Molecular Rectification Based on Asymmetrical Molecule-Electrode Contact

Instead of asymmetric molecular substitution, we investigate the molecular rectification originating from asymmetrical electrode−molecule contacts using density functional theory combined with the nonequilibrium Green’s function method. In the porphyrin-based molecular junction, we employ an additional thiol group on the left side to enhance the electronic coupling. The transportation of these contact-asymmetrical junctions shows obvious rectification, implying that the asymmetrical interface modification is feasible in the design of molecular diodes. The theoretical calculations show that the rectification ratio is about 2.6 in the large bias range from 0.6 to 1.2 V. We give an interpretation using the alignment of the molecular orbital levels to the biased electrodes in the metal−molecule−metal junction. This highlights the fact that contact asymmetry is a significant factor to be considered when evaluating nanoelectrical junctions incorporating single molecules.