Tuning the Direction of Rectification by Adjusting the Location of the Bipyridyl Group in Alkanethiolate Molecular Diodes

Controlling and optimizing the performance of molecular electronic devices is a great challenge in molecular electronics. By using a first-principles method, here we show that rectifying direction and rectification performance of molecular diodes, consisting of a single bipyridyl-embedded alkanethiolate molecule sandwiched between two parallel Ag(111) electrodes, can be precisely controlled by placing the bipyridyl group at different locations of the alkanethiolate backbone. The analysis reveals that the monotonic shift of energy levels of frontier molecular orbitals induced by the electrostatic effect of external bias voltage on the strongly localized wave functions is responsible for the features of rectification. The spatial distributions of frontier molecular orbitals are highly dependent on the location of the bipyridyl group. Hence, varying the bipyridyl location in the alkanethiolate backbone essentially changes the evolving behavior of the frontier molecular orbitals under external bias voltages. This work is helpful for the rational design of molecular diodes.