Iijima et al. (ref. 3,4), then reported Transmission Electron Microscopy (TEM) observations of hollow graphitic tubes, nanotubes, and the large-scale nanotube synthesis by Ebbesen and Ajayan (ref. 5), has seen an explosive research effort into these unusual graphitic forms.
TEM observations of graphitic tubes synthesised using pyrolytic methods, (e.g. pyrolysis of acetylene over cobalt) and their derivatives have revealed fullerene geometries with closed cage arrangements, and also other "oddly" shaped fullerene structures such as "bucky-onions".
It is generally recognised that C60, which consists of pentagons and hexagons formed with carbon atoms, the pentagons play an essential role in creating the convex plane. By wrapping a cylinder in a sheet of graphite we get nanotubes, and in a study of the growth of nanotubes, Iijima found that heptagons of carbon atoms appear in the negatively curved surface. The positive curvature is achieved by inserting pentagons into the graphite sheet. This can be visualised below, where the pentagonal carbons are shown in red and the heptagonal carbons are shown in yellow. Structures incorporating pentagons, hexagons and heptagons include torroidal and helical forms.
Theoretical studies (ref. 6) have shown that many of the new graphitic arrangements are very stable, in some cases more that C60 itself, and related structures appear naturally in certain graphitic materials. One derivative, helical in structure, is analagous to that seen in crystal growth affected by screw disslocations.
|Hypothetical single walled helical nanotube|
Section of helical nanotube, pentagons in red, heptagons in yellow.
Special thanks must go to Mauricio Terrones and
Dr. Paul Birkett of the Sussex Fullerene Group, University of Sussex, for providing the helical structure.
(c) Wyn Locke. 24 May, 1996.