Page 23 - Carbon Nanotubes

14 D. T. COLBERT and R. E. SMALLEY
In the absence of an electric field, the dome-closed With expanding knowledge about the ways nano-
conformation must be the most stable tip structure, tubes form and behave, and as their special properties
even when spot-welds are considered, since only the are increasingly probed, the time is fast approaching
perfectly dome-closed tip has no dangling bonds (Le., when nanotubes can be put to novel uses. Their size
it is a true hemifullerene). At the 3000°C temperature and electrical properties suggest their use as nano-
of the arc, the rate of tip annealing should be so fast probes, for instance, as nanoelectrodes for probing the
that it is sure to find its most stable structure (i.e., to chemistry of living cells on the nanometer scale. The
close as a dome). Clear evidence of this facile closure atomic wire may be an unrivaled cold field emission
is the fact that virtually all nanotubes found in the arc source of coherent electrons. Such potential uses of-
deposit are dome-closed. (Even stronger evidence is fer the prospect of opening up new worlds of investi-
the observation of only dome-closed nanotubes made gation into previously unapproachable domains.
at 1200°C by the oven laser vaporization method.)
Such considerations constituted the original motiva- Acknowledgements-This work was supported by the Office
tion for the electric field hypothesis. of Naval Research, the National Science Foundation, the
Armed with these results, a direct test of the hy- Robert A. Welch Foundation, and used equipment designed
pothesis using a single mounted nanotube in our vac- for study of fullerene-encapsulated catalysts supported by the
uum apparatus was sensible. A dome-closed nanotube Department of Energy, Division of Chemical Sciences.
harvested from the arc deposit gave inactivated state
behavior at -75 V bias. Maintaining the bias voltage
at -75 V, the nanotube was irradiated for about 30 sec- REFERENCES
onds with sufficient intensity to sublime some carbon
from the tip (-3000°C). Now the nanotube exhibited 1. T. W. Ebbesen, Ann. Rev. Mat. Sei. 24, 235 (1994); S.
Iijima, P. M. Ajayan, and T. Ichihashi, Phys. Rev. Lett.
typical activated emission behavior, indicating an open 69, 3100 (1992); Y. Saito, T. Yoshikawa, M. Inagaki, M.
tip from which long carbon chains were pulled, con- Tomita, and T. Hayashi, Chem. Phys. Lett. 204, 277
stituting the emitting structures described in section 3 (1993).
above. When the nanotube was reheated at 0 V bias, 2. D. T. Colbert et a/., Science 266, 1218 (1994).
the tip was re-closed. Subsequent heating to 3000°C 3. S. Iijima, Nature 354, 56 (1991).
at -75 V bias re-opened the tip. These results can only 4. T. W. Ebbesen and P. M. Ajayan, Nature 358, 220
(1992).
be explained by the electric field’s providing the nec- 5. D. Ugarte, Chem. Phys. Lett. 198,596 (1992); D. Ugarte,
essary stabilization to keep the tip open. Nature 359, 707 (1992).
The structure calculations on single-walled tubes 6. R. E. Srnalley, Mat Sci. Eng. B19, 1 (1993).
show that the stabilizing effect of the field is at most 7. T. W. Ebbesen, P. M. Ajayan, H. Hiura, and K.
Tanigaki, Nature 367, 519 (1994).
about 10% of that required to lower the energy of the 8. A. G. Rinzler, J. H. Hafner, P. Nikolaev, D. T. Colbert,
open below that of the closed tip, before reaching un- and R. E. Srnalley, MRS Proceedings 359 (1995).
realistically strong field strengths. However, with our 9. A. G. Rinzler et al., in preparation.
enhanced understanding of the structure of nanotube 10. T. Guo et al., submitted for publication.
tips, much of the energy lowering is achieved by the 11. J. Jund, S. G. Kim, and D. Tomanek, in preparation;
C. Xu and G. E. Scuseria, in preparation.
adatom spot-welds (not included in the calculations), 12. L. Lou, P. Nordlander, and R. E. Srnalley, Phys. Rev.
leaving less of an energy gap for the electric field ef- Lett. (in press).
fect to bridge. We emphasize that spot-welds alone 13. A. Maiti, C. J. Brabec, C. M. Roland, and J. Bernholc,
cannot be sufficient to render the open tip stable rel- Phys. Rev. Lett. 13, 2468 (1994).
ative to the dome-closed one, since the latter structure 14. R. E. Haufler et al., Mat. Res. Symp. Proc. 206, 621
(1 991).
is the only known way to eliminate all the energetically 15. R. E. Srnalley, Acct. Chem. Res. 25, 98 (1992).
costly dangling bonds. An electric field is necessary.