Greatly Improving The Rectifying Performance Of Single-Molecule Diodes Through Molecular Structure Design And Electrode Material Optimization

Exploring and optimizing the factors that influence the rectification performance of single-molecule diodes is one of the most important approaches to pursue high-performance molecular diodes. By using the first-principles method, effects of electrode materials (i.e., Ag versus Pt) and the conjugated head group (i.e., Fc versus Fc- C???C-Fc, Fc = ferrocenyl) on the rectification properties of ? - ? type single-molecule diodes, which here are comprised of an alkanethiolate backbone (HS(CH2)15, abbreviated as SC15) and a conjugated head group, have been investigated. It is found that, compared to Fc head group and Ag electrode, Fc- C???C-Fc head group and Pt electrode can greatly enhance the rectification performance of the single-molecule diodes. More specifically, the maximum value of R for SC15Fc- C???C-Fc can reach 4.2 x 103 on Ag electrode and 3.4 x 104 on Pt electrode, which are respectively about 30 times larger than the counterparts of SC15Fc. Meanwhile, the maximum values of R for SC15Fc and SC15Fc- C???C-Fc on Pt electrode are about 1 order of magnitude larger than those on Ag electrode. Detailed analysis attributes the former to the evident increase in the number of electron tunneling channels around EF when the Fc group is replaced by Fc- C???C-Fc, while the latter is due to the energy shift of electron tunneling channels towards EF on Pt electrode and the stronger coupling strength of S?Pt than S?Ag. Our results suggest that there is still large room for optimizing the rectification performance of alkanethiolate-based ? - ? type single-molecule diodes and can also help to understand the recent experimental observations [Nat. Nanotechnol. 2017, 12, 797?803].