Investigating graphene/metal interfaces by
time-resolved non linear photoemission
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan
Graphene: the future in a pencil trace!
2004:
Graphene is
discovered
K. Novoselov A. Geim
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
What is Graphene?
Graphene is a single layer of carbon atoms packed in hexagonal (honeycomb) lattice.
Graphene band structure
K
K
E. Y. Andrei et al., Rep. Prog. Phys. 75
(2012) 056501
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene’s properties
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
The aims in graphene production
K.S. Novoselov et al., Nature 490,192 (2012).
Aims:
• Transfer of graphene layer from
growing substrate;
• Fabrication of very large and
extremely well-defined
graphene sheets.
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metal
investigation
Investigating
graphene/metal
interface
Strategy: Investigation of the coupling of graphene with substrate.
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Investigating
Graphene/metal
graphene/metal
investigation
interface
Strategy: Investigation of the coupling of graphene with substrate.
y
x
z
Silvia Tognolini
The metal surface has a “sentinel”
that monitors every change in the
surroundings.
First Year Workshop, 15 October 2013, Milan.
Investigating
Graphene/metal
graphene/metal
investigation
interface
Strategy: Investigation of the coupling of graphene with substrate;
The “sentinel”, interacting with
graphene sheet, can change his
properties (mass or position).
Also in function of the distance
of graphene layer!
y
x
z
Silvia Tognolini
The metal surface has a “sentinel”
that monitors every change in the
surroundings.
First Year Workshop, 15 October 2013, Milan.
Investigating
Graphene/metal
graphene/metal
investigation
interface
Strategy: Investigation of the coupling of graphene with substrate;
The “sentinel”, interacting with
graphene sheet, can change his
properties (mass or position).
Also in function of the distance
of graphene layer!
Attention: graphene presents a
dual series of “sentinels”!
y
x
z
Silvia Tognolini
The metal surface has a “sentinel”
that monitors every change in the
surroundings.
First Year Workshop, 15 October 2013, Milan.
The image potential states
= The Image Potential State (IPS)
z
- Empty states;
- Localized in the z direction outside
the sample surface;
- Electrons are quasi-free in the plane
parallel to the sample surface;
IPS is a very sensitive tool
for understanding the
coupling of graphene
with the substrate.
- Interactions may result in a modified
IPS electron mass m* or binding energy;
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
How is possible to investigate the IPSs?
NONLINEAR PHOTOEMISSION :
hn < F
(Femtosecond Light Pulses)
Electron properties of UNOCCUPIED
STATES above the Fermi level.
Time-Resolved NL-PE
e–
hν
probe
Evac
e– Δt
hν
empty states
Φ
Electron dynamics and charge transfer
processes.
Efermi
occupied states
pump
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metal
investigation
Graphene
IPSs: from
literature…
Free standing graphene:
 Two series of IPSs at Γ point of BZ;
 The wave functions of these states
have opposite parity with respect to
the reflection at the graphene plane;
V.M. Silkin et al., PRB 80, 121408R 2009.(ab initio
calculations.)
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metal
investigation
Graphene
IPSs: from
literature…
Free standing graphene:
 Two series of IPSs at Γ point of BZ;
 The wave functions of these states
have opposite parity with respect to
the reflection at the graphene plane;
V.M. Silkin et al., PRB 80, 121408R 2009.(ab initio
calculations.)
Graphene on metal substrate
1- IPS  spatial expansion towards the
metal interface region;
1+ IPS  larger weight towards the vacuum.
Borca et al. PRL 105, 036804 (2010).
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metal
investigation
IPSs at graphene/metal
interface:
from literature…
Batzill et al., Surf. Science Rep. 67, 83 (2012).
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metal
investigation
IPSs at graphene/metal
interface:
from literature…
Graphene on Iridium (111)
Batzill et al., Surf. Science Rep. 67, 83 (2012).
D. Niesner et al., Phys. Rev. B 85, 081402
(2012).
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metal
investigation
IPSs at graphene/metal
interface:
from literature…
Graphene on Iridium (111)
Batzill et al., Surf. Science Rep. 67, 83 (2012).
Graphene on Ruthenium (0001)
W. Moritz et al., PRL 104, 136102 (2010).
D. Niesner et al., Phys. Rev. B 85, 081402
(2012).
N. Armbrust et al., PRL. 108, 056801 (2012)
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metalinterface:
investigation
IPSs at graphene/metal
our results…
Graphene on Cu(111)
hn = 4.10 eV @ k//=0
d bands
Cu(111) band structure
IPS n=1 Cu(111)
IPS
Surface state
(occupied)
n=0 Cu(111)
S.Pagliara et al., Surface Science 600, 4290 (2006) .
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metalinterface:
investigation
IPSs at graphene/metal
our results…
Graphene on Polycrystalline Cu foil
1+ Graphene IPS
hn = 4.10 eV @ k//=0
P-polarization
NL-PES performed on graphene grown
on different Cu substrates allows us to:
•
identify graphene IPS;
•
distinguish the contributions of the
substrate from those of the graphene;
These samples are kindly provided by the group of P. Rudolf of Groningen University.
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
Graphene/metal
investigation
Future outlook
• Non-linear photoemission measurements at graphene/metal interfaces:
- weakly interacting system: Gr/Ir(111) (in collaboration with C.Mariani,
Università La Sapienza, Roma);
- strongly interacting system: Gr/Ni(111) (in collaboration with C.Cepek,
IOM-TASC, Trieste);
• NL-photoemission measurements at molecule/metal interfaces, as porphyrins
and fullerenes on Ag(100) (in collaboration with M.Sambi, Università degli
studi di Padova);
• Time-Resolved NL-photoemission in order to investigate:
- the electron dynamics of the image potential states;
- the photoinduced charge transfer from metal substrates to overlayer.
Thanks for your attention!!
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.
The image potential states
Interaction between an electron outside of a
surface and the polarization it induces at that
surface.
Electron energy < EVAC
Forbidden bulk band gap at the
metal surface
The electron is trapped in the potential
well built by the image potential and the
crystal potential.
Silvia Tognolini
First Year Workshop, 15 October 2013, Milan.