsaw, Warsaw, Poland Kacper.grodecki@itme.edu.pl It is commonly accepted that properties of epitaxial graphene (EG) grown on SiC are determined by interaction with substrate. It was found, that hydrogen intercalation of EG grown on SiC(0001) substrates by sublimation is a promising method to increase the mobility of carriers [1]. As verified by Raman spectroscopy [2] sublimation grown samples show much stronger interaction with the SiC substrate than epitaxial graphene grown using Chemical Vapor Deposition (CVD) method [3]. In order to achieve better understanding of the effects induced by hydrogenation strain analysis for graphene grown by Si sublimation and CVD technique was performed. In this study random fluctuations of the G and the 2D peaks were used to analyze strain nature in epitaxial graphene grown on terraces of the SiC(0001) substrate. Two types of EG samples grown on 4H-SiC(0001) have been investigated: graphene grown by Si sublimation and grown by CVD. Micro-Raman mapping were used to analyze the influence of strain fluctuation on the relative shifts of the G and 2D peaks. The data were collected from terraces avoiding regions of multisteps. It was found that of the G peak position plotted as a function of the 2D band position shows linear correlation with the slope of 0.42 and 0.37 for EG grown by Si sublimation and by CVD, respectively. It was already shown in previous studies the correlation of the Raman G peak versus the 2D peak position provides information about the nature of stress existing in graphene [4,5]. It can be deduced, that in the case of freestanding material, uniaxial strain induces a shift of the Raman G peak being of about 0,33 of the 2D peak position change [4]. On the other hand, in the case of biaxialy strained samples the average energy shift of the Raman G peak is of about 0.45 of the 2D peak position change. Our results indicate that graphene grown by Si sublimation predominantly biaxialy strained, whereas for CVD grown samples uniaxial component is dominant. Important change of the measured slopes takes place after hydrogenation. The slope of the G versus the 2D peak position is strongly reduced for the Si sublimated EG from the value of 0.42 to 0.29. Reduction of the measured slope indicates that after hydrogenation the dominant stress becomes the axial one. This result is in agreement with a common believe that hydrogenation process causes decoupling of the graphene from the SiC substrate. The remaining uniaxial strain is the most likely resulting from influence of steps on terraces regions. For the CVD grown EG the corresponding slope is only slightly reduced from 0,37 to 0.33. This small change of slopes indicates that CVD grown layers are not so strongly bound to the SiC substrates before hydrogenation. In this case hydrogenation process only slightly reduces residual biaxial strain of the CVD grown layers, keeping as a dominant the uniaxial part of the strain most likely resulting from the influence of steps. [1] F. Speck, J. Jobst, F. Fromm, M. Ostler, D. Waldmann, M. Hundhausen, H. B. Weber, and Th. Seyller Appl. Phys. Lett. 99, 122106 (2011) [2] K. Grodecki, J. A. Blaszczyk, W. Strupinski, A. Wysmolek, R. StÄ&#x2122;pniewski, A. Drabinska, M. Sochacki, A. Dominiak, and J. M. Baranowski J. Appl. Phys. 11, 114307 (2012)

[3] W. Strupinski, K. Grodecki, A. Wysmolek, R. Stepniewski, . Szkopek, . . Gaskell, A. Gr neis, D. Haberer, R. Bozek, J. Krupka, and J. M. Baranowski Nano Lett, 11, 1786 (2011) [4] T. M. G. Mohiuddin, A. Lombardo, R. R. Nair, A. Bonetti, G. Savini, R. Jalil, N. Bonini, D. M. Basko, C. Galiotis, N. Marzari, K. S. Novoselov, A. K. Geim, and A. C. Ferrari Phys. Rev. B 79, 205433 (2009) [5] J. Zabel, R. R. Nair, A. Ott, T. Georgiou, A. K. Geim, K. S. Novoselov, and C. Casiraghi Nano Lett., 12, 617(2012)

Making large area graphene field-effect transistors (FETs) without polymer resists 1

M. Gurram , E.H.Huisman , X. Zhang , J.J. van den Berg , B.L. Feringa and B.J. van Wees

Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands 2 Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands m.gurram@student.rug.nl

The conventional way to produce field-effect transistors (FETs) is by lithographic techniques using polymer resists as a patterning mask. This technique has also been successfully used to make graphene FETs out of small flakes of exfoliated graphene [1] and large area graphene such as graphene grown using chemical vapour deposition [2]. However, the polymer resists usually leave residues reducing the electronic performance of the FET [3]. Here we report a simple and reliable process to fabricate large 2 area graphene FETs (1 mm ) without using polymer resist. Our procedure can be used as a tool to quickly map the electronic properties of large area graphene on insulating substrates. The recipe is schematically depicted in figure 1. We use two reactive ion etching steps to pattern a sheet of graphene into the desired shape. Subsequently, we deposit source and drain contacts using shadow2 mask e-gun evaporation in vacuum. We applied our recipe to a 1 cm graphene sheet on silicon/silicon dioxide wafer [4] resulting in 12 FETs of 1mmx1mm without employing any polymer resist. The electrical 0 characterization of the prepared graphene FETs is carried out after annealing the devices at 130 C -5 overnight in a vacuum of 10 mbar. After annealing, the charge neutrality point is observed to be close to 0 volts applied to the back gate electrode (figure 2), indicating that thermal annealing has successfully removed dopants.

References [1] Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 5696, 666-669. [2] Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 5932, 1312-1314. [3] Goossens, A. M.; Calado, V. E.; Barreiro, A.; Watanabe, K.; Taniguchi, T.; Vandersypen, L. M. K. Appl. Phys. Lett. 2012, 7, 073110. [4] Samples were obtained from Graphene Supermarket Inc. The graphene was grown using chemical vapour deposition on copper and subsequently transferred to a Si/SiO 2 wafer.

Figure 1: Schematics of the preparation of large area graphene (1 mm ) field-effect transistors using shadow-mask etching and evaporation.

Resistance (KOhm)

16 14 12 10 8 6 4 -40

-20

Gate Voltage (V) 2

Figure 2: Gate spectroscopy traces of eight large area graphene (1 mm ) FETs fabricated using shadowmask etching/evaporation without using polymer resist. The traces were recorded after annealing 0 overnight at 130 C in vacuum. The variation in resistance of the FETs is attributed to macroscopic deformations such as cracks in the CVD-graphene.

Computation of Intrinsic Mechanical Properties of Single and Double Layer Graphene a

Balázs Hajgató , Songül Güryel , Yves Dauphin , Jean-Marie Blarion , Gregory Van Lier , b a a Hans E. Miltner , Frank De Proft , Paul Geerlings a) Free University of Brussels - Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium b) SOLVAY S.A., Innovation Center, rue de Ransbeek, 310, 1120 Brussels, Belgium hajgato@vub.ac.be Abstract Until the second trimester of the late century only two ordered forms of carbon were known to scientists, namely diamond, with its perfect crystal structure, and graphite, also crystalline but black and flaky and not at all transparent. Besides those ordered forms, also coal, coke, soot, lampblack, and the many kinds of charcoal were known. The graphite structure reflects its properties, since it is made up of sheets of carbon atoms arranged in a hexagonal lattice, like a honeycomb of fused benzene rings, and with weak bonding between adjacent sheets. This means that graphite easily forms flakes where the sheets can slide over each other, providing use of graphite as a lubricant, and resulting good electrical conductivity. But it is only in 2007 that researchers in Manchester found a way to mechanically peel 1

single two-dimensional sheets from three-dimensional graphite crystals . Graphene is the name given to this flat monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice. 1

Since the first experimental analysis , graphene has recently gained significant attention. In particular, its excellent mechanical properties are an important advantage for the practical applications of graphene. These mechanical properties have extensively been investigated, and in particular, the Young’s Modulus has been predicted using a range of experimental and theoretical approaches. On the experimental side, by ultrasonic, sonic resonance, and static test methods, Blakslee et. al.

reported a Young’s modulus of 1.06 TPa for bulk pyrolytic graphite, which has been highly ordered by 3

annealing under compressive stress. Frank et al. measured the modulus for a stack of graphene sheets (less than five layers) to be 0.5 TPa using an atomic force microscope. More recently, by nanoindenting the centre of a free-standing monolayer graphene membrane with an atomic force 4

microscope, Lee et al. measured the Young’s modulus as 1.0 TPa, assuming the thickness of graphene to be 0.335 nm. Many theoretical and computational studies have also been performed to investigate the mechanical 5

properties of graphene, for example, the pioneering study by Van Lier et al. , using super-molecular approach. There are numerous theoretical studies using a super-molecular approach, to calculate 6

mechanical properties, however the number of infinite (periodic) calculations is very scarce . Stressstrain curves are seldom reported, and other mechanical properties for example bending modulus were not investigated up to now. In this study, the in-plane and out-of-plane Young’s modulus, the out-of-plane shear modulus, and the bending modus of single and double layer graphene have been theoretically investigated using Periodic Boundary Condition (PBC) Density Functional Theory (DFT) with the PBE, HSEh1PBE, and M06L 7-9

functionals in conjunction with the 6-31G* and the 3-21G basis sets . The out-of-plane mechanical properties are compared to the corresponding mechanical properties of graphite. The unit-cell size and

shape dependence as well as the directional dependencies of the mechanical properties have been also investigated. Some of the calculated strain-stress curves are also reported (see figures). References [1] A. K. Geim, and K. S. Novoselov, Nature Materials, 6 (2007) 183–191. [2] O. L. Blakslee, D. G. Proctor, E. J. Seldin, G. B. Spence, T. Weng, J. Appl. Phys. 41 (1970) 3373– 3382. [3] I. W. Frank, D. M. Tanenbaum, A. M. van der Zande, P. L. McEuen, J. Vac. Sci. Technol. B 25 (2007) 2558–2561. [4] C. Lee, X. Wei, J. W. Kysar, J. Hone, Science 321 (2008) 385–388. [5] G. Van Lier, C. Van Alsenoy, V. Van Doren, P. Geerlings, Chem. Phys. Lett. 326 (2000) 181–185. [6] K. N. Kudin, G. E. Scuseria, B. I. Yakobson, Phys. Rev. B, 64 (2001) 235406. [7] B. Hajgató, S. Güryel, Y. Dauphin, J-M. Blairon, H. E. Miltner, G. Van Lier, F. De Proft, P. Geerlings, J. Phys. Chem. C, 116 (2012) 22608. [8] S. Güryel, B. Hajgató,Y. Dauphin, J-M. Blairon, H. E. Miltner, F. De Proft, P. Geerlings, G. Van Lier, Phys. Chem. Chem. Phys., 15 (2013) 659. [9] B. Hajgató, S. Güryel, Y. Dauphin, J-M. Blairon, H. E. Miltner, G. Van Lier, F. De Proft, P. Geerlings, “Out-of-plane shear and out-of-plane Young's modulus of double- layer graphene”, Chem. Phys. Lett., Submitted. Figures

Stress-Strain curves for double-layer graphene, using all employed methods (only 3-21G basis set). Both the errors between the minimum and maximum in percentage and the number of different levels of theoretical calculations (# DATA) are displayed on the right axis.

CATALYST FREE PECVD NANOCRYSTALLINE GRAPHENE ON INSULATOR 1

1,3

Zainidi Haji Abdul Hamid , Cigang Xu , Marek E. Schmidt , Hiroshi Mizuta , Mike Cooke , 1 Harold M. H. Chong 1)

Nano Research Group, Electronics and Computer Science, Faculty of Physical and Applied Science, University of Southampton, Highfield, SO17 1BJ, Southampton, United Kingdom 2) Oxford Instruments Plasma Technology, North End, Yatton, Bristol, United Kingdom 3) School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Ishikawa, 923-1292, Japan Email: zhah1g11@ecs.soton.ac.uk

The unique electrical and optical properties of graphene have attracted application as transparent electrodes, transistors and photodetectors. However, the small flake size of mechanical exfoliated graphene has limited its scalability to large area production. Recently, large area graphene is produced through chemical vapour deposition (CVD) and epitaxial growth method [1]. CVD of graphene on insulator or metal-catalyst is considered low cost and process flexible. This is because graphene film quality and deposition rate can be controlled through process parameters such as temperature, gas flow rate and gas mixture. Large area CVD graphene on metal-catalyst such as Ni or Cu is amongst other established method [2] but film transfer and metal etching can ultimately affect device performance. Therefore, the research attention is now focussed on catalyst free CVD graphene process on insulator materials. In this work, we present a plasma based CVD method of depositing large area graphene on SiO 2 surface at different temperature and RF power. The plasma enhanced chemical vapour deposition (PECVD) approach produces nanocrystalline graphene (NCG), which has potential application as transparent conductors and sensor devices. The deposition process is carried out using the Oxford Instrument Agile 1000 system and on 300 nm thick PECVD SiO2 on 150 mm diameter silicon wafer. The gas mixture ratio of H2:CH4 is 1:1.2, plasma RF of 100 W and the deposition temperature is done ° ° ° ° at 900 C, 950 C and 1000 C. The deposition rate of the NCG deposited at 900 C is 1.2 nm/min, ° ° 950 C is 1.8 nm/min and 1000 C is 1.9 nm/min. The rate is determined by measuring the NCG thickness using white light ellipsometer, as shown in Figure 1. The NCG film is then analysed using Raman spectroscopy with 532 nm excitation laser wavelength and the distinct D, G and broad 2D bands of the NCG have been observed in Figure 2. The nanocrystalline nature of the graphene is -1 -1 characterised by the split of the D (1350 cm ) and G (1594 cm ) band, with strong D peak signifying 2 -1 disorder in the sp carbon lattice. The 2D (2694 cm ) band is identified with the thickness and -1 crystalline nature of the graphene and the combination D+G (2946 cm ) band of graphitic disorder in the film [10]. By varying the RF power and introducing LF power of 300 W and 100 W to the process, we are able to demonstrate an increase in the deposition rate to > 3.8 nm/min whilst maintaining the NCG Raman characteristics. Preliminary optical transmission measurement has shown > 88 % transparency from 200 nm to 800 nm for all NCG wafers. Future work is to attempt deposition at lower ° ° temperature range from 550 C to 900 C and improvement of the D and 2D Raman peaks. Acknowledgement The author would like to acknowledge the support of the Government of Brunei scholarship, Southampton Nanofabrication Centre and Oxford Instrument for the support of the project. References [1]

Samuel Grandthyll, Stefan Gsell, Michael Weinl, Matthias Schreck, Stefan Hufner and Frank Muller, J. Phys. Condens. Matter., vol. 24, pp. 314204, (2012).

[2] [3]

Ivan Vlassiouk, Pasquale Fulvio, Harry Meyer, Nick Lavrik, Sheng Dai, Panos Datskos, Sergei Smirnov, Carbon, vol. 54, pp 58-67 (2013). R. Saito, M. Hofmann, G. Dresselhaus, A. Jorio, M. S. Dresselhaus, Adv. in Phys., vol. 60, no. 3, pp. 413-550, (2011).

Figures

Figure 1. Ellipsometer thickness and uniformity measurement of NCG film on 300 nm SiO2 on silicon wafer. The NCG film is deposited at 900 째C.

Figure 2. Raman spectra of NCG film 째 deposited at 900 째C (blue), 950 C (red) and -1 1000 째C (green). The D peak is 1350 cm , G -1 -1 peak is 1594 cm , 2D peak is 2694 cm and -1 D+G peak is 2946 cm .

Band gap engineering graphene on Ir(111) by hydrogenation and controlled patterning of graphene by hydrogen desorption Line Kyhl Hansen1,2,*, Jill A. Miwa 1,2, R. Balog 1,2, L. Nilsson 1,2, Liv Hornekær1,2 1

iNANO, Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

line1@inano.au.dk The interest in graphene on metal substrates has been increasing over the last decade due to overwhelming prospects in numerous applications such as, nano-sized electrical components. Graphene has remarkable electronic properties, in particular, ballistic transport of the charge carriers leads to the highest electronic conductivity of known substrates at room temperature [1]. This high conductivity is desirable for high speed transistors; however the on/off ratio for intrinsic graphene is too low for using it in real applications [2]. Opening a tunable band gap in the graphene band structure is a means of overcoming this problem, but this is a challenging task. We present measurements showing how patterned hydrogenation mediated by the Moiré structure of graphene on Ir(111) opens a band gap. This is explained by confinement effects and breaking the translational symmetry of the real, as well as, the reciprocal sublattices of the graphene [3]. The band structure was investigated by angle-resolved photoemission spectroscopy (ARPES) and the electronic density of both graphene on Ir(111) and hydrogenated graphene on Ir(111) was studied by scanning tunneling spectroscopy (STS) and the results agreed with density functional theory (DFT) calculations [3]. The patterned hydrogenation of graphene leads to a band gap opening around the Fermi level of at least 450 meV which is seen in the ARPES measurements presented in Figure 1. The size of the band gap increases with coverage (see Figures 1b and 1c). If one assumes that the band gap is symmetric around the Fermi level, the patterned hydrogenation has given rise to a band gap of approximately 900 meV. For comparison silicon has a band gap of 1.1eV. Thus patterned hydrogenation is a way of functionalizing graphene to optimize its usefulness in device applications. STM images of hydrogenated graphene on Ir(111) are shown in Figure 2 imaged at different sample bias voltages at absolute values greater than 0.5V. The hydrogen clusters are imaged as donut shapes following the honeycomb Moiré pattern of the graphene. Here we measure the change in the electronic structure of the hydrogenated graphene by imaging the same area with different biases. The fact that the middle of the hydrogen clusters shift from dark to bright with increasing bias voltage, could indicate the presence of a gap in these areas. An estimate of the band gap is 1000 meV as measured at the centers of the hydrogenated regions. The presence of localized states, due to e.g. hydrogen vacancies, makes the evaluation of the size of the gap uncertain. However, investigations by STS will be carried out in order to confirm the size of the local band gap opening. Furthermore we plan to evaluate the size of the gap in the areas between the hydrogenated regions. These regions will be accessible by STS and we expect interesting electronic properties due to lateral confinement effects imposed by the hydrogenated and non-hydrogenated regions [3]. Finally we performed controlled hydrogen desorption using the scanning tunneling microscope tip. In Figure 3a it is clearly seen that a well-defined patch of clean graphene has been revealed. This allows us to directly measure both the hydrogen terminated and clean graphene regions on the sample with the same tip. Moreover, this method demonstrates STM as a strong tool for the direct patterning of devices in graphene at the nanoscale [4].

1. 2. 3. 4.

Geim, A.K. and K.S. Novoselov, The rise of graphene. Nature Materials, 2007. 6(3): p. 183-191. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669. Balog, R., et al., Bandgap opening in graphene induced by patterned hydrogen adsorption. Nature Materials, 2010. 9(4): p. 315-319. Fuechsle, M., et al., A single-atom transistor. Nature Nanotechnology, 2012. 7(4): p. 242-246.

Figure 1 đ??&#x2026;-band images of graphene obtained by ARPES a) clean graphene on Ir(111) b) graphene on Ir(111) exposed to 30s dose of atomic hydrogen c) graphene on Ir(111) exposed to 50 s dose of atomic hydrogen [3].

Figure 2 Full coverage of hydrogen on graphene on Ir(111) 100Ă&#x2026;x100Ă&#x2026; A) +657.2mV, +0.370nA B) +1314.4mV, +0.370nA C) -657mV, 0.370nA, D) 100Ă&#x2026;x100Ă&#x2026; i: -423mV, -0.360nA ii: -1314.4mV, -0.350nA

Figure 3 Desorption of hydrogen on graphene on Ir(111) A) 300Ă&#x2026;x300Ă&#x2026;, -449mV, -0.490nA B) 150Ă&#x2026;x150Ă&#x2026;, -449mV, -0.470nA, a high resolution image of the area marked with blue in A).

Raman scattering of suspended graphene enhanced by plasmonic Au nano-structures Sebastian Heeg†, Roberto Fernandez-Garcia‡, Antonios Oikonomou¶, Fred Schedin§, Rohit Narula†, Stefan A. Maier‡, Aravind Vijayaraghavan¶,§, and Stephanie Reich† †Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany ‡Department of Physics, Imperial College London, London SW7 2AZ, U.K. ¶School of Computer Science, The University of Manchester, Manchester M13 9PL, U.K. §Centre for Mesoscience and Nanotechnology, The University of Manchester, Manchester M13 9PL, U.K.

sebastian.heeg@physik.fu-berlin.de Abstract Enhanced Raman scattering has become a spectacular application in the field of plasmonics. It combines the generation of highly localized light fields at metal-dielectric interfaces with the variety of properties that can be obtained by Raman spectroscopy. We demonstrate a graphene Raman enhancement of up to 103 arising from a nanoscale cavity between two closely spaced Au nano-disks [1]. Graphene is suspended between the two disks and partially extends into the cavity, which is schematically depicted in Figure 1. The suspended graphene is under tensile strain, which is induced by the double structure partially elevating the graphene. The resulting phonon mode softening allows for a clear identification of the enhanced Raman signals arising from the plasmonic hotspot (red spectrum) between the disks in comparison to unperturbed graphene (black spectrum) as depicted in Figure 2. Spatially resolved Raman measurements reveal that the enhancement in the cavity is localized in dimensions one order of magnitude smaller than the wavelength of the excitation. Upon rotating the polarization of the excitation, we switch from localized cavity enhancement to the dots acting as two approximately separate plasmonic particles. As a result, the near field localization in the cavity is lifted and the enhancement drops by a factor of 20. From a plasmonics point of view, graphene is used a Raman active, two-dimensional membrane that serves as a detection channel of the local near-field distribution. Raman enhancement in strained graphene can be used to characterize plasmonic enhancement arising from any variety of nano-structure geometries. From the perspective of Raman scattering of graphene, the nano-structures induce local strain and simultaneously provide the means of local detection. The induced strain configuration can neither be achieved by uniaxial nor biaxial strain. Figures Figure 1

Figure 2

References [1] Heeg, S. et al. Polarized Plasmonic Enhancement by Au Nanostructures Probed through Raman Scattering of Suspended Graphene. Nano Lett. 13, 301–308 (2013).

Reduction of metal-graphene contact resistance by direct growth of graphene over metal

Seul Ki Hong, Seung Min Song, Taek Yong Kim and Byung Jin Cho* Dept. of Electrical Engineering, KAIST, 291 Daehak-Ro, Yuseong-gu, Daejeon, Korea, 305-701 bjcho@kaist.edu

Abstract Graphene is a zero-band gap semiconductor and has emerged as a promising candidate for device application due to its superior electrical performance [1, 2]. Despite of the electrical potential of graphene, the contact resistance between metal and graphene can be a major limiting factor of device performance. So understanding the metal-graphene contact is great scientific and technological importance. However, despite its importance, there have been few reports about understanding the dominant factors that control the contact resistance between graphene and metal. The detailed properties and method to improve the contact with graphene and metal have not yet been explored. Here we demonstrate a new method to reduce the contact resistance between transferred graphene and metal using CVD grown graphene as a buffer layer. Graphene has direct contact with electrode metal in a conventional contact, but in our method, there is another synthesized graphene layer between metal and graphene, and it can reduce the contact resistance between them. Metal-graphene contact resistance has been widely measured by the transfer length methods (TLM)[3-5] or through a graphene field effect transistor (FET). Using both approaches, we measured the total resistance between electrodes in a TLM pattern or in a graphene FET and extracted the contact resistance. The contact resistance of proposed structure is nearly half of conventional structure. Moreover, CVD growth graphene has more similar resistivity with Cu and higher adhesion energy than transferred graphene on Cu. The results indicate that the CVD growth graphene is hardly contact with Cu comparing with transferred graphene, which can make the charge transfer between electrode and channel in device easily.

References [1] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V and Firsov A A Science (2004). 306 666 [2] Schedin F, Geim A K, Morozov S V, Hill E W, Blake P, Katsnelson M I and Novoselov K S 2007 Nat. Material (2007). 6 652 [3] Danneau R, Wu F, Craciun M F, Russo S, Tomi M Y, Salmilehto J, Morpurgo A F and Hakonen P J Phys. Rev. Lett (2008). 100 196802 [4] Blake P, Yang R, Morozov S V, Schedin F, Ponomarenko L A, Zhukov A A, Nair R R, Grigorieva I V, Novoselov K S and Geim A K Solid. State. Commun (2009). 149 1068 [5] Venugopal A, Colombo L and Vogel E M Appl. Phys. Lett (2010). 96 013512

Figures (a)

SiO2

PR deposition & patterning

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SiO2 Etch by BOE

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SiO2 pattern

Cu deposition (Thermal evaporator)

Synthesis Graphene Cu

Graphene

SiO2 Si Substrate

Cu lift off by Acetone

Cu deposited

Embedded Cu

Graphene growth by CVD

Embedded Cu structure

Graphene growth pattern

Figure 1. (a) Fabrication process of embedded Cu substrate for chemical vapor deposition (CVD) graphene growth. (b) The improvement of metal-graphene contact structure using CVD growth graphene as buffer layer.

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Figure 2. (a) Contact resistance as a function of gate bias, showing the gate bias and contact structure dependence. The inset graph shows contact resistance results at gate biases of -2 V and -1.7 V. (b) Deriving contact resistance using the TLM pattern.

Oral Contribution

Large-scale exfoliation of Molybdenum Disulphide in solvent mixtures R. C. T. Howe, F. Torrisi, F. Tomarchio, S. Mignuzzi, A. C. Ferrari, T. Hasan Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, UK rcth2@cam.ac.uk

We demonstrate preparation of high quality dispersions of single- and few-layer MoS2 using mild sonication of bulk MoS2 in an alcohol-water binary solvent mixture. The key issue for dispersion is ensuring that the solvent properties, including the surface energy and the Hansen solubility parameters, are well matched to those of the layered material [1, 2, 3]. In this case, exfoliation becomes energetically favourable, and sonication becomes sufficient to overcome the van der Waals forces between the layers, leading to stable dispersions of single- and few-layer 2-dimensional (2d) crystal flakes (Fig. 1). This can be typically achieved through the use of single solvents, e.g. NMethylpyrrolidone (NMP) [1, 2] or via the addition of surfactants [1, 3]. The latter is useful as it can allow the use of solvents with poorer solubility parameter matching, but with more desirable properties, such as lower boiling point and lower toxicity. However, this requires removal of surfactants at a later stage. The use of binary-solvent mixtures has many of the advantages of the aqueous-surfactant based dispersions, while not introducing the need to remove surfactants [4]. We sonicate bulk MoS2 crystals in 50/50 vol% water-alcohol mixture; Fig. 1. Following sonication, the dispersions are centrifuged to remove un-exfoliated material, and then characterised by optical absorption spectroscopy, Raman and photoluminescence spectroscopy, transmission electron and atomic force microscopy. These reveal a concentration of up to 0.02g/l, with average lateral flake size ~2 microns. This is lower than concentrations previously reported with solvents such as NMP [1], although with comparable flake size. When compared to organic solvents, the increased ease of o o handling and the lower boiling point of the solvent mixtures (~80 C [4] against ~202 C for NMP [5]) makes these dispersions suited for coating or printing on polymeric substrates.

References [1] Coleman, J. N. et al., Science, 331 (2011), 568-571. [2] Cunningham, G. et al., ACS Nano, 6 (2012), 3468-3480. [3] Bonaccorso, F. et al., Mat Today, 15 (2012), 564-589. [4] Oâ&#x20AC;&#x2122;Neill, A. et al., Journal of Physical Chemistry C, 115 (2011), 5422-5428 [5] http://www.sigmaaldrich.com/catalog/product/sial/494496

Figures

Fig. 1: Liquid phase exfoliation of MoS2: a) MoS2 flake is added to b) solvent/solvent mixture. c) The mixture is ultrasonicated, producing a dispersion with varying flake thickness d) The dispersion is centrifuged to sediment unexfoliated MoS2. e) The supernatant is enriched with mono- and few-layer flakes.

(Two page abstract format: including figures and references. Please follow the model below.) Transparent flexible photosensors based on graphene-metal hybrid structures Ya-Ping Hsieh*, Mario Hofmann and Chih-Han Yen Graduate of Institute of Opto-Mechatronics National Chung Cheng University, Chiayi, Taiwan, ROC yphsieh@ccu.edu.tw Abstract (Arial 10) Graphene, a single atomic layer of carbon, has potential as a transparent and flexible optoelectronic sensor due to its high carrier mobility and low carrier concentration. Because of its semi-metallic band structure, grapheneâ&#x20AC;&#x2122;s adsorption constant covers over a wide range of illumination wavelength, ranging from infrared to visible light[1]. Thus, applications in broadband light detection in communication and sensing is envisioned[2]. However, graphene's optoelectronic response is limited. The main challenge, however, arise from grapheneâ&#x20AC;&#x2122;s small interaction with light. Firstly, due to its high transparency, graphene only interacts with 2.3% of the incident photons. Additionally, these photons only generate electrons with a 6â&#x20AC;&#x201C;16% internal quantum efficiency [1]. These two factors result in a very small photoresponse of pristine graphene[1]. Improving the absorption and conversion of light on graphene are therefore of great importance for the application of this material in photosensors. Current approaches include the use of functionalized graphene oxide and photosensitizers in contact with grpahene[3-5]. These strategies have been shown to result in higher photocurrent gain but are hampered by a slow current response and exhibit a decreased detection bandwidth. In this work, we introduce a novel, simple approach to produce CVD graphene-metal hybrid structures which could be transparent and flexible. Due to a new sensing mechanism, photocurrent response of 100x~10000x can be observed as shown in Fig. 1(a), which represents a significant increase over existing graphene based sensors. The response time constant is significantly lower than previously reports for metal decorated graphene as shown in Fig 1(b). Based on the presented detailed study of the mechanism responsible for this enhanced performance, we suggest novel applications that make the novel graphene-metal hybrid structures promising for optoelectronic products. References [1] Mueller T., F.N.A. Xia, et al., Nat Photonics, 4(2010) 297. [2] Xia F., T. Mueller, et al., Nature nanotechnology, 4(2009)839. [3] Konstantatos G., M. Badioli, et al., Nature nanotechnology, 7(2012)363. [4] Zhang D., L. Gan, et al., Adv Mater,24 (2012)2715. [5] Chang-Jian S.K., J.R. Ho, et al., Aip Advances, 2(2012)022104.

Figures

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(b)

Figure 1. The response and react time are great the normal devices (a)About two to four order photo-response under 5mW of white light (b) the time photoresponse shows the sharp current amplitude representing a fast response.

Nitrogen-Doped Graphene: Novel Growth and Plasma Doping Kun-Ping Huang, Yu-Wen Chi, Yu-Tin Lin, and Chih-Chen Chang ITRI, No. 195, Sec 4 Chung Hsing Rd. Chutung, Hsinchu, Taiwan R. O. C. kphuang@itri.org.tw Abstract (Arial 10) 1 We present a microwave plasma torch (MPT) method which uses hydrocarbon gas source as precursor to synthesis grapheen sheets. Growing graphene experiments are carried out in an atmospheric-pressure microwave (2.45 GHz) plasma reactor (figure 1). Reaction gas mixture (Ar:CH4 = 2000:3 flow ratio) is introduced into the quartz tube. The Raman data (figure 2) and TEM image (figure 3) show the graphene sheets are less than three layers. Electron cyclotron resonance (ECR) system has higher plasma density and lower plasma potential which are suitable to do nitrogen plasma doping. -4 Nitrogen plasma doping experiments are carried in 7 x 10 torr for 5 minutes without heating. XPS data (figure 4) shows the graphene sheets can achieve 8.1 at% of nitrogen easily. References [1] Albert Dato, Velimir Radmilovic, Zonghoon Lee, Jonathan Phillips, and Michael Frenklach, Nano Lett. 8, (2008) 2012.

Figures

Figure 1. Schematic of the atmospheric-pressure microwave plasma reactor used to synthesize graphene.

2D G D

Figure 2. Raman spectroscopy characterization.

Figure 3. Typical TEM image of graphene sheet grown by MPT.

Figure 4. XPS survey for as-synthesized N-graphene sheet.

Graphene Capture Using Nanowebs for Sensor Applications 1,3

Chi P. Huynh , Stephen C. Hawkins , Kallista Sears , Mustafa Musameh , Yen Truong , Amanda 1 2 3 Barnard , Tim Williams and George P. Simon 1. CSIRO Materials Science and Engineering, Clayton, Victoria 3168, Australia 2. Monash Centre for Electron Microscopy, Monash University, Clayton Victoria 3168, Australia 3. Department of Materials Engineering, Monash University, Clayton Victoria 3168, Australia Chi.huynh@csiro.au Abstract Graphene (G) has attracted considerable attention because of its excellent physical properties and potential electronic and spintronic applications. Synthesis is most commonly by chemical vapor deposition (CVD) and surface precipitation methods on transition metals such as copper (Cu) and nickel (Ni). To capture the G layer without damage or distortion, it is first coated with a supporting material such as a polymer and the metal etched away from below. The supported graphene is then transferred to another substrate such as glass or silicon and the supporting layer is removed. The advantage of these methods is that large areas of defect-free G can be obtained. The most common supporting materials are poly(methylmethacrylate) (PMMA) or polydimethylsiloxane 1 (PDMS). However, it is difficult to remove the polymer support after transfer. The R2R (Roll to Roll) transfer using thermal release tapes is particularly efficient for flexible target substrates. Nevertheless, this method is not suitable for rigid substrate such as ITO, glass or silicon. We have developed novel transfer techniques using Nanowebs for the first time as supporting structures which allow CVD graphene to be transferred directly onto chosen polymers or substrates for certain applications without the use of PMMA or PDMS coatings. We use materials such as directly spinnable carbon nanotube (DSCNT) webs and electrospun (E-spin) nanowebs of any required polymer. The nanowebs are deposited directly onto the graphene/metal substrate and are able to efficiently support the Graphene during etching. 2

The DSCNT was prepared according to our published methods . From one to eight layers of highly aligned web was used to recover the Graphene. E-spun nanoweb was prepared as previously 3 reported using PVDF for this study. Fig. 1 illustrates DSCNT and E-spun nanoweb respectively being deposited onto the Graphene sheet. These techniques can be used to fabricate a number of devices such as flexible electronics, solar cells, batteries, sensors etc. To test the effectiveness of graphene captured using nanowebs we’ve fabricated sensors made from CNT-web- (8 layers) and electrospun-supported Graphene for biosensor. We have obtained sheet resistance of 240 Ω/ sq for DSCNT, 600 Ω/ sq for Graphene, 156 Ω/ sq for G on DSCNT webs (8 layers) and 1070 Ω/ sq for G on PVDF nanowebs. Fig 2a and b show cyclic voltammetry results using potassium ferrocyanide as redox probe. Higher electron transfer rates for CNT and E-spun supported Graphene were achieved when used to modify electrodes compared with bare glassy carbon electrodes. We also capture G with a single layer of CNT web and fabricated organic bulk heterojunction devices for transparent electrode solar cell devices. Table 1 shows that G captured by PMMA has maximum efficiency of 1.64%, while DSCNT web (1 layer) has 1.84%. Significant improvement was found for device from G on DSCNT web. Although the transparency was lower,it is still within the acceptable range for transparent solar cells.

References [1] Bae S, Kim H, Lee Y, Xu X, Park J, Zheng Y et al. Nature Nanotechnology, 5 (2010) 574 – 8. [2] Huynh CP, Hawkins SC. Carbon,48 (2010) 1105–15 [3] Truong YB, Glattauer V, Lang G, Hands K, Kyratzis IL, Werkmeister JA and Ramshaw JAM. Biomed Mater. 5 (2010) 25005 (1-7)

Figures

Figure 1. DSCNT and E-spun nanoweb is being deposited over the Graphene sheet respectively (from left to right)

Figure 2. Cyclic voltammetry using K4Fe(CN)6 for G on DSCNT webs (a) and G captured by E-spun nanowebs (b)

Solar cell samples ITO Graphene captured using PMMA DSCNT Web Graphene on DSCNT web (no PMMA)

Max efficiency (%) Transparency 3.3 85.00 1.64 77 1.84 80 2.25 68

Table 1. Maximum efficiency (%) and transparency for organic bulk heterojunction devices

Stacking-Dependent Superstructures and Taxonomy at Armchair Interfaces of Bilayer/Trilayer Graphene Asieh Kazemi1,2, Simon Crampin1,2, and Adelina Ilie1,2 1Centre

for Graphene Science, 2Department of Physics, University of Bath, Bath, BA2 7AY, UK a.ilie@bath.ac.uk

We study quantum interference (QI) phenomena at bilayer-trilayer armchair interfaces, for different stacking sequences. The resulting QI patterns are discussed in terms of pattern taxonomy for graphene systems. Visualization using scanning tunneling microscopy (STM) and theoretical calculations (by STM image calculations and density functional theory) provide direct evidence that near armchair edges electron behavior is dominated by the “hard” edge, where the layer is abruptly truncated, as opposed to the “soft” edges, where layers continue across the boundary. Intervalley reflection causes universal quenching of the wavefunction with a periodicity of three C atoms, while the exact interference patterns depend on the stacking sequence and appear to be robust to disorder and chemical terminations. Lateral interfaces within multi-stacked graphene systems can provide unique system-specific opportunities for wave-function engineering to be exploited in devices employing quantum-interference.

Figure 1: (a) Pattern taxonomy for stacked graphene systems at armchair edges/interfaces; STM image simulations of mono-bilayer and bilayer-trilayer armchair interfaces. (b) Experimental STM images at a bilayer-trilayer interface, showing a change in stacking on the top layer of the trilayer side; magnified regions show patterns that are interpreted based on scattering processes and the given taxonomy.

Temperature-gradient-induced charge and spin currents in graphene with resonance levels Michał Inglot, V. K. Dugaev Department of Physics, Rzeszów University of Technology Al. Powstańców Warszawy 6, 35-959 Rzeszów, Poland ming@prz.edu.pl Abstract Charge and spin caloritronic effects attracted recently a lot of attention as a new step in enhancing functionality of spintronic devices [1,2]. From this point of view, graphene is probably the most important material for investigations, that has been already proved by studying its numerous very unusual electronic and transport properties. Not only ‘classical’ one-layer graphene but also two-layer [3,4], three- and multi-layer [5] graphene structures have been already studied as new perspective thermoelectric materials [6,7]. In our work we consider charge and spin thermoelectric effects in a one-layer graphene with certain choice of impurities creating resonance levels. We use the model with a short-range impurity potential characterized by parameter related to the type of impurity in the graphene lattice or at the surface of graphene. Especially important for the characterization of impurity states are spin-orbit interaction effects, even in the case when they are rather weak [9]. In our calculations of impurity states we used a standard T-matrix approach for the resonant states near K and K’ Dirac points in the Dirac energy spectrum of low-energy electron excitations. To calculate the charge and spin thermoelectric current we use the kinetic equation method, correspondingly modified to account for pseudospin and spin-dependence of the electron distribution function. The key point of are calculation is the electron relaxation time, which includes the effect of resonant scattering from impurities. It strongly modifies both momentum and spin relaxation in graphene. Figure 1 demonstrates the variation of electron relaxation time for different values of the parameter V0 and different impurity density N. Impurities influence the thermoelectric Seebeck constant α and thermoelectric current induced by the temperature gradient [8]. As we show in Fig. 2, the localized states affect significantly the thermoelectric constant . For positive chemical potential µ, only resonant states with positive energies (correspondingly, negative impurity potential, V0 < 0) increase the Seebeck current. The value of constant α depends on temperature, and for T = 10K it is smaller, whereas at T = 300K it can be as high as α > 0.15 V/K. We also show the charge current (Fig.3) as a function of chemical potential for different temperatures T and impurity concentration N. We find that the thermoelectric constant can be larger than it was reported before [6]. Taking into account heat conductivity by electrons without phonons, we evaluated the figure of merit ZT. We also take into account intrinsic spin-orbit interaction which modifies resonant states in graphene [8]. This interaction gives us a possibility of generation of the spin-polarized current in graphene by the temperature gradient. Another problem which we consider in this work is the magnetization induced by the temperature gradient in graphene with Rashba spin-orbit interaction. Strong Rashba interaction is possible for graphene on substrate In this case we use Green’s function formalism to find the magnetization. Also, in this case we take into account resonant state generating by impurities for the calculations of relaxation times. We find an influence of resonant states on the magnetization in graphene. This influence is especially strong when impurities enhance the local spin-orbit interaction. This work is supported by the National Science Center in Poland as the project No. DEC2011/01/N/ST3/00394 for years 2011-2014. References [1] T. Kikkawa, K. Uchida, Y. Shiomi, Phys. Rev. Lett. 110, 067207 (2013) [2] S.Y. Huang, W.G. Wang, S.F. Lee, J. Kwo, C.L. Chien, Phys. Rev. Lett. 107, 216604 (2011) [3] E.H. Hwang, E. Rossi, S. Das Sarma, Phys. Rev. B 80, 235415 (2009) [4] C.R. Wang, W.S. Lu, L. Hao, W.L. Lee, T.-K. Lee, Phys. Rev. Lett. 107, 186602 (2011) [5] L. Hao, T.K. Lee, Phys. Rev. B 82, 245415 (2010) [6] D. Dragoman, M. Dragomana, Appl. Phys. Lett. 91, 203116 (2007) [7] L. Zhu, R. Ma, L. Sheng, M. Liu, D.N. Sheng, Phys. Rev. Lett. 104, 076804 (2010) [8] M. Inglot, V.K. Dugaev, J. Appl. Phys. 109, 123709 (2011) [9] D. Wang, J. Shi, Phys. Rev. B 83, 113403 (2011)

Fig.1 Electron relaxation time ď ´ (ď Ľ ) for different impurity potential V0 and different impurity concentrations N

Fig. 2 Thermoelectric current coefficient vs. chemical potential for different impurity potential V0 - (a) V0>0, (b) V0<0.

Fig. 3 Seebeck coefficient vs. chemical potential for different impurity potential (a) T=300 K, (b) T=10 K.

Diamond-like carbon and nanodiamond layers as a platform for supporting graphene Fang Zhao, Thuong Thuong Ngyen, Glenn C. Tyrrell* and Richard B. Jackman London Centre for Nanotechnology and Department of Electronic and Electrical and Engineering, University College London, 17-19 Gordon Street, London, WC1H 0AH, UK. *Applied Scintillation Technologies Ltd, 8 Roydonbury Industrial Estate, Horsecroft Road, Harlow, Essex, CM19 5BZ, UK r.jackman@ucl.ac.uk Inevitably the substrate that attaches to a material such as graphene will influence the electronic properties of the carbon monolayer. The majority of reports concerning the electronic properties of chemical vapour deposited (CVD) graphene that has been transferred to an insulating substrate for electronic characterization concern the use of SiO2-Si, due to the widespread availability of this material system. A more ideal platform for the graphene could be another carbon form such as diamond and favourable reports has appeared [1], and our own group have investigated the influence of terminating chemical groups on the diamond substrate on the graphene layers electrical properties [2]. However, the limited area of single crystal diamond materials is an obvious limitation for the deployment of this material as a graphene substrate. A potential alternative, diamond-like carbon (DLC) can be deposited at room temperature on a wide range of materials over large areas. A report on the use of DLC as a graphene support has appeared [3], but no investigation into the effect of surface terminations of the DLC on the resultant graphene properties has been reported. We initiated such a study; initial results from this work are extremely encouraging, in terms of Hall effect measurements, AFM, SEM, XPS and Raman Spectroscopy which are reported here. In addition, the use of detonation nanodiamond (ND) layers attached by conventional seeding methods to a range of substrates, again over large areas, has been studied for this application. CVD produced monolayer graphene was transferred onto DLC and ND coated substrates that had been previously subjected to differing chemical and plasma treatments to lend them differing surface terminating groups. For example, monolayer attachments of H and O were investigated, in all cases leading to a p-type graphene layers. Stark differences in the electrical character of the resultant graphene-DLC or ND-heterostructure were observed. For example, it was found that higher carrier mobility values cannot be simply associated with lower carrier densities (as they are in conventional semiconductor systems). Rather, each chemisorbed species give rise to a unique character. In the case 2 of H terminations, the maximum p-type mobility arose (more than 500 cm /Vs), allied to the lowest carrier concentration, but the carrier concentration rose noticeably for O terminations, with only a modest decrease in mobility. In additional to chemical changes (XPS), surface roughness changes (AFM) associated with differing termination treatments also led to different mobility values. These results will be discussed in terms of the possible surface-transfer effects that may be occurring within the DLC-terminating group graphene heterostructures, and the potential use of this approach for engineering tunable electrical properties. References [[1] Jie Yu and co-workers, Nano Lett. 2012, DOI: 10.1021/nl204545q [2] Fang Zhao et al Presented at the International Conference on Diamond and Related materials, Granada, Spain, September 2012 [3] Yanqing Wu et al, Nature 472, 74â&#x20AC;&#x201C;78 (2011)

Direct exfoliation of graphite in water solutions 1

J. Jagiełło , M. Zdrojek , M. Aksienionek , J. Judek , K. Librant , R. Koziński , L. Lipińska

Institute of Electronic Materials Technology, Wólczyńska 133, 01-919 Warsaw, Poland Faculty of Physics of Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland

joanna.jagiello@itme.edu.pl Abstract Graphene, because of its unique electrical properties, is the material with a wide range of potential applications, e.g. in supercapacitors, transistors and batteries. There are two main criterions, that should be fulfilled to make the process economic: quality and amount of obtained graphene. CVD method provide material with good electrical conductivity but it cannot be a mass production. On the other hand, chemical reduction of graphene oxide is the most quantitatively effective, but such obtained sheets have poor electrical conductivity. The most perspective way for the electronic applications seems to be direct chemical exfoliation of graphite. Such process is carried out in organic solutions or in water with surfactants and must be ultrasound-assisted. These molecules penetrate the layers in graphite causing the separation of graphene sheets and preventing from aggregation. The one serious disadvantage of direct exfoliation is the usage of toxic organic solvents [1]. There are some experiments with water-solution exfoliation of graphite [2]. In this work the results of direct exfoliation of expanded graphite are presented. Medium for the process were water solutions of chitosan dissolved in two different solutions and with sodium deoksycholate (SDC). These are non-toxic compounds. As a consequence, graphene composites with different morphologies were obtained (Fig.1. a, b). Medium for the exfoliation has significant influence on the thickness of graphene sheets, electrical properties and the amount of defects in graphene structure, what can be seen on Raman spectra. The best sheet resistance of obtained samples was below 1 Ω/sq. Raman measurements indicated no or small amount of defects in this few-layer graphene. AFM images prove good separation of graphene layers from graphite and the thickness of platelets obtained by exfoliation was about 2 nm (Fig. 2). References [1] Changqing Liu et al., J. of Supercritical Fluids, 63 (2012) 99-104 [2] H. Yang et al., Carbon 53 (2013) 357-365

Figures a

Fig.1 SEM images of composite of grahene/chitosan dissolved in two different solutions (a- dried, bimposed on the silica plate).

Fig.2 AFM profile of graphite exfoliated with SDC.

High-throughput, automatic and roboust identification of graphene and similar 2D thin-film materials in digital images. (Jessen, 2012) Bjarke Jessen, MSc, bsoje@nanotech.dtu.dk Technical University of Denmark, Kongens Lyngby, Denmark The experimental isolation of a single-layer of graphene by the Manchester group in 2004 has paved the way for an entire family of new truly two-dimensional (2D) materials including graphene, hexagonal boron nitride, and molybdenum disulphide. While they are truly fascinating materials each in its own right, a major bottleneck in using these materials for research purpose lies in the need for manual identification after mechanical exfoliation, where especially single-layer identification can be challenging due to low visibility. Furthermore, since the identification is manual different researchers often does not find the same flakes on the same wafers, thus lowering the effective yield. To overcome this bottleneck there is thus a need for an automated, simple, reliable, and efficient way of identifying graphene and other thin-film materials in a digital image. The contrast of 2D materials can be accurately modelled with the optical transfer matrix method (P. Blake, 2007). This yields the contrast as a function of wavelength and often serves as a guideline for researchers to choose an appropriate substrate. To describe the colours in a digital image, namely red, green and blue (RGB), it is necessary to couple this to the actual microscope used through the numerical aperture, light bulb spectra and charge coupled device (CCD) sensor spectral sensitivities (Guild, 1931). This provides an accurate framework for predicting the actual RGB values of the pixels corresponding to a given 2D material, as illustrated with single-layer graphene in figure 1. Having obtained digital images of a wafer with e.g. graphene it is possible to accurately extract the pixels with graphene though dynamic background detection and careful application of image processing filters such as median smoothing, dilate, erode, and thesholding. This enables the identification of parts of an image containing e.g. single-layer graphene while removing false-positives such as graphite shadows, dirt, tape residues, etc, as shown in figure 2. Applying an edge-detection algorithm it is possible to extract quantitative information such as size, shape, along with defects and rips, allowing automatic selection of suitable flakes for further processing. Having accurate, digital representations of graphene flakes also allow for direct import into CAD programs significantly easing the design phase of device fabrication. In summary it has been explained how the contrast of 2D thin film materials such as graphene, hexagonal boron nitride and molybdenum disulphide can be accurately mapped into the RGB colour-space as represented in digital images. By the application of dynamic background detection, various image processing filters and an edge-detection algorithm it is possible to automatically and with a high-throughput accurately detect individual layers of 2D thin film materials while gaining quantitative data about various properties such as size, shape, number of layers etc. This greatly speeds up basic research using novel 2D materials as the individual flakes moves from being scarce to being a commodity resource. Finally, the method only requires an optical microscope with a CCD camera and a programmable XY stage, relatively cheap equipment which are already found in most research labs.

Bibliography Jessen, B. S. (2012). Patent No. 12180234.2. Denmark. Guild, T. S. (1931). The C.I.E. colorimetric standards and their use. Transactions of the Optical Society, 73. P. Blake, E. W. (2007). Making graphene visible. Applied Physics Letters, 063124.

Figures

Figure 1 - Red, green and blue pixel-contrast of single-layer graphene on a SiO2/Si substrate as a function of varying thickness of SiO2. The dashed line is at 90 nm SiO2, a commonly used thickness for identifying graphene.

Figure 2 â&#x20AC;&#x201C;(a) Image of single- and bi-layer graphene on a substrate of 90 nm SiO2 on Si. The image also contains graphite, shadows, index marks and residues, all typical to samples with mechanically exfoliated graphene. (b) digital representation of single- and bi-layer graphene after post processing of the digital image. False-negatives are removed, and the final shape is detected with an edgedetection algorithm.

Contribution (Oral/Poster/Keynote) Monolayer transition metal dichalcogenide based field-effect transistors David Jiménez Departament d'Enginyeria Electrònica, Escola d'Enginyeria, Universitat Autònoma de Barcelona, Spain david.jimenez@uab.es

A great deal of interest in two-dimensional materials analogues of graphene has appeared among the scientific community since the demonstration of isolated 2D atomic plane crystals from bulk crystals [1]. Dimensionality is key for the definition of material properties and the same chemical compound can exhibit dramatically different properties depending on whether it is arranged in dots (0D), wires (1D), sheets (2D) or bulk (3D) crystal structure. Notably, experimental studies of 2D atomic crystals were lacking until recently because of the difficulty in their identification [1]. Representative of this class are the 2D monolayer of transition metal dichalcogenides (TMD) with a chemical formula MX2, where M stands for a transition metal and X for Se, S, or Te. The potential of this family of layered materials for flexible electronics was proposed by Podzorov et al., who demonstrate an ambipolar WSe2 p-FET with a hole mobility comparable to silicon (500 cm2/V-s) [2]. The electronic properties of TMDs vary from semiconducting (e.g., WSe2) to superconducting (e.g., NbSe2). The semiconducting monolayer TMDs, like MoS2, MoSe2, MoTe2, WS2, and WSe2 are predicted to exhibit a direct gap in the range of 1–2 eV [3]. The wide gap together with a promising ability to scale to short gate lengths because of the optimum electrostatic control of the channel, by virtue of its thinness, make monolayer TMDs very promising for low power switching and optoelectronics applications. The first 2D crystal based FET relying on a semiconducting analogue of graphene was demonstrated using a monolayer MoS2 as the active channel [4]. Low power switching with an ION/IOFF108 and subthreshold swing (SS) of 74 mV/decade at room temperature, was experimentally measured. More recently, a monolayer p-type WSe2 FET with an optimum SS  60 mV/decade and ION/IOFF >106 was demonstrated [5]. To boost the development of 2D-material based transistor technology, modeling of the electrical characteristics is essential to cover aspects as device design optimization, projection of performances, and exploration of low-power switching circuits. Some models aimed to explore the performance limits of monolayer TMD transistors have been reported assuming ballistic transport. However, the behavior of state-of-the art devices is far from ballistic and a drift-diffusion transport regime seems more appropriate for channel lengths well above the carrier mean free path. In this context, I propose a model for the current-voltage (I-V) characteristics of monolayer TMD FETs, based on the drift-diffusion theory. As a previous step a surface potential model, accounting for the 2D density-of-states (DOS2D) of monolayer TMDs, is proposed. I will consider that carriers are free to move parallel to the TMD sheet. However their motion is restricted in the perpendicular direction because the strong quantum confinement. The DOS2D has a profound impact on the quantum capacitance, which is essentially different from that of a nanowire (1D) or a bulk (3D) material. Analytical expressions are derived for both the surface potential and drain current covering both subthreshold and above threshold operation regions. The model has been assessed by means of experimental data (Fig. 1) and used to make some predictions on the existing tradeoff between the ON-current and ON/OFF current ratio.

Contribution (Oral/Poster/Keynote) References [1] K. S. Novoselov et al., “Two-dimensional atomic crystals,” PNAS vol. 102, no. 30, pp. 10451-10453 (2005). [2] V. Podzorov et al. “ High-mobility field-effect transistors based on transition metal dichalcogenides,” Appl. Phys. Lett. 84, no. 17, pp. 3301-3303 (2004). [3] W. S. Yun et al., “Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-MX2 semiconductors (M=Mo, W; X=S, Se, Te),” Phys. Rev. B, vol. 85, 033305 (2012). [4] B. Radisavljevic et al., “Single-layer MoS2 transistors,” Nature Nanotech., vol. 6, pp. 147-150 (2011). [5] H. Fang et al., “High-performance single layered WSe2 p-FETs with chemically doped contacts,” Nano Lett., vol. 12, pp 3788–3792 (2012).

Figure 1. Transfer (a) and output (b) characteristics obtained from the analytical model (solid lines) compared with experimental results from Ref. [5] (symbols). Inset (a): cross section of the dual-gate monolayer TMD transistor. Inset (b): equivalent capacitive circuit.

Electronic properties of Twisted Bilayer Graphene with vacancies J.-F. Jobidon (1), L. Magaud (1) (1) Institut Néel, CNRS & UJF, Grenoble, France jean-francois.jobidon@grenoble.cnrs.fr

Abstract Graphene bilayers (BLG), with AA or AB (Bernal) stacking have been known for several years[1]. AA stacking corresponds to a rotational angle of 0˚ while AB stacking corresponds to an angle of 60˚, but in fact, nothing prevents us to build systems with arbitrary angle between 0˚ and 60˚. Indeed it has been observed from experiments that graphene layers can be rotated on some substrates[2-4] with an angle between successive layers that ranges from 0˚ to 60˚. These rotations create moiré patterns that are observed on STM images[2]. These rotated (twisted) bilayer graphene (TBLG) differ both from single layer graphene (SLG) and graphite and their electronic structure is governed by the rotation angle. While a large rotation angle (θ ∼ 30◦ ) results in two decoupled graphene layers[5-12] with a linear dispersion near the Dirac points, carriers in a twisted graphene bilayer evolve from Dirac fermions to strongly localized electrons (or holes) when the rotation angle is decreased toward very small angles[6] (or increased toward 60˚, the system is symmetric with respect to θ = 30˚, Fig. 1). At the intersection between two Dirac cones originating from two different layers, the interaction between the states can open a gap and saddle points appear (Fig. 2). At 2D such saddle points give rise to van Hove singularities (named E+ and E− in Fig. 2) characterized by sharp peaks in the density of states (DOS). As θ is brought closer to 0˚, the two van Hove singularities fall closer to the Dirac point and eventually merge for θ close to 2˚. The van Hove singularities have been recently observed in different systems[4, 13] confirming these theoretical predictions. When the two van Hove singularities merge, the DOS shows a sharp peak at the Dirac point (fig. 2b). States belonging to this peak are localized in the AA region of the moiré pattern[6]. We want to test the robustness of such systems when confronted by the environment. Such environmental factors include doping, strength, defects, vacancies. Here we will focus on vacancies for angles as small as possible permitted by the ab-initio DFT simulations. In the small rotational angle limit, the number of atoms scales like 1/θ2 So in order to keep the simulation time reasonably small, we were limited by systems with few hundreds of atoms which give us systems with 508 atoms and θ = 5.08◦ . We compare how a simple vacancy in different systems (SLG, BLG, TBLG) and different vacancy sites (AA, AB, TOP and HOLLOW) can modify the electronic properties.

References [1] S. Latil and L. Henrard. Phys. Rev. Lett., 97, 036803, 2007. [2] F. Varchon, P. Mallet, L. Magaud, and J.-Y. Veuillen. Phys. Rev. B 77, 165415, 2008. [3] G. Li, A. Luican, J. M. B. Lopes dos Santos, A. H. Castro Neto, A. Reina, J. Kong, and E. Y. Andrei. Nat. Phys., 6, 109, 2010. [4] A Luican, Guohong Li, A Reina, J Kong, R Nair, Konstantin S Novoselov, Andre K Geim, and E Andrei. Phys. Rev. Lett., 106, 126802, 2011. [5] J. Hass, F. Varchon, J. E. Millan-Otoya, M. Sprinkle, N. Sharma, W. de Heer, C. Berger, P. N. First, L. Magaud, and E. H. Conrad. Phys. Rev. Lett., 100, 125504, 2008. [6] G. Trambly de Laissardière, D. Mayou, and L Magaud. Nano Lett., 10, 804, 2010. [7] G. Trambly de Laissardière, D. Mayou, and L. Magaud. Phys. Rev. B, 86, 125413, 2012. [8] J. M. B. Lopes dos Santos, N. M. R. Peres, and A. H. Castro Neto. Phys. Rev. Lett., 99, 256802, 2007. [9] J. Lopes dos Santos, N. Peres, and A. H. Castro Neto. Phys. Rev. B, 86, 155449, 2012. [10] E. Suárez Morell, J. Correa, P. Vargas, M. Pacheco, and Z. Barticevic. Phys. Rev. B, 82, 121407, 2010. [11] S. Latil, V. Meunier, and L. Henrard. Phys. Rev. B, 76, 201402, 2007. [12] R. Bistritzer and A. H. MacDonald. PNAS, 108, 12233, 2011. [13] I. Brihuega, P. Mallet, H. González-Herrero, G. Trambly de Laissardière, M. Ugeda, L. Magaud, J. Gómez-Rodríguez, F. Yndurain, and J. Y. Veuillen. Phys. Rev. Lett., 109, 196802, 2012.

Figures

Figure 1 â&#x20AC;&#x201C; Velocity ratio of bilayer and monolayer graphene for a commensurate bilayer cell versus rotation angle. Circles : DFT calculations ; crosses : TB calculations ; line : theoretical prediction of Lopez dos Santos et al.

Figure 2 â&#x20AC;&#x201C; Van Hove singularities of a (6,7) bilayer. a) schematic drawing, b) (6,7) band structure and total DOS.

Spatially resolved electronic characterization of hydrogenated graphene by scanning tunneling spectroscopy 1

Jakob Jørgensen , Richard Balog , Liv Hornekær

1,2

iNANO, Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark jakobj88@inano.au.dk

Abstract Graphene has attracted great research interest in the past years due to its extraordinary mechanical [1] and electronic [2] properties. Graphene has a very high electrical conductivity due to extremely high carrier mobility governed by ballistic carrier transport, i.e. transport without scattering over macroscopic distances. The linear band dispersion of graphene close to the Dirac points results in electrons behaving as massless Dirac fermions [2]. Due to the aforementioned properties, graphene is a promising candidate for fabrication of ultrafast transistors but since graphene is a semimetal, the on/off ratio of current switching is inefficient. Therefore many different mechanisms have been proposed on how to modify graphene into semionductor by opening a bandgap at Dirac point. One possible route to achieve this is by hydrogenation of graphene. We have successfully demonstrated, that a bandgap of at least 450 meV [3] is induced in graphene on Ir(111), which is sufficient for application in electronic devices. The poster will present preliminary data of LDOS mapping of hydrogenated graphene on Ir(111). The hydrogenation of graphene has already been investigates extensively by our group. Graphene on Ir(111) possesses a superstructure (Moiré structure) due to lattice mismatch between the substrate and graphene. Hydrogen does not adsorb with same probability to all sites in the Moiré - it has been shown that at lower coverages hydrogen preferentially binds to the fcc and hcp parts of the moiré ,fig. 1. In these reagions the density functional theory (DFT) calculations show that the most stable structure is obtained when a C atom binds alternately to H and Ir. This result can also be understood from geometrical considerations since the 3 optimal geometry for sp hybridization is tetrahedral with angles of between each bond. Since the formation of a C-Ir bond stabilizes H adsorption, only regions where a C atom is directly above an Ir atom are hydrogenated, which explains why hydrogen is adsorbed in a periodic fashion until high coverage. When hydrogen is adsorped to the surface a local graphane-like structure is created which, according to calculations is insulating [4]. Our hypothesis is that the periodic modulation with insulating regions leads to confinements of electrons and hence opens a bandgap [6]. This hypothesis has been challenged by other studies where a bandgap was observed by hydrogenation without periodic modulation [5]. We propose to investigate this experimentally by atomic resolution mapping of the LDOS using low temperature STS measurements which should shed light on this matter. Future work includes LDOS mapping of hydrogenated graphene on several different substrates which do not show the periodic modulation, e.g. SiC(0001) and Pt(100). We have demonstrated that these substrates show different hydrogen adsorption scheme compared to Ir(111): At very low coverages, graphene on SiC is geometrically modulated due to interactions with the underlying buffer layer. It has been shown that hydrogen preferentially binds to convex areas at low coverage resulting in quasi periodic adsorption following the SiC reconstruction as imaged in fig. 2a [6]. At higher coverage, disordered hydrogen clusters are observed, fig. 2b. Adsorption on Pt(100), on the other hand, shows no site preference and disordered hydrogen structures are observed, fig. 3 [7]. By investigating the electronic behaviour of these systems, we should be able to comment on whether the periodic arrangement plays an important role for the bandgap opening or not.

References 1.

Lee, C., et al., Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science, 2008. 321(5887): p. 385-388.

2. 3. 4. 5. 6. 7.

Geim, A.K. and K.S. Novoselov, The rise of graphene. Nature Materials, 2007. 6(3): p. 183-191. Balog, R., et al., Bandgap opening in graphene induced by patterned hydrogen adsorption. Nature Materials, 2010. 9(4): p. 315-319. Chandrachud, P., et al., A systematic study of electronic structure from graphene to graphane. Journal of Physics Condensed Matter, 2010. 22(46). Haberer, D., et al., Tunable band gap in hydrogenated quasi-free-standing graphene. Nano Letters, 2010. 10(9): p. 3360-3366. Balog, R., et al., Atomic hydrogen adsorbate structures on graphene. Journal of the American Chemical Society, 2009. 131(25): p. 8744-8745. Nilsson, L., et al., Graphene coatings: Probing the limits of the one atom thick protection layer. ACS Nano, 2012. 6(11): p. 10258-10266.

Figures

Figure 1 Hydrogenated graphene on Ir(111) a) Clean graphene where the MoirĂŠ is easily observed, b) at low coverage H binds in dimers at fcc and hcp sites. When increasing the coverage c), ring shaped clusters are formed and finally at highest coverage d), e) these merge to form elongated H clusters. f) is fourier transform of e) shows that the MoirĂŠ periodicity is preserved.

Figure 2: STM images of hydrogenated graphene on SiC, a) at low coverage hydrogen binds as ortho or para dimers at sites following the resonstruction of SiC, b) when the coverage is increased no site preference is observed. Hydrogen binds in larger disordered clusters

Figure 3: STM images of hydrogenated graphene on Pt(100) both at low a) and high b) coverage no ordering of hydrogen adsorption is seen

A Synoptic Look on Characterization Techniques of Graphene and its Derivatives and Analogues Gerald Kada, Jing-jiang Yu, Shijie Wu, Christian Rankl, Jining Xie, Ferry Kienberger Agilent Technologies, Nanoscale Sciences Division, 4330 W Chandler Blvd, Chandler, AZ, USA Gerald_Kada@Agilent.com Abstract 2 Graphene consisting of a monolayer of sp -bonded carbon atoms is a relatively new member of the carbon family. But owing to its unique structure, exceptional electrical, optical and mechanical properties, graphene is a rapidly rising star on the horizon of materials science and condensed matter physics [1]. Since its discovery, optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and micro-Raman spectroscopy have been widely employed to investigate its optical properties, determine its thickness, resolve its atomic arrangement, and detect its film quality, respectively. We have applied several of those imaging and characterization techniques to investigate surface morphology and electronic properties of graphene, selected derivatives like graphene oxide (GO) or silanized graphene, graphene-metal nanoparticle composites as well as graphene analogues like hexagonal boron-nitride (h-BN) a) Imaging graphene with a low beam voltage field emission scanning electron microscope (LV FE-SEM) is very promising because of its unique combination of high resolution, a small bean/specimen interaction volume, enhanced contrasts and the capability of revealing more surface details [2]. Three typical contrasts in SEM imaging of graphene including surface roughness contrast, edge contrast, and thickness contrast will be discussed in detail (see fig. 1). b) An Atomic force microscopy (AFM)-based technique called Kelvin force microscopy (KFM) is an experimental means to investigate the local properties of both single-layer graphene (SLG) as well as few-layer graphene (FLG) and graphene oxide. The effect of film thickness on the surface potential is detected and quantitative measurements are readily obtained (see fig. 2). Other AFM based techniques like current sensing AFM or electrostatic force microscopy (EFM) are applied as well on graphene, GO, and h-BN, respectively [3]. c) A unique and novel AFM-based technique is using a combination of near-field electromagnetic field in the microwave regime to probe both local topographical and mechanical properties with a scanning AFM tip, and simultaneously recording capacitance, dopant profile changes and impurities of underlying layers of material [4]. This technique is called scanning microwave microscopy (SMM), and first applications on detecting differences in electronic properties of graphene oxide and boron nitride will be introduced to this scientific audience. d) Micro-Raman spectroscopy has been used already to distinguish layers of SLG from few layers of graphene. We will demonstrate the use of a combination of a micro-Raman inverted light microscope in combination with an AFM in order to correlate both Raman maps and topographical or mechanical maps of the very same area [5].

References [1] A.K. Geim and K.S. Novoselov, Nature Materials, 6 (2007) 183. [2] J. Xie and J.P. Spellas, Agilent Technologies Application Note (2012), 5991-0782EN. [3] S.S. Datta, D.R. Strachan, E.J. Mele, A.T.C. Johnson, Nano Letters, 9 (2009), 7. [4] H.P. Huber, I. Humer, M. Hochleitner, M. Fenner, M. Moertelmaier, C. Rankl, A. Imtiaz, T. M. Wallis, H. Tanbakuchi, P. Hinterdorfer, P. Kabos, J. Smoliner, J. J. Kopanski, and F. Kienberger, J Applied Physics, 111 (2012), 014301. [5] R.D. Rodriguez, E. Sheremet, S. M端ller, O.D. Gordan, A. Villabona, S. Schulze,M. Hietschold, D.R.T. Zahn, Rev Sci Instr, 83 (2012), 123708.

Figures

Figure 1 left) A typical SEM micrograph of CVD synthesized graphene on Cu foil; right) magnified image of the yellow box in a) showing different contrast in certain areas; right bottom) the intensity profile along the yellow line in the top right image clearly displaying four plateaus representing monolayer, bilayer, trilayer and quadrilayer graphene.

Figure 2 Topography (left), Capacitance gradient (dC/dZ; middle), and Surface Potential (Right) images recorded in single-pass Kelvin Force Microscopy (KFM) of few-layer graphene on silica surface.

Deposition of ultra-thin graphene-like coatings by PECVD methods Reinhard Kaindl, Jeanine Pichler, Roland Fischer, Georg Jakopic, Wolfgang Waldhauser JOANNEUM RESEARCH, Leobner Straße 94, A-8712 Niklasdorf, Austria reinhard.kaindl@joanneum.at Abstract Since its first theoretical descriptions in the 50’s of the last century and experimental evidence in 2005 a large number of production processes for graphene have been proposed. However, only a small part of them is interesting in terms of up-scaling into a cheap and reliable industrial production. Plasma enhanced chemical vapor deposition (PECVD) offers this potential but only few reports about successful synthesis exist [1,2]. This contribution presents some results of thin and ultra-thin diamond-like carbon coatings (DLC), deposited and modified by PECVD with the aim to produce graphene-like structures on top of Si and Si/SiO2 wafers. The coatings were deposited on standard single side polished <100>, P/Bor doped, 525±25 µm thick Si wafers, with and without 100 nm thermal SiO2 layer. Prior to deposition the wafer were cleaned chemically in an ultrasonic cleaner with aceton and ethanol and then by argon plasma (nominal purity >99.999%), using an ALS340L anode layer source from Veeco Instruments (Woodbury, NY, USA) with a rotating carrousel, at 3 kV accelerating voltage and 30 sccm gas flow rate. The coatings were deposited by employing the same ion source with 20 sccm C2H2 (nominal purity >99.96%) and 1 and 3 kV voltages and subsequently etched in Ar plasma at 1 and 3 kV. Deposition times were varied within 1.5 to 60 minutes, etching times from 30 seconds to 40 minutes. Several coatings were deposited statically at deposition and etching temperatures of 400°C and reduced deposition and etching times (5 – 15 seconds). Some coatings were tempered in a nitrogen-filled tubular furnace at 300° and 800°C for 8 hours, 15 minutes, respectively. The coating thickness, measured by profilometry and ellipsometry, ranges between 10 and 92 nm. The measured optical constants of the coatings, namely n (refraction index, Fig. 1) and k (absorption coefficient) in the range 300 - 1000 nm, are most similar to the characteristics of graphite and graphene [3,4] when deposited at 3 kV for 60 minutes, and ion etched at 800 eV for 40 minutes. Raman spectroscopy of the as-deposited and the etched and tempered DLC coatings revealed a slight increase in the wavenumber of the G-band and the intensity ratio of the D- and G-band ID/IG, accompanied by a decrease in full width at half maximum (FWHM) of the G-band (Fig. 2). The effect was much more pronounced for the coatings deposited at higher temperatures and the tempered coatings. Such spectral changes can be interpreted as transformation from DLC coatings to 2 nanocrystalline graphite, accompanied by increasing structural order and crystallite size of sp coordinated carbon [5]. The smaller effect of plasma treatment might result from the relatively high ion etching energies of 800 to 1500 eV, increasing rather the structural disorder and crystallite size of the 3 coatings instead of selectively removing comparable lower stable sp coordinated carbon, as it was expected.

References [1] Qi, Zheng, Zheng, Wang, Tian, Applied Surface Science, 257 (2011) 6531. [2] Tyurnina, Tsukagoshi, Hiura, Obraztsov, Carbon, 52 (2013) 49. [3] Kravets, Grigorenko, Nair, Blake, Anissimova, Novoselov, Geim, Physical Review B, 81 (2010) 155413. [4] Weber, Calado, van de Sanden, Applied Physics Letters, 97 (2010) 091904. [5] Ferrari, Robertson, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences, 362 (2004) 2477.

Figures

Figure 1: Refraction index n of three plasma-etched DLC coatings (solid, dotted, dashed lines), legend indicates deposition/etching time. Chain-dotted line shows as deposited DLC coating. Graphene and graphite are represented by circles and crosses.

Figure 2: Full width at half maximum of the Raman G-band (FWHM G) of DLC coatings, deposited at 3 kV accelerating voltage, at various deposition and tempering temperatures and ion etching energies. FWHM G of graphene and graphite are given for comparison.

Graphene biosensor based on functionalized hydrophobins Markku Kainlauri, Sanna Arpiainen, Katri Kurppa, Miika Soikkeli, David Gunnarsson, Mika Prunnila, Markus Linder, Jouni Ahopelto VTT Technical Research Centre of Finland, P.O.Box 1000 FI-02044 VTT, Espoo, Finland markku.kainlauri@vtt.fi Abstract Graphene field effect transistors (GFET) are sensitive to the variations in the charge density in the vicinity of the channel and because most biomolecules are charged, the detection is mostly label free. However, biorecognition can only be achieved by selective binding of the analyte, which requires functionalization of graphene surface with antibodies, DNA, peptides or proteins. As the defect free graphene surface is inert and the formation of covalent bonds hinders the electronic properties of graphene, many schemes for non-covalent binding have been developed, such as physisorption of aromatic molecules [1], thiol functionalization of nanoparticles [2] or peptide functionalization [3,4]. In covalent binding the sensing surface is usually graphene oxide (GO) or graphene damaged with oxygen or ammonia plasma treatments and electrical performance is sacrificed for increased binding. Our approach is based on the functionalization of the graphene by engineered hydrophobin proteins, which self-assemble on hydrophobic surfaces to an ordered monomolecular layer with known orientation. Hydrophobins are protein amphiphiles having a hydrophobic patch in one end. Hydrophobins attach to hydrophobic substrates such as graphite or graphene [5]. Hydrophobins have been used to exfoliate thin graphene flakes from graphite and the binding of the protein on graphene was demonstrated with N-cysteine funtionalised hydrophobin (NCys-HFBI) layer on the graphene to which selective binding of mercaptosuccinic acid treated Au nanoparticles occurs [6]. Here we present a graphene FET biosensor with surface functionalization by tailored hydrophobic protein HFBI-ZE having a ZE-zipper amino acid chain and an analyte ZR-zipper amino acid (pI 11.7) which binds to the ZE (pI 4.1). The graphene sensor was fabricated on a highly doped (p-type) Si wafer with 300 nm SiO2 on top by transferring CVD grown graphene to the SiO2 surface. Graphene was then patterned using optical lithography and O2-plasma. Graphene contacts were fabricated using lift-off and evaporation of Ti and Au with a thickness of 5 nm and 50 nm respectively. Protective ALD Al2O3 was deposited on the chip and holes were etched on to the contact pads and graphene channel. Pt was deposited on liquid electrode pads using lift off and evaporation. The chip was wire bonded to a chip carrier that was attached to a circuit board having electrical connections and fluidistic cell support mechanism. The fluidistic cell was fabricated from PDMS (SYLGARD 184) by using a mold followed by attachment of flexible tubes for fluid transport. No adhesion promotion such as O2-plasma or corona discharge was used on the PDMS and the sensor chip because graphene is etched by the plasma. Instead, mechanical clamping was used to attach the PDMS fluidistic cell to the sensor. A computer controlled syringe pump was used to feed the protein and buffer solutions into the system. Figure 1A shows the schematic of the measurement setup. In the experiments the liquid gate potential (Vgate) was sweeped and the liquid potential (VL) was measured using high ohmic voltage preamplifier. Electrical characterization of the sensor was conducted in sodium phosphate buffer solutions having concentration of 0.1 M and a pH of 7. First the response of the clean sensor was measured in buffer. Next, a solution containing 100 Âľg/ml of HFBI-ZE protein was introduced to the system and was allowed to form a monolayer on the graphene. After flushing with buffer solution the response was recorded. Last, a solution containing 10 Âľg/ml of ZR-protein was introduced to the system to see the effects of binding. Figure 1B shows the measured graphene resistance plotted against the measured potential of the electrolyte VL. A clear shift in the resistance vs. VL curve is observed after the graphene surface has been covered with HFBI-ZE. After applying the ZR zipper protein analyte the curve shifts again producing a clear bio-response (see Fig. 1B). Shifting of the resistance curve due to different protein coatings can be explained by gating effect caused by the charges in the proteins. HFBI-ZE has a negative charge at pH 7 moving the resistance curve and the Dirac peak to the right in Fig. 1B. The binding of the protein can also cause shifts in the curves due to interactions at the graphene interface. ZE-protein, on the other hand, is positively charged at pH 7. Therefore, binding of ZE should move the resistance curve to the left on VL axis, which is precisely the case Fig. 1B.

To summarize, we have fabricated a graphene bio-sensor and a fluidic setup and measured the effect of graphene surface functionalization with HFBI-ZE protein and demonstrated the selective binding of the analyte ZR protein. Effect of the charged proteins and the binding of analyte molecule can be seen in the shift of the resistance curve and Dirac peak measured against the reference liquid electrode and can be explained by the gating induced due to charged molecules.

References [1] Ohno, Y., Maehashi, K.; Matsumoto, K., J. Am. Chem. Soc. 132 (2010) 18012−18013. [2] Mao, S., Lu, G., Yu, K., Bo, Z., Chen, J., Adv. Mater. 22 (2010) 3521–3526. [3] Cui, Y., Kim, S. N., Jones, S. E., Wissler, L. L., Naik, R. R., McAlpine, M. C., Nano Lett. 10 (2010) 4559−4565 [4] Mannoor, M. S., Tao, H., Clayton, J. D., Sengupta, A., Kaplan, D. L., Naik, R. R., Verma, N., Omenetto, F. G., McAlpine, M. C., Nat. Commun. 3 (2012) 763. [5] Linder, M. B., Szilvay, G. R.,Nakari-Setälä, T., Penttilä, M. E., FEMS Microbiology Reviews 29 (2005) 877–896. [6] Laaksonen, P., Kainlauri, M., Laaksonen, T., Shchepetov, A., Jiang, H., Ahopelto, J., and Linder, M. B., Angew. Chem. Int. Ed., 49, (2010), 1–5 [7] Kainlauri, M., Kurppa, K., Soikkeli, M., Arpiainen, S., Prunnila, M., Gunnarsson D., Linder M., Ahopelto J., To be published (2013).

Figures

Figure 1 A) GFET biosensor setup and B) measured resistance (R) as a function of liquid potential (VL). Clean graphene (blue) functionalized with HFBI-ZE protein before (red) and after (green) ZR zipper binding, which induces a 15 % shift in R. Measurement was done in buffer solution having a pH of 7. [7]

Fabrication of Graphene Double Quantum Dot Devices by He-ion Beam Milling Nima Kalhor1, Stuart A. Boden1, Shuojin Hang1, Zakaria Moktadir1 and Hiroshi Mizuta1,2 1

Electronics and Computer Science, University of Southampton, Highfield, Southampton, SO17 1BJ, UK School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), Ishikawa 923-1292, Japan nk1d09@ecs.soton.ac.uk Abstract Graphene, a single layer of carbon atoms arranged in a honeycomb pattern, is considered as a wonder material due to its remarkable properties. Small spin-orbit and hyperfine interactions have been theoretically predicted [1,2], which make graphene a promising option for Quantum Information Technologies (QIT) and spin qubit embodiment. 2

Currently, e-beam lithography (EBL) followed by reactive ion etching (RIE) is the most established method for fabricating graphene devices. However, proximity effects, e-beam spot size, undercutting of resists during etching and uneven distribution of resist layer (thickness-wise), which may be caused by the presence of graphite pieces, metallic alignment marks, and roughness of the SiO2 substrate on the sample surface, can limit the resolution of devices fabricated by this method. Helium-ion microscopy (HIM) is a new surface imaging technique that involves scanning a focused beam of helium ions across a sample surface and generating an image from the emitted secondary electrons (SE). An atomically sharp and extremely bright source, combined with the larger momentum (and so smaller de Broglie wavelength) of helium ions compared to electrons, enables a sub-nanometer probe size at the sample surface and high resolution imaging, below 0.35 nm in some systems [3-5]. In this work, we demonstrate a novel fabrication method by combining the EBL and HIM milling to fabricate high resolution GDQDs devices. Helium ion milling is then used to pattern the flakes with intricate DQDs devices, with sub 10 nm resolution and high fidelity. This hybrid fabrication approach could pave the way to a better understanding and more detailed study of graphene quantum devices. Figure 1 compares one of our GDQDs devices fabricated by standard e-beam lithography and RIE etch (Figure 1a) and a high resolution DQDs pattern milled on a monolayer flake by HIM milling (Figure 1b). Graphene flakes were produced by mechanical exfoliation and were transferred onto highly doped Si substrates with a 295 nm-thick SiO2 top layer. EBL, metallization and lift-off processes were employed to fabricate metal contacts onto the deposited flakes. To minimize e-beam induced defects in the graphene flakes, a ~ 460 nm-thick layer of Methyl Methacryllate (MMA) 8.5 resist was used. The soft nature of MMA resist allows the use of a low e-beam base dose (110 mC/cm2), at an acceleration voltage of 100 kV. Metallization of the samples was carried out by evaporation of Ti/Au (5 nm/55 nm), followed by lift-off. EBL and RIE was then employed to introduce isolation lines on the flakes, separating the metal contacts and leaving an area of approximately 500 nm Ă&#x2014; 500 nm for the final HIM milling step (Figure 1c). For the second lithography step (to define the isolation lines), a ~40 nm-thick layer of PMMA 495K resist was exposed by a constant e-beam dose of 190 mC/cm2. The exposed patterns were then transferred onto the flakes by RIE etch in Ar/O2 (4:1) gas flow with RF power of 35 W. HIM milling was carried out using a OrionPlusTM helium ion microscope (Carl Zeiss) operated with a beam current of 1 pA at an accelerating voltage of 30 kV. Initial dose tests on pristine graphene showed a He-dose of 0.63 nC/mm2 is adequate to mill desired patterns on a monolayer graphene. However, milling using the same dose was not successful when applied to e-beam processed samples. The milled patterns were not so clearly defined compared to those produced on pristine flakes and there was evidence of accumulation rather than removal of material in the scanned area. This was due to residues remaining on the surface following the exposure of graphene to resists and solvents during e-beam processing [6]. An annealing process was therefore developed whereby e-beam processed samples were annealed at ~320Â°C in 1.3 L/min forming gas flow (6% H2 and 94% N2) after 2 h in an atmospheric furnace. This step reduces surface contaminants sufficient to allow successful HIM milling of DQDs patterns (Figure 2a), using a dose of 0.63 nC/mm2. Results from room temperature measurements of the forward (ISD) and the reverse (IDS) source-drain current through a device with the same dimensions as the device shown in Figure 2a, as a function of the applied source-drain voltage (VSD) are presented in Figure 2b. For a bias voltage of 5 mV the current through the device was 130 pA with almost no current leakage from the channel and the DQDs. To confirm complete milling of the graphene exposed to the He beam and therefore successful isolation of all side gates, the device was measured for possible gate leakage between the side-gates and the channel. The currents measured between all side gates lie within the noise levels of the measurement equipment (in fA range, Figure 2c) and so can be considered to be negligible, indicating successful

isolation of all side gates. This proved that accurate alignment between the e-beam pattern and the HIM pattern was achieved and the developed milling conditions were adequate to completely mill the flake with the desired pattern with a high yield. This work demonstrates that HIM milling has the potential to enable fine-scale fabrication of nanoelectronic devices in graphene and could ultimately pave the way towards observation of Coulomb blockade at room temperature in GQDs devices. References [1] N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van Wees, Nature 448 (2007) 571574. [2] H. Min, E. J. Hill, N. A. Sinitsyn, B. R. Sahu, L. Kleinman, and A. H. MacDonald. PRB 74 (2006) 165310. [3] D. C. Bell, M. C. Lemme, L. A. Stern, J. R. Williams, and C. M. Marcus, Nanotechnology 20 (2009) 455301. [4] C. A. Sanford, L. Stern, L. Barris, L. Farkas, M. DiManna, R. Mello, D. J. Maas, P. F. A. Alkemade, J. Vac. Sci. Technol. B 27(6) (2009) 2660-2667. [5] D. C. Bell, M. C. Lemme, L. A. Stern, C. M. Marcus, J. Vac. Sci. Technol. B. 27(6) (2009) 27552758. [6] M. Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer, and E. D. Williams, Nano. Lett 7 (2007) 16431648. Figures (a)

(b)

(c)

Figure 1. HIM SE imaging: (a) A GDQDs device fabricated by standard e-beam lithography and RIE etch; (b) A high resolution DQDs pattern milled on a monolayer flake by HIM milling. The distances between the side-gates and the QDs are less than 8 nm; (c) The isolation lines fabricated by EBL and RIE on monolayer graphene flake. (a) (b) (c)

Figure 2. (a) A DQD device milled into a metal-contacted graphene flake using a helium ion dose of 0.63 nC/Âľm2, with the design of HIM milling pattern to define DQD device (upper right inset); (b) I-V measurement results for source-drain channel; (c) Gate leakage I-V measurement results.

Contribution (Oral/Poster/Keynote)

Clean and efficient solvothermal deoxidation of alkyl-functionalized graphene sub-oxide dispersions with carbon monoxide in organic solvents b)

Zhi-Li Chen , Fong-Yu Kam , Jie Song , Chen Hu , Loke-Yuen Wong , Geok-Kieng Lim

b,c)

and Lay-Lay

a,b)

Chua a

Department of Chemistry, National University of Singapore, Lower Kent Ridge Road, S117543, Singapore b Department of Physics, National University of Singapore, Lower Kent Ridge Road, S117542, Singapore c DSO National Laboratories, 20 Science Park Drive, Science Park I, S118230, Singapore a0030326@nus.edu.sg; chmcll@nus.edu.sg

We report an efficient solvothermal process to achieve deep deoxidation of octadecylamine functionalized sub-stoichiometric graphene oxides (sub-GOx)

[1]

in organic solvents. An initial average carbon oxidation

state of ca. 0.6 (i.e., 0.6 OH per basal-plane C) could be reduced to ca. 0.15, while retaining a critical density of the alkyl chains for solvent processability, ca. 0.02 chains per basal-plane C. The products can thus be characterized as single-sheet dispersions of alkyl-functionalized disordered “graphenes”. The oxidation state was characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and Raman spectroscopy. We found a strong solvent effect in the relative rates of alkylchain degrafting and/or chain scission versus deoxidation and regraphenization. The desired deoxidation and regraphenization is promoted at higher temperatures and by the use of aprotic amide solvents. Furthermore we found that carbon monoxide (CO) was a remarkably efficient fugitive deoxidizer. A key advantage of this solvothermal deoxidation process is the resultant dispersion of conductive graphenes can be directly used for printing, coating or formulating into nanocomposites, without further purification or −1

heat treatment. Thin films of the deoxidized sub-GOx give dc conductivities of up to 40 S cm , which is the highest, reported to date for solvent-processable graphene derivatives.

These films show

temperature independent conductivity that suggests its conductivity is largely limited by tunneling between the alkyl-chain spacers.

References [1] a) L.-L. Chua, S. Wang, P.-J. Chia, L. Chen, L.-H. Zhao, W. Chen, A. T.-S. Wee and P. K.-H. Ho, J. Chem. Phys. 2008, 129, 114702; b) S. Wang, P.-J. Chia, L.-L. Chua, L.-H. Zhao, R.-Q. Png, S. Sivaramakrishnan, M. Zhou, R. G.-S. Goh, R. H. Friend, A. T.-S. Wee and P. K.-H. Ho, Adv. Mater. 2008, 20, 3440-3446.

Contribution (Oral/Poster/Keynote)

Figures

C1s

N1s NMP 150ºC

Photoemission intensity

DMF 150ºC DMAc 150ºC TCB 150ºC DCB 150ºC solid-state 150ºC

Cα Cβ 282

284

Cγ C -alkyl-sub-GOx 18

286 288 290 Binding energy (eV)

396

292

398 400 402 Binding energy (eV)

404

Figure 1. X-ray photoelectron spectroscopy of the deoxidized sub-GOx obtained in various solvents. Left: C1s corelevel spectra. Right: N1s core-level spectra. A pristine film of octadecylamine functionalized sub-GOx before and after heat treatment at 150°C is shown for comparison.

(b) 0.7 0.6

0.6

Absorbance @532nm

0.7 Heat treatment @150°C 0.2mg/mL in TCB

0.5 0.4 0.3 0.2

0.2 mg mL−1 sat’d CO in TCB

0.5 0.4

12h

0.3

0.2 start state 0.1

4 6 8 Reduction time (h)

0.0

TCB normal bp

(a)

50 75 100 125 150 200 Heat-treatment temperature (°C)

Figure 2a. Effect of CO on deoxidation of sub-GOx in 1,2,4-trichlorobenzene, measured by absorbance of the dispersion at 532-nm wavelength. (a) Time dependence with and without CO bubbled through the solvent at 1 bar. (b) Temperature−time−transformation plot for the deoxidation of sub-GOx in the presence of CO, showing temperature-dependent limiting behavior. Without CO, the absorbance after 12 h is ca. 0.4 at 200°C, 0.3 at 150°C, −1 and 0.23 (unchanged) at room temperature. Cell path length, 2.0 mm. Dispersion concentration, 0.2 mg mL .

Contribution (Oral/Poster/Keynote)

(a)

(b)

7000 5000 3000

100µm

(d)

(e)

1000

10µm

(f)

4cm Figure 3. Excellent processability of alkyl-functionalized sub-GOx and deoxidized graphene materials. (a) −1 Uniform jetting through print head nozzles. Inset: Photograph of 0.3 mg mL C18-alkylamine functionalized subGOx in chlorobenzene used for printing. (b) Printed arrays on hexamethyldisilazane-treated 300-nm-thick SiO2/ Si wafers. Inset: Zoom-in image of a single printed droplet film, showing uniform interference color. (c) MicroRaman of a printed droplet film, showing uniform G band intensity. (d) Printed arrays on source−drain electrode patterns. Inset: Zoom-in image of single droplet film on an electrode pattern. (e) Printed lines on a commercial plastic foil, with increasing graphene density from left to right. (f) Spray-coated film on an A4-size commercial plastic foil, shown here over a light background with “NUS ONDL” wordings to show film uniformity.

Direct synthesis of a two-dimensional graphene grating on a dielectric substrate Tommi Kaplas, Martti Silvennoinen, Kimmo Päiväsaari and Yuri Svirko University of Eastern Finland, Department of Physics and Mathematics, P.O. Box-111, FI-80101 Joensuu, Finland tommi.kaplas@uef.fi Abstract Graphene has recently found a number of applications in photonic and optoelectronic componets including transparent electrodes, saturable absorbers, ultrafast transistors and optical modulators [1]. However, until now, incorporation of graphene into photonic and optoelectronic devices requires its transferring from metallic catalyst used for its synthesis to an insulator or semiconductor substrate. Moreover in order to achieve monolithic integration graphene should be deposited on the prescribed location on the Si/SiO2 wafer. Here we propose a technique for position-selective graphene growth directly on the pre-patterned dielectric substrate. In the experiment, we formed an array of holes with diameter of about 4 µm and the period of 10 µm on a quartz substrate by femtosecond laser ablation. This pre-patterned substrate was covered by a 200 nm thick copper film and was employed as a substrate in the chemical vapor deposition (CVD) process (see Ref. [2] for details). During the CVD process the Cu film liquidizes and forms a network prescribed by the morphology of the substrate and the temperature, while a few layer graphene will grow both on copper-vacuum and copper-silica interfaces [2,3]. Thus the graphene network will be “imprinted” on the silica substrate and can be revealed by removing copper by plasma and wet etching. The fabricated graphene network was characterized by scanning electron microscope and Raman spectroscopy. In the Fig. 1 the Cu receding is presented without (1a) and with (1b) the surface modification. Without patterning the Cu layer is receded randomly, but on top of the hole grating the copper recedes by well controlled manner and form an uniform network structure. A typical Raman spectrum, measured from the graphene network revealed beneath Cu, shows that the quality of the graphene is comparable to the graphene samples reported in Refs. [2] and [3]. In conclusion we demonstrate a transfer free technique in order to deposit graphene to a prescribed location on dielectric substrate without post patterning. This technique can be extended by modifying the substrate by other means (e.f. by e-beam lithography or nanoimprinting). It can be also employed for introducing graphene elements into optical gratings or planar waveguides.

References [1] F. Bonaccorso, Z. Sun, T. Hasan and A.C. Ferrari, Nat. Photon., 4 (2010) 611-622. [2] T. Kaplas, D. Sharma and Y. Svirko, Carbon, 50(4) (2012) 1503-1509. [3] C.Y. Su, A.Y. Lu, C.Y. Wu, Y.T. Li, K.K. Liu, W. Zhang, et al., Nano Lett., 11 (2011), 3612–3616.

Figures

Fig. 1. A quartz sample, coated with Cu thin film, after the CVD process a) without patterning and b) with ablation holes. c) When the Cu remains are removed by wet etching the graphene network is revealed.

Fig. 2. A typical Raman spectrum measured from graphene network and Lorentzian peak fitting.

Graphene halides: properties and applications 1

1,2

Frantisek Karlicky, Athanasios B. Bourlinos,

Michal Otyepka and Radek Zboril

Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, 17. listopadu 12, 771 46 Olomouc, Czech Republic 2 Institute of Materials Science, NCSR "Demokritos", g. Paraskevi Attikis, 15310 Athens, Greece frantisek.karlicky@upol.cz

Abstract Graphene, a single-layer carbon sheet, has been regarded as one of the most promising candidates for next generation electronic materials due to its extremely high mobility of electrical carrier. However, graphene lacks a band-gap around Fermi level, which is defining concept for semiconductor materials and essential for controlling the conductivity by electronic means [1]. One of the ways to bandgap opening, i.e. covalently modified graphene derivatives prepared by attachment of hydrogen, halogens or other atoms have been of broad interest for their potential applications (e.g. in electronic devices) in the last few years [2]. The relative simplicity of atomic adsorbates allows them to be also well described by theoretical calculations. Here we review recent progress on graphene halides field and focus on structural and vibrational properties [3-5], which are simultaneously fingerprints of considered materials, whereas electronic, optical, thermodynamical and mechanical properties [5-8] of graphene halides are important from application perspective. E.g. the zero band gap of graphene is opened by hydrogenation and halogenation and strongly depends on the chemical composition of mixed graphene halides. The stability of graphene halides decreases sharply with increasing size of the halogen atom [7,8]. Periodic hybrid DFT calculations on graphane and stoichiometrically halogenated graphene derivatives predict that fluorographene is most stable 2D insulator, whereas graphene iodide spontaneously decomposes in agreement with results obtained in our experimental laboratories [3,4]. In terms of band gap and stability, promising materials are suggested, e.g., for (opto)electronics applications, because band gaps of such derivatives are similar to those of conventional semiconductors, and they are expected to be stable under ambient conditions (Figure 1). References [1] Schwierz F., Nature Nanotech. 5 (2010) 487 [2] Georgakilas V., Otyepka M., Bourlinos AB., Chandra V., Kim N., Kemp KC., Hobza P., Zboril R., Kim KS., Chem. Rev. 112 (2012) 6156 [3] Bourlinos AB., Safarova K., Siskova K., Zboril R., Carbon 50 (2012) 1425 [4] Zboril R., Karlicky F., Bourlinos AB., Steriotis TA., Stubos AK., Georgakilas V., Safarova K., Jancik D., Trapalis C., Otyepka M., Small 6 (2010) 2885 [5] Nair RR., Ren WC., Jalil R., Riaz I., Kravets VG., Britnell L., Blake P., Schedin F., Mayorov AS., Yuan SJ., Katsnelson MI., Cheng HM., Strupinski W., Bulusheva LG., Okotrub AV., Grigorieva IV., Grigorenko AN., Novoselov KS., Geim AK., Small 6 (2010) 2877 [6] Bourlinos AB., Bakandritsos A., Liaros N., Couris S., Safarova K., Otyepka M., Zboril R., Chem. Phys. Lett. 543 (2012) 101 [7] Karlicky F., Zboril R., Otyepka M., J. Chem. Phys. 137 (2012) 034709 [8] Karlicky F., Otyepka M., submitted to J. Phys. Chem. C Figures

Figure 1: Schematic view of selected promising compounds whose stability is expected (carbon, fluorine, chlorine, and bromine atoms are shown as black, blue, green, and red spheres, respectively).

Liquid phase exfoliation of graphite in alcohols 1

J. Kastner , I. Gnatiuk , B. Unterauer , I. Bergmair , O. Lorret , G. Hesser , K. Hingerl , D. Holzinger , 1 M. Mühlberger 1

Functional Surfaces and Nanostructures, Profactor GmbH, Im Stadtgut A2, Steyr-Gleink, 4407, Austria 2 TIGER Coatings GmbH & Co KG, Negrellistrasse 36, Wels, 4600, Austria 3 Center of Surface- and Nanoanalytics, Johannes Kepler University, Altenbergerstrasse 69, 4040 Linz, Austria julia.kastner@profactor.at, iurii.gnatiuk@tiger-coatings.com

Abstract Graphene can be produced by methods like growth on metal substrates by Chemical Vapor Deposition (CVD) or annealing SiC substrates [1]. However, for industrial applications also Liquid Phase Exfoliation (LPE) is a prominent way for a high-yield production of graphene flakes [2]. In this process a dispersion of graphene in a solvent is obtained that can be used as ink for inkjet printing. The top electrode in organic photovoltaic (OPV) solar cells is usually silver deposited by thermal evaporation [3]. Our goal is to replace the silver grid by an inkjet printed graphene pattern, since graphene is transparent with 4 -1 conductivity up to ~10 S cm [4] and less expensive than silver. A common solvent for LPE of graphite is N-methyl-pyrrolidone (NMP) [5]. However, it is forbidden to use with most of the print-heads, obviously, due to its high dissolution power. Moreover the reproduction toxicity of NMP can reduce the working place safety. In this study we will compare exfoliation in different alcohols and glycols like isopropyl alcohol, 1-butanol, 1-pentanol, hexylene glycol, 1,2- and 1,3-propanediol. We tested the exfoliation of graphite by ultrasonication in pure solvents and mixtures. After ultracentrifugation the graphite dispersions are characterized by Raman, SEM, AFM and compared with already published results of LPE in other organic solvents [6]. Figure 1 shows a stable dispersion of few layer graphene in 1-pentanol. Furthermore surface tension and viscosity of the graphene dispersions are measured to be optimized for inkjet printing. The authors acknowledge the MEM4WIN project (FP7-NMP, No. 314578) for funding parts of this work.

References [1] F. Bonaccorso, A. Lombardo, T. Hasan, Z. Sun, L. Colombo, A.C. Ferrari, Materials Today, 15 (2012) 564-589. [2] Y. Hernandez, V. Nicolosi, M. Lotya, F.M. Blighe, Z. Sun, S. De, I.T. McGovern, B. Holland, M. Byrne, Y. Gun’ko, J. Boland, P. Niraj, G. Duesberg, S. Krishnamurti, R. Goodhue, J. Hutchinson, V. Scardaci, A.C. Ferrari, J.N. Coleman, Nature Nanotechnology, 3 (2008) 563-568. [3] B. Ma, C.H. Woo, Y. Miyamoto, J.M.J. Fréchet, Chem. Mater., 21 (2009) 1413-1417. [4] P. Blake, P.D. Brimicombe, R.R. Nair, T.J. Booth, D. Jiang, F. Schedin, L.A. Ponomarenko, S.V. Morozov, H.F. Gleeson, E.W. Hill, A.K. Geim, K.S. Novoselov, Nano Lett., 8 (2008) 1704-1708. [5] F. Torrisi, T. Hasan, W. Wu, Z. Sun, A. Lombardo, T.S. Kulmala, G.-W. Hsieh, S. Jung, F. Bonaccorso, P.J. Paul, D. Chu, A.C. Ferrari, ACS Nano, 6 (2012) 2992-3006. [6] M. Lotya, Y. Hernandez, P.J. King, R.J. Smith, V. Nicolosi, L.S. Karlsson, F.M. Blighe, S. De, Z. Wang, I.T. McGovern, G.S. Duesberg, J.N. Coleman, J. Am. Chem. Soc., 131 (2009) 3611-3620.

Figures

Figure 1 SEM image of graphene/graphite flakes on Si/SiO2 substrate. Inset: 10 ml-Vial with dispersed flakes in 1-pentanol.

Effects of Plasma Treatment on Interfacial Adhesion between CVD-grown Graphene and Polymeric Substrate Kwang-Seop Kim, Kyungmin Jo, Dae-Hoon Lee, Woo-Suk Kang, Jae-Hyun Kim, Hak-Joo Lee Nano-Convergence Mechanical Systems Research Division, Korea Institute of Machinery & Materials (KIMM), 156 Gajungbukno, Yuseong-gu, Daejeon, 305-343, KOREA kskim@kimm.re.kr Abstract Graphene, one atomic layer of carbon, has drawn much attention due to its exceptional electrical, 1 optical, thermal, and mechanical properties . Graphene based conductive film is regarded as the most promising application for commercialization in the field of transparent electrodes for touch panel, flexible 2-3 display, and solar cell applications . Large-area graphene can be synthesized by a chemical vapor deposition (CVD) process and the graphene is easily transferred onto various substrates, usually on a 4-5 polymeric film for a flexible conductive film . In most applications, graphene should be transferred on a target substrate and the adhesion between graphene and the underlying substrate is critical because 6 the adhesion can significantly affect the reliability and performance of the application devices . When the adhesion between graphene and the underlying substrate is weak, the graphene can be easily delaminated and fractured by external disturbance during production process and operation. In the case of graphene based large area conductive films, one of the bottlenecks in their commercialization is the reliability of graphene conductive films under high humidity and high temperature. In this study, we investigated the effects of plasma treatments on the adhesion between CVD-grown graphene and a polymeric substrate. Polyethylene terephthalate (PET) substrate with good optical properties was used as polymer substrate and Ar, O2, and NH3 plasma was treated on the PET surface. The plasma treatment on PET was carried out in vacuum under low power conditions. After each plasma treatment, the treated surface was analyzed using a contact angle meter, atomic force microscope (AFM), an X-ray photoelectron spectroscopy (XPS). The results show that the plasma treatment did not change the surface morphology of PET but the surface energy drastically increased by introducing reactive group on the surface. CVD-grown graphene was transferred on the plasma-treated 5 PET through a roll-to-roll dry transfer process using a thermal release tape . In order to examine the durability of the graphene electrode, friction test was carried out on the graphene on PET samples using a home-built microtribometer for microscale contact. The friction force was measured under constant normal load during sliding and the wear track was analyzed using an optical microscope and AFM. As a counterpart material in the test, a laser-quality fused silica plano-convex lens was used. In addition, adhesion test was performed to directly investigate the effect of the plasma treatment on the adhesion between the graphene and PET. For the adhesion test, the graphene was transferred on the fused silica lens and the adhesion force between the lens coated with graphene and the plasma treated PET samples were measured. The results show that the plasma treatment can remarkably tune the adhesion properties between graphene and PET substrate, which leads to increase the durability of graphene transferred PET film. The optimization of plasma treatment and transfer conditions could provide the high quality and durable graphene based flexible electrode in the market.

References [1] Geim, A. K., Graphene: status and prospects. Science 324 (2009), 1530-1534. [2] Zhu, Y., Murali, S., Cai, W., Li, X., Suk, J. W., Potts, J. R., Ruoff, R. S., Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 22 (2010), 3906â&#x20AC;&#x201C;3924. [3] Novoselov, K. S., Falâ&#x20AC;&#x2122;ko, V. I., Colombo, L., Gellert, P. R., Schwab, M. G. & Kim, K. A roadmap for graphene. Nature 490 (2012), 192-200.

[4] Li, X., Cai, W., An, J., Kim, S., Nah, J., Yang, D., Piner, R., Velamakanni, A., Jung, I., Tutuc, E. et al. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 324 (2009), 1312â&#x20AC;&#x201C;1314. [5] Bae, S., Kim, H., Lee, Y., Xu, X., Park, J - S., Zheng, Y., Balakrishnan, J., Lei, T., Kim, H. R., Song, Y. I. et al. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 5 (2010), 574â&#x20AC;&#x201C;578. [6] Yan, C., Kim, K. S., Lee, S. K., Bae, S. H., Hong, B. H., Kim, J. H., Lee, H. J., Ahn, J. H., Mechanical and environmental stability of polymer thin-film-coated graphene, ACS Nano. 6 (2012), 2096-2103.

Figures

Figure 1. Photograph of home-built microtribometer and schematic of friction test

Friction force, Ff [mN]

100

Bare PET Cu-Gr/PET Ni-Gr/PET

80 60 40 20 0 -20

Time [s] Figure 2. Friction force measured during the friction test

Photocurrent in tri-layer graphene photodevices 1

Minjung Kim , Ho Ang Yoon , Jung Cheol Kim , Sun Keun Choi , Sang Wook Lee , and Hyeonsik 1 Cheong 1

Department of physics, Sogang University, 121-742, Seoul, South of Korea Division of Quantum Phases and Devices, School of Physics, Konkuk University, 143-701, Seoul, South of Korea

hcheong@sogang.ac.kr Abstract Tri-layer graphene can have two stacking orders, ABA (Bernal), and ABC (Rhombohedral) staking, which have different energy band structures near the K point. Optical and electrical properties of the boundary between ABA and ABC stacking are interesting in studying the fundamental science of graphene. The Raman spectra of ABA- and ABC- stacked tri-layer graphene have been reported[1,2], but photocurrent in those tri-layer has not been studied. We measured photocurrent in ABA- and ABCstacked tri-layer graphene photodevices as a function of the back-gate bias and the incident polarization. The tri-layer graphene photodevices were fabricated by depositing Pd/Au electrodes on exfoliated trilayer graphene on SiO2-covered silicon substrates by using e-beam lithography. The ABA and ABC stackings in tri-layer graphene were confirmed by the 2D band of the Raman spectrum. Photocurrent images were taken by scanning a focused laser beam across the photodevice. Raman spectra and photocurrent images were taken simultaneously in order to identify the exact position of the photocurrent, so that we can match the photocurrent and the stacking order where the photocurrent is measured. References [1] C. H. Lui et al., Nano lett., 11 (2011) 164. [2] C. Cong et al., ACS Nano, 5 (2011) 8760. Figure

Low-temperature photocarrier dynamics in single-layer MoS2 flakes T. Korn, G. Plechinger, S. Heydrich, M. Hirmer, F.-X. Schrettenbrunner, D. Weiss, J. Eroms, and C. Schüller Institut für Experimentelle und Angewandte Physik, Universität Regensburg, D-93040 Regensburg, Germany Tobias.korn@physik.uni-regensburg.de

Abstract The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become an efficient emitter of photoluminescence [1,2] as it is thinned to a single layer, indicating a transition to a direct-gap semiconductor due to confinement effects. The material is a layered structure of weakly coupled, covalently bonded two-dimensional sheets, and it can be prepared, just as graphene, using chemical or mechanical exfoliation techniques. Here, we present temperature-dependent and time-resolved photoluminescence (PL) studies [3] of single-layer MoS2 flakes, some of which have been covered with thin dielectric layers (HfO 2 or Al2O3) using atomic layer deposition (ALD). The flakes are prepared from bulk MoS2 by transparent tape liftoff. For initial characterization, optical, atomic-force and Raman microscopy are used (Fig.1). We observe that the PL peak position of individual single-layer MoS2 flakes fluctuates by about 25 meV at room temperature. Mild annealing at 150°C in vacuum leads to a homogenization of the PL peak position. For as-prepared flakes, we clearly see two PL peaks at low temperatures, which we may assign to bound and free exciton transitions (Fig. 2a). Oxide-covered flakes only show the free exciton peak, indicating that bound excitons form due to adsorbates at the flake surface (Fig. 2b). [4]. Temperaturedependent PL measurements indicate that the oxide-covered flakes are strained due to different thermal expansion coefficients of MoS2 and the oxides. This observation also indicates that the flakes strongly adhere to the oxide layers. Strain strongly influences the band structure in single-layer dichalcogenides and may even cause a metal-insulator-transition [5]. In time-resolved PL measurements using highly nonresonant, pulsed excitation, we observe very fast photocarrier recombination on the few-ps timescale at low temperatures, with increasing photocarrier lifetimes at higher temperatures due to increased exciton-phonon scattering (Fig. 2c). Excitons which are scattered out of the light cone by phonons first have to relax their momentum before they are able to recombine radiatively. Financial support by the DFG via GRK 1570, SFB689, SPP1285, and KO3612/1-1 is gratefully acknowledged. References [1] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Phys. Rev. Lett. 105(2010), 136805 [2] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, and F. Wang, Nano Letters 10 (2010), 1271 [3] T. Korn, S. Heydrich, M. Hirmer, J. Schmutzler, and C. Schüller, Appl. Phys. Lett., 99(2011), 102109 [4] G. Plechinger, F.-X. Schrettenbrunner, J. Eroms, D. Weiss, C. Schüller, and T. Korn, Phys. Status Solidi Rapid Research Letters 6(2012), 126 [5] W. S. Yun, S. W. Han, S. C. Hong, I. G. Kim, and J. D. Lee, Phys. Rev. B 85(2012), 033305 Figures

Fig. 1: (a) Optical micrograph of a thin MoS2 flake. (b) AFM scan of area marked in (a) by orange 1 square. (c) False color plot of the intensity of the MoS2 E 2g Raman mode.

Fig. 2: (a) Temperature-dependent PL spectra of single-layer MoS2. (b) PL spectra of bare and oxidecovered single-layer MoS2 measured at low temperatures. (c) Temperature-dependent Time-resolved PL traces of single-layer MoS2.

Preparation and characterization of reduced graphene oxide deposited on Si/SiO 2 wafer by rod coating technique Rafał Koziński, Lech Dobrzański, Krzysztof Librant, Natarajan Satish, Andrzej Kozłowski, Zbigniew Wiliński, Krzysztof Góra, Ludwika Lipińska. Institute of Electronic Materials Technology, st. Wólczyńska 133, 01-919 Warsaw, Poland rafal.kozinski@itme.edu.pl Abstract Conducting and transparent large area graphene films can find many application such as touch screens, antennas, circuits, or as resistivity heaters. Here we present easy, fast and scalable method of producing graphene oxide films by rod coating method. Graphene oxide films were deposited on Si 2” wafer with thermally grown 200nm thick SiO 2 layer. The graphene oxide layers were reduced in the RTP oven in an Ar/H2 atmosphere at temperatures of 700, 800, 900 and 1000°C. The annealing time was 3 min. The reduction process was followed by lithography and isolation etching in oxygen plasma. Finally the pattern of electrical contacts was defined in subsequent photolithography step followed by Ti/Au metallization evaporation and lift-off. The TLM structures were measured across 2” wafer and the basic statistics out of 100 measured devices for each reduction process were calculated. All devices exhibited excellent linear I-V characteristics. In table I one can see evolution of layer resistivity with increase of reduction temperature. Tab. 1 Resistivity of reduced GO layers Temperature of reduction Sheet resistivity remarks kΩ/sqr 700 125 800 44.8 900 19 First symptoms of delamination 1000 46 Delamination and peeling The thickness of layer was measured with mechanical surface profiler. Its value does not depend on reduction temperature and equals 6-8 nm. The Raman spectra were measured for each reduction process and two strong peaks D and G were observed along with small 2D peak. The FWHM of the D modes is reduced as temperature increases while G band FWHM stays at the same level. This trend is continuous and exhibits no saturation. We attribute D band width evolution to the defect annealing (i.e. oxygen reduction). 2D band is single mode graphene like for 800°C reduction process and becomes wide - graphite like for higher temperatures. This trend we attribute to interaction of carbon layers in a stack which is preferred when reduction degree of GO increases. Further carrier transport test are planned to explore RGO feasibility for fabrication of cheap devices on liberally chosen substrates.

Modification of graphene properties with plasmonic nanostructures

1,2

Aleksandra Krajewska , Iwona Pasternak , Bartosz Bartosewicz , Bartlomiej J. Jankiewicz , 1,3 2 1 Tymoteusz Ciuk , Zygmunt Mierczyk and Wlodek Strupinski 1) Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland, 2) Institute of Optoelectronics, Military University of Technology, Gen. S. Kaliskiego 2, 00-908 Warsaw, Poland 3) Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland aleksandra.krajewska@itme.edu.pl Abstract Graphene, due to its unique mechanical, electric, magnetic and thermal properties, has a great potential for many applications including electronics and sensor technology. When it comes to its applications in optics and photonics, relatively inefficient interaction of graphene with light may become a limiting factor. The solution to this problem may be combination of graphene with plasmonic nanostructures such as noble metal nanoparticles, which could enhance the optical properties of graphene [1]. In this work we present results of the studies on influence of CVD graphene modification with plasmonic nanostructures on its physical properties. The CVD graphene was grown on copper foil and it was transferred onto a high-resistivity Si/SiO2 substrate with or without noble metal nanostructures deposited on its surface. The preparation of the silver and gold nanoparticles was carried out by reduction of silver and gold salts with NaBH4 [2] and sodium citrate [3], respectively. The hybrid structures with various configurations of graphene and plasmonic nanostructures were fabricated. The structure and morphology of modified graphene were characterized by using Raman spectroscopy and SEM imaging. Optical properties of modified graphene were investigated by using UV-Vis-NIR spectroscopy. The resistance of modified with noble metal nanostructures and unmodified graphene were measured by using a contactless method employing a single-post dielectric resonator operating at microwave frequencies [4]. References [1] A. N. Grigorenko, M. Polini, K. S. Novoselov, Nat. Photonics, 6 (2012) 749–758. [2] P.C. Lee, D. Meisel, J. Phys. Chem., 86 (1982) 3391–3395. [3] J Turkevich, Gold Bull, 18 (1985) 125–131. [4] J. Krupka, W. Strupinski, J. Nanosci. Nanotechnol., 11 (2011) 3358–3362.

Figures

Fig. 1. SEM image of Graphene-Ag particles (A) and Graphene-Au particles (B).

Functional multilayer films formed with reduced graphene oxide T. Kruk1, A. Pajor-Świerzy1, L. Szyk-Warszyńska1, R. Socha1, M. Kolasińska-Sojka1, R. Wendelbo2, P. Warszyński1 1

Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland 2 Abalonyx AS, Forskningsveien 1, 0314 Oslo, Norway nckruk@cyf-kr.edu.pl

Graphene - a new material consisting of single layer of sp2 – bonded carbon atoms with unique two-dimensional nanostructure attracts attention of many specialists from various branches of science. Since its discovery in 2004 [1], graphene has emerged as the “material of the future” due to its unique nanostructure and electrical, thermal and mechanical properties [2,3]. It is considered as a promising material for application in various technological fields such as transparent conductive films [4], solar cells [5], gas storage media [6] or next generation of electronic devices [7]. Sequential adsorption of charged nanoobjects, as polyelectrolytes, micelles, nanoparticles or proteins (known also as the “Layer by layer” (LbL) deposition method) [8], is considered as one of the most promising techniques of surface modification and formation of highly tailored functional thin films for the wide range of possible applications, including: selective membranes, biosensors and drug delivery systems. As graphene is a hydrophobic material, it cannot be directly used for construction of multilayer films with the LbL method. Therefore, in this work we have proposed to use suspension of graphene oxide (GO) for formation of such films and its subsequent reduction to the reduced graphene oxide (rGO). Since GO is negatively charged it can be easily suspended in water and used as an anionic layer for the film formation. For the formation of multilayer films we deposited sequentially, at various substrates (gold, polyimide), layers of polycation polyethylene imine (PEI) or poly(allylamine hydrochloride) (PAH) and GO. Then we compared several reduction methods to turn GO into reduced graphene oxide, e.g. thermal reduction, UV irradiation, chemical reduction by hydrazine and electrochemical reduction. We noticed that the thermal reduction of GO above the temperature 180oC is the most effective process leading to formation of sp2-bonded carbon atoms. The examination of XPS spectra indicated that after the reduction the ratio of the sp2 carbon increased to c.a. 80 at.%. The atomic ratios of carbon to oxygen for GO before and after reduction determined by XPS were corroborated by EDS analysis. The structure and properties of the films before and after reduction were investigated also using ellipsometry, ATRFTIR spectroscopy, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). By measuring surface conductivity with the four point method we showed that using the proposed method it is possible to obtain ultrathin conductive films on quartz and polyimide (PI) plates. Formation of such films on PI allows creating flexible electrodes, which can find applications in biomedicine as disposable, electroactive sensors. The same methodology was applied to obtain the electroactive films containing PAH/Prussian blue nanoparticles/rGO at the gold electrode surface. We demonstrated that such sensor layers can be used for the electrochemical detection of hydrogen peroxide. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science, 306 (2004) 666-669. [2] J.S. Bunch, A.v.d.Z. Arend, S.V. Scott, W.F. Ian, M.T. David, M.P. Jeevak, G.C. Harold, L. M. Paul, Science, 315 (2007) 490-493. [3] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nanotechnol., 3 (2008) 270-274. [4] S. Watcharotone, D.A. Dikin, S. Stankovich, R. Piner, I. Jung, G.H.B. Dommett, et al. Nano Lett. 7 (2007) 1888. [5] X. Wang, L.J. Zhi, N. Tsao, Z. Tomovic, J.L. Li, K. Mullen, Angew Chem Int Ed. 47 ( 2008) 2990. [6] C. Sealy, Nano Today 4(2009) 6. [7] C. Gomez-Navarro, R.T. Weitz, A.M. Bittner, M. Scolari, A. Mews, M. Burghard, et al. Nano Lett. 7 (2007) 3499. [8] Multilayer Thin Films Sequential Assembly of Nanocomposite Materials, G. Decher, J. Schlenoff, (Eds.), ISBN 978-3-527-31648-9 - Wiley-VCH 2011, Weinheim.

Water-borne polymer/reduced graphene oxide adhesives Gracia Patricia Leal Wilhelma, Alejandro José Arzac Peñaa, Radmila Tomovskaa,b a

POLYMAT, University of the Basque Country UPV/EHU, Joxe Mari Korta Center, Avda. Tolosa, 72, 20018 Donostia-San Sebastián, Spain b IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain Contact@E-mail: graciapatricia.leal@ehu.es

The main objective of the present work is synthesis of water borne adhesives reinforced with reduced graphene oxide (rGO) platelets. For preparation of these composites the technique of emulsion mixing of water borne polymer dispersions (latexes) with stable rGO aqueous dispersions was used. The water borne latexes composed of poly (butyl acrylate/methyl methacrylate) (pBA/MMA) with adhesive formulation and with 40% solid content were synthesized by semicontinuous seeded emulsion polymerization using 4,4 azobis cyanopentanoic acid or potassium persulphate as initiators and sodium lauryl sulphate or Dowfax as emulsifiers. After thorough characterization of polymer microstructure they were mixed with stable rGO aqueous dispersions, obtained by hydrazine reduction of GO in presence of polyvinyl pyrrolidone or polyacrylic acid. The composite films were prepared from hybrid latexes, by water evaporation at standard atmospheric conditions. The composite films formed have shown a lot of irregularities and presence of bubbles. The possible cause was found to be the presence of ammonium salts, formed during the reduction process. Thus, before mixing, the rGO dispersions were dialyzed in order to remove them. The hybrid latexes formed from dialyzed rGO dispersions have shown to be stable without any precipitation or phase separation. The content of rGO was varied and the mechanical properties and electrical conductivity of these composite adhesives were determined.

Tight-binding Molecular Dynamics Simulation Study on Defect Structures in Graphene: Analysis of TEM images Gun-Do Lee1, Euijoon Yoon1, Cai-Zhuang Wang2, and Kai-Ming Ho2, and Jamie Warner3 1

Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea 2 Ames Laboratory-USDOE and Department of Physics, Iowa State University, IA 50011, USA 3

Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK

gdlee@snu.ac.kr Abstract Transmission electron microscopy technique has been developed to study the structure of materials. Two dimensional materials, such as graphene, have been an effective subject to test the TEM technique. However, it sometimes faces the limitation in the detection of short-time dynamical processes such as defect creation and reconstruction. Simulation methods are useful to elucidate detailed atomic processes which could hard to be observed in experiments even with the state-of-the-art microscope techniques. While ab initio simulation method are quite accurate, it focused only on total energy and band structure calculations due to the high cost in performing molecular dynamics simulation. Classical molecular dynamics simulations are sometimes questionable in their accuracy because such simulations do not contain quantum mechanical interactions. In order to overcome the limitations in the above two types of simulation methods, tight-binding molecular dynamics (TBMD) simulation method is very useful in the respect of accuracy and computational cost. The environment dependent tight-binding carbon potential is employed and it gives accurate results as those obtained by ab initio calculation for various defects structures. The TBMD simulations reveal interesting mechanisms for the formation and reconstruction of vacancy [1, 2], dislocation [3 - 5], and grain-boundary in graphene, which are also confirmed by ab initio total energy calculations. In this talk, I also discuss how we can analyze the TEM images of graphene defect structures and find the unseen processes by the aid of TBMD simulation method.

References [1] G.-D. Lee, C. Z. Wang, E. Yoon, N.-M. Hwang, D.-Y. Kim, K. M. Ho, Phys Rev Lett 95, 205501 (2005) [2] Y. Kim, J. Ihm, E. Yoon, and G.-D. Lee Phys. Rev. B 84, 075445 (2011). [3] B. W. Jeong, J. Ihm, and G.-D. Lee, Phys. Rev. B 78, 165403 (2008).

[4] G.-D. Lee, E. Yoon, N.-M. Hwang, C. Z. Wang, K. M. Ho, Appl. Phys. Lett. 102, 021603 (2013) [5] J. H. Warner, E. R. Margine, M. Mukai, A. W. Robertson, F. Giustino, and A. I. Kirkland, Science 2012, 337, 209

Figures

Figure1: Snapshots from the TBMD simulation for development into dislocation from vacancy defects

Figure2: Snapshots from TBMD simulations for dimer evaporation from graphene grain boundary. Numbers show the trajectory of identical atoms.

Doniach Diagram of Disordered Graphene Hyunyong Lee, Stefan Kettemann Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea hyunyongrhee@postech.ac.kr Abstract (Arial 10) We derive the quantum phase diagram of disordered electron systems as function of the concentration of magnetic impurities n and the local exchange coupling J in disordered grphene. Using the Kernel Polynomial Method (KPM), we have calculated the distribution of RKKY interaction and Kondo temperature as well as their ratio, JRKKY/TK . We find a sharp cut-off in the wide distribution of the ratio which allows us to define a critical density of magnetic impurities nc below which Kondo screening wins at all sites of the system in disordered graphene above the critical exchange coupling Jc above which there is no more than one free magnetic moment in the whole sample. We find that Jc does not

depends on the disorder strength and the magnetic coupled phase is more stable against Kondo screening but is more easily destroyed by disorder, because of pseudo gap at Dirac point. References [1] A. WeiĂ&#x;e, G. Wellein, A. Alvermann, and H. Fehske, Rev. Mod. Phys. 78, 275 (2006) [2] H. Lee, J. Kim, E. R. Mucciolo, G. Bouzerar, and S. Kettemann, PRB 85, 075420 (2012) [3] H. Lee, E. R. Mucciolo, G. Bouzerar, and S. Kettemann, PRB 85, 205427 (2012) [4] H. Lee, and S. Kettemann, arXiv:1211.1734 (2013)

Figures

Simulation of Electronic Transport in Quasi-Amorphous Graphene. 1,2

3,4

Aurelien Lherbier , Stephan Roche , Oscar. A. Restrepo , Arnaud Delcorte , Yann-Michel Niquet , 1,2 and Jean-Christophe Charlier 1

Universite catholique de Louvain (UCL), Institute of Condensed Matter and Nanoscience (IMCN), NAPS, Chemin des etoiles 8, 1348 Louvain-la-Neuve, Belgium 2 European Theoretical Spectroscopy Facility (ETSF) 3 CIN2 (ICN-CSIC) and Universitat Autonoma de Barcelona (UAB), Catalan Institute of Nanotechnology, Campus UAB, 08193 Bellatera (Barcelona), Spain 4 Institucio Catalana de Recerca i Estudis Avancats (ICREA), 08070 Barcelona, Spain 5 Universite catholique de Louvain (UCL), Institute of Condensed Matter and Nanoscience (IMCN), BSMA, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium 6 CEA-UJF, INAC, SP2M/L_Sim, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France aurelien.lherbier@uclouvain.be Abstract Graphene, a one atom-thick layer of carbon arranged in a honeycomb lattice, has sparked out intense research activities from both experimental and theoretical sides since almost a decade now. The striking properties of graphene in various fields, such as mechanical, thermal, or electronic transport properties, are intrinsically related to its two-dimensional aspect and to its bipartite honeycomb lattice structure yielding both to the peculiar electronics of Dirac Fermions and pseudo-spin symmetry. From the electronic transport point of view, clean graphene samples exhibit particularly long coherence length and high electronic mobility both interesting for devices applications in nanoelecronics. Graphene provide simultaneously a genuine playground for fundamental researches such as exploration of Anderson (anti-)localization phenomena in two-dimensional systems. In this presentation, simulations of electronic transport in quasi-amorphous graphene structures will be exposed. Employing tight-binding models, and using a real-space order-N Kubo-Greenwood method [1-2], the transport properties of quasi-amorphous graphene structures are computed. The impact of a huge amount of various structural defects disrupting the ideal honeycomb lattice is thus investigated. Starting from a randomized graphene plane, molecular dynamics simulations are conducted to obtain highly defective graphene structures exhibiting both domains of amorphous graphene [3-5] and reconstructed pristine graphene areas (Fig.1). A careful analysis of the transport properties is performed through the Kubo-Greenwood formalism. Structural defects are found to induce strong resonant scattering states at different energies depending on their nature [6-8], inducing extremely short mean free paths and low mobilities. At low temperatures and in the coherent transport regime, large contributions of quantum interferences driving to localization phenomena are predicted. Actually, in regards to the results obtained, such quasi-amorphous graphene structures are predicted to behave as a strong two-dimensional Anderson insulator material [9], which could be experimentally confirmed by the magneto transport measurements at low temperatures for instance.

References [1] S. Roche, D. Mayou, Phys. Rev. Lett. 79 (1997) 2518 [2] A. Lherbier, X. Blase, Y.M. Niquet, F. Triozon, S. Roche, Phys. Rev. Lett 101 (2008) 036808 [3] J. Kotakoski, A.V. Krasheninnikov, U. Kaiser, J.C. Meyer, Phys. Rev. Lett 106 (2011) 105505 [4] V. Kapko, D.A. Drabold, M.F. Thorpe, Phys. Status Solidi B 247 (2010) 1197 [5] E. Holmstrรถm, J. Fransson, O. Eriksson, R. Lizรกrraga, B. Sanyal, S. Bhandary, M.I. Katsnelson, Phys. Rev. B 84 (2011) 205414 [6] T.O. Wehling, S. Yuan, A.I. Lichtenstein, A.K. Geim, M.I. Katsnelson, Phys. Rev. Lett. 105 (2010) 056802 [7] Y.V. Skrypnyk, V.M. Loktev, Phys. Rev. B 82 (2010) 085436 [8] A. Lherbier, S.M.-M. Dubois, X. Declerck, S. Roche, Y.M. Niquet, J.-C. Charlier, Phys. Rev. Lett. 106 (2011) 046803 [9] A. Lherbier, S. Roche, O.A. Restrepo, Y.M. Niquet, A. Delcorte, J.-C. Charlier, submitted for publication (2013).

Figures

Figure 1: Highly defective graphene (HDG) structure: (a) Randomized graphene sample, (b) Structurally optimized model of HDG, (c) short list of structural defects observed.

Direct Growth of Large Area Graphene on Si/SiO2 substrate from Sputtered Carbon/Nickel Films 1

1,2

Genhua Pan , Mark Heath , Bing Li , David Horsell , M. Lesley Wears , Shakil Awan Laith Al-Taan 1

Wolfson Nanomaterials & Devices Laboratory, Faculty of Science and Technology, University of Plymouth, Devon, PL4 8AA, UK

College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, UK G.Pan@plymouth.ac.uk

Abstract Graphene is a highly promising material for numerous potential applications, ranging from electronics and photonics to sensors and composites [1-2]. To exploit these applications at an industrial level requires large scale growth of high quality graphene on device compatible substrates. To date, this has mainly been achieved via chemical vapour deposition (CVD) [3] and single crystal SiC epitaxial [4] growth routes. CVD graphene has been synthesised on various metal substrates such as nickel [5, 6] and copper [3], but the need to transfer it to different substrates for device fabrication has so far hindered its up-scaling to roll-to-roll production methods. Epitaxially grown graphene has been demonstrated to be a viable route for the production of electronic devices, such as field effect transistors[7]; however, SiC wafers are expensive and, unless SiC is required in the device, again graphene needs to be transferred. Graphene has also been synthesised via the rapid thermal processing (RTP) route with Ni Layer atop of single crystal SiC substrate at a temperatures ranging o o from 1100 C [8] down to 750 C [9]. Large area single or few layer graphene were also grown via the RTP route using amorphous SiC or carbon films and metal layers deposited by evaporation [10] or sputtering [11]. However, in all those growth techniques, transfer of graphene from metal substrate to insulator is required for device applications. Here we show the direct growth of large area graphene on Si/SiO2 substrates from sputtered amorphous carbon or SiC films either atop or underneath a Ni layer using in-situ or ex-situ RTP in the o temperature range from 650 to 1000 C. We have found that for samples with thick Ni layers (>100nm), graphene grew on the top surface of the stack, in close contact with the Ni or Ni-silicide and suspended in the liquid solution after the etching of the Ni-Silicide in HCl. However, when thinner Ni and carbon layers were used, the graphene remained on the original substrate (Si/SiO2) after the Ni-Silicide is etched away. Figure 1 is a summary of the typical properties of the graphene. Raman spectra typical of high quality exfoliated monolayer graphene [12] were obtained for samples under optimised conditions. A fast cooling rate was found to be essential to the formation of monolayer graphene. Samples with Ni atop SiC or carbon produced the best monolayer graphene spectra with ~40% surface area coverage as shown by Raman mapping, whereas samples with Ni below SiC or carbon produced poorer quality graphene but with 99% coverage. The results present a potential route for the production of large area graphene directly on Si/SiO2 insulating wafers, which could be a critical step forward for the integration of graphene into modern semiconductor device process flows. Growth mechanism and detailed characterisation of graphene by Raman, optical, AFM, and electrical transport measurement will be presented at the conference. References [1] Geim A. K., “Graphene: Status and Prospects”, Science, 324 (2009),1530-1534. [2] Geim A. K, Novoselov K. S., “The rise of graphene”, Nat Mater., 6 (2007), 183-191. [3] Li XS, Cai WW, An JH, Kim S, Nah J, Yang DX, et al. “Large-Area Synthesis of HighQuality and Uniform Graphene Films on Copper Foils”, Science, 324 (2009),1312-1314. [4] Berger C, Song ZM, Li TB, Li XB, Ogbazghi AY, Feng R, et al. "Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics”, J Phys Chem B. 108 (2004),19912-19916. [5] Yu Q, Lian J, Siriponglert S, Li H, Chen YP, Pei S-S., “Graphene segregated on Ni surfaces and transferred to insulators”, Appl. Phys Lett., 93 (2008),113103-3.

[6] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al., “Large-scale pattern growth of graphene films for stretchable transparent electrodes”, Nature 457 (2009), 706-710. [7] Moon JS, Curtis D, Hu M, Wong D, McGuire C, Campbell PM, et al., “Epitaxial-Graphene RF FieldEffect Transistors on Si-Face 6H-SiC Substrates”, IEEE Electron Device Letters, 30 (2009), 650-652. [8] Hofrichter J, Szafranek BuN, Otto M, Echtermeyer TJ, Baus M, Majerus A, et al., “Synthesis of Graphene on Silicon Dioxide by a Solid Carbon Source”, Nano Lett. 10 (2009), 36-42. [9] Juang Z-Y, Wu C-Y, Lo C-W, Chen W-Y, Huang C-F, Hwang J-C, et al., “Synthesis of graphene on silicon carbide substrates at low temperature”, Carbon, 47 (2009), 2026-2031. [10] Zheng M, Takei K, Hsia B, Fang H, Zhang X, et al, “Metal-catalyzed crystallization of amorphous carbon to graphene”, APPLIED PHYSICS LETTERS 96 (2010), 063110. [11] Orofeo CM., Ago H, Hu B, and Tsuji M, “Synthesis of Large Area, Homogeneous, Single Layer Graphene Films by Annealing Amorphous Carbon on Co and Ni”, Nano Res 4 (2011), 531–540. [12] Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, et al. Raman Spectrum of Graphene and Graphene Layers. Physical Review Letters. 97 (2006):187401.

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Figure 1. Summary of properties of graphene. (a) Raman spectra of samples processed with different RTP process conditions. (b) Raman mapping of FWHM of the 2D band for a sample area of 750μmx500μm. (c) AFM image of graphene on Si/SiO2 substrate after etching of Ni-Silicide in HCl, inset is a Raman spectrum of the sample. (d) and (e), Camera shots showing distinctive areas with and without graphene after RTP. (f) Electrical characteristics of a back-gated graphene field effect transistor, inset is an optical microscope picture of the device.

Systematic Study of Photo-thermal Chemical Vapor Deposition of Graphene on Copper Changfeng Li, Wonjae Kim, Juha Riikonen, and Harri Lipsanen Aalto University, Department of Micro- and Nanosciences, Micronova, P.O.Box 3500, FI-00076 AALTO, Espoo, Finland changfeng.li@aalto.fi Chemical vapor deposition (CVD) is considered as one of the most promising methods for the synthesis of high-quality single-layer graphene [1]. An alternative for typically used tubular furnaces with resistive heating is photo-thermal chemical vapor deposition (PTCVD) which is based on rapid thermal process (RTP). The PTCVD system used in this work includes a cold-wall reactor, pyrometer enabling real-time temperature measurement, and halogen lamps as a heat source. Compared to traditional tubular furnaces, the required growth time for single-layer graphene is significantly reduced in PTCVD due to very high growth rate. Typical growth time of graphene films utilizing tubular furnaces is around 20 - 30 minutes [2]. In contrast, uniform graphene films can be synthesized on copper using PTCVD in 15 - 60 seconds depending on growth parameters. Various PTCVD process parameters were adjusted to assess their effects on the quality of graphene. The growth temperature plays a key role in graphene synthesis due to the hydrocarbon dissociation on the surface of the copper catalyst. Graphene films fabricated at different temperatures were examined by scanning electron microscopy (SEM) and confocal µ-Raman microscopy. The gas ratio (CH4: H2) has also significant influence on the quality of graphene. For example, the areal density of flakes (adlayer domains) can be decreased using a lower gas ratio. PTCVD was found to produce high-quality single-layer graphene at 935 - 950 °C using a growth time of 60 s with a gas ratio of 4 in low pressure (~10 mbar). The influence of the cooling rate from growth temperature was also studied by varying the rate between 3 °C/s and 15 °C/s. The results indicated that the areal density of flakes on graphene can be reduced by increasing the cooling rate. However, the size of the flakes seemed to be invariable although new flakes nucleated during a long cooling stage, resulting even in flake merging. Typical Raman histograms show intensity ratio of the D and G bands (ID/IG) around 0.2, which corresponds to very low defect density [3, 4]. The quality of the film was further improved with higher cooling rate as defect density (ID/IG ratio) was decreased and I2D/IG ratio increased. References [1] Xuesong Li, Weiwei Cai, Jinho An, Seyoung Kim, Junghyo Nah, Dongxing Yang, Richard Piner, Aruna Velamakanni, Inhwa Jung, Emanuel Tutuc, Sanjay K. Banerjee, Luigi Colombo, Rodney S. Ruoff, Science, 5932 (2009) 1312. [2] Dale A. C. Brownson and Craig E. Banks, Physical Chemistry Chemical Physics, 23 (2012), 8264. [3] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Physics Report, 5-6 (2009) 51. [4] L. G. Cancado, A. Jorio, E. H. Martins Ferreira, F. Stavale, C. A. Achete, R. B. Capaz, M. V. O. Moutinho, A. Lombardo, T. S. Kulmala, and A. C. Ferrari, Nano Letters, 8 (2011), 3190.

Electron Diffusion in Electrodynamic Response of Graphene Hua-Min Li, Euyheon Hwang, and Won Jong Yoo Samsung-SKKU Graphene Center (SSGC), SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 440-746, Korea euyheon@skku.edu, yoowj@skku.edu Abstract Carrier transport in graphene always occurs in electrostatic equilibrium due to the low density of states (DOS) near the charge neutral point (Dirac point), and the carrier response is assumed to respond instantaneously to relatively slow environmental fluctuations [1-4]. However, in the presence of environmental fluctuations that introduce changes on the timescale of the electrodynamic response time, graphene cannot be assumed to respond instantaneously. A lag in the graphene response can degrade the efficiency of carrier transport relative to electrostatic equilibrium conditions. In this work, we investigate the carrier diffusion induced by a non-uniform carrier concentration and measure the diffusion time in graphene. For the first time, the electron diffusion times were measured from the electrodynamic response of a single-layer graphene field-effect transistor (SLG-FET). The application of a negative gate voltage (VG) electrodynamically introduced a local excess of electron density into the graphene channel, and diffusion from the high-concentration region to the low-concentration region was characterized by measuring the time evolution of the drain current (ID). In a dark environment, the electron diffusion times in graphene were inversely proportional to the temperature. Electron diffusion required a relatively long diffusion time (4 s) to achieve electrostatic equilibrium at low temperatures (100 K); however, the extra energy provided by the photons makes carriers diffuse fast under illuminated conditions, indicating the presence of a strong photoresponse in graphene. A back-gate SLG-FET was fabricated, in which the SLG was synthesized via metal-catalyzed chemical vapor deposition (CVD) and its Dirac point was located at the VG of 160 V. An electrodynamic theory of electron diffusion was proposed. For a VG of zero, the Fermi level of graphene in the channel is equilibrated with the source and drain. The electrodynamic response of graphene, however, is characterized by an instantaneous downward shift in the Fermi level with the Dirac point energy, upon application of a positive VG, thereby producing a temporary shortage of electrons in the channel. The drift of electrons moving from the source to the drain rapidly compensates for this shortage of electrons, and the Fermi level shifts back to the electrostatic equilibrium. For comparison purposes, the Fermi level instantaneously shifted to higher values upon formation of a temporary excess of electrons under the application of a negative VG. The extra electrons diffused toward the source and drain to restore the Fermi level to electrostatic equilibrium. The carrier diffusion induced by the excess electrons under a negative VG showed a strong photoresponse under illumination conditions. This response contrasted with the absence of a photoresponse under electron shortage conditions produced under a positive VG. The absence of carrier diffusion under laser illumination substituted a slow transition, with a transition time of 0.25 s under dark conditions, with a fast transition, with a transition time of 0.04 s under illumination conditions. All the electrons in the channel then become “hot” under laser illumination (655 nm wavelength) due to the introduction of additional kinetic energy (1.89 eV) provided by the photons. In this case, the distribution of electrons in the channel corresponded to a non-equilibrium state where no electrodynamic disturbances were present in the Fermi level and the electron diffusion in the channel ceased. The carrier diffusion time, which was defined as the time required to equilibrate the Fermi level after disruption by the application of VG, was obtained from the time evolution of ID. The diffusion time under a negative VG increased significantly to 4 s at 100 K from less than 1 s at room temperature. As a comparison, the time evolution of the channel current under a positive VG did not vary significantly with temperature. The time evolution of the normalized ID at VG = –40 V was obtained for various temperatures. The diffusion time as a function of temperature was calculated, which was inversely proportional to the temperature, in agreement with the diffusion theory. In summary, for the first time, electron diffusion in graphene was measured based on the electrodynamic response of an SLG-FET. We measured the electron diffusion time from the time evolution of the channel current. The photoresponse and temperature dependence of the channel current were also measured. Electron diffusion with a relatively long diffusion time reduced the channel current under dark conditions. In contrast, the electron diffusion was absent under photo illumination

conditions, resulting in a strong photoresponse in graphene. The diffusion time was inversely proportional to the temperature, in agreement with the theory. Because the electron diffusion properties in graphene could be easily controlled through the temperature and light illumination conditions, these findings may be applicable to the development of future electronics and photo-electronic devices based on graphene. The effects are particularly useful in high-frequency device applications.

References [1] [2] [3] [4]

K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature, 438 (2005) 197. Y. Zhang, Y.-W. Tan, H. L. Stormer, and P. Kim, Nature, 438 (2005) 201. A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, Rev. Mod. Phys., 81 (2009) 109. S. Das Sarma, S. Adam, E. H. Hwang, and E. Rossi, Rev. Mod. Phys., 83 (2011) 407.

Mechanical cleaning of graphene using an atomic force microscope and chemical cleaning by Cl-based solvents Niclas Lindvall, Alexey Kalabukhov, and August Yurgens Chalmers University of Technology, SE-41296, Gothenburg, Sweden niclas.lindvall@chalmers.se Being essentially two surfaces and no bulk, graphene is easily affected by its environment. To achieve clean graphene devices with controlled doping is in many cases essential in order to bring graphene from the lab to real devices. Surface adsorbents play an important role on the properties of a graphene device. Polymers used in microfabrication are one of the dominant sources of such adsorbents. Traditional solvents are not sufficient to completely remove those contaminants. In order to efficiently remove them, we use two different approaches. First, we use the tip of an atomic force microscope (AFM) to mechanically clean graphene devices and obtain atomically smooth graphene. By appropriate choice of the AFM cantilever and contact force, a root mean square roughness of around 0.1 nm, less than that of our pristine SiO2, can be achieved. In addition, we observe a shift of the Dirac voltage towards zero and improved carrier mobility. This method is convenient for single devices of limited size. For cleaning large area graphene cleaning we investigate a second approach, the use of Cl-based solvents, such as chloroform and dichloroethane. The result is highly dependent on the polymer used and the baking conditions. We compare the results of mechanical- and chemical cleaning approaches. References [1] N. Lindvall, A. Kalabukhov, and A. Yurgens, J Appl Phys, 111 (2012) 064904-064904. [2] N. Lindvall, J. Sun, G. Abdul, and A. Yurgens, Micro Nano Lett, 7 (2012) 749-752. [3] J. Svensson, N. Lindahl, H. Yun, M. Seo, D. Midtvedt, Y. Tarakanov, N. Lindvall, O. Nerushev, J. Kinaret, S. Lee, and E. E. B. Campbell, Nano Lett, 11 (2011) 3569-3575. [4] N. Lindahl, D. Midtvedt, J. Svensson, O. A. Nerushev, N. Lindvall, A. Isacsson, and E. E. B. Campbell, Nano Lett, 12 (2012) 3526-3531.

Graphene based heterojunction solar cells Laura Lancellotti(1), Nicola Lisi(2), Eugenia Bobeico(1), Marco Della Noce(1), Paola Delli Veneri(1), Salvatore Del Sorbo(1), Theodoros Dikonimos(2), Girolamo Di Francia(1), Rossella Giorgi(2), Alberto Mittiga(2), Tiziana Polichetti(1), Filiberto Ricciardella(1) (1)

ENEA, Res Ctr Portici, P.le E. Fermi 1, 80055 Portici (NA), Italy (2) ENEA, Res Ctr Casaccia, I-00123 Rome, Italy nicola.lisi@enea.it

Thanks to properties like excellent optical transmittance, low resistance and high mechanical and chemical stabilities [1], graphene has a great potentiality in the field of solar energy [2]. Tongay et al. [3] were the first to anticipate applications where single layer graphene is directly contacted to a semiconductor substrate; later, the formation of graphene/semiconductor Schottky barriers was experimentally verified, opening the interesting scenery of graphene based solar cells [4, 5]. Application of graphene in this field is a very open question and requires investigation on different aspects of the material and the devices. In this frame, we have fabricated and characterized devices based on graphene/silicon heterojunction, with simple planar thin-film geometry (Fig. 1), in order to inspect the properties of the material and its interaction with the semiconductor. The graphene sheets used in this work have been grown by chemical vapor deposition (CVD) on copper foils [6]. Raman spectroscopy performed on our samples (Fig.2) has shown a few layers graphene (FLG) structure. The rectifying properties of the devices have been verified through dark current-voltage (I-V) measurements (Fig.3). An estimation of the Schottky barrier height (SBH) has been given using the thermionic-emission based diode equation and extrapolating, to zero bias, the dark saturation current density. The obtained value of SBH is 0.69 eV consistent with a FLG [7], in agreement with Raman results. As shown in Fig. 3, we have also used capacitance-voltage (C-V) measurements plotted in the form 1/C2 vs. V, where V is the reverse bias voltage, to characterize our junctions at room temperature. Linear extrapolation (dotted line) to the intercept with the abscissa identifies the built-in potential, V bi, which is related to SBH. The SBH values extracted from the C−V measurements is 0.75eV. We note that this value is slightly higher than the value extracted from I-V analysis. This trend might be attributed to the fact that I−V and C−V provide complementary techniques for determining the SBH. While I−V measurements manifest current transport processes across the graphene/Si and give informations about the lowest SBH, capacitance measurements probe the space charge region of the Schottky junction and provide an average SBH at the interface. External quantum efficiencies (EQE) have also been measured (Fig.4) to evaluate the charge separation and collection, as a result of the Schottky barrier formed at the interface graphene/silicon. Our devices show an EQE values over 50% for wavelengths in the range 400 nm<λ<950 nm with 60% peaks, indicating significant electron-hole pair generation and the subsequent charge collection by the corresponding electrodes. These values, are in line with the best results presented in literature[8]. Short circuit current (Jsc) extracted from EQE reaches values of 23mA/cm 2. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666. [2] Y.H. Hu, H. Wang, B. Hu, ChemSusChem 3 (2010) 782. [3] S. Tongay, T. Schumann, A.F. Hebard, Appl. Phys. Lett. 95 (2009) 222103. [4] B.X. Li, H. Zhu, K. Wang, A. Cao, J. Wei, C. Li, Y. Jia, Z. Li, X. Li, D. Wu, Adv. Mat. 22 (2010) 2743. [5] C.C. Chen, M. Aykol, C.C. Chang, A.F.J. Levi, S.B. Cronin, Nano Lett. 11 (2011) 1863. [6] R. Giorgi, Th. Dikonimos, M. Falconieri, S. Gagliardi, L. Giorgi, N. Lisi, P. Morales, L. Pilloni, and E. Salernitano Proceedings of GraphITA 2011, L.Ottaviano-V.Morandi Eds., Carbon nanostructures, Springer-Verlag (2012) 109 [7] L. Lancellotti, T. Polichetti, F. Ricciardella, O. Tari, S. Gnanapragasam, S. Daliento, G. Di Francia, Thin Solid Films 522 (2012) 390. [8] X. Miao, S. Tongay, M. Petterson, K. Berke, A. Rinzler, B. Appleton, A. Hebard, Nano Lett. 12 (2012) 2745.

Figures

(a) (b) Fig.1: Schematic illustration of graphene/Si heterojunction (a) and photograph of the frontal part of the device (b): the yellow window is the gold contact, the shadow visible on gold is graphene film, the central grey zone is the active area of the device where graphene contacts silicon and the dark zone is SiO2/Si. 8000 7000

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Enhanced Magnetic Anisotropy in Low Dimensional Metal-Organic Layers Deposited on Graphene/Ir(111) Simone Lisi, Pierluigi Gargiani, Mattia Scardamaglia, Carlo Mariani and Maria Grazia Betti University of Rome “La Sapienza” , Piazzale Aldo Moro 2, Rome, Italy Simone.lisi@roma1.infn.it Abstract Graphene (Gr) interaction with metal surfaces and with magnetic atoms is of great technological importance not only because of widely demonstrated large-scale productions of high-quality Gr on metal substrates, but also for the preparation of spin-polarized contacts in Gr-based devices and spintronic units. A fascinating perspective is to study arrays of size-selected magnetic molecular systems comprising a single magnetic atom embedded in an organic frame in interaction with the Gr-metal support. Self-assembling of nanosized magnetic architectures is attractive for basic investigations as well as for device applications, when the magnetic anisotropy can be controlled. Gr grown on transition metal (TM) surfaces presents different degrees of interaction and a long-scale ordered moiré reconstruction at the nanoscale [1]. The Ir(111) surface allows to have a supported Gr layer and a low interaction, leaving almost unchanged its intrinsic electronic properties, while granting samples of high quality and stability with a small moiré corrugation [1]. Thus, Gr/Ir can be used as an efficient buffer layer, allowing ordered molecular self-assembly [2-3] and electronic decoupling between the adsorbate and the underlying Ir metal [4]. TM magnetic atoms deposited on Gr generally assemble in clusters [5, 6], while when they are embedded into an organic cage can be assembled as a regular array of magnetic centers at the nanoscale [2-3]. The metal-phthalocyanines (MPcs, M-C 32H16N8) are organo-metallic molecules with a metal atom embedded in the center of an organic environment, in a nearly D4h symmetry (Fig.1). The organic environment allows self-assembling on metal-supported Gr, with the formation of planar ordered layers [2-3, 7]. The central TM atom in the free molecule presents a bonding with transfer of two electrons to the surrounding N atoms, with incomplete non-degenerate d-shell, due to the symmetry lowering imposed by the D4h ligand field (Fig.1).The Hund's rule is broken for this configuration and the spin and orbital moments can be heavily quenched. However, this is not the case for FePc, where the spin and orbital magnetic moments are still present [8]. We present an experimental study of the spin and orbital configuration of a single-layer (SL) of FePc grown on Gr/Ir(111), by X-ray magnetic circular dichroism (XMCD). The sample is kept in a high magnetic field (5T) at liquid helium temperature. Near-edge absorption fine-structure (NEXAFS) measurements are taken at the Fe-L2,3 edges, by means of circularly polarized X-ray radiation with helicity parallel or antiparallel to the magnetic field. The resulting spectra can be related, via suitable sum rules, to the effective spin and orbital moments of the initial state of the system [9]. We explored the dependence on the X-ray incidence angle to shed light on the spatial localization of the magnetic moments of the FePc SL. The FePc single layer is long range ordered on the Gr sheet and the interaction with the Gr slightly influences the electronic molecular states [6]. The Gr results a buffer layer and the FePc are uncoupled with the underlying Ir surface [6]. The orbital and effective spin magnetic moments of the central Fe ion are unquenched (Fig. 2). Furthermore, the effective spin and orbital spatial anisotropy results enhanced with respect to those of a FePc thick film. Due to the relation between orbital moment and magnetic anisotropy and the long-range order, this configuration allows to the formation of a spin network with a preferential magnetic axis parallel to the molecular plane. References [1] A.B. Preobrajenski, May Ling Ng, A.S. Vinogradov, N. Mårtensson, Physical Review B, 78 (2008) 073401. [2] J. Mao, H. Zhang, Y. Jiang, Y. Pan, M. Gao, W. Xiao,H.-J. Gao, J. Am. Chem. Soc., 131 (2009)14136 [3] S.K. Hämäläinen, M. Stepanova, R. Drost, P. Liljeroth, J. Lahtinen, J. Sainio, J. Phys. Chem. C, 116 (2012) 20433. [4] Mattia Scardamaglia, Simone Lisi, Silvano Lizzit, Alessandro Baraldi, Rossana Larciprete, Carlo Mariani, Maria Grazia Betti, J. Phys. Chem. C, in press, DOI:10.1021/jp308861b [5] C. Vo-Van, S. Schumacher, J. Coraux, V. Sessi, O. Fruchart, N.B. Brookes, P. Ohresser, T. Michely, Appl. Phys. Lett., 99 (2011) 142504. [6] A. Cavallin, M. Pozzo, C. Africh, A. Baraldi, E. Vesselli, C. Dri, G. Comelli, R. Larciprete, P. Lacovig, S. Lizzit, et al., ACS Nano, 6 (2012) 3034.

[7] M. Scardamaglia, G. Forte, S. Lizzit, A. Baraldi, P. Lacovig, R. Larciprete, C. Mariani, M.G. Betti, J. Nanopart. Res., 13 (2011) 6013. [8] J. Bartolomé, F. Bartolomé, L.M. García, G. Filoti, T. Gredig, C.N. Colesniuc, I.K. Schuller, and J.C. Cezar, Phys. Rev. B, 81 (2010) 195405. [9] B.T. Thole, Paolo Carra, F. Sette, G. van der Laan, Phys. Rev. Lett., 68 (1992) 1943. Figures

Fig. 1: a) Sketch of FePc molecule C32H16FeN8 (hydrogen atoms omitted); b) Scheme of the ligand field effect, reducing the symmetry of the environment from spherical (O 3) to octahedral (Oh) and to square planar (D4h), on the 3d orbitals of the central Fe ion. The subsequent occupation leads to a spin S=1 state.

Fig. 2: XAS (left) and XMCD (center) data of the Fe L 23-edges for TF FePc (top) and SL FePc on Gr/Ir(111) (bottom). Green lines label light with 0° incidence angle and blue lines label light with 70° incidence angle. Dotted lines correspond to parallel circularly polarized light and solid line to antiparallel circularly polarized light. The XMCD spectra are normalized to the integral of the sum signal resulting from parallel and atiparallel spectra for direct comparison with the sum rules [9]. Right: estimated orbital (solid circles) and effective spin (empty circles) magnetic moments as a function of the radiation incidence angle.

Graphene-nanopillars photodetector 1

Antonio Lombardo , Alan Colli , Tim J. Echtermeyer , Shakil A. Awan , Ravi S. Sundaram , Silvia 1 Milana, Andrea C. Ferrari 1

Cambridge Graphene Centre, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK Nokia Research Centre, Cambridge, UK, 21 JJ Thomson Avenue, Cambridge CB3 0FA, UK al515@cam.ac.uk

Abstract Graphene-based photodetectors exhibit good quantum efficiency [1], fast response[2] and broadband operation [2,3]. In order to convert the incident light into a detectable electric signal, photo-generated carriers must be separated by an electric field. This is usually achieved by exploiting the built-in potential created at the graphene-contact interface by the difference in work function of metal and graphene [1,2,4]. However, the active area of these devices (i.e. where carrier separation occurs) is limited to the region in close proximity to the metal contacts (~0.2Îźm[4]), while carriers generated elsewhere recombine without contributing to the external photocurrent [4]. Here we fabricate graphenenanopillars photodetectors, where graphene is suspend onto vertically-aligned metallic nano-pillars (Fig. 1a). We fabricate high aspect-ratio silicon nanostructures via deep reactive ion etching by alternating etching and passivation steps [5]. By tuning the etching/passivation parameters we produce vertically aligned silicon pillars, usually referred as black Si or Si grass [6]. Pillars are subsequently coated with Ni by magnetron sputtering. This configuration allows us to extend the active area of the photodetector. Each nano-pillar constitutes a point-like contact (Fig. 1b) and locally induces a built-in field. Such field is exploited to separate the photo-generated charges, then collected by the nearby metallic pillar and the external electrode (yellow contact in Fig 1a). Therefore, a net voltage (up to 1.5V/W) develops between the substrate (to which pillars are connected) and the external electrode. In this geometry, the whole graphene area suspended over the pillars is active both for light absorption and for the collection of photogenerated charge. This allows us to reach responsivities up to 2 orders of magnitude higher than standard graphene photodetectors.

References [1] E.J.H. Lee, K. Balasubramanian, R.T. Weitz, M. Burghards, K. Kern, Nature Nanotech, 3 (2008), 486 [2] F. Xia, T, Mueller, Y.M. Lin, A. Valdes-Garcia, P. Avouris, Nature Nanotech. 4 (2009), 839 [3] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Nature Photon. 4 (2010), 611 [4] T. Mueller, F. Xia, P. Avouris, Nature Photon. 4 (2010), 297 [5] Y.Q. Fu, A. Colli, A. Fasoli, J.K. Luo, A.J. Flewitt, A.C. Ferrari, W.I. Milne, J. Vac. Sci. Technol. B 27 (2009), 1520 [6] M. Stubenrauch, M. Fischer, C. Kremin, S. Stoebenau, A. Albrecht, O. Nagel, J. Micromech. Microeng. 16 (2006), S82-S87

Figure 1: schematic of photodetector and measuring set-up: graphene is partially suspended on vertically aligned pillars. A laser is scanned across the graphene surface and photovoltage is recorded between the substrate (in ohmic contact with pillars) and the electrode on graphene on SiO2.

Langmuir-Blodgett and Langmuir-Schaefer Films of GRAnPH®: The Role of Oxidative Debris. López-Díaz, D(1); S. Blanco(2); P. Merino(2); A. Pérez(2); C. Merino(2); M.M Velázquez(1) (1) Dpto. de Química Física. Facultad de Ciencias Químicas. Universidad de Salamanca (Spain). (2) Grupo Antolin Ingeniería SA – GRAnPH Nanotech. Burgos (Spain). dld@usal.es It has recently published that graphene oxide (GO) obtained from graphite by Hummers’ oxidation is composed by nanoplatelets, named as GOP and oxidative debris (OD)1. OD seems to be composed by highly oxidative aromatic molecules coming from rupture of the sheets and they are adsorbed on the top of the nanoplatelets. The existence of OD modifies the physical and chemical properties of the material, such us, solubility in water, oxidation degree, electrical conductivity1 etc. We present results corresponding to the study of the effect of the oxidative structures on the properties of graphene oxide GRAnPH® prepared by oxidation of Carbon Nanofibers (GANF®) supplied by Grupo Antolín. To analyze the quality of purified material, GOP, UV-Vis and XPS spectroscopies, X-Ray Diffraction and Zeta Potential measurements have been used. The results show a red shift in UV-Vis characteristic of the material reduction and the increase of 20% in the percentage of Csp2 when the impurities are removed. Moreover, a decrease in the interlayer distance and a decrease of the zeta potential values have been found. All these results indicate that purified GRAnPH® graphene oxide presents properties similar to the reduced graphene oxide (RGO). So that, the purified process renders samples with similar properties to the reduced graphene oxide by conventional methods. A great variety of applications involving graphene materials require its deposition onto wafers. Spin coating or Drop Casting methods do not result the best techniques because they have low reproducibility and drop evaporation produces stacking and agglomeration of the material. To improve the wafer recovery and decrease the material agglomeration, we suggest Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) methodologies. The morphology of different films was studied by AFM and FE-SEM. Results show that the LB technique renders a higher recovery than the LS one. In addition, the morphology and recovery can be related with the nature of the wafer and the composition of the GRAnPH®2. Bibliography 1. J. P. Rourke, P. A. Pandey, J. J. Moore, M. Bates, I. A. Kinloch, R.J. Young, N.R. Wilson; Angew. Chem. 50 (2011) 3173. 2. Martín-García, B.; Velázquez, M.M.; Rossella, F.; Bellani, V.; Diez, E.; Fierro, J.L.G.; PerezHernández, J.A.; Hernández-Toro, J.; Claramunt, S.; Cirera, A; Chem.Phys.Chem. 13 (2012) 3682.

Acknowledgement Ministerio de Economía y Competitividad, IPT-2012-0429-420000, and CDTI, IDI-20111312, for financial support. The Center for Ultrashort Ultraintense Pulsed Lasers (CLPU) and Dr. José Luis García Fierro (Instituto de Catálisis y Petroleoquímica, CSIC) for AFM and XPS measurements, respectively.

Charge, spin currents and magnetism in Graphene nanoislands 1

Alejandro López , Nelson Bolivar

1, 2

, Ernesto Medina

1, 2

, Bertrand Berche

1, 2

Laboratorio de Física Estadística, Centro de Física, Instituto Venezolano de Investigaciones Científicas, Km 11 Panamericana, Caracas, Venezuela Groupe de Physique Statistique, Institut Jean Lamour, Universite de Lorraine, 54506 Vandoevre-lesNancy Cedex, France ernesto@ivic.gob.ve Abstract: Recently, very precise experimental techniques[1] have been developed to build graphene nano-island structures using either scanning tunneling microscope lithography or catalytic cutting using Fe atoms for multilayer or single layer graphene. Using a tight binding model of graphene, we address the problem of persistent charge and spin currents and their magnetic signatures in disk-like and corbino nanoislands. Graphene is described by the Hamiltonian[2,3]

(

)

iφ H = t ∑ e ij ci†c j + iΔ ∑ ν ij ci†s z c j + iλ R ∑ ci† s × dˆij c j ij

including the kinetic energy, both intrinsic (Δ) and Rashba (λR) spin-orbit interactions and an external magnetic field (hopping phase factor). We only consider weak fields the Zeeman contribution is ignored. Our tight-binding model generates hexagonal graphene nanoislands[4] with several diameters that we use as template. The selection of two radii, the minor radius and the major radius in the Corbino geometry, determines the carbon atoms that the program will extract of the template (Figure 1.a). Cutting the template and excluding the sites with coordination number less than two, we obtain disk or annular geometries with armchair and zigzag edges (Figure 1.b). We compute the eigenstates of electrons subject to the boundary conditions of Graphene sheets in disk and Corbino geometries (see Figure 2), which retain the symmetry of the island. A linearization[2] of the Hamiltonian in the vicinity of the Fermi energy is also considered, to assess the regimes of validity of the continuum approximation as compared to the lattice description. The linear dispersion of graphene produces charge persistent currents induced by a weak external magnetic field in step-like patterns when spin-orbit interactions are small. For stronger interactions, persistent charge currents alternate between smooth steps and sharp steps due to the opening of a gap and well defined level crossings respectively. Figure 3 shows the persistent charge and spin currents, for the continuum approximation as a function of the Rashba spin-orbit interaction λR for a finite value of the intrinsic term Δ. As can be assessed from the figure it is possible to control persistent currents by the application of perpendicular electric fields that modulate the Rashba interaction. References [1] L. Chi, Z. Xu, L. Wang, G. Gao, F. Ding, F. Kelly, I. Yakobso, and P. Ajayan, Nano Research 1, (2008) 116-122. [2] C. L. Kane, and E. J. Mele, Phys. Rev. Lett. 95, (2005) 146802. [3] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Nature, 438 (2005) 197-200. [4] J. Fernandez-Rossier, J. J. Palacios, Phys. Rev. Lett. 99 (2007) 177204.

Figures

Figure 1: Construction scheme of a graphene Corbino, a) Selection of the minor and major radii that determine the cut of the template, b) The resulting graphene Corbino after the cut with armchair and zigzag edges.

Figure 2: Tight binding ground state (left panel) and an excited state (right panel) on a Corbino geometry.

Figure 2: Persistent charge (left panel) and spin current (right panel) for the continuum model and for

Î&#x201D; =0.5. The spin current is non-zero when the Rashba interaction is turned on, its value depending on the ground state as a function of the SO parameters.

Palladium clusters anchored on graphene vacancies: A density functional study María J. López, Iván Cabria, Julio A. Alonso Dpto. Física Teórica, Atómica y Óptica, Universidad de Valladolid, 47011 Valladolid, Spain maria.lopez@fta.uva.es Abstract Palladium adatoms bind relatively weakly to the surface of pristine graphene, with a binding energy much lower (about three times smaller) than the cohesive energy of bulk palladium. A similar behavior has been found for many transition, alkaline-earth, and noble metal adatoms on graphene, exception made of the alkaline metals. In a previous work [1] we have shown that Pd atoms deposited on graphene have a strong tendency to nucleate and form three dimensional clusters. This is a consequence of the stronger Pd-Pd interactions as compared to the Pd-C interaction. The clusters are bound to the graphene surface with adsorption energies ranging from 0.68 to 1.26 eV for Pd n clusters in the range n = 1 – 6. The small binding energy of Pd clusters to graphene, might be an important drawback for technical applications of Pd doped graphene (or other Pd doped graphene based carbon materials) in catalysis, sensing or for hydrogen storage [2]. However, it has been shown that metal atoms bind stronger to defects in graphene such as vacancies, di-vacancies, graphene edges, etc. We have performed Density Functional calculations to investigate the adsorption of Pd atoms on graphene single vacancies and the tendency of Pd adatoms to nucleate and form clusters around those defects. A Pd atom binds strongly to a graphene vacancy with a binding energy of 5.1 eV. This energy has to be compared with the binding energy of 1.1 eV of a Pd atom on pristine graphene, showing a strong preference of Pd for the defects in graphene. Pd atoms are much larger than C atoms and, therefore, Pd does not fit in the vacant site left out by a C atom on the graphene layer. Pd attaches to the three C atoms around the vacancy at about 1.6 Å above the graphene layer. We have found that Pd atoms deposited on graphene have a strong tendency to form three dimensional clusters around graphene vacancies. The Pd clusters build up around the first Pd atom attached to the vacancy. As an example, the Figure shows a Pd4 tetrahedral cluster attached to a graphene vacancy. Free Pdn clusters (n = 2 – 6) exhibit a magnetic moment of 2 B. The adsorption of these clusters on the surface of pristine graphene preserves their magnetic moments, except for Pd 2 and Pd5 whose magnetic moments are changed to zero. We have found that Pdn clusters attached to a graphene vacancy have moments equal to zero, probably due to the stronger interaction of the cluster with the vacancy. The structural, binding, and electronic properties of Pdn clusters (n = 1 – 6) attached to a graphene vacancy will be discussed in detail. The most salient result of this study is that defects in graphene, in particular single vacancies, behave as attraction centers for Pd atoms and clusters. The atoms and clusters get anchored to the vacancies with significant binding energies, much higher than the corresponding binding energies to pristine graphene. This will be relevant for possible applications of Pd doped carbon materials. References [1] I. Cabria, M.J. López and J.A. Alonso, Phys. Rev. B, 81 (2010) 035403. [2] I. Cabria, M.J. López, S. Fraile and J.A. Alonso, J Phys. Chem. C, 116 (2012) 21179.

Figures

Fig. Top and side view of a tetrahedral Pd4 cluster attached to a vacancy of graphene.

Step like surface potential on few layered graphene oxide López-Polín G., Jaafar M. , Gómez-Navarro C. , Gómez-Herrero J. Dpto. Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049, Madrid, Spain guillermo.lopezpolin@uam.es Abstract Graphite oxide (GO) and its derivatives have attracted much interest recently as a possible route for the large-scale production of graphene [1]. Due to its two dimensional nature, mechanical resilience [2] and tunable conductivity [3] it is being tested for a variety of application that, among others, include, transparent conducting films, sensors, transistors and double layered capacitors. Many of these applications involve deposition of graphene oxide films on either metal or insulating supports. We report surface potential maps of few layered graphene oxide ﬁlms on different metallic substrates. Kelvin probe force microscopy images reveal that the surface potential decreases in steps with increasing number of layers on the substrate until ﬁve layers are reached, where it saturates to a constant value [4]. Electrostatic Force Microscopy proves to be a fast and suitable technique to quickly determine the coverage of GO. We also present a detailed study on how environmental conditions influence these measurements. References [1] Park, S., and Ruoff, R.S., Nature Nanotechnology, 10 (2009) 1038. [2] Gomez- Navarro, C., Burghard, M., and Kern, K., Nano Letters, 8(7) (2008) 2045. [3] Jung, I., Dikin, D. A., Piner, R. D., and Ruoff, R. S., Nano Letters, 8 (12) (2008) 4283. [4] Jaafar, M., López-Polín, G., Gómez-Navarro, C. and Gómez-Herrero, J., Applied Physics Letters, 101 (2012) 263109. Figures

mV 15165.27mV

HOPG

3 ML 2 ML

4 ML

150

Surface Potential (mV)

-366.11 mV mV -510

Substrate

0 -150

1ML

2ML

-300

3ML 4ML

-450 0

80 L ( m)

Figure 1. (a) Surface potential map of GO deposited on HOPG. Equipotential areas corresponding to different coverages can be appreciated. (b) Proﬁle performed on the surface potential map following regions with different coverages (The red line in (a) indicates schematically the paths followed to perform the proﬁle)

120

Graphene to Enhance Chemical and Optical Stability of Plasmonic Properties of Silver based Plasmonic- and Meta-Materials 1

Maria Losurdo, Iris Bergmair, Maria M Giangregorio, Kurt Hingerl, Giovanni Bruno

Institute of Inorganic Methodologies and of Plasmas, IMIP-CNR, via Orabona 4, 70126 Bari, Italy 2 Profactor GmbH, Im Stadtgut A2 | 4407 Steyr-Gleink | Austria 3 Center for Surface Nanoanalytics, Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria maria.losurdo@ba.imip.cnr.it Graphene, with its extensive π-electron conjugation and delocalization giving rise to the extreme physical strength and chemical inertness, is a very versatile material that can be used for a number of applications in electronics, sensing, catalysis, batteries, photovoltaics, and recently also in plasmonics. A very recent work has proposed plasmon polaritons—coupled excitations of photons and charge carriers—in graphene as a promising way to achieve electric control of light. Furthermore, the tuning of the plasmon is possible through electrical or chemical modiﬁcation of the charge carrier density [1]. On the other hand, silver is an appropriate plasmonic material for the visible range. A main drawback of silver comes from its chemical instability leading to silver oxidation in time, with shift and quenching of optical resonances [2]. In this plasmonic perspective, we address the following question: what is the potential of combining together the attractive materials of graphene and silver nanostructures for plasmonic- and metamaterials? The study of corrosion/oxidation processes and their inhibition by organic inhibitors is a very active field of research. Many factors can contribute to the inhibition effect, such as gas permeability and electron density of inhibiting atoms and orbital character of donating electrons. The pioneering work from Ruoff [3] showed that graphene grown on metals (copper and nickel) effectively suppresses metal oxidation by posing a high energy barrier to diffusion of oxygen. Indeed, when graphene is transferred to a metal, it is interesting to explore how the electronic properties of the graphene/metal hybrid change. Understanding the electronic interaction between graphene and Ag is important from many points of view and for many applications. As an example, even in explaining the SERS and variation of the Raman signals of graphene coupled to Ag, the splitting of the G band can be attributed to the interaction between Ag and graphene, which induces a change in the graphene electronic structure [4]. Here, we present a study on graphene-coated silver plasmonic nanostructures, which are ultrathin films, nanoparticles arrays and 3D fishnet nanostructures. High-quality, single-layer graphene (SLG) was synthesized by chemical vapour deposition (CVD) on copper foil and transferred to the silver nanostructures. Ag nanoparticles are formed by evaporation of silver while Ag periodic fishnets are fabricated by nanoimprint lithography (Figure 1). A novel type of hybrids functional materials, which are oxidation resistant and with stable plasmon resonance in the visible range over almost a year of air exposure are enabled. The interface chemistry and electronic phenomena of charge transfer underpinning the coupling of graphene with silver gratings and fishnet structures is extensively characterized chemically and optically by X-ray photoelectron spectroscopy, Raman spectroscopy, and spectroscopic ellipsometry, in conjunction with microscopies of SEM and AFM also in Kelvin mode to probe local changes of surface potential. (c)

(a)

(b)

1200 1400 1600 1800 2000 2200 2400 2600 2800 -1

Wavenumber (cm )

Figure 1. Scanning electron microscope (SEM) images of a silver fihnet structure fabricated by nanoimprint lithography to have optical resonances in the visible range (a) as deposited and (b) with graphene transferred on it; (c) Raman spectra acquired in two different points of the graphene/Ag fishnet hybrid using a 514 nm laser with a power of ∼1mW/μm a 10 s integration time, and one accumulation.

After 6 months of air exposure

As Ag was deposited

Ag-SiO2

Ag-Graphene AgO

AgO Ag2O Ag

AgO

Ag AgO

Figure 2. XPS spectra of Ag3d for the Ag fishnet on SiO2 without and with graphene ontop measured soon after fabrication (top) and after 6 months of air exposure (bottom).

We demonstrate that electron transfer from graphene can activate reduction of silver oxides to metallic silver in a dynamic way and preserve temporally stable silver plasmonic properties. A chemical model for the electron transfer from graphene to silver is given to rationalize the oxidation resistance behavior of silver-coupled-to-graphene and the new chemical/optical properties of the hybrid [Figure 2]. Specifically, the Ag/SiO2/Si sample shows fitcomponents due to Ag, Ag2O and AgO, indicating a fast surface oxidation soon after deposition; after a week of air exposure, the only component detected is AgO, and it stays after a month the same, indicating complete oxidation of the Ag silver layer. Conversely, for the Ag/graphene, a significant part of silver stays as metallic silver even after six months of air exposure. A similar reduction of Ag-oxide to metallic Ag has also been verified for 30 nm Ag nanoparticles passivated by graphene.

In this case, the graphene coupled to 30 nm Ag nanoparticles results in an increase of the localized plasmon resonance (LSPR) of silver/graphene, which remains stable and intense over months of air exposure. Differently, a complete damping of the plasmon resonance occurs on a similar Ag nanoparticles sample without graphene (Figure 3). Applications of this research on graphene/metals will likely have a large impact in a variety of fields, such as optical metrology, SERS sensors, renewable energy, metamaterials, high transparent and electrically conductive thin films research, through the development of novel graphene/silver composites. Consequently, the graphene passivated silver fishnet and nanoparticles can provide much more stable SERS substrates. Here, we also show the effectiveness of the new Ag/graphene hybrids as SERS substrates and sensors using benzyl mercaptane and hemin as the probe molecules.

Graphene/Ag NPs on Al2O3 Graphene/Ag NPs on Al2O3 After 4 months air exposure

LSPR

1 Ä_i

Ag NPs on Al2O3

Ag NPs on Al2O3 After 4 months air exposure

0 2

4 5 Photon Energy (eV)

Figure 3. Spectra of the extinction coefficient of 30nm Ag nanoparticles deposited on sapphire with (top spectra) and without (bottom spectra) graphene ontop. The 30nm Ag NPs have a LSPR at 3 eV that is enhanced by graphene and stays stable over 4 months of air exposure, whereas it completely dampens for the nacked Ag NPs. (Blue curves refer to “as-fabricated” sample, while red curves are after 4 months of air exposure. The insets are the 2mx2m AFM images of graphene-passivated (top) and naked Ag NPs

The SERS enhancement mechanism of the SLG-coated substrates is discussed, and the different enhancement factors (EFs) between graphene-coated nanoparticles and nanoholes are understood based on the different morphologies of graphene on the two substrates using numerical simulation. Ultimately, the performance of a novel sensor based on Hemin/graphene/silver to detection of the nitric oxide, NO, is demonstrated. Acknowledgements. We acknowledge funding of the FP7 European project NIM-NIL (GA 228637) www.nimnil.org References [1] F. H. L. Koppens, et al. *Graphene Plasmonics: A Platform for Strong LightMatter Interactions, Nano Lett. 11 (2011) 3370. [2] M. Losurdo et al. “Enhancing chemical and optical stability of silver nanostructures” J. Phys. Chem. C, 116 (2012), 23004. [3] J. Lee, K. S. Novoselov, et al. Interaction between Metal and Graphene: Dependence on the Layer Number of Graphene ACS Nano 5 (2011) 608. [4] A. Varykhalov, et al. Phys. Rev. B 82 (2010) 121101.

Growth of nanocrystalline graphene layers on various dielectric surfaces by CVD a

M. Lukosius , M. H. Zoellner , G. Lippert, J. Dabrowski , W. Mehr , X. Wang , M. Arens and G. a Lupina a

IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany b SENTECH Instruments, Schwarzschildstr. 2, 12489 Berlin, Germamy lukosius@ihp-microelectronics.com

Abstract The discovery of graphene, a two dimensional carbon sheet with unique structure and therefore electronic quality, has triggered the investigations of its potential applications in microelectronic as well as in photonic devices [1,2]. At this point, for the successful integration of graphene into various devices, an appropriate deposition technique is required in order to grow uniform, large area and high quality layers. Up to date, two common graphene synthesis methods, like decomposition of SiC and chemical vapor deposition (CVD) on Cu have been established [3,4]. However, these methods have some severe limitations due to the high temperatures or large area SiC wafers that are required for the SiC approach, or the necessity to transfer graphene from Cu to the target substrates, which causes wrinkles, holes or etching residuals to name a few. These drawbacks strongly limit the integration of graphene into the Si microelectronics. Therefore, the biggest challenge here is to grow continuous, defect-free graphene layers directly on the target substrates at low temperatures (<1000°C). Particularly, a direct growth of graphene on dielectric substrates is of a great interest for high frequency applications, where graphene has to be embedded between dielectrics [5]. A number of considerable efforts have been made to grow graphene directly on substrates like SiO2, Si3N4 or Al2O3 [6-8]. However, continuous and high quality conductive graphene films are hard to obtain since the growth mechanism and therefore the graphitization process of the graphene on dielectrics is still not well understood. We have recently obtained high quality micrometer scale graphene layers on dielectric mica substrates below 1000 °C by Molecular beam epitaxy (MBE) using solid carbon source [9]. Raman spectra of graphene grown directly on mica show a negligible D mode suggesting a very high crystal quality as indicated in Fig.1. In the present work, we examined the possibility to grow graphene films on several dielectric substrates at various temperatures by employing low pressure CVD technique. Oxides, such as PrOx or CeOx (which are known to possess some catalytic activity to hydrocarbons) and CMOS compatible dielectrics such as HfO2, SiO2 and Si3N4 have been investigated in this study. The depositions were performed at 800 – 1000 °C using C2H4 as reactive gas at the pressures of 0.1 mbar. 4 inch Si wafers, covered with mentioned oxides were used as substrate materials. PrOx and CeOX films were grown at room temperature by MBE and then transferred to the CVD chamber without breaking the vacuum. On the other hand, HfO2, Si3N4 or SiO2 were externally deposited by the CVD technique. A typical deposition time was 30 minutes. Carbon content was determined by X-ray Photoelectron spectroscopy (XPS), whereas the quality of the deposited carbon was then evaluated by Raman spectroscopy. In order to check the initial amount of carbon, all investigated dielectrics were examined by XPS before the CVD depositions. The carbon 1s signals of selected oxides are presented in Fig.2 and Fig. 3. As black line in Fig. 2 indicates, as grown amorphous CeOx films had no carbon contamination, since they were deposited by the MBE and then directly transferred to the XPS chamber without breaking the vacuum. The exact same result was observed for as grown PrOx films (not shown). On the contrary, the as grown HfO2 films have already contained carbon, as can be seen in Fig.3. This effect was also expected, since HfO2 as well as Si3N4 were grown externally by CVD. The evolution of carbon content on CeOx was further analyzed after exposing the films to C2H4 at 800-1000 °C. As can be seen in Fig. 3, the highest C 1s signal was observed for the samples grown at 1000 °C, although some carbon was catalytically grown on CeOx (as well as on PrOx) even at the temperatures as low as 700 °C (not shown). In the case of the depositions on HfO2, the reduction of the C 1s signal was observed as indicated in the Fig.3 (red line). This can be explained by the fact that carbon, which had been absorbed on the surface, was removed at 1000 °C and then the graphitic carbon was grown from the C2H4 precursor. In addition, after the deposition, the position of the C 1s signal moves the lower binding energies towards elemental carbon. Raman measurements of the grown carbon on PrOx, HfO2 and Si3N4, are summarized in Fig. 4 and Fig. 5. As can be seen, nanocrystalline graphene can be grown at the deposition temperature of 1000 °C on PrOx (CeOx not shown here) and HfO2 dielectric substrates, whereas no crystalline features where detected on SiO2 and Si3N4. These observations might lead to the suggestion that oxygen vacancies could affect the catalytic growth of the graphene on some dielectrics. A short possible graphitization mechanism is sketched in Fig. 6.

References [1] A. K. Geim, K. S. Novoselov, Nature Materials, 6 (2007) 183. [2] K. S. Novoselov, V. I. Falko, L. Colombo, P. R. Gellert, M. Schwab, K. Kim, Nature, 490 (2012) 192. [3] W. A. de Heer, C. Berger, M. Ruan, E. Conrad, Proc. National Academy of Science (2011). [4] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, R. S. Ruoff, Science, 324 (2009) 1312. [5] W. Mehr, J. Dabrowski, C. Scheytt, G. Lippert, Y. Xie, G. Lupina Electron Dev. Lett., 33 (2012) 691. [6] A. Ismach, C. Fruzgalski, S. Penwell, M. Zheng, A. Javey, Y. Zhang, Nano Lett., 10 (2010) 1542. [7] J. Sun, N. Lindvall, M. T. Cole, K. B. K. Teo, A. Yurgens, Appl. Phys. Lett. 98 (2011) 252107. [8] Y. Miyasaka, A. Nakamura, J. Temmyo, Jpn. J. Appl. Phys., 50 (2011) 04DH12. [9] G. Lippert, J. Dabrowski, J. Maultzsch, M. C. Lemme, W. Mehr, G. Lupina, Carbon 52 (2013) 40. Figures

. Fig.1. Raman spectra of high quality micrometer range graphene on biotite mica substrate.

Fig.3. XPS spectra of carbon 1s line in HfO2 for as grown layers (black line) and after Carbon deposition at 1000 째C (red) line.

Fig.5. Raman spectra after carbon growth on HfO2 (green line) and Si3N4 (black line).

Fig.2. The evolution of carbon 1s signal on CeOx for as grown film (black line) and after exposure to C2H4 gas at 800 째C (blue line), 900 째C (cyan line) and 1000 째C (red line).

Fig.4. Raman spectra of nanocrystalline graphene grown at different temperatures on PrOx substrates.

Fig.6. A possible mechanism for catalytic graphitization.

Graphene-Organosilane Memory Cell Based on Charge Transfer Hysteresis Hongming Lv, Huaqiang Wu, Ke Xiao, Ning Deng, He Qian Institute of Microelectronics, Tsinghua University, Beijing, China 10084 wuhq@tsinghua.edu.cn Abstract One of the most striking features of graphene is that the electrons/holes are confined to a plane of atomic thickness, making graphene devices sensitive to the surrounding environment. In this study, a prototype memory cell based on charge transfer between graphene and organosilane self-assembled monolayer (SAM) has been demonstrated. 3-Aminpropyltriethoxysilane (NH2-silane) has been used to modify SiO2 substrates, which has been proven to enhance graphene transfer integrity due to stronger Van der Waal’s force[1]. To apply this organosilane, substrates with 90 nm thermally grown SiO2 were immersed in 2 wt % NH2-silane ethonal solution. During the process, silane groups reacted with hydroxyl groups and are chemically bonded to SiO2 surface[2], as shown in Fig.1(a). Graphene in this study was synthesized by APCVD method and transferred by electrochemical bubbling technique [3]. Raman spectrum of graphene on NH2-silane SAM is shown in Fig. 2. The absence of D-band proves high quality of the graphene without defects, and high peak intensity ratio of G-band to 2D-band indicates charge carrier doping induced by proximity effect of NH2-silane molecules [2]. Bottom gate graphene field effect transistors (GFET) with channel dimension of 300μm*250μm were fabricated on those treated substrates, as illustrated in Fig. 1(b). Source/Drain electrodes, Ti/Au (5nm/45nm), were evaporated through shadow mask and graphene channel was patterned by conventional photolithography followed by oxygen plasma etching. This type of device patterning is easily applicable and contaminations from photoresist residues are reduced. Electrical characterization of these bottom gate GFETs shows significant and reproducible hysteresis in conductance. Fig. 3 compares transfer curves of GFETs with and without NH2-silane interface engineering. With gate voltage sweeping speed of 2.1V/s, the hysteresis window in NH2-silane case is 36V in -50V-to-50V range, which is 8 times larger than the counterpart on untreated SiO2. Hysteresis phenomenon is explained by electron transfer between graphene and trapping sites in dielectric underneath. Significant enhancement of hysteresis in NH2-silane case is attributed to the charge trapping/detrapping properties of electron lone pairs of nitrogen atoms[4]. At zero bottom gate voltage, a 4X change in graphene channel resistance is observed. Reproducibility of transfer curve is further studied by comparing its minor shifts under ambient atmosphere and in vacuum. Fig. 4(a) shows that transfer curves shift in positive direction under ambient atmosphere, which is unfavorable from memory window point of view. In vacuum, however, transfer curves are very stable, with Dirac points slightly heading zero and electron/hole branches getting more symmetric, as shown in Fig. 4(b). These results indicate that absorbates from the air cause nonnegligible p-doping for the exposed graphene device, while electro-cleaning effect in vaccum produces favorable stabilized memory characteristics. For better memory performance, air absorbates are to be avoided via packaging in future memory cell fabrication. Even under ambient atmosphere, memory window remains above 1.6X over 100 sweeping cycles, as shown in Fig. 5. Pulse program/erase was applied to those devices. A stable 2X memory window is observed. Those memory effects are more sensitive to pulse height rather than width, as shown in Fig. 6. Memory cell functionality was further examined through retention test. Fig. 7 shows that the resistance difference between high and low values decays slowly with time. In summary, we demonstrated graphene device with nonvolatile memory effects which show significant promise as a prototype for flexible and transparent memory applications. Further device optimization, trapping layer design and packaging would greatly improve the memory performances. References [1] H. M. Lv, H. Q. Wu, K. Xiao, W. N. Zhu, H. L. Xu, Z. Y. Zhang and H. Qian, Unpublished work. [2] K. Yokota, K. Takai, and T. Enoki, Nano Lett., 9 (2011) 3669–3675. [3] L. B. Gao, W. C. Ren, H. L. Xu, L. Jin, Z. X. Wang, T. Ma, L. P. Ma, Z. Y. Zhang, Q. Fu, L. M. Peng, X. H. Bao and H. M. Cheng, Nat. Commun., 3 (2012) 699. [4] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa and Y. Iwasa, Nature Materials, 3 (2004) 317-322.

Fig. 1. (a) Schematic of graphene transfer with NH2-silane interface engineering. (b) Optical image of the as-fabricated device.

Fig. 2. (a) Raman spectrum of graphene/NH2-silane SAM.

Fig. 4. GFET transfer curve shifts under ambient atmosphere (a) and in vaccum (b).

Fig. 6. Channel resistance change in pulse program/erase mode. Pulse width and height is kept constant in (a) and (b), respectively.

Fig. 7. Channel resistance versus time for two different charge states. The resistance difference decays slowly with time as the stored charge decays.

Fig. 3. Transfer curve of GFETs with (a) and without (b) NH2-silane interface engineer.

Fig. 5. DC sweep cycling endurance under ambient atmosphere (a) and in vaccum (b).

Electronic and magnetic properties of Crn and Con clusters on nanographenes Ángel Mañanes, Ángela Calleja, and Tomás Alonso-Lanza Departamento de Física Moderna, Universidad de Cantabria, 39005 Santander, Spain angel.mananes@unican.es We present Density Functional calculations of the electronic properties of small aggregates of two different transition metals, Crn and Con (n=1, 2, 4), interacting with small nanographenes with zigzag boundaries. We have considered two different triangular nanographenes (TNG), C46H18 and C33H15, which present a net magnetic moment in their ground state of 3 and 4 µB, respectively, with an intense spin localization in the zigzag edges, and a hexagonal nanographene C54H18 (HNG) which has zero magnetic moment in its ground state and a locally compensated spin structure. The free clusters Cr2 and Cr4 present an antiferromagnetic coupling with zero total magnetic moment; on the other hand Co2 and Co4 have a ferromagnetic structure with 4 µB and 10 µB magnetic moments, respectively. For the combined systems, we discuss the equilibrium geometries and the binding and formation energies, the electronic densities of states (DOS) and the presence of different gaps for alpha and beta electrons, the structure of the edge orbitals which present p-d hybridization, the total magnetic moment, the distribution of the atomic magnetic moments and the charge and magnetic moment transfer between the metallic cluster and the nanographene. We pay special attention to the changes induced by the metallic clusters on the magnetic properties of the nanographene. In the case of Crn, we have considered the interaction with the two different TNGs already mentioned. For the Con aggregates we analyze the interaction with the HNG. The work was motivated by recent experiments on the interaction between Cr and Fe atoms with a free standing graphene structure [1] and by previous calculations of the electronic and magnetic properties of iron aggregates encapsulated inside finite zigzag carbon nanotubes [2]. Single Cr atoms prefer “top” positions over the C atoms of the TNGs. The large alpha and beta gaps (around 2 eV) of the free TNGs are strongly modified by the interaction with Cr n: For Cr2 and Cr4 the resulting combined system is a half-metal with zero beta gap. We analyze the d-pz hybridization which is present in those molecular orbitals of the combined systems which are close to the Fermi level. The combined systems Crn—TNG present a net total magnetic moment, which is, in general, smaller than the value corresponding to the isolated systems. The contribution to the magnetic moment due to the TNG structure is lower than the corresponding magnetic moment of the free nanographene; however there is also a net magnetic moment contribution from the Crn aggregate. In Figure 1 we present the equilibrium structure and the net spin density distribution in the case Cr4C46H18, a system that has the same magnetic moment as the free TNG, M=4 µB. The spin structure in Figure 1 (b) shows both the spin concentration in the zigzag edges and the antiferromagnetic coupling of the Cr atoms. In the case of cobalt clusters, we have analyzed first the equilibrium of Co atom in the centre of the HNG, in a hollow location. The total magnetic moment is reduced from 3 µB for the isolated atom to 1 µB. For the dimer, Co2, the equilibrium is obtained for the dimer vertically located in the central hollow location with zero total magnetic moment for the combined system. The free cobalt dimer present ferromagnetic coupling and it has M=4 µB, so a strong reduction of the total magnetization is obtained due to the interaction of the Co2 with the HNG. This reduction is due to the antiferromagnetic coupling between the two Co atoms which are now separated by 2.37 Å instead of the 2 Å equilibrium distance of the free dimer. In the case of Co4 the equilibrium structure is given in Figure 2 (a). There is a reduction of the total magnetic moment from 10 to 6 µB. However, in this case the coupling of the Co atoms is ferromagnetic and they dominate completely the total magnetic moment of the system. The DOS is given in Figure 2 (b), which presents a complete d-alpha band under the Fermi level, and d-p hybridization, on some levels, Work supported by MICINN, Spain (Projects MAT2008-06483-C03-03 and MAT2011-22781). A. C. acknowledges the support of a grant (“Iniciación a la Investigación”) of the University of Cantabria. [1] R. Zan, U. Bangert, Q. Ramasse, K. S. Novoselov, Nano Lett., 11 (2011) 1087 [2] F. I. Horga, A. Mañanes, M. J. López, and J. A. Alonso, Phys. Rev. B 87 (2013) 85402

(b)

(a)

M= 4 μB

Figure 1: (a) Equilibrium geometry for Cr4C46H18 (TNG). The distances are in Å. The total magnetic moment M=4 B is identical to the corresponding to the free TNG. (b) Net spin density (ρβ−ρα). In the color scale, red indicates dominant alpha spin, blue dominant beta spin, and green spin compensation.

M= 6 μB

(a)

(b)

Figure 2: (a) Atomic magnetic moments for Co4C54H18 as a function of their distance to the centre of the HNG. Co ( ), C ( , ) and H ( ). The values (+) are for Co in the free cluster. For C and H the values are multiplied by 100. M is the total magnetic moment. (b) Electronic density of states (DOS) for majority (upper panel) and minority spin (lower panel). Red lines d states, green lines p states, and black lines s states. The horizontal arrows indicate the alpha and beta electronic gaps.

Optical properties of graphene flakes from time-dependent density functional calculations F. Marchesin1,2,a), P. Koval1,2, D. Foerster3, D. Sánchez-Portal1,2 1

Centro de Física de Materiales CFM-MPC, Centro Mixto CSIC-UPV/EHU, Paseo Manuel de Lardizabal 5, E20018 San Sebastián, Spain 2 Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, E-20018 San Sebastián, Spain 3 CPMOH, University of Bordeaux 1, 351 Cours de la Liberation, 33405, Talence, France a) Electronic mail: federico_marchesin1@ehu.es Abstract The unique properties of graphene[1], such as large conductivity, high mechanical strength, high thermal stability, tunable optical properties[2], have led to an intensive study since it was first isolated in 2004. It has become one of the most promising materials for applications such as optical signal processing and quantum information. Part of this interest stems from the possibility to control the properties of graphene by means of chemical and physical modifications. Apart from extended graphene structures, also graphene quantum dots, antidots, nanoribbons and moebius strips have attracted the interest of the scientific community. In this work we focused our attention on the optical properties of hydrogen saturated and chemically modified graphene nanoflakes. We investigated in detail how the optical properties depend on the geometry and the size of the flakes. Although there are some previous works on similar subjects, they mainly use simple TightBinding (TB) models. In order to overcome restrictions of TB models we use ab-initio methods of electronic structure theory. We performed our calculation with an efficient Time-Dependent Density Functional Theory code[3] based on the use of basis set of localized functions (atomic orbitals). In conjunction with the ab-initio SIESTA package[4] this allows to study flakes containing more than a thousand atoms using much less severe approximations than those in the TB models. We have chosen hydrogen-saturated hexagonal and rectangular planar flakes to characterize size and shape dependence of optical absorption. The spectra of all the flakes show two main groups of peaks, π peak and σ peak, which are blueshifted as the size of the flakes decreases. A typical spectrum can be seen in fig.1(a). At low frequency range the optical absorption is determined by the type of edges. In particular, we quantified the optical gap which is defined as the lowest-frequency resonance position. In fig.1(b) the dependence of the optical gap on the number of the valence electrons in the system for zigzag-hexagonal, armchair-hexagonal and rectangular flakes (which contain both edge types) can be observed. The optical gap is smaller for zigzaghexagonal flakes than for the corresponding armchair flakes. However, we do not observe a complete closure of the gap for the explored sizes. In marked contrast, rectangular flakes with more than 250 valence electrons already show a negligible gap. Moreover, we found that chemically functionalized edges affect the absorption spectrum. We saturated the carbon flakes with oxygen, fluorine and hydroxyl groups. The obtained results open the way for tuning optical properties by changing the edge functionalization. In fig.1(c) the shift of the first resonances as a function of the chemical modification of the flake. Oxygen functionalization provides clearly distinct optical response from hydrogen and fluorine. Hydrogen and fluorine functionalization show similar effects. We also explored the spatial distribution of the density change induced by an external electric field with a given frequency. In particular, the plots of the imaginary part of that density change show which parts of the flake are responsible for an enhanced optical absorption. An example of a density change plot is gives in fig.1(d). We hope that these results can improve our understanding of graphene optical properties and for future applications in the field of optical signal processing and quantum information. References [1]A. K. Geim, SCIENCE 5934, (2009), 1530-1534 [2]Jianing Chen et al., NATURE 7405, (2012), 77-81 [3]P. Koval, D. Foerster and O. Coulaud, J. Chem. Theory Comput., 9, (2010), 2654-2668 [4]José M. Soler, Emilio Artacho, Julian D. Gale, Alberto García, Javier Junquera, Pablo Ordejón and Daniel Sánchez- Portal, J. Phys.: Condens. Matter, 11, (2002), 2745-2779

Figures

Figure 1: (a) Absorption spectrum of a hydrogen-saturated armchair (AC) graphene flake containing 222 carbon atoms. Ď&#x20AC; and Ď&#x192; plasmonic features are characteristic for all grapheme flakes we studied. (b) Optical gap dependence on the number of valence electrons in the flake for different flake shapes. Optical gap of hexagonal flakes decreases steady, while optical gap of rectangular flakes quickly becomes negligible. (c) Absorption spectrum at low frequencies of hexagonal zigzag (ZZ) flakes saturated with hydrogen, oxygen and fluorine. Oxygen functionalization leads to a broader absorption which is weaker than in hydrogen- and fluorineterminated flakes. (d) Example of density change plot which highlights the flake's parts with the highest absorption corresponding to an applied field with a frequency in the range 0-2eV for a ZZ-hexagonal and AChexagonal flake. Edges contribute stronger to the density change in case of ZZ flake.

Flexible graphene device for lighting LEDs 1

1,2

1,3

J. Martinez , D. J. Choi , S. Shrestha , T. Valero , A. Bosca , J. Pedros , F. Calle 1 ISOM - UPM, E.T.S.I.Telecomunicacion, Madrid, Spain 2 Division of Materials Science and Engineering, Hanyang University, Seoul, Korea 3 Institute of engineering, Tribhuvan Univ, Pulchowk,Lalitpur, Nepal. javier.martinez@upm.es

Abstract Graphene, has attracted increasing attention in recent years [1] due to its excellent mechanical, 2 -1 optical and electrical properties. Its high theoretical surface area (2630 m g ) and high electrical conductivity make it an attractive material for many industrial applications [2]. Also is a flexible transparent material that can be used for solar cells, light emitting diodes (LEDs, OLEDs), touchscreens and LCD displays [3].And in the near future, its flexibility will let to create foldable and wearable devices[4]. A layer of graphene can be prepared by several techniques: by mechanical exfoliation from graphite, by precipitation on a silicon carbide surface, by reduction of exfoliated graphene oxide, and by chemical vapor deposition growth on Cu or Ni. The most used one is the CVD, and the synthesized graphene is commonly grown on a flat metal foil or thin film. This method provides high quality graphene, and can also fabricate 3D graphene structures using metallic foams. The large area and porosity of this 3D graphene structures, makes them an ideal material for flexible 2 electronics. In order to create this structures, we used a 1 x1 cm Ni foam as the catalytic template to form graphene layers by plasma-enhanced CVD (PECVD). After this step, the Ni was removed by a wet etching in HCl acid, obtaining a soft graphene foam with the same porous size. Figure 1 shows an image of the foam by scanning electron microscopy (SEM). This graphene foam has a very high conductivity and can used for flexible electronics. The graphene foams were coated partially with PMMA for mechanical stability and sealed inside a plastic container with an electrolyte and two electrical contacts. This flexible device can store energy when is polarized by a positive bias and can light up several commercial LEDs as it is shown in the Figure 2.

Acknowledgements. This work has been partially supported by Ministerio de EconomĂa y Competitividad (Project No. TEC 2010-19511) and technical advice from Repsol.

References [1] B. Luo, S. Liu, L. Zhi, Small 8 (2012) 630. [2] M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Nano Letter, 8 (2008) 3498. [3] X. Cao, Y. Shi, W. Shi, G. Lu, X. Huang, Q. Yan, Q. Zhang, and H. Zhang, Small 7 (2011) 3163. [4] M. F. El-Kady, V. Strong, S. Dubin, R. B. Kaner, Science 335 (2012) 1326

Figures

Figure 1. SEM picture of the graphene 3D foam

Figure 2. Picture of the flexible device lighting up 3 LEDs

Theoretical study of electronic properties of graphene on a BN monolayer substrate Rafael Martínez-Gordillo1, Frank Ortmann2, Stephan Roche2, Miguel Pruneda1 1

Centre d'Investigació en Nanciència i Nanotecnologia, CIN2. (CSIC-ICN), 08193 Bellaterra, Barcelona, Spain 2 CIN2 (ICN-CSIC) and Universitat Autonoma de Barcelona, Catalan Institute of Nanotechnology, Campus UAB, 08193, Bellaterra, Spain rmartinez@cin2.es Abstract To take advantage of the electronic properties of graphene in new technology, it is important to consider the effect of the substrate over which graphene is grown as this has an important impact in the structure, symmetry, appearance of charge impurities and dopping effect [1,2,3]. Boron nitride (BN) is an insulator that offers improved properties as a substrate for graphene, keeping the intrinsic properties like the high mobility [4] and semimetallicity [5] to an acceptable degree, also avoiding ripples and charge inhomogeneities [6]. Even though, the coupling between boron nitride layer and graphene is weak, the variations in the potential felt by graphene due to the underlying BN have important consequences. The slightly mismatch in lattice parameter and the rotation angle formed by the two lattices give rise to miré patterns (Figure 1) that have been experimentally observed. The weak modulated potential of the underlying BN-sheet creates new Dirac points at energies above and below the Fermi level [7], having an important impact in the density of states, and thus, giving place to new physics not yet well understood. In this work, the effect of BN monolayer used as a substrate for graphene is analyzed by means of first-principles calculations using DFT as implemented in the SIESTA package [8]. We considered three rotation angles between BN and graphene lattices using three different supercell sizes. For all the studied systems the calculated DOS show dips at the positions of the new Dirac cones as reported experimentally in Ref. [7]. The position of these dips depends on the periodicity of the moirés λ, getting closer to the Fermi level as the λ increases, although the energy of these Direc points do not fully agree with the perturbative expression given in Ref. [7]. Also, the depth of these features increases as the periodicity of the potential increases. Although a depletion is found, the depth is smaller than what is seen by STM or by modeling the system with tight-binding (TB) parameters [7]. We suggest that one possible explanation for this is that the STM tip applies pressure on the sample decreasing the interlayer distance. We compared our results to the TB model proposed by Sachs et al. [9] that describes the behavior of the biggest cell of the moiré pattern (with a rotation angle between both lattices of 0º). This model considers only graphene, with a modulation term in the on-site energy, so that the BN part is not explicitly treated in the hamiltonian. This term describes the change in the potential felt by graphene due to BN lowering and rising depending on the local conformation of the moiré pattern. By considering this

TB model, we can simulate larger systems to explore the effect of disorder on the morié potential and the impact on mesoscopic transport. To represent impurities and structural dislocations we used Anderson disorder, that is added as a random uncorrelated change in the on-site energy. We show that the underlying moiré pattern has important consequences on the electronic properties.

References [1] M. Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer, E. D. Williams, Nano Letters, 7 (2007) 1643. [2] E. H. Hwang, S. Adam, S. Das Sarma, Phys. Rev. Lett., 98 (2007) 186806. [3] Y.-J. Kang, J. Kang, K. J. Chang, Phys. Rev. B, 78 (2008) 115404. [4] C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard, J. Hone, Nature Nanotech., 5 (2010) 722. [5] J. Xue, J. Sanchez-Yamagishi, D. Bulmash, P. Jacquod, A. Deshpande, K. Watanabe, T. Tanigushi, P. Jarillo-Herrero, B. J. LeRoy, Nature Material, 10 (2011) 282. [6] R. Decker, Y. Wang, V. W. Brar, W. Regan, H.-Z. Tsai, Q. Wu, W. Gannett, A. Zettl, M. F. Crommie, Nano Letters, 11 (2011) 2291. [7] M. Yankowitz, J. Xue, D. Cormode, J. D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, P. JarilloHerrero, P. Jacquod, B. J. LeRoy, Nature Physics, 8 (2012) 382. [8] J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón, D. Sánchez-Portal, J. Phys.: Condens. Matter, 14 (2002) 2745. [9] B. Sachs, T. O. Wehling, M. I. Katsnelson, A. I. Lichtenstein, Phys. Rev. B, 84 (2011) 195414. Figures

Figure 1. Construction of the moiré pattern with graphene lattice parameter a and BN lattice parameter a+δ. The rotation angle φ determines the size of the supercell.

Rational edge functionalization of zigzag graphene nanoribbons for band gap opening and induced aromaticity patterns. Francisco Martin-Martinez, Stijn Fias, Gregory Van Lier, Frank De Proft, Paul Geerlings Vrije Universiteit Brussel (VUB), Research Group General Chemistry (ALGC), Pleinlaan 2 B-1050, Brussels, Belgium fmartinm@vub.ac.be Tuning the band gap of graphene nanoribbons by chemical edge functionalization is a promising 1 approach towards future electronic devices based on graphene. The band gap is closely related to the 2 aromaticity distribution and therefore tailoring the aromaticity patterns is a rational way for controlling the band gap. In the present work, a way to open a band gap on zigzag graphene nanoribbons by 3 selective edge functionalization is shown. Based on Clarâ&#x20AC;&#x2122;s Sextet Theory, additional hydrogen at the edges, oxygen and tailored edges are suggested as possible ways for inducing aromaticity patterns and therefore opening the band gap. It demonstrates that rational tuning of the band gap can be performed, allowing the application of graphene nanoribbons in semiconducting devices. The electronic structure and the aromaticity distribution are studied using DFT calculations and through a series of delocalisation and geometry analysis methods, like the six-centre index (SCI) and the mean bond length (MBL) 4 geometry descriptor. References [1] D. Selli, M. Baldoni, A. Sgamellotti and F. Mercuri, Nanoscale, 4 (2012) 1350. [2] F.J. Martin-Martinez, S. Fias. G. Van Lier, F. De Proft, P. Geerlings, Chem. Eur. J., 18 (2012) 6183. [3] E. Clar, The Aromatic Sextet, Wiley, London, 1972. [4] P. Bultinck, R.Ponec, S. Van Damme, J Phys Org Chem 18, (2005) 706. [5] F.J. Martin-Martinez, S. Melchor, J.A. Dobado, Org. Lett. 10 (2008) 1991.

Gate-tunable Polarization Effect on Photocurrent at Graphene-Metal Interfaces Mathieu Massicotte, Klaas-Jan Tielrooij, Ivan Nikitskiy, Michela Badioli, Gabriele Navickaite and Frank Koppens ICFO - Institut de Ci茅ncies Fot贸niques, Mediterranean Technology Park, Castelldefels (Barcelona) 08860, Spain mathieu.massicotte@icfo.es

Abstract Graphene holds the potential to be a fast and efficient light-to-current converter thanks to its high carrier mobility, its strong carrier-carrier interaction [1] and the long lifetime of its hot carriers [2]. Although it has been clearly shown that hot carriers dominate the photoresponse in a graphene p-n junction [2], the photocurrent (PC) generation mechanism at graphene-metal (GM) interfaces is still debated. In particular, the observation of polarization dependent photocurrent at GM junctions has been interpreted as the result of photovoltaic photocurrent [3]. Here we study the effect of light polarization on the photocurrent at the GM interface and away from it. Several field-effect phototransistors made of mechanically exfoliated single- and bilayer were investigated under vacuum by scanning photocurrent microscopy. Spatial mapping of the photocurrent (Figure 1a) reveals strong light polarization dependence (~30% contrast) of the photocurrent at GM interfaces, with a maximum when the polarization is perpendicular to the interface (Figure 1b). Photocurrent measured away from the contact shows no polarization dependence, thus indicating the role of the metal contact. Interestingly, we find that the polarization contrast and even the photocurrent sign depend on the backgate voltage controlling the Fermi energy (Figure 2). These non-trivial effects provide new insight into photocurrent generation mechanism at GM junctions.

References [1] Tielrooij K. J., et al. Nature Physics, 2564 (2013) [2] Garbor N. M., et al. Science, 334, 6056 (2011) pp. 648-652 [3] Echtermeyer T, et al. Nature Com. 2, 458 (2011)

Functionalisation of Graphene Surfaces with Downstream Plasma Treatments

1,2

Niall McEvoy , Hugo Nolan , Nanjundan Ashok Kumar , Ehsan Rezvani , Toby Hallam , Georg S. Duesberg

1, 2*

1 - CRANN, Trinity College Dublin, Dublin 2, Ireland 2 - School of Chemistry, Trinity College Dublin, Dublin 2, Ireland nmcevoy@tcd.ie Abstract

Graphene has garnered much interest from the research community due to its unique physical, structural and electronic properties. It has been linked with applications in next generation electronics, sensors and energy storage to name but a few. Central to the realisation of these applications is control over the quality and surface chemistry of graphene. Plasma treatments allow for the introduction of controlled levels of functionalities onto surfaces without the need for wet chemical steps. This makes it a clean, green technique, compatible with industrial processes. Downstream plasmas generate a high density of ionized species, but induced surface damage is minimised due to the absence of an applied bias. Such treatments have previously been reported for adding functionalities to the surface of thin pyrolytic carbon films [1, 2] and for post transfer cleaning of graphene [3]. We report on an adjustable process for the functionalisation of graphene surfaces with a downstream plasma source [4]. The parameters of oxygen plasma treatments are modified such that oxygenated functionalities can be added to the surface of graphene films prepared by chemical vapour deposition in a controlled manner. The nature of induced defects is investigated thoroughly using Raman and x-ray photoelectron spectroscopy. A massive change in the surface properties is observed through the use of contact angle and electrochemical measurements. We propose the usage of such plasma treatments to facilitate the addition of further functional groups to the surface of graphene. The incorporation of nitrogen in to the graphene lattice by substitution of oxygenated functional groups is demonstrated outlining the validity of this approach for further functionalisation. We show that similar plasma treatments can be applied to other forms of graphene, including graphene oxide powder, allowing for dry functionalisation on a large scale. Preliminary studies indicate that such powders have potential applicability for solar cells and electrocatalysis.

References [1] G. P. Keeley, N. McEvoy, S. Kumar, N. Peltekis, M. Mausser and G. S. Duesberg, Electrochemistry communications, 8 (2010) 1034-1036 [2] N. McEvoy, N. Peltekis, S. Kumar, E. Rezvani, H. Nolan, G. P. Keeley, W. J. Blau, and G. S. Duesberg, Carbon, 3 (2012) 1216-1226 [3] N. Peltekis, S. Kumar, N. McEvoy, K. Lee, A. Weidlich and G. S. Duesberg, Carbon, 2 (2012), 395403 [3] N. McEvoy, H. Nolan, N. A. Kumar, T. Hallam, and G. S. Duesberg, Carbon, (2012), accepted for publication, DOI: 10.1016/j.carbon.2012.11.040

Figure 1: Left – Schematic of graphene plasma treatment, Right – contact angle measurements on as grown and plasma treated graphene samples outlining increased hydrophilicty following treatment.

Figure 2: Left – Raman spectra of pristine, oxygen functionalised and partially nitrogen functionalised graphene. Right – corresponding X-ray photoelectron spectra.

Fine tuning of graphene-metal adhesion by surface alloying 1,2

3,4

E.Miniussi , D. Alfè , M. Pozzo , S. Günther , P. Lacovig , S. Lizzit , R.Larciprete , B. Santos 6 6 6 1,2 Burgos , T.O. Menteş , A. Locatelli , and A. Baraldi * 1

Physics Department and CENMAT, University of Trieste, Via Valerio 2, I-34127 Trieste, Italy.

IOM-CNR, Laboratorio TASC, S.S. 14 Km 163.5, I-34149 Trieste, Italy.

Department of Earth Sciences, Department of Physics and Astronomy, TYC@UCL, and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom. 4

IOM-CNR, DEMOCRITOS National Simulation Centre, I-34100 Trieste, Italy.

Department Chemie, Tecnische Universität München, Lichtenbergststrasse 4, D-85748 München, Germany. 6

Sincrotrone Trieste S.C.p.A., Strada Statale 14 Km 163.5, 34149 Trieste, Italy.

CNR-Institute for Complex Systems, via Fosso del Cavaliere 100, I-00133 Roma, Italy.

*Correspondence to: alessandro.baraldi@elettra.trieste.it

Abstract Controlling the adhesion between graphene and the metal support requires a thorough understanding of the physical mechanisms responsible for the different degrees of interaction. This is a key step towards the establishment of graphene-based technologies and for the development of graphene heterostuctures in novel electrical and mechanical devices. It is well known, in fact, that the coupling between supported graphene (GR) and the substrate is responsible for a number of properties of GR-based materials, including the electromechanical properties, contact resistance, and ultrastrong adhesion as well as the electronic transport and heat dissipation in nanoelectronic devices. Several methods, such as the choice of the substrate (1,2), the intercalation of adspecies (2-6) or the creation of an oxide buffer layer (7), have been proven as a possible means of controlling the graphene-metal interaction, though not ad libitum. These approaches, in fact, do not enable to precisely control the graphene-substrate coupling strength and come with some significant drawbacks. Here we show that bimetallic surface alloying provides a viable route for governing the interaction between graphene and metal through the selective choice of the elemental composition of the surface alloy. This concept is illustrated by characterizing the properties of graphene on a model PtRu surface alloy on Ru(0001), with Pt concentrations ranging from 0 to 50%. The specific choice of these two metal relies on the fact that Pt and Ru stand out as two model examples of weakly and strongly interacting substrates, respectively. In fact, GR develops just a weak coupling to Pt, as reflected in the almost flat morphology of the C layer. On the other hand, a strong bonding has been observed for GR/Ru(0001), leading to a significant GR corrugation. Our study was conducted with a multidisciplinary approach, combining a range of experimental techniques –high energy resolution core level Photoemission Spectroscopy, Low Energy Electron Diffraction and Low Energy Electron Microscopy- and state-of-the-art DFT calculations Our results show that the progressive increase of the Pt content in the surface alloy leads to a gradual detachment of graphene from the substrate, which results from the modification of the carbon orbital hybridization promoted by Pt. Alloying is also found to affect the growth mode and the morphology of graphene, which is strongly corrugated on bare Ru but becomes flat at a Pt coverage of 50%. Our work is the proof of concept that the employment of binary surface alloys, which are used in many areas of materials science, can provide an unprecedented tool to selectively manipulate the adhesion between GR and the metal. The proposed method can be readily extended to a range supports, thus opening the way to a full tunability of the graphene-substrate interaction.

References [1] A.B. Preobrajenski, M.L. Ng, A.S. Vinogradov, N. M책rtensson, Phys. Rev. B 78, 073401 (2008). [2] E. Miniussi et al., Phys. Rev. Lett. 106, 216101 (2011). [3] C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, U. Starke, Phys. Rev. Lett. 103, 246804 (2009). [4] A. Bostwick et al., Science 328, 999 (2010). [5] P. Sutter, J.T. Sadowski, E.A. Sutter, J. A. Chem. Soc. 132, 8175 (2010). [6] A.M. Shikin, G.V. Prudnikova, V. K. Adamchuk, F. Moresco, K.H. Rieder, Phys. Rev. B 62, 13202 (2000). [7] S. Lizzit et al., Nano Lett., 12, 4503 (2012). [8] J. Wintterlin, M.-L. Bocquet, Surf. Sci. 603, 1841 (2009). .

Raman and AFM signature of flame-formed carbonaceous 2-D nano-flakes Patrizia Minutoloa, Alexander Santamariaa,b, Mario Commodoa, Gianluigi De Falcoc, Andrea D’Annac a

Istituto di Ricerche sulla Combusitone, P.le V. Tecchio 80, 80125, Napoli – Italy Institute of Chemistry, University of Antioquia, A.A. 1226, Medellín - Colombia c Dip. di Ingegneria Chimica, dei Materiali e della Produzione Industriale - Università Federico II, P.le Tecchio 80, 80125, Napoli, Italy anddanna@unina.it, minutolo@irc.cnr.it b

Abstract Flames and combustion devices operated in rich hydrocarbon fuel condition produce a large variety of carbonaceous compounds spanning from low and high molecular weight gas-phase polycyclic aromatic hydrocarbons (PAHs) to 2-D partially aromatic nanometric size compounds and solid soot particles. All these species present different molecular weights and sizes, from few nanometers, 2-3 nm, up to hundreds of nanometers, chemical and physical characteristics, optical and electronic properties. Both their amount and chemical characteristics are strongly related to the combustion conditions. Indeed, differences in the fuel chemical composition, as well as in the type of the combustion process, i.e. diffusive or premixed, laminar or turbulent, can result on different carbonaceous products. Moreover, other parameter such as temperature, residence time and pressure, can also be used as controller parameters in order to get specific carbon products. The proper choice of the “flame reactor” can, in fact, generate carbonaceous compound having different sizes, micro and nanostructure, light absorption and emission properties. These latter ones are related to the chemical and structural conformation of the formed nanoparticles as well as on their electronic properties, i.e. HOMO-LUMO levels position. The understanding of the mechanisms responsible for the formation, in rich fuel flames, of the broad set of carbon compounds, from planar PAH molecules to solid state 3-D carbonaceous nanostructures, has been the subject of numerous studies over the last decades [1Libro Capri]. Although these studies were driven by the necessity of more efficient combustion processes and by the necessity of reducing such compounds from the emission of combustion devices, they have lead to a wide knowledge on the chemical kinetic reactions governing the formation and growth of such species, as well as on modelling capabilities and experimental methods allowing to tailor the carbonaceous “by-products” properties by changing the flame synthesis parameters thus allowing a new synthesis method for specific carbon species. We have produced carbon compounds in laminar hydrocarbon premixed flames burning at atmospheric pressure. Flame products were sampled from the flame by means of a dilution probe operated with nitrogen and collected on-line on a quartz filter. The filters were then analysed by a confocal Raman microscope (Horiba XploRA with laser=532 nm). Morphological analyses were performed by Atomic Force Microscopy, AFM, (NT-MDT NTEGRA prima). To this aim, carbon compounds were collected by thermophoresis, inserting a cold substrate in the flame by means of a pneumatic actuator that assures fast sampling times (about 30 ms). The short sampling time allowed collecting isolated species; freshly cleaved mica was used as substrate to have an atomically flat background in AFM images. Flames operated with fuel/air mixture slightly richer than the stoichiometric value are blue colored and produce 2-D carbon structure. In fig. 1-left is reported a typical AFM image of an area of 500 nmx500nm, while the height profile across the green line is reported on the right, fig. 1-right. During the sampling time of 30 ms a large number of carbon nano-flakes is collected on the mica substrate. The height of such species is from 0.35 to about 1 nm, as indication of a carbon compound made of a monoatomic to a few carbon layers. The in-plane shape is roughly circular. The Raman spectrum of the carbon nano-flakes sampled from a blue-luminosity flame is reported in fig. 2 also, in fig. 3 the Raman bands of the first order and second order part of the spectrum are compared to the one measured in HOPG. Raman spectrum of flame carbon nano-flakes is characterized by strong signal due to disorder. A broad D band appear close to 1350 cm-1 while the G and D’ bands merge in one peak. A fitting procedure of this peak with two Lorentian lineshape show that the G peak is shifted to larger wavenumber respect to the G band of HOPG due to the nanosize of graphitic island [2] and the D’ band, at 1620 cm-1, is more intense than G. The strong D and D’ signals probably originate by the large amount of edges in our sample being the probed region composed by a many nano-flakes [3]. The Raman spectrum in the overtone and combination bands region add further information on the sample. The position of 2D band confirms the bi-dimensional nature of the flakes. In fact, the center of the band is significantly shifted

towards the lower frequency zone respect the signal of the 2D band from HOPG, closer to the position of a single layer graphene [2]. Nevertheless, differently from a pure graphene the band is much broader, and less intense. Furthermore, the appearance of relevant overtone of G and D’ lines and an evident G+D combination band indicate that beside the disorder due to edges, also lattice distortion can be present. References [1] Bockhorn H., D’Anna A., Sarofim A.F., Wang H. (Eds.), “Combustion Generated Fine Carbonaceus Particles” KIT Scientific Publishing, Karlsruhe, 2009. [2] Ferrari, A. Solid state communication, 143 (2007) 47-57 [3] Pimenta, M. A. Dresselhaus G., Dresselhaus M., S., Cancado, L., G., PCCP 9, (2007) 1276-1291 Figures

Fig 1 (Left) AFM image of carbon flakes, sampled from blue-luminosity flames, deposited on mica substrate, and (right) height profile along the vertical green nine over the two of the particles.

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Fig. 2 Raman spectrum of carbon flakes sampled from blue-luminosity flames.

Fig. 3. Raman spectrum of carbon flakes sampled from blue-luminosity flames and HOPG in the first order and second order regions.

In situ growth of graphene within SiC ceramics by spark plasma sintering P. Miranzoa, C. Ramíreza, B. Román-Mansoa, H. R. Gutiérrezc, M. Terronesd,e, C. Ocalb, M. I. Osendia, M. Belmontea a

Institute of Ceramics and Glass (ICV-CSIC) Campus Cantoblanco, 28049 Madrid, Spain. Institut de Ciència de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Spain. c Department of Physics & Astronomy, University of Louisville, Louisville, KY 40292, USA. d Department of Physics, The Pennsylvania State University, University Park, PA 16802, USA. e Research Center for Exotic Nanocarbons (JST), Shinshu University, Nagano 380-853, Japan.

pmiranzo@icv.csic.es Abstract Over the last few years, there has been a growing interest in the development of new ceramics with enhanced mechanical properties and novel electrical and thermal functionalities. The outstanding electronic and physico-chemical properties of graphene make it ideal filler in the fabrication of conducting and robust ceramic composites. In this context, recent studies on graphene/ceramic nanocomposites have demonstrated outstanding improvements in the mechanical and electrical properties of alumina and silicon nitride ceramics [1-3] by the introduction of graphene nanoplatelets (GNPs) or graphene oxide (GO). At present, this type of graphene/ceramic composites have only been produced by mixing dispersions of GNPs or GO with ceramic powders in solvents, followed by a densification at high temperatures. Unfortunately, these experiments may lead to both graphene agglomeration (forming aggregated flakes) and its structural degradation. Although silicon carbide (SiC) is one of the most appropriate ceramics for structural applications due to its exceptional thermomechanical properties at high temperatures [4], no work has been reported on the fabrication of SiC matrix composites containing graphene. It is also important to point out that the fast generation of largearea and homogenous epitaxial grown graphene on SiC single crystals by thermal annealing have been reported [5,6]. In this work, a novel single step approach for manufacturing electrically conducting and well dispersed graphene/SiC nanocomposites with enhanced mechanical and electrical properties is explored. Epitaxial graphene (EG) was in-situ grown within either α- or β-phase SiC ceramics during their densification by Spark Plasma Sintering (SPS). The in situ graphene growth mechanism is believed to be due to the simultaneous actions of the high temperature, the electric current passing through the graphite dies and specimen (Joule heating), and the partial vacuum, all of them involved in the SPS process. The presence of few- layer epitaxial graphene (FLG) located between the grain boundaries of the bulk SiC has been confirmed using Raman spectroscopy combined with high resolution transmission electron microscopy (HRTEM) and conductive-scanning force microscopy (C-SFM). Raman spectroscopy mappings on the SiC nanocomposites (Fig. 1, left side) showed the presence of about 4 vol. % graphite-like structures dispersed within the SiC matrix. These structures were also observed on the fracture surface of the specimens using SEM. HRTEM examination showed that few-layer graphene domains were generated at grain boundaries, the number of layers being related with the crystal orientation (Fig. 1), whereas conducting and topographic SFM studies established a clear correlation between those FLG and conducting regions in the composites [7]. The developed conducting graphene network significantly enhanced the electrical and mechanical performance of SiC, reaching electrical conductivity values as high as 1.02 x 102 S•m-1 and fracture toughness increases of 55%. The approach presented here offers unprecedented opportunities for the fast manufacturing of graphene/SiC nanocomposites precluding the handling of potentially hazardous nanostructures and it widens possible applications, including micro- or nano-electromechanical systems, brakes, microturbines or micro-rotors.

References [1] Fan Y, Wang L, Li J, Li J, Sun S, Chen F, Chen L, Jiang W. Carbon 48 (2010) 1743-1749 [2] Walker LS, Marotto VR, Rafiee MA, Koratkar N, Corral EL. ACS Nano (2011; 5: 3182-3190.

[3] Ramirez C, Figueiredo FM, Miranzo P, Poza P, Osendi MI. Graphene nanoplatelet/silicon nitride composites with high electrical conductivity. Carbon 2012; 50: 3607-3615. [4] Roewer G, Herzog U, Trommer K, Mueller E, Fruehauf S. Silicon carbide‐a survey of synthetic approaches, properties and applications. Struct Bond 2002; 101: 59-135. [5]. Emtsev KV, Bostwick A, Horn K, Jobst J, Kellogg GL, Ley L, McChesney JL, Ohta T, Reshanov SA, Röhrl J, Rotenberg E, Schmid AK, Waldmann D, Weber HB, Seyller T. Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nat Mater 2009; 8: 203-207. [6] Yannopoulos SN, Siokou A, Nasikas NK, Dracopoulos V, Ravani F, Papatheodorou GN. CO2-laserinduced growth of epitaxial graphene on 6H-SiC(0001). Adv Funct Mater 2012; 22: 113-120. [7] P. Miranzo, C. Ramírez, B. Román-Manso, H. R. Gutiérrez, M. Terrones, C. Ocal, M. I. Osendi, M. Belmonte. J. Eur. Ceram. Soc. (2013). In press (Ref. JECS_9068]

I2D/IG

Figure 1. FESEM micrograph of the fracture surface of a graphene/α-SiC composite where multilayered graphene flakes are shown bending along grain boundaries, HRTEM examination showing fewlayer graphene domains at different grain boundaries and Raman spectroscopy mappings on scanned areas of 10 μm x 10 μm.

Frictional mechanisms in few-layer graphene 1

1,2

M.C. Righi , M. Reguzzoni , A. Fasolino , Mattia Sacchi , Elisa Molinari

1,2

Istituto Nanoscienze, CNR - Consiglio Nazionale delle Ricerche, I-41125 Modena, Italy Dept of Physics, Informatics and Mathematics, University of Modena & Reggio, I-41125 Modena, Italy 3 Institute for Molecules and Materials, Radboud University Nijmegen, NL-6525 Nijmegen, NL Corresponding author: mcrighi@unimore.it

In micro- and nano-scale applications the thickness of a solid lubricant represents a very important factor to preserve relevant interactions (e.g. magnetic) between the sliding materials. Graphene --a revolutionary material for its known electronic and mechanical propertiesâ&#x20AC;&#x201D; has great potential also in this context as the thinnest solid lubricant. The fundamental mechanisms governing friction are however not yet clear, and important issues are still open e.g. concerning the role of multiple graphene layers [1]. We investigate the friction mechanisms in multilayer graphene films by calculating the potential energy surfaces (PES) from first-principles [2], and by simulating the motion of a model tip on the films by classical molecular dynamics [1]. From the ab-initio PES we derive an analytical expression the describes the interaction energy between two graphene layers vs their relative position. Thanks to its formal simplicity, the proposed model allows for an immediate interpretation of the interlayer binding and the potential corrugation. The latter plays a crucial role in determining the intrinsic resistance to interlayer sliding and controls the frictional behaviour under load (fig. 1). We show that the dominant mechanism in these ď °-bonded systems is the increase in Pauli repulsion with load, while the effect of van der Waals adhesion is negligible. To understand the role of N-layer graphene films, we evaluate both the PES modifications as a function of N [3] and the onset of mechanisms of energy dissipation due to interlayer motions during finite temperature simulations [1]. We find that a sliding tip on a supported few-layer film induces both out-ofplane (fig. 2) and in-plane deformations, which increase with the number of layers in the film. We elucidate a new frictional mechanism connected with shear layer motions.

[1] M. Reguzzoni, A. Fasolino, E. Molinari, M.C. Righi, J. Phys. Chem. C 16, 21104 (2012), and references therein. [2] M. Reguzzoni, A. Fasolino, E. Molinari, M.C. Righi, Phys. Rev. B 86, 245424 (2012). [3] M.C. Righi et al, to be published.

Fig. 1. PES function for bilayer graphene, represented in two dimensions for two different values of the interlayer separation z. The bilayer configurations corresponding to the PES stationary points are illustrated in the top part of the picture.

Fig. 2. Equilibrated structure of a 4-layer graphene film. The colour code indicates the deviation of the particle height from that of the layer center of mass. Blue corresponds to 0.4 Å and red to – 0.9 Å.

Probing the distribution of crystallographic charges in graphene membranes : electron holography measurements and DFT modeling 1

1,2

Vittorio Morandi , Luca Ortolani , Cristian Degli Esposti Boschi , Raffaello Mazzaro , 1 1 3 Giulio Paolo Veronese , Rita Rizzoli , Etienne Snoeck 1

CNR-IMM Section of Bologna, via Gobetti 101, I-40129 Bologna, Italy Chemistry Dept. ‘‘G. Ciamician’’, Bologna University, via Selmi 2, I-40126 Bologna, Italy 3 CNRS-CEMES, 29 Rue Jeanne Marvig, F-31055 Toulouse, France morandi@bo.imm.cnr.it

Abstract Graphene is a fascinating new material [1], and its peculiar properties hold promises for a great technological impact [2]. Around the Fermi energy, the band structure of graphene presents six conical points where the energy dispersion is perfectly linear in the momentum. For this reason, low energy excitations in graphene exhibit a charge conjugation between electrons and holes, and those carriers can be described as a 2D gas of effective massless Dirac fermions [3]. Unfortunately, upon stacking to form a Few-Graphenes-Crystal (FGC), the weak interlayer interaction could induce small valence charge redistribution in the crystal lattice, suppressing the linear dispersion in the band-structure [4]. In particular, in turbostratic FGCs, the interlayer charge redistribution, and hence the electronic structure of the crystal, depend on the rotation angle between the graphenes. For certain orientations, when the lattices are commensurable, graphenes electronically decouple, and the FGC behaves like an individual monolayer [5]. Using transmission electron holography [6], we investigated the redistribution of electronic crystal charges in a turbostratic FGC, showing that, when the lattice of adjacent layers becomes commensurable, each graphene within a turbostratic FGC contributes independently to the total phase shift of the electronic wave-front, with no visible interlayer effect, hence confirming that each layer is electronically decoupled [7]. In this paper we will show improved results achieved on Chemical Vapor Deposition (CVD) grown graphene membranes folded over itself (see Fig. 1), as reconstructed elsewhere [8], confirming the electronic decoupling of turbostratic commensurate stacked membranes (see Fig. 2). At the same time, the combination of the 3D reconstruction methodology reported in Ref. 8, together with the results reported here on the very same flake, opens interesting capabilities to a combined structural-electrical characterization of graphene membranes based on Transmission Electron Microscopy (TEM) interferometric techniques. Moreover, we will show that electronic density and internal potential energy experienced by an electron can be computed by an ab-initio approach based on Density Functional Theory with high accuracy on single and multiple graphene layers. The computed phase-shift values are in very good agreement with the ones measured by electron holography (see Fig. 3), strengthening the perspectives of the application of the technique, as well as of the computational approach, to more complicated and interesting systems, like functionalized graphene layers. References [1] K. Novoselov et al., Science, 306 (2004) 666. [2] A.K. Geim et al., Nat. Mater., 6, (2007) 183. [3] A.H.C. Neto et al., Rev. Mod. Phys., 81, (2009) 109. [4] S. Latil et al., Phys. Rev. Lett., 97, (2006) 036803. [5] J. Hass et al., Phys. Rev. Lett., 100, (2008) 125504. [6] H. Lichte et al., Ann. Rev. Mat. Res., 37, (2007) 539. [7] L. Ortolani et al., Carbon, 49, (2011) 1423. [8] L. Ortolani et al., Nano Lett. 12, (2012) 5207.

Figures

Figure 1: High resolution TEM image, obtained at 100 keV, showing a single CVD-grown graphene flake folded over itself, as reported in the scheme reported on the right (see Ref.8 for details). The two graphene lattice are rotated by 21.7, as shown by the FFT reported in the inset in the bottom left corner, making the stack turbostratic but commensurate.

Figure 2: Reconstructed phase map for the folded flake of Fig.1. On the right (top) is reported the phase profile of the region highlighted by the white box, showing a total phase shift induced by the two graphenes traversed by the electron beam of 0.11 ± 0.01 rad., therefore each graphene plane shift the electron phase by 0.055 rad., as shown by the scheme on the bottom right corner.

Figure 3: 3D representation (left) and contour map (right) of the integrated (along z-axis) calculated potential for a (single cell) single layer graphene. From the value of the integrated potential it is possible to calculate the phase shift for the given beam energy (Δφ = (π/λ)<IV>). The simulated internal potential should be convoluted to take into account the instrumental resolution. The calculated phase for each graphene layer is 0.06 rad.

Contribution (Poster)

Carbon-Conducting nanostructures to promote high density Si nanowires growth 1

A. Gómez-Martínez , F. Márquez , E. Elizalde , C. Morant

Dpto. de Física Aplicada, Universidad Autónoma de Madrid, Cantoblanco 28049 Madrid, Spain IPCAR, Institute for Physical Chemical Applied Research, School of Science and Technology, University of Turabo, 00778PR, USA c.morant@uam.es

Due to the critical demand for high-energy, compact Li-ion batteries are increasingly driven by technologic industry for the operation of multiple telecommunications, computer and entertainment devices, as well as clean transportation vehicles [1]. Nowadays, a point of great interest is to find an -1

alternative material with a Li storage capacity higher than that of graphite (372 mAh g ) that is the actual common commercial anode-material. Among the several candidates, silicon has much research -1

attention because it exhibits the highest known theoretical capacity, i.e., 4200 mAh g [2]. The ultrahigh capacity of silicon stems from the fact that in the electrochemical reaction 4.4 Li atoms are incorporated by each Si atom, compared with the insertion mechanism of graphite. However, incorporating a huge +

amount of Li inevitably causes gigantic changes in volume and an intense lattice strain; consequently, particles are pulverized and electrical connections are lost. Another limitation to the high performance of silicon comes from its semiconductive character that hinders the electrode redox process and electronic diffusion, compared with the good conductivity of the graphite. In this study, we present a simple procedure to avoid both difficulties that are present when silicon is used as anode material. By one hand, instead of bulk silicon, Si nanowires (SiNWs) are synthesized because of the 1-D nanostructures can better accommodate the changes in volume and mechanic strain thanks to their large elasticity. On the other hand, electronic conduction can be improved by the addition of different carbon-conductive nanostructures on the anode electrode. Herein, we report the use of carbon nanostructures as supporting material in the synthesis of SiNWs that promotes a high density of nanowires growth. We use crystalline Si (100) and other substrates, covered by the nanostructures and a thin film of gold. The silicon nanowires were synthesized by thermal treatment (900 ºC) at ambient conditions with a flux of hydrogen and argon during 30 min. The details of this method have recently been reported [3]. It is necessary for the nanowire growth that the substrate has an adequate roughness; in this way, during the thermal treatment, the gold film becomes nanoparticles which catalyze the nanowires formation. For this purpose, silica nanospheres were the first nanomaterial used as supporting for the synthesis of SiNWs. The silica nanospheres were fabricated by controlled hydrolysis of alkyl following the StöberFink-Bohn method [4]. The curved surface of these nanostructures makes gold film agglomerates in particles of nanometric size by superficial stresses and these nanoparticles catalyze the nanowires growth following a vapor-liquid-solid (VLS) process (Fig. 1). In light of these positive results, we introduced a new improvement, changing silica nanospheres by carbon spheres that were synthesized according to a process of dehydration and condensation of sugar [5]. Due to the low conductivity of the SiNWs, the introduction of a conductive material on the interface of silicon nanowires would allow the electron transfer during the charge/discharge cycles. From the viewpoint of electronic conduction, we moved in the direction to include graphene and carbon nanofibers as supporting material for the SiNWs synthesis process. These materials are massless charge carriers with high mobility. The results obtained were successful and a high density of SiNws was obtained by using these carbon-conducting materials onto the substrates (Fig. 2).

Contribution (Poster) A full characterization has been performed with the synthesized SiNWs, which includes Atomic Force Microscopy (AFM), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results of this work demonstrate the favorable role of inclusion carbon-conducting nanostructures in the substrates for SiNWs growth. We show the improvement of them for silicon nanowire synthesis, as well as the good electron conductivity of the electrode. References [1] M. Armand, J. M. Tarascon, Nature 451 (2008) 652–657. [2] C. K. Chan et al, Nanotechnol. 3 (2008) 31–35. [3] C. Morant, A. Gómez-Martínez, T. Campo, E. Elizalde, F. Márquez. “Procedure for obtaining silicon nanowires, in absence of silicon gas source, on different substrates”, patent (2013) [4] W. Stöber, A. Fink, E. Bohn, Journal of Colloid and Interface Science, 26 (1968) 62-69 [5] Huang, Qing Xuejie, Carbon 39 (2001) 2211-2214. Figure 1

FESEM images of carbon spheres after a thermal treatment at 900ºC by using 5 nm of Au as catalyst. a) Au nanoparticles formation in absence of H2 gas b) SiNWs synthesis from Au nanoparticles

Figure 2

FESEM images of SiNWs growth on Si substrates using carbon-conducting nanomaterials as supports: a) Carbon nanofibers b) Graphene

Proximity influence of adjacent fields on extended states: Schrödinger nonlocality as a probe for quantization of macroscopic quantities and application to novel magnetoelectronic devices

Konstantinos Moulopoulos University of Cyprus, Department of Physics, Nicosia, Cyprus cos@ucy.ac.cy Abstract: A novel effect in Planar Physics is reported, that shows that, under certain conditions, interesting physics may occur outside magnetic or (time-dependent) electric field regions: A proximity influence of a static magnetic field on adjacent regions in flat 2D space is shown to be a natural consequence of the Aharonov-Bohm effect combined with the nonexistence of magnetic monopoles. This influence is confirmed through a recent theory[1,2] that goes beyond the standard Dirac phase factor (and that incorporates wavefunction-phase-nonlocalities) and affects numerous results in the literature on extended (and open) arrangements with inhomogeneous magnetic fields: under certain conditions, there seems to be a gauge-ambiguity remaining, that has been overlooked in all previous works. The deep origin of this annoying feature is explained and it is shown that it can be removed when outside (remote) fluxes are properly quantized. This theory therefore suggests natural ways to eliminate the artificial effect for confined systems (closed manifolds), and it has a direct generalization even to cases with effective magnetic monopoles present, leading to quantization of macroscopic quantities (response functions) in a wide range of systems of current interest. Examples include application of such phase-nonlocality (i) to a spherical geometry, that leads to the standard Dirac quantization of magnetic monopoles without further topological or gauge considerations, (ii) to a cylindrical configuration, by additionally invoking Axion Electrodynamics, that naturally leads to fractional quantization of dyons (the Witten effect), as well as quantization of “Witten current”, leading in turn to 2 the quantization of Hall conductance, either (a) in whole, or (b) in half integral units of e /h (corresponding (a) to conventional Quantum Hall Effect systems, and (b) to the recently predicted exotic magnetoelectric phenomena in time-reversal-symmetric topological insulators, respectively). A similar consideration with adjacent electric fields (that vary with time) leads to the possibility of manufacturing of interesting quantum devices (that properly utilize the proximity influence to induce Integral Quantum Hall Effect and other topological phenomena in novel time-dependent ways from outside the system). For tdependent fields, the time-derivative of the above phase-nonlocalities is shown to be directly related to very recent considerations of Berry and Shukla[3] on “curl forces” that are spatially confined in classical systems, giving simultaneously their quantum generalization (or quantization). Finally, a combination of proximity influence with proper Lorentz boosts can also induce Aharonov-Casher edge states, thereby inducing spin-physics such as Quantum Spin Hall Effect, starting from purely orbital considerations.

References [1] K. Moulopoulos, Journal of Modern Physics, 2 (2011) 1250-1271. [2] K. Moulopoulos, Journal of Physics A, 43 (2010) Art. No.354019. [3] M.V. Berry and P. Shukla, Journal of Physics A, 45 Art. No.305201.

Electroactive Carbon Nanotube and Graphene-based Materials: Processing into Macroscopic Electrode Materials of Different Shapes Edgar Muñoz, J. David Núñez, Ana M. Benito and Wolfgang K. Maser Instituto de Carboquímica ICB-CSIC, C/Miguel Luesma Castán 4, 50018 Zaragoza, Spain wmaser@icb.csic.es

Abstract Carbon nanotubes and graphene are fundamental nanoscale objects exhibiting a series of exceptional physical and chemical properties. Being lightweight, mechanically and thermally robust of high environmental stability, electrically conducting, and having a very high surface, makes them promising candidates for the development of novel electroactive electrode materials of special interest for applications in electrochemical sensing, energy storage and artificial muscles. Of uppermost interest for this purpose is their assembly into different macroscopic shapes ranging from thin coatings towards fibers. We here present our latest advances in the processing of carbon nanotubes [1] and graphene [2, 3] and respective composites using intrinsically conducting polymers [4]. Focus lays on the development of stable and homogeneous dispersions and their processing into different macroscopic forms ranging from supported films and coatings to fibers and free-standing paper-like materials thus underlining the wide range of potential technological applications for these functional materials References [1] P. Jimenez, P. Castell, R. Sainz, A. Ansón, M.T. Martínez, J. Phys. Chem. B 114 (2010) 1579 [2] C. Vallés, J.D. Núñez, A.M. Benito, W.K. Maser, Carbon 50 (2011) 835. [3] R. Hernandez, J. Riu, J. Boback, C. Vallés, P. Jimenez, A.M. Benito, W.K. Maser, F. Xavier Rius, J. Phys. Chem. C 116 (2012), 22570 [4] C. Vallés, P. Jiménez, E. Muñoz, A.M. Benito, W.K. Maser, J. Phys. Chem. C 115 (2011) 10468

Acknowledgements: Financial support from Spanish Ministry MINECO under project MAT2010-15026, CSIC under project 201080E124, and the Government of Aragon and the European Social Fund under project DGA-FSE-T66 CNN is acknowledged.

Figures:

Figure 1: Polianiline Composite Dispersions: a) Emeraldine base with CNTs, b) Emeraldine Salt with reduced graphene oxide (RGO), c) Emeraldine Base with graphene oxide (GO).

Figure 2: Processing of polyaniline (emeraldine base)-CNT/RGO dispersions into b) coatings, c) freestanding films and d) fibers

Figure 3: Processing of CNT into free-standing paper-like materials

Figure 4: Processing of GO paper-like material into flexible conductive RGO paper-like material.

Tunneling Current through Vertical Heterostructures Composed of Graphene and Atomically Thin MoS2 Insulators 1

Nojoon Myoung , Kyungchul Seo , Seung Joo Lee , Gukhyung Ihm

1 Department of Physics, Chungnam National University, Daejeon 305-764, Korea 2 Quantum-functional Semiconductor Research Center, Dongguk University, Seoul 100-715, Korea toughnj@cnu.ac.kr Abstract Graphene has been considered to be a promising material for future electronics due to its extraordinary properties such as high carrier mobility[1], thermal conductivity[2], and strong break strength[3]. Although the extremely high electrical conductivity makes graphene a candidate for replacing silicon-based electronics, Klein tunneling causes that electrical transport of Dirac fermions is insensitive to electrostatic potentials, resulting low current on/off ratio of graphene based field-effecttransistors[4,5,6]. This is a serious problem for the prospect of graphene-based electronics. In order to increase the current on/off ratio, there have been many attempts to open a band gap by studying graphene nanoribbons (GNRs)[7,8,9], mechanically strained graphene[10], and bilayer graphene[11]. However, it has been revealed that, in those case, the electronic quality tends to be reduced compared to pristine graphene. Therefore, it is worthwhile to obtain high current on/off ratio in graphene-based devices without the reduction in the sample quality of graphene. Very recently, increasing interest has been focused on an alternative graphene device structure by using quantum tunneling. In a graphene/silicon junction, large current on/off ratio was achieved by controlling Schottky barrer at the interface[12]. However, for graphene deposited on silicon substrate, carrier mobility is expected to decrease because of carrier inhomogeneity induced by the substrate[13]. Meanwhile, L. Britnell et al. reported the possibility of a graphene field-effect-transistor based on vertical heterostructures with atomically thin insulating barriers, hexagonal boron nitride (hBN) and molybdenum disulfide (MoS2)[14]. hBN has gained burgeoning interest as a material for use in graphene deiveces because the encapsulation of graphene by hBN maintains the high electronic quality of pristine graphene[15]. In spite of this advantage, the large band of hBN (~5.97 eV[16]) causes an insufficient current on/off ratio. The larger on/off ratio was observed in graphene heterostructures with MoS2, instead of hBN, owing to its smaller bandgap compared to hBN. Therfore, alongside the experimental observation of the significant on/off ratio in the graphene heterostructure with MoS2, it is natural to investigate possible functional devices utilizing its advantages for electronic applications. Herein, calculations of the tunneling current density for graphene heterostructures with MoS2 are presented. The purpose of this study is not only to enhance the performances of existing graphene fieldeffect-transistors based on the vertical heterostructure[14] with MoS2 but also to add utilities to them. Our proposed structures consists of an atomically thin MoS2 layer sandwiched by two graphene sheets as shown in Fig. 1. The MoS2 layer in the heterostructure becomes a tunneling barrier for Dirac fermions in a graphene sheet, and both graphene sheets play the role of high-quality source and drain electrodes. Dirac fermions experience the direct bandgap near K-valley of the MoS2 because of the momentum conservation in the lateral plane, neglecting electron-phonon scattering processes. The transmission probability through the barrier is calculated by using WKB approximation; ,

(1)

Where is the thickness of the barrier and is the potential energy of the barrier as a function of . This approximation is valid for the direct tunneling regime, i.e., has to be less than . As Britnell et.al established[14], the tunnel barrier, , is used, where is the barrier height for incident Dirac fermions. Based on it, the tunneling current through the MoS2 insulating barrier can be obtained by , (2) where is the unit of current density with electric charge of carriers, , and the characteristic length of the system . Here, and are density of states of graphene and the FermiDirac distribution for source ( ) and drain ( ) graphene electrodes on both sides of the MoS2 layer, respectively, as shown in Fig. 1(a) and (b). The equilibrium chemical potential is defined as . Without a bias voltage, , applied between the source and drain graphene electrodes, tunneling current from Eq. (2) must be zero. The novel utility of our heterostructures, which is found in this study, is threefold. First, the distinct asymmetric current-voltage characteristics of graphene/MoS2/graphene implies that it could be used as a field-effect-diode which is operated by a gate voltage. Second, it is expected that the tunneling current exhibits interesting divergent behavior for a graphene/MoS2/GNR heterostructure. This result has great

potential for use of the divergent current peaks. Furthermore, the existent of magnetism in few-layer MoS2[17,18,19] leads to spin-polarized current in the graphene heterostructures. In this case, the graphene heterostructure could be a perfect spin-filter only for holes with the electron-hole asymmetric spin-splitting of magnetic MoS2[20]. Such tunneling phenomena may offer more potential applications in graphene-based electronics. References [1] K. S. Novoselov, et.al, Nature (London), 438 (2005) 197. [2] S. Ghosh, et.al, Nat. Mater. 9 (2010) 555. [3] I. Frank, et.al, J. Vac. Sci. Technol. 25 (2007) 2558. [4] P. Avouris, et.al, Nat. Nanotechnol. 2 (2007) 605. [5] F. Schweirz, et.al, Nat. Nanotechnol. 5 (2010) 487. [6] A. K. Geim, Science. 324 (2009) 1530. [7] V. Barone, et.al, Nano. Lett. 6 (2006) 2748. [8] Y. Son, et.al, Nature (London) 444 (2006) 347. [9] L. Brey, et.al, Phys. Rev. B 73 (2006) 235411. [10] G. Gui, et.al, Phys. Rev. B 78 (2008) 075435. [11] Y. Zhang, et.al, Nature (London) 459 (2009) 820. [12] H. Yang, et.al, Science. 336 (2012) 1140. [13] J. Sabio, et.al, Phys. Rev. B 77 (2008) 195409. [14] L. Britnell, et.al, Science 335 (2012) 947. [15] C. R. Dean, et.al, Nat. Nanotechnol. 5 (2010) 722. [16] J. Zhao, et.al, J. Mater. Chem. 22 (2012) 9343. [17] Y. Li, et.al, J. Am. Chem. Soc. 130 (2008) 16739. [18] C. Ataca, et.al, Phys. Chem. C 115 (2011) 3934. [19] Z. Wang, et.al, J. Am. Chem. Soc. 132 (2010) 13840 [20] E. S. Kadantsev, et.al, Solid State Commun. 152 (2012) 909.. Figures

Fig. 1 Energetic diagrams of the systems considered in this study; (a) graphene/MoS2/graphene and (b) graphene/MoS2/AGNR. (c) Schematic diagram of the model fabrication.

Identification of Functional Groups in Brodie Graphite Oxide at Different Degrees of Oxidation 1

1,2

O. Papaianina , M. Savoskin , A. Vdovichenko , R. Mysyk , M. Rodygin , I. Nosyrev , O. Abakumov , 2 2 O. Bondarchuk , T. Rojo 1

Litvinenko Institute of Physical Organic and Coal Chemistry, NAS of Ukraine, R.Luxemburg 70, 83114, Donetsk, Ukraine 2 CIC ENERGIGUNE, Parque Tecnológico de Álava, Albert Einstein 48, ED.CIC, 01510 Miñano, Spain rmysyk@cicenergigune.com

Abstract Graphite oxide (GO) is a non-stoichiometric material with a layered structure obtained by strong oxidation of graphite through the Brodie or Hammers methods, and is an indispensable intermediate product in the most common methods for preparing graphene [1]. In the known structural models of GO, the surface functionality is believed to be composed of hydroxyl, epoxy, and carboxyl groups and, less commonly, also quinoid groups directly bound to graphene layers. However, GO is known to exhibit features such as high oxidative strength and explosive decomposition at 150-200 0С, which cannot be explained if only those functional groups are considered [2]. To provide better insight into the functionality of graphite oxide, we synthesized GO of different oxidation degrees by varying the amounts of KClO3 according to the modified Brodie method [3]. The total amount of hydroperoxy and peroxy groups was evaluated on the basis of the iodine released by graphite 0 oxide during 24 h from the solution of potassium iodide and diluted sulfuric acid at 60 С. The amount of hydroperoxy groups was determined similarly but using NaI/propan-2-ol, and water was measured using a Karl Fischer titration. 13C solid-state NMR experiments were conducted using a Bruker Avance II 400 instrument (13C 100 MHz, 1H 400 MHz). The samples were packed in 4 mm ZrO2 rotors and rotated at a MAS rate of 5-12 kHz. 13C cross-polarization magic-angle spinning NMR and the direct pulse method were employed. FTIR/ATR spectra were acquired using Bruker Vertex 70 instrument. Survey and deconvoluted high resolution C1s and O1s XPS spectra were recorded using PHOIBOS 150 analyzer (SPECS) and monochromated Al Kα X-ray source. The combined data analysis of 13C solid-state NMR spectroscopy, FTIR, XPS and specific chemical tests has led us to the conclusion that a major fraction of oxygen in Brodie GO is contained in hydroxyl, hydroperoxy and peroxy groups. The quantitative analysis of GO at different degrees of oxidation allowed us to propose two stages in the formation of GO, the first being the hydroxylation of graphene layers with 4-6 hydroxyl groups per coronene fragment (C24) while the second is the build-up of hydroxyl, hydroperoxy and peroxy groups simultaneously involving a significant aromaticity loss in graphene layers. A remarkable result is that the presence of epoxy groups was not evidenced in Brodie GO whereas they are believed to be an abundant part of oxygencontaining groups in widely accepted GO model structures. Finally, a model for Brodie GO (see Figure 1) is suggested on the basis of the collective analysis of all the physical and chemical data. References [1] Novoselov K.S., Geim A.K., Morozov S.V. et al., Science, 306 (2004) 666. [2] Rodríguez A.M., Jiménez P.S.V., Carbon, 24 (1986) 163. [3] Brodie B.C., Phil. Trans. R. Soc. Lond., 149 (1859) 249.

Figures OH O

HO OH

OH HO

HO O

HO OH

OH O

OHO

HO OH O HO HO

OH OH

O HO

HO OH HO

OH HO

O HO

HO OH

HO OH HO

O OH O

HO OH

O O

OH HO

OH O

Figure 1. The suggested structure of a single-layer fragment in Brodie graphite oxide

Functionalization of Epitaxial Graphene by gold intercalation M.N. Nair, T. Jiang, M. Cranney, F. Vonau, D. Aubel, M.L. Bocquet and L.Simon. Institut de Sciences des Matériaux de Mulhouse LRC 7228-CNRS, 4, rue des frères Lumière, 68093 Mulhouse, France ENS Lyon, Laboratoire de Chimie, UMR 5182 CNRS 46, Allée d'Italie 69364 Lyon CEDEX 07, France contact@maya.narayanan-nair@uha.fr

The epitaxial graphene formed on silicon carbide substrate is obtained by the annealing of the substrate at temperatures up to 1200°C. On the silicon terminated face of the hexagonal SiC(0001), it is formed a graphene layer partially covalently bonded to the substrate called buffer layer (BL) above which the actual graphene layer which shows the expected linear dispersion is formed. Epitaxial Graphene (EG) exhibits n type doping which is induced by the substrate. Inspired by the works of I. Gierz et. al who proposed a simple way to shift Fermi level and induce p type doping by deposition of gold atoms on top of graphene [1], we have performed detailed Scanning Tunneling Microscopy studies of the deposition of gold atoms under ultra_high vacuum conditions and revealed that gold atoms can intercalate between the buffer layer and monolayer graphene in different forms depending on the specific preparation procedure. An inhomogeneous intercalation of gold atoms (DP) or continuous atomic thin film (FP) was first observed [2] then more exotic forms of intercalation such as stripes (SP) as shown in figure below, were also recently observed. Using Fourier transform scanning tunnelling spectroscopy [3], confirmed by Angle-resolved photoemission spectroscopy measurements [4], we have shown that the band structure of the upper monolayer graphene is modified by the inhomogeneous intercalation. Despite the intercalation process, the relativistic character of the quasiparticles is preserved and moreover showed a higher Fermi velocity than that for pristine graphene. The band structure is modified around the Van Hove singularity which shows a large extension [4]. With the help of periodic DFT calculations we discussed the position of the intercalated gold atoms between the ML and BL graphene (or under the BL) and showed the expected STM images and the DOS modifications.

References: [1] I. Gierz et al, Nano Letters, 8 (2008) 4603 [2] B. Premlal et al, APL, 94 (2009) 263115 [3] M. Cranney et al, EPL, 91 (2010) 66004 [4] M.N.Nair et al, Phys.Rev.B, 85 (2012) 245421

Figures: Figure shows the STM images of three different phases: FP (Film phase), DP(Diluted phase) and SP( Stripes phase)

Bottom-up Solution Synthesis of Long Graphene Nanoribbons with High Structural Definition Akimitsu Narita, Xinliang Feng, Klaus Müllen Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz, Germany narita@mpip-mainz.mpg.de Graphene nanoribbons (GNRs), namely nano-stripes of graphene, are attracting increasing attention as 1 highly promising candidates for new generation semiconductor materials. Theoretical and experimental studies have revealed that quantum confinement and edge effects impart GNRs with semiconducting properties, i.e. with a finite bandgap. The magnitude of the bandgap depends critically on the width and 2 edge structures. While top-down methods such as lithographical patterning of graphene and unzipping 3,4 of carbon nanotubes cannot produce structurally defined GNRs, especially resulting in undefined edge structures, we have developed a bottom-up synthetic approach via intramolecular cyclodehydrogenation, namely “graphitization” and “planarization”, of three-dimensional polyphenylene 5,6,7 precursors, which allowed the fabrication of GNRs with a variety of highly defined lateral structures. However, it has been challenging to synthesize well-extended (>100 nm) GNRs with high structural definition. In this study we employed Diels-Alder polymerization instead of previously used Suzuki and Yamamoto polymerization for synthesizing the polyphenylene precursors of GNRs, and achieved unprecedentedly high weight-average molecular weight (Mw) of up to 600000 g/mol based on size exclusion chromatography analysis. This Mw value corresponds to the longitudinal length of as long as ca. 600 nm for the resulting GNR 1 with lateral width of ~1 nm (Figure 1). Further, long alkyl chains densely installed on the periphery rendered the GNRs dispersible in standard organic solvents such as tetrahydrofuran and chlorobenzene, allowing characterizations in dispersions as well as solution processing. Characterization by infrared, Raman, and UV–vis absorption spectroscopies as well as investigation of model systems proved the efficiency of the “graphitization” and homogeneity of the GNRs (Figure 2). The optical bandgap of GNR 1 was reveled to be 1.88 eV based on the absorption edge, which was in good agreement with the estimated bandgap of 2.04 eV obtained by density 8 functional theory (DFT) calculations. Moreover, scanning probe microscope demonstrated the formation of neatly organized self-assembled monolayers on HOPG, indicating high solution processability of GNR 1. Applying the same synthetic strategy, we have also fabricated laterally extended GNR 2 with the width of ~2 nm (Figure 1). The efficient formation of GNR 2 was corroborated by infrared, Raman, and UV–vis absorption spectroscopies in the same manner as the characterization of GNR 1. The dodecyl chains at the peripheral positions imparted slight dispersibility to GNR 2, which allowed spectroscopic analyses in dispersion. Notably, the UV–vis absorption spectrum showed broad absorbance extending to the near infrared region with the optical bandgap of as low as 1.24 eV. This 8 value was consistent with the DFT-calculated bandgap of 1.18 eV, indicating high structural identity of GNR 2. These results further demonstrated the controllability of the bandgap of GNRs by changing the lateral width. Such structurally defined and solution processable GNRs with open and controllable bandgap are highly promising candidates for the application in next-generation optoelectronic devices, including field-effect transistors and solar cells. References [1] X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science 319 (2008) 1229. [2] M. Han, B. Özyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett. 98 (2007) 206805. [3] D. V. Kosynkin, A. L. Higginbotham, A. Sinitskii, J. R. Lomeda, A. Dimiev, B. K. Price, J. M. Tour, Nature 458 (2009) 872. [4] L. Jiao, L. Zhang, X. Wang, G. Diankov, H. Dai, Nature 458 (2009) 877. [5] J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. Feng, K. Müllen, R. Fasel, Nature 466 (2010) 470. [6] L. Chen, Y. Hernandez, X. Feng, K. Müllen, Angew. Chem., Int. Ed. 51 (2012) 7640. [7] M. G. Schwab, A. Narita, Y. Hernandez, T. Balandina, K. S. Mali, S. De Feyter, X. Feng, K. Müllen, J. Am. Chem. Soc. 134 (2012) 18169. [8] S. Osella, A. Narita, M. G. Schwab, Y. Hernandez, X. L. Feng, K. Müllen, D. Beljonne, ACS Nano 6 (2012) 5539.

Figures

C12H25

1.13 nm 0.69 nm C12H25 C12H25

C12H25

C12H25 2

1.98 nm 1.54 nm C12H25 C12H25

C12H25

Figure 1. Chemical structures of GNRs 1 and 2.

0.5

1319 1593

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2914 3190 2639

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2 400

Absorbance (a.u.)

Intensity (a.u.)

550

1200 2000 2800 Wave number (cm–1)

3600

400 500 600 700 800 900 10001100 Wave length (nm)

Figure 2. (a) Raman spectrum of GNR 1 measured at 488 nm on a powder sample with laser power below 1 mW. (b) UV–vis absorption spectrum of GNR 1 in NMP.

Electrical detection of spin precession in high-quality freely-suspended graphene devices Neumann, Ingmar

1, 2

; Van De Vondel, Joris ; Bridoux, German ; Costache, Marius V. ; Valenzuela, 1, 3 Sergio O.

1. Catalan Institute of Nanotechnology (ICN-CIN2), Bellaterra, Barcelona, Spain. 2. Universitat Autònoma de Barcelona, Bellaterra, Barcelona, Spain. 3. Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Barcelona, Spain.

Abstract We achieve spin injection and detection in freely-suspended graphene using cobalt electrodes and a nonlocal spin-valve geometry [1]. The devices are fabricated with a single electron-beam-resist (PMMA) process that minimizes both the fabrication steps and the number of (aggressive) chemicals used, greatly reducing contamination and increasing the yield of high-quality, mechanically stable devices. As4 -1 -1 grown devices can present mobilities exceeding 10 cm2 V s at room temperature and, because the contacts deposited on graphene are only exposed to acetone and isopropanol, the method is compatible with almost any contacting material. We study spin accumulation and spin precession in our nonlocal spin valves with large spin accumulation signals of the order of 10 Ω. Fitting of Hanle spin precession data (Figure) in bilayer and multilayer graphene yields a spin relaxation time of ~ 125-250 ps and a spin diffusion length of 1.7-1.9 µm at room temperature.

References [1] Neumann, Ingmar; Van De Vondel, Joris; Bridoux, German; Costache, Marius V.; Alzina Francesc; Sotomayor Torres, Clivia M.; Valenzuela, Sergio O., Small, 9 (2013) 156–160

Figures

Hybrid graphene–quantum dot phototransistors with ultrahigh gain Ivan Nikitskiy, Louis Gaudreau, Michela Badioli, Johann Osmond, Maria Bernechea, F. Pelayo Garcia de Arquer, Fabio Gatti, Gerasimos Konstantatos and Frank H. L. Koppens ICFO – The Institute of Photonic Sciences, Av. Carl Friedrich Gauss 3, 08860 Castelldefels (Barcelona), Spain ivan.nikitskiy@icfo.es Abstract Graphene is an attractive material for optoelectronics and photodetection applications because it offers a broad spectral bandwidth and fast response times. However, weak light absorption and the absence of a gain mechanism that can generate multiple charge carriers from one incident photon have limited the responsivity of graphene -based photodetectors. We have developed a novel hybrid graphene–quantum dot phototransistor that exhibits ultrahigh photodetection gain and high quantum efficiency, enabling high-sensitivity and gate-tunable photodetection.[1] The key functionality of this light-activated transistor is provided by a layer of strongly light-absorbing and spectrally tunable colloidal quantum dots, from which photogenerated charges can transfer to graphene, while oppositely charged carriers remain trapped in the quantum-dot layer. The main feature of the device is its ultrahigh gain, which originates from the high carrier mobility of the graphene and the recirculation of charge carriers during the lifetime of the carriers that remain trapped in the PbS quantum dots. The unique electronic properties of graphene offer a gate-tunable carrier density and polarity that enable us to tune the sensitivity and operating speed of the detector. We exploit this to maximize the photoconductive gain or to fully reduce it to zero, which is useful for pixelated imaging applications, where the implementation of nanoscale local gates enables a locally tunable photoresponse. References [1] Gerasimos Konstantatos, Michela Badioli, Louis Gaudreau, Johann Osmond, Maria Bernechea, F. Pelayo Garcia de Arquer, Fabio Gatti and Frank H. L. Koppens, Nature Nanotechnology, 7 (2012) 363– 368.

Room temperature Plasma-Assisted Simultaneous Reduction and Nitrogen Doping of Graphene Oxide Nanosheets for Energy Applications 1,2

1,2

Hugo Nolan , Niall McEvoy , Nanjundan Ashok Kumar , Ehsan Rezvani , Richard L. Doyle , Michael 2 1,2* E. G. Lyons , Georg S. Duesberg 1: CRANN, Trinity College Dublin, Dublin 2, Ireland 2: School of Chemistry, Trinity College Dublin, Dublin 2, Ireland nolanhc@tcd.ie Abstract Recent activity in the research community has seen much interest in graphene and related materials. This material, with its unique physical, chemical, and electronic properties has seen many studies conducted with potential applications in sensing, electronics and energy storage and conversion. Of significant importance to the realisation of these applications and devices is the cheap production of large scale quantities of graphene material. The chemical doping of graphene with heteroatoms has been shown to enhance the performance of graphene in many of these applications. In particular, nitrogen-doping of graphene has been widely demonstrated to improve its performance in a range of applications such as energy conversion and storage and sensing. This study presents an environmentally benign and scalable route for the production of gram scale quantities of nitrogen-doped graphene using a downstream microwave plasma source. Simultaneous reduction and doping of graphene oxide is achieved and the process negates the need for high temperatures and toxic solvents associated with existing methods. Material characterisation, including XPS, FTIR and Raman spectroscopy, demonstrates significant reduction in oxygen content of the parent material with a nitrogen doping level up to 5.8 at. %. This gas-phase, room temperature process is completely dry and, thus, minimizes re-aggregation of graphene flakes which is typically associated with liquid phase reduction methods. Nitrogen-doped graphene has been proposed as an alternative to expensive platinum/carbon materials used as electrode materials for oxygen reduction in hydrogen fuel 1 cells . Here, preliminary results are presented which demonstrate the potential of our nitrogen-doped graphene in this application. Results indicate that the oxygen reduction mechanism proceeds via a four electron pathway; the most energetically favourable route towards oxygen reduction.

References [1] L. Qu, Y. Liu, J.-B. Baek, and L. Dai, ACS Nano, 4 (2010), 1321â&#x20AC;&#x201C;1326 [2] N. McEvoy, H. Nolan, A. K. Nanjundan, T. Hallam, and G. S. Duesberg, Carbon,(2012), In press, DOI: 10.1016/j.carbon.2012.11.040

Figures

Figure 1: (a) XPS data showing the significant reduction in oxygen content after exposing graphene oxide to the plasma treatment; as evidenced by the smaller oxygen 1s peak for N-doped graphene. Also of note is the introduction of the characteristic nitrogen 1s peak in the doped material. The indium peaks are due to sample mounting procedures and may be ignored. (b) Cyclic voltammograms for N-doped graphene employed as the working electrode in a three electrode set-up in both N2- saturated and O2-saturated 1M NaOH. (c) Linear sweep voltammograms of the same material measured using a rotating ring disc electrode system. Currents associated with the reduction of oxygen (black) and oxidation of hydrogen (red) are shown; indicating a four electron oxygen reduction mechanism.

Processing of Graphene Oxide – Carbon Nanotubes Hybrids for Supercapacitor Electrodes 1,3

J.D. Núñez , A.M. Benito , A.L.M. Reddy , T.N. Narayanan , P.M. Ajayan , W.K. Maser 1

Instituto de Carboquímica ICB-CSIC, Department of Chemical Processes and Nanotechnology Zaragoza, Spain

Rice University, Department of Mechanical Engineering & Materials Science, Houston, Texas 3

Centro de Estudios Avanzados de Cuba, La Habana, Cuba dnunez@icb.csic.es

Abstract Graphene oxide (GO) is currently the most prominent precursor for mass production of graphene towards feasible industrial applications. GO has a lot of potential due to its large area of functional groups, mechanical strength, low-cost synthesis and simply processing into freestanding electrodes [1]. However GO is actually a non-conducting material, thus additives or partial chemical-thermal reduction is needed to achieve reliable electrochemical sensing and robust energy storage materials. Lately, it was assumed that mixture of carbon nanotubes (CNT) and graphene materials leads to synergism for enhanced electrochemical hybrids, which may open a wide range of nanomaterial architectures with superior performance [2,3]. However, wisely control of nanostructure assembly during processing seems to be key to improve its electrochemical properties. In this work, pure GO and CNT membranes along with three routes of fabrication of buckypapers hybrids (GOBucky) were obtained by vacuum assisted filtration using different dispersion treatments: All samples were structural and physico-chemical extensively characterized by TEM, FESEM, XRD, Raman, XPS, TGA, and BET, Fig. 1. Thickness, GO to CNT ratio, and gentle temperature reduction treatment are critical parameters to achieve higher specific capacitance. Direct correlation between electrochemical performance (Fig. 2) and type of processing applied was found, which could increase up to one order of magnitude higher after partial reduction treatment. Finally, hybrids’ charge storage improvement is explained taking into account nanostructure assembly, dispersion stability, specific superficial area and conductivity for each type of processing route.

References [1] Jian Li, Xiaoqian Cheng, Alexey Shashurin, Michael Keidar, Graphene, 1, 2012, 1-13 [2] Zhen-Dong Huang et al, Carbon, 50, 2012, 4239–4251 [3] Sun Hwa Lee, Duck Hyun Lee, Won Jun Lee, Sang Ouk Kim, Adv. Funct. Mater, XX, 2011, 1–17

Figures Fig. 1. Buckypapers hybrids (GOBucky) obtained by vacuum assisted filtration using different dispersion treatments. P1

Fig. 2

Acknowledgement Financial support from Ministry of Economy and Competition (MINECO) under project MAT2010-15026 and CSIC under 201080E124 is acknowledged. J.D.N. would like to thank CSIC for his PhD-grant JAEPRE_09_01155 and travel grant 2012ESTCSIC-7945.

Weak localization and Raman Study of Graphene Antidot Lattices obtained by Crystallographically Anisotropic Etching Florian Oberhuber, Stefan Blien, Stefanie Heydrich, Tobias Korn, Christian Schüller, Dieter Weiss, and Jonathan Eroms Institute for Experimental and Applied Physics, University of Regensburg, Universitaetsstr. 31, D-93053 Regensburg, Germany florian.oberhuber@physik.uni-regensburg.de Abstract We report about the crystallographically anisotropic etching of exfoliated graphene on SiO2 substrates by applying a high-temperature etching mechanism (800°C and argon atmosphere) that was demonstrated to eliminate carbon atoms located on armchair sites, thus leading to zigzag edges [1]. Before exposing samples to this carbothermal reaction, they were patterned with circular antidots (diameter d ~ 40nm) by electron-beam lithography and reactive-ion etching. In the subsequent carbothermal etching step, the predefined holes evolved into larger hexagonal antidots (d ~ 100150nm), as shown in the inset of Fig.1. We will discuss a sample preconditioning procedure which is essential to etch single-layer graphene in an anisotropic manner, however not for bi- and multilayer samples. We investigated a set of single-layer graphene samples, patterned with square lattices of different constants a of hexagonal antidots with different diameters d, and compare them to graphene patterned with lattices of circular holes, investigated previously [2,3]. First, we compare samples by analyzing the weak localization peak in electron transport at low temperatures. By fits of the data to the model put forward in [4], we extract the phase coherence length and the lengths for inter- and intravalley scattering. The intervalley scattering length Li increases linearly with the length a-d, the spacing between neighboring holes, as shown in Fig.1. The process of intervalley scattering is related to the presence of armchair edges in graphene. The large scatter in the observed Li’s for different hexagonal antidot samples points to differences in the ratio of the number of armchair to zigzag sites. The scatter must therefore stem from variations in the quality of the anisotropic etching process. The samples with the highest Li’s for corresponding values of a-d are those with hexagonal antidots. For this reason our interpretation is that our anisotropic etching process creates hexagonal holes with edges along the zigzag orientation. We think that the edges of hexagonal antidot samples with lower Li values must be rough on an atomic scale, since this creates intervalley scattering. This, however, cannot by judged adequately by SEM imaging. Second, the set of antidot samples was characterized by Raman spectroscopy, focusing on the D and G peak. The D-Peak is related to intervalley scattering and therefore to the presence of armchair edges. In a systematic study, we compare the intensity ratios between the D and the G peak for samples patterned with different antidot lattice constants and diameters. We expect the D peak to be proportional to the length of the circumference and to the number N of antidots within the laser spot, and, therefore, to Nd. The G peak is expected to be proportional to the area N(a² - πd²/4) (cf. inset Fig.2). The data shown in Fig.2 show a reasonable correlation between the patterned antidot lattice structure and the D/G intensity ratio for samples with circular antidots. For the hexagonal antidots there is again considerable scatter in the data. However, the smallest D/G intensity ratios are obtained for samples with hexagonal antidots. The interpretation of the weak localization data is consistent with the Raman data. Furthermore, we analyze the correlation between Raman and weak localization measurements with regard to the intervalley scattering mechanism. In Fig.3 the values for the D/G intensity ratios of different antidot samples are plotted against the intervalley scattering times Li, acquired for each sample. The plot shows a clear correlation between the D/G intensity ratio and Li. In addition to the above mentioned studies of graphene antidot lattices, we demonstrate the influence of the anisotropic high-temperature etching reaction on the properties of graphene by showing a series of Raman spectra, acquired between consecutive sample preparation steps.

References [1] P. Nemes-Incze, G. Magda, K. Kamarás, and L.P. Biró, Nano Research, 2 (2010) 110. [2] J. Eroms and D. Weiss, New Journal of Physics, 11 (2009) 095021. [3] S. Heydrich, M. Hirmer, C. Preis, T. Korn, J. Eroms, D. Weiss, and C. Schüller, Applied Physics Letters, 97 (2010) 043113. [4] E. McCann, K. Kechedzhi, V.I. Fal’ko, H. Suzuura, T. Ando, and B.L. Altshuler, Physical Review Letters, 97 (2006) 146805.

Figures Figure 1: The intervalley scattering lengths deduced from weak localization measurements at temperatures of 1.6K for samples patterned with circular antidots and hexagonal holes obtained by high-temperature etching are depicted by the blue circles and the red hexagons, respectively. The blue and the red line show linear fits to blue and the red data points, respectively. The inset shows a SEM image of graphene that was patterned with a quadratic lattice of antidots and anisotropically etched in the high-temperature etching reaction.

Figure 2: The D/G peak intensity ratios deduced from the peak areas in Raman spectra for samples patterned with circular antidots and hexagonal holes obtained by high-temperature etching are depicted by the blue circles and the red hexagons, respectively. The inset shows in orange and dark blue colour the areas of the Raman active zones for the D and the G peak within each unit cell of the antidot lattice, respectively.

Figure 3: The D/G peak intensity ratios for samples with circular and hexagonal antidots correlates with the corresponding intervalley scattering lengths Li acquired for specific samples (blue circles and red hexagons, respectively). The insets show the different Raman active zones for the D and the G peak for circular and hexagonal holes in orange and dark blue colour, respectively.

Novel Method of Graphene Flake Coating on Flexible Substrate to Reduce Sheet Resistance 1

JongSik Oh , SungHee Kim and GeunYoung Yeom

1, 2

Department of Advanced Materials Engineering, Sungkyunkwan University, Suwon 440-726, South Korea 2 SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 440-746, South Korea ojs2k@skku.edu Abstract Indium Tin Oxide (ITO) is one of the most widely used transparent conducting oxides because of its electrical conductivity and optical transparency. However, ITO is a ceramic material making production very expensive, brittle, prone to cracking and therefore of limited use on flexible substrates. In addition, indium is a well kwon metal diffusion material, which leads to a degradation of performance over time at Organic Light-Emitting Diode (OLED). Since graphene has superior high intrinsic mobility, high Youngâ&#x20AC;&#x2122;s modulus, thermal conductivity and high optical transmittance, it has attracted tremendous attention as a replacement material of ITO. Unfortunately, synthesis of graphene is limited to the surface of the catalyst. Thus using these graphene for flexible substrate require transfer step and give rise to low-yield and low-throughput. In recent days, in order to obtain high-yield and high-throughput graphene, many researchers have studied about reduced Graphene Oxide (rGO), which has been reported as a promising synthesis method to address these issues. rGOs are usually obtained in large quantities by an oxidation and reduction process of graphite flakes. Firstly, graphite flakes are oxidized to graphite oxide (GO) by various oxidants. GOs are heavily decorated oxygen-containing group like â&#x2C6;&#x2019;OH and >O functionality, which not only expand the interlayer spacing between graphite flakes but also weaken the van der Waals forces between layers. To restore nature of graphene properties, thermal and chemical reduction process have been followed. However, as a matter of fact, while rGOs could be easily realized on plastic substrate, due to the chemical and thermal annealing reduction process which imparts formation of unsaturated and conjugated carbon atoms, electrical conductivity is dramatically changed. In this study, the graphene flakes solution was prepared in isopropyl alcohol by using sonicator and homogenizer without any surfactant to avoid mobility reduction and Vdirac shift. This novel effort meets all functional criteria for a manufacturing graphene films, as it is chemically and thermally inert due to the elimination of oxidation and reduction process, possible to apply TCFs. In order to form the uniform graphene films on PET substrate, we used spray-coating and roll-milling systems. By using spray coating, we could obtain uniformly distributed graphene flakes on the PET substrate but droplets activate and guide graphene flakes into folded structure. While graphene flakes with strong interlayer can form large structures, indivisual flakes could fold into a variety of 3-dimensional structures. Thus, we tried mechanical force along 2-dimensional direction and found that the individual graphene flakes turned into flat and inter-connected each other. Graphene flake film could get 20 % higher transparency and 98.1 % lower sheet resistance using by this novel method. Furthermore, surface uniformity and morphology of graphene flake film are improved during press-milling process, which are very important to electrical conductivity and device performance. As a result, we could discover the potential of graphene flake film as a replacement of ITO electrode.

References [1] I.K. Moon, J.H. Lee, R.S. Ruoff, H.Y. Lee, Nature Communications, 1 (2010) 1-6. [2] D. Chen, H. Feng, J.H. Li, Chemical Reviews, 112 (2012) 6027-6053 [3] K.S. Choi, Y.S. Park, K.C. Kwon, J.H. Kim, C.K. Kim, S.Y. Kim, K.H. Hong, J.L. Lee, Journal of The Electrochemical Society, 158 (2011) J231-J235

Figures

Figure 1. Uniformity and morphology improvement of graphene film after press/rolling process

Fugure 2. Press/rolling effect on transparency of graphene film

Multimode Fabry-Perot interference in suspended graphene 1

M. Oksanen , A. Uppstu , A. Laitinen , D. J. Cox , A. Harju , M. F. Craciun , S. Russo , P. Hakonen 1

O. V. Lounasmaa Laboratory, School of Science, P. O. Box 15100, FI-00076 AALTO Department of Applied Physics, School of Science, P. O. Box 11100, FI-00076 AALTO 3 Centre for Graphene Science, CEMPS,University of Exeter, Exeter EX4 4QF, UK mika.oksanen@aalto.fi

We report Fabry-Perot type interferences in high-mobility suspended graphene with both shot noise and conductance measurements. Due to the long phase-coherence length in carbon nanomaterials, resonant tunnelling may occur even with relatively long samples. Liang et. al. [1] observed evenly spaced electron-resonances of ΔE ~ hvF/2L, where h is Plank’s constant, vF is the Fermi velocity and L is sample length – with single-wall carbon nanotubes. Recent experiments on suspended, exfoliated graphene have demonstrated mobilities exceeding 200 000 cm2/Vs [2, 3], which facilitate studies of wavelike transport in graphene. Indeed, oscillations in the conductivity of graphene have been observed, indicating coherent transport and Fabry-Perot like resonances [3, 4]. The observed interference structure is more involved than in SWCNTs, which is caused by the two dimensional nature of graphene, which allows more complex interfering paths. Besides extended, high-quality samples, Fabry-Perot resonances have been investigated under very narrow top gates [5] so that the cavity length is on the order of the mean free path of the carriers and the interference is confined between bordering pn interfaces. The conditions for Fabry-Perot resonances in rectangular graphene sheets with nonperfect contacts were recently analyzed by Gunlycke and White [6] who showed that, under certain conditions evenly spaced groups of resonances, separated by ΔE ~ hvF/2L, can emerge. These collective resonances originate owing to simultaneous participation of modes in nonequivalent channels that are facilitated by transversely quantized states with small energy separation. Such collective resonances should not be confused with the ordinary two-channel Fabry-Perot resonances observed in single-wall carbon nanotubes. Conductance and shot noise measurements were used to analyse the transport resonances generated by contacts as well as pn (nn’) junctions. Differential conductance shows faint Fabry-Perot patterns emerging, by taking the derivative of the conductance the visibility is improved (Fig. 1 a)). The separation of the maxima is 4 meV in gate and 8 mV in bias. Because of the negative gate polarity used nn’ interfaces are formed, and at positive gate voltages pn interfaces are formed which present a more visible structure. The Fourier transform of the data was taken at high positive and negative chemical potential (Fig. 1b)). Analysis of the Fourier transform shows peaks corresponding to the different contribution of interferences from the metallic leads and pn (nn’) interfaces. Shot noise shows similar results, and the analysis agrees with the findings from the conductance.

References [1] W. Liang, M. Bockrath, D. Bozovic and H. Park, Nature 411 (2001) 665 [2] K.I. Bolotin, K.J. Sikes, J. Hone, H.L. Stormer, and P. Kim, Phys. Rev. Lett. 101 (2008) 096802 [3] X. Du, I. Skachko, A. Barker, and E.Y. Andrei, Nat. Nanotechnol. 3, (2008) 491 [4] F. Miao, S. Wijeratne, Y. Zhang, U. C. Coskun, W. Bao, and C. N. Lau, Science 317, (2007) 1530 [5] A. F. Young and P. Kim, Nat. Phys 5, (2009) 222 [6] D. Gunlycke and C. T. White, Appl. Phys. Lett. 93 (2008) 122106

Figures

Figure 1. a) Taking the derivative of the conductance with respect to chemical potential gives better visibility of the resonance structure. b) Fourier transform of the data within the rectangle, where the solid lines indicate resonances in the center region of the sample (between pn interfaces), and the dashed lines indicate resonances from the metal contacts.

Growth of graphene nanoislands on a Ni(111) surface 1

2,3

M. Ollé , G. Ceballos , D. Serrate , A. Mugarza and P. Gambardella

1,4

ICN, Catalan Institute of Nanotechnology, Campus de la UAB 08193 Bellaterra (Barcelona) Spain

INA-LMA, University of Zaragoza, 50018 (Zaragoza), Spain

Dpto. Física Materia Condensada, University of zaragoza, 50009 (Zaragoza), Spain)

Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain

Marc.olle@icn.cat Abstract: Since they first time a layer of graphene was isolated in 2004 [1], the interest in this material increased exponentially. The main attraction of this material lies in their unusual and surprising electronic, mechanical and magnetic properties such as the anomalous quantum Hall effect, the absence of electronic localization, high optical transparency, the high electrical conductivity, flexibility and high mechanical strength. These properties make graphene a very promising material for applications in electronics and spintronics, and therefore it is necessary to control the growth and properties at the nanoscale. The growth of graphene layers on the nickel surface by decomposition of hydrocarbons is interesting for three main reasons. On the one hand, the lattice constant of the surface of Ni(111) coincides almost perfectly with the lattice constant of graphene, which allows it to grow in a (1×1) structure. Moreover, due to the catalytic effect of nickel surface, it is an autoterminated reaction, i.e., the reaction stops once the graphene monolayer is formed avoiding the growth of multilayers [2]. Finally, the Nickel is a ferromagnetic material, which opens the door to applications in spintronics. Progress in the manufacture of low dimensional structures such as graphene nanoribbons has been reported [3]. It shows that the electronic properties of graphene change in a non-trivial way going to nanoscopic dimensions mainly due to the contribution of edge effects. In this work we study by STM the growth of graphene on a Ni (111) surface by decomposition of hydrocarbons [4]. By varying parameters such as the dosage of hydrocarbon, reaction time and temperature was possible to obtain in a reproducible manner a wide range of coverages. For low coverages the carbon atoms organize themselves into graphene nanoislands whose size and density are related to the reaction parameters. These nanoislands, initially with irregular shapes, may, by further thermal treatment selectively acquire a triangular (Fig. 1) or hexagonal shape both with zig-zag edges. In the case of triangular islands all edges have the same packaging with the nickel surface

underneath while in the case of hexagonal islands edges change alternatively the packaging. The optimum conditions to obtain nanoislands of particular size and shape are studied by systematic variation of the parameters of the hydrocarbon decomposition reaction and the thermal diffusion of carbon on the surface.

Bibliography:

[1] K.S. Novoselov et al, Science, 306, 666-669 (2004). [2] J. Wintterlin, M.-L. Bocquet, Surface Science, 603, 1841–1852, (2009). [3] M.Y. Han et al., Phys. Rev. Lett. 98, 206805 (2007). [4] M. Ollé, G. Ceballos, D. Serrate and P. Gambardella, Nano Lett. 12, 4431−4436 (2012).

Figure 1: STM images of triangular and hexagonal graphene nanoisland on a Ni(111) surface.

Transfer-Free Electrical Insulation of Epitaxial Graphene from its Metal Substrate Fabrizio Orlando,(1) Silvano Lizzit,(2) Rosanna Larciprete,(3) Paolo Lacovig,(2) Matteo Dalmiglio,(2) Alessandro Baraldi,(1) Lauge Gammelgaard,(4) Lucas Barreto,(5) Marco Bianchi,(5) Edward Perkins,(5) and Philip Hofmann,(5) (1) Physics Department and CENMAT, University of Trieste, Via Valerio 2, 34127 Trieste, Italy and IOM-CNR Laboratorio TASC, Area Science Park, 34149 Trieste, Italy (2) Elettra-Sincrotrone Trieste, S.S. 14 Km 163.5, 34149 Trieste, Italy (3) CNR-Institute for Complex Systems, Via Fosso del Cavaliere 100, 00133 Roma, Italy (4) Capres A/S, 2800 Kgs. Lyngby, Denmark (5) Department of Physics and Astronomy, Interdisciplinary Nanoscience Center, Aarhus University, 8000 Aarhus C, Denmark fabrizio.orlando@elettra.trieste.it

Abstract The remarkable properties of graphene, such as the very high carrier mobility at room temperature, tolerance to high temperature and inertness, make it the most promising candidate for future nanoelectronics. Several methods have been developed to produce graphene layers of various dimensions and quality, which, however, hardly match the requirements for mass production of electronic devices. Exfoliation-based techniques are very expensive, time-consuming and produce small flakes or graphene of poor quality. The most common way to obtain extended, high-quality, single graphene layers is the epitaxial growth on transition-metal surfaces. However, the graphene/metal interface has the disadvantage of a conductive substrate, that makes the conduction through graphene irrelevant. In order to face this problem we have developed a novel transfer-free method to electrically insulate epitaxial graphene from the metal substrate it is grown on. This is achieved by growing in situ an insulating SiO2 layer of the desired thickness directly below the graphene layer, through a stepwise reaction between intercalated silicon and oxygen [1]. Firstly, epitaxial graphene is grown on a Ru(0001) crystal surface. Subsequent exposure of the sample to a flux of Si at 720 K results in Si intercalation and in the formation of a Ru silicide layer below graphene. Intercalation of oxygen [2] at T=630 K rapidly oxidizes the silicide producing a thin SiO2 layer over an oxygen covered Ru surface. By following the entire process by high resolution fast-XPS measurements we establish that graphene does not react with O2 and that during the decomposition of the Ru silicide oxygen binds exclusively to Si [3]. At the end of the process, we proved electrical insulation of the graphene layer from the Ru substrate by performing lateral transport measurements with a microscopic 12 point probe, showing a resistance value characteristic of two-dimensional systems. The transfer-free method developed in the present work shows that element intercalation can be exploited for the synthesis of materials below graphene, which might have wide application in graphene research and nanotechnology devices fabrication.

References [1] S. Lizzit, R. Larciprete, P. Lacovig, M. Dalmiglio, F. Orlando, A. Baraldi, L. Gammelgaard, L. Barreto, M. Bianchi, E. Perkins, and Ph. Hofmann, Nano Letters, 12 (2012) 4503. Highlighted in Nature Nanotechnology, 7 (2012) 613. [2] R. Larciprete, S. Ulstrup, P. Lacovig, M. Dalmiglio, M. Bianchi, F. Mazzola, L. HornekĂŚr, F. Orlando, A. Baraldi, Ph. Hofmann, and S. Lizzit, ACS Nano 6 (2012) 9551. Highlighted in Nature Materials, 12 (2013) 3. [3] R. Larciprete, S. Fabris, T. Sun, P. Lacovig, A. Baraldi, and S. Lizzit, Journal of the American Chemical Society, 133 (2012) 17315.

Application of thermally conductive plastics and a novel coating in Solar Thermal Panel Collectors (THERMALCOND). Raquel LLorens- Chiralt1, Amaya Ortega1, Julio Gómez2, Jon Lea3 1

AIMPLAS, Plastic Research Institute,C/Gustave Eiffel, 4, 46980 Paterna,SPAIN 2 AVANZARE, C/jardines 5 ( Pol.Ind. Lentiscares) 26370 Navarrete (La Rioja),SPAIN 3 Smithers RAPRA, Shawbury, Shrewbury, Shorpshire SY4 4NR,ENGLAND rllorens@aimplas.es

Abstract The main goal of ThermalCond project has been to develop a new family of low cost polyolefin based components (sheets, pipes and fittings) to be used in the manufacture of flat-plate solar thermal collectors. These components are a viable alternative to current collector´s metallic components. However, due to the current limitations of thermoplastic materials (low thermal conductivity and solar absorption) two main approaches have been followed in this project. New thermally conductive nanocomposites for sheets, pipes and fittings have been developed using conductive nanostructured materials such as graphene. The resulting compounds retain the processing and increase the performance properties of the original polymer. On the other hand, a novel and specific surface coating based on molecular self assembly (SAM) technology has been developed which enhances the conversion efficiency of the entire wavelength spectrum of solar incident into heat energy. These developments allow a novel low cost and low weight components design´s with enhanced thermal conductivity and high solar energy absorption to develop high energy thermal collector designs. The use of plastics components instead of metallic ones offers additional advantages: folding and easy assembling structures design, reduced cost of material and manufacturing, weight reduction, corrosion resistance, low friction coefficient (less pump energy consumption), prevent theft or vandalism (due the low cost of components in comparison with copper). The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 262647" References

[1] L. Vayssieres: On solar hydrogen and nanotechnology. Ed. New York, NY: John Wiley & Sons, Inc.; 2009 [2] P. Mukhopadhyay and R.K. Gupta, trends and Frontiers in Graphene-Based polymer Nanocomposites, Plastics Engineering, January 2011. [3] S.K. Park,S. H. Kim, Jin T. Hwang, Carboxylated. Journal of Applied Polymer Science, Vol. 109, (2008) 388 [4] S.P. Bao, S.C. Tjong, Materials Science and Engineering .A 485 (2008) 508. [5] Y. Xu, D.D.L Chung, C. Mroz, Composites, Part A 32 (2011) 1749.

Figures

Figure 1. Thermal prototype compare with comercial solar panel.

Figure2. Lateral view: ThermalCond collector (left); commercial collector (right).

Silicon nitride composites containing graphene nanostructures M. Isabel Osendi, Cristina Ramirez, P. Miranzo, M. Belmonte CSIC, Institute of Ceramics and Glass. Campus Cantoblanco, Madrid 28049. Spain miosendi@icv.csic.es Abstract Ceramic composites containing graphene nanostructures have not raised as much expectation as polymer composites, although the inclusion of graphene in ceramics has shown important effects on several properties of these new composites, and with the added benefit of multi-functionality. In particular, an increase in toughness (KIC) from 2.8 to 6.6 MPa.m1/2 for Si3N4/graphene1 composites has been reported and electrical conductivities of 5709 S.m-1 were observed in Al2O3/graphene2 composites. Ceramic-graphene composites are mainly fabricated by mixing fine ceramic powders (normally under 1 m) and graphene flakes. The graphene flakes in the composites show a range of thickness from few nm to100nm for the so-called graphene nanoplatelets (GNP), and much thinner flakes when graphene oxide (GO) is used as starting nanostructure.3 The more common densification sintering method is the spark plasma sintering that allows very fast sintering rates and consolidation at lower temperatures than more conventional methods. The characteristics of the technique – application of load and pulsed direct current on the graphite die containing the compound- induce orientation of the flakes in the composite microstructure, as evidenced in the fracture surface image of Fig.1. This arrangement of the graphene flakes produces a certain degree of anisotropy in the properties.4,5 We will show that high electrical conductivity and exceptional mechanical properties are achieved for Si3N4/graphene composites by an appropriate selection of the graphene source. Electrical conductivities of 4000 S.m-1 -13 orders of magnitude higher than the monolithic material- and a percolation threshold of 0.075 are observed for Si3N4/GNP composites, but electric percolation is accomplished for much lower GO volume contents (0.04). The electrical conductivity attained gives an important attribute to the material, such as the possibility of being shaped using electrical discharge machining (EDM).6 Si3N4 ceramics are hard materials, which are costly and difficult to machine and require the use of diamond tools; therefore, the possibility of using EDM is one interesting utility provided by the graphene network. The influence of the graphene source is also evidenced in the KIC of these composites. In fact, KIC of 10 MPa.m1/2 has been measured for composites fabricated from reduced GO, which is the highest reported to date for Si3N4-graphene composites, whereas a more modest toughening effect was measured in alike composites containing GNPs (4.1 MPa.m1/2). Graphene flakes also have an interesting influence on the composite tribology. Decreases in the friction coefficient and the wear volume (56 %) for high contact loads are observed when compared to the plain material. This effect is mostly related to the lubricant effect caused by the buildup of graphene layers –detached from the composite- on the wear track (Fig. 2). The improved wear behavior of the composites may find application in gasoline direct injection engines.7 Summarizing, graphene-Si3N4 composites show excellent mechanical, tribological and electrical properties, in conjunction with an outstanding wear behavior, all these characteristics make possible their application in many fields, such as automotive industry, MEMS, turbines, etc.

References [1] Walker LS, Marotto VR, Rafiee MA, Koratkar N, Corral EL. ACS nano, 5 (2011) 3182. [2] Fan Y, Wang L, Li J, Sun S, Chen F, Chen L, et al. Carbon, 48 (2010) 1743. [3] Ramírez C,Vega-Diaz SM, Morelos-Gómez A, Figueiredo FM, Mauricio Terrones M, Osendi MI, Belmonte M, Miranzo P. DOI: 10.1016/j.carbon.2013.02.015. [4] Ramírez C, Garzón L, Miranzo P, Osendi MI, Ocal C. Carbon, 49 (2011) 3873. [5] Ramirez C, Figueiredo F, Miranzo P, Poza P, Osendi M I. Carbon, 50 (2012) 3607. [6] Malek O, González-Julián J, Vleugels J, Vanderauwer W, Lauwers B, Belmonte M. Materials Today, 14 (2011) 496.

[7] Gonzalez-Julian J, Schneider J, Pilar Miranzo P, Osendi MI, Belmonte M. J. Am. Ceram. Soc. 94 (2011) 2542.

Figures

Figure 1: Field emission scanning electron micrograph of a Si3N4-GNP composite showing GNP orientation

Figure 2: Presence of graphene flakes in the composite wear track

Non-covalent Interactions to Graphene: Theory and Experiment a

Michal Otyepka, Petr Lazar, František Karlický, Eva Otyepková, Pavel Hobza, c Mingdong Dong, a

a,b

Radek Zbořil,

Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc, Czech Republic b Institute of Organic Chemistry and Biochemistry, Prague, Czech Republic c iNANO, Aarhus University, Aarhus, Denmark michal.otyepka@upol.cz

Abstract Graphene is a two-dimensional π-conjugated material having extraordinary physical properties, which makes its a perspective material in catalysis, energy storage, nano(opto)electronics and sensor applications.[1,2] The application potential of graphene can be enormously enhanced by its covalent and non-covalent functionalization.[3] An exact quantification of interaction between graphene and guest molecules as well as thorough understanding of the nature of interaction between graphene and guest molecules have not been yet achieved. We analysed nature of interaction between Ag, Au, Pd, Pt metal atoms and clusters to benzene, coronene and graphene, by quantum chemical calculations [4,5] The silver atom is bound weakly by London dispersion forces, while interaction of palladium and platinum is significantly stronger and involves some covalent character. Involvement of relativistic effects is required for a reasonable description of interactions involving Au and Pt. We measured the interaction force between metalized AFM tips (Cu, Ag, Au, Pt and naturally Si were considered, Figure 1) and graphene and correlated the results with theoretical calculations carried out by density functional theory (DFT). The theoretically calculated interaction forces agree with the experimental data, only when non-local electron correlation and exact Hartree-Fock electron exchange is explicitly treated.[6] The AFM tip covered by copper displayed the highest affinity to graphene among the metals considered. We also quantified the adsorption enthalpies between graphene and several organic molecules and compared the experimental results with theoretical calculations carried out by DFT and spin-component scaled MP2 calculations. References [1] Novoselov KS et al. Science 306, (2004) 666. [2] Novoselov KS et al. Nature 490, (2012) 192. [3] Georgakilas V et al. Chem. Rev. 112, (2012) 6156. [4] Granatier J et al. J. Chem. Theory and Comput. 7, (2011) 3743 [5] Granatier J et al. J. Phys. Chem. C 116, (2012) 14151. [6] Lazar P et al. ACS Nano, in press (DOI : 10.1021/nn305608a) Figure 1 C

A: The interaction force between graphene and metallized AFM tip was measured by dynamic AFM at ambient conditions. B The model used for theoretical calculations involves graphene (green) and metal tetrahedron (blue). C: Comparison betwee measured and calculated forces. Adopted from Ref. [6].

Multilayer polyelectrolyte films formed with Prussian Blue nanoparticles and reduced graphene oxide as a sensitive tool for H2O2 detection 1

A. Pajor-Świerzy , T. Kruk , L. Szyk-Warszyńska , R. Socha , R. Wendelbo , 1 P. Warszyński 1

Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland 2 Abalonyx AS, Forskningsveien 1, 0314 Oslo, Norway ncpajor@cyf-kr.edu.pl 2

Graphene is a new material consisting of single layer of sp – bonded carbon atoms. Since its discovery in 2004 [1] graphene has emerged as the “material of the future” due to its unique nanostructure and electrical, thermal and mechanical properties [2,3]. Graphene is now considered as a promising material for application in various technological fields such as transparent conductive films [4], solar cells [5], gas storage media [6], and next generation of electronic devices [7]. Prussian Blue (PB), which is the prototype of mixed-valence transition metal hexacyanoferrates was initially used as a blue pigment only. In 1978 Neff reported that PB can be deposited onto an electrode surface to produce an electroactive coating [8]. In particular the PB films can be used in electrochemical sensors and biosensors because of its catalytic properties toward the detection of hydrogen peroxide [9]. PB nanoparticles with the size 10 nm and ζ-potential –50 mV were synthesized by the reaction of -2 FeCl3 and K4[Fe(CN)6] in the presence of 10 HCl. PB nanoparticles were immobilized in polyelectrolyte (PE) multilayers using the layer-by-layer (LbL) method [10]. Additionally the graphene oxide (GO) was deposited from its aqueous suspension to form additional layers of the film. We found that the thermal o 2 reduction of GO at the temperature 180 C is the effective processes leading to formation of sp -bonded 2 carbon atoms. The examination of XPS spectra indicated that after the ratio of the sp carbon increased to c.a. 80 at.%. Since, on the other hand the PB nanoparticles remained stable at that temperature; we applied thermal reduction to turn graphene oxide embedded in the multilayer films into its reduced form (rGO). The electric conductivity of films after the reduction was analyzed by the four point surface conductivity measurements, whereas cyclic voltamperometry was used to determine the electroactive properties of multilayer films containing both Prussian blue and rGO. We noticed that in the presence of rGO sheets the intensity of the redox current of PB embedded in the multilayer films markedly increased due to enhancement of electron transport to the gold electrode surface. In the presence of hydrogen peroxide characteristic peaks from reduction of H2O2 to OH ions and oxidation to O2 molecules appeared. We found a linear correlation between that redox current density with the number of deposited PB/rGO layers and concentration of hydrogen peroxide in the solution. Therefore, the electrodes covered with composite polyelectrolyte/PB nanoparticles/rGO film enable in-situ sensing of H2O2. Presence of rGO in the multilayer structure increases ca. 30 times the sensitivity for hydrogen peroxide with respect to films containing polyelectrolyte and the same number of PB layers only. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Since, 306 (2004) 666-669. [2] J.S. Bunch, A.v.d.Z. Arend, S.V. Scott, W.F. Ian, M.T. David, M.P. Jeevak, G.C. Harold, L. M. Paul, Science, 315 (2007) 490-493. [3] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nanotechnol., 3 (2008) 270-274. [4] S. Watcharotone, D.A. Dikin, S. Stankovich, R. Piner, I. Jung, G.H.B. Dommett, et al. Nano Lett. 7 (2007) 1888. [5] X. Wang, L.J. Zhi, N. Tsao, Z. Tomovic, J.L. Li, K. Mullen, Angew Chem Int Ed. 47 ( 2008) 2990. [6] C. Sealy, Nano Today 4(2009) 6. [7] C. Gomez-Navarro, R.T. Weitz, A.M. Bittner, M. Scolari, A. Mews, M. Burghard, et al. Nano Lett. 7 (2007) 3499. [8] V.D. Neff, J. Electrochem.Soc. 128 (1978) 886. [9] A.A. Karyakin, E.A. Kotel’nikova, L.V. Lukachova, E.E. Karyakina, J. Wang, Anal. Chem. 74 (2002) 1597. [10] Decher, G.; Hong, J.D.; Schmitt, J. Thin Solid Films 210/211 (1992) 831-838.

Direct Growth of Large Area Graphene on Si/SiO2 substrate from Sputtered Carbon/Nickel Films 1

1,2

Genhua Pan , Mark Heath , David Horsell , M. Lesley Wears , Bing Li , Shakil Awan Laith Al-Taan 1

Wolfson Nanomaterials & Devices Laboratory, Faculty of Science and Technology, University of Plymouth, Devon, PL4 8AA, UK

College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, UK G.Pan@plymouth.ac.uk

2 m

Preparation of graphene and graphene-metal nanoparticle hybrids with enhanced catalytic activity by reduction of graphite oxide with efficient natural bioreductants J. I. Paredes, M. J. Fernández-Merino, S. Villar-Rodil, P. Solís-Fernández, L. Guardia, R. García, A. Martínez-Alonso, J. M. D. Tascón Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain paredes@incar.csic.es Abstract The implementation of green approaches towards the preparation of graphene-based materials with enhanced functionality from graphite oxide requires the use of appropriate reductants in place of the widely employed but highly toxic hydrazine. Vitamin C has been previously shown to efficiently deoxygenate graphene oxide in aqueous and organic dispersion [ 1]. In the present work, a large number of environmentally friendly, natural antioxidants (mostly vitamins, amino acids and organic acids) has been tested for their ability to reduce graphene oxide. Specifically, vitamin B6 (in two forms: pyridoxine and pyridoxamine dihydrochloride) and vitamin B2 [(-)-riboflavin and riboflavin 5’-monophosphate salt hydrate; the organic acids citric acid, fumaric acid, L-malic acid, and Ltartaric acid; a representative set of the 22 naturally occurring amino acids, as well as two peptides, namely: L-arginine, L-asparagine, L-carnosine (dipeptide), glycine, L-glutamic acid, L-gluthatione (tripeptide), L-histidine, L-methionine, L-phenylalanine, L-tryptophan, and L-tyrosine have been investigated. By establishing a protocol to systematically compare and optimize their performance, several new efficient bioreductants of graphene oxide have been identified, namely, pyridoxine and pyridoxamine (vitamin B6), riboflavin (vitamin B2), as well as the amino acids arginine, histidine and tryptophan. Indeed, as clearly seen in Fig. 1a, treatment of graphene oxide with any of these bioreductants prompted a significant decrease in the relative contribution of oxygen-bonded carbon to the XPS C1s signal, especially of carbon single-bonded to oxygen, which possesses a binding energy (BE) of 286.6 eV. The reduced sheets did not agglomerate and were kept as individual single-layer objects in their corresponding dispersions, even several months after the reduction treatment was carried out. Fig. 1b illustrates this point with some representative AFM images obtained from dispersions reduced with glutathione (b) and pyridoxamine (c). The reduced sheets are constituted by irregular (polygonal) objects a few to several hundred nm in lateral size and ~1 nm of apparent thickness, which indicates that they indeed correspond to single-layer sheets [2]. The aforementioned biomolecules were also successfully used to prepare reduced graphene oxide-silver nanoparticle hybrids (RGO-Ag NPs) by the simultaneous reduction of graphene oxide and Ag(I) (in the form of AgNO3). The grayish tinge of diluted RGO dispersions (inset to Fig. 2a, right cuvette) was replaced by a yellowish tone when graphene oxide and AgNO3 were co-reduced (inset to Fig. 2a, left cuvette). UV-vis absorption spectra revealed the appearance of a feature at ~420 nm for the latter (Fig. 2a), due to the surface plasmon resonance band characteristic of metallic Ag nanostructures [3]. The RGO sheets were decorated with either white (AFM, Fig. 2b) or black (bright-field STEM, Fig. 2c) dots, which were never observed for RGO sheets obtained in the absence of AgNO3 (AFM, Fig. 1b and c), and were therefore attributed to the Ag NPs. These were exclusively associated to the sheets, without stand-alone nanoparticles being generated (Fig. 2b and c). From the AFM images, the density of Ag NPs on the RGO sheets was estimated to be typically between a few and several m-2. The hybrids prepared with pyridoxamine exhibited a combination of long-term colloidal stability and exceptionally high catalytic activity among silver nanoparticle-based catalysts in the reduction of pnitrophenol with NaBH4 (see Fig. 3 for results on the reaction kinetics). Thus, in addition to expanding substantially the number of green reductants available for the deoxygenation of graphene oxide, the present results underline the idea that a proper selection of bioreductant can be relevant to achieve graphene-based materials with improved performance. References

[1] M. J. Fernández-Merino, L. Guardia, J. I. Paredes, S. Villar-Rodil, P. Solís-Fernández, A. MartínezAlonso, J. M. D. Tascón, J. Phys. Chem. C 114 (2010) 6426-6432. [2] P. Solís-Fernández, J. I. Paredes, S. Villar-Rodil, A. Martínez-Alonso, J. M. D.Tascón, Carbon 48 (2010) 2657-2660. [3] D. D. Evanoff Jr., G. Chumanov, ChemPhysChem 6 (2005) 1221-1231.

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Figure 1. (a) Normalized, high resolution XPS C1s core level spectra for unreduced graphene oxide (orange) and graphene oxide reduced with pyridoxamine (red), B2 (fluorescent green), B2 salt (yellow), arginine (blue), glutathione (pink), histidine (black), tryptophan (olive green) and glucose (wine). (b) AFM images of graphene oxide sheets reduced with (b) glutathione, and (c) pyridoxamine. c)

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Figure 3. (a) UV-vis absorption spectra of p-nitrophenoxide ion (dark yellow), and p-aminophenoxide ion (violet). The absorption peak at 400 nm of p-nitrophenoxide is used to monitor its conversion to paminophenoxide by reduction with NaBH4. (b) Plot of absorbance at 400 nm for the reduction of pnitrophenoxide with NaBH4 catalysed with RGO-Ag NP hybrid prepared with pyridoxamine (black squares) and with stand-alone, citrate-capped Ag NPs prepared by NaBH4 reduction of AgNO3 following standard procedures (green squares). The experimental kinetic profiles could be well fitted to exponential decay functions, which are shown as overlaid red and orange lines, respectively. Experimental conditions: [p-nitrophenol] = 0.06 mM; [NaBH4] = 0.018 M; [Ag NP] ~0.2 mg mL-1.

Electrical property degradation of graphene FET by water diffusion beneath graphene channels Joonkyu Park, Wonsuk Jung, Ju Yeon Woo, Sehyun Shin, Chang-Soo Han School of Mechanical Engineering, Korea University, Anam, Seongbuk, Seoul 136-701, Korea cshan@korea.ac.kr Abstract Despite of being discovered as a perfect 2 dimensional material with a high carrier mobility which stems from the linear energy-momentum dispersion in a low energy level[1, 2], many engineers casted doubt on using graphene in modern sciences since its size realized on silicon dioxide substrate had been too small to apply conventional semiconductor fabrication for mass production. Afterward a technique of making a wafer size large area graphene was developed by using chemical vapor deposition method.[3] There are still, however, obstacles when transferring it to certain substrates in terms of residue, adhesion and ripples.[4-6] Most of all, electrical property degradations such as the charge inhomogeneity caused by electron-hole puddles[7] and the partial doping which is usually found to be a p-type are introduced by water even in ambient condition.[8] There is an experimental paper regarding its physical property alterations by using atomic force microscopy when water diffuses in between graphene and substrate though the edges or defects of the exfoliated graphene sheets.[9] Judging from this paper, we expect there are also similar changes in electrical properties of graphene based devices under uses of water. Graphene FET devices with three terminals (source, drain and gate) of which channel widths and lengths are fixed at 2 um and ranging from 5 to 50 um, respectively, were fabricated. We placed them in a thermo-hygrostat making its condition 80% relative humidity under 70 degrees Celsius at most. In relatively high humidity condition, water diffuses faster than usual, and it shows clear results in instabilities of electrical property. Therefore, not only for making reliable graphene devices under water permeation but also for applications of them performing in water, for example, humidity sensors, we need to enhance the adhesion between graphene and substrate or develop improved passivation techniques.

References [1] NOVOSELOV AKGAKS, nature materials, 6 (2007) 183 [2] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Nature, 438 (2005) 197 [3] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Ahn JH, et al. Nature, 457 (2009) 706 [4] Gibertini M, Tomadin A, Polini M, Fasolino A, Katsnelson MI, Physical Review B, 8 (2010) 1 [5] Lin YC, Lu CC, Yeh CH, Jin C, Suenaga K, Chiu PW, Nano Lett, 12 (2012) 414 [6] Sen D, Novoselov KS, Reis PM, Buehler MJ, Small, 6 (2010) 1108 [7] Martin J, Akerman N, Ulbricht G, Lohmann T, Smet JH, von Klitzing K, et al. Nature Physics, 4 (2007) 144 [8] T. O. Wehling KSN, S. V. Morozov, E. E. Vdovin,, M. I. Katsnelson AKGaAIL, Nano Lett, 8 (2007) 173 [9] Lee MJ, Choi JS, Kim J-S, Byun I-S, Lee DH, Ryu S, et al. Nano Research, 5 (2012) 710

On the mechanical deformation of single and multilayer graphene

Georgia Tsoukleri1 , John Parthenios 1,4 , Kostas Papagelis1,2 , Otakar Frank3 , Kostya Novoselov4 , Costas Galiotis1,2 1 Institute of Chemical Engineering Sciences - Foundation of Research and Technology Hellas, 26504 Patras, Greece 2 Department of Materials Science, University of Patras, 26504 Patras, Greece 3 J. Heyrovsky Institute of Physical Chemistry of the AS CR, v.v.i., Prague 8, Czech Republic 4 School of Physics and Astronomy, University of Manchester, Manchester, UK A key issue to most applications involving graphene is its mechanical response under various stress/ strain states. The electronic band structure of bi- and tri-layer graphene differs remarkably from that of a monolayer resulting in materials with different electronic properties, suitable for next-generation optoelectronics and post-silicon nanoelectronics. However, their mechanical properties that can significantly alter their electronic properties are not well documented so far. In this work, we present an experimental study on single, bi- and tri-layer graphene flakes under uniaxial tensile and compressive strain, for low levels of strain (up to 1.5%). Graphene layers were loaded by employing a polymeric cantilever beam assembly, where the graphene flakes are embedded into the polymer beam. The mechanical response of graphene is monitored by simultaneous Raman measurements by means of the frequency shift of the G and 2D optical phonons, and their strain rates are determined. The results can be used to quantify the amount of uniaxial strain, providing a fundamental tool for measuring load transfer in graphene based nanocomposites. References [1] O. Frank, M. Bouša, I. Riaz, R. Jalil, K. S. Novoselov, G. Tsoukleri, J. Parthenios, L. Kavan, K. Papagelis* and C. Galiotis, “Phonon and structural changes in deformed Bernal stacked bilayer graphene”, Nano Letters 12, 687-693 (2012). [2] O. Frank G. Tsoukleri, I. Riaz, K. Papagelis J. Parthenios, A.C. Ferrari, A. K. Geim, K. S. Novoselov and C. Galiotis, “Development of a universal stress sensor for graphene and carbon fibres”, Nature Communications 2:255 doi: 10.1038/ncomms1247 (2011). [3] O. Frank, M. Mohr, J. Maultzsch, C. Thomsen, I. Riaz, R. Jalil, K. S. Novoselov, G. Tsoukleri, J. Parthenios, K. Papagelis*, L. Kavan and C. Galiotis, “Raman 2D-band splitting in graphene: theory and experiment”, ACS-Nano 5, 2231-2239 (2011). [4] O. Frank G. Tsoukleri, J. Parthenios, K. Papagelis, I. Riaz,R. Jalil, K. S. Novoselov and C. [5] G. Tsoukleri, J. Parthenios, K. Papagelis, R. Jalil, A. C. Ferrari, A. K. Geim, K. S. Novoselov and C. Galiotis. “Subjecting a graphene monolayer to tension and compression”, Small 2, 2397-2402 (2009).

Electrochemically Exfoliated Graphene as Solution Processable, Highly-Conductive Electrodes for Organic Electronics Khaled Parvez, Dr. Rongjin Li, Prof. Xinliang Feng, Prof. Klaus Müllen Max Planck Institute for Polymer Research, 55128 Mainz, Germany parvez@mpip-mainz.mpg.de Abstract Solution processable thin layer graphene is an intriguing nanomaterial with tremendous potential for electronic applications. In this work, we demonstrate that electrochemical exfoliation of graphite furnishes graphene sheets in high quality and high yield. The electrochemically exfoliated graphene (EG) has a large sheet size (~ 10 μm), high C/O ratio of 12.3 and low sheet resistance (4.8 kΩ/□) which comparable to that of CVD graphene. Due to the solution-processability (~ 1 mg/mL in DMF) of such graphene sheets, large and homogeneous graphene films can be fabricated on both rigid and flexible substrates by vacuum filtration and subsequent transfer to the desired substrates. The resulting graphene films exhibit sheet resistances of 4.1 and 2.4 kΩ/□ with transmittances of 85% and 73%, respectively. Patterned graphene films can serve as high-performance source/drain (S/D) electrodes for OFETs. an serve as high-performance source/drain (S/D) electrodes for OFETs. References [1] X.Wang, L. Zhi, K. Müllen, Nano Lett. 8 (2008) 323-327. [2] P. Matyba, H. Yamaguchi, G. Eda, M. Chhowalla, L. Edman, N.D. Robinson, ACS Nano 4 (2010) 637-642. [3] S. Pang, H.N. Tsao, X. Feng, K. Müllen, Adv. Mater. 21 (2009) 1-4. [4] A. Reina, X.T. Jia, J. Ho, D. Nezich, H.B. Son, V. Bulovic, Nano Lett. 9 (2009) 30-35.

Figures

Figure caption: (a) Schematic illustration of the electrochemical exfoliation of graphite, (b) AFM image of the exfoliated graphene sheet and, (c) photograph of the patterned graphene electrodes on PET substrate and filter paper (inset).

Comparison of CVD graphene grown on copper foil and PVD copper 1

1,2

I. Pasternak , K. Grodecki , A. Piatkowska , P. Caban and W. Strupinski 1

Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland 2 Faculty of Physics, University of Warsaw, Hoza 69, 00-681 Warsaw, Poland iwona.pasternak@itme.edu.pl

Abstract One of the most effective methods for obtaining graphene for low-cost and large-scale applications is its synthesis by CVD performed on the surface of copper. The bonding of a single graphene layer to a metal surface depends on factors such as the metal surface itself, the quality of the substrate and the grain size [1]. At present, foil is treated as the most suitable and, at the same time the most frequently used Cu substrate for graphene growth. Nevertheless, new substrates ought to be introduced and the quality of the obtained graphene layers has to be enhanced. Sputtered Cu films on insulating substrates have emerged as a promising alternative. In this work we draw a comparison between graphene grown by the CVD method on 12Âľm thick copper foil and on sputter-deposited copper on a Si/SiO2 substrate. We collect information on the properties of graphene films transferred from different copper substrates onto dielectric substrates. We show that the grain size in the case of sputtered Cu films is much smaller than for Cu foil. In addition, we present the micro-Raman maps of graphene, which reveal that these characteristics cause significant changes in the 2D band position for graphene on both substrates (Figure 1). Moreover, we demonstrate differences in the crystallographic orientation of the copper grains on Cu foils and sputtered Cu films (Figure 2). We note that the PVD Cu film is highly textured and the preferred grain orientation is (111). We present how these features influence the quality of the grown and transferred graphene films. The assessment of the properties of graphene grown on both Cu substrates and transferred graphene has been performed with Raman spectroscopy, confirming the formation of graphitic structures, as well as AFM and SEM imaging, showing the morphology of the graphene/Cu interaction. References [1] Gang Hee Han, Fethullah Gunes, Jung Jun Bae, Eun Sung Kim, Seung Jin Chae, Hyeon-Jin Shin, Jae-Young Choi, Didier Pribat, and Young Hee Lee, Nano Lett. 11 (2011) 4144. Figures

Fig. 1. Raman maps of 2D band position for graphene on a) Cu foil and b) PVD Cu film.

Fig. 2. EBSD maps of Cu grains distributions of a) PVD Cu film and d) Cu foil. Inverse pole figure EBSD maps of b) PVD Cu film and e) Cu foil. Stereographic projection for c) PVD Cu film and f) Cu foil.

Synthesis and characterization of graphenes containing S, N and P. Electrochemical activity toward the oxygen reduction reaction (a) (b) (a) (a) Elena Pastor , Natalia Monge , Juan Carlos Calderón Gomez , Luis Miguel Gavidia , Gabriel (b) (b) Planes ,Gustavo Marcelo Morales (a) (b)

Universidad de La Laguna, Astrofísico F. Sanchez s/n, La Laguna (38026), Tenerife, España Universidad Nacional de Río Cuarto, Ruta Nac. 36, Km 601, Rio Cuarto (X5804BYA), Argentina epastor@ull.es

Abstract The discovery of graphene, a single-layer carbon atoms densely packed into a two-dimensional honeycomb lattice, has opened up a new field based in bidimensional materials (2D) [1].The driving forces in the field are the potential technologies associated with graphene and its derivatives, such as nanoelectronics, biolabeling, sensors, energy storage and catalysis [2]. The great interest in the use of graphene as catalysts has its origin in three properties: large surface area, very good conductivity and high mobility of charges. In addition, the graphitic structure gives to the graphene based materials a high chemical and mechanical stability. Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. The development of the oxygen electrode catalysts with high activity at low cost remains a great challenge despite tremendous efforts. The non-noble metal catalysts based on pyrolyzed carbon and nitrogen precursors have shown good performance toward the oxygen reduction reaction (ORR) [3]. In graphene, the substitution of carbon by heteroatoms or the covalent bonding of functional moieties containing heteroatoms modifies the electronic structure, hydrophobic character and chemical affinity, providing a way to tailor the catalytic properties. Therefore, graphene could be a good system to correlate its catalytic reactivity with chemical properties. Graphene oxide (GO) and reduced graphene oxide (RGO) can be synthesized from graphite using low cost and easily scalable methods (Scheme 1) [4] and then chemically modified [5]. The present work reports the preparation and characterization of modified GO and RGO with the final purpose to study its use as catalyst in the ORR. The synthesis of GO was performed by a modified Hummers method [6]. RGO was obtained by hydrothermal treatment of GO at 90, 150, 200 and 300 °C. Graphene used as reference was synthesized by electrochemical exfoliation of HOPG in aqueous solution [7]. GO and RGO were doped with nitrogen, sulfur and phosphorous by different methods. The materials were purified by a combination of several centrifugation and dialysis steps. Finally, graphenes were characterized by FTIR, Raman, AFM, TEM and EDS-SEM.

Preliminary measurements realized in acid and basic media show good catalytic activity toward the ORR, in agreement with literature data,.

References [1]a) Geim, A.K. Science 324 (2009) 1530. b) Rao, C.N.R.; Sood, A.K.; Subrahmanyam, K.S.; Govindaraj, A. Angew. Chem. Int. Ed 48 (2009) 7752. [2]a) Stine, R.; Mulvaney, S.P.; Robinson, J.T.; Tamanaha, C.R.; Sheehan, P.E. Analytical Chemistry 85 (2013) 509. b) Feng, L.; Wu, L.; Qu, X. Adv. Mat. 25 (2013) 168. c) Kuila, T.; Mishra, A.K.; Khanra, P.; Kim, N.H.; Lee, J.H. Nanoscale 5 (2013) 52. [3]a) Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J. Energy & Environmental Science 4 (2011) 3167. b) Li, W., Wu, J., Higgins, D.C., Choi, J.-Y., Chen, Z. ACS Catalysis 2 (2012) 2761. [4]a) Dreyer, D.R., Ruoff, R.S., Bielawski, C.W. Angew. Chem. Int. Ed 49 (2010) 9336. b) Guo, S., Dong, S. Chem. Soc. Rev 40 (2011) 2644. c) Singh, V., Joung, D., Zhai, L., Das, S., Khondaker, S.I., Seal, S. Prog. Mater.Sci. 56 (2011) 1178. [5] a) Loh, K.P.; Bao, Q.; Ang, P.K.; Yang, J. J. Mat. Chem. 20 (2010) 2277. b) Dreyer, D.R.; Park, S.; Bielawski, C.W.; Ruoff, R.S.; Chem. Soc. Rev. 39 (2010) 228. [6] Hummers, Wm. S., Jr.; Offeman, R. E. J. Am. Chem. Soc. 80 (1958) 1339. [7] Morales, G.M.; Schifani, P.; Ellis, G.; Ballesteros, C.; Gerardo, M.; Barbero C.; Salavagione, H.J. Carbon, 49 (2011) 2809.

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Scheme 1: A) graphene and B) graphene oxide and reduced graphene oxide [5]. Acknowledgement Financial support from the Spanish Ministry of Science and Innovation (MICINN) and Argentine Ministry of Science, Technology and Productive Innovation (MINCYT) (PRI-AIBAR-2011-1307 and CTQ201128913-C02-02), FONCYT-PICT-2011-1701 and CONICET-PIP 2010-2012 GI are gratefully acknowledged. G.M. Morales and G. Planes are permanent research fellows of CONICET.

Contribution (Poster)

Comparison of different Graphene Materials and their Electrochemical Application 1

Wendy Patterson* , Alexander ZĂśpfl , Masoumeh Sisakthi , Thomas Hirsch , 1 2 1 Otto S. Wolfbeis , Christoph Strunk , Frank-Michael Matysik 1

Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg 2 Institute of Experimental and Applied Physics, Micro- and Nanophysics UniversitĂ¤tsstrasse 31, 93053 Regensburg, Germany wendy.patterson@chemie.uni-regensburg.de

Carbon materials are extensively used in electrochemistry due to their wide electrode potential window, low cost, chemical stability, and electrocatalytic activity for various redox reactions [1]. In particular, graphene is popular because of its high electrical conductivity and its potential in terms of device miniaturization, and also in lowering sensor detection limits, and is proving to be a versatile material for many applications. The most common synthesis routes to obtain graphene are the Scotch-tape method, chemical vapor deposition (CVD), and chemical preparation of reduced graphene oxide (rGO). All of these methods require varying amounts of effort, and result in materials differing in size, quality and uniformity of coverage.

We have systematically evaluated and compared each type of these graphene materials using microscopy to study their varying morphology, with Raman spectroscopy to obtain chemical and structural information, and with electrochemical methods to investigate electron transfer characteristics. The density of defects has a considerable effect on the electronic properties of graphene, with a higher concentration of defects resulting in lower electrical conductivity. Such defects are visible in the Raman spectra. Whereas graphene flakes obtained by the Scotch-tape method are nearly defect-free, the CVD graphene contains structural defects, and rGO is ill-defined as can be seen from the Raman spectra (Fig. 1).

For electrochemical applications, it is important to transfer the graphene to an insulating substrate, and also to provide electrical contacts. Each of the preparation methods require different transfer methods, each varying in effort. CVD graphene was transferred by a standard stamping process [2] onto electrodes with preexisting electrical contacts. The Scotch-tape graphene was not transferred, but rather the electrical contacts were applied afterwards by a laborious electron beam method. In contrast, since rGO can be dispersed in water, easy transfer to any substrate or preexisting electrode was possible by spin coating or drop casting. Characterization by Raman spectroscopy and microscopy reveals better layer uniformity for the CVD graphene as compared to rGO and Scotch-tape derived graphene. In this work, electrochemical characterization is also presented. Furthermore, the electrocatalytic activity towards H2O2 was investigated. Graphene has excellent potential for wide spread sensor applications due to its exceptional structural, electrical, electrochemical, optical and mechanical properties. However, practical considerations such as differences in the ease of synthesis, transfer, and electrode construction for the various types of graphene must be considered for future widespread industrial applications and mass production.

Contribution (Poster)

The research was supported by Deutsche Forschungsgesellschaft (GRK 1570). References

[1]

M. Zhou, Y. Zhai, S. Dong, Anal. Chem. 81, 5603â&#x20AC;&#x201C;5613 (2009)

[2]

X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, R. S. Ruoff, Nano Lett. 9 (12), 4359â&#x20AC;&#x201C;4363 (2009)

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Chemical vapour deposition of graphene on 3-dimensional metal foams for energy-storage applications 1,2

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J. Pedrós , K. B. K. Teo , J. Martínez , S. Álvarez-García , A. Boscá , A. de Andrés , and F. Calle 1

Instituto de Sistemas Optoelectrónicos y Microtecnología, Universidad Politécnica de Madrid, Av. Complutense s/n, Madrid 28040, Spain 2 CEI Campus Moncloa, UCM-UPM, Av. Complutense s/n, Madrid 28040, Spain 3 AIXTRON Ltd, Anderson Road, Swavesey, Cambridge CB24 4FQ, United Kingdom 4 Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Sor Juana Inés de la Cruz 3, Madrid 28049, Spain j.pedros@upm.es

Several applications rely on the integration of 2-dimensional (2D) graphene sheets into 3D structures while keeping their unique properties, such as the high surface area and electrical conductivity. Metal oxide nanoparticles are typically mixed with the graphene sheets to avoid their stacking and to functionalize their surface, forming composites. A novel approach to fabricate graphene/metal oxide composites uses 3D Ni foams as templates where the graphene is grown by chemical vapour deposition (CVD) [1]. After removing the metal scaffold, the free-standing graphene foams can be coated with an electro-deposited layer of metal oxide [2] or with metal oxide nanoparticles dispersed in a solution [3]. These graphene 3D networks are being investigated, for example, as advanced electrode materials for sensors [4] and energy storage devices, such as supercapacitors [2] and lithium ion batteries [3]. In this communication, we report on the properties of graphene grown by CVD and plasmaenhanced CVD (PECVD) on 3D Ni and Cu foams using CH4 or C2H2 as precursors. The temperature, pressure, and plasma conditions were varied to study their effect on the graphene properties. The graphene/metal foam structures have been characterized by Raman spectroscopy and scanning electron microscopy (SEM). The structural characteristics and the homogeneity of the graphene coating are discussed in terms of the growth conditions. We demonstrate an accurate control of the number of graphene layers by tuning the deposition conditions, not only by CVD at standard temperatures (T~ 1000ºC) but also by PECVD at reduced temperatures as low as 700ºC. Figure 1 presents the Raman spectra and SEM images of graphene layers grown by PECVD on Ni foams at 600ºC, 700ºC, and 800ºC. The quality and number of layers evolves with temperature as follows. At T = 600ºC (Fig. 1(a)) the D and D' Raman peaks indicate the presence of defects in the graphene coating, that are associated to the formation of nanocrystals, as confirmed by the textured morphology observed by SEM (Fig. 1(c)). At T = 800ºC (Fig. 1(g)) the 2D Raman peak presents a graphite-like shape, indicating a large number of layers. However, at T = 700ºC the Raman spectrum (Fig. 1(d)) indicates that defectfree bilayer (or few-layer) graphene is obtained. SEM images indicate an almost continuous coverage of the foam surface (Fig. 1(e)), with some influence from its grain structure. The optimum temperature of the PECVD process presented is much lower than that previously reported for CVD graphene foams [14]. Acknowledgements. This work has been partially supported by Repsol (Programme Inspire) and Ministerio de Economía y Competitividad (Project No. TEC 2010-19511). J. Pedrós acknowledges the support from the Moncloa Campus of International Excellence (UCM-UPM, ISOM).

References [1] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, and H. Cheng, Nature Mater. 10 (2011) 424. [2] X. Cao, Y. Shi, W. Shi, G. Lu, X. Huang, Q. Yan, Q. Zhang, and H. Zhang, Small 7 (2011) 3163. [3] N. Li, Z. Chen, W. Ren, F. Li, and H. Cheng, PNAS 109 (2012) 17360. [4] F. Yavari, Z. Chen, A. V. Thomas, W. Ren, H. Cheng, and N. Koratkar, Sci. Rep. 1 (2011) 166.

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Divergence of the Thermal Conductivity in Uniaxially Strained Graphene Luiz Felipe C. Pereira, Davide Donadio Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz, Germany pereira@mpip-mainz.mpg.de Abstract Unique heat transport properties make graphene a strong candidate to applications in future thermal management devices. At room temperature its thermal conductivity is dominated by phonons and values as large as 5000 W/m-K have been measured for suspended graphene at ≈ 300 K [1]. Graphene is also a very interesting candidate for phononics and a graphene-based thermal diode has been proposed [2]. However, in order to build functional devices it is necessary to manipulate and tailor phonon transport properties, which requires a deep understanding of the behavior of phonons in graphene. We perform extensive equilibrium molecular dynamics simulations, based on the Tersoff interatomic potential, to understanding the mechanism of heat transport in suspended graphene under various conditions. We show that the thermal conductivity of unstrained graphene, calculated from the fluctuations of the heat current at equilibrium, is finite and converges with size at finite temperature. Studying size convergence we demonstrate that low-frequency out-of-plane vibrational modes act as scatterers and limit the thermal conductivity to a large but finite value. We then show that the thermal conductivity of an extended periodic graphene model under uniaxial tensile strain diverges logarithmically with the size of the model, when strain exceeds a threshold value of 2%. Tensile strain changes the dispersion relations of phonons in graphene, including a linearization of the originally quadratic dispersion of out-of-plane modes. An analysis of phonon populations and lifetimes shows that the divergent behavior is caused by changes in the occupation of low-frequency out-of-plane phonons and an increase in their lifetimes due to strain. Furthermore, the divergence observed in our simulations would lead to a strong size dependence in experimental measurements of thermal conductivity, which allows for a direct verification. In view of the recent fabrication of graphene with predefined concentrations of carbon isotopes [3], we also investigate the effect of isotopic mass disorder on the thermal conductivity of unstrained and strained graphene. Our simulation results show that even for the highest concentration of 13C isotopes (50%), the divergence of the thermal conductivity remains. Finally, we investigate the thermal conductivity of unstrained and strained functionalized graphene. References [1] A. A. Balandin, Nature Mater. 10 (2011) 569. [2] J. Hu, X. Ruan, and Y. P. Chen, Nano Lett. 9 (2009) 2730. [3] W. Cai, A. L. Moore, Y. Zhu, X. Li, S. Chen, L. Shi, and R. S. Ruoff, Nano Lett. 10 (2010) 1645.

Direct growth of graphene films on Si(111) Pham Thanh Trung1, Frédéric Joucken1, Jessica Campos-Delgado2, Jean-Pierre Raskin2, and Robert Sporken1 1

Research Center in Physics of Matter and Radiation (PMR), University of Namur (FUNDP), 61 Rue de Bruxelles, 5000 Namur, Belgium. 2

Université Catholique de Louvain (UCL), Institute of Information and Communication Technologies, Electronics and Applied Mathematics (ICTEAM), 4 Avenue Georges Lemaître, 1348 Louvain-la-Neuve,Belgium. Email: phamtha@fundp.ac.be Abstract: The preparation of graphene, a 2D sheet of carbon atoms arranged in a honey-comb structure on a suitable substrate, has attracted enormous attention in the scientific community during the last ten years [1-2]. Si(111) 7×7 might be one of the most attractive candidates because of its integration in the silicon technology [3]. The observation of honey-comb lattice of free-standing graphene after growing graphitic carbon films on Si(111) substrate through the deposition of a buffer layer with various thicknesses of amorphous carbon at room temperature has been demonstrated [4] (Figure 1). This indicates a high potential for graphene formation on silicon wafers. However, the surface roughness as well as the small size of the crystallites is still not suitable for further applications. In this poster, we present a significant improvement in the direct growth of graphene on Si(111) 7×7 surface under appropriate conditions through the deposition of carbon atoms using electron beam evaporation of a graphite rod. Our experimental results confirm that the effect of substrate temperature plays a very important role to the quality of graphene films. The structural properties of the samples are investigated by Auger Electron Spectroscopy (AES), X-ray Photoemission Spectroscopy (XPS), Reflection High Energy Electron Diffraction (RHEED), Raman Spectroscopy (RS) and Scanning Tunneling Microscopy (STM). References: [1] A. K. Geim and K. S. Novoselov, The rise of graphene, Nature materials, Vol. 6 (2007). [2] Th. Seyller, A. Bostwick, K. V. Emtsev, K. Horn, L. Ley, J. L. McChesney, T. Ohta, J. D. Riley, E. Rotenberg, and F. Speck, phys. stat. sol. (b) 245, No. 7, 1436–1446 (2008). [3] Francesco Bonaccorso, Antonio Lombardo, Tawfique Hasan, Zhipei Sun, Luigi Colombo, and Andrea C. Ferrari, Materialstoday, vol 15, No.12, 564-589 (2012) [4] Pham Thanh Trung, Frederic Joucken, Jessica Campos-Delgado, Jean-Pierre Raskin, Benoit Hackens, and Robert Sporken, Appl. Phys. Lett. 102, 013118 (2013). Figures:

Figure 1: Atomic resolution STM images of graphene on Si(111) of 10×10nm2 (VSample = -0.2V, IT = 10nA) with an inset is a corresponding FFT image that exhibits diffraction pattern of hexagonal film structure.

1.5nm

In-situ TEM study of hexagonal holes in graphene growing from defects in oxygen atmosphere Filippo Pizzocchero, Tim Booth and Peter Bøggild DTU Nanotech, Ørsteds Plads b.344 , Kgs. Lyngby, Denmark filippo.pizzocchero@nanotech.dtu.dk Abstract The recent improvements in the quality of the graphene growth [1-2] have narrowed down the differences in terms of performances between artificially produced and mechanically exfoliated graphene. In particular, Chemical Vapor Deposition (CVD) seems to represent the most suitable technique for the implementation of graphene films as components in standard cleanroom fabrication processes. In order to preserve and exploit the outstanding properties of graphene, great care has to be spent in preserving the structure and smoothness of the edges, crucial parameters for the performances of most of the graphene-based devices [3]. For this reason, a large scale patterning technique able to preserve the crystallographic orientation and quality of the graphene edges would be ideal. Catalytic oxidation of graphene by metal nanoparticles [4-5] appears to be an interesting candidate for such a purpose. The intrinsic stochastic nature of the motion of the particles [5] poses fundamental limitations on the control and reproducibility of the etched channels and fabricated devices. The anisotropic oxidation of graphene can overcome these limitations. Defects in graphene develop into hexagonal shapes at high temperatures (around 800 °C) in oxygen rich atmosphere [6] or in presence of hydrogen plasma [7-8]. The edges of these hexagons are parallel to the <100> crystallographic direction in the graphene lattice, the so-called zigzag (ZZ) direction [6-8]. All the previous reports on this phenomenon were performed ex-situ and on a support, typically silicon oxide [6-8]. In this work we report, for the first time, the in-situ observation of oxidation of suspended graphene inside a FEI Titan Environmental Transmission Electron Microscope (E-TEM). The evolution of the defects in the graphene flake is studied in a large temperature range (room temperature – 1000 °C) and with different oxygen pressures. This results in a wide ensemble of behaviors of the holes, which grow as hexagons only in a specific temperature range (400 °C – 700 °C), while presenting a more isotropic dynamics at higher temperatures. The measure of the velocity of the receding edges is carefully evaluated with a statistical approach of the discrete removal of carbon atoms. The resulting energy barrier is compared with Density Functional Theory (DFT) calculations, which help to explain the cause of the exclusively ZZ edges of the hexagons. High resolution (HR) TEM images makes possible to estimate the roughness of the produced holes, which results to smaller than 1 nm.

References [1] L. Tao et al. - J. Phys. Chem. C, 116 (45), 2012, 24068–24074 [2] S. Chen et al. – Advanced Materials, Pre-print Online Version, 2013 [3] K. A. Ritter and J.W. Lyding - Nature Materials 8, 2009, 235-242 [4] L. Ci et al. - Nano Res. 1, 2008, 116-122 [5] Booth et al. - Nano Lett., 11 (7), 2011, 2689–2692 [6] X. Wang and H. Dai - Nat. Chem. 2, 201, 661–665 [7] Z. Shi et al. - Advanced Materials Vol. 23, Issue 27, 2011, 3061–3065 [8] L. Xie , L. Jiao , and H. Dai - J. Am. Chem. Soc., 132 (42), 2010, 14751–14753

Figures

Fig. 1 – a-d TEM images of the evolution of beam-induced defects in suspended graphene at 600 °C and 10-2 mbar oxygen atmosphere. The scale bar is 50nm and the time between two consecutive frames is 30s.

Excitonic characteristics of singlelayer MoS2 G. Plechinger, S. Heydrich, J. Eroms, D. Weiss, C. Schüller and T. Korn Institut für Experimentelle und Angewandte Physik, Universität Regensburg, D-93040 Regensburg, Germany gerd.plechinger@physik.uni-regensburg.de Abstract Complementary to the gapless material graphene, the transition-metal dichalcogenide molybdenite (MoS2) is a promising two-dimensional layered semiconductor for future ultrathin nanoelectronic and optoelectronic devices. Recent experiments have revealed the transition from an indirect to a direct-gap semiconductor, going from the bulk material to the monolayer regime [1]. Subnanometer thickness, large direct bandgap in the visible range and ultrafast carrier dynamics make singlelayer MoS 2 interesting for devices like transistors, ultrafast optical switches or photovoltaic applications. Here, we present photoluminescence and Raman measurements on single- and few-layer MoS2 flakes. The samples are produced by the well-known transparent tape liftoff method from natural molybdenite. To identify monolayer regions on the substrate, we first characterize them in an optical microscope. For further determination of the layer number, we apply scanning Raman spectroscopy and atomic force microscopy [2, 3]. The observation of a frequency shift in the Raman spectrum of the interlayer shear mode induced by a change in the layer number allows us to use a mapping of this mode frequency for the assignment of the number of layers in few-layer samples (fig. 1). Via photoluminescence (PL) measurements on the singlelayer regions of our samples, we investigate the different behavior of the so-called A and B excitons. These bound electron-hole-pairs arise from transitions from the spin-orbit split valence band to the conduction band at the K-point of the Brillouin zone with energies of 1.8 eV (A) and 2.0 eV (B). We see a difference in the temperature-induced energy shift for the two excitons, going from 4 K to room temperature. We also infer an energy transfer between A and B excitons dependent on the excitation power. Recently, coupled spin-valley-physics in monolayer MoS2 due to symmetry reasons was predicted [4]. Based on an inverse spin occupation of the split valence band at the K + and the K- valley in the Brillouin zone, a lot of interesting new effects are expected using the valley index as a new degree of freedom in this system. We can see the conservation of the helicity of the incoming laser light in the PL if the excitation energy lies between the A and B exciton energy, which might be a basic effect of coupled spin-valley-physics (fig. 2). Furthermore, we could generate monolayer regions out of few-layer flakes via intense focused laser radiation. Thereby, monolayer MoS2 samples can be designed in shape and size as desired. Financial support by the DFG via GRK 1570, SFB689, SPP1285, and KO3612/1-1 and by the DBU is gratefully acknowledged.

References [1] K. F. Mak, C. Lee, J. Hone, J. Shan und T. F. Heinz, Phys. Rev. Lett. 105 (2010) 136805. [2] G. Plechinger, S. Heydrich, J. Eroms, D. Weiss, C. Schüller und T. Korn, Appl. Phys. Lett. 101 (2012) 101906. [3] T. Korn, S. Heydrich, M. Hirmer, J. Schmutzler und C. Schüller, Appl. Phys. Lett. 99 (2011) 102109. [4] D. Xiao, G.-B. Liu, W. Feng, X. Xu und W. Yao, Phys. Rev. Lett. 108 (2012) 196802.

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Fig. 1: (a) Raman mapping of the frequency difference of the E 2g and the A1g mode. Purple color indicates a monolayer region. (b) Raman mapping of the interlayer shear mode on the same sample as in (a). The frequency of this mode is strongly dependent on the layer number and is therefore suitable for sample characterization.

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Fig. 2: (a) Photoluminescence spectra of singlelayer MoS2 at 20 K under circularly polarized near resonant excitation and helicity dependent detection. We see a conservation of the helicity of the incoming laser light. (b) Degree of circular polarization under circularly polarized near resonant excitation for sample temperatures from 4K up to 180K.