Page 102 - Carbon Nanotubes

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92 A. FONSECA al.

Fig. 7. (a) Torus Csz0 formed by 10 (9,O)-(5,5) knees, (b) torus Cgo0 formed by 10 (9,O)-(5,5) knees
extended by adding one circumferential ring on both sides.

knees. On the inner equatorial circle of the torus at the connecting meridian between two successive
made from the N, knees, each pair of abutting fractions of a torus, then its equator will no longer
heptagons sharing a common bond is separated from be a plane. In Fig. 10, single bond shifts in two
the pair of heptagons on either side by one bond. (9,O)-( 53) knees are represented. The repeated intro-
This can also be observed on Fig. 7(a). duction of such bond shifts will lead to a helix.
Several bond shifts can also be introduced at the
2.5 From torus to helix same knee connection. If there is a long straight
As seen in the previous section, if two identical tubule segment joining two knees, several rotation
knees of the (L,O)-(L’,L’) family are connected bond shifts can also be introduced at different places
together symmetrically with respect to a connecting of that segment. All of these bond shifts can be
plane, and if this connecting process is continued present in the same helix. The helix will be regular
while maintaining the knees in a common plane, the or irregular depending on the periodic or random
structure obtained will close to a torus which will be occurrence of such bond shifts in the straight sections.
completed after 10 fractional turns (Figs 7 and 9). The pitch and diameter of a regular coiled tubule
However, if a rotational bond shift is introduced will be determined by the length of the straight