SUBSTRATE EFFECTS ON THE STRUCTURE OF LANGMUIR-BLODGETT MONOLAYERS OFCADMIUM 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 La ngmuir-Blodgett (LB) films of cadmium arachidate deposited onto variou s substrates, silver, hydrogen terminated silicon, and silicon oxide. In contrast to other techniques (like infrared spectroscopy) NEXAFS sp ectroscopy allows for a direct comparison of molecular orientations in LB films deposited on the different substrate materials and reveals t hat the different substrates induce strongly different structural prop erties of the deposited films, although preparation conditions were id entical. Whereas for the Ag substrate the monolayers show a virtually perpendicular orientation of the alkyl chains with respect to the surf ace, tilted and/or disordered chains were found for the differently pr etreated silicon substrates. Films with three or more layers, however, show virtually perpendicular orientation of the alkyl chains in the o utermost layers irrespective of the substrate, indicating that substra te effects are mainly limited to the first LB layer.