Ab initio and empirical studies on the asymmetry of molecular current-voltage characteristics

We perform theoretical calculations of the tunnelling current through various small organic molecules sandwiched between gold electrodes by using both a tunnel barrier model and an ab initio transport code. The height of the tunnelling barrier is taken to be the work function of gold as modified by the adsorbed molecule and calculated from an ab initio electronic structure code. The current-voltage characteristics of these molecules are compared. Asymmetry is introduced into the system in two ways: an asymmetric molecule and a gap between the molecule and the right electrode. The latter is a realistic situation in scanning probe experiments. The asymmetry is also realized in the tunnel barrier model by two distinct work functions on the left and right electrodes. Significant asymmetry is observed in the ab initio i (V) curves. The tunnel barrier i (V) curves show much less pronounced asymmetry. The relative sizes of the currents through the molecules are compared. In addition, the performance of theWKB approximation is compared to the results obtained from the exact Schrodinger solution to the tunnelling barrier problem.