Panels (a), (b), and (c) are plots of the electronic density of states projected onto the p-orbitals of carbon atoms A0, B1, and A2, respectively, when a hydrogen atom is adsorbed at carbon atom A0. Blue lines indicate spin-up states and red lines indicate spin-down states. Panel (d) is a plot of the spin-polarization of lattice atoms #1 to #14, the positions of which are indicated in the inset. Atom #8 is the adsorbent carbon atom for the hydrogen atom. Hole sites at α, β, and γ positions relative to the A0 carbon atom are labelled in the figure. In our coadsorption calculations, the dimer is adsorbed at the hole sites with the circled labels.
The spin-polarization of the carbon atoms #3 to #14 in Fig. 1(d) when a transition metal dimer is singly adsorbed at the hole site is denoted by α. The magnetization of the carbon atoms around the hole site is in the same direction as that of the Ni2 dimer, but in the opposite direction to that for the Fe2, Co2, and FeCo dimers.
Electronic density of states projected onto the d-orbitals of the singly adsorbed dimer (left panels) and onto the p-orbitals of the carbon atoms (right panels) at the hole site that the dimer is adsorbed in. The Fermi level is set at zero for the energy axis. Blue lines indicate spin-up and red lines indicate spin-down states. These DOS illustrate the resonance peaks for both spins in the vicinity of the Fermi level. These DOS should be compared to Fig. 5 to see the effect of coadsorption on the electronic states.
The spin-polarization of the carbon atoms #1 to #14 denoted by small hexagons in Fig. 1(d) for all dimer species in coadsorption sites α′, β, and γ. Note that these results are not expected to be “symmetrical” about atom #8; for instance, atom #5 and atom #11 are not equivalent.
Electronic density of states projected onto the d-orbitals of the γ-coadsorbed dimer (left panels) and onto the p-orbitals of the carbon atoms (right panels) at the hole site that the dimer is adsorbed in. The Fermi level is set at zero for the energy axis. Blue lines indicate spin-up states and red lines indicate spin-down states. These data should be compared to the graphs in Fig. 3 for the singly adsorbed dimer, and illustrate the interactions between the dimer states and the spin-polarized lattice states that arise from hydrogen adsorption. Arrows in the left panels indicate the dimer component of the spin-up state near the Fermi level, while arrows in the right panels indicate the lattice components of the spin-down states near the Fermi level.
Structures of coadsorbed species Fe2 and FeCo to illustrate the bonding geometry for the α′, β, and γ configurations. The hydrogen, adsorbent carbon, iron, and cobalt atoms are represented by white, green, red, and blue balls, respectively.
Singly adsorbed hydrogen atom: adsorption energy E1H (eV), C–H bond length (Å), and the relaxation (Å) of the adsorption site carbon atom away from the graphene lattice.
Singly adsorbed transition metal dimers: adsorption energies E1M (eV), dimer bond lengths (Å), and magnetic moments of upper/lower dimer atom (μB). Values in bracket for Fe2 are from the 6 × 6 supercell calculations in this work. In Ref. 14 , dimers are adsorbed parallel to the graphene plane.
Coadsorption energies E2H and E2M in units of eV for each stable configuration.
Coadsorbed hydrogen adatom: C–H bond length (Å), tilt angle (°) between C–H bond and normal to the graphene plane, and the puckering of adsorbent carbon atom away from graphene plane (Å).
Coadsorbed dimer magnetic moment for the upper/lower dimer atom (μB), dimer bond length (Å), and angle θ (°) between dimer axis and the normal to the graphene plane. Values in row 2 are for the singly adsorbed dimer. The magnetic moments for the FeCo dimer from 6 × 6 supercell calculations are also included in brackets.
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