Orbital selective coupling between Ni adatoms and graphene Dirac electrons

We investigate the coupling of Ni monomers and trimers to graphene by means of atomically resolved scanning tunneling microscopy (STM). The precise adsorption site of the adatoms is determined experimentally. STM images reveal characteristic nodal structures above the Ni adatoms and trimers on graphene. First-principles calculations combined with symmetry considerations explain our experimental results by an orbitally controlled interaction of the adatoms and clusters with the Dirac electrons in graphene.

Figure 1

(a) Topographic image of Ni monomers on graphene. Tunneling parameters: $U=200$ mV and $I=0.3$ nA. (b) STM topography indicating the adatom's position on graphene. $U=200$ mV and $I=0.3$ nA. The scan size is $3\mathrm{nm}\times 3$ nm. (c) Simulated LDOS in the vacuum above a single Ni adatom on graphene (left) and adsorption geometry of the adatom (right). The energy range between the Fermi level and 0.1 eV has been integrated. The LDOS has been calculated at a height of 4.0 Å above the graphene sheet, which corresponds to 2.4 Å above the adatom.

Figure 2

(a) STS spectra measured on the center of the Ni adatom and bare graphene. Stabilization parameters: $U=1$ V and $I=0.4$ nA. (b) Simulation of the electronic structure of Ni monomers in different electronic configurations performed in GGA and GGA + U. For the calculations shown here the Ni atoms were placed above the center of a carbon hexagon and an electron doping level of $2\times {10}^{13}$ cm${}^{-2}$ has been taken into account in the LDOS simulations.

Figure 3

(a) STS spectra of Ni adatom, adatom's edge, and graphene. Tunneling parameters are $U=150$ mV and $I=0.3$ nA. The inset shows a topographic image of graphene coated by Ni adatoms ($U=150$ mV and $I=0.3$ nA). (b) and (c) are spatial maps of the tunneling conductance of the same area presented in the inset of (a). The inset in (b) presents a magnified $\mathrm{d}I/\mathrm{d}U$ map of an adatom and the locations where the spectra were measured. The scan size is $1.7\text{nm}\times 1.4$ nm.

Figure 4

(a) Constant current image of a Ni trimer and corresponding atomic structure in (d) ($U=150$ mV and $I=0.3$ nA). (b) and (c) $\mathrm{d}I/\mathrm{d}U$ maps of the Ni trimer taken at $-$55 and $-$1.0 meV, respectively. (e) and (f) calculated LDOS maps on the trimer for A- and E-symmetry states.