Chemisorption and Chemical Reaction Effects on the Resistivity of Ultrathin Gold Films at the Liquid−Solid Interface

Abstract

Ultrathin gold films, with thicknesses between the onset of conductivity (d ∼ 5 nm) and the electron mean free path (d ∼ 80 nm), display surface-sensitive resistivities, which have been exploited to follow the adsorption and desorption of molecular monolayers at the metal−solution interface with high precision. For nominal Au film thicknesses (d ∼ 40 nm), strongly chemisorbed thiolate monolayers increase the resistivity of the thin Au films by ∼4%, but weakly adsorbed species, such as pyridine or phenolate at open circuit, induce no observable change in the Au film resistance. Resistivity measurements implemented with a high-stability current source and high-precision digital voltmeter sampling at 1 Hz resulted in 3σ uncertainties in alkanethiolate coverage of 1.4 × 10^-4 monolayer. Surface plasmon resonance measurements, performed simultaneously with resistivity measurements, indicate that changes in resistivity vary monotonically with coverage with three distinct regions: a low-coverage region of heightened adsorbate mobility, an intermediate-coverage region with generally linear behavior, and a chain length-dependent saturation region at high coverages. Resistivity measurements were also capable of reproducibly following the chemical state of the Au surface through a complex set of redox manipulations, demonstrating the versatility of this simple measurement.