Studies of interfacial layers between 4H-SiC (0 0 0 1) and graphene

The region between epitaxial graphene and the SiC substrate has been investigated. 4H-SiC (0 0 0 1) samples were annealed in a high temperature molecular beam epitaxy system at temperatures between 1100 and 1700 °C. The interfacial layers between the pristine SiC and the graphene layers were studied by X-ray photoelectron spectroscopy. Graphene was found to grow on the SiC surface at temperatures above 1200 °C. Below this temperature, however, sp³ bonded carbon layers were formed with a constant atomic Si concentration. C1s and Si2p core level spectra of the graphene samples suggest that the interface layer we observe has a high carbon concentration and its thickness increases during the graphitization process. A significant concentration of Si atoms is trapped in the interface layer and their concentration also increases during graphitization.     

Low-energy excitations of graphene on Ru(0 0 0 1)

The structure and the acoustic phonon branches of graphene on Ru(0 0 0 1) have been experimentally investigated with helium atom scattering (HAS) and analyzed by means of density functional theory (DFT) including Grimme dispersion forces. In-plane interactions are unaffected by the interaction with the substrate. The energy of 16 meV for the vertical rigid vibration of graphene against the Ru(0 0 0 1) surface layer indicates an interlayer effective force constant about five times larger than in graphite. The Rayleigh mode observed for graphene/Ru(0 0 0 1) is almost identical to the one measured on clean Ru(0 0 0 1). This is accounted for by the strong bonding to the substrate, which also explains the previously reported high reflectivity to He atoms of this system. Finally, we report the observation of an additional acoustic branch, closely corresponding to the one already observed by HAS in graphite, which cannot be ascribed to any phonon mode and suggests a possible plasmonic origin.     

Formation of high-quality quasi-free-standing bilayer graphene on SiC(0 0 0 1) by oxygen intercalation upon annealing in air

We report on the conversion of epitaxial monolayer graphene on SiC(0 0 0 1) into decoupled bilayer graphene by performing an annealing step in air. We prove by Raman scattering and photoemission experiments that it has structural and electronic properties that characterize its quasi-free-standing nature. The (6√3 × 6√3)R30° buffer layer underneath the monolayer graphene loses its covalent bonding to the substrate and is converted into a graphene layer due to the oxidation of the SiC surface. The oxygen reacts with the SiC surface without inducing defects in the topmost carbon layers. The high-quality bilayer graphene obtained after air annealing is p-doped and homogeneous over a large area.     

Growth of single and multi-layer graphene on Ir(1 0 0)

We report on the high temperature chemical vapor deposition of ethylene on Ir(1 0 0) and the resulting development of single and multi-layer graphene films. By employing X-ray photoemission electron spectromicroscopy, low energy electron microscopy and related microprobe methods, we investigate nucleation and growth of graphene as a function of the concentration of the chemisorbed carbon lattice gas. Further, we characterize the morphology and crystal structure of graphene as a function of temperature, revealing subtle changes in bonding occurring upon cooling from growth to room temperature. We also identify conditions to grow multi-layer flakes. Their thickness, unambiguously determined through the analysis of the intensity of the Ir 4f and C 1s emission, is correlated to the electron reflectivity at very low kinetic energy. The effective attenuation length of electrons in few-layer graphene is estimated to be 4.4 and 8.4 Å at kinetic energies of 116 and 338 eV, respectively.     

Large area buffer-free graphene on non-polar (0 0 1) cubic silicon carbide

Graphene is, due to its extraordinary properties, a promising material for future electronic applications. A common process for the production of large area epitaxial graphene is a high temperature annealing process of atomically flat surfaces from hexagonal silicon carbide. This procedure is very promising but has the drawback of the formation of a buffer layer consisting of a graphene-like sheet, which is covalently bound to the substrate. This buffer layer degenerates the properties of the graphene above and needs to be avoided.We are presenting the combination of a high temperature process for the graphene production with a newly developed substrate of (0 0 1)-oriented cubic silicon carbide. This combination is a promising candidate to be able to supply large area homogenous epitaxial graphene on silicon carbide without a buffer layer.We are presenting the new substrate and first samples of epitaxial graphene on them. Results are shown using low energy electron microscopy and diffraction, photoelectron angular distribution and X-ray photoemission spectroscopy. All these measurements indicate the successful growth of a buffer free few layer graphene on a cubic silicon carbide surface. On our large area samples also the epitaxial relationship between the cubic substrate and the hexagonal graphene could be clarified.     

Multiscale investigation of graphene layers on 6H-SiC(000-1)

In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-μm scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the μm scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.     

Buffer layer free graphene on SiC(0 0 0 1) via interface oxidation in water vapor

Intercalation of various elements has become a popular technique to decouple the buffer layer of epitaxial graphene on SiC(0 0 0 1) from the substrate. Among many other elements, oxygen can be used to passivate the SiC interface, causing the buffer layer to transform into graphene. Here, we study a gentle oxidation of the interface by annealing buffer layer and monolayer graphene samples in water vapor. X-ray photoelectron spectroscopy demonstrates the decoupling of the buffer layer from the SiC substrate. Raman spectroscopy is utilized to investigate a possible introduction of defects. Angle-resolved photoemission spectroscopy shows that the electronic structure of the water vapor treated samples. Low-energy electron microscopy (LEEM) measurements demonstrate that the decoupling takes place without changes in the surface morphology. The LEEM reflectivity spectra are discussed in terms of two different interpretations.     

Controllable growth of single-layer graphene on a Pd(1 1 1) substrate

Using a surface segregation technique, single-layer graphene can be grown on a carbon-doped Pd(1 1 1) substrate. The growth was monitored and visualized using Auger electron spectroscopy, X-ray photoelectron spectroscopy, Raman microscopy, atomic force microscopy and scanning tunneling microscopy. Appropriate adjustment of annealing parameters enables controllable growth of single-layer graphene islands and homogeneous, wafer-scale, single-layer graphene. The chemical state of the C 1s peak from X-ray photoelectron spectroscopy indicates there is almost no charge transfer between graphene and the Pd(1 1 1) substrate, suggesting weak graphene–substrate interaction. These findings show surface segregation to be an effective method for synthesizing large-scale graphene for fundamental research as well as potential applications.     

Revealing the atomic structure of the buffer layer between SiC(0 0 0 1) and epitaxial graphene

On the SiC(0 0 0 1) surface (the silicon face of SiC), epitaxial graphene is obtained by sublimation of Si from the substrate. The graphene film is separated from the bulk by a carbon-rich interface layer (hereafter called the buffer layer) which in part covalently binds to the substrate. Its structural and electronic properties are currently under debate. In the present work we report scanning tunneling microscopy (STM) studies of the buffer layer and of quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling the buffer layer from the SiC(0 0 0 1) substrate by means of hydrogen intercalation. Atomic resolution STM images of the buffer layer reveal that, within the periodic structural corrugation of this interfacial layer, the arrangement of atoms is topologically identical to that of graphene. After hydrogen intercalation, we show that the resulting QFMLG is relieved from the periodic corrugation and presents no detectable defect sites.     

Mono- and few-layer nanocrystalline graphene grown on Al2O3(0 0 0 1) by molecular beam epitaxy

We report on the growth of nanocrystalline graphene on c-plane Al₂O₃ substrates by molecular beam epitaxy. Graphene films are grown by carbon evaporation from a highly-oriented-pyrolytic-graphite filament and cover the entire surface of two-inch wafers. The structural quality of the material (degree of crystallinity) is investigated in detail by Raman spectroscopy and is revealed to be strongly dependent on the growth temperature and time. We observe that adjacent graphene layers grow parallel to each other and to the substrate surface with domains sizes larger than 30 nm. Transmission electron microscopy confirms the planarity of the nanocrystalline films and X-ray photoelectron spectroscopy proves the predominant sp² nature of the grown layers. Transport measurements reveal that the layers are p-type doped with mobility values up to 140 cm²/Vs at room temperature. The present results demonstrate the potential of molecular beam epitaxy as a technique for realizing the controlled growth of graphene (mono- and few-layer) over large areas directly on an insulating substrate.     

Heteroepitaxial growth and characterization of 3C-SiC films on on-axis Si (1 1 0) substrates by LPCVD

Cubic SiC (3C-SiC) film has been deposited on Si (1 1 0) substrate by the low pressure chemical vapor deposition (LPCVD) with gas sources of SiH₄, C₃H₈ and carrier gas of H₂. The 3C-SiC crystalline film can be confirmed through the observations using reflection high-energy electron diffraction (RHEED) images. The X-ray diffraction (XRD) pattern and the rocking curve indicate that the (1 1 1) plane of SiC film is parallel to the surface of the Si (1 1 0) substrate and the film is of high crystallinity. The results of the field emission scanning electron microscope (FESEM) images show that the film has smooth surface morphology. Transmitted electron diffraction (TED) pattern and high resolution transmission electron microscope (HRTEM) image further confirm the high quality of the film.     

A first-principles study on the role of hydrogen in early stage of graphene growth during the CH4 dissociation on Cu(1 1 1) and Ni(1 1 1) surfaces

The influence of hydrogen for CH₄ dissociation on Cu(1 1 1) and Ni(1 1 1) surfaces has been investigated by using the density functional theory. The two possible reactions, i.e. H-abstraction reaction (CH_x + H → CH_x−1 + H₂) and direct dehydrogenation reaction (CH_x + H → CH_x−1 + 2H), are studied. Our results show that H-abstraction reaction has higher energy barrier than direct dehydrogenation reaction on Cu(1 1 1), while for Ni(1 1 1), only the direct dehydrogenation reaction is observed. The microkinetic analysis supports that H-abstraction reaction is less competitive than the direct dehydrogenation reaction at broad coverage of H atom on Cu(1 1 1) surface. The major intermediate changes from CH to CH₃ on Cu(1 1 1) and Ni(1 1 1) with the increase of H₂ partial pressure. Furthermore, the behavior of free C atoms on both clean and H pre-adsorbed metal surfaces is discussed. The adsorbed H atom hinders the polymerization of the C atoms on Cu(1 1 1), resulting in sufficient time for C relaxed to the most stable site and further lead to a prefect graphene pattern formation, while H atom has little effect on such process for Ni(1 1 1).     

Initial growth of heteroepitaxial diamond on Ir (0 0 1)/MgO (0 0 1) substrates using antenna-edge-type microwave plasma assisted chemical vapor deposition

Initial growth of heteroepitaxial diamond on Ir (0 0 1)/MgO (0 0 1) was investigated by scanning electron microscopy, reflection high-energy electron diffraction (RHEED) and atomic force microscopy. Bias-enhanced nucleation (BEN) was performed by antenna-edge-type microwave plasma assisted chemical vapor deposition. In BEN, diamond crystallites nucleated and grew along the [−1 1 0] and [1 1 0] directions of iridium. Diamond was likely to nucleate on protruded iridium areas. After BEN, in addition to the diamond diffraction spots, iridium bulk diffraction spots, which were not observed before BEN, were observed by RHEED. The iridium surface appeared to be protruded and changed by the high ion current density in BEN. Under [0 0 1] selective growth conditions, diamond crystallites, which were less than 10 nm in diameter, were etched by H₂ plasma. Diamond nucleated areas corresponded to the surface ridges of iridium along the [−1 1 0] and [1 1 0] directions at 10–40 nm intervals before BEN.     

EELS study of the epitaxial graphene/Ni(1 1 1) and graphene/Au/Ni(1 1 1) systems

We have performed electron energy-loss spectroscopy (EELS) studies of Ni(1 1 1), graphene/Ni(1 1 1), and the graphene/Au/Ni(1 1 1) intercalation-like system at different primary electron energies. A reduced parabolic dispersion of the π plasmon excitation for the graphene/Ni(1 1 1) system is observed compared to that for bulk pristine and intercalated graphite and to linear for free graphene, reflecting the strong changes in the electronic structure of graphene on Ni(1 1 1) relative to free-standing graphene. We have also found that intercalation of gold underneath a graphene layer on Ni(1 1 1) leads to the disappearance of the EELS spectral features which are characteristic of the graphene/Ni(1 1 1) interface. At the same time the shift of the π plasmon to the lower loss-energies is observed, indicating the transition of initial system of strongly bonded graphene on Ni(1 1 1) to a quasi free-standing-like graphene state.     

Kinetics and thermodynamics of carbon segregation and graphene growth on Ru(0 0 0 1)

We measure the concentration of carbon adatoms on the Ru(0 0 0 1) surface that are in equilibrium with C atoms in the crystal’s bulk by monitoring the electron reflectivity of the surface while imaging. During cooling from high temperature, C atoms segregate to the Ru surface, causing graphene islands to nucleate. Using low-energy electron microscopy (LEEM), we measure the growth rate of individual graphene islands and, simultaneously, the local concentration of C adatoms on the surface. We find that graphene growth is fed by the supersaturated, two-dimensional gas of C adatoms rather than by direct exchange between the bulk C and the graphene. At long times, the rate at which C diffuses from the bulk to the surface controls the graphene growth rate. The competition among C in three states - dissolved in Ru, as an adatom, and in graphene - is quantified and discussed. The adatom segregation enthalpy determined by applying the simple Langmuir-McLean model to the temperature-dependent equilibrium concentration seriously disagrees with the value calculated from first-principles. This discrepancy suggests that the assumption in the model of non-interacting C is not valid.     

Catalytic conversion of olefins on supported cubic platinum nanoparticles: Selectivity of (1 0 0) versus (1 1 1) surfaces

The surface chemistry and catalytic conversion of cis- and trans-2-butenes on platinum (1 0 0) facets were characterized via surface-science and catalytic experiments. Temperature-programed desorption studies on Pt(1 0 0) single crystals pointed to the higher hydrogenation probability of the trans isomer at the expense of a lower extent of CC double-bond isomerization. To test these trends under catalytic conditions, shape selective catalysts were prepared by dispersing cubic platinum colloidal nanoparticles (which expose only (1 0 0) facets) onto a high-surface-area silica xerogel support. Infrared absorption spectroscopy and transmission electron microscopy were used to determine the conditions needed to remove the organic surfactants without loosing the original narrow size distribution and cubic shape of the original metal nanoparticles. Catalytic kinetic measurements with these materials corroborated the surface-science predictions, and pointed to a switch in isomerization selectivity from preferential cis-to-trans conversion with Pt(1 0 0) surfaces to the reverse trans-to-cis reaction with Pt(1 1 1) facets.     

Surface chemistry of Ni induced graphite formation on the 6H-SiC (0 0 0 1) surface and its implications for graphene synthesis

Ni films ranging in thickness from 0.4 nm to 50 nm were deposited by evaporation onto terraced SiC (0 0 0 1) substrates at room temperature and annealed at 700 °C. The resulting changes in surface composition and morphology were characterized using Auger electron spectroscopy and atomic force microscopy. In all cases, graphitic films dominate the surface chemistry. There appears to be three different thickness dependent morphology regimes. For the thinnest Ni films (0.4 nm), there is a uniform carbon-overlayer. For slightly thicker Ni films (0.6-9.6 nm), clustering and platelet formation are observed, and for still thicker films (50 nm), the platelets give way to hillocks. Within the platelet regime, there is a critical thickness at which surface roughening occurs. These results reveal a potential parametric window in which graphene may be produced and harvested.