Chemistry Reference
In-Depth Information
For short-range interactions
impact scattering
occurs. Within this scattering mech-
anism, electrons are diffused in every possible solid angle, even beyond the incidence
plane. Both perpendicular and parallel component of the wave-vector (with respect
to the sample normal) are not conserved. As a consequence of such complexity, the-
ory which describes such interactions is not deeply developed. The cross section is
defined as (Ibach and Mills
1982
):
mE
p
cos
2
θ
s
2
π
2
d
σ
d
=
2
h
2
cos
θ
i
|
M
|
(3.6)
¯
where
M
is the matrix element for the transition and m is the mass of electrons. The
cross section presents only minimal changes with scattering angle.
3.2.3
HREEL Spectrometer
We used an instrument manufactured by SPECS GmbH under licence of
Forschungszentrum Jülich, Germany, distributed worldwide under the name DELTA
0.4. This spectrometer, designed by prof. Harald Ibach and shown in Figs.
3.8
-
3.9
,
is constituted by a two-step monochromator and by a rotating analyser which the pe-
culiarity of having 151
◦
cylindrical deflectors. This results in an ultimate resolution
of 0.5 meV and a significant increase of intensity in the high resolution range.
The basic concept of the DELTA 0.5 spectrometer is a fixed double stage
monochromator and a rotatable single stage analyser. In order to allow the prob-
ing of the largest possible fraction of the surface BZ the maximum rotation angle of
the DELTA 0.5 analyzer stage was increased to
>
90
◦
. The impact energy is variable
from 0 to 250 eV.
3.3
Phonon Modes of Epitaxial Graphene
3.3.1
Structure of Graphene on Pt(111)
The graphene-metal interface is a model system where the interaction between the
graphene
-bands and the metal bands can be investigated. This has relevance to
contacting of graphene with metal electrodes. The carbon hybridization and also the
epitaxial relationship with the metal substrate were proposed to influence the contact
transmittance (Nemec et al.
2006
) and cause local doping of graphene (Giovannetti
et al.
2008
). A variety of situations regarding the interplay of graphene and its metal
support is realized depending on the support material. One type with strong bonding
between metal graphene and a typical metal-graphene distance smaller then a metal-
metal bond (around 1.4-2.2 Å) and one with weak bonds and a typical metal-graphene
distance which is much larger then the bonds between the substrate atoms (around
3.2-3.7 Å).
π
