Optical properties of C-60 nanocrystals embedded in a SiO matrix

Photoluminescence spectra of (C-60)(n) nanocrystals (1000 <n < 10 000) in a SiO matrix have been investigated in order to study electronic and vibroni c states of mesoscopic phase of C-60 crystals. We have found that both vibr onic bands, which are derived from the gerade- and ungerade-parity vibratio nal modes, obviously appear in the spectrum of the C-60 nanocrystals at 80 K. Results indicate that Herzberg-Teller transitions become strong with a d ecrease in the size of the crystal. It has been also found that Herzberg-Te ller vibronic bands become relatively dominant with an increase in the temp erature compared to Jahn-Teller vibronic bands. We analyzed the data by the temperature dependence of quantum efficiency, and found that the thermal a ctivation energy for nonradiative relaxation can be classified into two gro ups; 190 and 90 meV for the Herzberg- and Jahn-Teller vibronic bands, respe ctively. This indicates that two kinds of electronic origins coexist in the C-60 nanocrystals at a temperature above 80 K. (C) 2001 American Institute of Physics.