The conductance of a single 1,4-diisocyanatobenzene molecule sandwiched between twosingle-walled carbon nanotube (SWCNT) electrodes are studied using a fully self-consistentab initio approach which combines nonequilibrium Green's function formalism with densityfunctional theory calculations. Several metallic zigzag and armchair SWCNTs with differentdiameters are used as electrodes; dangling bonds at their open ends are terminated with hydrogenatoms. Within the energy range of a few eV of the Fermi energy, all the SWCNT electrodes couplestrongly only with the frontier molecular orbitals that are related to nonlocal π bonds. Although thechirality of SWCNT electrodes has significant influences on this coupling and thus the molecularconductance, the diameter of electrodes, the distance, and the torsion angle between electrodes haveonly minor influences on the conductance, showing the advantage of using SWCNTs as theelectrodes for molecular electronic devices.

Two popular models of the gold-4,4 bipyridine (44BPD)-gold molecular junction, i.e., the direct contact of the 44BPD molecule with the Au(111) surface and the intermediary contact through one extra gold atom on each side, were studied using density functional theory calculations under periodic boundary conditions. The relative position of the Fermi level is changed by the extra gold atom from well below the LUMO(lowest unoccupied molecular orbital) of the 44BPD molecule in the direct contact model to within the energy range of theLUMOin the intermediary contact model, indicating that the local structure of the contact can significantly affect the conducting characteristics of the junction. The dependence of the molecule–electrode interaction on the interface structure was also investigated in details.

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 Green's 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.

Reversible conductance transitions are demonstrated on the molecular scale in a complex of 3-nitrobenzal malononitrile and 1,4-phenylenediamine, by application of local electric field pulses. Both macroscopic and local current-voltage (I-V) measurements show similar electrical bistability behavior. The mechanism of the electrical bistability is discussed.