Page 86 - Carbon Nanotubes

Topological and sp3 defect structures in nanotubes 15
inwards, layer by layer[21,22]. If there were smaller
domains along the cylindrical part, their edges would
be expected to react very fast to oxidation, contrary
to observation. Second, ESR studies[23] do not reveal
any strong signal from dangling bonds and other de-
fects, which would be expected from the numerous
edges in the paper-machk model.
To try to clarify this issue, we recently analyzed
crude nanotube samples and purified nanotubes be-
fore and after annealing them at high temperature[20].
It is well known that defects can be annealed away at
high temperatures (ca. 285OOC). The annealing effect
was very significant on the ESR properties, indicating
clearly the presence of defects in the nanotubes[20].
However, our nanotubes do not fit the defect struc-
ture proposed in the paper-machi model for the rea-
Fig. 4. Hexagonal network of graphite and the 4 different
pairs of carbon atoms across which the sp3-like defect line son discussed in the previous paragraph. Considering
may form[l8]. the types of possible defects (see part 2), the presence
of either a large number of pentagon/heptagon pairs
in the nanotubes and/or polygonal nanotubes, as ob-
served by Liu and Cowley[12,13], could possibly ac-
posed that nanotubes were composed of pieces of gra- count for these results. Both the 5/7 pairs and the
phitic sheets stuck together in a paper-machi model. edges of the polygon would significantly perturb the
The problem with this model is that it is not consis- electronic properties of the nanotubes and could be
tent with two other observations. First, when nano- annealed away at very high temperatures. The sensi-
tubes are oxidized they are consumed from the tip tivity of these defects to oxidation is unknown.
In attempting to reconcile these results with those
of other studies, one is limited by the variation in sam-
ple quality from one study to another. For instance,
lb =C IESR measurements undertaken on bulk samples in
three different laboratories shoq7 very different re-
sults[19,23,24]. As we have pointed out elsewhere, the
quantity of nanotubes (and their quality) varies from
a few percent to over 60% of the crude samples, de-
pending on the current control and the extent of cool-
ing in the carbon arc apparatus[l]. The type and
distribution of defects might also be strongly affected
by the conditions during nanotube production. The ef-
fect of pressure on the spacing between the graphene
sheets observed by Zhou et al. argues most strongly
in favor of the particles in the sample having a non-
closed structure[l9]. Harris et af. actually observe that
nanoparticles in these samples sometimes do not form
closed structures[25]. It would be interesting to repeat
the pressure study on purified nanotubes before and
after annealing with samples of various origins. This
should give significant information on the nature of
the defects. The results taken before annealing will,
no doubt, vary depending on where and how the sam-
ple was prepared. The results after sufficient anneal-
ing should be consistent and independent of sample
origin.

4. CONCLUSION
The issue of defects in nanotubes is very important
in interpreting the observed properties of nanotubes.
For instance, electronic and magnetic properties will
Fig. 5. Conformations of the 4 types of defect lines that can be significantly altered as is already clear from obser-
occur in the graphene sheet[l8]. vation of the conduction electron spin resonance[20,23].