SUBSTRATE EFFECTS ON THE STRUCTURE OF LANGMUIR-BLODGETT MONOLAYERS OF CADMIUM ARACHIDATE USING X-RAY-ABSORPTION SPECTROSCOPY

NEXAFS (near edge X-ray absorption fine structure) spectroscopy at the C 1s edge has been employed to study the molecular orientations in Langmuir-Blodgett (LB) films of cadmium arachidate deposited onto various substrates, silver, hydrogen terminated silicon, and silicon oxide. In contrast to other techniques (like infrared spectroscopy) NEXAFS spectroscopy allows for a direct comparison of molecular orientations in LB films deposited on the different substrate materials and reveals that the different substrates induce strongly different structural properties of the deposited films, although preparation conditions were identical. Whereas for the Ag substrate the monolayers show a virtually perpendicular orientation of the alkyl chains with respect to the surface, tilted and/or disordered chains were found for the differently pretreated silicon substrates. Films with three or more layers, however, show virtually perpendicular orientation of the alkyl chains in the outermost layers irrespective of the substrate, indicating that substrate effects are mainly limited to the first LB layer.