Berlin 2012 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 53: Graphene IV

O 53.4: Vortrag

Mittwoch, 28. März 2012, 17:30–17:45, MA 041

Graphene on 3C-SiC(111): Ab initio study of structure and stability including van der Waals effects — •Lydia Nemec, Volker Blum, Patrick Rinke, and Matthias Scheffler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, D-14195 Berlin

SiC is a widely used substrate for the growth of mono- and few-layer graphene. By Si sublimation, large ordered areas of graphene form on the Si side as (6 √3 × 6 √3)-R30^∘ (“6√3”) commensurate periodic films [1]. In experiment, a first, partially covalently attached “buffer layer” (BL) 6√3 carbon layer can be grown in a small temperature window, followed by a quasi-freestanding 6√3 monolayer of graphene (GBL) at higher temperatures, and finally, few-layer graphene. What is not a priori clear is whether the BL and GBL phases are the result of some (narrow) thermodynamic equilibrium condition, and how the substrate affects the structure (monolayer graphene corrugation). Both are natural questions for theory, but a challenge for two reasons: (1) system size (up to ∼ 2000 atoms in this work) and (2) van der Waals interactions, not accounted for by most standard density functionals. We show that the BL and GBL phases are in fact equilibrium phases within all-electron density functional theory (DFT) including van der Waals effects (PBE+vdW [2]). The equilibrium geometry emerges as a direct prediction, including the significant graphene layer corrugation. We also show that smaller-cell approximants may miss some geometric features, and could erroneously stabilize buffer layer defects due to artificially induced strain. [1] K. V. Emtsev et al., Nature Materials 8, 203 (2009). [2] A. Tkatchenko, M. Scheffler, PRL 102, 073005 (2009).