Biomedical Engineering Reference
In-Depth Information
5.7
Summary and Concluding Remarks
We discussed in this chapter the stability and likelihood of the
presence of small hydrogen clusters on a face of graphene with a
coverage of
≈ 0.06. The most stable adsorbed structures among
groups of two and three H atoms were found to be the configurations
in which the hydrogen atoms are immediately next to each other,
i.e., in the closest possible clustering geometries. In both cases,
the adsorption per H atom is stronger in comparison with the
adsorption of the isolated H atom on graphene, and so a favorable
cluster formation of H on the graphene surface is ascertained.
Other stably adsorbed configurations have been explicitly pointed
out. These results show that the H-graphene system is an excellent
example of substrate effects being much more important, compared
with direct adsorbate interaction, in determining the nature of
grouping/ordering of adsorbed species on a surface. In all cases,
the importance of reconstruction to reach stable configurations has
been found. The trend of adjacent-adsorption configurations with
the strongest bound structures on graphene may still hold for the
next larger clusters of bound hydrogen, but this trend is not expected
to hold if bound hydrogen is infinitely increased, unless both faces of
a graphene sheet are hydrogenated. It should be further noted that
in the graphite case, inter-sheet interactions may be important in
determining the relative stability of these small hydrogen clusters
on the surface, and so results presented here may not necessarily
be identical.
A hydrogen atom on graphene has been shown to be easily
identified through its effects on graphene electronic states near
the Fermi level. A comparison with effects brought about by pairs
having H-H separations less than 2.7 Å, shows clear differences that
make it possible to discriminate the atom from the pairs at a very
local level. The adsorption of hydrogen was confirmed to disrupt
the
θ
π
network near the Fermi level of graphene in a rather extended
range, an event that affects the material's reactivity to subsequent
hydrogen, and hence promotes cluster formation. Calculations for
the closest pairing using both faces of graphene suggest not only
more strongly adsorbed states, but facile processes for the entry of
subsequently adsorbing hydrogen. The ensuing fully hydrogenated
material is stable, and has a structure greatly distinct from a pristine
graphene.
