Electronic transport properties of molecular bipyridine junctions: Effects of isomer and contact structures

The effects of contact structures and isomers on the current-voltage characteristics of molecular bipyridine junctions have been investigated by means of a generalized quantum chemical approach based on the elastic scattering Green's function method. For the four isomers 4,4'-bipyridine, 2,2'-bipyridine, 2,4'-bipyridine, and 2,6'-bipyridine under investigation, the 2,4'- bipyridine junction is found to be the poorest conductor owing to its nonsymmetrical arrangement. Numerical simulations of the 4,4'-bipyridine junction show that the contact structures between molecule and metallic electrodes have a noticeable effect on the electron transport characteristics of molecular junctions. The shortening of the distance between two metallic electrodes results in a stronger coupling and lower potential barrier between them, leading to larger conductance. The external electric fields cause the charge redistribution within molecule and lead to the presence of resistivity dipoles at the molecule-metal interface, and the bias voltages are mainly dropped there. The nonlinear charge transport effect on the current and conductance of 4,4'-bipyridine molecular junctions at the lower-bias region is presented. The experimental measurement has been reproduced by the theoretical calculations for a well-defined structure.