Graphene layers have been found in various configurations, including multisheet stacks whose properties quickly converge to bulk graphite, as overlayers which grow on metal surfaces, as well as single-sheet graphene crystals recently isolated by cleavage of graphite [1]. Graphene has attracted a lot of interest due to its structure, which consists of carbon atoms strongly bound into two dimensional honeycomb lattice, with much weaker interaction in the third dimension. The peculiar atomic and electronic structure demands specialized theoretical approaches. We have performed ab-initio calculations of the structure and electronic properties of several graphene systems in the Density Functional Theory with GGA functionals complemented with the long range correlation effects, i.e. the van der Waals interaction, using recently developed approaches. In one approach the coefficients of the asymptotic interaction are deduced starting from the calculated electronic densities. We have previously used this approach to study the adsorption of Xe monolayers on metals [2]. We have also applied the DFT functional in which the usual GGA correlation (local and semi-local) is modified to include fully non-local effects in a "seamless" fashion [3]. We have considered in particular the graphene overlayer on Ir(111), where STM experiments show moiré patterns indicating an adsorption with large periodicity due to a small lattice mismatch between graphene and the substrate [4] leading to interesting periodic dependence of the adlayer separation, bonding strength and reactivity. We have also calculated the properties of bilayer and multilayer graphene stacks in different configurations. We discuss the results for various structural properties and electronic structure of the calculated systems.
[1] A.K. Geim and K.S. Novoselov, The rise of graphene, cond-mat/0702595 (preprint).
[2] P. Laziæ, Ž. Crljen, R. Brako, and B. Gumhalter, Phys. Rev. B 72, 245407 (2005).
[3] M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004).
[4] A.T. N'Diaye, S. Bleikamp, P.J. Feibelman, and T. Michely, Phys. Rev. Lett. 97, 215501 (2006).
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