Near-edge X-ray absorption fine structure studies of oriented molecular chains in polyethylene and Langmuir-Blodgett monolayers on Si(111)

Abstract

C K shell near-edge X-ray absorption fine structure (NEXAFS) spectra of oriented hydrocarbon chains were measured in polyethylene and single Langmuir-Blodgett monolayers of arachidic acid, the calcium salt of arachidic acid, the cadmium salts of arachidic, octadecylfumaric and tetradecylbenzoic acids and a semifluorinated fatty acid on oxidized Si(111) surfaces. The observed NEXAFS resonances have been assigned to transitions to excited states localized on individual bonds, the presence of functional groups has been detected and the orientation of molecular segments relative to the surface has been determined. From a detailed analysis of the angular dependence of the various spectral peaks, the hydrocarbon chains of the cadmium arachidate monolayer were estimated to lie within 15° of the surface normal and the hydrocarbon chains of the calcium arachidate monolayer were estimated to be tilted by 33° ± 5° from the surface normal, while the arachidic acid film was not ordered at all. The determined chain orientations are discussed in terms of the surface bonding geometry of the terminal carboxylate group and the lateral interactions between the zigzag hydrocarbon chains. Similarly, a fumarate monolayer was found to be oriented perpendicular to the substrate while the benzoate and the semifluorinated fatty acid salt were more tilted. © 1988.