Optical properties of C60 nanocrystals embedded in a SiO matrix

Abstract

Photoluminescence spectra of (C60)n nanocrystals (1000<n<10 000) in a SiO matrix have been investigated in order to study electronic and vibronic states of mesoscopic phase of C60 crystals. We have found that both vibronic bands, which are derived from the gerade- and ungerade-parity vibrational modes, obviously appear in the spectrum of the C60 nanocrystals at 80 K. Results indicate that Herzberg–Teller transitions become strong with a decrease in the size of the crystal. It has been also found that Herzberg–Teller vibronic bands become relatively dominant with an increase in the temperature compared to Jahn–Teller vibronic bands. We analyzed the data by the temperature dependence of quantum efficiency, and found that the thermal activation energy for nonradiative relaxation can be classified into two groups; 190 and 90 meV for the Herzberg– and Jahn–Teller vibronic bands, respectively. This indicates that two kinds of electronic origins coexist in the C60 nanocrystals at a temperature above 80 K.