Investigation of a Vapor-Deposited Thin Silica Film: Morphological and Spectral Characterization

Surface modification reactions by organosilicon compounds have demonstrated great success in a wide variety of applications. However, they are of limited usefulness in that they only proceed appreciably on surfaces that have an abundance of reactive hydroxyl groups, thus preventing their application to some materials of technological relevance, such as plastics and polymers. A process capable of depositing a surface rich in reactive hydroxyl groups onto a wide variety of substrates could potentially enable the extension of organosilane surface modification reactions to new materials, but conventional processes for depositing oxide layers require temperatures that are too high for most polymers and plastics. It has been shown that silica layers can be deposited from the vapor-phase hydrolysis of tetrachlorosilane at room temperature, but little if any work has been done to characterize the resulting films. In this work, ellipsometry, atomic force microscopy, and Fourier transform infrared spectroscopy are employed to study the characteristics of films formed from this process. Interestingly, very different film morphologies can be obtained by changing key processing parameters. Furthermore, isotopic exchange experiments and dehydration studies show that the surfaces of the silica films obtained by this method are composed entirely of hydrogen-bonded silanol groups and do not exhibit any freely vibrating surface silanol groups, a result that is in contrast with conventionally prepared silica materials. Still, this layer has been shown to behave very similarly to conventional silica materials with respect to surface reactions. Finally, infrared spectral data and contact angle data demonstrate that this method can be employed to deposit silica layers onto poly(methyl methacrylate) and polystyrene surfaces.