Local electronic properties of graphene flakes on noble metal surfaces

Abstract

This thesis examines possible routes for the preparation of graphene nanostructures on metal substrates and performs structural and electronic characterizations using scanning tunneling microcopy and spectroscopy. Investigations of graphene nanostructures necessitate the use of a suitable graphene-substrate combination, which allows for a controlled in situ preparation of small and well-shaped graphene nanostructures. The choice of a graphene-substrate combination with weak interaction in order to prevent the destruction of monolayer graphene properties is inevitable.Within this work graphene layers and graphene nanostructures are grown using well-established procedures based on thermal decomposition of hydrocarbons on Ir(111) and Rh(111) surfaces. Implementing intercalation - the insertion of additional material between graphene and substrate - allows for a tailoring of interactions between graphene and the substrate. In the first part of this work the intercalation of Fe and Ni is investigated. Graphene on Fe and Ni surfaces represents a system with strong interaction. The intercalation of submonolayers of Fe and Ni allows for the investigation of binding strength variations due to intercalation within one sample. A moiré superstructure of graphene on metal surfaces leads to a local modulation of the binding strength, which was found to influence the arrangement of intercalated material considerably. The studied systems furthermore give an insight into the intercalation processes at the atomic scale.For an electronic decoupling of graphene from the substrate in the second part of the work, intercalation of noble metals was implemented. Graphene flakes which become electronically decoupled by Au and Ag were investigated using low temperature scanning tunneling microscopy and spectroscopy. A substantial decrease of graphene-substrate interactions compared to other graphene/metal systems was found. Graphene on Au and Ag substrates exhibits characteristic local density of states modulations at edges and defects indicative of quasiparticle scattering in graphene.For the characterization of the electronic properties local density of states maps were measured using scanning tunneling spectroscopy. The maps were subsequently Fourier-transformed and analyzed in reciprocal space. The detected quasiparticle scattering vectors allow for a precise discrimination between scattering within the Au(111) surface state and between states in graphene. Graphene on Au in particular shows a linear dispersion relation within the accessible energy range. Additional scattering between the two electronic systems of graphene and the Au(111) surface state was identified and used for a determination of the Rashba splitting in Au(111) using scanning tunneling spectroscopy.Quasiparticle scattering between graphene states was studied in confined, elongated graphene flakes on Au and Ag. Additional scattering vectors compared to infinite graphene were found and confinement as the origin of the additional scattering was confirmed. The confinement effects exist also in large systems up to 100 x100 nm2.