Berlin 2012 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 27: Graphene II
O 27.3: Vortrag
Dienstag, 27. März 2012, 11:00–11:15, MA 041
Phase Coexistence of Clusters and Islands: Europium on Graphene — Daniel F. Förster1, Tim O. Wehling2, •Stefan Schumacher1, Achim Rosch3, and Thomas Michely1 — 1II. Physikalisches Institut, Universität zu Köln, D-50937 Köln — 21. Institut für Theoretische Physik, Universität Hamburg, D-20355 Hamburg — 3Institut für Theoretische Physik, Universität zu Köln, D-50937 Köln
The adsorption and equilibrium surface phases of Eu on graphene on Ir(111) are investigated in the temperature range from 35 K to 400 K and for coverages ranging from a small fraction of a saturated monolayer to the second layer by scanning tunnelling microscopy (STM). Using density functional theory (DFT) including the 4f-shell Coulomb interactions and modelling of the electronic interactions, excellent agreement with the experimental results for the equilibrium adsorbate phase, adsorbate diffusion and work function is obtained. Most remarkable, at 300 K in an intermediate coverage range a phase of uniformly distributed Eu clusters (size 10 - 20 atoms) coexists in two dimensional equilibrium with large Eu-islands in a (√3×√3)R30∘ structure. We argue that the formation of the cluster phase is driven by the interplay of three effects: First, the metallic Eu-Eu binding leads to the local stability of (√3×√3)R30∘ structures. Second, electrons lower their kinetic energy by leaving the Eu clusters, thereby doping graphene. Third, the Coulomb energy penalty associated with the charge transfer from Eu to graphene is strongly reduced for smaller clusters.