Page 173 - Carbon Nanotubes

164 D. UGARTE

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Fig. 1. High-resolution electron micrographs of graphitic particles: (a) as obtained from the electric arc
deposit, they display a well-defined faceted structure and a large inner hollow space, (b) the same parti-
cles after being subjected to intense electron irradiation (note the remarkable spherical shape and the dis-
appearance of the central empty space); dark lines represent graphitic layers.

is always very small, displaying a size very close to in the 0.2-1 pm range, (b) small particles (<0.2 pm)
C60 (20.7-1 nm) (see Fig. 2). also spherical in shape, but formed by concentric gra-
Onion-like graphitic clusters have also been gener- phitic layers. Unfortunately, no detailed HREM study
ated by other methods: (a) shock-wave treatment of of these particles was undertaken, but an apparent
carbon soot[l6]; (b) carbon deposits generated in a size effect connecting diamond and graphitic particles
plasma torch[l7], (c) laser melting of carbon within was insinuated. Recently, an interesting experiment
a high-pressure cell (50-300 kbar)[l8]. For these three also revealed a relation between ultradispersed dia-
cases, the reported graphitic particles display a sphe- monds (3-6 nm in diameter) and onion-like graphitic
roidal shape. particles. The nanometer diamond sample was obtained
In particular, the laser melting experiment produced by a chemical purification of detonation soot. The an-
two well-differentiated populations of carbon clusters: nealing treatment (1 100-1 500°C) of nanodiamonds
(a) spheroidal diamond particles with a radial texture generates onion-like graphitic particles with a remark-