Measuring the Tilt Angle of ODTMS Self-Assembled Monolayers on Al Oxide Surfaces

Adsorbed octadecyltrimethoxysilane (ODTMS) on native aluminium oxide substrates have been studied with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in order to determine the average tilt angle of the surface film. Previous experiments have shown that the time-dependant adsorption isotherm of ODTMS on Al exhibits oscillatory adsorption. In this work we present the findings of our tilt angle studies for Al samples dipped in a 0.75% ODTMS solution at various times during oscillatory adsorption. It clearly can be seen that the alignment of the surface film is related to the coverage, hence for a minimum in the film coverage (at 65 s dip time) the tilt angle of the ODTMS film is found to be ∼50° relative to the surface normal. Below the minimum in the film coverage (at 30 s dip time) the tilt angle has been found to be ∼41° whereas for samples dipped above the minimum film coverage (at 80 and 95 s) the angles are ∼46° and ∼45° respectively. These measurements indicate a strong correlation between film alignment and film coverage during oscillatory adsorption.