Chemistry Reference
In-Depth Information
graphene have recently been studied for the case of graphene deposited on SiC(0001)
(Langer et al.
2010
; Tegenkamp et al.
2011
) and Ir(111) (Langer et al.
2011
). The
understanding of plasmonic excitations of graphene plays a key role in tailoring the
properties of novel graphene-based devices (Bostwick et al.
2010
).
Indeed, many of the peculiar graphene's properties are related to its electronic
collective excitations (Yuan et al.
2011
; Yan et al.
2011
), even if their understanding
is still missing. In particular, it is essential to shed the light on plasmon modes in
graphene/metal interfaces to understand dynamical processes and screening in such
systems.
The electronic structure of MLG on Pt(111) resembles that of isolated graphene
(Sutter et al.
2009
). In particular, the linear dispersion of
π
bands in the so-called
Dirac cones, which gives rise to many manifestations of massless Dirac fermions,
is preserved. ARPES experiments do not show any remarkable hybridization of
graphene
π
states with metal
d
states. They just represent a superposition of graphene
and metal-derived states, with minimal interaction between them. The MLG on
Pt(111) is hole doped by charge transfer to the Pt substrate (Gao et al.
2010
). The
Fermi energy
E
F
of the graphene layer shifts 0.30
0.15 eV below the Dirac-energy
crossing point of the bands, with the Fermi wave vector
k
F
=
±
0.09 Å
−
1
. Epitax-
ial graphene on Pt(111) thus behaves as an ideal 2D system, sustaining a purely
2DEG system whose collective excitations (plasmon modes) are able to propagate
along the sheet. The dielectric response of the 2DEG system is determined by plas-
mon dispersion, which could be measured by high-resolution electron energy loss
spectroscopy.
The 2D plasmon, characterized by its square-root-like dispersion, has been pre-
dicted (Stern
1967
) and observed in metal layers on semiconductors (Nagao et al.
2001a
,
b
). On the other hand, the ASP with a linear dispersion was demonstrated to
exist on semiconductor quantum wells with two interacting quantum well minibands
(Chen et al.
1989
). Successively, ASP has been experimentally revealed on Be(0001)
(Diaconescu et al.
2007
) and on noble-metal surfaces (Park and Palmer
2010
; Pohl
et al.
2010
). The acoustic-like dispersion is a consequence of the combination of the
nonlocality of the 3D response and the spill-out of the 3D electron density into the
vacuum, both providing incomplete screening of the 2D electron-density oscillations.
Previous measurements on MLG/SiC(0001) showed a nonlinear dispersion for
the sheet plasmon in MLG. Such behaviour could be described by the Stern's model
(Stern
1967
). It is interesting to study the behaviour of collective excitations of MLG
grown on a metal substrate in order to shed light on the screening mechanisms of the
sheet plasmon in the presence of an underlying metal substrate. Present measure-
ments by HREELS show a linear dispersion for the sheet plasmon in MLG/Pt(111).
Our results indicate that the sheet Plasmon of MLG survives up to a high energy,
i.e., 3 eV. This is a consequence of the fact that intraband excitations have negligible
influence on the propagation of the plasmon mode. On the other hand, the disper-
sion curve of the sheet Plasmon overlaps with the continuum of interband transitions
above the Fermi wave vector. This broadens the plasmon peak but does not cause its
disappearance, in contrast with the behaviour found for ordinary sheet plasmons in
2DEG and ASP.
