Electronic Superstructures Observed by Scanning Tunneling Microscopy on Graphene Sheet in Vicinity of Nano-Defects of Ir
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Two-dimensional electronic and atomic superstructures (SS) on monolayer graphite (graphene) grown on a clean Ir(111) surface were observed near substrate nano-defect sites by scanning tunneling microscopy. These SS were found rotated 30° with respect to the undistorted areas (away from defects sites) of the graphene layer with a periodicity of ∼(√3 × √3) a (a ⳱ 2.46 Å) and decay over a distance of few nanometers away from the defects. The observed SS were not due to real surface reconstruction, rather they just had an electronic nature.
I. INTRODUCTION
The studies of adsorption, surface diffusion, and intercalation effects on metal surfaces covered with graphite layers were most intensive in the middle of the 1980s and at the beginning of 1990s.1–4 The typical methods used were thermal desorption, Auger spectrometry, x-ray photoelectron spectroscopy, and mass spectrometry. These techniques happen to reveal formation of graphite layers, adsorption mechanisms, and intercalation of atoms between substrate and the grown layers, providing kinetics of the process with the involved thermodynamic potentials. A major drawback and limitation of these techniques lies in the fact that no precise understanding of physical and chemical properties of the individual atoms could be resolved. To handle this task, scanning tunneling microscope (STM) has great potential in its microscopic and spectroscopic modes and has extensively been used to study graphite coverage on different metallic substrates.5–8 There were also some studies reporting superstrucure (SS) on a graphite surface near defect sites, such as steps, holes, and adsorbed particles in a graphite net.9–13 In this paper we show that SS has been observed on monolayer graphite (graphene) on an Ir(111) surface in vicinity of the substrate defect sites. The observed SS have been found to have a periodicity of √3 times the graphite lattice constant and oriented 30° upward as compared with diagonal rows of undisturbed graphene sheet. a)
Present address: Lash Miller Chemical Labs., Dept. of Chemistry, University of Toronto, 80 St George Street, M5S 1A1, Toronto, Ontario, Canada. e-mail: [email protected] b) Present address: Groupe de Physique des Etats Condencés, UMR CNRS 6631, Marseille, France. DOI: 10.1557/JMR.2004.0138 1058
J. Mater. Res., Vol. 19, No. 4, Apr 2004
Such SS decays over a distance of a few nanometers like earlier reports for SS on bulk graphite.9–13 The electronic-perturbation and electron-wave-interference models9,10 explain only some of the features of SS. To fully interpret and understand the physics of these SS, one needs more experimental data and more theoretical analysis. We show that the observed SS is due to electronic effects from superposition of the graphene sheet and a periodic modulation of the surface charge density localized in the surrounding regions of the defect sites [beneath graphene sheet on Ir(111) surface]. There are reports on the appearance of SS on graphite (0001) surface near defects like holes, steps, or adsorbed metal par
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