Geometry and electronic structure of iridium adsorbed on graphene

We report investigation of the geometry and electronic structure of iridium atoms adsorbed onto graphene through a combined experimental and theoretical study. Ir atoms were deposited onto a flake of graphene on a Pt(111) surface and found to form clusters even at low temperatures. The areal density of the observed clusters on the graphene flake suggests the clusters are most likely pairs of Ir atoms. Theoretical ab initio density functional (DFT) calculations indicate that these Ir dimers are oriented horizontally, near neighboring “bridge” sites of the graphene lattice, as this configuration has the strongest adsorption energy of all high-symmetry configurations for the Ir dimer. A large peak in the local density of states (LDOS) at the Dirac point energy was measured via scanning tunneling spectroscopy, and this result is reproduced by a DFT calculation of the LDOS. The peak at the Dirac point energy is found to be from the Ir $s$ and $p$ states. The LDOS in the monomer case was also calculated, and is found to significantly differ from the experimentally determined data, further supporting the hypothesis of low-temperature clustering.

Figure 1

(a) STM topography of Ir clusters on bare Pt(111) and graphene/Pt(111). The graphene flake is within the green dashed contour, and the Ir clusters under consideration are within the yellow dotted rectangle ($V_{\mathrm{s}}=0.33$ V, $I=1$ nA). (b) The $dI/dV$ map of the same region acquired simultaneously with (a). (c) $dI/dV$ spectra for the Ir clusters indicated in the top-left inset. The top-right inset is the $dI/dV$ spectrum for the bare graphene flake away from the Ir clusters ($dI/dV$ parameters: $V_{\mathrm{rms}}=4$ mV, wiggle freq. = 2140 Hz, initial $I=1$ nA). The estimated Dirac point is indicated by the vertical dashed red line. (d) The number of clusters as a function of the area of the graphene island. The black dash-dotted line has a slope with a density corresponding to one Ir atom per cluster. The red dashed line corresponds to two Ir atoms per cluster.

Figure 2

Sketch of different adsorption sites: bridge ($B^1$, $B^2$), hollow ($H^1$, $H^2$), and top ($T^1$, $T^2$). In the monomer and vertical dimer cases, only $B^1$, $H^1$, and $T^1$ are considered and are referred to as $B$, $H$, and $T$, respectively.

Figure 3

Vertical configuration of the Ir dimer over the hollow site. The distance of the bottom-most Ir atom to the graphene plane is $h_b$.

Figure 4

Comparison between the calculated DOS for pristine graphene and for a single Ir adatom on graphene, in a $4\times 4$ supercell. The DOS for the supercell is averaged over the primitive cells.

Figure 5

The calculated LDOS for a single Ir adatom on graphene, calculated 4 Å above the Ir atom.

Figure 6

The calculated PDOS for a single Ir adatom on graphene at energies relative to the Dirac point energy.

Figure 7

Comparison of the calculated DOS for a pristine sheet of graphene and an Ir dimer adsorbed onto graphene, in a $5\times 5$ supercell. The DOS for the supercell is averaged over the primitive cells.

Figure 8

The calculated LDOS 4 Å above an Ir dimer on graphene at energies relative to the Dirac point energy.

Figure 9

The calculated PDOS for an Ir dimer on graphene at energies relative to the Dirac point energy.

Figure 10

The calculated PDOS from the sum of Ir $6s$ and $6p$ orbitals, compared to the total LDOS for an Ir dimer on graphene at energies relative to the Dirac point energy. The curves are normalized so that the central peaks of the LDOS and PDOS have equal heights.

Figure 11

Comparison of experimentally determined $dI/dV$ for an Ir cluster on graphene/Pt(111) (solid black curve) and calculated LDOS for an Ir dimer on graphene (dashed red curve) at energies relative to the Dirac point energy.