Growth through controlled adsorption of ferromagnetic elements such as Fe, Co and Ni on two-dimensional silicene provides an alternative route for silicon-based spintronics. Plane wave DFT calculations show that Fe, Co and Ni adatoms are strongly chemisorbed via strong sigma bonds, with adsorption energies (1.55 - 2.29 eV) that are two to six times greater compared to adsorption on graphene. All adatoms adsorb more strongly at the hole site than at the atom site, with Ni adsorbing strongest. Of the dimer configurations investigated, the hole – hole, b-atom – hole, vertically stacked at hole, vertically stacked at b-atom and bridge sites were found to be stable. The Co and Ni dimers are most stable when adsorbed in the hole-hole configuration while the Fe dimer is most stable when adsorbed in the atom-hole configuration. Metal-to-silicene and interconfigurational s-to-d electron transfer processes underpin the trends observed in adsorption energies and magnetic moments for both adatoms and dimers. Adsorption of these metals induces a small band gap at the Dirac Cone. In particular Co adatom adsorption at the hole site induces the largest spin-polarized band gaps of 0.70 eV (spin-up) and 0.28 eV (spin-down) making it a potential material candidate for spintronics applications.
Plane-wave density functional theory is used to investigate the impact of hydrogen passivation of the p(2×2) reconstructed Ge1−xSnx surface on Sn segregation, aggregation, and distribution. On a clean surface, Sn preferentially segregates to the surface layer, with surface coverages of 25%, 50%, and 100% for total Sn concentrations of 2.5%, 5.0%, and 10.0%, respectively. In contrast, a hydrogen passivated surface increases interlayer migration of Sn to subsurface layers, in particular, to the third layer from the surface, and results in surface coverages of 0%, 0%, and 50% corresponding to Sn concentrations of 2.5%, 5.0%, and 10.0%, respectively. Hydrogen transfer from a Ge-capped surface to the one enriched with increasing Sn surface coverage is also an unfavorable process. The presence of hydrogen therefore reduces the surface energy by passivating the reactive dangling bonds and enhancing Sn interlayer migration to the subsurface layers. For both clean and hydrogenated surfaces, aggregation of Sn at the surface layer is also not favored. We explain these results by considering bond enthalpies and the enthalpies of hydrogenation for various surface reactions. Our results thus point to reduced Sn segregation to the surface in a Ge1−xSnx epitaxial thin film if CVD growth, using hydride precursors in the hydrogen limited growth regime, is used. This would lead to a more abrupt interface and is consistent with recent experimental observation. Hydrogenation is therefore a promising method for controlling and manipulating elemental population of Sn in a Ge1−xSnx epitaxial thin film.
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 µ B 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 826 H. Johll et al.graphene. Therefore, the accuracy of these calculations will depend rather strongly on how adequately the 3d-4s exchange correlation inter-action is treated.
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