Page 81 - Carbon Nanotubes

70 K. SATTLER
armchair shapes) to match their corresponding cones and are
unlikely to form. This explains why only the 19.2"
cones have been observed in our experiment.
Carbon cones are peculiar mesoscopic objects.
They are characterized by a continuous transition
from fullerene to graphite through a tubular-like in-
termedium. The dimensionality changes gradually as
the cone opens. It resembles a 0-D cluster at the apex,
c
then proceeds to a 1-D 'pipe' and finally approaches
a 2-D layer. The cabon cones may have complex band
structures and fascinating charge transport properties,
apex
from insulating at the apex to metallic at the base.
They might be used as building units in future nano-
scale electronics devices.
zigzag
Acknowledgements-Financial support from the National
Fig. 9. Ball-and-stick model for a 19.2" fullerene cone. The Science Foundation, Grant No. DMR-9106374, is gratefully
back part of the cone is identical to the front part displayed acknowledged.
in the figure, due to the mirror symmetry. The network is in
'armchair' and 'zigzag' configurations, at the upper and lower
sides, respectively. The apex of the cone is a fullerene-type
cap containing five pentagons. REFERENCES
1. S. Iijima, Nature 354, 56 (1991). T. W. Ebbesen and P.
easily show that moving a 'pitch' (the distance between M. Ajayan, Nature 358, 220 (1992).
two equivalent sites in the network) along any closure 2. D. Ugarte, Nature 359, 707 (1992).
3. M. Ge and K. Sattler, Chem. Phys. Lett. 220, 192 (1994).
line of the network leads to another identical cone. For 4. M. Ge and K. Sattler, Science 260, 515 (1993).
the 19.2" cone, a hexagon row changes its 'local heli- 5. T. W. Ebbesen, H. Hiura, J. Fijita, Y. Ochiai, S. Mat-
cal' direction at half a turn around the cone axis and sui, and K. Tanigaki, Chem. Phys. Lett. 209,83 (1993).
comes back after a full turn, due to its mirror symme- 6. M. J. Gallagher, D. Chen, B. P. Jakobsen, D. Sand, L.
D. Lamb, E A. Tinker, J. Jiao, D. R. Huffman, S. Ser-
try in respect to its axis. The other four cones, with aphin, and D. Zhou, Surf. Sci. Lett. 281, L335 (1993).
larger opening angles, have Dnd (n = 2,3,4,5) symme- 7. Z. Zhang and Ch. M. Lieber, Appl. Phys. Lett. 62,2792
try along the axis. The 'local helicity' changes its di- (1993).
rection at each of their symmetry planes. 8. R. Hoeper, R. K. Workman, D. Chen, D. Sarid, T.
Yadav, J. C. Withers, and R. 0. Loutfy, Surf. Sci. 311,
As fullerenes, tubes, and cones are produced in the L371 (1994).
vapor phase we consider all three structures being 9. K. Sattler, Znt. J. Mod. Phys. B 6, 3603 (1992).
originated by a similar-type nucleation seed, a small 10. J. Xhie, K. Sattler, N. Venkateswaran, and M. Ge, Phys.
curved carbon sheet composed .of hexagons and pent- Rev. B 47, 15835 (1993).
agons. The number of pentagons (m) in this fullerene- 11. J. Xhie, K. Sattler, U. Mueller, G. Raina, and N.
Venkateswaran, Phys. Rev. B 43, 8917 (1991).
type (m-P) seed determines its shape. Continuing growth 12. M. Ge, K. Sattler, J. Xhie, and N. Venkateswaran, In
of an alternating pentagodhexagon (516) network Novel forms of carbon (Edited by C. L. Renschler, J.
leads to the formation of C60 (and higher fullerenes). Pouch, and D. Cox) Mat. Res. SOC. Proc. 270, 109
If however, after the Cb0 hemisphere is completed, (1992).
growth continues rather as a graphitic (6/6) network, 13. R. Saito, M. Fujita, G. Dresselhaus, and M. S. Dressel-
haus, Appl. Phys. Lett. 60, 2204 (1992).
a tubule is formed. If graphitic growth progresses 14. M. Ge and K. Sattler, Appl. Phys. Lett. 65, 2284 (1994).
from seeds containing one to five pentagons, fuller- 15. D. Tomanek, S. G. Louie, H. J. Mamin, D. W. Abra-
ene cones can be formed. ham, R. E. Thomson, E. Ganz, and J. Clarke, Phys.
Rev. B 35, 7790 (1987).
The shape of the 5-P seed is closest to spherical 16. J. M. Soler, A. M. Baro, N. Garcia, and H. Rohrer,
among the five possible seeds. Also, its opening an- Phys. Rev. Lett. 57, 444 (1986).
gle matches well with the 19.2" graphitic cone. There- 17. N. Hamada, S. Samada, and A. Oshiyama, Phys. Rev.
fore, continuing growth of a graphitic network can Lett. 68, 1579 (1992).
proceed from the 5-P seed, without considerable strain 18. M. S. Dresselhaus, G. Dresselhaus, and R. Saito, Phys.
Rev. B 45, 6234 (1992).
in the transition region. The 2-P, 3-P, and 4-P seeds 19. G. Tibbetts, J. Cryst. Growth 66, 632 (1984).
would induce higher strain (due to their nonspherical 20. M. Ge and K. Sattler, Appl. Phys. Lett. 64, 710 (1994).