Page 17 - Carbon Nanotubes

Pyrolytic carbon nanotubes from vapor-grown carbon fibers
The toroidal structures show interesting changes
in morphology as they become larger-at least at the
lip. The hypothetical small toroidal structure shown
in Fig. 1 lb is actually quite smooth and has an essen-
tially rounded structure[24]. As the structures become
larger, the strain tends to focus in the regions near the
pentagons and heptagons, and this results in more
prominent localized cusps and saddle points. Rather
elegant toroidal structures with Dnh and Dnd symme-
try are produced, depending on whether the various
paired heptagodpentagon sets which lie at opposite
ends of the tube are aligned or are offset. In general,
they probably lie is fairly randomly disposed positions.
Chiral structures can be produced by off-setting the
pentagons and heptagons. In the D5d structure shown
in Fig. 11 which was developed for the basic study, the
walls are fluted between the heptagons at opposite
ends of the inner tube and the pentagons of the outer
wall rim[l7]. It is interesting to note that in the com-
puter images the localized cusping leads to variations
in the smoothness of the image generated by the rim,
though it still appears to be quite elliptical when
viewed at an angle[ 171. The observed image appears
to exhibit variations that are consistent with the local- c B
ized cusps as the model predicts. k
In this study, we note that epitaxial graphitization Spinale-shape model
is achieved by heat treatment of the apparently mainly
amorphous material which surrounds a single-walled Fig. 12. As-grown PCNTs with partially thickened spindle
shape (a) and the proposed structural model for spindle par-
nanotube[ 171. As well as bulk graphitization, localized ticles including 12 pentagons in hexagon cage (b).
hemi-toroidal structures that connect adjacent walls
have been identified and appear to be fairly common
in this type of material. This type of infrastructure ber of pentagons as required by variants of Euler’s
may be important as it suggests that double walls may Law. Hypothetical structural models for these spin-
form fairly readily. Indeed, the observations suggest dles are depicted in Fig. 12. It is possible that simi-
that pure carbon rim-sealed structures may be readily lar two-stage growth processes occur in the case of
produced by heat treatment, suggesting that the future ACNTs but, in general, the secondary growth appears
fabrication of stabilized double-walled nanoscale to be intrinsically highly epitaxial. This may be be-
graphite tubes in which dangling bonds have been cause in the ACNT growth case only carbon atoms are
eliminated is a feasible objective. It will be interesting involved and there are fewer (non-graphitizing) alter-
to prove the relative reactivities of these structures for native accretion pathways available. It is likely that
their possible future applications in nanoscale devices epitaxial growth control factors will be rather weak
(e.g., as quantum wire supports). Although the cur- when secondary deposition is very fast, and so thin
vatures of the rims appear to be quite tight, it is clear layers may result in poorly ordered graphitic structure
from the abundance of loop images observed, that the in the thicker sections. It appears that graphitization
occurrence of such turnovers between concentric cylin- of this secondary deposit that occurs upon heat treat-
ders with a gap spacing close to the standard graphite ment may be partly responsible for the fine structure
interlayer spacing is relatively common. Interestingly, such as compartmentalization, as well as basic tip
the edges of the toroidal structures appear to be readily morphology[ 171.
visible and this has allowed us to confirm the relation-
ship between opposing loops. Bulges in the loops of
the kind observed are simulated theoretically[ 171. 7. VGCFs DERIVED FROM NANOTUBES
Once one layer has formed (the primary nanotube In Fig. 13 is shown the 002 lattice images of an “as-
core), further secondary layers appear to deposit with formed” very thin VGCF. The innermost core diam-
various degrees of epitaxial coherence. When inhomo- eter (ca. 20 nm as indicated by arrows) has two layers;
geneous deposition occurs in PCNTs, the thickening it is rather straight and appears to be the primary
has a characteristic spindle shape, which may be a nanotube. The outer carbon layers, with diameters ca.
consequence of non-carbon impurities which impede 3-4 nm, are quite uniformly stacked parallel to the
graphitization (see below)- this is not the case for central core with 0.35 nm spacing. From the difference
ACNTs were growth takes place in an essentially all- in structure as well as the special features in the me-
carbon atmosphere, except, of course, for the rare gas. chanical strength (as in Fig. 7) it might appear possi-
These spindles probably include the appropriate num- ble that the two intrinsically different types of material