Structure and properties of thermally annealed fullerene films

The effects of postdeposition thermal annealing on the structure and properties of C60 films are investigated. Non-oxygenated C60 films are heated at 200°C under vacuum. UV/visible absorption spectroscopy shows a decrease in the intensities of the allowed transitions and a red-shift of up to 50 run. IR analyses show no indication of chemical alteration of the sample. X-ray studies reveal cubic symmetry in the unannealed thin films, while the annealed films show a mixture of face-centred cubic and hexagonal close-packed phases and a higher degree of crystallinity. Room-temperature Raman studies in the region of the pentagonal pinch mode reveal the presence of two components. The first component, at 1468 cm−1, is photochemically unstable, as is the pristine material. The second component, positioned at 1464 cm−1, remains stable under prolonged low-intensity illumination. The effects of thermal annealing of oxygen-free films are contrasted to those of thermal treatment under oxygen and it is concluded that the modifications are structural in nature.