X-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy have been used to study the time-dependent adsorption and molecular orientation behaviour of octadecyltrimethoxysilane (ODTMS) on native aluminium oxide surfaces. By measuring the adsorption isotherm using XPS, we show that ODTMS molecules exhibit oscillatory adsorption. The oscillatory adsorption behaviour for ODTMS is analogous to that observed for its simpler short-chain 'cousin' - propyltrimethoxysilane (PTMS) - and suggests that the length of the functional alkyl chain on an organosilane does not have a significant influence upon the oscillatory adsorption mechanism. The oscillation in the ODTMS adsorption isotherm shows a maximum and a minimum in coverage at an adsorption time of &SIM; 30 and &SIM; 65 s, respectively, for a 0.75% ODTMS solution in a 90% ethanol-10% water mixture at pH 4. The time-dependent orientation behaviour of the ODTMS molecules during adsorption was examined using angular-dependent carbon K-edge NEXAFS spectroscopy. We show that the alignment of the ODTMS film changes systematically with deposition time and appears to be correlated with coverage measurements obtained using XPS. In particular, by combining the XPS and NEXAFS results we demonstrate that the minimum ODTMS coverage corresponds to a film whose alignment appears to be predominantly randomized.
Surface and Interface Analysis Vol. 37, no. 5, p. 472-477