Adsorbate (N2+, Mg)-Induced Changes in Electronic and Structural Properties of Graphene

Abstract

The research activities on graphene have been fervent for past decade due mainly to its unique physics opening a new paradigm for relativistic condensed matter physics with a promising possibility for various industrial applications especially in next generation electronics device. In order to control the Dirac point (DP) of graphene where the conduction and valence bands cross each other and modify the band structure, we have investigated the changes in the π-band of graphene by implanting non-metal atoms and/or by adsorbing metal atoms on single layer graphene (SLG) formed either on SiC(0001) or on Ni(111) substrate. We have adopted the combination of angle resolved photoemission spectroscopy (ARPES) and high resolution core level spectroscopy (HRCLS) as a major experimental tool to examine changes in electronic properties of graphene. In order to probe the structural changes in graphene, we have also used scanning tunneling microscopy (STM). We also carried out density functional theory (DFT) band calculations for these metal-added SLG systems in order to understand the basic driving forces of the changes in collaboration with theory groups. As the first system studied in this thesis, we have doped energetic nitrogen cations (N2+) on SLG to find any dopant-induced changes in the π-band or in the local atomic bonding structures. We find that such a doping of nitrogen ions effectively modifies the local N-C bonding structures and the π-band of graphene critically depending on the ion energy Ek (100 eV ≤ 500 eV). With increasing Ek, we find a phase transformation of the N-C bonding structures from a graphitic phase where nitrogen substitutes carbon to a pyridinic phase where nitrogen loses one of its bonding arms, with a critical energy Ekc = 100 eV that separates the two phases. The N2+-induced changes in the π-band with varying Ek indicate an n-doping effect in the graphitic phases for Ek < Ekc but a p-doping effect for the pyridinic graphene for Ek > Ekc. We further show that one may control the electron charge density of graphene by two orders of magnitude by varying Ek of N2+ ions within the energy range adopted. Our DFT-based band calculations reproduce the distinct doping effects observed in the π-band of the N2+-doped graphene and provide an orbital origin of the different doping types. We thus demonstrate that the doping type and electron number density in the N2+ ion-doped SLG can be artificially fine-controlled by adjusting the kinetic energy of incoming N2+ ions. As the second system, we report changes in the electronic and structural properties induced by Mg adatoms on SLG formed on Ni(111) substrate. We find the presence of several Mg-induced superstructures depending on Mg coverage and a strong metallic parabolic band near the Fermi level at the Γ-point of the Brillouin zone. We find that Mg adatoms intercalate initially to lift the SLG from the Ni substrate to produce a well-defined π-band of SLG, and then the parabolic band appears upon adding extra Mg atoms on the Mg-intercalated SLG. Our Fourier-transformed STM images obtained from these systems reveal the presence of superstructures, a 2√3×2√3 phase for the intercalated Mg layer below the SLG and then a √7×√7 phase for the Mg overlayer formed on the Mg-intercalated SLG. We discuss physical implications of these superstructures and the associated parabolic band in terms of a possible graphene-based two-dimensional superconductivity.