Structural and chemical modifications in Cu-supported C-60 thin films exposed to an atmosphere of air or iodine

Structural and chemical changes in C-60 thin films, grown on a Cu substrate and exposed to air or I-2 atmosphere, at room temperature, were monitored by X-ray diffraction., Auger electron and X-ray photoelectron spectroscopy. Exposure to air is demonstrated to result not only in oxygen diffusion but also in a counter diffusion of the metal from the substrate into the C-60 matrix. In particular, 10 months of air-exposure led to the presence of Cu and O atoms at all depths over the sample and the formation of a complex wh ich we refer to as CuxOyC60. The new phase is quasi-stable at room temperat ure, but a relatively short thermal annealing at 150 degreesC destroys it a nd restores the initial C-60 FCC lattice. After 10 min of exposure of the C u-supported C-60 films to an I-2 atmosphere the Cu sub-layer disappears com pletely and macroscopic amounts of a stable CuI phase are formed over the e ntire thickness of the C-60 film. To explain the results a model of chemica lly-induced counter electro-diffusion is proposed. We propose the possible usefulness of this counter diffusion approach for tailoring C-60-based mate rials doped with various compounds in the form of both phase-separated comp osites and solid solutions (intercalated fullerides). (C) 2001 Elsevier Sci ence B.V. All rights reserved.