Structural changes of fullerene by heat-treatment up to graphitization temperature

Structural changes of the C-60 and C-70 mixture during heat-treatment up to 2400 degrees C were studied by observing the carbonized disk of the fullerene with Raman spectroscopy, X-ray diffraction, FE-SEM, TEM, and AFM/STM. The fullerene lost its five-membered ring and its fee crystal structure by heat-treatment at 800 degrees C, as revealed by the Raman spectra and X-ray diffraction, forming hexagonal planes which were randomly arranged by 1300 degrees C. Further heat-treatment allowed some stacking of layers which grow to dominate, reducing the randomly oriented planes. The graphitized temperature up to 2400 degrees C provided a very sharp peak at 26 degrees, suggesting formation of stable turbostratic layers. The TEM characterized turbostratic stacking of 3 to 4 layers. A series of observation under AFM/STM and TEM indicate the crystal of the fullerene, amorphous grain of the hexagonal planes, and the hollow sphere are all in the same range of size around 10-20 nm. Such microdomains induced micro-roughness as observed by FE-SEM on the surface of the carbon disk. Superstructure of hexagonal plane was observed on the surface. A kind of solid state carbonization of the fullerene is suggested to maintain the dimensions of its crystal into the spherical microdomain, even if its marked structural changes take place within the unit. (C) 1997 Elsevier Science Ltd.