Controlling Rectification Performance by Tuning Molecule–Electrode Coupling Strength in Ferrocenyl-Undecanethiolate Molecular Diodes

Controlling and optimizing rectifying performance of single-molecule diodes remain a formidable challenge in the field of molectronics. As is known, molecule–electrode interfaces play a critical role in determining the charge transport properties and functionalities of molecular devices. Definitely, the molecule–electrode interfaces should also be paid special attention in designing molecular diodes with high performance. Recent experiments showed that rectification ratios of ferrocenyl-alkanethiolate diodes can vary by orders of magnitude through controlling the materials and surface topography of bottom electrode, orientation of ferrocenyl group with respect to the top electrode, and orderliness of the self-assembled monolayers. Here, we theoretically investigate the effects of the orientation (i.e., tilt angle α) of ferrocenyl group with respect to the electrode surface on the rectifying performance of ferrocenyl-undecanethiolate single-molecule diodes based on first-principles calculations. It is revealed that rectification ratios of the diodes are dramatically modulated by 2 orders of magnitude when α is varied. Further analysis shows that the conducting molecular orbitals move away from the Fermi energy because of enhanced coupling between the molecule and right electrode when α decreases. This is found to be responsible for the considerable reduction in rectifying performance of the diodes. Our results suggest that the coupling strength between the molecule and right electrode is of great importance on the rectification performance of ferrocenyl-alkanethiolate diodes and a weaker coupling strength is in favor of improvement of rectification ratios. Hence, our results provide a design principle for high-performance molecular diodes based on SCnFc or structurally similar systems.