Charge Transport in Strongly Coupled Molecular Junctions: “In-Phase” and “Out-of-Phase” Contribution to Electron Tunneling

We report a first-principles study on the evolution of nonequilibrium charge transport in a two-terminal molecular-scale device with the increase in the length of the molecular wire built out of cubane oligomers. In particular, for wires of three different lengths, we look into the relative contribution of the “in-phase” and the “out-of-phase” components of the total electronic current under the influence of an external bias. In the low-bias regime, the “out-of-phase” contribution to the total current is minimal and the “in-phase” elastic tunneling of the electrons is responsible for the net electronic current. This is true, irrespective of the length of the molecular spacer. In this regime, the current–voltage characteristics follow Ohm’s law and the conductance of the wires is found to decrease exponentially with the increase in length, which is in agreement with experimental results. However, after a certain “offset” voltage, the current increases nonlinearly with bias and the “out-of-phase” tunneling ...