Biomedical Engineering Reference
In-Depth Information
π
breaks the resonance of the original
-network at the Fermi level of
clean graphene.
An obvious manifestation of this effect is that every other p
z
orbital on the C lattice is skipped by the wave function, i.e., the
electronic state has maxima only in the graphene sublattice, which
does not include the receiving C atom, aside from the maximum at
the location of the H atom itself. This causes the original honeycomb
lattice to form a triangular one. In contrast to it, the charge density
distributions for all three systems do not discriminate graphene
C atoms and show that both the puckering up of the receiving
carbon atom and the presence of hydrogen adsorbate itself should
contribute to the observable topography change over the position of
the adsorbed atom, which should be relevant, for example, in atomic
force microscopy measurements
The H atom proximity to its images in the periodic model used
here, unfortunately makes us unable to comment more on larger
superstructures associated with the adsorption. But it is not of
much significance in this discussion. We limit the analysis to the
immediate vicinity of the adsorbed atom, which would be more
useful in identifying the adsorbed structures as intermediate to high
adsorbate coverage. The corresponding simulated STM image for
H atom adsorption is shown in Fig. 5.12a, which can be compared
with a high-resolution image attributed to the adsorbed D (type a) in
Ref. [13]
In that report, it was assumed that a D atom was at the center
of a bright spot on the STM plot. It should be noted that H and D
are electronically equivalent. The simulation reproduces the image
from experiment satisfactorily in terms of size and general shape
and symmetry. The tripod extensions of the central spot in the
simulation come from p
orbitals from third-nearest neighboring
(para) C atoms (shown as the black dots in the schematics of
Fig. 5.13). Figure 5.12a can be contrasted with Fig. 5.13a, which
concerns exactly the same system but with a surface scan calculated
on the basis of total electron density
z
(r). We can see here that a
probe that follows a total electron density isosurface does not
discriminate between the graphene sublattices, although the
achievement of such level resolutions to identify H adatoms may
prove challenging. While the H atom adsorption on graphene
exhibits a distinct threefold symmetry, the pairs, due to their
nature, have to break such symmetry completely. A similar means
n
