Density-functional study of tilt-angle and temperature-dependent conductance in biphenyl dithiol single-molecule junctions

In the framework of a density-functional-based method, we study electron transport through several biphenyl-derived dithiol molecules connected to gold electrodes. The molecules differ in the degree of conjugation of the pi-electron system, which is controlled by the choice of the side groups. The low-temperature conductance depends on the tilt angle phi between the two phenyl ring units and follows closely a cos(2) phi law consistent with an effective pi-orbital coupling model. Tilting the phenyl rings from a planar conformation to one with perpendicular rings decreases the conductance by roughly 2 orders of magnitude. These findings are in agreement with experiments. We further study the temperature dependence of both the conductance and its fluctuations and find qualitative differences between the investigated molecules. The temperature dependence arises from thermal smearing in the leads and also from a thermal average over different contact configurations. Our results suggest that the variations of the conductance due to the latter effect can be reduced by an appropriate design of the molecule.