Biomedical Engineering Reference
In-Depth Information
defects are characterized by the addition of the gas adsorbate and
the rehybridized region, as we saw with the continuous graphene
sheet and diamond (111) surface. In this section we will examine the
(localized) effect the chemisorption of individual H atoms has upon
the structure of single-walled CNTs, and use the method described
above to characterize the extent of the rehybridization surrounding
the adsorption site on tubes of different chiralities [55]. From this
perspective, we will consider each individual H adsorption event
as an isolated sp
network. In this case,
continued hydrogenation of a CNT will either result in clustering of
sp
3
-point defect on the sp
2
3
sites to create an extended defect, or it will simply increase the
density of isolated sp
3
-point defects.
In the following examples, the adsorption of hydrogen was
simulated on the outer wall (exohedral) of the (6,6) and (9,0) single-
walled CNTs. Beginning with pristine, fully relaxed CNTs these
structures were exposed to groups of one, two, and three hydrogen
atoms, chemisorbed in different adsorption configurations, to
simulate the effects of isolation adsorption or adsorbate clustering.
Clustering phenomena presumes that the defective region introduced
by a single adsorbate will represent a preferred site for subsequent
adsorption. Therefore, multiple hydrogen atoms are adsorbed onto
carbon atoms which were second-nearest neighbors with respect
to one another, thereby forming clusters of C-H bonds akin to
the structure of a (111):H surface of diamond. In the cases where
three hydrogen atoms were absorbed, two different configurations
were examined. The first configuration involves the adsorption of
hydrogen atoms around the base of a triangular pyramid, formed
by three carbon atoms bound to the same carbon in the center
(pyramid-configuration). The second configuration involves the
adsorption of the hydrogen atoms equidistant around one of the
six-member rings in the tube wall (ring-configuration). Diagrams
of these configurations, denoted as 3(P) and 3(R), are shown in
Fig. 7.6a and b, respectively.
The effects of H adsorption upon the structure of the CNTs are
shown in Figs. 7.7 and 7.8 for the (6,6) and (9,0) CNTs, respectively
[55], by generating the sp
2
iso-surfaces (by applying an iso-surface
to the ECD at a fractional value of 93%) to highlight the hybridization
of the carbon bonds in the structure. In Figs. 7.7a and 7.8a, there
are no H adsorbates, and the total bonding iso-surfaces are shown.
In Figs. 7.7b and 7.8b, as there are no hydrogen adsorbates, and
