Surface modification of C-60 by ion irradiation studied with photoelectron spectroscopy

Abstract

Thin films of C-60 were subjected to irradiation by low energy Ar+ ions (E<2 keV) and the changes in the surface initiated by ion impact were monitored in situ with photoelectron spectroscopy in the ultraviolet (UPS) and x-ray regime (XPS). The ion irradiation leads to damage of the C-60 cages and accumulation of defects in the surface regions. This is expressed in the UPS spectra by a gradual decrease in the intensity of the sharp features associated with the valence band of C-60 and the appearance of additional states close to the Fermi energy. An analysis of the UPS-valence band spectra shows that the destruction of the cages is incomplete. The formation of defects and structural changes in the surface layer trigger a shift of the C-60 related features in the valence band spectra and the C 1s core level by 0.4 eV to smaller binding energies. This is commensurate with a shift and pinning of the Fermi level in the gap. The C 1s peak parameters derived from a deconvolution of the peak likewise reflect the partial cage destruction, and the deconvolution indicates the presence of a single component. This might point to the presence of a structurally rather homogeneous surface layer, but different explanations for this experimental result can be envisaged and are discussed. (C) 2002 American Institute of Physics.