1. Silvia Tognolini First Year Workshop, 15 October 2013, Milan Investigating graphene/metal interfaces by time - resolved non linear photoemission

2. 2004: Graphene is discovered Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene: the future in a pencil trace! K. Novoselov A. Geim

3. 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

4. Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene’s properties

5. 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 :

6. 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

7. 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

8. 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.

9. 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.

10. 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.

11. 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. hνhν hνhν occupied states empty states e–e– Φ E vac E fermi e–e– pump probe ΔtΔt

12. Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene/metal investigation Graphene 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;

13. Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene/metal investigation Graphene 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.

14. 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).

15. Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene/metal investigation IPSs 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)

16. Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene/metal investigation IPSs 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).

17. Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene/metal investigation IPSs 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) IPS Surface state (occupied) d bands

18. Silvia Tognolini First Year Workshop, 15 October 2013, Milan. Graphene/metal investigation IPSs at graphene/metal interface: our results… Graphene on Polycrystalline Cu foil h = 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; 1 + Graphene IPS These samples are kindly provided by the group of P. Rudolf of Groningen University.

19. 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!!

20. 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 < E VAC 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.