Transport characteristics of a single-layer graphene field-effect transistor grown on 4H-silicon carbide

We have investigated transport characteristics of epitaxial graphene grown on semi-insulating silicon-face 4H-silicon carbide (SiC) substrate by thermal decomposition method in relatively high N₂ pressure atmosphere. We have succeeded in forming 1–2 layers of graphene on SiC in controlled manner. The surface morphology of formed graphene was analyzed by atomic force microscopy (AFM), low-energy electron diffraction (LEED) and low-energy electron microscope (LEEM). We have confirmed single-layer graphene growth in average by this method. Top-gated, single-layer graphene field-effect transistors (FETs) were fabricated on epitaxial graphene grown on 4H-SiC. Increased on/off ratio of nearly 100 at low temperature and extremely small minimum conductance (0.018–0.3 in 4 e²/h) in gated Hall-bar samples suggest possible band-gap opening of single-layer epitaxial graphene grown on Si-face SiC.     

Carbon nanosheets by microwave plasma enhanced chemical vapor deposition in CH4-Ar system

We employ a new gas mixture of CH₄-Ar to fabricate carbon nanosheets by microwave plasma enhanced chemical vapor deposition at the growth temperature of less than 500 °C. The catalyst-free nanosheets possess flower-like structures with a large amount of sharp edges, which consist of a few layers of graphene sheets according to the observation by transmission electron microscopy. These high-quality carbon nanosheets demonstrated a faster electron transfer between the electrolyte and the nanosheet surface, due to their edge defects and graphene structures.     

Graphene layer number dependent size distribution of silver nanoparticles

We observe that silver atoms deposited by thermal evaporation deposition onto n-layer graphene films condense upon annealing to form nanoparticles with an average diameter and density that is determined by the layer numbers of graphene films. The optical microscopy and Raman spectroscopy were utilized to identify the number of the graphene layers and the SEM (scanning electron microscopy) was used to observe the morphologies of the particles. Systematic analysis revealed that the average sizes of the nanoparticles increased with the number of graphene layers. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver nanoparticles are related to the surface free energy and surface diffusion of the n-layer graphenes.     

Direct measurement of the growth mode of graphene on SiC(0001) and SiC(0001¯)

We have determined the growth mode of graphene on SiC(0001) and SiC(0001ˉ) using ultrathin, isotopically labeled Si(13)C "marker layers" grown epitaxially on the Si(12)C surfaces. Few-layer graphene overlayers were formed via thermal decomposition at elevated temperature. For both surface terminations (Si face and C face), we find that the (13)C is located mainly in the outermost graphene layers, indicating that, during decomposition, new graphene layers form underneath existing ones.     

Control of the degree of surface graphitization on 3C-SiC(100)/Si(100)

The current method of growing graphene by thermal decomposition of 3C-SiC(100) on silicon substrates is technologically attractive. Here, we investigate the evolution of the surface graphitization as a function of the synthesis temperature. We establish that the carbon enrichment of the surface is characterized by a clear modulation of the surface potential and structuration. The structural properties analysis of the graphene layers by low energy electron diffraction and micro-Raman spectroscopy demonstrate a graphitization of the surface.     

Diamond-like C2H nanolayer, diamane: Simulation of the structure and properties

We consider a new C₂H nanostructure based on bilayer graphene transformed under the covalent bond of hydrogen atoms adsorbed on its external surface, as well as compounds of carbon atoms located opposite each other in neighboring layers. They constitute a “film” of the 〈111〉 diamond with a thickness of less than 1 nm, which is called diamane. The energy characteristics and electron spectra of diamane, graphene, and diamond are calculated using the density functional theory and are compared with each other. The effective Young’s moduli and destruction thresholds of diamane and graphene membranes are determined by the molecular dynamics method. It is shown that C₂H diamane is more stable than CH graphane, its dielectric “gap” is narrower than the band gap of bulk diamond (by 0.8 eV) and graphane (by 0.3 eV), and is harder and more brittle than the latter.     

Surface refinement and electronic properties of graphene layers grown on copper substrate: An XPS, UPS and EELS study

The present work focuses on the assessment of two surface treatment procedures employed under ultra high vacuum conditions in order to obtain atomically clean graphene layers without disrupting the morphology and the two dimensional character of the films. Graphene layers grown by chemical vapor deposition on polycrystalline Cu were stepwise annealed up to 750 °C or treated by mild Ar⁺ sputtering. The effectiveness of both methods and the changes that they induce on the surface morphology and electronic structure of the films were systematically studied by X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. Ultraviolet photoelectron spectroscopy was employed for the study of the electronic properties of the as received sample and in combination with the work function measurements, indicated the hybridization of the C-π network with Cu d-orbitals. Mild Ar⁺ sputtering sessions were found to disrupt the sp2 network and cause amorphisation of the graphitic carbon. Annealing between 300 °C and 450 °C under ultra high vacuum proved to be an effective and lenient way for achieving an atomically clean graphene surface. At higher temperatures the rigid structure of graphene does not follow the expansion of the copper substrate leading to the graphene/Cu interface breakdown and possibly to further rippling of the graphene layers leaving bare areas of cooper substrate.     

Scalable synthesis of graphene on single crystal Ir(111) films

We have investigated single crystal Ir(111) films grown heteroepitaxially on Si(111) wafers with yttria-stabilized zirconia (YSZ) buffer layers as possible substrates for an up-scalable synthesis of graphene. Graphene was grown by chemical vapor deposition (CVD) of ethylene. As surface analytical techniques we have used scanning tunneling microscopy (STM), low-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The mosaic spread of the metal films was below 0.2° similar to or even below that of standard Ir bulk single crystals, and the films were basically twin-free. The film surfaces could be improved by annealing so that they attained the perfection of bulk single crystals. Depending on the CVD conditions a lattice-aligned graphene layer or a film consisting of different rotational domains were obtained. STM data of the non-rotated phase and of the phases rotated by 14° and 19° were acquired. The quality of the graphene was comparable to graphene grown on bulk Ir(111) single crystals.     

Structural analysis of PTCDA monolayers on epitaxial graphene with ultra-high vacuum scanning tunneling microscopy and high-resolution X-ray reflectivity

Epitaxial graphene, grown by thermal decomposition of the SiC (0001) surface, is a promising material for future applications due to its unique and superlative electronic properties. However, the innate chemical passivity of graphene presents challenges for integration with other materials for device applications. Here, we present structural characterization of epitaxial graphene functionalized by the organic semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). A combination of ultra-high vacuum scanning tunneling microscopy (STM) and high-resolution X-ray reflectivity (XRR) is used to extract lateral and vertical structures of 0, 1, and 2 monolayer (ML) PTCDA on epitaxial graphene. Both Fienup-based phase-retrieval algorithms and model-based least-squares analyses of the XRR data are used to extract an electron density profile that is interpreted in terms of a stacking sequence of molecular layers with specific interlayer spacings. Features in the STM and XRR analysis indicate long-range molecular ordering and weak π–π* interactions binding PTCDA molecules to the graphene surface. The high degree of both lateral and vertical ordering of the self-assembled film demonstrates PTCDA functionalization as a viable route for templating graphene for the growth and deposition of additional materials required for next-generation electronics and sensors.     

Laser scribing on HOPG for graphene stamp printing on silicon wafer

Highly oriented pyrolytic graphite (HOPG) was scribed by pulsed laser beam to produce square patterns. Patterning of HOPG surface facilitates the detachment of graphene layers during contact printing. Direct HOPG-to-substrate and glue-assisted stamp printing of a few-layers graphene was compared. Printed graphene sheets were visualized by optical and scanning electron microscopy. The number of graphene layers was measured by atomic force microscopy. Glue-assisted stamp printing allows printing relatively large graphene sheets (40×40 μm) onto a silicon wafer. The presented method is easier to implement and is more flexible than the majority of existing ways of placing graphene sheets onto a substrate.     

Rashba effect in the graphene/ni(111) system

We report on angle-resolved photoemission studies of the electronic pi states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the pi states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived pi band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the pi band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.     

Energy transfer rate in double-layer graphene systems: Linear regime

We theoretically investigate the energy transfer phenomenon in a double-layer graphene (DLG) system. We use the balance equation approach in linear regime and random phase approximation screening function to obtain energy transfer rates at different electron temperatures, densities and interlayer spacings. We find that the rate of energy transfer in the DLG is qualitatively similar to that obtained in the double-layer two-dimensional electron gas but its values are an order of magnitude greater. Also, at large electron temperature differences between two graphene layers, the electron density dependence of energy transfer is significantly different, particularly in case of unequal electron densities.     

Effect of surface hydrogenation on the topological properties of multilayer graphene

We have investigated effects of surface hydrogenation on the topological properties of multilayer graphene by using density functional theory calculations and a tight-binding model. Hydrogen adsorption on a dimer site of a surface layer decouples the surface layer from the rest of the layers. Hydrogen adsorption on a nondimer site introduces a band mixing between the hydrogenated graphene and the rest of the graphene layers. The valley Hall effects and spin-valley-resolved Chern numbers of multilayer graphene, calculated as a function of the sublattice potential and the potential perpendicular to the layers, was found to be sensitive to details of inversion symmetry-breaking potentials. While the topological invariant depends on the adsorption site and spin polarization, surface-hydrogenated M-layer graphene was found to be topologically equivalent to (M-1)-layer graphene under inversion symmetry-breaking potentials regardless of the adsorption site.     

Raman Spectrum of Epitaxial Graphene on SiC （0001） by Pulsed Electron Irradiation

We report new Raman features of epitaxial graphene （EG） on Si-face 4H-SiC prepared by pulsed electron irradiation （PEI）. With increasing graphene layers, frequencies of G and 2D peaks show blue-shifts and approach those of bulk highly-oriented pyrolytic graphite. It is indicated that the EG is slightly tension strained and tends to be strain-free. Meanwhile, single Lorentzian line shapes are well fitted to the 2D peaks of EG on SiC（O001） and their full widths at half maximum decrease with the increasing graphene layers, which indicates that the multilayer EG on Si-face can also contain turbostratic stacking by our PEI route instead of only AB Bernal stacking by a traditional thermal annealing method. It is worth noting that the stacking style plays an important role on the charge carrier mobility. Therefore our findings will be a candidate for growing quality graphene with high carrier mobility both on the Si- and C-terminated SiC substrate. Mechanisms behind the features are studied and discussed.     

Near-edge x-ray absorption fine-structure investigation of graphene

We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single-layer graphene we observe a splitting of the pi resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.     

Ultrafast relaxation of excited Dirac fermions in epitaxial graphene using optical differential transmission spectroscopy

We investigate the ultrafast relaxation dynamics of hot Dirac fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission spectroscopy. We observe differential transmission spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, the electron cooling is determined by electron-acoustic phonon scattering, found to occur on the time scale of 1 ps for highly doped layers, and 4-11 ps in undoped layers. The spectra also provide strong evidence for the multilayer structure and doping profile of thermally grown epitaxial graphene on SiC.     

Secondary electron emission characteristics of graphene films with copper substrate

For modern and future circular accelerators, especially high-intensity proton synchrotrons or colliders, the electron cloud effect is a key issue. So, in order to reduce the electron cloud effect, exploring very low secondary electron yield (SEY) material or coating used in vacuum tubes becomes necessary. In this article, we studied the SEY characteristics of graphene films with different thicknesses which were deposited on copper substrates using chemical vapor deposition. The SEY tests were done at temperatures of 25℃ and vacuum pressure of (2-6)×10^-9 torr. The properties of the deposited graphene films were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The SEY curves show that the number of graphene layers has a great effect on the SEY of graphene films. The maximum SEY of graphene films decreases with the increase of the number of layers. The maximum SEY of 6-8 layers of graphene film is 1.25. These results have a great significance for next-generation particle accelerators.     

Signatures of epitaxial graphene grown on Si-terminated 6H-SiC (0 0 0 1)

Epitaxial graphene layers thermally grown on Si-terminated 6H-SiC (0 0 0 1) have been probed using Auger electron spectroscopy, Raman microspectroscopy, and scanning tunneling microscopy (STM). The average multilayer graphene thickness is determined by attenuation of the Si (L₂₃VV) and C (KVV) Auger electron signals. Systematic changes in the Raman spectra are observed as the film thickness increases from one to three layers. The most striking observation is a large increase in the intensity of the Raman 2D-band (overtone of the D-band and also known as the G′-band) for samples with a mean thickness of more than ∼1.5 graphene layers. Correlating this information with STM images, we show that the first graphene layer imaged by STM produces very little 2D intensity, but the second imaged layer shows a single-Lorentzian 2D peak near 2750 cm⁻¹, similar to spectra acquired from single-layer micromechanically cleaved graphene (CG). The 4-10 cm⁻¹ higher frequency shift of the G peak relative to CG can be associated with charge exchange with the underlying SiC substrate and the formation of finite size domains of graphene. The much greater (41-50 cm⁻¹) blue shift observed for the 2D-band may be correlated with these domains and compressive strain.     

Direct growth of graphene on gallium nitride using C2H2 as carbon source

Growing graphene on gallium nitride (GaN) at temperatures greater than 900°C is a challenge that must be overcome to obtain high quality of GaN epi-layers. We successfully met this challenge using C₂H₂ as the carbon source. We demonstrated that graphene can be grown both on copper and directly on GaN epi-layers. The Raman spectra indicated that the graphene films were about 4–5 layers thick. Meanwhile, the effects of the growth temperature on the growth of the graphene films were systematically studied, and 830°C was found to be the optimum growth temperature. We successfully grew high-quality graphene films directly on gallium nitride.     

Electronic structure of the Au-intercalated graphene/Ni(111) surface

We report the electronic structure of the Au-intercalated graphene/Ni(111) surface using angle-resolved photoemission spectroscopy and low energy electron diffraction. The graphene/Ni(111) shows no Dirac cone near the Fermi level and a relatively broad C 1s core level spectrum probably due to the broken sublattice symmetry in the graphene on the Ni(111) substrate. When Au atoms are intercalated between them, the characteristic Dirac cone is completely recovered near the Fermi level and the C 1s spectrum becomes sharper with the appearance of a 10?×?10 superstructure. The fully Au-intercalated graphene/Ni(111) surface shows a p-type character with a hole pocket of ～0.034?Å^?1 diameter at the Fermi level. When the surface is doped with Na and K, a clear energy gap of ～0.4?eV is visible irrespective of alkali metal.