Synthesis and electronic Properties of atomically well-defined Graphene Nanostructures

The carbon allotrope graphene is the first member of a new family of two-dimensional materials which have roused the interest of scientists worldwide. Comprised of only a single layer of carbon atoms arranged on a honeycomb lattice, graphene has a peculiar electronic structure with linearly dispersing bands forming two Dirac cones at the Fermi level. These give rise to several unusual quantum effects such as Klein tunnelling and the integer quantum Hall effect as well as a high electron mobility through suppressed back scattering of charge carriers. Graphene has therefor been heralded as a replacement material for silicon in a new generation of low-power electronic devices. In order to compete with silicon electronics however, the size of gates, leads, and channels in graphene devices must be brought down into the nanometer range. At these length scales, the boundaries of the graphene structures will have a great influence on their properties and performance as a large fraction of atoms occupy edge sites. This thesis is concerned with the experimental realisation of atomically well-defined graphene systems and the study of their electronic properties by scanning tunnelling microscopy. The electronic properties of graphene are first introduced by means of a tight binding model, focusing on graphene nanoribbons as model systems. It is shown that both the size of the system and the symmetry of its edge have a significant influence on the electronic properties of small graphene structures. After these considerations, some approaches to realise well-defined graphene nanosystems are discussed and data on their electronic properties is presented. The major part of the experimental section is concerned with the zig-zag boundary of graphene and the resulting edge state, which is predicted to enable spintronics and valleytronics in all carbon nanostructures. Two attempts to produce stable zig-zag edges of high quality through self-assembly by chemical vapour deposition driven epitaxy of graphene and hexagonal boron nitride on an Ir(111) and Ni(111) surface are discussed. Atomically precise interfaces of over 150 nm length could be obtained, but no signature of the edge state is found on the as-grown samples. A further experiment is presented where the intercalation of gold under a sample grown on Ir(111) was used to decouple the heterostructures from their support and the edge state could be observed. Finally, data on narrow armchair graphene nanoribbons is presented. Numerical methods predict a nearly vanishing band gap for the structure considered here and they have hence been proposed as connections in graphene devices. The ribbons are self-assembled form molecular precursors by an Ullmann type reaction in vacuo on an Au(111) surface. Their electronic structure is examined by means of scanning tunnelling spectroscopy and a band gap of 0.1 eV could be extrapolated for ribbons of over ca. 6 nm length.