The effect of light on the hardness of C 60 crystals was investigated. The hardness increased after the crystals were illuminated with white light. Such a photo-illumination hardening was effective in a temperature range of 298-380 K. The photopolymerized C 60 molecules near the crystal surface were responsible for the hardening. The anomalous temperature dependence of the hardness at temperatures higher than 300 K, which we previously have observed, is explained as due to the photo-illumination hardening.
The photoinduced indentation hardening of C60 crystals was investigated in ultraviolet-visible light. The hardening strongly depended on the wavelength of the illumination light: the magnitude increased with increasing wavelength, and then reached a maximum at 700 nm. This indicated that red light is more effective than ultraviolet light in the hardening. The wavelength dependence of the hardening is discussed in terms of an interplay between the effective depth of the photoinduced polymerization and the penetration depth of the indenter.
The growth of the shells of C60 crystals was carried out under various conditions. The detailed structures of the grown shells were investigated by transmission electron microscopy and Raman spectroscopy. The shells were formed during thermal sublimation of the C60 crystals, which were irradiated with white light in air. The shells were mainly composed of a kind of amorphous carbon. From these results, it is suggested that the oxygen-induced disintegration of C60 cages is responsible for the shell formation.
The microhardness of C70 crystals was investigated in the temperature range of 295–425 K. The hardness gradually decreased with increasing temperature. The photoinduced hardening was observed after the long-time illumination of light. The hardening reached the maximum near the phase transition temperature of 348 K. These results are discussed and compared with those of C60 crystals.
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