Atomic Structure and Crystalline Order of Graphene-Supported Ir Nanoparticle Lattices

We present the atomic structure of Ir nanoparticles with 1.5 nm diameter at half height and three layers average height grown on $\mathrm{\text{graphene}}/\mathrm{Ir}(111)$. Using surface x-ray diffraction, we demonstrate that Ir nanoparticles on $\mathrm{\text{graphene}}/\mathrm{Ir}(111)$ form a crystallographic superlattice with high perfection. The superlattice arrangement allows us to obtain detailed information on the atomic structure of the nanoparticles themselves, such as size, shape, internal layer stacking and strain. Our experiments disclose that the nanoparticles reside epitaxially on top of the graphene moiré structure on Ir(111), resulting in significant lateral compressive intraparticle strain. Normal incidence x-ray standing wave experiments deliver additional information on the particle formation induced restructuring of the graphene layer.

Figure 1

(a) Reciprocal lattice sketch: thick (red) lines: crystal truncation rods (CTRs) from the Ir substrate (bulk Bragg reflections are indicated by closed circles), thin (black) lines: graphene rods, (blue) ellipses: reciprocal lattice positions, where additional in-plane reflections were observed after Ir deposition. (b) LEED pattern at 65 eV from the pristine graphene layer. (c) reciprocal space map of the ($H=0$, $K$, $L$) plane before (left) and after (right) Ir deposition. (d) $K$ scans at $L=0.85$ along the dashed lines in (c). (Red) open circles: before Ir deposition, (blue) filled circles: after Ir deposition. The dashed line is a fit of the broad component (see text).

Figure 2

Experimental x-ray structure factor magnitudes (open circles) together with error bars. The solid and dashed lines represent fits to the data and model calculations, respectively, as discussed in the text.

Figure 3

(a) Top view of the structural model, the moiré unit cell is indicated by the white line. High symmetry sites are marked, where the center of the carbon rings is located either at an fcc or on-top site with respect to the substrate. The center of the nanoparticles is located at the hcp position of the carbon rings. The $ACB$ stacked particles are rotated by 60° with respect to the $ABC$ stacked particles (not shown). (b) Cross-sectional side view of the particles together with relaxation values. Not all C atoms are shown for clarity. The Ir(111) bulk layer spacing is 2.216Å.

Figure 4

C 1s XSW photoelectron yields of 0.63 ML C/Ir(111) normalized to the yield without Bragg reflected beam (dashed lines). Open circles: pristine graphene; filled circles: graphene $+0.9$ ML Ir; squares: normalized reflectivity. The curves are shifted for clarity. Solid lines represent fits to the data. Inset: C 1s core level line. Before (after) Ir deposition: open (filled) circles, respectively. Solid lines: fit with one Gaussian, Shirley background subtracted.