Page 80 - Carbon Nanotubes

STM of carbon nanotubes and nanocones 69

Fig. 6. A closer view of the disturbed area of the bundle in Fig. 7. A (244 A) STM image of two fullerene cones.
Fig. 5; the concentric nature of the tubes is shown. The outer-
most tubes are broken and the adjacent inner tubes are
complete.
is a hexagon network, while the apex contains five
pentagons. The 19.2" cone has mirror symmetry
through a plane which bisects the 'armchair' and 'zig-
intertube interaction is weaker than the intratube in- zag' hexagon rows.
teraction. This might be the reason for bundle forma- It is interesting that both carbon tubules and cones
tion in the vapor phase. After a certain diameter is have graphene networks. A honeycomb lattice with-
reached for a single tube, growth of adjacent tubes out inclusion of pentagons forms both structures.
might be energetically favorable over the addition of However, their surface nets are configured differently.
further concentric graphene shells, leading to the gen- The graphitic tubule is characterized by its diameter
eration of bundles. and its helicity, and the graphitic cone is entirely char-
acterized by its cone angle. Helicity is not defined for
6. CONES the graphitic cone. The hexagon rows are rather ar-
ranged in helical-like fashion locally. Such 'local he-
Nanocones of carbon are found[3] in some areas licity' varies monotonously along the axis direction of
on the substrate together with tubes and other the cone, as the curvature gradually changes. One can
mesoscopic structures. In Fig. 7 two carbon cones are
displayed. For both cones we measure opening angles
of 19.0 f 0.5". The cones are 240 A and 130 A long.
Strikingly, all the observed cones (as many as 10 in a
(800 A)2 area) have nearly identical cone angles - 19".
At the cone bases, flat or rounded terminations
were found. The large cone in Fig. 7 shows a sharp
edge at the base, which suggests that it is open. The
small cone in this image appears closed by a spherical-
shaped cap.
We can model a cone by rolling a sector of a sheet
around its apex and joining the two open sides. If the
sheet is periodically textured, matching the structure
at the closure line is required to form a complete net-
work, leading to a set of discrete opening angles. The
higher the symmetry of the network, the larger the set.
In the case of a honeycomb structure, the sectors with
angles of n x (2p/6) (n = 1,2,3,4,5) can satisfy per-
fect matching. Each cone angle is determined by the
corresponding sector. The possible cone angles are
19.2", 38.9", 60", 86.6", and 123.6", as illustrated in
Fig. 8. Only the 19.2" angle was observed for all the
cones in our experiment. A ball-and-stick model of the Fig. 8. The five possible graphitic cones, with cone angles
19.2" fullerene cone is shown in Fig. 9. The body part of 19.2", 38.9", 60", 86.6", and 123.6".