Switching of conductance in a molecular wire: Role of junction geometry, interfacial distance, and conformational change

Achieving atomic level control at the metal–molecule interface in a single molecule conductance measurement is a daunting challenge. An equally important issue is the lack of atomic level structural information of the interface, which makes the theoretical interpretation of observed conductance much harder; conductance sensitively depends upon the junction geometry. In this article, we report a junction dependent conductance study in a ruthenium–bis(terpyridine) molecular device, which has been fabricated and characterized ( J. Am. Chem. Soc. 2008, 130, 2553) using a scanning tunneling microscope. An ensemble of device structures is created by varying metal–molecule binding sites, the orientation of the molecule at the interface, interfacial distances, and conformational change within the molecule to study junction dependent effects. An orbital dependent density functional theory in conjunction with a parameter free, single particle Green’s function approach is used to study the current–voltage (I–V) characteristics. For the ONTOP junction geometry, our results show a sharp increase in current at a threshold voltage (Vth). The current is found to be relatively small (OFF state) for bias range below the threshold value. As we approach the weakly coupled regime, a drop in Vth is found; following a sharp increase in current at Vth, a current plateau (ON state) is observed with the increase of bias beyond ∼Vth. A similar nonlinear I–V curve with a current switching feature is reported by the experiment. An analysis of bias dependent transmission and orbital characters of participating eigen-channels is presented to understand the origin of distinct I–V features observed in strongly and weakly coupled junctions