Temperature dependence of the band gap in C60 crystals

The temperature dependence (13–300 K) of the optical-allowed direct band gap Eg(T) deduced from the transmission and photoconductivity spectra in high-quality C60 crystals is presented. A strong temperature-dependent Eg(T) is observed and attributed to the electron–phonon interaction via the localized C60 phonon modes. In addition, a discontinuous change for dEg/dT around 249 K is reported due to the orientational order/disorder phase transition. Finally, three discrete localized bands are observed and the photoconduction for these localized bands is discussed in terms of photothermal ionization.