An EPR study of as-grown and photoinduced defects in C₆₀ thin films

EPR active species were found in irradiated C₆₀ thin films at room temperature both as-grown and after irradiation by light. The samples exhibit a singlet Lorentzian line with g = 2.0026 ± 0.0002 and ΔH_PP = 0.124 ± 0.005 mT which may be attributed to holes localized at rapidly rotating C₆₀ molecules, i.e. C₆₀⁺ free radicals. The bulk density of the C₆₀⁺ radicals in the as-grown samples depends on the conditions of the sample preparation and reaches 1 spin per 10 000 molecules for fast-grown films. Annealing at 570 K leads to a rapid decrease in that density. Irradiation of the C₆₀ thin films by visible and UV light leads to a significant increase in the number of the same C₆₀⁺ radicals. The total number of photoinduced defects depends on the light intensity and time of exposure. The irradiation by visible light is active only in the presence of oxygen whereas the UV irradiation acts also when the sample is kept in an inert atmosphere. At ambient conditions the C₆₀⁺ radicals are metastable with life time of several months.