Soft Hedgehogs on Coarse Carpets: A Molecular Simulation Study of Capped Nanocrystals Interacting with Self-Assembled Monolayers of Alkylthiols on a Gold (111) Surface

Abstract

We report atomistic simulations of alkylthiol capped gold nanocrystals (NCs) adsorbed on a gold (111) surface that is covered with a self-assembled monolayer (SAM) of alkylthiols. For these systems, we have computed the potential of mean force as a function of NC−surface distance for alkylthiols of varying tail length (butanethiol−dodecanethiol), temperature (250−400 K), and NC size (1.8−2.7 nm). The length of the thiol in the SAM determines alone the equilibrium distance between the NC and the surface. The equilibrium distance does not depend on the length of the capping molecule on a NC. We explain this finding by the efficient packing of capping molecules on top of the SAM. Conversely, the depth of the potential well only depends on the length of the capping molecule on the NC, and this dependence is linear. The well depth does not depend on the length of the thiol in the SAM. The structure of the SAM and of the capping layer on the NC is described using appropriate order parameters. We also study the behavior of systems where either the capping layer of the NC or the SAM on a flat surface is absent. Unlike capped NCs, bare NCs diffuse a few angstroms into the SAM. When a SAM is not present, the capping layer does not protect a NC from sintering with the surface.