Figure 1

With ball-model sizes proportional to the atomic radii and balls positioned according to LDA optimization, a side view of a graphene layer pinned to a four-layer Ir(111) slab by a periodic array of four-Ir-atom flat islands. The balls representing Ir atoms are white. Under Ir islands, the C atoms (gray) are in chemical contact with the Ir slab’s top atomic layer, but nowhere else.Reuse & Permissions

Figure 2

Four unit cells of the model graphene moiré on Ir(111) decorated by LDA-optimized: (a) flat four Ir-atom islands and (b) trigonal pyramid islands. Ir atoms are white. C atom heights relative to the average height of the top Ir layer correspond to the grayscale bar. Rings of depressed C atoms surrounding the flat islands of panel a manifest a stronger, induced C interaction with underlying metal than under the pyramidal islands of panel b.Reuse & Permissions

Figure 3

(Color) Left panel: Computed using the LDA and the $3\times 3$ SBZ sample, the graphene/Ir(111) unit cell with a four-atom flat Ir island on it in its lowest energy configuration. First, second, and third substrate layer Ir atoms are colored cyan, red, and green, respectively (cf. the inset side view). This coloring shows that the solid and dotted white circles encompass the hcp and fcc regions of the moiré. Atop regions, which are delimited by the black arcs, are in the cell corners. The balls representing Ir adatoms are white and labeled as $\mathit{\alpha }$, $\mathit{\beta }$, $\mathit{\gamma }$, and $\mathit{\delta }$ with radii reduced to reveal ad-Ir registry atop C atoms in fcc threefold hollows. Local island-induced depression of the graphene layer is shown by the C atom coloring; the color bar numerical scale denotes heights measured from the average for the first Ir atom layer. Right panel: A blow-up of the immediate island binding region, with atoms labeled to facilitate discussion of bond lengths and angles. Among the C atoms, $e$, $f$, $g$, $h$, $i$, $j$, and $k$ lie low and bind to substrate Ir atoms, among them, $X$, $Y$, and $Z$, while $a$, $b$, $c$, and $d$, lie high and bind to Ir adatoms $\mathit{\alpha }$, $\mathit{\beta }$, $\mathit{\gamma }$, and $\mathit{\delta }$. Distances and angles drawn from this figure indicate a locally $s{p}^3$ bonded structure.Reuse & Permissions

Figure 4

Top and side views illustrating an optimized four-atom island geometry in the atop region of the graphene moiré. Small white balls represent Ir adatoms. Gray balls, representing C atoms, lie in threefold hollow sites relative to the substrate. Large white balls visible in the left-hand panel through the rings of the C atoms are the substrate Ir atoms. There is no hint of $s{p}^3$ bonding. The Ir adatoms bridge C atoms. The graphene sheet is unbuckled (contrast Fig. 4), makes no chemical contact with the substrate, and indeed, lies higher above the metal substrate where the island resides.Reuse & Permissions

Figure 5

(Color) Top views of 19 Ir atom islands centered in the fcc (left panel) and hcp (right panel) regions of the graphene/Ir(111) moiré. First, second, and third substrate layer Ir atoms are colored cyan, red and green, respectively (cf. the inset side view in Fig. 3). This coloring shows that the solid and dotted white circles encompass the hcp and fcc regions of the moiré. The balls representing Ir adatoms are white, with radii reduced to reveal the ad-Ir registry atop C-atoms in threefold hollows. The color bar denotes heights measured from the average for the first Ir atom layer.Reuse & Permissions

Figure 6

(Color) Top view of a 19 Ir adatom island, a step away from being centered in the hcp region of the moiré. The color scheme is the same as in Figs. 3, 5. Note the low lying C atoms to the lower left of the island. This “trail” of improved bonding to the substrate is a clue to the characteristics of the best island binding geometries.Reuse & Permissions

Figure 7

An optimized Ir trimer island on the moiré. The small white balls represent Ir adatoms, the gray balls represent C atoms, and the large white balls are substrate Ir atoms. Compare the bonding here to that of the tetramer shown in Fig. 3.Reuse & Permissions

Figure 8

Optimized geometries of an isolated Ir adatom and of an Ir ad-dimer in the hcp region of the moiré. The small white balls represent adatoms, the gray balls represent C atoms, and the large white balls are substrate Ir atoms. Compare the bonding here to that of the tetramer shown in Fig. 3 and the trimer shown in Fig. 7.Reuse & Permissions