Page 68 - Carbon Nanotubes

Carbon nanotubes with single-layer walls 57
growth occurs under extreme conditions of tempera- high (1600°C) temperature[ll]. The amorphous car-
ture and carbon density. The nanotubes produced also bon soot particles, however, are difficult to remove,
have very different characteristics. Therefore, we ex- and the oxidative approach used with some success to
pect that their formation mechanism will be quite dif- isolate multilayer tubes[58] seems to destroy the single-
ferent. It is possible that instead of growth occurring layer tubes. Measurement of the mechanical, optical,
at a metal particle interface, as has been proposed for electrical, and magnetic properties requires a clean
VOCF, urchin particles[& 171, and long single-walled sample to interpret the data unambiguously. Tests of
nanotubes[5], a mechanism more akin to those pro- the dependence of electrical conductivity and mechan-
posed for the growth of multilayer nanotubes on the ical strength on the tube diameter should be done, and
cathode tip in an all-carbon environment may be in- may soon be feasible with the availability of nanotubes
volved[21,26,29,54]. In that case, it has been suggested with a wide range of diameters.
that growth occurs at the free end of the nanotube, The unique properties of single-layer carbon nano-
which protrudes out into the carbon plasma. tubes will continue to inspire scientists in diverse fields
Some features of the arc process are known and are to explore their properties and possible applications.
relevant to growth models for single-layer nanotubes. Defect-free nanotubes are predicted to have very high
Earlier isotope labelling analyses of fullerene forma- tensile strength. A theoretical calculatior, of the elastic
tion shows that fullerenes formed in the arc are built constant for single-layer nanotubes[52] gives a result
up from atomic carbon[55-57]. Also, the production consistent with a simple estimate based on the c, elas-
of nanotubes does not seem to depend on whether tic constant of graphite (cI1 = 1.06 TPa). Using this
metal oxide or pure metal is used in the graphite an- constant, one finds a force constant of 350 Nt/(m of
ode. These results imply that both the metal and the edge) for graphene sheet. Multiplying this value by the
carbon are completely atomized under the arc condi- circumference of a 1.3 nm diameter nanotube gives
tions, and that both the catalytic species and nano- an elastic constant of 1.45 x Nt for such a tube.
tubes must be built up from atoms or atomic ions. Macroscopically, a bundle of these tubes 25 pms in di-
This fact, together with the consistency of the diam- ameter would support a 1-kg weight at a strain of 3%.
eters of the single-layer nanotubes produced in the gas In comparison, a steel wire of that diameter would
phase by transition metal catalysts, suggests a model break under a load of about 50 gm. When nanotubes
where small catalytic particles rapidly assemble in a re- are assembled into crystalline bundles, the elastic mod-
gion of high carbon density. Single-layer tubules nu- ulus does not decrease linearly with tube diameter but,
cleate and grow very rapidly on these particles as soon rather, it remains constant for tube diameters between
as they reach a critical size, leading to the relatively 3 and 6 nm, suggesting the strength-to-weight ratio
narrow diameter distributions observed. If nucleation of the crystal increases as the tube diameter increases
of additional layers is slow, the rapid drops in temper- [23]. The anisotropy inherent in the extreme aspect
ature and carbon density as the tubes move away from ratios characteristic of these fibers is an important
the arc could turn off the growth processes before feature, particularly if they can be aligned. Ab initio
multilayers can form. calculations show that these nanotubes could be one-
dimensional electric conductors or semiconductors,
depending on their diameter and helicity[36,39,59].
6. FUTURE DIRECTIONS
Other applications of carbon nanotubes have been
Experimental research on single-layer nanotubes is proposed in areas that range widely, from physics,
still in a very early stage. Understanding the growth chemistry, and materials to biology. Examples, such
mechanism of these nanotubes remains a great chal- as hydrogen storage media, nanowire templates, scan-
lenge for scientists working in this area. Not even the ning tunneling microscopy tips, catalyst supports, seeds
nature of the catalytically active species has been es- for growing carbon fibers, batteries materials, reinforc-
tablished to date. Developing better controlled systems ing fillings in concrete, etc. provide ample motivation
than standard arc reactors will be necessary to allow for further research on this pseudo-one-dimensional
the dependence of tube growth on the various impor- form of carbon.
tant parameters to be isolated. The temperature and
the carbon and metal densities are obvious examples Acknowledgement-This research is partially supported by
of such parameters. In the arc plasma, they are highly the NSF (ASC-9217368) and by the Materials and Molecu-
coupled and extremely inhomogeneous. Knowledge of lar Simulation Center. We thank J. Vazquez for help with the
SEM imaging of nanotubes, G. Gorman and R. Savoy for
the growth mechanism will possibly allow us to opti- X-ray analysis, and M. S. de Vries for mass spectrometry.
mize the fabrication scheme and the characteristics of
the nanotubes.
A second key problem is to devise means to sepa- REFERENCES
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