Johann Coraux

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.

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.

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.

Growth of epitaxial graphene: Theory and experiment

A detailed review of the literature for the last 5-10 years on epitaxial growth of graphene is presented. Both experimental and theoretical aspects related to growth on transition metals and on silicon carbide are thoroughly reviewed. Thermodynamic and kinetic aspects of growth on all these materials, where possible, are discussed. To make this text useful for a wider audience, a range of important experimental techniques that have been used over the last decade to grow (e.g. CVD, TPG and segregation) and characterize (STM, LEEM, etc.) graphene are reviewed, and a critical survey of the most important theoretical techniques is given. Finally, we critically discuss various unsolved problems related to growth and its mechanism which we believe require proper attention in future research.

In?situ observation of stress relaxation in epitaxial graphene

Upon cooling, branched line defects develop in epitaxial graphene grown at high temperature on Pt(111) and Ir(111). Using atomically resolved scanning tunneling microscopy we demonstrate that these defects are wrinkles in the graphene layer, i.e. stripes of partially delaminated graphene. With low energy electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ. Upon temperature cycling we observe hysteresis in the appearance and disappearance of the wrinkles. Simultaneously with wrinkle formation a change in bright field imaging intensity of adjacent areas and a shift in the moire spot positions for micro diffraction of such areas takes place. The stress relieved by wrinkle formation results from the mismatch in thermal expansion coefficients of graphene and the substrate. A simple one-dimensional model taking into account the energies related to strain, delamination and bending of graphene is in qualitative agreement with our observations.

Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire

Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e., much higher than on metal thin films used so far.

Selecting a single orientation for millimeter sized graphene sheets

We have used low energy electron microscopy and photo emission electron microscopy to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99% composed of aligned domains. Precovering the substrate with a high density of graphene nuclei prior to CVD yields pure films of aligned domains extending over millimeters. Such films can be used to prepare cluster-graphene hybrid materials for catalysis or nanomagnetism and can potentially be combined with lift-off techniques to yield high-quality, graphene based, electronic devices.