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

Abstract

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).

Introduction

The discovery of C₆₀ [1], a third allotrope of carbon in addition to the more familiar diamond and graphite, together with an easy method to produce macroscopic quantities of the material [2] have generated enormous interest in many areas of physics, chemistry and material science.

At room temperature, solid C₆₀ is a molecular crystal with entire C₆₀ molecules occupying the lattice sites of a face-centered cubic (FCC) structure [3]. Crystals and thin films of pristine C₆₀ are found to exhibit semiconductor-like behavior in their optical and electronic properties, while the electronic properties of doped fullerenes can be “tuned” to produce semiconductors, conductors and even superconductors [4].

The most commonly practiced kind of doping is intercalation: whereby dopants are located between the C₆₀ molecules in the interstitial positions of the host crystal structure. Intercalation may occur as a spontaneous process or it can be induced by an external stimulus, like vapor pressure or an electric field applied to the sample [5]. There has been a considerable research effort to study M₃C₆₀ compounds (where M is an alkali or alkali-earth metal) since the discovery of superconductivity in these compounds [6]. C₆₀-metal interactions can be classified according to whether they form compounds (like alkali- or alkali-earth fullerides) with charge transfer from the metal-donor to the C₆₀-electron-acceptor, or phase-separated solids [7]. For the latter, no solid solution is formed with metals (other than alkali- or alkali-earth metals), owing to their usually high cohesive energy. However, limited charge transfer between most metals and C₆₀ is still possible, since their work functions have low values [8]. The C₆₀-metal interaction, with emphasis on the possibility of tailoring new C₆₀-metal compounds or composites, is one of the main subjects in fullerene research today [5].

Given the large size of the interstitial sites in the C₆₀ crystal (the corresponding voids are more than 4 Å in diameter) molecular oxygen from air readily diffuses into this solid even at room temperature [9]. The perturbing effect of such spontaneous intercalation on the electronic properties of C₆₀ single crystals and thin films (in particular, a drastic reduction of both dark and photo-conductivity) has been reported by many research teams [10], [11], [12], [13], [14], [15], [16]. However, some positive effects due to the spontaneous intercalation of C₆₀ thin films with oxygen have been reported too. Wen et al. [17] revealed that C₆₀ films exposed to air, at room temperature, for 1 day or longer start to emit broad, intense white light under laser irradiation. Recently [18], we demonstrated that lengthy exposure to air at room temperature of Cu-supported C₆₀ thin films results not only in oxygen diffusion but also in a counter diffusion of metal from the substrate into the C₆₀ matrix. We referred to this multi-step process as “chemically-induced counter electro-diffusion” (CICED) and suggested to use the counter diffusion approach to produce new C₆₀-based materials. In the present paper, motivated by this interest, we report on a detailed study of the structural and chemical transformations in Cu-supported C₆₀ films, as promoted by their room-temperature-exposure to air or I₂ atmospheres.