ABSORPTION-SPECTRA OF C-60 AND C-70 THIN-FILMS

The spectra of C-60 and C-70 thin films over a wide energy range (0.6 to 6.5 eV) are measured by transmission spectroscopy and photothermal deflection spectroscopy (PDS), and the optical absorption coefficients are obtained. The optical transitions for the C-60 and C-70 thin film s are analyzed according to the molecular orbital model. The weak abso rption spectra of the fullerenes are similar to that of amorphous sili con. The optical energy gaps are given by Tauc's plots as 1.75 and 1.6 5 eV for C-60 and C-70 thin films, respectively. The disorders in the fullerene films, which resulted in band-tail state or defect state, ar e indicated by Urbach edge and sub-gap absorption. It is the disorder that brought the difficulty in determination of the energy gap for ful lerenes. The effects of the deflection medium and substrate on the wea k absorption spectra of fullerene films are also discussed.