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 prepare d by adsorption from dilute solutions under dry inert gas conditions a nd the hydrocarbon chain orientation in the monolayer films has been i nvestigated by external reflection infrared spectroscopy. A quantitati ve analysis of the monolayer reflection spectra based on a spectral si mulation and fit procedure reveals that the hydrocarbon chains in thes e 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 intensiti es in the monolayer reflection spectra, which further supports an esse ntial conservation of an ordered film structure with a constant chain tilt angle within this series of adsorbate compounds. A slight increas e of structural disorder with decreasing chain length was indicated, h owever, by peak shifts and band broadenings of the CH stretching absor ptions and has been ascribed to an increased amount of conformationall y disordered hydrocarbon chains. Significant changes in the film struc ture 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, crystal line domains. The long-chain compounds, on the other hand, yielded ide ntical film structures under both atmospheric and dry inert gas condit ions.