Graphene-supported metal clusters: A fs two photon photoemission and scanning tunneling microscopy study

During the last decade, the growth of graphene on metal single crystal surfaces has attracted considerable attention. This substrate system provides an ideal template for the ordered preparation of regular metal cluster arrays. Our new experimental setup enables time-resolved measurements due to a femtosecond laser system on the one hand and surface analysis via scanning tunnelling microscopy on the other hand. In this contribution the bound unoccupied electronic state structure of an Ir(111)/graphene surface covered by differently sized and spaced Ir clusters was investigated by means of femtosecond two-photon photoemission spectroscopy. The cluster lattice was found to affect the image potential states of the substrate to an extent, which is surprisingly large compared to the graphene area covered by Ir clusters. This effect can be related to the influence of the cluster lattice on the screening of the image state electron trapped in front of the surface. Furthermore, lateral confinement effects could be observed due to the decreasing extension of bare graphene areas.