Electrochromic phenomena in fullerene thin films

In recent years many scientists have devoted a great deal of attention to study of the conductive and superconductive properties of alkali-metal-doped C,, [l-31. On exposure to alkali vapor, the conductivity of A$, (A = K, Rb, Cs) first increases and then decreases depending on the degree of doping x [4,51. These studies were done immediately after the synthesis of C, by KrHtschmer et al. [61. The changes in electronic structure of C, induced by doping with alkaline metals have been investigated by several techniques [7-91 and recently optical absorption spectra of these systems have been reported [lo121. A change in color of C,, thin films from yellowbrown (undoped state) to silver-black when doped with alkali metals had already been observed [5]. Electrochromism can be generally defined as the phenomenon of reversible color change brought about by the passage of an electric current. The current causes a chemical change, an oxidation or reduction reaction, which is accompanied by a change in the optical absorption of the material. Among inorganic electrochromic materials, WO, [13-X] and NiO, [15,17], have been widely studied. There are also some organic materials such as conductive polymers [18,191 which present electrochromism when they are oxidized or reduced. All these materials undergo an electrochemical doping process where the injection or ejection of electrons is compensated for by the insertion or deinsertion of ions to maintain electroneutrality. Recently, Chabre et al. have reported interesting results