Amina Kimouche

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.

Homogeneous Optical and Electronic Properties of Graphene Due to the Suppression of Multilayer Patches During CVD on Copper Foils

By limiting the carbon segregation at the copper surface defects, a pulsed chemical vapor deposition method for single layer graphene growth is shown to inhibit the formation of few-layer regions, leading to a fully single-layered graphene homogeneous at the centimeter scale. Graphene field-effect devices obtained after transfer of pulsed grown graphene on oxidized silicon exhibit mobilities above 5000 cm^2.V^-1.s^-1.

Strain Relaxation in CVD Graphene: Wrinkling with Shear Lag

We measure uniaxial strain fields in the vicinity of edges and wrinkles in graphene prepared by chemical vapor deposition (CVD), by combining microscopy techniques and local vibrational characterization. These strain fields have magnitudes of several tenths of a percent and extend across micrometer distances. The nonlinear shear-lag model remarkably captures these strain fields in terms of the graphene-substrate interaction and provides a complete understanding of strain-relieving wrinkles in graphene for any level of graphene-substrate coherency.

Cobalt intercalation at the graphene/iridium(111) interface: influence of rotational domains, wrinkles and atomic steps

Using low-energy electron microscopy, we study Co intercalation under graphene grown on Ir(111). Depending on the rotational domain of graphene on which it is deposited, Co is found intercalated at different locations. While intercalated Co is observed preferentially at the substrate step edges below certain rotational domains, it is mostly found close to wrinkles below other domains. These results indicate that curved regions (near substrate atomic steps and wrinkles) of the graphene sheet facilitate Co intercalation and suggest that the strength of the graphene/Ir interaction determines which pathway is energetically more favorable.

Disorder and screening in decoupled graphene on a metallic substrate

Graphene on a dielectric substrate exhibits spatial doping inhomogeneities, forming electronhole puddles. Understanding and controlling the latter is of crucial importance for unraveling many of graphene’s fundamental properties at the Dirac point. Here we show the coexistence and correlation of charge puddles and topographic ripples in graphene decoupled from the metallic substrate it was grown on. The analysis of interferences of Dirac fermion-like electrons yields a linear dispersion relation, indicating that graphene on a metal can recover its intrinsic electronic properties. 1 ar X iv :1 30 4. 11 83 v3 [ co nd -m at .m es -h al l] 1 J ul 2 01 4 The study of electron-hole puddles in graphene has so far relied on graphene sheets isolated by mechanical exfoliation of graphite on dielectric substrates such as SiO2. The origin of these has been subject to debate, as different studies have pointed to either charged impurities between graphene and SiO2 as sources of the puddles , others invoking in addition the mixing of the π and σ orbitals due to local curvature. In this context, the limited knowledge about the graphene/SiO2 interface and the ensuing low graphene mobility calls for the use of other substrates. More generally, experiments based on different dielectric environments, that is, different strengths of charged impurities’ screening, have been performed. They however showed no significant influence of the substrate dielectric constant on the graphene electronic properties, thereby questioning the role of charged impurities for the puddle formation (see also). On metallic supports, the origin of charge disorder might be very different. Periodic ripples, arising from the lattice parameter mismatch between graphene and most transition metal surfaces, were for instance related to the puddles in graphene on Ru(0001). It however turned out that charge carriers in this system do not exhibit Dirac fermion-like properties, due to a strong hybridization between the 4d Ru and pz C orbitals . Even in less strongly coupled systems, interaction between graphene’s conduction/valence bands with surface states of the metal cannot be excluded. A linear dispersion relation in the electronic band structure at the Brillouin zone corners was recovered in graphene on Ru(0001) intercalated with an atomic layer of oxygen below the graphene. This layer unfortunately suppresses the graphene ripples, which prevents from addressing the possible relationship between puddles and topography. In this letter we report on a STM/STS study of corrugated graphene lying on an Ir metallic substrate. The analysis of the quasi-particle interference pattern reveals the linear dispersion relation of the graphene band structure, and demonstrates the absence of hybridization with the Ir substrate. Despite the immediate proximity of the metal that acts as an electrostatic screening plate, we observe electron-hole puddles close to the charge neutrality point, akin to those observed on graphene on dielectric substrates. Furthermore, the topographic images of the ripples exhibit strong spatial correlations with charge density inhomogeneities, suggesting electron-hole puddles of a new origin. The studied sample is graphene prepared by chemical vapor deposition on epitaxial Ir(111) under ultra-high vacuum (UHV), as described in. Exposure to air can decou-

Intercalating cobalt between graphene and iridium (111): Spatially dependent kinetics from the edges

Using low-energy electron microscopy, we image in real time the intercalation of a cobalt monolayer between graphene and the (111) surface of iridium. Our measurements reveal that the edges of a graphene flake represent an energy barrier to intercalation. Based on a simple description of the growth kinetics, we estimate this energy barrier and find small, but substantial, local variations. These local variations suggest a possible influence of the graphene orientation with respect to its substrate and of the graphene edge termination on the energy value of the barrier height. Besides, our measurements show that intercalated cobalt is energetically more favorable than cobalt on bare iridium, indicating a surfactant role of graphene.

Graphene as a Mechanically Active, Deformable Two-Dimensional Surfactant

In crystal growth, surfactants are additive molecules used in dilute amount or as dense, permeable layers to control surface morphologies. We investigate the properties of a strikingly different surfactant: a 2D and covalent layer with close atomic packing, graphene. Using in situ, real-time electron microscopy, scanning tunneling microscopy, kinetic Monte Carlo simulations, and continuum mechanics calculations, we reveal why metallic atomic layers can grow in a 2D manner below an impermeable graphene membrane. Upon metal growth, graphene dynamically opens nanochannels called wrinkles, facilitating mass transport while at the same time storing and releasing elastic energy via lattice distortions. Graphene thus behaves as a mechanically active, deformable surfactant. The wrinkle-driven mass transport of the metallic layer intercalated between graphene and the substrate is observed for two graphene-based systems, characterized by different physicochemical interactions, between graphene and the substrate and between the intercalated material and graphene. The deformable surfactant character of graphene that we unveil should then apply to a broad variety of species, opening new avenues for using graphene as a 2D surfactant forcing the growth of flat films, nanostructures, and unconventional crystalline phases.