STRUCTURE OF ALKYLSILOXANE MONOLAYERS ON SILICON SURFACES INVESTIGATED BY EXTERNAL REFLECTION INFRARED-SPECTROSCOPY

Self-assembled monolayers of homologous alkyltrichlorosilanes (CH3(CH2)(n)SiCl3, n = 10, 13-17) on bulk silicon substrates have been prepared by adsorption from dilute solutions under dry inert gas conditions and the hydrocarbon chain orientation in the monolayer films has been investigated by external reflection infrared spectroscopy. A quantitative analysis of the monolayer reflection spectra based on a spectral simulation and fit procedure reveals that the hydrocarbon chains in these monolayers are tilted by an average of 8 degrees toward the surface normal. A linear relationship was found between the hydrocarbon chain length of the adsorbate molecules and the nu(CH2) absorption intensities in the monolayer reflection spectra, which further supports an essential conservation of an ordered film structure with a constant chain tilt angle within this series of adsorbate compounds. A slight increase of structural disorder with decreasing chain length was indicated, however, by peak shifts and band broadenings of the CH stretching absorptions and has been ascribed to an increased amount of conformationally disordered hydrocarbon chains. Significant changes in the film structure were observed for monolayers of short-chain compounds (n = 10, n = 13) prepared under ambient atmospheric conditions. The films consist of isotropically disordered, liquid-like regions and ordered, crystalline domains. The long-chain compounds, on the other hand, yielded identical film structures under both atmospheric and dry inert gas conditions.