Large Rectification Ratio of up to 106 for Conjugation-Group-Terminated Undecanethiolate Single-Molecule Diodes on Pt Electrodes

Designing and fabricating high-performance single-molecule diodes is always a great pursuit in the field of molecular electronics. For this aim, here, we theoretically design an organic molecule (designated as HSC11BIPY–CC–BIPY), which is composed of a π conjugated BIPY–CC–BIPY group (BIPY is the abbreviation of the 4-methyl-2,2′-bipyridyl group and −CC– stands for the ethynylene group) and a long alkyl chain (undecanethiol, abbreviated as HSC11). Then, the corresponding electronic transport properties of the HSC11BIPY–CC–BIPY molecule are investigated by using the nonequilibrium Green's function method in combination with the density functional theory. It is found that the proposed molecule manifests a significant rectifying effect and the rectification ratio is as large as 305 on a Ag electrode and is about 3.4 times larger than that of its analogue previously reported, for which the BIPY–CC–BIPY group is replaced by a BIPY group instead. More interestingly, when the Ag electrodes are changed to Pt electrodes, the rectification ratio is dramatically improved to 6 orders of magnitude and reaches 8.26 × 106. Detailed analysis reveals that when the BIPY group is replaced by the BIPY–CC–BIPY group, there is a significant increase in the number of available channels for tunneling electrons in the proximity of Fermi energy (E F). Also, these electron tunneling channels get much closer to E F on the Pt electrode than that on the Ag electrode, induced by a stronger coupling strength between the molecule and Pt electrode, which leads to a boost in the rectification performance. This work demonstrates the importance and feasibility of the strategy for designing high-performance σ–π-type single-molecule diodes by optimizing the conjugated terminal group and the metal electrode.