silvia tognolini first year workshop, 15 october 2013, milan investigating graphene/metal interfaces...

Silvia Tognolini

First Year Workshop, 15 October 2013, Milan

Investigating graphene/metal Investigating graphene/metal interfaces by timeinterfaces by time--resolved non resolved non

linear photoemissionlinear photoemission

2004: Graphene is discovered

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene: the future in a pencil trace!

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

• Transfer of graphene layer from growing substrate;

• Fabrication of very large and extremely well-defined graphene sheets.

K.S. Novoselov et al., Nature 490,192 (2012).

Aims:

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigation

Strategy: Investigation of the coupling of graphene with substrate.

Investigating graphene/metal interface

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigation

Strategy: Investigation of the coupling of graphene with substrate.

x

y

z

The metal surface has a “sentinel” that monitors every change in the surroundings.

Investigating graphene/metal interface

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigation

Strategy: Investigation of the coupling of graphene with substrate;

x

y

z

The “sentinel”, interacting with

graphene sheet, can change his

properties (mass or position).

Also in function of the distance of graphene

layer!

Investigating graphene/metal interface

The metal surface has a “sentinel” that monitors every change in the surroundings.

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigation

Strategy: Investigation of the coupling of graphene with substrate;

x

y

z

The “sentinel”, interacting with

graphene sheet, can change his

properties (mass or position).

Also in function of the distance of graphene

layer!

Investigating graphene/metal interface

Attention: graphene presents a dual series of “sentinels”!

The metal surface has a “sentinel” that monitors every change in the surroundings.

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

The image potential states

= The Image Potential State (IPS)

- Empty states;

- Localized in the z direction outside

the sample surface;

- Electrons are quasi-free in the plane

parallel to the sample surface;

- Interactions may result in a modified

IPS electron mass m* or binding energy;

z

IPS is a very sensitive tool for understanding the coupling of graphene with the substrate.

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

How is possible to investigate the IPSs?

NONLINEAR PHOTOEMISSION : h <

(Femtosecond Light Pulses)

Electron properties of UNOCCUPIED

STATES above the Fermi level.

Time-Resolved NL-PE

Electron dynamics and charge transfer processes.

hν

hν

occupied states

empty states

e–

Φ

Evac

Efermi

e–

pump

probeΔt

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigationGraphene IPSs: from literature…

V.M. Silkin et al., PRB 80, 121408R 2009.(ab initio calculations.)

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;

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigationGraphene IPSs: from literature…

V.M. Silkin et al., PRB 80, 121408R 2009.(ab initio calculations.)

Borca et al. PRL 105, 036804 (2010).

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;

Graphene on metal substrate

1- IPS spatial expansion towards the metal interface region;

1+ IPS larger weight towards the vacuum.

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigationIPSs 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 investigationIPSs at graphene/metal interface: from literature…

D. Niesner et al., Phys. Rev. B 85, 081402 (2012).

Batzill et al., Surf. Science Rep. 67, 83 (2012).

Graphene on Iridium (111)

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigationIPSs at graphene/metal interface: from literature…

D. Niesner et al., Phys. Rev. B 85, 081402 (2012).

Batzill et al., Surf. Science Rep. 67, 83 (2012).

Graphene on Iridium (111)

Graphene on Ruthenium (0001)

N. Armbrust et al., PRL. 108, 056801 (2012)

W. Moritz et al., PRL 104, 136102 (2010).

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigationIPSs at graphene/metal interface: our results…

Graphene on Cu(111)

Cu(111) band structure

S.Pagliara et al., Surface Science 600, 4290 (2006) .

h = 4.10 eV @ k//=0

n=0 Cu(111)

IPS n=1 Cu(111)

IPSSurface state(occupied)

d bands

Silvia Tognolini First Year Workshop, 15 October 2013, Milan.

Graphene/metal investigationIPSs at graphene/metal interface: our results…

Graphene on Polycrystalline Cu foil

h = 4.10 eV @ k//=0P-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;

1+ Graphene IPS

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 investigationFuture 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!!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.