INFLUENCE OF OXYGEN IMPURITIES ON ELECTRICAL-PROPERTIES OF FULLERENE C-60

We present high temperature dc, ac and contactless microwave conductivity results on solid-state C-60 (films and crystals) from room temperature up to 850 K. Heating pristine samples, which were exposed to the ambient atmosphere, under dynamic vacuum at first leads to a reduction of the electrical resistance and finally, above approximate to 700 K, to an increase in the resistance. The decrease is ascribed to oxygen desorption and the increase to the chemical reactivity of residual chemisorbed oxygen with the C-60 host molecules, respectively. Samples, annealed above 800 K, display a reversible temperature dependence of the resistance. The high temperature regime of their resistance exhibits an activated behaviour with an universal activation energy of 2E(a) = 1.85 +/- 0.04 eV for crystals and films, which is identical to the HOMO-LUMO splitting of the C-60-molecules.