Page 83 - Carbon Nanotubes

12 T. W. EBBESEN and T. TAKADA

Fig. 1. Five examples of nanotubes showing evidence of defects in their structure (p: pentagon, h: hep-
tagon, d: dislocation); see text (the scale bars equal 10 nm).

work to close the structure. The addition of one hep- (notice the different spacing between the layers on the
tagon (n7) to the nanotube will require the presence left and right-hand side of the nanotube).
of 13 pentagons to close the structure (and so forth) Another common defect appears to be the aniline
because they induce opposite 60" disclinations in the structure that is formed by attaching a pentagon and
surface. Although the presence of pentagons (ns) and a heptagon to each other. Their presence is hard to de-
heptagons (n,) in nanotubes[9,10] is clear from the tect directly because they create only a small local de-
disclinations observed in their structures (Fig. la), we formation in the width of the nanotube. However,
are not aware of any evidence for larger or smaller cy- from time to time, when a very large number of them
cles (probably because the strain would be too great). are accidentally aligned, the nanotube becomes grad-
A single heptagon or pentagon can be thought of ually thicker and thicker, as shown in Fig. 1 (b). The
as point defects and their properties have been calcu- existence of such tubes indicates that such pairs are
lated[l I]. Typical nanotubes don't have large numbers probably much more common in nanotubes, but that
of these defects, except close to the tips. However, the they normally go undetected because they cancel each
point defects polygonize the tip of the nanotubes, as other out (random alignment). The frequency of oc-
shown in Fig. 2. This might also favor the polygonal- currence of these aligned 5/7 pairs can be estimated
ization of the entire length of the nanotube as illus- to be about 1 per 3 nm from the change in the diame-
trated by the dotted lines in Fig. 2. Liu and Cowley ter of the tube. Randomly aligned 5/7 pairs should be
have shown that a large fraction of nanotubes are po- present at even higher frequencies, seriously affecting
lygonized in the core[ 12,131. This will undoubtedly have the nanotube properties. Various aspects of such pairs
significant effects on their properties due to local re- have been discussed from a theoretical point of view
hybridization, as will be discussed in the next section. in the literature[l4,15]. In particular, it has been
The nanotube in Fig. 1 (e) appears to be polygonized pointed out by Saito et a1.[14] that such defect pairs