Page 120 - Carbon Nanotubes

Hemi-toroidal networks in pyrolytic carbon nanotubes 109
becomes focused in the regions where the pentagons are unlikely to form very often under the conditions
and heptagons lie, producing localised cusps and sad- discussed here. However, the present results do sug-
dle points that can be seen relatively clearly in Fig. 3. gest that, in the future, nanoscale engineering tech-
Fascinating toroidal structures with Doh and Dnd sym- niques may be developed in which the graphene edges
metry are produced, depending on whether the p/h of open-ended double-walled tubes, produced by con-
pairs at opposite ends of the tube are directly aligned trolled oxidation or otherwise, may be cauterised by
or are offset by 27r/2n. In Fig. 3 the symmetry is DSh. rim-seals which link the inner and outer tubes; this
Chiral structures are produced by off-setting the pent- may be one viable way of overcoming any dangling
agon from the heptagons[24]. In a D5d structure, the bond reactivity that would otherwise preclude the use
walls are fluted between heptagons at opposite ends of these structures in nanoscale devices. In this way,
of the inner tube and the pentagons of the outer walls. a long (or high) fully toroidal structure might be
It is interesting to note that, in the computer images, formed.
the cusps give rise to variations in the smoothness of
the more-or-less elliptical image generated by the rim
when viewed at an angle. The observed image (b-b’) Acknowledgement-We thank Raz Abeysinghe, Lawrence
Dunne, Thomas Ebbeson, Mauricio Terrones, and David
exhibits variations consistent with the localised cusp- Walton for their help. We are also grateful to the SERC and
ing that our model predicts will occur. the Royal Society for financial support.

3. DISCUSSION
REFERENCES
In this study, we note that epitaxial graphitisation
is achieved by heat treatment of the apparently mainly 1. S. Iijima, Nature 354, 56 (1991).
amorphous material which surrounds a nanotube[20]. 2. W. Kratschmer, L. Lamb, K. Fostiropoulos, and D.
Huffman, Nature 347, 354 (1990).
As well as bulk graphitisation, localised hemi-toroidal 3. T. W. Ebbeson and P. M. Ajayan, Nature 358, 220
structures that connect adjacent walls appear to form (1 992).
readily. This type of structure may be important as it 4. M. Endo and H. W. Kroto, L Phys. Chem. 96, 6941
suggests that double-walled, closed structures may be ( 1992).
produced by heat treatment, so forming stable nano- 5. S. Iijima, P. M. Ajayan, and T. Ichihashi, Phys. Rev.
Letts. 69, 3100 (1992).
scale graphite tubes in which dangling bonds have 6. G. Ulmer, E. E. B. Cambell, R. Kuhnle, H-G. Busmann,
been eliminated. It will be most interesting to probe and I. V. Hertel, Chem. Phys. Letts. 182, 114 (1991).
the relative reactivity of these structures for the future 7. S. W. McElvany, M. M. Ross, N. S. Goroff, and E
of nanoscale devices, such as quantum wire supports. Diederich, Science 259, 1594 (1993).
Although the curvatures of the rims appear to be quite 8. H. W. Kroto, J. R. Heath, S. C. O’Brien, R. E Curl, and
R. E. Smalley, Nature 354, 359 (1985).
tight, it is clear from the loop images that the occur- 9. H. W. Kroto, Nature 329, 529 (1977).
rence of turnovers between concentric cylinders with 10. H. W. Kroto and K. G. McKay, Nature 331, 328 (1988).
a gap spacing of the standard graphite interlayer spac- 11. H. W. Kroto, Chem. Brit. 26, 40 (1990).
ing is relatively common. Interestingly, the edges of 12. T. Guo, M. D. Diener, Y. Chai, M. J. Alford, R. E.
Haufler, S. M. McClure, T. Ohno, J. H. Weaver, G. E.
the rims appear to be readily visible and this has al- Scuseria, and R. E. Smalley, Science 257, 1661 (1992).
lowed us to confirm the relationship between oppos- 13. L. D. Lamb, D. R. Huffman, R. K. Workman, S. How-
ing loops. Bulges in the loops of the kind observed at ells, T. Chen, D. Sarid, and R. F. Ziolo, Science 255,
1413 (1991).
d’ in Fig. 2 are simulated theoretically as indicated in 14. H. Terrones and A. L. Mackay, In The Fuilerenes (Ed-
Fig. 6. ited by H. W. Kroto, J. E. Fischer, and D. E. Cox), pp.
During the course of the study on the simplest ar- 113-122. Pergamon, Oxford (1993).
chetypal twin-walled structures in PCNT material de- 15. A. L. Mackay and H. Terrones, In TheFullerenes (Ed-
scribed here, the observation of Iijima et al.[5] on ited by H. W. Kroto and D. R. M. Walton), Cambridge
University Press (1993).
related multi-walled hemi-toroidal structures in ACNTs 16. T. Lenosky, X. Gonze, M. P. Teter, and V. Elser, Na-
appeared. It will be most interesting to probe the dif- ture 355, 333 (1992).
ferences in the formation process involved. 17. L. A. Chernozatonskii, Phys. Letts. A. 170, 37 (1992).
The question might be addressed as to whether the 18. S. Ihara, S. Itoh, and Y. Kitakami, Phys. Rev. B 47,
hemispheroidal structures observed here may provide 13908 (1993).
an indication that complete toroids may be formed 19. S. Itoh, S. Ihara, and Y. Kitakami, Phys. Rev. B 47,
1703 (1993).
from graphite. The structures appear to form as a re- 20. A. Sarkar, H. W. Kroto, and M. Endo, in prep.
sult of optimal graphitisation of two adjacent concen- 21. M. Endo, K. Takeuchi, S. Igarishi, M. Shiraishi, and
tric graphene tubes, when further extension growth is H. W. Kroto, J. Phys. Chem. Solids 54, 1841 (1993).
no longer feasible (Lea, as in the case shown in Fig. 2 22. S. Iijima, Phys. Rev. Letts. 21, 3100 (1992).
23. S. Iijima, Mat. Sci. and Eng. B19, 172 (1993).
at b-b‘ where a four-walled tube section must reduce 24. U. Heinen, H. W. Kroto, A. Sarkar, and M. Terrones,
to a two-walled one.) This suggests that perfect toroids in prep.