AVS 60th International Symposium and Exhibition
    Graphene and Other 2D Materials Focus Topic Wednesday Sessions
       Session GR-WeM

Paper GR-WeM2
Energies and Lifetimes of Unoccupied Electronic States of Graphene on the Ru(0001) Surface

Wednesday, October 30, 2013, 8:20 am, Room 104 B

Session: 2D Materials on Metallic Substrates
Presenter: J. Guedde, Philipps-University Marburg, Germany
Authors: J. Guedde, Philipps-University Marburg, Germany
N. Armbrust, Philipps-University Marburg, Germany
P. Jakob, Philipps-University Marburg, Germany
U. Hoefer, Philipps-University Marburg, Germany
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We present the results of an investigation of energies, dispersion and lifetimes of initially unoccupied electronic states between Fermi and vacuum level of single graphene layers on Ru(0001) obtained by means of time- and angle-resolved two-photon photoelectron spectroscopy (2PPE) [1]. Graphene/Ru(0001) is a particular interesting system because of a relative strong geometrical corrugation of the graphene layer in form of a periodically rippled moiré superstructure caused by the interaction with the Ru surface. The high in-plane polarizability of the graphene sheet and the dissimilar graphene-Ru distances in the hill and valley areas of the moiré superstructure lead to a number of novelties referring to image-potential and interface states as compared to other adsorbate systems on metal surfaces.

In the experiment we identify three image-potential states close to the vaccum level and two interface states at energies of 0.91 and 2.58 eV above the Fermi level. The most strongly bound, short lived and least dispersing image-potential state is suggested to have some quantum-well character with a large amplitude below the graphene hills. The two other image-potential states are attributed to a series of slightly decoupled states. Their lifetimes and dispersions are indicative of almost free-moving electrons, predominantly located in the interconnected valley areas. The two interface states most likely originate both from a surface resonance of Ru(0001) and exhibit different confinement and energy shifts in the valley and hill areas.

These results will be compared with model calculations using a parametrized one-dimensional model potential for a varying graphene/metal distance. It will be shown that the strong attraction of graphene for electrons in combination with its low workfunction lead to the situation that the image-potential states typically originate from the graphene and are modified by the presence of the metal surface. This is in strong contrast to insulating adsorbates on metal surfaces where the adsorbate layer modifies the image-potential states of the metal. The observed interface states, on the other hand, are both assigned to a modified surface resonance of Ru. The different confinement in the hill and valey areas result in distinct energy shift of this resonance that resembles the Shockley surface state observed on the (111) surfaces of the noble metals. This will be underlined by comparing the present results with the energy shift observed for the Shockley surface state of the Ag(111) surface upon adsorption of polycyclic aromatic hydrocarbon layers.

[1] N. Armbrust, J. Güdde, P. Jakob, and U. Höfer, Phys. Rev. Lett. 108, 056801 (2012).