Low temperature photoluminescence of C_(60) thin films of different structures

摘要

In this paper results of a study of fullerite C_(60) vacuum deposited thin films low temperature photoluminescence (PL) are provided. The two kinds of the films on a NaCl substrate were selected for the investigation: the nonoriented polycrystalline films with the ultimate dispersibility degree and with the microcrystals average size about 5 nm and the films with good structural ordering and with the average microcrystals size about 30-40 nm. For the first time the PL spectra of the free thin films of the second kind were studied. The PL spectra were measured at 5 K covering the spectral range from 1.5 to 1.85 eV. All PL spectra were fitted to four Gaussians peaks at 1.78; 1.71; 1.6 and 1.52 eV. Probable versions of the crystal excited states are considered. It is likely that the PL peak at 1.71 eV corresponds to the radiative recombination of a self-trapped Frenkel exciton with a contribution of a charge-transfer character and the PL peak at 1.78 eV corresponds to the radiative recombination of the excited X-traps. The nature of the X-traps is discussed. The X-trap Pl peak at 1.78 eV is considerably larger for oriented films than for nonoriented ones. It is assumed that this phenomenon is stipulated by peculiarities of exciton transport and localization within the microcrystals of films.