Zekan Qian

An efficient nonequilibrium Green’s function formalism combined with density functional theory approach for calculating electron transport properties of molecular devices with quasi-one-dimensional electrodes

An efficient self-consistent approach combining the nonequilibrium Greens function formalism with density functional theory is developed to calculate electron transport properties of molecular devices with quasi-one-dimensional (1D) electrodes. Two problems associated with the low dimensionality of the 1D electrodes, i.e., the nonequilibrium state and the uncertain boundary conditions for the electrostatic potential, are circumvented by introducing the reflectionless boundary conditions at the electrode-contact interfaces and the zero electric field boundary conditions at the electrode-molecule interfaces. Three prototypical systems, respectively, an ideal ballistic conductor, a high resistance tunnel junction, and a molecular device, are investigated to illustrate the accuracy and efficiency of our approach.

Analysis on the contribution of molecular orbitals to the conductance of molecular electronic devices

We present a theoretical approach which allows one to extract the orbital contribution to the conductance of molecular electronic devices. This is achieved by calculating the scattering wave functions after the Hamiltonian matrix of the extended molecule is obtained from a self-consistent calculation that combines the nonequilibrium Greens function formalism with density functional theory employing a finite basis of local atomic orbitals. As an example, the contribution of molecular orbitals to the conductance of a model system consisting of a 4,4-bipyridine molecule connected to two semi-infinite gold monatomic chains is explored, illustrating the capability of our approach.

An accurate and efficient self-consistent approach for calculating electron transport through molecular electronic devices: including the corrections of electrodes

A self-consistent ab initio approach for calculating electron transport through molecular electronic devices is developed. It is based on density functional theory (DFT) calculations and the Greens function technique employing a finite basis of local orbitals. The device is rigorously separated into the extended molecule region and the electrode region. In the DFT part calculating the Hamiltonian matrix of the extended molecule from its density matrix, the electrostatic correction induced by electrodes and the exchange?correlation correction due to the spatial diffuseness of localized basis functions are included. Our approach is efficient and accurate, with a controllable error to deal with such open systems. A one-dimensional infinite gold monatomic chain, whose electronic structure can be known from conventional DFT calculations with periodic boundary conditions (PBCs), is employed to validate the accuracy of our approach. With both corrections, our result for the gold chain at equilibrium is in excellent agreement with the PBC DFT result. We find that, for the gold chain, the exchange?correlation correction is more significant than the electrostatic correction.

First-principles calculation on the conductance of a single 1,4-diisocyanatobenzene molecule with single-walled carbon nanotubes as the electrodes

The conductance of a single 1,4-diisocyanatobenzene molecule sandwiched between two single-walled carbon nanotube (SWCNT) electrodes are studied using a fully self-consistent ab initio approach which combines nonequilibrium Greens function formalism with density functional theory calculations. Several metallic zigzag and armchair SWCNTs with different diameters are used as electrodes; dangling bonds at their open ends are terminated with hydrogen atoms. Within the energy range of a few eV of the Fermi energy, all the SWCNT electrodes couple strongly only with the frontier molecular orbitals that are related to nonlocal pi bonds. Although the chirality of SWCNT electrodes has significant influences on this coupling and thus the molecular conductance, the diameter of electrodes, the distance, and the torsion angle between electrodes have only minor influences on the conductance, showing the advantage of using SWCNTs as the electrodes for molecular electronic devices.

First-principles calculation on the zero-bias conductance of a gold/1,4-diaminobenzene/gold molecular junction

The conductance of a Au/1,4-diaminobenzene/Au molecular junction is investigated using a fully self-consistent ab initio technique which combines the non-equilibrium Greens function formalism with density functional theory. The transmission coefficient at the Fermi level is calculated to be 0.028, in reasonable quantitative agreement with the recent break junction experiments (0.0064). Transport is mainly through the highest occupied molecular orbital (HOMO) and the HOMO-2 state of the 1,4-diaminobenzene molecule. These states are all π conjugated orbitals formed from the p-orbitals of the two nitrogen atoms and a π-type orbital on the benzene backbone. Our calculations also demonstrate that these frontier molecular states form two σ-type bonds with the s-orbitals of the gold adatoms, which is helpful in reducing the dependence of the junction conductance over the anchoring geometry.

Effects of spin–orbit coupling on the conductance of molecules contacted with gold electrodes

The effects of spin-orbit coupling on the conductance of molecular devices made with Au electrodes are investigated using a fully self-consistent ab initio approach, which combines the non-equilibrium Greens function formalism with density functional theory. In general, we find that the extent to which spin-orbit interaction affects the transport depends on the specific materials system investigated and on the dimensionality of the electrodes. For one-dimensional electrodes contacting benzene-dithiol molecules the spin-orbit coupling induces changes in the low-bias conductance up to about 20%. These originate mostly from changes in the electrode band structure. In contrast when three-dimensional electrodes are used, the bands near the Fermi level are only weakly modified by spin-orbit coupling and most of the variations are due to symmetry changes at the molecule-electrode interface. For this reason strongly coupled systems, such as Au atomic nanowires sandwiched between Au (100) surfaces and benzene-dithiol molecules bonded at the Au (111) hollow site, are rather insensitive to spin-orbit effects. In contrast, in junctions where the coupling between the molecule and the electrodes is weaker, as in the case of benzene-dithiol bonded to Au (111) at adatom positions, the transmission coefficient at the Fermi level can be modified by as much as 14%.

Effect of the continuity of the π conjugation on the conductance of ruthenium-octene-ruthenium molecular junctions

The conductance of a family of ruthenium-octene-ruthenium molecular junctions with different pi conjugation are investigated using a fully self-consistent ab initio approach which combines the nonequilibrium Greens function formalism with density functional theory. Our calculations demonstrate that the continuity of the pi conjugation in the contact region as well as along the molecular backbone affects the junction conductance significantly, showing the advantage of using the ruthenium-carbon double bond as the linkage of conjugated organic molecules.