Li-Ion Intercalation, Rectification, and Solid Electrolyte Interphase in Molecular Tunnel Junctions

This paper describes Li-ion intercalation into a pyrenyl-terminated self-assembled monolayer (SAM) on gold, inspired by the graphite anode in a Li-ion battery, and its effect on tunneling performance in a molecular junction incorporating the SAM. As the concentration of the Li-ion precursor ([LiPF6]) increased from 0 to 10-2 M, the rectification ratio increased to ∼102. Further experiments revealed that the intercalation-induced changes in the orientation of PYR group and in the HOMO energy level account for the enhanced rectification. Treatment with high concentrations of LiPF6 (from 10-2 to 100 M) yielded a considerable solid electrolyte interphase (SEI), mainly composed of LiF, on the surface of the SAM, resulting in the disappearance of rectification. This was attributed to renormalization of the HOMO level back to that of the intact SAM, caused by the SEI layer. Our work demonstrates the interplay among Li-ion intercalation, SEI, and tunneling in the molecular junction, benefiting the research of molecular electronics as well as SAM-based batteries.