REFLECTION-ABSORPTION INFRARED-SPECTROSCOPY OF SELF-ASSEMBLED MONOLAYERS ON GOLD AND SILICON SURFACES

Reflection absorption infrared spectroscopy is used to investigate the structure of self-assembled monolayers of dioctadecyl-disulfide (ODS) on gold and octadecyltrichlorosilane (OTS) on silicon surfaces and to determine the relative influence of the substrate's optical properties and the molecular orientation of the adsorbate on the spectral characteristics of ordered monolayer films. Comparable sensitivity is achieved in the monolayer spectra on both surfaces despite the large difference in the absolute reflected intensities. The peak positions of the CH-stretching absorptions are identical for ODS and OTS monolayers and are characteristic of a crystalline, densely-packed film structure. The orientation of the hydrocarbon chains on the surface is determined by a quantitative spectral analysis aided by a computer simulation and fit procedure. A tilt angle of 33 degrees between the hydrocarbon chain axis and the surface normal is found for ODS monolayers on gold. An almost perpendicular orientation (8 degrees tilt), on the other hand, is determined from the OTS monolayer spectra on silicon. The complex absorption profiles in the latter spectra, consisting of superimposed positive and negative absorptions, are accurately reproduced in the simulated spectra. They result from the excitation of vibrational modes both parallel and perpendicular to the silicon surface, which cause reflectivity changes of the OTS/silicon adsorbate system in opposite directions. Comparatively simple, ''transmission-like'' spectra are obtained for ODS monolayers on gold, in which the band intensities are governed by the well-known metal surface selection rule.