X-ray absorption spectroscopy study of pulsed-laser-evaporated amorphous carbon films

Abstract

Amorphous carbon (a-C) films obtained by pulsed-laser ablation of graphite have been investigated by X-ray Absorption Spectroscopy (XAS). The onset of 1s →σ* transitions in the films lies in the gap between theπ* andσ* bands in graphite and very close to the absorption edge of diamond, indicating a high content ofsp3 hybridization. A sharp feature at this onset is observed and assigned to a core exciton insp3-hybridized disordered C atoms. Its shift of 0.5 eV with respect to the core exciton in diamond is probably due to a higher localization of the excited electron induced by disorder. A small peak coming from C−H bonds at the surface is observed and its intensity inereases with the amount ofsp3-hybridized atoms in the sample. This can be easily explained by associating a higher amount of dangling bonds at the surface to a highersp3 content. Polarization-dependent XAS measurements show that the angular distribution of these C−H bonds has a mean value close to the normal to the surface.