Biomedical Engineering Reference
In-Depth Information
Compiling a catalog of all possible CNT structures under all
possible conditions is an enormous undertaking, and is (at this
stage) still beyond the routine capabilities of most experimental
laboratories. This is partially because the adsorption and re-
hybridization events occurring as a result of gas adsorption take place
at the limits of our experimental resolution, and partially because
the complete description of a realistic
is nontrivial.
However, at the most fundamental level we can begin to approximate
the surroundings of any nanomaterial by assuming it is dominated
by the characteristic temperature, pressure, charge (including pH),
and chemistry, which includes the supersaturation of different
gas adsorbates. The latter is most relevant to CNTs exposed to the
atmosphere, and is also closely related to the deliberate exposure of
CNTs to any other types of gaseous environment such as the plasma
treatments mentioned briefly above.
Based on this assumption an alternative approach is to turn to
computational
environment
to understand how and why these
adsorption-related structural changes are occurring. Unfortunately
when we consult the literature we find that although growth models
for chemical vapor deposition (CVD) synthesis of CNTs in hydrogen
have been proposed over the years [38-41], far less attention has
been given to the affect of gases on the stability of pre-grown CNTs.
Computational studies of the adsorption of hydrogen on CNTs [33, 34,
42, 43] and planar graphene [44] have also been undertaken before,
but such investigations have tended to focus more on the implications
of full or partial monolayer coverage (with particular patterning), or
differences between isotopic species. The questions remain: (i) what
happens when we allow for anisotropic or inhomogeneous adsorption,
and go beyond the generic passivation afforded by hydrogen? (ii)
what happens in the case of other gas species, or if pre-grown CNTs
are exposed to gaseous mixtures such as dry or humid air?
With this motivation in mind, this chapter outlines a theoretical
framework for studying the stability of CNTs exposed to gases. We will
begin by examining the issue of re-hybridization, using computational
techniques, and explore the transformation from sp
virtual experiments
2
3
carbon
via the relamination of an annealed diamond (111) surface exposed
to atomic hydrogen. These methods will then be applied to CNTs, and
the relationship between adsorbate, adsorption, and chirality will be
addressed. In the next section, a general analytical model to describe
the energetics and relative stability of CNTs in the presence of gases
to sp
