Selective formation of molecular junctions with high and low conductance states by tuning the velocity of electrode displacement.

A single-molecule junction of 1,4-di(4-pyridyl)benzene (DPB) was prepared in a nano-gap between two Au electrodes using the scanning tunnelling microscopy-based break junction method (STM-BJ). Electric conductance and current versus bias voltage (I-V) measurements during the pulling and pushing processes of DPB single-molecule junctions revealed that high (H) and low (L) conductance states formed in both the pulling and pushing processes. Analysis of the I-V curves based on a single-level model indicated that the difference in conductivity of the H and L states mainly arises from high and low metal-molecule electric coupling in the junction. We demonstrated the controllable formation of H and L conductance states by simply tuning the velocity of electrode displacement in the pushing process. In the pulling process, both H and L states formed regardless of the velocity (v) of electrode displacement, while in the pushing process, H and L states could be selectively fabricated by using low (v = 16 nm s-1) and high (v = 64 nm s-1) velocities of displacement, respectively. This study provides a simple approach to selectively fabricate high and low conductance states by fine tuning of the electrode displacement.