In Situ Observation of a Self-Assembled Monolayer Formation of Octadecyltrimethoxysilane on a Silicon Oxide Surface Using a High-Speed Atomic Force Microscope

Abstract

The formation mechanism of a self-assembled monolayer (SAM) of octadecyltrimethoxysilane on a silicon oxide surface in reaction is studied in situ by using a high-speed atomic force microscope that has a time resolution of 2 s per frame. The SAM formation of a silane coupling reagent on silicon is known to comprise three development stages of nucleation, growth, and coalescence. In the present study, the first nucleation stage is found to have dynamical processes: a molecular cluster attached to the substrate works as a reaction base, on which additional reactive molecules are in a bind/unbind equilibrium. In this time period, the cluster needs a long time to develop in diameter. Once a domain of ca. 30 nm in diameter is formed, the reaction rate is changed, which is dominated by the rim length of the domain. This implies that the weakly adsorbing limit approximation on the substrate surface can be employed. Another important point is that the molecular domains generate a SAM like an occupied sheet of tiles, and each tile is connected to the substrate by a few feet. In fact, a molecular tile can easily be removed by applying soft air plasma leaving the rest of the tiles of highly packed molecules, which is confirmed by infrared p-polarized external reflection spectroscopy.