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Nanotube diameters (nm)

Fig. 19.8. Histograms of the single-wall nanotube diameter distribution for (a) Fe-catalyzed nanotubes [19.33] and (b) Co (4%)-catalyzed nanotubes [19.31].

19.2.3. Tubule Caps and Chirality

Most carbon nanotubes are capped by carbon shells that fit continuously on the ends of the long cylinder. This subsection reviews some of the properties of these caps. The number of possible caps containing only hexagons and pentagons that can be continuously attached to each carbon nanotube defined by (n, m) is given in Fig. 19.2(b), and is found conveniently by using the concept of projection mapping discussed in ยง3.3. In the enumeration given in Fig. 19.2(b), permutations of a given cap are not counted and only caps satisfying the isolated pentagon rule are included. From this figure we can conclude that the armchair tubule (5,5) which joins smoothly to a C60 hemisphere is the smallest diameter tubule that can be capped. The (9,0) zigzag tubule, which also joins smoothly to a C60 hemisphere, is the next smallest diameter tubule. Moving further out in diameter, we come to tubule (6,5) which is the smallest diameter chiral tubule for which there exists a unique cap containing only pentagons and hexagons, satisfying the isolated pentagon rule. In contrast, the chiral tubule (7,5) can be joined to b

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