Thomas Michely

Structural coherency of graphene on Ir(111).

Low-pressure chemical vapor deposition allows one to grow high structural quality monolayer graphene on Ir(111). Using scanning tunneling microscopy, we show that graphene prepared this way exhibits remarkably large-scale continuity of its carbon rows over terraces and step edges. The graphene layer contains only a very low density of defects. These are zero-dimensional defects, edge dislocation cores consisting of heptagon-pentagon pairs of carbon atom rings, which we relate to small-angle in-plane tilt boundaries in the graphene. We quantitatively examined the bending of graphene across Ir step edges. The corresponding radius of curvature compares to typical radii of thin single-wall carbon nanotubes.

Two-dimensional Ir cluster lattice on a graphene Moiré on Ir(111)

Lattices of Ir clusters have been grown by vapor phase deposition on graphene moirés on Ir(111). The clusters are highly ordered, and spatially and thermally stable below 500 K. Their narrow size distribution is tunable from 4 to about 130 atoms. A model for cluster binding to the graphene is presented based on scanning tunneling microscopy and density functional theory. The proposed binding mechanism suggests that similar cluster lattices might be grown of materials other than Ir.

STM investigation of single layer graphite structures produced on Pt(111) by hydrocarbon decomposition

STM has provided new insight into the nucleation, growth and nature of the graphite layer formed on Pt(111) by hydrocarbon decomposition. Annealing an ethylene covered surface to 800 K results in the formation of small (about 20–30 A in diameter) graphite islands which are initially uniformly distributed over the surface. With further annealing above 1000 K, the graphite is observed to accumulate, forming a layer at the lower step edges and also forming large, regularly shaped islands on the terraces. It has been determined that hydrocarbon decomposition at elevated temperatures results in formation of a single layer of graphite on the Pt surface. It is interesting to note that in the STM images of this single layer of graphite only three of the six carbon atoms in the graphite lattice are visible. This result cannot be accounted for by the usual explanation given in terms of inequivalent carbons atoms for the bulk graphite surface. Superstructures with periodicities varying up to 22 A are evident on the graphite areas and are due to a higher order commensurability of the graphite and Pt lattices at different relative rotations.

Growth of graphene on Ir(111)

Catalytic decomposition of hydrocarbons on transition metals attracts a renewed interest as a route toward high-quality graphene prepared in a reproducible manner. Here we employ two growth methods for graphene on Ir(111), namely room temperature adsorption and thermal decomposition at 870–1470 K (temperature programmed growth (TPG)) as well as direct exposure of the hot substrate at 870–1320 K (chemical vapor deposition (CVD)). The temperature- and exposure-dependent growth of graphene is investigated in detail by scanning tunneling microscopy. TPG is found to yield compact graphene islands bounded by C zigzag edges. The island size may be tuned from a few to a couple of tens of nanometers through Smoluchowski ripening. In the CVD growth, the carbon in ethene molecules arriving on the Ir surface is found to convert with probability near unity to graphene. The temperature-dependent nucleation, interaction with steps and coalescence of graphene islands are analyzed and a consistent model for CVD growth is developed.

Structure of epitaxial graphene on Ir(111)

A graphene monolayer has been prepared on an Ir(111) single crystal via pyrolytic cleavage of ethylene (C2H4). The resulting superstructure has been examined with scanning tunneling microscopy (STM) and low energy electron diffraction. It has been identified as a well aligned, incommensurate (9.32?9.32) pattern, which is described as a moir?. This pattern shows three distinct regions resulting from different local configurations of the carbon adlayer with respect to the Ir-substrate. These regions are imaged differently by STM and differ strongly in their ability to bind metal deposits.

Dirac Cones and Minigaps for Graphene on Ir(111)

Epitaxial graphene on Ir(111) prepared in excellent structural quality is investigated by angle-resolved photoelectron spectroscopy. It clearly displays a Dirac cone with the Dirac point shifted only slightly above the Fermi level. The moiré resulting from the overlaid graphene and Ir(111) surface lattices imposes a superperiodic potential giving rise to Dirac cone replicas and the opening of minigaps in the band structure.

Graphene on Ir(111): physisorption with chemical modulation.

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.

The homoepitaxial growth of Pt on Pt(111) studied with STM

The homoepitaxial growth of Pt on Pt(111) has been investigated by STM and the results have been compared to recent thermal He scattering (TEAS) data obtained on the same system. Additional information on the growth modes is obtained and the real space aspect of the growing surface, which results in TEAS and RHEED oscillations is evidenced. The three different growth modes, including the reentrant layer-by-layer growth at low temperatures, are confirmed. The limited diffusion along the adatom island edges, which causes their fractal aspect with dendritic structures, appears to play a significant role in the appearance of the low temperature layer-by-layer growth.

Temperature dependence of the sputtering morphology of Pt(111)

Abstract The morphology of the Pt(111) surface after sputtering with 600 eV Ar + ions as a function of the annealing temperature, during and after sputtering, has been investigated by scanning tunneling microscopy. The data confirm the transition from pit formation to layer-by-layer removal regime and emphasize the role of thermally created adatoms and/or vacancies in the mass transport between atomic layers. The measurements show further that the shape of the vacancy islands upon annealing above 700 K is equilibrium-like. The application of the Wulff construction to the two-dimensional vacancy islands allows the determination of the ratio of the free energies of the two types of step edges which bound the islands.

A versatile fabrication method for cluster superlattices

On the graphene moire on Ir(111) a variety of highly perfect cluster superlattices can be grown as shown 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 and 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 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.