Chemistry Reference
In-Depth Information
Fig. 3.11
HREEL spectrum
of MLG/Pt(111), recorded
with an impinging energy of 4
eV and in specular scattering
conditions. In the inset the
LEED pattern recorded with
E
p
=
74.7 eV is reported
3.3.2
Phonon Dispersion
The dynamics of atoms at surfaces plays an important role in many chemical and
physical processes. In particular, lattice vibrations can afford essential information
on many physical properties, such as thermal expansion, heat capacity, sound veloc-
ity, magnetic forces, and thermal conductivity (Chis and Benedek
2011
). Recently,
phonon modes of graphene sheets are attracting much attention (Allard and Wirtz
2010
) as they influence many of the novel and unusual properties of graphene (Efetov
and Kim
2010
).
Due to the high energy of phonon modes in graphitic materials, several techniques
as inelastic helium atom scattering can not be used. Infrared or Raman spectroscopy
can not probe the whole Brillouin zone, while neutron or inelastic ion scattering
are not always applicable. Among various detection methods for phonon dispersion,
HREELS is a powerful tool to gain knowledge on phonons in graphene.
Present HREELS measurements show the dispersion relation of phonon modes
in MLG/Pt(111). Results have been compared with recent calculations in Ref.
(Karssemeijer and Fasolino
2011
).
The energy resolution of the spectrometer was degraded to 4 meV so as to increase
the signal-to-noise ratio of loss peaks. Dispersion of the loss peaks, i.e., E
loss
(
q
||
), was
measured by moving the analyzer while keeping the sample and the monochromator
in a fixed position (See Chap. 2). To measure the dispersion relation, values for
the parameters E
p
, impinging energy and
θ
i
, the incident angle, were chosen so
as to obtain the highest signal-to-noise ratio. The primary beam energy used for
the dispersion, E
p
=
20 eV, provided, in fact, the best compromise among surface
sensitivity, the highest cross-section for mode excitation and
q
||
resolution.
All measurements were made at room temperature.
