Low temperature growth of SnO2 nanowires by Resistive thermal evaporation and their application in Methanol sensing R. Rakesh Kumar, K. Narasimha Rao and K.Rajanna Department of Instrumentation and Applied Physics, Indian Institute of Science Bangalore – 560 012, India
OBJECTIVE
CHARACTERIZATION
To grow Tin oxide (SnO2) nanowires by resistive thermal evaporation
technique
at
low
temperatures
by
VLS
mechanism and testing their Methanol sensing
Micro structural and morphology: FESEM (FEI SIRION),Transmission electron microscopy(TEM) ( Technai F-30) Crystallinity: XRD (Bruker D8 advance X- ray diffractometer), Photo luminiscence (LabRAM HR instrument with CCD detector, illuminated by 325 nm line of He-Cd laser) Composition: XPS (Multilab 2000 Thermo Scientific)
Vapor Liquid Solid (VLS) Mechanism Vapor
Vapor (V)
nanowire
UV response: Current - Voltage (I-V) measurements were performed by applying the bias voltage using a source meter (model : Keithley-2400) and the current was measured through an electrometer (model: Keithley-6514) controlled by LABVIEW (National Instruments).
Solid (S) nanowire
I Metal
catalyst
Advantages of VLS mechanism
II III Alloy Nucleation Liquid(L)
X-ray Photoelectron Spectroscopy
R E S U L T S
IV
Fig 4 XPS patterns of the as-grown SnO2 NWs. (a) Survey spectrum (b)High resolution core level spectrum of Sn 3d.
Scanning Electron Microscopy (SEM)
Growth
X-ray Diffraction
Control on diameter of nanowires ( by defining catalyst size) Selective area nanowires growth (by placing catalyst particles)
(b)
(a)
Grow Variety nanowires (SnO2, Si, Ge, GaN ,InP, Zno etc..)
The survey spectrum reveals the existence of Sn, Au, O, and C elements. C peak was due the to exposure of the sample to the atmosphere. The peaks centered at 486.6 eV and 495.1eV were assigned to Sn 3d5/2, and Sn3d3/2 of Sn+4 ion in SnO2, respectively.
500 nm
Nanowire hetrojunctions , core shell structures( SnO2 / ZnO, Si/Ge)
Application of SnO2 NWs Gas sensor of many gases
Super hydrophobic surfaces
Li – ion rechargeable batteries
Field emitters
UV photo detectors
Dye based solar cells
(c) 200 nm
E X P E R I M E N T A L
Fig. 1 (a) High and (b) Low magnification SEM images of SnO2 nanowires (Au tipped on
Thermo couple
the surface is marked with red arrows) (c) Cross sectional view of the SnO2 nanowires on Si (d) XRD pattern of the as grown SnO2 nanowires on Si substrate, (inset) High
Substrate holder
20 cm
Set up
Length approximately : 2-3 µm, diameter 30-100 nm Nanowire growth rate: 100 nm/min, aspect ration of nanowires 100:1
shutter Crystal monitor
The measured XRD reflections have been indexed as (111), (101) and (221) planes of the tetragonal rutile structure of the SnO2 (JCPDS card # 77-0448)
Transmission Electron Microscopy(TEM)
Sn evaporation boat
Shutter open/close
(a)
(b)
vacuum pump Diffusion pump, rotary pump combination with Liquid Nitrogen trap
Absence of Methanol
Thermal evaporation of Sn in presence of Oxygen
Cleaned in an ultrasonic bath with acetone, de-ionized (D.I) water and finally with a dilute HF solution for 3 min to remove the native oxide layer.
Deposition parameters Gold deposition and formation of gold droplets We have employed BALTEC SCD 500 sputter coating system to deposit gold film of 3 nm thickness. The distance between the substrate and gold target is fixed at 10 cm to obtain uniform gold film. Au coated substrates were annealed in situ at 450º C under high vacuum for 15 min prior to the growth of SnO2 NWs to form gold droplets
Sn evaporation (Nanowires growth) Substrate temperature :4500C Rate of deposition : 0.2 nm/s Vacuum : 4×10-4 mbar (PO2 ) Time of nanowires growth : 15 min Thickness monitoring
Thickness and the rate of deposition were monitored by quartz crystal monitor. Advantages of Thermal evaporation Simple economic PVD technique Less expensive Non ultra high vacuum technique with broad industrial applications Precise control of deposition parameters (rate of evaporation)
Au Intensity (a. u.)
Deposition method
100 nm
O
Sn
(c)
Cu Cu
Au
(d)
Au
Intensity (a. u.)
Pumping system
Cleaning
Fig 5 Figure.3 (a) – (b) Temperature modulation and concentration modulation of the methanol sensing studies (c) Typical resistance variation at optimum operating temperature during the methanol exposure
magnification SEM image of gold droplets formed Si substrate after annealing of gold film of 3 nm thick at 450 °C for 15 min
Substrates
Oxygen
(d)
2µm
NANOWIRES GROWTH Substrate heater
2µm
1µm
Transparent conducing electrodes
Methanol Sensing
The resistance of SnO2 nanowires film increases in the presence of oxygen ambiance. O2(gas) O2 (adsorbed) Methanol O2 (adsorbed) + eO2- (adsorbed) O2-(adsorbed) + e2O-(adsorbed)
Presence of Methanol O
Sn
During the sensing process, methanol undergoes dehydrogenation resulting in the release free electrons and there by varying the film resistance. As a result of this, the resistance of sensing material decreases during sensing. Reaction mechanism as follows
Cu
CH3OH + O-(adsorb)---------- CH2O+ H2O+ e-
Energy (KeV)
Energy (KeV)
Figure 2 (a) TEM image of single nanowire (b) HRTEM image of the nanowire , EDX Spectrum as recorded on (c) tip (d)wire The spacing between the lattice fringes is found to be 0.26 nm, which is well in accordance with the d spacing of the (101) plane of SnO2.
Photo luminescence A very broad emission peak centered at 556 nm is attributed to oxygen vacancies. The oxygen vacancies create defect states in the middle of the band gap. These defect levels trap electrons from the valence band and contribute to the luminescence
CH3OH + O2- (adsorb)---------- HCOOH+ H2O+ e-
C O N C L U S I O N S Tin oxide (SnO2) nanowires were grown on Si substrates by thermal evaporation of Sn at 4×10-4 mbar oxygen partial pressure and at 450 °C substrate temperature SEM studies show that the nanowires are oriented mostly vertical direction on the substrate, and the growth is observed to follow VLS mechanism. XRD, XPS studies show that the grown SnO2 nanowires are crystalline in nature and pure. TEM studies confirmed the single crystalline nature of the nanowires SnO2 NWs films showed reproducible response to Methanol
REFERNCES Fig 3 Room temperature PL spectrum of the SnO2 NWs
ACKNOWLEDGEMENTS
The authors sincerely acknowledge the DST- Advanced Facility for Microscopy and Microanalysis(AFMM), Indian Institute of Science.
1.
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2.
Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, and C. Zhou, Adv.Mater. 15 1754 (2003). R.Rakesh Kumar, M. Parmar, K. Narasimha Rao, K.Rajanna, A R Phani, Sciri. Mater. 68 408 (2013).
3.
R. Rakesh Kumar Department of Instrumentation and Applied Physics Indian Institute of science ,Bangalore Email: rakeshr@isu.iisc.ernet.in, Rakesh.rajaboina@gmail.com http://isu.iisc.ernet.in/~rakeshr/ Ph: 080-22933190.
NANOPOSTER 2013 - 3rd Virtual Nanotechnology Poster Conference 9th – 13TH Sept, 2013.