Molecular Conductivity

Molecular electronics are one of promising candidates for future electronic devices. One typical model is sandwiching a single molecule(usually conjugated molecule) between two mesoscopic electrodes. However, both experiments and theories have great difficulties to understand conducting mechanism. The detailed molecular configuration of studied system in experiments is unclear where single molecule can be viewed as one particular kind of defects. The stability of this system is also a big issue since it is easy to break during the measurements by STM tips. Density functional theory(NEGF) and Hartree-Fock(HF) theory combined with non-equilibrium Green's function method have been used to simulate the model system under Landauer picture, however they do not agree with experimental results very well. Some important problems, such as electron-phonon coupling and non-accurate molecular configuration, are not treated very well. We try to employ time-dependent density functional theory(TDDFT) and Vanderbilt's ultrasoft pseudopotentials(USPP) to study electron transport through the model system, which allows people to go beyond linear response region of DFT-NEGF and include electron-phonon effect. Our preliminary results show that TDDFT calculation in the rigid band approximation for zero-bias case is similar to NEGF and complex band structure method's calculation. Later we will include electron-phonon coupling, voltage/current boundary condition and other nontrivial effects.

In terms of computational cost, plane wave basis-set is much more expensive than localized orbitals. Recently we have developed a set of quasiatomic orbitals(QO) which combines Kohn-Sham occupied and unoccupied orbitals via certain localization strategy from plane-wave DFT calculations. These localized orbitals allow us to capture the range of electronic structure exactly. Therefore they can be applied to NEGF to study linear response regime of electrical conductance. By comparing the detailed results from QO-NEGF and TDDFT, we hope to find the difference between these two theoretical approaches and compare with experiments, then finally predict the existence of negative differential resistance(NDR) in various systems.

Figure: (a) The one on left-top shows Fermi electron from gold nanowire transmits through benzene-(1,4)-dithiolate (BDT) junction from TDDFT calculation. (b) The one on the right-top shows optical absorption spectrum of benzene molecule from TDDFT gives a sharp peak of pi-pi* transition around 7eV and a broad peak of sigma-sigma* transition above 9eV. (c) Figure on left-bottom is one of chemical-bonding environment-dependent quasiatomic orbitals for boron atoms in superconductor magnesium diboride(MgB2). (d) Figure on right-bottom shows the corresponding Fermi surface for MgB2 based on these QOs.

This project is supported by DARPA-ONR funding agency.

Reference:

Time-dependent density functional theory with ultrasoft pseudopotentials: Real-time electron propagation across a molecular junction
Xiaofeng Qian, Ju Li, Xi Lin and S. Yip, Physical Review B 73, 035408 (2006).

Quasiatomic orbitals for ab initio tight-binding analysis
Xiaofeng Qian, Ju Li, Liang Qi, Cai-Zhuang Wang, Tzu-Liang Chan, Yong-Xin Yao, Kai-Ming Ho, and Sidney Yip. Physical Review B 78, 245112 (2008).