Theory of scanning electrochemical microscopy (SECM) as a probe of surface conductivity

Abstract

Scanning electrochemical microscopy (SECM) can be used in the feedback mode as a ‘contactless’ method for probing the conductivity of ultra-thin films. With this new technique a sample is bathed with one form of a reversible redox (Red/Ox) couple, e.g. Red, and an ultramicroelectrode (UME), located close to the film surface is used to locally convert Red to Ox by diffusion-limited oxidation. Ox diffuses from the UME to the film where it may undergo conversion back to Red. This process is driven by a potential gradient, which depends on the surface conductivity. A model is developed for the SECM feedback process from which the conductivity of the film can be extracted. Two routes arising from the model are considered: an approximate method which allows an order of magnitude estimate of the film conductivity and a more involved approach, which can be used with greater accuracy. The merits of the two approaches are assessed by analysing experimental data for the effect of surface pressure on the conductivity of a polyaniline monolayer at a water/air interface.