Density functional theory study of Fe, Co, and Ni adatoms and dimers adsorbed on graphene

Metal clusters have been investigated rather intensely for both fundamental and technological reasons. In this work we report the results of plane-wave density functional theory calculations of Fe, Co, and Ni adatoms and dimers adsorbed on graphene. We study both homonuclear and heteronuclear dimers, and the latter includes mixed dimers of Fe, Co, and Ni along with dimers of these elements with Pt. Our work is motivated by the fundamental interest in their configurational and magnetic properties. We calculated the adsorption site, the structure and relative stabilities of various adsorption configurations, the band structures, the atomic projected electronic density of states, and the magnetic moments of the adatoms and dimers. Contrary to previous work, our results show that adatoms bind weakly to graphene with binding energies ranging from 0.2 to 1.4 eV depending on the adsorption site and species. For both homonuclear and heteronuclear dimers the binding energies per atom are lower than the respective adatom cases, ranging from 0.1 to 0.5 eV per metal atom. The most strongly bound configurations for all the dimers studied are those with the dimer axis (nearly) perpendicular to the graphene plane and bound at the hole site. These configurations, which, to our knowledge, have not been considered in previous work, also turn out to have the largest enhancement of the magnetic moment at least for the atom farther from the graphene. The binding energies of these most strongly bound dimers are dependent on three factors, namely, the interconfigurational energy change in the dimer atom farther from graphene upon desorption, the charge transfer from the dimer to the graphene, and the adsorption site favored by the atom closer to the graphene sheet. The first factor is dominant for all the dimers studied here except for CoPt and NiPt. The relatively high electronegativity of Pt affects the character of the charge transfer from the dimer to graphene. In most of the dimers we investigated, charge is transferred almost exclusively from the dimer atom closer to the graphene except for heteronuclear dimers with Pt where charge is also transferred between the two dimer atoms upon adsorption. Thus, our calculations of the electronic structure allow us to understand the trends in binding energy and the magnetic moment in these dimers.