CHARACTERIZATION OF OCTADECYLSILANE AND STEARIC-ACID LAYERS ON AL2O3 SURFACES BY RAMAN-SPECTROSCOPY

Results are presented which demonstrate the use of Raman spectroscopy for the characterization of octadecylsilane, octadecyldimethylsilane, and stearic acid layers on Al2O3 surfaces without the use of surface enhancement. Unlike previous attempts to study adsorbates at Al2O3 surfaces, an enhancing Ag adlayer was not used. These results represent the first report of the use of Raman spectroscopy for the characterization of alkylsilane and stearic acid layers on Al2O3 surfaces. The spectral data are interpreted in terms of alkyl chain conformation within these layers. Raman data from the V(C-C) and V(C-H) spectral regions suggest that the alkyl chains comprising the octadecylsilane layer are more disordered than similar films formed on silica. The octadecyldimethylsilane monolayers are less ordered, suggesting an even greater concentration of gauche conformations in the alkyl chains of these films. The observed increase in disorder in films formed from octadecyldimethylsilane is attributed to the influence of the bulky methyl groups on bonding density which prevent the alkyl chains from packing at their van der Waals radii. The Raman spectral data from self-assembled films formed from stearic acid suggest that these films are more ordered than the similar films from alkylchlorosilanes. The increase in order is believed to be due to the bonding orientation of the carbonyl group at the Al2O3 surface.