Chemistry Reference
In-Depth Information
Fig. 3.15
HREEL spectra for
MLG/Pt(111) as a function of
the scattering angle around
the high-symmetry point K.
The incident angle is fixed to
86.0
◦
with respect to the
sample normal
MLG/Pt(111) loss spectra around the high-symmetry point K show (Fig.
3.15
)a
softening by about 30 meV of the TO mode (characterized by
A
1
symmetry), which
reaches a minimal loss energy at K (172 meV).
The impinging energy is 20 eV. All measurements have been carried out at room
temperature.
Figures
3.16
and
3.17
report the dispersion relation of the HOB as a function of
q
||
around the K and
¯
symmetry points, respectively. The most striking feature of these
dispersions is the discontinuity in the derivative of the HOB (
A
1
and
E
2
g
, respec-
tively) at K and
¯
,
which is a direct evidence of the existence of KAs. This should
be put in relationship with the abrupt change in the screening of lattice vibrations
by conduction electrons. On the other hand, for MLG/Ni(111), the dispersion of the
HOB is almost flat at both K and
¯
symmetry points. This means that the interaction
with the substrate leads to a complete suppression of KAs, as a consequence of the
strong hybridization of the graphene
π-
bands with Ni d-bands (Giovannetti et al.
2008
). In fact, the hybridization induces around K the appearance of a “gap” of
almost 4 eV between unoccupied and occupied
π
-bands.
3.4
Electronic Collective Excitations in Epitaxial Graphene
3.4.1
General Consideration on Plasmons in Graphene
Low-energy collective excitations in graphene are attracting much interest in recent
years (Apalkov et al.
2007
) as they influence many of the peculiar properties of
graphene samples. In particular, the dispersion and damping of plasmons in epitaxial
