The nonlocal van der Waals density functional approach is applied to calculate the binding of graphene to Ir(111). The precise agreement of the calculated mean height h = 3.41  Å of the C atoms with their mean height h = (3.38±0.04)  Å as measured by the x-ray standing wave technique provides a benchmark for the applicability of the nonlocal functional. We find bonding of graphene to Ir(111) to be due to the van der Waals interaction with an antibonding average contribution from chemical interaction. Despite its globally repulsive character, in certain areas of the large graphene moiré unit cell charge accumulation between Ir substrate and graphene C atoms is observed, signaling a weak covalent bond formation.

On the graphene moiré on Ir(111) a variety of highly perfect cluster superlattices can be grown as shown is for Ir, Pt, W, and Re. Even materials that do not form cluster superlattices upon room temperature deposition may be grown into such by low temperature deposition or the application of cluster seeding through Ir as shown for Au, AuIr, FeIr. Criteria for the suitability of a material to form a superlattice are given and largely confirmed. It is proven that at least Pt and Ir even form epitaxial cluster superlattices. The temperature stability of the cluster superlattices is investigated and understood on the basis of positional fluctuations of the clusters around their sites of minimum potential energy. The binding sites of Ir, Pt, W and Re cluster superlattices are determined and the ability to cover samples macroscopically with a variety of superlattices is demonstrated.

Using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) we resolve the temperature-, time-, and flake size-dependent intercalation phases of oxygen underneath graphene on Ir(111) formed upon exposure to molecular oxygen. Through the applied pressure of molecular oxygen the atomic oxygen created on the bare Ir terraces is driven underneath graphene flakes. The importance of substrate steps and of the unbinding of graphene flake edges from the substrate for the intercalation is identified. With the use of CO titration to selectively remove oxygen from the bare Ir terraces the energetics of intercalation is uncovered. Cluster decoration techniques are used as an efficient tool to visualize intercalation processes in real space.

Here we show that it is possible to intercalate CO under graphene grown on Ir(111) already at room temperature when CO pressures in the millibar regime are used. From the interplay of X-ray photoelectron spectroscopy and scanning tunneling microscopy we conclude that the intercalated CO adsorption structure is similar to the (3√3 × 3√3)R30°) adsorption structure that is formed on Ir(111) upon exposure to ∼1 mbar of CO. Further, density functional theory calculations reveal that the structural and electronic properties of CO-intercalated graphene are similar to p-doped freestanding graphene. Finally we characterize nonintercalated stripes and islands that we always observe in the CO-intercalated graphene. We observe these nonintercalated areas predominately in HCP and FCC areas near step edges and suggest that stress release in graphene is the driving force for their formation, while the weak chemical bonds in HCP and FCC areas are the reason for their area selectivity.

Regular Pt cluster arrays grown on the moiré template formed by graphene on Ir(111) were tested for their stability with respect to CO gas exposure. Cluster stability and adsorption-induced processes were analyzed as a function of cluster size, with in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Small clusters containing fewer than 10 atoms were unstable upon CO adsorption. They sintered through Smoluchowski ripening-cluster diffusion and coalescence-rather than the frequently reported Ostwald ripening mediated by metal-adsorbate complexes. Larger clusters remained immobile upon CO adsorption but became more three-dimensional. Careful analysis of the experimental data complemented by ab initio density functional theory calculations provides insight into the origin of the CO-induced Pt cluster ripening and shape transformations.