Structural alteration and chemical stability of heat treated C₆₀ films

The effects of postdeposition annealing of C₆₀ films on the structure and chemical stability of the solid-state material are investigated. An initial study of the change of the photoconductive response during a heating cycle reveals an anomalous irreversible increase of the photocurrent. In a subsequent study, non-oxygenated polycrystalline C₆₀ films are subject to postdeposition heating at 200°C under vacuum. IR analyses reveal no new bond formation, indicating no chemical alteration of the sample. Ultra Violet/Visible absorption spectroscopy shows a decrease in the intensities of the allowed transitions and a significant red-shift of up to 50nm. In Scanning Electron Microscopy, larger grains and island growth are observed. X-ray diffraction shows the presence of mixed phases identified as cubic and hexagonal, which is a closer packed lattice, not seen in unheated films. Time of illumination dependent, low intensity room temperature Raman studies of the pentagonal pinch mode reveal the presence of two spectral components, which are associated with the two phases observed in X-ray diffraction. The intensity of the first component identified with the 1468cm^-1 Raman line decreases with time. The intensity of the second spectral component identified with the 1464cm^-1 Raman line remains stable under prolonged low intensity illumination. The first component is seen to be highly unstable and undergoes a light induced chemical transformation, while the second appears largely insensitive to prolonged illumination. The results suggest that the effect of film annealing is structural in nature and leads to a repacking of the lattice.