Biomedical Engineering Reference
In-Depth Information
Table 5.1 values clearly show that adding a third H atom to the
surface, no matter how far/close it is to a preadsorbed pair, is never
an endothermic process. However, a more important question to
answer here is would it be more favorable to have a cluster of three,
or a pair and single H atom separately adsorbed on graphene?
Table 5.1 can be used straightforwardly to provide answers to this: if
E
then the trimer is more
stable than a separately adsorbed pair and atom. From this criterion
it can be seen that most of the trimers are unlikely to dissociate into
a pair and an atom on the surface, especially when one considers the
reported high barriers for H atom diffusion. These results once again
affirm the likelihood of clustering of hydrogen atom adsorbates on
graphene. Besides, while we expect a low probability for meta pairs
existing on graphene, it is shown here that a third H atom, adsorbing
nearby, in most cases stabilizes the pair.
Comparing the stabilities of trimers with that of pairs, another
trend is evident. The most stable pairs, the ortho and para pairs are
the components of the most stable trimers in the set considered in
this study. The configuration to1, for example, is a combination of
two ortho pairs and one meta pair. It may be useful to note that it is
geometrically impossible to have a trimer of purely odd pairs. Less
stable trimers such as tm3 contain the stable pair p3 or p4, which,
while less stable than the ortho and para pairs, are nonetheless
favored pairing geometries. The least stable trimers are entirely
comprised of the three least stable pairs discussed above, and thus
one may actually obtain a good qualitatively estimate regarding the
stability of adsorbed groups of three, based on the knowledge of pair
and single atom adsorption.
As a final note of this section, the threefold-symmetric
configurations of three hydrogen atoms are shown to be the least
stable, and so STM structures with threefold/sixfold symmetry are
not likely originating from closely clustered H trimers. This fact is
advantageous for more precise H adatom identification; this issue is
discussed in more detail in the next section.
(or
E
,
E
) for a given trimer is less than
E
o
m
p
iso
5.5
Effects of Adsorbed Hydrogen on the
Electronic States of Graphene
Detecting and discriminating singly adsorbed and small groups
of adsorbates is the essential step in order to know how to reach
