The Georgia Tech Materials Science and Engineering Center (MRSEC) welcomes Dr. Harald Brune, a professor at the Ecole Polytechnique Fédérale de Lausanne, on "Band Gap Engineering and Real Space Structure of Graphene Mono- and Bilayers on Metal Surfaces."

Abstract:
Graphene forms moiré structures on lattice mismatched close-packed metal surfaces. These structures involve periodic transitions between three stacking areas. Graphene is most loosely bound in the one where the C-rings are centered on metal atoms and therefore these stacking areas are expected to exhibit an electronic structure coming close to the one of free standing graphene. Indeed sharp linear bands forming Dirac cones have been observed for g-monolayers on Ir(111), but hitherto not on Ru(0001), where only the second layer displayed the characteristic electronic structure of graphene.  We present angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling microscopy (STM) results on the electronic and real-space structure of graphene mono- and bilayers on Ru(0001) and of graphene monolayers on Ir(111). We find that long-range ordered graphene monolayers on Ru(0001) display sharp Dirac cones. The lateral positions of the C-ring centers in the first monolayer show strong distortions from a hexagonal lattice. Therefore the moiré structure is not the beating of two laterally rigid hexagonal lattices, instead, graphene optimizes its binding to the substrate by strongly adapting the C-C distances. The moiré of g/Ir(111) gives rise to six-fold symmetric replica around the K-point. A super-lattice of Ir islands grown on-top introduces a strongly increases the amplitude of the periodic electron potential leading to three-fold symmetry, a band gap opening and to strongly asymmetric group velocities. In the case of Ru, very similar growth temperatures and identical ethylene dosage give rise to strongly different g coverage and long-range order such that these samples are best assessed by the presented combination of morphology and electronic structure characterization.