Page 16 - Carbon Nanotubes

6 M. ENDO et al.

It is proposed that during the growth of primary
tubule cores, carbon atoms, diameters, and longer lin-
ear clusters are continuously incorporated into the ac-
tive sites, which almost certainly lie in the vicinity of
the pentagons in the end caps, effectively creating he-
lical arrays of consecutive hexagons in the tube wall
as shown in Fig. 10a,b[9,11]. Sequential addition of
2 carbon atoms at a time to the wall of the helix re-
sults in a cap that is indistinguishable other than by
rotation[ll,l2]. Thus, if carbon is ingested into the
cap and wholesale rearrangement occurs to allow the
new atoms to “knit” smoothly into’ the wall, the cap
can be considered as effectively fluid and to move
in a screw-like motion leaving the base of the wall
stationary- though growing by insertion of an essen-
tially uniform thread of carbon atoms to generate a
helical array of hexagons in the wall. The example
shown in Fig. 10a results in a cylinder that has a di-
ameter (ca. 1 nm) and a 22-carbon atom repeat cycle
and a single hexagon screw pitch - the smallest arche-
typal (isolated pentagon) example of a graphene nano-
tube helix. Though this model generates a tubule that
is rather smaller than is usually the case for the PCNTs
observed in this study (the simplest of which have di-
ameters > 2-3 nm), the results are of general semi-
quantitative validity. Figure 10b,c shows the growth
mechanism diagrammatically from a side view. When
the tip is covered by further deposition of aromatic
layers, it is possible that a templating effect occurs to
form the new secondary surface involving pentagons
in the hexagonal network. Such a process would ex-
plain the laminated or stacked-cup-like morphology
observed.
In the case of single-walled nanotubes, it has been
recognized recently that transition metal particles play
a role in the initial filament growth process[23]. ACNTs
and PCNTs have many similarities but, as the vapor-
growth method for PCNTs allows greater control of
the growth process, it promises to facilitate applica-
tions more readily and is thus becoming the preferred
method of production. Fig. 11. The sealed tip of a PCNT heat treated at 2800°C
with a toroidal structure (T) and, (b) molecular graphics im-
ages of archetypal flattened toroidal model at different orien-
tations and the corresponding simulated TEM images.
6. CHARACTERISTIC TOROIDAL AND
SPINDLE-LIKE STRUCTURES OF PCNTS
In Fig. 1 la is shown an HRTEM image of part of the basis of archetypal double-walled nanotubes[24].
the end of a PCNTs. The initial material consisted of As the orientation changes, we note that the HRTEM
a single-walled nanotube upon which bi-conical interference pattern associated with the rim changes
spindle-like growth can be seen at the tip. Originally, from a line to an ellipse and the loop structures at the
this tip showed no apparent structure in the HRTEM apices remain relatively distinct. The oval patterns in
image at the as-grown state, suggesting that it might the observed and simulated HRTEM image (Fig. 1 lb)
consist largely of some form of “amorphous” carbon. are consistent with one another. For this preliminary
After a second stage of heat treatment at 280O0C, the investigation a symmetric (rather than helical) wall
amorphous sheaths graphitize to a very large degree, configuration was used for simplicity. Hemi-toroidal
producing multi-walled graphite nanotubes that tend connection of the inner and outer tubes with helical
to be sealed off with caps at points where the spindle- structured walls requires somewhat more complicated
like formations are the thinnest. The sealed-off end re- dispositions of the 5/6/7 rings in the lip region. The
gion of one such PCNT with a hemi-toroidal shape is general validity of the conclusions drawn here are,
shown in Fig. 1 la. however, not affected. Initial studies of the problem
In Fig. 1 lb are depicted sets of molecular graph- indicated that linking between the inner and outer
ics images of flattened toroidal structures which are walls is not, in general, a hindered process.