Electronic properties of self-assembled monolayers on Au(111) studied by electrical force spectroscopy

Abstract

Localized voltage dependent capacitive currents through different self-assembled monolayers on Au(111) surfaces were studied by a spectroscopic method in a modified atomic force microscope that detects surface potentials and $\ensuremath{\partial}C/\ensuremath{\partial}z$ data as functions of an applied modulated and biased voltage between tip and sample. On flame-annealed Au surfaces, the $\ensuremath{\partial}C/\ensuremath{\partial}z$ signal is completely voltage independent. The signal amplitudes on 2-amino-alkanethiol layers change with both ac and dc voltage. The $\ensuremath{\partial}C/\ensuremath{\partial}{z(U}_{\mathrm{dc}})$ data obtained represent the energy-dependent density of states of the investigated surface layer. Moreover, the charges of ligand stabilized clusters and the dependence of the clusters' electrical properties on their size were reproduced. The results are discussed in connection with schematic energy models and replacement circuits.