Biomedical Engineering Reference
In-Depth Information
four directions. Several factors may contribute to these differences,
including surface unit cell size (pair adsorption coverage) and
density of the used iso-surfaces states, as well as electronic states
chosen with respect to the Fermi level. On the other hand, the meta
pair reproduces the cloaked sublattice effect found in the atomic
adsorption. These two pairs have been shown to differ significantly
in terms of adsorption strength and are found here to differ in the
way they alter the graphene electronic states (and thus how they
would look like under the STM).
The features of the electronic state at elevated positions, marked
by dashed contours in Fig. 5.10d-k, suggest that the exact positions
of adsorbed H atoms are at the centers of the bright spots. Hence,
based on these results one may actually say that the adsorbed
hydrogen atom on graphene is not really transparent to STM. This
of course works under the important assumption that the hydrogen
atom remains localized on its adsorption position, which assumption
should not be unreasonable considering the high barriers involved
in chemisorbed H hopping and the standard use of D atoms in the
related experimental work. An equally important concern is: can
H atoms in the close pairings be resolved? The states in Fig. 5.10g
and j do not answer in the negative, though the actual experimental
conditions may not be ideal. If at this scale individual hydrogen atom
resolution is poor, one can at least expect that more elongated bright
spots at specific orientations for the closely spaced adsorbed pairs,
as in Fig. 5.12b and c, and the surrounding C atom contributions to
the STM image could provide supporting clues.
On an actual multilayer graphite system without defects, it is
well known that the electronic differences on A and B site atoms
(otherwise known as α and β sites) are very significant, to the point
that it is common to observe under STM only the surface B site atoms
(carbon atoms that do not have neighbors in adjacent sheets) [31].
This is confirmed by the straightforward calculations using two-
atom basis for graphene (four atoms for the bilayer graphene), as
shown in Fig. 5.14.
Here, the case of a single graphene sheet shows the hexagonal
arrangement of carbon atoms, but the bilayer graphene model clearly
shows the difference between A and B carbon atoms, as all the A
carbon atoms have gone missing. One may then wonder about what
should happen when a second perturbation — from an adsorbed
H atom — is present. Similar calculations using a two-layer system
