Chemistry Reference
In-Depth Information
point of the Brillouin zone of graphitic systems. Recently, it has been demonstrated
(Yan et al.
2011
) that in the presence of a free-electron substrate the plasmonic
excitation of the graphene sheet are nearly completely quenched, as a consequence
of the dynamical Coulomb interaction between induced charges in the substrate and
graphene. However, in the case of graphene on Ni(111), the
plasmon was found
to exist (Generalov and Dedkov
2012
). It is thus interesting to characterize the
π
π
plasmon mode also in the case of a graphene sheet weakly bonded to the metal
substrate, as occurs for MLG/Pt(111).
HREELS measurements show a quadratic dispersion for the
plasmon in
MLG/Pt(111), in contrast with results obtained for MLG/6H-SiC(0001) (Lu et al.
2009
) and in agreement with very recent findings for MLG/Ni(111) (Generalov
and Dedkov
2012
). However, the quadratic coefficient of the dispersion relation of
π
π
8 with respect to the case of
MLG/Ni(111) while it is similar to the value reported for graphite.
Moreover, we found that the plasmon peak is blue-shifted by about 1.5 eV with
respect to free-standing graphene and MLG/6H-SiC(0001). The presence of the metal
substrate also decreases the lifetime of the plasmonic excitation, as evidenced by a
careful analysis of its damping processes.
To measure the dispersion relation, primary beam energies, Ep
plasmon in MLG/Pt(111) is higher by a factor
∼
=
30-70 eV, were
used. Spectra recorded for MLG on Pt(111) with the substrate oriented in the
¯
−
M
direction are reported in Fig.
3.21
a (for a primary energy Ep of 70 eV) and 3.21b
(for Ep
30 eV).
It is worth mentioning that, due to the very weak intensity of loss peaks (
=
10
−
4
with respect to the intensity of the elastic peak), an acquisition time of several hours
has been required for each spectrum to reach a sufficient signal-to-noise ratio. All
measurements were made at room temperature.
A peak showing clear dispersion from 6.2 to 8.2 eV has been recorded as a
function of the parallel momentum transfer
q
||
. It has been assigned to
≈
plasmon of
graphene, in agreement with previous (Lu et al.
2009
; Generalov and Dedkov
2012
;
Kramberger et al.
2008
; Rosei et al.
1984
) works.
The intensity of the backscattering yield around the
π
plasmon energy versus the
off-specular angle clearly demonstrates that the plasmon mode has a dipolar nature
because it is nearly peaked in the specular direction (Rocca
1995
; Politano et al.
2009
).
The measured dispersion curve E
loss
(
q
||
) in Fig.
3.22
was fitted by a second-order
polynomial given by:
π
h
2
m
q
//
+
α
¯
+
Aq
//
=
E
loss
(0)
E
loss
(
q
//
)
=
E
loss
(0)
A
2
,
α
Where
E
loss
(
0
)
=
(
6
.
2
±
0
.
1
)eV
,
A
=
(
4
.
1
±
0
.
2
)eV
·
=
0
.
53
.
