The energy gaps of fullerene C60 and C70 determined from the temperature dependent microwave conductivity

The dielectric constants\(\tilde \varepsilon = \varepsilon ' + i\varepsilon ''\) of C60 and C70 powders have been measured in the temperature range from 150 K to 900 K using a microwave absorption method. From the temperature dependence of e′′ the “electrical” band gaps are found to be 1.86±0.1 eV for C60 and 1.57±0.1 eV for C70. Our method is not influenced by the possible generation of excitons. The results are discussed and related to theoretical predictions.