Charge-state lifetimes of single molecules on ultrathin insulating films

TL;DR

This study measures the charge-state lifetimes of individual molecules on ultrathin insulating films using STM, revealing how substrate and film thickness influence charge dynamics crucial for understanding molecular electroluminescence.

Contribution

It introduces a method to directly measure charge-state lifetimes of single molecules on insulating films, highlighting the impact of substrate and film properties on charge dynamics.

Findings

Charge-state lifetimes vary with film thickness and substrate.

Level alignment affects charge-state stability.

Charge dynamics influence STM-induced electroluminescence.

Abstract

In scanning tunneling microscopy (STM) experiments of molecules on insulating films, tunneling through molecular resonances implies transiently charging the molecule. The transition back to the charge ground state by tunneling through the insulating film is crucial, for example, for understanding STM-induced electroluminescence. Here, using STM, we report on the charge-state lifetimes of individual molecules adsorbed on NaCl films of different thicknesses on Cu(111) and Au(111). To that end, we approached the tip to the molecule at resonant tunnel conditions up to a regime where charge transport was limited by tunneling through the NaCl film. The resulting saturation of tunnel current is a direct measure of the molecule's charge-state lifetime, thus providing a means to study charge dynamics and, thereby, exciton dynamics. Comparison of anion and cation lifetimes on different substrates reveals the critical role of the level alignment with the insulator's conduction and valence band, and the metal-insulator interface state.