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.     

Temperature and face dependent copper–graphene interactions

The interaction between graphene and metals represents an important issue for the large-area preparation of graphene, graphene transfer and the contact quality in graphene devices. We demonstrate a simple method for estimating and manipulating the level of interaction between graphene and copper single crystals through heat treatment, at temperatures from 298 K to 1073 K. We performed in-situ Raman spectroscopy showing Cu face-specific behavior of the overlying graphene during the heat treatment. On Cu(1 1 1) the interaction is consistent with theoretical predictions and remains stable, whereas on Cu(1 0 0) and Cu(1 1 0), the initially very weak interaction and charge transfer can be tuned by heating. Our results also suggest that graphene grown on Cu(1 0 0) and Cu(1 1 0) is detached from the copper substrate, thereby possibly enabling an easier graphene transfer process as compared to Cu(1 1 1).     

Interaction between graphene and copper substrate: The role of lattice orientation

We present a comprehensive study of graphene grown by chemical vapor deposition on copper single crystals with exposed (1 0 0), (1 1 0) and (1 1 1) faces. Direct examination of the as-grown graphene by Raman spectroscopy using a range of visible excitation energies and microRaman mapping shows distinct strain and doping levels for individual Cu surfaces. Comparison of results from Raman mapping with X-ray diffraction techniques and atomic force microscopy shows it is neither the crystal quality nor the surface topography responsible for the specific strain and doping values, but it is the Cu lattice orientation itself. We also report an exceptionally narrow Raman 2D band width caused by the interaction between graphene and metallic substrate. The appearance of this extremely narrow 2D band with full-width-at-half maximum (FWHM) as low as 16 cm⁻¹ is correlated with flat and undoped regions on the Cu(1 0 0) and (1 1 0) surfaces. The generally compressed (∼0.3% of strain) and n-doped (Fermi level shift of ∼250 meV) graphene on Cu(1 1 1) shows the 2D band FWHM minimum of ∼20 cm⁻¹. In contrast, graphene grown on Cu foil under the same conditions reflects the heterogeneity of the polycrystalline surface and its 2D band is accordingly broader with FWHM >24 cm⁻¹.     

Synthesis of well-aligned millimeter-sized tetragon-shaped graphene domains by tuning the copper substrate orientation

We present a simple processing method for synthesizing well-aligned millimeter-sized tetragon-shaped graphene domains on a polycrystalline copper substrate via low-pressure chemical vapor deposition. The tetragonal shape is achieved simply by wet loading the copper substrate with processing conditions previously used for the growth of millimeter-sized hexagon-shaped graphene domains. Electron backscatter diffraction (EBSD) shows that the wet loaded copper substrate is uniformly textured with a surface plane between Cu (1 0 0) and Cu (1 1 0). The in-plane rotation of the crystalline orientation across the Cu grains is very small. However, the EBSD showed that the surface orientation of the dry loaded substrate is close to the (1 1 1) crystal plane. The different surface orientation of the wet and dry loaded samples is attributed to the different surface oxygen concentration, which changes the relative stability of the (0 0 1), (1 1 0), and (1 1 1) plane during copper sublimation and recrystallization. These results provide an approach to tune the surface crystal orientation and thus the shape and orientation of the graphene domains.     

Graphene etching controlled by atomic structures on the substrate surface

We etched graphene on a sapphire (1 ?1 0 2) surface using the reaction between graphene and hydrogen catalyzed by metal nanoparticles. To investigate effects of the atomic structure of the sapphire substrate on graphene etching, we used sapphire substrate with as-polished, air-annealed, and step-ordered surfaces. We investigated the relationship between the atomic arrangement of sapphire and graphene etching and found that graphene is selectively etched in the [1 ?1 0 ?1] direction of sapphire. This indicates that atomic structure of the sapphire surface can be used as a template to control graphene etching. By combining the transfer method for graphene sheets grown on metal substrates with the present etching technique, graphene nanoribbons can be fabricated at a wafer level.     

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.     

Periodically rippled graphene on Ru(0 0 0 1): A template for site-selective adsorption of hydrogen dimers via water splitting and hydrogen-spillover at room temperature

The interaction of water with periodically rippled graphene deposited on Ru(0 0 0 1) and nearly-flat graphene/Pt(1 1 1) has been investigated by using high-resolution electron energy loss spectroscopy. Graphene samples were exposed to ambient air humidity as well as to water molecules under controlled conditions in vacuum. In both cases, loss measurements show that water molecules dosed at room temperature dissociate giving rise to C–H bonds. We suggest that water molecules intercalate under graphene and are split by the underlying metal catalyst. On the lattice-mismatched graphene/Ru(0 0 0 1) interface, the corrugation of the graphene overlayer induces site selectivity for H adsorption in ortho and para dimers. On the other hand, no dimer formation occurs for the nearly-flat, multidomain graphene/Pt(1 1 1) interface.     

On the intercalation of CO molecules in ultra-high vacuum conditions underneath graphene epitaxially grown on metal substrates

We have studied the interaction of CO with epitaxial graphene on Pt(1 1 1) and Ru(0 0 0 1) by means of high-resolution electron energy loss spectroscopy measurements. Our experiments unambiguously demonstrate that in ultra-high vacuum conditions CO does not intercalate underneath the graphene monolayer supported on Pt(1 1 1) and Ru(0 0 0 1). For submonolayer coverages of graphene on Pt(1 1 1), CO adsorption occurs only in Pt on-top sites while bridge sites, usually populated on the clean Pt(1 1 1) surface, are inhibited. Instead, we find that epitaxial graphene is rather reactive toward water molecules.     

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.     

Exfoliated graphene-supported Pt and Pt-based alloys as electrocatalysts for direct methanol fuel cells

To greatly improve the electrocatalytic activity for methanol oxidation, high-quality exfoliated graphene decorated with uniform Pt nanocrystals (NCs) (3 nm) have been prepared by a very simple, low-cost and environmentally benign process. During the entire process, no surfactant and no halide ions were involved, which not only enabled very clean surface of Pt/graphene leading to excellent conductivity, but also greatly improved the electrocatalyst tolerance to carbon monoxide poisoning (Pt/graphene, I_f/I_b = 1.197), compared to commercial Pt/C (I_f/I_b = 0.893) catalysts. To maximize the electrocatalytic performance and minimize the amount of precious Pt, Pt–M/graphene (M = Pd, Co) hybrids have also been prepared, and these hybrids have much larger electrochemically active surface areas (ECSA), which are 4 (PtPd/graphene) and 3.3 (PtCo/graphene) times those of commercial Pt/C. The PtPd/graphene and PtCo/graphene hybrids also have remarkably increased activity toward methanol oxidation (I_f/I_b = 1.218 and 1.558). Furthermore, density functional theory (DFT) simulations demonstrate that an electronic interaction occurred between Pt atoms and graphene, indicating that graphene substrate plays a crucial role in regulating the electron structure of attached Pt atom, which confirmed that the increased efficiency of methanol oxidation was due to the synergetic effects of the hybrid structure.     

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.     

Heteroepitaxial nucleation and growth of graphene nanowalls on silicon

Heteroepitaxial nucleation of {0 0 2} graphene sheets on {1 1 1} facets of plasma treated (1 0 0) silicon by direct-current plasma enhanced chemical vapor deposition in methane–hydrogen gas mixtures is confirmed by high-resolution transmission electron microscopy. Lattice mismatch by 12% is compensated by tilting the graphene {0 0 2} with respect to silicon {1 1 1} and matching the silicon lattice with fewer graphene layers. The interlayer spacing of graphene sheets near the silicon surface is 0.355 nm, which is larger than that of AB stacked graphite and confirmed as AA stacked graphitic phase. Subsequent growth of standing graphene nanowalls is characterized by scanning electron microscopy and Raman scattering (633 and 514 nm excitation). The Raman peaks of D-band, G-band, and 2D-band are discussed in correlation with SEM images of graphene nanowalls. A strong Raman peak corresponding to silicon–hydrogen stretch vibration is detected by 633 nm excitation at the early stage of graphene nucleation, indicating the silicon substrate etched by hydrogen plasma. With these analyses, the growth mechanism is also proposed in this paper.     

Electronic and structural properties of graphene-based metal-semiconducting heterostructures engineered by silicon intercalation

The initial decoupling of the (6√3 × 6√3)R30° buffer layer also called zero layer graphene (ZLG) on 6H-SiC(0 0 0 1) by Si intercalation has been investigated by means of high resolution photoemission spectroscopy (HRPES) and microscopy imaging techniques. A combination of complementary techniques has shown that the annealing above 700 °C of amorphous Si deposited on ZLG leads to the diffusion of the silicon over the surface. Two competing processes are then observed. Part of the silicon contributes to a progressive decoupling of the ZLG from the substrate (partial decoupling) while the rest agglomerates at the surface to form oriented silicon clusters. After sequences of Si deposition, followed by annealing at 750 °C, complete decoupling is observed into quasi-free standing monolayer (ML) graphene. Investigation of the evolution of the C1s and Si2p core levels during the intermediate states shows that the appearance of the graphene contribution coincides with the creation of an extra SiC bulk component, indicating their electronic decoupling. At partial decoupling of the ZLG, we have the coexistence of structurally linked metal-semiconducting materials presenting mutual electronic interactions and composed of nanometric metal-semiconducting heterojunctions.     

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.     

Probing graphene interfaces with secondary electrons

The empty states of graphene and graphene adsorbed on nickel (1 1 1) are studied using a plane-wave pseudo-potential method based on local density functional theory. The analysis is used to assign spectroscopic features observed in secondary electron emission experiments either to the graphene sheet or to the nickel substrate. The calculations provide insights into the key role of the graphene–substrate interaction, through the hybridization between carbon and nickel states, and indicate secondary electron emission as an efficient probe for interfaces made of two dimensional crystals.     

Towards the perfect graphene membrane? – Improvement and limits during formation of high quality graphene grown on Cu-foils

We investigated the structure and crystalline quality of monolayer graphene grown by hydrogen and methane chemical vapor deposition (CVD) on polycrystalline Cu foils. Our data show that the high temperature hydrogen pretreatment of the Cu foil has to be performed at a sufficiently high H₂ pressure in order to avoid graphene (g) formation already during the pretreatment, which limits the achievable domain size during subsequent growth in the CH₄/H₂ mixture. Methane–hydrogen CVD sustains g growth but induces the faceting of the Cu substrate. Characterization by low energy electron microscopy evidenced a staircase Cu substrate morphology of alternating (4 1 0) and (1 0 0) planes interrupted by (n 1 1) type facets. The g flakes cover the staircase shaped support as a coherent layer. The polycrystalline film mostly contains rotational domains that are preferentially, but not strictly, aligned with respect to the stepped support surface. The substrate induced corrugated morphology occurs also underneath large single crystalline flakes and is transferred to suspended membranes, produced by etching the Cu underneath the graphene. Thus, membranes manufactured from g-Cu are non flat. This explains their reported softened elastic response and the formation of so called nanorippled graphene after transfer from the Cu support which deteriorates its electrical conductivity.     

Growth of large-area non-polar ZnO film without constraint to substrate using oblique-angle sputtering deposition

Non-polar ZnO thin film with high crystal quality is grown on a glass substrate using one-step oblique-angle deposition. Cross-sectional transmission electron microscopy images and selected area electron diffraction patterns reveal that the film is constructed as a stack of grains from the bottom to the top with the [0 0 0 2] axis gradually titled from a vertical to a nearly horizontal orientation with respect to the substrate. The (0 0 0 2) pole figure exhibits a continuous angle distribution in the ψ direction with the most concentration at approximately ψ = 18° and ? = 0°. Strong anisotropic effects in local electronic structure were observed for the highly oriented ZnO surface rod by angle-dependent X-ray absorption near-edge structure measurements. The structure also exhibits polarization that depends on Raman scattering.     

A theoretical analysis of the surface dependent binding,peeling and folding of graphene on single crystal copper

The binding, peeling and folding behavior of graphene on different surfaces of single crystal copper were examined theoretically. We show that the binding energy is the highest on the Cu(1 1 1), and follows the order of Cu(1 1 1) > (1 0 0) > (1 1 0) > (1 1 2). Conventional theory is capable of capturing the dynamic process of graphene peeling seen from molecular dynamics simulations. We show that the number of graphene layers on Cu surfaces could be distinguished by performing simple peeling tests. Further investigation of the folding/unfolding of graphene on Cu surfaces shows that Cu(1 1 1) favors the growth of monolayer graphene. These observations on the interaction between graphene with single crystal Cu surfaces might provide guidelines for improving graphene fabrication.