Influence of applied potential on the impedance of alkanethiol SAMs

Self-assembled monolayers are generally considered to behave as dielectric layers with a capacitance that is dependent on the monolayer thickness and the relative permittivity that is determined by the hydrocarbon tail. We show that the impedance response of alkanethiol-modified gold surfaces can be modeled as a parallel network consisting of the capacitance and resistance of the monolayer over a wide potential range. At potentials positive to -0.3 V (Ag/AgCl), the monolayer resistance is greater than 10⁶ Ω cm²; however, at more negative potentials, the monolayer resistance decreases exponentially with potential with an inverse slope of about 250 mV. Over the same potential range, the monolayer capacitance is independent of potential. Although the same behavior is observed on ultrasmooth, template-stripped gold, the resistance at any potential is larger than for evaporated gold. The progressive increase in permeability of the monolayer is associated with an increase in electric field at potentials negative to the potential of zero charge.