Biomedical Engineering Reference
In-Depth Information
The fragmentation of this iso-surface as the percentage of
surface hydrogenation is increased is indicative of a systematic
change in hybridization. We also see “bow-tie” iso-surfaces, which
are characteristic of sp
bonds. The implication of these results are
summarized in Fig. 7.5, which plots the number of interlayer bonds
(determined via the electronic charge density profiles shown in
Figs. 7.2 and 7.3) as a function of the percentage of surface
hydrogenation. Here it is apparent that each impinging hydrogen
atom does not necessarily induce a single corresponding interlayer
C-C bond beneath. Two
2 +
x
s occur at 25-33.3% and 50-58.7%
surface hydrogenation. At these levels of hydrogenation the formation
of interlayer bonds adjacent to the impinging hydrogen atoms causes
the formation of additional interlayer bonds at neighboring sites
(without adjacent H atoms).
cascade
(a)
(b)
Figure 7.5
(a) The percentage of bonds between the diamond surface
and the reattaching graphene layer, 0% implies complete
separation, and 100% complete reattachment, and (b) the
average C-H bond energy (open symbols) and the potential
energy associated with the dangling sp
3
bonds (closed
symbols). Reproduced with permission from Ref. [12].
Copyright American Scientific Publishing, 2005.
A more detailed examination of the structure of the reattaching
graphene layer and the energetics of the system at each stage reveals
that these cascades occur when two distinct energetic barriers are
breached. The first, when the coverage of hydrogen is still low, is
related to the re-hybridization of the sheet/layer. When sufficient
hydrogen is adsorbed, the sheet/layer buckles and a transition from
sp
2
2 +
x
hybridization occurs. The atoms within the sheet/layer
are still predominantly three-fold coordinated, but they are aliphatic
to sp
