Page 15 - Carbon Nanotubes

Pyrolytic carbon nanotubes from vapor-grown carbon fibers

(c) e =60.0° (d) e =38.9' (e) e =i9.2'

e = 180- (360/ n )cos-' [l - (n/6)] [" I
(n : number of pentagons)
Fig. 9. The possible tip structure with cone shape, in which Growth direction
the pentagons are included. As a function of the number of
pentagons, the cone shape changes. The shaded one with 19.2" 4
tip angle is the most frequently observed in PCNTs.

have an opening angle of ca. 20". Such caps may,
however, be of five possible opening angles (e.g., from
112.9" to 19.2") depending on the number of pentag-
onal disclinations clustered at the tip of the cone, as
indicated in Fig. 9[8]. Hexagons in individual tube
walls are, in general, arranged in a helical disposition
with variable pitches. It is worth noting that the small-
est angle (19.2') that can involve five pentagons is
most frequently observed in such samples. It is fre- chiralsaucture
quently observed that PCNTs exhibit a spindle-shaped
structure at the tube head, as shown in Fig. 8b. Fig. 10. Growth mechanism proposed for the helical
nanotubes (a) and helicity (b), and the model that gives the
bridge and laminated tip structure (c).
5. GROWTH MODEL OF PCNTs
In the case of the PCNTs considered here, the
growth temperature is much lower than that for small closed cage fullerenes. Based on the observation
ACNTs, and no electric fields, which might influence of open-ended tubes, Iijima et a1.[13] have discussed
the growth of ACNTs, are present. It is possible that a plausible alternative way in which such tubules might
different growth mechanisms apply to PCNT and possibly grow. The closed cap growth mechanism ef-
ACNT growth and this should be taken into consid- fectively involves the addition of extended chains of
eration. As mentioned previously, one plausible mech- sp carbon atoms to the periphery of the asymmetric
anism for nanotube growth involves the insertion of 6-pentagon cap, of the kind whose Schlegel diagram
small carbon species C,, (n = 1,2,3 . . .) into a closed is depicted in Fig. loa, and results in a hexagonal
fullerene cap (Fig. loa-c)[ll]. Such a mechanism is re- graphene cylinder wall in which the added atoms are
lated to the processes that Ulmer et a1.[20] and McE1- arranged in a helical disposition[9,1 I] similar to that
vaney et a1.[21] have discovered for the growth of observed first by Iijima[l].