Origins of asymmetric charge transport properties of weakly coupled molecular junctions

Understanding charge transport properties of a molecular junction plays a crucial role in the possible application of such devices. In this work we present a theoretical study on the transport properties of molecular transistors in the sequential tunneling regime. Special attention is paid to the origins of the asymmetries in the charge transport properties as presented by the commonly measured conductance stability diagrams (two-dimensional plots of the differential conductance against gate and bias voltages). It is found that both the external factors, including the bias coupling constant and the coupling strength between the molecule and the electrodes, and the intrinsic molecular properties such as the frequency differences between different charging states, the anharmonicity of the potential energy surface, and the mode-mixing effect can lead to asymmetric conductance stability diagrams. Especially, we show that the uneven bias coupling between the molecule and the two electrodes can result in strong current rectifications. These results successfully reproduce the change in rectification directions of molecular diodes as observed in a recent experiment [L. Yuan et al., Nat. Commun. 6, 6324 (2015)]. The differences in the asymmetric stability diagram caused by the studied factors are also discussed, which helps us to correctly interpret the experimental results.