Biomedical Engineering Reference
In-Depth Information
closest pairing, p1. The extensions at the sides of the simulated
bright spot can be made more prominent due to the close proximity
of adjacent adsorbed pair images in the periodic model.
The calculations suggest that the characteristic feature in
Fig. 5.13b should then hold for atomic adsorbates on the graphite
basal plane, no matter which type of site (A or B) the H atom is
attached onto. However, one can easily see two factors which could
tell these two adsites apart: (1) a 60° rotation of the prominent
structure, following the different orientations of the substrate
threefold symmetries at two inequivalent sites (a similar conclusion
is pointed out for a vacancy-induced superlattice in Ref. [27]); (2) a
more disrupted substrate atom visibility for B-site adsorption. An
adsorbed atom's strong electronic perturbation becomes weaker as
one moves farther from the adsorbate location, and so one comes
at a point where the effects of the subsurface layers would be more
dominant, showing what is expected from a clean graphite surface.
The interesting case comes with B-site adsorption, as the visibility
of an A-site C atom is enhanced by the presence of the adsorbed H,
creating a region centered at the adsite wherein the C atom visibility
should be different with respect to the surrounding clean surface.
The size of this region depends on the relative strengths of the two
perturbations to the single-layer graphene electronic states, but its
mere presence should already be of practical use.
Lastly, it is also interesting to see if STM is powerful enough to
detect subsurface hydrogen in graphite, since a proof for hydrogen
entry inside the graphitic structures is needed. In particular, here we
focus on the hydrogen atoms, adsorbed just below a graphene sheet.
A simulation, performed on the opposite face of the system used
for Fig. 5.12a using similar parameters, is shown in Fig. 5.13d. The
results show a much less pronounced feature for the unpopulated
face, but it is important to note that (1) the honeycomb lattice is
still incomplete, and (2) accessing electronic states farther from the
Fermi level might improve the prominence of the central feature.
The electronic effects of H adsorption also provide fundamental
insights on clustering stability. More precisely, the location of the
maxima in Fig. 5.10d, g, and j are not casually the most preferred
locations for the additional H atoms in order to create a strongly
adsorbed pair from an initially adsorbed H atom, or a strongly
adsorbed trimer from initially adsorbed ortho and meta H pairs.
This is in agreement with a recent study which also tackled H pairing
