Page 129 - Carbon Nanotubes

Properties of buckytubes and derivatives 119

Fig. 11. (a) and (b) are HREM images of the deposited rods produced by the glow discharge and by the
conventional arc discharge, respectively.

polygonal and cone-shaped caps are formed by incor- nanoparticles. An effort to promote the growth of car-
porating pentagons into the hexagonal network. He bon nanotubes and eliminate the formation of carbon
speculated that the formation of the pentagons may nanoparticles is presently underway.
result from a depletion of carbon in plasma near the
end of cathode[2]. Our experimental results offer ev-
idence for the above speculation. Fluctuations of car- 4. CONCLUSIONS
bon species caused by a discontinuous arc discharge In this article, we have reported the structural,
may be responsible for the formation of short tubes magnetic, and transport properties of bundles of
with the caps, consisting of pentagons and other buckytubes produced by an arc discharge. By adjust-
defects. ing the arc mode into a stable glow discharge, evenly
Based on these experimental results, one can spec- spaced and parallel buckybundles with diameters up to
ulate on the influence of the arc mode on the yield and 200 pm have been synthesized. The magnetic suscep-
distribution of the bundles. For the glow discharge, tibility of a bulk sample of buckybundles is -10.75 x
the plasma is continuous, homogeneous, and stable. emu/g for the magnetic field parallel to the bun-
In other words, the temperature distribution, the elec- dle axes, which is approximately 1.1 times the perpen-
tric field which keeps growing tube tips open[47], and dicular value and 30 times larger than that of c60.
the availability of carbon species (atoms, ions, and The magnetoresistance (MR) and Hall coefficient mea-
radicals) are continuous, homogeneous, and stable surements on the buckybundles show a negative MR
over the entire central region of the cathode. Accord- at low temperature, a positive MR at a temperature
ingly, a high yield and better quality buckytubes above 60K, and a conductivity which increases ap-
should occur over the entire central region of the cath- proximately linearly with temperature. Our results
ode. These are consistent with what we observed in show that a buckybundle may best be described as a
Fig. 9 (a), Fig. 10, and Fig. 11 (a). For the conven- semimetal.
tional arc discharge, we can speculate that the arc
starts at a sharp edge near the point of closest ap- Acknowledgements-We are grateful to A. Patashinski for
proach, and after vaporizing this region it jumps to useful discussions. This work was performed under the sup-
what then becomes the next point of closest approach port of NSF grant #9320520 and DMR-9357513 (NYI award
(usually within about a radius of the arc area), and so for VPD). The use of MRC central facilities supported by
on. The arc wanders around on the surface of the end NSF is gratefully acknowledged.
of the anode, leading, on the average, to a discontin-
uous evaporation process and an instability of the elec- REFERENCES
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