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.     

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.     

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.     

Epitaxial growth of large-area single-layer graphene over Cu(1 1 1)/sapphire by atmospheric pressure CVD

We report the atmospheric pressure chemical vapor deposition (CVD) growth of single-layer graphene over a crystalline Cu(1 1 1) film heteroepitaxially deposited on c-plane sapphire. Orientation-controlled, epitaxial single-layer graphene is achieved over the Cu(1 1 1) film on sapphire, while a polycrystalline Cu film deposited on a Si wafer gives non-uniform graphene with multi-layer flakes. Moreover, the CVD temperature is found to affect the quality and orientation of graphene grown on the Cu/sapphire substrates. The CVD growth at 1000 °C gives high-quality epitaxial single-layer graphene whose orientation of hexagonal lattice matches with the Cu(1 1 1) lattice which is determined by the sapphire’s crystallographic direction. At lower CVD temperature of 900 °C, low-quality graphene with enhanced Raman D band is obtained, and it showed two different orientations of the hexagonal lattice; one matches with the Cu lattice and another rotated by 30°. Carbon isotope-labeling experiment indicates rapid exchange of the surface-adsorbed and gas-supplied carbon atoms at the higher temperature, resulting in the highly crystallized graphene with energetically most stable orientation consistent with the underlying Cu(1 1 1) lattice.     

Evolution of epitaxial graphene layers on 3C SiC/Si (1 1 1) as a function of annealing temperature in UHV

The growth of graphene on SiC/Si substrates is an appealing alternative to the growth on bulk SiC for cost reduction and to better integrate the material with Si based electronic devices. In this paper, we present a thorough in situ study of the growth of epitaxial graphene on 3C SiC (1 1 1)/Si (1 1 1) substrates via high temperature annealing (ranging from 1125 to 1375 °C) in ultra high vacuum (UHV). The quality and number of graphene layers have been investigated by using X-ray Photoelectron Spectroscopy (XPS), while the surface characterization have been studied by Scanning Tunnelling Microscopy (STM). Ex-situ Raman spectroscopy measurements confirm our findings, which demonstrate the exponential dependence of the number of graphene layers on the annealing temperature.     

Spin polarization of single-layer graphene epitaxially grown on Ni(1 1 1) thin film

The spin-resolved electronic structure of graphene on Ni(1 1 1) was investigated using spin-polarized metastable deexcitation spectroscopy (SPMDS). Graphene was grown epitaxially on a Ni(1 1 1) single-crystalline surface using the ultra high vacuum chemical vapor deposition technique with benzene vapor as a precursor. At 50 L (5 × 10⁻⁵ Torr s), a single epitaxial layer of graphene was formed, but no further growth was observed at higher exposure. The spin-summed spectrum of graphene/Ni(1 1 1) had a new peak at the Fermi level and three weak features corresponding to the molecular orbitals of graphene. Spin asymmetry analysis of the SPMDS spectra revealed that the spin polarization of the electronic states shown by the new peak was parallel to the majority spin of the Ni substrate. The appearance and spin polarization of the new electronic states are discussed in terms of the hybridization of graphene π orbitals and Ni d orbitals.     

Combined UHV and ambient pressure studies of 1,3-butadiene adsorption and reaction on Pd(1 1 1) by GC,IRAS and XPS

The hydrogenation of 1,3-butadiene on Pd(1 1 1) at 300 K was studied at atmospheric pressure by infrared reflection absorption spectroscopy (IRAS) and gas chromatography (GC). Kinetic measurements showed 1-butene, trans-2-butene and cis-2-butene as primary products. Once 1,3-butadiene had been completely consumed, 1-butene was re-adsorbed on the surface producing trans-/cis-2-butene through isomerization and n-butane through hydrogenation. These results were corroborated by in situ IRAS spectroscopy. Post-reaction analysis by X-ray photoelectron spectroscopy (XPS) in the C1s region revealed a band at 284.2 eV, corresponding to adsorbed butadiene and/or carbonaceous deposits. Quantification of this peak revealed a total carbon coverage of 0.3 ML. Nevertheless, deactivation due to carbon deposition was a minor effect under our reaction conditions, as indicated by the kinetics of the subsequent butene hydrogenation reaction. Temperature-dependent XPS experiments after butadiene adsorption at 100 K indicated a high stability of the diene molecule with hardly any desorption and/or decomposition up to 500 K. Above this temperature, butadiene decomposed to carbon species that eventually dissolved in the Pd bulk above 700 K.     

Growth of silicon carbide on hydrogen-terminated Si (001) 1 × 1 surfaces using dimethyl- and monomethyl silane

A problem of long standing in the high-temperature growth of 3C–SiC on Si has been the formation of pits at the SiC/Si interface. The research described in this paper addresses this problem through the use of organosilicon growth precursors and explores issues related to the mechanisms of pit formation. Specifically, this paper reports studies of 3C–SiC growth at 800 °C on hydrogen-terminated Si (001) 1 × 1 surfaces using dimethyl- and monomethyl silane under gas source molecular beam epitaxy conditions. In-situ analysis of the films by Auger electron spectroscopy and reflection high-energy electron diffraction along with ex-situ scanning electron microscopy and atomic force microscopy were used to gain insight concerning the details of the growth mechanism. The experimental variables, in addition to the growth species, included molecular flux and growth time. For growth using both dimethyl- and monomethyl silane, clear evidence for the out-diffusion, segregation, and participation of substrate Si in the growth process was found. The molecular flux at the substrate surface was found to play a key role in determining the microstructure of the initial SiC layers and the SiC/Si interface. The effective sticking coefficient for both gas species was found to be on the order of ∼ 0.13. Conditions under which interfacial pits may be eliminated are identified.     

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.     

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.     

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.     

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.     

A simple method to synthesize graphene at 633 K by dechlorination of hexachlorobenzene on Cu foils

A modified chemical vapor deposition method to synthesize graphene at 360 °C is described. Hexachlorobenzene (HCB) was used as carbon source, and copper foils were used as not only the substrates for graphene deposition but also the catalyst to HCB dechlorination. The possible growth mechanism was investigated using X-ray photoelectron spectroscopy. Enhancement of HCB dechlorination by copper played a key role in synthesis of graphene at such a low temperature.     

The production of large bilayer hexagonal graphene domains by a two-step growth process of segregation and surface-catalytic chemical vapor deposition

A novel two-step process combining surface catalyzed process with segregation growth was used to prepare single crystal hexagonal bilayer graphene domains on Cu metal substrates by ambient pressure chemical vapor deposition. Carbon atoms are first dissolved into the quasi-melting Cu metal at 1080 °C and then segregated on the Cu surface to form nucleation centers of single-layer graphene during cooling. The graphene crystallites spontaneously act as templates to induce the carbon atoms to form hexagonal bilayer graphene domains. The bilayer graphene domains are size-tunable by controlling the growth conditions. The yield of the bilayer graphene is over 90% and the defect-free domains reach ～100 μm in size, greater than the reported single-layer domains.     

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.     

Ethylene conversion to ethylidyne on Pd(1 1 1) and Pt(1 1 1): A first-principles-based kinetic Monte Carlo study

We present kinetic Monte Carlo simulations of ethylene conversion to ethylidyne on Pd(1 1 1) and Pt(1 1 1) surfaces, on the basis of reaction enthalpies and barriers obtained from periodic density functional calculations. We considered three possible mechanisms encompassing four different intermediates, ethyl, vinyl, ethylidene, and vinylidene. Our simulations predict that the most plausible pathway on both surfaces is ethylene → vinyl → vinylidene → ethylidyne. In contrast to earlier suggestions that the dehydrogenation to vinyl is rate-limiting on Pt(1 1 1), we found the hydrogenation of vinylidene to ethylidyne to be crucial on this surface. On Pd(1 1 1), the initial dehydrogenation of ethylene is rate-limiting. Hence, vinylidene species accumulate on Pt(1 1 1), while all intermediates on Pd(1 1 1) convert rapidly to ethylidyne without accumulation. The simulated apparent activation energies for the formation of ethylidyne on Pd(1 1 1), 94 kJ mol^?1, and on Pt(1 1 1), 65 kJ mol^?1, agree well with experimental results.     

Synthesis of palladium nanoparticles on carbon nanotubes and graphene for the chemoselective hydrogenation of para-chloronitrobenzene

We have studied the synthesis of palladium nanoparticles over carbon nanotubes (Pd/CNT) and graphene (Pd/G) and we have tested their catalytic performance in the liquid phase chemoselective hydrogenation of para-chloronitrobenzene at room temperature. The catalysts were characterized by N₂ adsorption/desorption isotherms, TEM, X-ray diffraction, infrared and X-ray photoelectron spectroscopy and ICP-OES. The palladium particle size on Pd/G (3.4 nm) and Pd/CNT (2.8 nm) was similar though the deposition was higher on Pd/G. Pd/CNT was more active which can be ascribed to the different surface area and electronic properties of the Pd nanoparticles over CNT, while the selectivity was 100% to the corresponding haloaniline over both catalysts and they were quite stable upon recycling.     

Suzuki–Miyaura reaction catalyzed by graphene oxide supported palladium nanoparticles

Pd supported on polyamine modified graphene oxide (GO-NH₂-Pd^2 +) was fabricated for the first time. The prepared catalyst was characterized by transmission electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy and infrared spectroscopy. The catalytic activity of the prepared catalyst was investigated by employing Suzuki–Miyaura coupling reaction as a model reaction. A series of biphenyl compounds were synthesized through the Suzuki–Miyaura reaction using GO-NH₂-Pd^2 + as catalyst. The yields of the products were in the range from 71% to 95%. The catalyst can be readily recovered and reused at least 10 consecutive cycles without significant loss its catalytic activity.     

Preferential {1 0 0} etching of boron-doped diamond electrodes and diamond particles by CO2 activation

The etching behavior of polycrystalline boron-doped diamond (BDD) electrodes and diamond particles with gaseous CO₂ at 800 and 900 °C was investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. Polycrystalline BDD (800 ppm), composed of a mixture of cubic {1 0 0} and triangular {1 1 1} orientated planes, was used so as to pursue the possibility of preferential etching by high temperature CO₂ treatment. Nanometer sized pits were observed on the {1 0 0} planes while no change was observable for the {1 1 1} planes when the activation temperature was 800 °C. The difference in the etching behavior by CO₂ with regard to the different planes was clarified using diamond particles and comparing with steam activation. The results demonstrate that CO₂ activation leads to preferential {1 0 0} etching, whereas steam-activation results in preferential {1 1 1} etching.