Onset of three-dimensional Ir islands on a graphene/Ir(111) template

In agreement with observation, local-density approximation (LDA) optimization of one- and two-layer clusters of Ir atoms, adsorbed periodically on a graphene/Ir(111) moiré, shows that the two-layer clusters only become favorable energetically once the clusters comprise as many as 26 adatoms. Heretofore it was known that the LDA predicts smaller islands to grow flat. In showing that the LDA captures the transition to three dimensionality, the present results support its use broadly to analyze Ir island formation on the graphene-covered metal.

Figure 1

(Color online) Two-dimensional islands containing 7-, 19-, and 26-Ir adatoms on the graphene-on-Ir(111) substrate. The Ir atoms of the first, second, and third metal layers are colored cyan, red, and green. The colors of the C atoms are brown (for the lowest, lying a little more than $2\text{Å}$ above the Ir surface layer) to yellow (for the highest, lying about $1.9\text{Å}$ higher). The spheres representing Ir adatoms are white, with sizes reduced to allow a view of where the adatoms reside. Thus, one can see that the islands of all three panels are adsorbed in the hcp region of the moiré, where red second-layer Ir atoms are visible through the C-atom hexagons (cf. Ref. 13). The black lines in each panel delimit the moiré unit cell.

Figure 2

(Color online) Three-dimensional islands containing 19-, 21-, and 26-Ir adatoms on the graphene-on-Ir(111) substrate. The Ir adatoms of the first, second, and third metal layers are colored cyan, red, and green. The colors of the C atoms are brown (for the lowest lying) to yellow (the highest). The clusters’ first-layer Ir adatoms are white and their second-layer atoms are orange. The black lines in each panel delimit the moiré unit cell.