Tim Wehling Title: Adsorbates on graphene: Interaction mechanisms and impurity states Abstract: We address adsorbate effects on the electronic properties of graphene. Firstly, different interaction mechanisms of impurities with graphene are introduced in terms of simple models and the occurrence of impurity induced midgap states is discussed. With this background, we consider realistic adsorbates and give an understanding of their effects based on density functional theory. Open-shell and inert impurities are shown to exhibit very different interaction mechanisms with graphene. The former impurities strongly interact with graphene and may bond covalently or ionically to it. Within the class of monovalent impurities a relation between chemical bonding, the local electronic structure and migration barriers for the impurities is illustrated. Covalent impurities with one chemically active electron (like H or OH) cause universal midgap states and exhibit migration barriers on the order of some 100meV to 1eV. This is in contrast to the ionically bond impurities with migration barriers of usually less than 0.1eV and about an order of magnitude less than their typical binding energies. An asymmetry between anions and cations regarding their adsorption sites and topology of their potential energy landscape is predicted. Inert impurities like H2O physisorb on graphene and do not cause direct doping. They can, however, contribute local dipole moments and cause electrostatically mediated doping involving graphene's substrate as reservoir for electrons or holes.