EFFECT OF JUNCTION SYMMETRY ON MOLECULAR CONDUCTANCE 1
2
V. K. Lamba, O. P. Garg
1
Gobal College of Engineering & Technology Khanpur Khui ; Punjab, India; 2RKSD College, Kaithal, India Email: lamba_vj@hotmail.com
In an effort to study the electronic states of anthracene-dithiol, and anthracene-dicyano , their molecular structures were How single-molecule devices operate and how its completely optimized by density functional theory (B3LYP) by characteristics can be controlled using the symmetry of an using (d, p) basis sets. The HOMO and LUMO energies and electrode molecule junction where two atoms from the HOMO-LUMO gaps are summarized in Table 1. The electronic molecule are anchored to two facing electrodes; states of the anthracene-dithiol, and anthracene-dicyano are Effect on conductance with molecule junction symmetry; clearly similar. Design Reliable computer simulations with quantitative HOMO (eV) LUMO (eV) Eg (eV) predictive power, i.e. Study of simple test systems. a -5.39 -1.25 4.14
Aim !
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Tool Used Virtual Nano Lab 11.8.2.
Model & Computational Method According to Landauer’s model, the conductance of a molecular junction composed of a molecule and two gold electrodes is
Where 2e2/h = Quantum conductance T = Transmission probability Ef = Fermi energy For calculating transmission probability, we employed the non-equilibrium Green’s function method combined with the Hückel molecular orbital method (NEGF-HMO). The transmission probability can be calculated
R
A
where G (G ) and GL (GR) represent the retarded (advanced) Green’s function and the broadening function for the left (right) electrode, respectively. We consider only the nearest-neighbor interactions between the electrode and the molecule
where I is the unit matrix and H is the Hamiltonian matrix of the molecule. In the NEGF-HMO method, the Hückel Hamiltonian matrix is employed for H. The parameters for the HMO calculations are set as a S = a C; ßC-S = 0.5 ßC-C, ßS-Au = 0.28 ßC-C, and ßAu-Au = 0.6 ßC-C, where a and ß are the Coulomb integral and resonance integral, respectively. When the Fermi level (EF) of the electrodes is between the energies of the highest occupied molecular orbital (eHOMO) and the lowest unoccupied molecular orbital (eLUMO), the transmission probability is qualitatively predicted from the contribution of the HOMO and LUMO in the isolated molecular Green’s function
b c d e f g h
5.52 5.62 -5.61 -5.39 -5.52 -5.74 -5.61
-1.25 -1.25 -1.28 -1.28 -1.28 -1.25 -1.28
4.27 4.37 4.33 4.11 4.24 4.49 4.33
ABSTRACT Molecular junction is a promising candidate for devices nowadays. Great effort has been devoted recently to understand the role of the symmetry in the transport properties of molecular junctions. However, these studies have been largely based on the analysis of the low-bias conductance, which does not allow to elucidate the exact influence of the symmetry in both the electronic structure and transport characteristics of the junctions. In this work we present a theoretical study of the transport properties, and how conductance changes with symmetry. Herein, we investigate an anthracene-dithiol, and anthracenea b single-molecule system in which sulphur and dicyano cyanoc group from the molecule are anchored to two d facinge gold electrodes. We have performed first principles calculations of the transport properties of f theseg molecules using a combination of density h functional theory and non-equilibrium Green's function techniques. Our computational results show that all anthracene isomers have similar energy gaps and molecular orbital levels, anchored with sulphur and cyano groups. The variation of conductance can be explained due to the difference in the frontier molecular orbital phases of the two anchored groups with the electrodes. Thus the orbital symmetry rule for charge transport in a molecular junction provides a rule for the design of molecules that exhibit known value of conductance
Molecular projected self-consistent Hamiltonian (MPSH) analysis at the DFT level of theory. (a) MPSH analysis for (a) molecule itself and (b)molecule and first-layer gold atoms.
Key Words: Nano-Junctions, NEGF, Anchors Group, Symmetry.
where CR,HOMO and CR,LUMO are the molecular-orbital expansion coefficient at a sulfur atom connected to the right electrode at the HOMO and LUMO, respectively, and the other terms are comparably assigned . This equation implies that when the signs of the two products CR,HOMOC*L,HOMO and CR,LUMOC*L,LUMO differ, Au single molecule Au junctions are symmetry allowed and exhibit high single-molecule conductance; when the signs of the two products are the same, the junctions are symmetry forbidden and exhibit low single-molecule conductance.To investigate orbital symmetry rule for electron transport,we selected anthracene-dithiol, and anthracene-dicyano molecule.
We explored the voltage dependence of single-molecule conductance by measuring current voltage characteristics at bias voltages of 0.8 to 0.8 V when the nanoelectrode gaps were fixed in the state where single-molecule conductance. The current voltage characteristics show single-molecule conductance for anthracene-dithiol, and anthracene-dicyano is in the order of e > f > d & h > g; a > b > c, respectively. We found that thel conductance does not followed the exponential decay law, although the exponential decrease in single-molecule conductance with increasing distance between sulfur atoms indicated electron tunneling. Exponential-distance scaling is known to fail for either of the two scenarios:(1) when electrode molecule coupling energy is strong compared to half of the HOMOLUMO gap or (2) when sitesite interaction in the molecule is sufficiently strong to drive the single-molecule junction close to resonance tunneling.
Conclusions we found that differences in the symmetry of anthracenedithiol, and anthracene-dicyano single-molecule junctions correspond strongly to differences in the single-molecule conductance of the junctions. Although all of the studied isomers have similar energy gaps and molecular orbital levels, the highest and lowest single-molecule conductances differ by a factor of 110. This large difference originates not from the difference in energy gaps and molecular orbital levels but rather from the difference in the frontier molecular orbital phases of the two sulfur atoms/ Cyano group anchored to the electrodes. Thus, we have demonstrated clearly that molecular orbital theory provides an orbital symmetry rule for describing electron transport in single-molecule junctions. Thus the orbital symmetry rule for electron transport in single-molecule junctions provides a guiding principle for the design of molecules that exhibit desirable levels of single-molecule conductance.
ACKNOWLEDGMENT We acknowledge the support by C.S.I.R. under Grant No. 22(0519)10/EMR-II. The authors would like to thank Dr. Swapan K. Pati, TSU Unit, JNCASR-India for useful discussions.