OPTICAL-ABSORPTION SPECTRA OF CARBON 60 THIN-FILMS FROM 0.4 TO 6.2 EV

The optical absorption spectrum for thin films of sublimed C60 is presented. Both transmission spectroscopy and photothermal deflection spectroscopy are used to obtain the spectrum over a wide energy range (0.4 to 6.2 eV) for the same film. The optical gap is almost-equal-to 1.6 eV and the corresponding transition appears to be optically forbidden given the weakness of the absorption. Structure is observed on the "gap edge" which can be attributed to vibronic transitions. The gap region can be described in terms used for amorphous semiconductors, having features such as an Urbach edge, and sub-gap defect ("dangling bond") absorption. The similarity suggests that the nature of C60 thin films may be analogous to amorphous silicon and carbon films.