Biomedical Engineering Reference
In-Depth Information
2
93% of the total ECD, the sp
bonds in carbon structures with
mixed hybridization may be viewed in isolation. This method
is more reliable than traditional methods based on measuring
ionic separation distances, but does require the calculation of the
complete electron charge density of the system in three dimensions,
which can be computationally demanding in some cases [12], and
does not consider differences in bond angles.
7.2.1
Adsorption and Rehybridization on Surfaces
We will now see how to use this technique, by examining the
relamination of an exfoliated diamond (111) surface. These
investigations were performed using a periodic (infinite) diamond
slab consisting of 12 atomic layers, with one graphene sheet above the
surface. The sheet was formed by annealing, and hence delaminating
a diamond (111) surface (consisting of the two uppermost atomic
layers) of a 14-atomic layer diamond slab. Atomic hydrogen was
adsorbed on the graphene sheet by randomly placing single H atoms
successively above the sheet (within the C-H interaction distance).
The structure was then fully relaxed after each adsorption, so that
the process proceeded adiabatically. Each relaxed configuration was
then tested for the existence of new bonds between the layers, by
recalculating the electronic charge density in the plane between
the graphene sheet and the diamond (111) surface. This is best
understood by examining Fig. 7.2, where the ECD is shown in a (110)
plane perpendicular to the surface; and Fig. 7.3, where the ECD is
shown in the plane parallel to the sheet above and surface below.
Of course, the forming of these interlayer bonds (in addition to
the C-H bonds formed as a result of the hydrogen adsorption), alter
the coordination of the C atoms within the upper layers. It is expected
that this change in coordination (as the graphene sheet becomes the
new upper (111) diamond surface) will also involve a change in the
hybridization of the C-C bonds within the sheet/surface. Using the
method outlined above for visualizing the hybridization of C-C bonds
the stepwise disappearance of the graphitic structure of the sheet
can be readily observed (see Fig. 7.4). Here we see a sp
2
iso-surface
applied to the plane of the upper graphene sheet/diamond surface,
for each percentage of surface hydrogenation. In the initial image,
when there are no hydrogen atoms present (0% hydrogenation),
the graphene sheet is entirely sp
2
-bonded. This is established by
