Biomedical Engineering Reference
In-Depth Information
saturation, as in the case of hydrogen adsorption on graphite.
Despite the difficulty associated with the size of the hydrogen atom,
structures ascribable to the presence of hydrogen in pairs or in
relative isolation have been recently reported in two independent
studies employing STM at high resolutions [13, 14]. Four distinct
pairs have been suggested, all involving odd-neighbor adsites, in
agreement with the results, presented in the previous section. One
point from Ref. [13], of particular interested for us, was the difficulty
of clearly recognize adsorbed pairs with atomic separations less
than that of a “para” pair, in view of the fact that the close proximity
of the atoms of these pairings casts noticeable doubts on clear
discrimination not only from each other, but also from an isolated
atomic adsorbate.
In this section, we comment on the surface electronic states
in the presence of adsorbed hydrogen and on their importance
for identification of adsorbed hydrogen structures [25] and for
providing fundamental knowledge on how graphene electronics
can be tailored by hydrogenation. Related theoretical descriptions
of defect-induced effects on the electronic states of graphite have
pointed out useful ways to describe the nature of point defects
on the surface [27-29], but have been primarily concerned with
vacancies, substitutions, or arbitrary adsorbates, so we believe
more specificity to H/D atoms effects is needed.
As important references, we include in this study is the adsorbed
ortho and meta hydrogen atom pairs (referred to as p1 and p6),
which are the pairs of the closest spacing. It should be further noted
that while simple stability calculations predict that p6 pair is not
likely to exist, especially when compared with p1, we do not refute
the mere possibility of its existence on the graphite surface.
Figure 5.10 shows two-dimensional cross-sections of represent-
ative electronic states near the Fermi level, shown through band-
decomposed charge densities
2
y
for pristine graphene and the
systems involving an H atom, an ortho pair, and a meta pair
adsorbed on graphene, respectively.
All carbon and hydrogen atom nuclei are frozen at their optimized
coordinates. To be consistent with STM measurement at surface-
positive bias voltages, we chose the lowest states for which
E
-
E
F
> 0, where
is the energy at the Fermi level. For reference, in the
lowermost panels the total charge density distributions are also
reported. Complementing the results shown in Fig. 5.10d, e, panels d
and e of Fig. 5.11 show the atomic orbital-projected density of states
for a free hydrogen atom and a component carbon atom of graphene,
E
F
