Impurities and Inhomogeneities in Nanoelectronic Systems

Abstract

The research on atomic scale solid state structures has developed into a highly dynamic field that is driven by fundamental questions as well as applications. In this thesis, the electronic properties of different nano scale systems are theoretically investigated and distinct physical effects determining their electronic structure will be encountered.

In the first part, effects of inhomogeneities and impurities in graphene are addressed by means of first-principles theory and analytical models. We give an explanation for an unexpected gap reported in recent scanning tunneling spectroscopy (STS) experiments on graphene. A particular type of electron-phonon coupling is shown to cause huge inelastic contributions in STS on graphene. As graphene exhibits long range ripples, we investigate the electronic properties of corrugated graphene. We show that quenched ripples induce pseudo-magnetic fields and that these can lead to the formation of flat bands near the Fermi level, which are destroyed upon annealing. Being crucial for any application, we then turn to impurity effects in graphene. Different interaction mechanisms of impurities with graphene are introduced and the requirements for impurity states in the vicinity of the Fermi level are worked out. We find that open-shell and inert impurities affect graphene very differently: the former interact directly with graphene, strongly hybridize, cause midgap states or become charged, whereas inert impurities usually physisorb and substrate mediated doping effects become important.

In the second part of this thesis, we address the electronic properties of magnetic ad-atoms and nanostructures on surfaces by combing density functional theory with many body approaches. For the model system of Ce ad-atoms on different transition metal surfaces, hybridization mechanisms are elucidated and recent photoemission experiments displaying a transition from localized to delocalized Ce 4f electrons upon changing the substrate are explained. Afterwards, we turn to the Kondo effect of Co atoms in different environments. In a joint STS and ab-initio theory investigation, we find that the Kondo temperature of Co atoms embedded in CoCu_n clusters on Cu(111) exhibits a nonmonotonic variation with the cluster size and demonstrate the importance of the local inhomogeneous electronic structure for correlation effects in small clusters.

Motivated by very recent experiments, we identify possible scenarios for the Kondo effect due to Co ad-atoms on graphene. We find that orbital physics controls the Kondo effect if Co is located in the center of a graphene hexagon. Characteristic signatures of the interplay of the orbital Co physics and the peculiar band-structure of graphene are predicted to occur in local probe experiments.