Surface Raman scattering of organically-modified metal and oxide surfaces.

Abstract

This research project was designed to investigate the surface chemistry of organosilanes at metal and oxide surfaces, specifically Ag, Au, SiO₂, and Al₂O₃. A combination of surface Raman spectroscopy, electrochemistry, scanning electron microscopy, x-ray photoelectron spectroscopy, and ellipsometry have been employed to study several important aspects of these films. Tetramethoxysilane (TMOS) and 1-propanethiol are used as models to provide detailed vibrational assignments for (3-mercaptopropyl)trimethoxysilane (3MPT). The vibrational assignments for 3MPT are used to assess the conformational order of 3MPT monolayers at Ag and Au surfaces before and after hydrolysis and condensation. Particular emphasis is given to the ν(Si-O), ν(C-H) modes, and the trans and gauche ν(C-S), ν(Si-C), and ν(C-C) modes in the frequency regions between ca. 500 to 1300 cm⁻¹ and ca. 2700 to 3000 cm⁻¹. The reactivity of the residual surface silanols and the wetting properties of the hydrolyzed 3MPT result in the surface acting as a modifying agent for the attachment of thin dielectric layers such as SiO₂ on Ag and Au surfaces. Adhesion of the thin SiO₂ films to the metal surfaces are characterized using standard qualitative and quantitative adhesion tests. Raman spectra of molecular assemblies of octadecylsilane layers, a model of reversed-phase liquid chromatographic stationary phase materials, are acquired using a thin silica gel/3MPT/Ag sandwich structure as the substrate. The Ag surface provides electromagnetic enhancement to the octadecylsilane overlayer. Spin coating a silica sol on the modified Ag surface results in the formation of a thin silica gel layer which contributes minimal fluorescence background to the Raman spectra. The spectra are interpreted in terms of the alkyl chain conformation on dry silica surfaces as well as in the presence of solvents commonly used in chromatography. The feasibility of acquiring Raman data from an Al/Al₂O₃ surface without an enhancing adlayer is demonstrated. Self-assembled films of octadecylsilane, octadecyldimethylsilane, and stearic acid were evaluated by Raman spectroscopy in terms of alkyl chain conformation. These data represent the first Raman spectra reported for alkylsilane layers on silica and Al₂O₃ surfaces without an enhancing adlayer and demonstrate the feasibility of acquiring conformational information from alkyl chains bound to oxide surfaces.