STRUCTURAL TRANSITIONS AND ELECTRICAL-CONDUCTIVITY OF C-60 FILMS AT HIGH-TEMPERATURE

X-ray diffraction analysis on C-60 films shows that besides fee phase, there also exists hcp phase, as well as a new crystalline phase with interplanar spacing (d-spacing) of planes parallel to the substrate 0. 95 nm. The new phase may relate to the intercrystalline packed C-60 mo lecules between fee crystallites. The room temperature electrical cond uctivity of C-60 films is found to be in the range of 10(-5)-10(-8) (O mega . cm)(-1). The room temperature conductivities of C-60 films anne aled at temperatures above 473 K are lower by one order of magnitude t han those at temperatures below 463 K. This is because the interconnec tion between the fee crystallites is weakened due to the disappearance of the new intercrystalline phase and the subsequent heightening of t he intercrystalline potential barrier. From the measurement on the con ductivity versus time when the film is maintained at a constant temper ature, we identified the increase of conductivity is the result of the decrease of hcp phase, while the decrease of conductivity is due to t he decrease of the new intercrystalline phase. Because the structures of the films become highly ordered, and defect states in the energy ba nd gap decrease on annealing at high temperature, the conductivity act ivation energy increases.