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Structure and properties of pure and mixed transition metal dimers on graphene

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Small Fe, Co and Ni clusters are known to exhibit high magnetic moments and are therefore investigated rather intensely for the purpose of developing novel magnetic materials with high magnetisation densities. The reduced bond density of these clusters compared to their respective bulk states, results in them being particularly sensitive to their environment. The choice of a substrate for these clusters is therefore of particular importance. Graphene has been shown to exhibit many novel phenomena and it is therefore interesting to investigate the suitability of using graphene as a support material for these metal clusters and if this would allow for an integration of technologies. In this paper, we report the results of plane–wave density functional theory (DFT) calculations of Fe, Co and Ni adatoms, and homonuclear and heteronuclear dimers, including mixing with Pt, adsorbed on graphene. We investigated the adsorption site structure, and stability, the projected density of states and electron populations, and magnetic moments. Calculations were performed using the Perdew–Burke–Ernzerhof (PBE) functional for the wavefunction with energy cutoffs of 40Ry and 480Ry for the wavefunction and density respectively. Brillouin zone sampling was performed with a Monkhorst–Pack grid of (8 ? 8 ? 1). We find that the adatoms bind weakly to graphene and that the magnetic moment of the most stable adatom configuration (the hole site configuration) is reduced by 2 ?
compared to the free adatom, and that the most stable dimer configuration is one where the dimer bond axis is oriented perpendicular to the graphene plane. The stability of the adatoms and dimers on graphene can be explained by considering the electronic interconfigurational energy change that accompanies the desorption of these clusters as well as the amount of charge transferred from cluster to graphene. Therefore, the accuracy of these calculations will depend rather strongly on how adequately the 3d–4s exchange correlation inter–action is treated.

Keywords: transition metal clusters, density functional theory, DFT, magnetic moments, graphene, nanotechnology, substrates, adatoms, dimers

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