Scanning Tunneling Spectroscopy on Graphene Nanostructures

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)

Graphene on weakly interacting metals: Dirac states versus surface states

We investigate the interplay between graphene and different, weakly interacting metal substrates by measuring the local density of states of the surface with scanning tunneling spectroscopy. Energy-resolved Friedel oscillations, confined states, and a prominent signal in point spectra are found after intercalating several monolayers of silver between graphene and Ir(111) and correspond to the shifted surface state of silver. These features outweigh spectroscopic signatures of graphene, which are retrieved when the amount of silver is reduced to one monolayer. Hence, suppressing the surface states of the metal substrate enhances the sensitivity to the Dirac states of quasi-free-standing graphene

Spin-Polarized Surface State in EuO(100)

High-quality films of the ferromagnetic semiconductor EuO are grown on epitaxial graphene on Ir(111) and investigated in situ with scanning tunneling microscopy and spectroscopy. Electron scattering at defects leads to standing-wave patterns, manifesting the existence of a surface state in EuO. The surface state is analyzed at different temperatures and energies. We observe a pronounced energy shift of the surface state when cooling down below the Curie temperature TC, which indicates a spin polarization of this state at low temperatures. The experimental results are in agreement with corresponding density functional theory calculation

Confinement of Dirac electrons in graphene quantum dots

We observe spatial confinement of Dirac states on epitaxial graphene quantum dots with low-temperature scanning tunneling microscopy after using oxygen as an intercalant to suppress the surface state of Ir(111) and to effectively decouple graphene from its metal substrate. We analyze the confined electronic states with a relativistic particle-in-a-box model and find a linear dispersion relation. The oxygen-intercalated graphene is p doped [ED=(0.64±0.07) eV] and has a Fermi velocity close to the one of free-standing graphene [vF=(0.96±0.07)×106 m/s]

Energy-Dependent Chirality Effects in Quasifree-Standing Graphene

We present direct experimental evidence of broken chirality in graphene by analyzing electron scattering processes at energies ranging from the linear (Dirac-like) to the strongly trigonally warped region. Furthermore, we are able to measure the energy of the van Hove singularity at the M point of the conduction band. Our data show a very good agreement with theoretical calculations for free-standing graphene. We identify a new intravalley scattering channel activated in case of a strongly trigonally warped constant energy contour, which is not suppressed by chirality. Finally, we compare our experimental findings with T-matrix simulations with and without the presence of a pseudomagnetic field and suggest that higher order electron hopping effects are a key factor in breaking the chirality near to the van Hove singularity

Step-induced faceting and related electronic effects for graphene on Ir(332)

Modifications of graphene's electronic band structure can be achieved through periodic bending strain and related potential in samples grown on stepped substrates, opening a viable route to implement the periodicity effects in this ultimate two-dimensional (2D) material. We studied graphene grown on stepped Ir(332), which can be benchmarked to a well-known graphene on flat Ir(111) recognized for a weak van der.Waals (vdW) interaction. The structural characterization indicated that graphene growth induces reversible, well defined faceting of iridium surface into alternating terraces and step bunches, while spectroscopy techniques revealed substantial changes of graphene's electronic structure. Crucially, highly concentrated Ir step edges, resulting in locally strong chemical bonding of graphene, introduce a dominant energy parameter which overwhelms the induced strain and presents a driving force for the surface faceting. This sets a general framework for the understanding of graphene mediated faceting of stepped substrates whenever the corresponding low index surface exhibits dominantly vdW interaction with graphene, which can be also supplemented to other 2D materials. Interestingly, the graphene band becomes pronouncedly anisotropic due to the presence of a periodic potential originating from steps, and lateral variation of the charge carrier concentration enabling a straightforward electronic band engineering in graphene. (C) 2016 Elsevier Ltd. All rights reserved

Ion Impacts on Graphene/Ir(111): Interface Channeling, Vacancy Funnels, and a Nanomesh

By combining ion beam experiments and atomistic simulations we study the production of defects in graphene on Ir(111) under grazing incidence of low energy Xe ions. We demonstrate that the ions are channeled in between graphene and the substrate, giving rise to chains of vacancy clusters with their edges bending down toward the substrate. These clusters self-organize to a graphene nanomesh via thermally activated diffusion as their formation energy varies within the graphene moiré supercell