Nanoscale Silicon Oxide Reduces Electron Transfer Kinetics of Surface-Bound Ferrocene Monolayers on Silicon

Abstract

Functionalizing Si with self-assembled monolayers (SAMs) paves the way for integrating the semiconducting properties of Si with the diverse properties of organic molecules. Highly packed SAMs such as those formed from alkyl chains protect Si from reoxidation in an ambient environment. Such monolayers have been largely considered oxide-free, but the effect of nanoscale reoxidation on the electrochemical kinetics of Si-based SAMs remains unknown. Here, we systematically study the effect of the oxide growth on the electrochemical charge-transfer kinetics of ferrocene-terminated SAMs on Si by exposing the surfaces to ambient conditions for controlled periods of time. X-ray photoelectron spectroscopy and atomic force microscopy revealed a gradual growth of silicon oxide (SiOx) on the surfaces over time. The oxide growth is accompanied by a decrease in the ferrocene surface coverage and a concomitant decrease in the electron transfer rate constant (ket) measured by electrochemical impedance spectroscopy. The drop in ket is attributed to a greater spacing between the ferrocene moieties induced by the surface oxide, which in turn blocks lateral electron transfer between neighboring ferrocene moieties. These findings explain the highly scattered literature data on electron transfer kinetics for monolayers on Si and have implications for the proper design of Si-based molecular electronic devices.