Spectroscopic characterization of alkylsilanes on metal and oxide surfaces

Abstract

The research projects described in this Dissertation were designed to investigate the interfacial chemistry of alkylsilanes on metal and oxide surfaces, specifically Ag and silica, respectively. A variety of surface analysis tools, including FTIR and Raman spectroscopies, electrochemistry, ellipsometry, and x-ray photoelectron spectroscopy, have been applied to study important aspects of bonded alkylsilane structures. Two model silica surfaces were designed and fabricated as substrates for the attachment of alkylsilanes. The first model silica surface employs self-assembled monolayers of hydrolyzed and condensed (3-mercaptopropyl)trimethoxysilane (3MPT) on Ag surfaces. The Ag substrate is used to provide a small enhancement to the Raman scattering at the interface. Octadecyltrichlorosilane (OTS) and dimethylchlorooctadecylsilane (DOS) are covalently bonded to these surfaces. Monolayers of OTS on 3MPT-modified Ag surfaces are ordered, while submonolayers of DOS, the maximum coverage achievable, are disordered. Results obtained from this study demonstrate the importance of van der Waals interactions and siloxane cross-linking in promoting an ordered alkylsilane structure. The second model silica surface studied is based on thin silica films prepared through sol-gel technology. These are prepared by spin-coating prehydrolyzed solutions of tetratmethoxysilane (TMOS) onto 3MPT-modified Ag surfaces. These surfaces are designed to contribute to an understanding of the partitioning process associated with alkylsilane stationary phases in reversed-phase liquid chromatography (RPLC). OTS layers covalently attached to such surfaces were studied previously in this laboratory. In this Dissertation, the sol-gel methodology used is improved to enable silica films in the ultrathin (< 100 Å) regime to be fabricated. These substrates allow study of model stationary phases of DOS. Both FTIR and Raman spectroscopies indicate that the DOS alkyl chains on silica are disordered, consistent with previous notions about monomeric alkylsilane stationary phases in RPLC. Further characterization of these thin silica films reveals them to be non-porous, dielectric, and homogeneous. Their dielectric strengths are found to be equivalent to or better than those from device-quality thermally grown silica. This research expands application of the well-defined sol-gel technology to the fabrication of ultrathin silica films that may be useful as insulating layers in the microelectronics and semiconductor industries.