Shape and Fantasy of Fullerenes
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MRS BULLETIN/NOVEMBER 1994
conditions, buckyonions are known to grow multilayered nanotubes from several places on the surface. The result is a composite structure that looks like a sea urchin (Figure Id).5 Similarly, catalytic action of some metal elements is known to cause regrowth of nanotubes from the cap of a nanotube along the direction of the host tube. This action produces a tubular structure containing nodes at intervals, resembling a bamboo trunk (Figure le).6 Both of the structures in Figures Id and le encapsulate metals. A recent masterpiece from the nano-factories is a chain of nanoparticles
(c)
Figure 1. Illustrations of various shapes of fullerenes: (a) tube with both ends closed by caps, (b) tube having differential diameter, (c) particle (buckyonion), (d) sea urchin, (e) bamboo, (f) beads, (g) spinning cone, (h) helical coil, and (i) connectors and tripod. Among these, (a) to (h) have been experimentally observed, while (i) is imaginary. The observed forms are usually multilayered, the interlayer distance being always equal to the interpianar distance in graphite of 3.6 A.
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Shape and Fantasy of Fullerenes
aligned like a string of beads or skewed dumplings (Figure If), observed in the cathodic deposit from the arc discharge of carbon rods containing nickel.63 In these forms, the distinction between tube and particle becomes rather obscure. Multilayered, pointed tubes shaped like spinning cones (Figure lg) are a remarkable result
Figure 2. Trunk of an old pine tree found near Toyohashi. While most of the bark sections are hexagonal, a pentagonal piece of bark appears in a place where the trunk is laterally expanded to produce a partially convex surface.
Figure 3. A bamboo vase containing a heptagonal pattern at the neck (from the Fujita Collection of Shapes).
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of the catalytic pyrolysis of hydrocarbons.7 Under different catalysts, coiled buckytubes (Figure lh) 8 have been found in the product of acetylene pyrolysis, producing great surprise among researchers. We have often seen helical coil structures in the atomistic world, but they are usually associated with DNA or proteins wherein the helix is maintained by hydrogen-bonding. There are, however, neither hydrogen bonds nor other anchoring mechanisms in the carbon tubes! How can we explain the formation of such a coiled form of carbon? If nanotubes are considered to be one of the construction materials in the nanoworld,1 joints and connectors for the tubes must be supplied to nanoworld plumbers. Various forms have already been designed 9 and their strengths theoretically checked (Figure li),M although not experimentally observed. So far we have not mentioned the fine details of the molecular surfaces of tubes and particles. Their surfaces are an extended network of mostly hexagonal rings of carbon atoms. The structure of graphite, which is known to be completely flat and rigid is, however, a hexagonal network. How can those varied forms, as shown in Figure 1, be accounted for?
Odd-Numbered Rings for Shaping Fullerenes The role of the
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