Biomedical Engineering Reference
In-Depth Information
be in most cases described by direct co-adsorbate interaction and
electronic screening effects, but the case of pairs on graphene is
one of the clearest examples of substrate effects being much more
important in determining the nature of clustering. In particular,
we have associated the reported stability trends with a high stress
due to the bonds involving both sp
2
3
carbon atoms. Certain
configurations, for example, leave the C atoms located between
strained receiving C atoms, leading to higher system energy.
Adsorbed groups of three hydrogen atoms are generally
stable: the
and sp
values are all negative. It means
that all adsorbed three-hydrogen groups are stable with respect
to hydrogen atoms located far from the graphene surface. If
E
and associated
E
ads
s
E
s
for a given trimer is less than
, then the trimer is more stable
compared to a system of three isolated H adsorbed on graphene.
In other words it would be more energetically favorable for the
hydrogen atoms to clump together than to separate from each
other on the surface, i.e., the net interaction shows an “attractive”
character. Only one trimer, tm1, is found unstable when compared
with separately adsorbed atoms. As in the cases of hydrogen atom
and pair on graphene, the extent of the sheet's reconstruction is
rather substantial. For the most stable trimer to1, which is also
the most closely grouped among the all considered geometries,
the receiving C atoms move all out from the plane, consequently
forming a local puckered-up island on the graphene sheet.
Comparing these findings with a pair adsorption [17], one can see
that most of the trimers are unlikely to dissociate into a pair and an
atom on the surface, especially when one considers the high barriers
for H atom diffusion. These results further affirm the likelihood of
clustering of hydrogen adatoms on graphene. Besides, while we
expect a low probability for meta pairs existing on graphene, it is
shown here that a third H adatom attaching nearby in most cases
stabilizes that pair. The general trend is that the clusters of three
have a stronger binding
E
iso
H atom compared to the pairs, and an
experimental work should be able to find it.
It is also worthwhile to mention the stabilities of adsorbed
hydrogen trimers in terms of component pairs. It has been earlier
shown that the “odd” ortho (p1) and para (p2) pairs are stable,
while “even” pairs such as the meta (p6) pair are higher in energy.
Enhanced reactivity for the formation of the former kind of pairs has
been previously suggested [16, 17, 24].
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