Modification of the electronic structure of graphene by quantum confinement and molecular self-assembly

Graphene has generated enormous interest after its discovery in 2004, both in the scientific community and recently even in industry. It has been highlighted as one of the strongest materials known with very high electrical and thermal conductivity. Much of the scientific interest in graphene is however related to its peculiar electronic structure. The valence and conduction bands of graphene touch at single points on the corners of its Brillouin zone. Near these points the dispersion is linear which makes the charge carriers behave like relativistic Dirac fermions with zero effective mass, which leads to many of its exceptional electronic properties. Graphene has already been used to build field-effect transistors, but due to the absence of a band gap the on-off ratios are very poor. In order for graphene to be used as a replacement for silicon in future electronics, a gap would need to be opened between its valence and conduction bands. In this thesis, two possible ways of modifying the graphene band structure are explored experimentally, namely quantum confinement and periodic potential modulations induced by self-assembled molecular layers. Quantum confinement of the graphene electronic states is studied on small graphene structures grown epitaxially on the (111) facet of iridium. The states are mapped by scanning tunneling spectrocopy and a continuum model using the Klein-Gordon equation is presented to model the states. The self-assembly of cobalt phthalocyanines is studied first on epitaxial graphene on iridium(111) by scanning tunneling microscopy and low energy electron diffraction. The effect of substrate roughness on the self-assembly is then investigated on graphene on silicon oxide and hexagonal boron nitride. Finally, the structure of the graphene on Ir(111) model system used in many of the measurements, is studied in detail by atomic force microscopy and dynamic low energy electron diffraction.