STM studies of electron transfer through single molecules at electrode-electrolyte interfaces

Abstract Electrochemical scanning tunnelling microscopy (EC-STM) is one of the most important tools in modern interfacial electrochemistry. Over several decades it has proven its worth by providing in-situ atomic and molecular scale visualisations of electrode surfaces, but its value goes far beyond topographic imaging. This article discusses the use of EC-STM as a tool for quantitatively studying mechanisms of electron transfer (ET) at electrode-electrolyte interfaces. In combination with theoretical modelling, it has been revealing mechanistic details of charge transfer across molecules at electrochemical interfaces which are of broad interest to a wide range of electrochemists. This article discusses different ways in which the EC-STM can be deployed to monitor charge flow through single molecules and thereby analyse ET mechanisms. Mechanisms covered range from superexchange to 2-step hopping ones such as the Kuznetsov Ulstrup (KU) mechanism. Literature examples of a variety of redox and non-redox molecular targets are used to describe how measurements of single molecule conductance as a function of electrode potential can be used to analyse charge transfer through single molecules. The review finishes by highlighting some of the most recent work and new developments which will ensure that EC-STM will continue to be an important tool for studying ET mechanisms at electrode-electrolyte interfaces.