Scanning Tunneling Spectroscopy on Graphene Nanostructures

Abstract

This thesis describes investigations on graphene nanostructures by the means of scanning tunneling microscopy (STM)and spectroscopy (STS) in ultra high vacuum at low temperature (5.5 K), focused on their electronic structure on the local scale. The experiments are based on structurally highly perfect epitaxial graphene on Ir(111) [gr/Ir(111)], but extend the range towards new graphene based nanomaterials. The first topic comprises the development of new nanomaterials which keep the structural coherency of epitaxial graphene on Ir(111) at a reduced electronic substrate interaction, in particular concerning graphene's quasi-relativistic Dirac particles. Therefor, we present the first study on graphene quantum dots (GQDs) on silver (gr/Ag). In STS, we observe the Ag(111) surface state on 15 ML of Ag on Ir(111), study its behavior in the presence of graphene, and discuss its role in the observation of Dirac electron confinement on GQDs. We find the surface state suppressed in 1 ML of Ag on Ir(111). In a next step we present an experimental advancement towards a system, where the metallic surface states are completely absent, namely oxygen covered Ir(111) [O/Ir(111)]. In an STS study, we discover new oxygen superstructures on iridium under graphene and two types of charge effects in the GQDs' local density of states (LDOS). We present the first unambiguous experimental observation of Dirac electron confinement on GQDs. We calculate the Dirac dispersion relation on the basis of our experimental data and confirm the efficient decoupling by DFT calculations and the direct observation of a Dirac feature in point spectroscopy and characteristic electron scattering processes. In addition to the benefit for the observation of Dirac confinement, our findings gain universal insight into the decoupling of graphene's electronic system from the metallic substrate by oxygen intercalation. The studies are extended towards the unoccupied surface state spectrum at high energies in form of image potential states (IPSs). For the first time we experimentally prove the size dependence of IPSs due to confinement on GQDs acting as a quantum well. We explain the occurrence of a strongly pronounced state, which is not the ground state, by an interplay of the LDOS and momentum conservation during tunneling. The positions of the IPSs can be tuned by chemical gating, which means the experimental realization of a quantum well tunable in both width and depth. We discuss the benefit of a direct measurement of the local workfunction for the determination of the local doping level in graphene intercalation compounds. In a next step we propose a route how to experimentally access the binding situation at the boundaries of GQDs on Ir(111), using the advanced technique of Inelastic Electron Tunneling Spectroscopy (IETS). Finally, we observe metallic features in the LDOS which are related to one dimensional defects in an extended monolayer of epitaxial graphene on Ir(111)