Spectroscopic studies of fullerene thin films and their composites

Photophysical properties of fullerene (C60 and C70) thin films were investigated by various spectroscopic measurements including absorption, luminescence, luminescence excitation, electric field induced luminescence quenching, and electroabsorption. These results were compared with photoconduction spectra, giving a picture of solid fullerene's excited states characterized by substantial charge transfer (CT) nature, that is, excited states delocalized over neighboring molecules (CT excitons) in the solid lattice. For C60 films, the CT exciton emerges around 540nm, while for C70 films, it was located at 690 and 535nm. Electroabsorption spectra of the two showed strong modulation at these same energies, and their spectral shapes were well fitted by the second derivative of the corresponding absorption spectra, another evidence for CT excitons. In a different context of work, an attempt was made to develop a new photonic material by the alternate deposition of C60/Si composite layers. We have found that, for appropriate Si content, the composite films emit strong white light under Ar ion laser excitation. The emission spectrum, after correction for the detecting system, was found to be very broad with no specific structures and increased monotonously from visible to 1.5μm, the limit of our measurement apparatus. Moreover, the spectral shape was well fitted by a Planck distribution, giving a surprisingly high black-body temperature of 1400-1600K. Also discussed are similarities and differences as compared with the previously reported white luminescence from C60 crystallites or powder and their implications.