Structural and chemical modifications in Cu-supported C₆₀ thin films exposed to an atmosphere of air or iodine

Structural and chemical changes in C₆₀ thin films, grown on a Cu substrate and exposed to air or I₂ 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 C₆₀ 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 which we refer to as Cu_xO_yC₆₀. The new phase is quasi-stable at room temperature, but a relatively short thermal annealing at 150°C destroys it and restores the initial C₆₀ FCC lattice. After 10 min of exposure of the Cu-supported C₆₀ films to an I₂ atmosphere the Cu sub-layer disappears completely and macroscopic amounts of a stable CuI phase are formed over the entire thickness of the C₆₀ film. To explain the results a model of chemically-induced counter electro-diffusion is proposed. We propose the possible usefulness of this counter diffusion approach for tailoring C₆₀ -based materials doped with various compounds in the form of both phase-separated composites and solid solutions (intercalated fullerides).