Role of molecular oxygen and other impurities in the electrical transport and dielectric properties of C 60 films

We have used dielectric spectroscopy to measure the frequency and temperature dependence of the lowfrequency ~0.5 mHz–100 kHz! complex dielectric function «(v) of thin C60 films. A small charge transfer occurs between the C60 and O2 molecules which occupy interstitial spaces of solid C60 exposed to oxygen ~or ambient air!. Due to the large size of the C60 molecules, this small charge transfer creates large dipole moments, which in turn are coupled to the applied ac electric field via a diffusion-controlled relaxation mechanism. This gives rise to a significant increase in the permittivity «1 accompanied by a broad dielectric loss peak «2 observed at ;10–100 Hz. With increasing oxygenation, the interstitial sites become nearly fully occupied, interstitial hopping is inhibited, and the loss peaks, together with the enhanced polarization, disappear. Both tracer and chemical diffusion coefficients for the C60/O2 system have been obtained purely from dielectric spectroscopy measurements. @S0163-1829~97!05423-4#