Page 160 - Carbon Nanotubes

150 Letters to the Editor

Figure 3. Lattice fringes LF 002 of buckled nanotube
particles.
follows: the tube, which is initially straight, is subjected
to bending during the preparation of the TEM grid. The
stress on the concave side of the tube results in buckling.
The buckling extends into the tube until the effect of the
stress on the tube is minimized. The effect of this
buckling on the graphene layers on the convex side is
that they are stretched and become flattened because this
is the only way to minimize damage. This extension
Figure 1. Lattice fringes LF 002 of nanotube parhcles.
results in a large coherent volume which causes the
observed increase in contrast. On the concave side of the
tube, damage is minimized by shortening the graphene
layer length in the formation of a buckling location.
We observe that compression and its associated
buckling instability only on the concave side of the tube,
but never on the convex side. This result suggests that it
is only necessary to consider the flexibility of the
graphene layers; and, thus, there is no need to invoke the
notion of defects due to the substitution of pentagons and
hexagons. In the latter case, we would expect to observe
the buckling phenomenon on both sides of the nanotube
upon bending. Thus, it is clear that further work must
be undertaken to study the flexibility of graphene layers
since, from the above results, it is possible to conclude
that graphene layers are not necessarily rigid and flat
entities. These entities do not present undulations or
various forms only as a result of the existence of atomic
andlor structural defects. The time has come to
discontinue the use of the description of graphene layers
based on rigid, coplanar chemical bonds (with 120'
angles)! A model of graphene layers which under
mechanical stress, for example, results in the
modification of bond angles and bond length values
Figure 2. Details of Figure 2 and an inset sketch induce observed curvature effects (without using any
illustrating what happens before and after traction. structural modifications such as pentagon substitution for