PNNL

Abstract

Alkylsiloxane self-assembled monolayers (SAMs) are used in the semi-conductor industry, and more recently as proxies for organics adsorbed on airborne mineral dust and on buildings and construction materials. A number of methods have been used for removing the SAM from the substrate after reaction or use, particularly plasmas or piranha (H2SO4:H2O2) solution. However, the impact of these cleaning methods on the chemistry of subsequently formed SAMs on the surface is not known. We report here AFM, XPS, Auger and FTIR studies of changes in the silicon substrate on repetitive deposition and removal of SAMs by these two methods. It is shown that a layer of silicon oxide is formed, and the surface becomes very irregular and roughened, particularly for the piranha treatment. This layer of silica impacts the structure of SAMs attached to it and can serve as a reservoir for trace gases that adsorb on it, potentially contributing to the subsequent reactions of the SAM. The implications for the use of such surfaces as a proxy for reactions of organics on airborne dust particles and on structures in the boundary layer are discussed.