Effect of molecular adsorption at the liquid–metal interface on electronic conductivity: the role of surface morphology

Abstract

Exposure of thin (20 nm≤ d≤80 nm) Au films to solutions of strongly interacting Lewis bases, e.g. organothiols, I −, results in adsorption which raises the resistivity of the metal film ∼2%. The relative ease of measuring resistivity via a four-point probe and the large signal-to-background ratio which characterizes the measurement makes electrical conduction measurements an attractive candidate for chemical sensors and studies of the liquid–solid interface. It is widely accepted that surface contributions to resistivity are determined both by electronic structure in the near-surface region and by morphology. In these experiments Au film sensitivity to molecular adsorption was varied by controlling surface roughness. Resistivity changes were measured on smooth annealed Au films, on the same films after etching with CN −, and then after subsequent treatment with I −. After each treatment the film morphology was characterized by atomic force microscopy (AFM), and the resulting power spectral densities were correlated with resistivity changes resulting from standard exposures to C 16H 33SH. These measurements were used to understand the role of morphology in determining the sensitivity of resistivity measurements to molecular adsorption. As expected CN − significantly roughened the annealed Au films, a change accompanied by an increase in sensitivity to adsorption. Subsequent I − exposure smoothed the films at all length scales, but surprisingly the sensitivity increased again. A two-site model is proposed which explains the results in terms of the competing effects of increased surface area and changes in surface site densities caused by chemical etching.