Transition-metal adatoms on graphene: Influence of local Coulomb interactions on chemical bonding and magnetic moments

We address the interaction of graphene with $3d$ transition-metal adatoms and the formation of localized magnetic moments by means of first-principles calculations. By comparing calculations within the generalized gradient approximation (GGA) to GGA $+$ $U$ we find that the electronic configuration and the adsorption geometries can be very sensitive to effects of local Coulomb interactions $U$ in the transition-metal $d$ orbitals. We find high-spin configurations being favorable for Cr and Mn adatoms independent of the functional. For Fe, Co, and Ni different electronic configurations are realized depending on the value of the local Coulomb interaction strength $U$. Chemical control over the spin of the adatoms by hydrogenation is demonstrated: NiH and CoH adsorbed to graphene exhibit spin $S=1/2$ and $S=1$, respectively.

Figure 1

Left: Schematic illustration of high-symmetry adsorption sites for Co adatoms on graphene (red) and paths connecting these sites (green). Middle: Total energy $E$ of graphene with a Co adatom as a function of the Co position along the path depicted in the left panel. Energies obtained with GGA, LDA, and GGA $+$ $U$ ($U=4$ eV, $J=0.9$ eV) are shown. The energy for Co at an h-site is defined as $E=0$. Right: Total magnetic moment of the graphene supercell with adsorbed Co as a function of the Co position.

Figure 2

Local orbitally and spin resolved density of states at the adatoms (Cr, Mn, Fe, Co, and Ni from top to bottom) and total density of states of the supercells obtained with GGA (left column) as well as GGA $+$ $U$ (right column, $U=4$ eV, $J=0.9$ eV). Spin-up is plotted on the positive ordinate, spin-down on the negative ordinate. The GGA $+$ $U$ high-spin solution shown for Ni is metastable at $U=4$ eV. All other LDOSs shown here belong to structures with lowest total energy.

Figure 3

Relaxed adsorption geometries of CoH on graphene as predicted by GGA (left) and GGA $+$ $U$ (right). For NiH, both, GGA and GGA $+$ $U$ yield h-site adsorption with virtually the same structure as obtained for CoH in GGA (left).

Figure 4

Local orbitally and spin resolved density of states for the hydrogenated adatoms CoH and NiH on graphene (from top to bottom) obtained with GGA (left column) as well as GGA $+$ $U$ (right column, $U=4$ eV, $J=0.9$ eV). Spin-up is plotted on the positive ordinate, spin-down on the negative ordinate.