Synthesis of graphene on SiC substrate via Ni-silicidation reactions

In this work, the features of graphene layers are studied with the aim of preparing the thinnest layers possible. The graphene layers were prepared by the annealing of Ni/SiC structures. The main advantage of this process is a relatively low temperature compared with the method of graphene epitaxial growth on SiC and short annealing times compared with the chemical vapor deposition method. We prepared graphene layers from several Ni/SiC structures in which the Ni layer thickness ranged from 1 to 200 nm. The parameters of the annealing process (temperature, rate of temperature increase, annealing time) were modified during the experiments. The formed graphene layers were analyzed by means of Raman spectroscopy. From the spectra, the basic parameters of graphene, such as the number of carbon layers and crystallinity, were determined. The annealing of the Ni(200 nm)/SiC structure at 1080 °C for 10 s, produced graphene in the form of 3-4 carbon monolayers. The value was verified by X-ray Photoelectron Spectroscopy (XPS). Good agreement was achieved in the results obtained using Raman spectroscopy and XPS.     

Growth of large-area single- and Bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces

We report graphene films composed mostly of one or two layers of graphene grown by controlled carbon precipitation on the surface of polycrystalline Ni thin films during atmospheric chemical vapor deposition (CVD). Controlling both the methane concentration during CVD and the substrate cooling rate during graphene growth can significantly improve the thickness uniformity. As a result, one- or two- layer graphene regions occupy up to 87% of the film area. Single layer coverage accounts for 5%–11% of the overall film. These regions expand across multiple grain boundaries of the underlying polycrystalline Ni film. The number density of sites with multilayer graphene/graphite (>2 layers) is reduced as the cooling rate decreases. These films can also be transferred to other substrates and their sizes are only limited by the sizes of the Ni film and the CVD chamber. Here, we demonstrate the formation of films as large as 1 in². These findings represent an important step towards the fabrication of large-scale high-quality graphene samples. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Synthesis of few-layered graphene by ion implantation of carbon in nickel thin films

The synthesis of few-layered graphene is performed by ion implantation of carbon species in thin nickel films, followed by high temperature annealing and quenching. Although ion implantation enables a precise control of the carbon content and of the uniformity of the in-plane carbon concentration in the Ni films before annealing, we observe thickness non-uniformities in the synthesized graphene layers after high temperature annealing. These non-uniformities are probably induced by the heterogeneous distribution/topography of the graphene nucleation sites on the Ni surface. Taken altogether, our results indicate that the number of graphene layers on top of Ni films is controlled by the nucleation process on the Ni surface rather than by the carbon content in the Ni film.     

Growth of few layer graphene from commercial C2 hydrocarbons at low temperature and controlled surface functionalization

Graphene is mostly grown from methane on copper foils at a high temperature about 1000°C. In this research, a commercial ethylene-acetylene-ethane mixture was used as a clean precursor for graphene synthesis on nickel foils in a chemical vapor deposition reactor at 750°C. Furthermore, controlled functionalization of graphene sheets was achieved via hydrothermal oxidation at moderate pressure and temperature using nitric acid. Broadened 2D band and G band frequencies in Raman spectra indicated that pristine graphene (PG) was of high quality with low defects. X-ray diffraction results confirmed that PG has five layers. Transmission electron microscopy and N₂ adsorption-desorption analyses affirmed that the graphene is of a good quality, large surface area (562 m²/g) and small pore size. Fourier transform infrared spectroscopy confirmed functionalization process performance. Thermogravimetric analysis affirmed that the thermal stability of PG was drastically decreased after the functionalization process.     

Graphene growth at the interface between Ni catalyst layer and SiO2/Si substrate

Graphene was synthesized deliberately at the interface between Ni film and SiO2/Si substrate as well as on top surface of Ni film using chemical vapor deposition (CVD) which is suitable for large-scale and low-cost synthesis of graphene. The carbon atom injected at the top surface of Ni film can penetrate and reach to the Ni/SiO2 interface for the formation of graphene. Once we have the graphene in between Ni film and SiO2/Si substrate, the substrate spontaneously provides insulating SiO2 layer and we may easily get graphene/SiO2/Si structure simply by discarding Ni film. This growth of graphene at the interface can exclude graphene transfer step for electronic application. Raman spectroscopy and optical microscopy show that graphene was successfully synthesized at the back of Ni film and the coverage of graphene varies with temperature and time of synthesis. The coverage of graphene at the interface depends on the amount of carbon atoms diffused into the back of Ni film.     

Low‐Temperature and Rapid Growth of Large Single‐Crystalline Graphene with Ethane

Future applications of graphene rely highly on the production of large‐area high‐quality graphene, especially large single‐crystalline graphene, due to the reduction of defects caused by grain boundaries. However, current large single‐crystalline graphene growing methodologies are suffering from low growth rate and as a result, industrial graphene production is always confronted by high energy consumption, which is primarily caused by high growth temperature and long growth time. Herein, a new growth condition achieved via ethane being the carbon feedstock to achieve low‐temperature yet rapid growth of large single‐crystalline graphene is reported. Ethane condition gives a growth rate about four times faster than methane, achieving about 420 µm min^?1 for the growth of sub‐centimeter graphene single crystals at temperature about 1000 °C. In addition, the temperature threshold to obtain graphene using ethane can be reduced to 750 °C, lower than the general growth temperature threshold (about 1000 °C) with methane on copper foil. Meanwhile ethane always keeps higher graphene growth rate than methane under the same growth temperature. This study demonstrates that ethane is indeed a potential carbon source for efficient growth of large single‐crystalline graphene, thus paves the way for graphene in high‐end electronical and optoelectronical applications.     

Thickness and chirality effects on tensile behavior of few-layer graphene by molecular dynamics simulations

The mechanical response of few-layer graphene (FLG), consisting of 2-7 atomic planes and bulk graphite is investigated by means of molecular dynamics simulations. By performing uniaxial tension tests at room temperature, the effects of number of atomic planes and chirality angle on the stress-strain response and deformation behavior of FLG were studied using the Tersoff potential. It was observed that by increasing of the FLG number of layers, the increase of bonding strength between neighboring layers reduce the elastic modulus and ultimate strength. It was found that, while the chirality angle of FLG showed a significant effect on the elastic modulus and ultimate tensile strength of two and three graphene layers, it turns to be less significant when the numbers of layers are more than four. Finally, by plotting the deformation behavior, it was concluded that FLGs present brittle performance.     

Synthesis of conducting transparent few-layer graphene directly on glass at 450 °C

Post-growth transfer and high growth temperature are two major hurdles that research has to overcome to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450?°C. Our few-layer graphene grows at the interface between a pre-deposited 200 nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after inkjet printing of electrical contacts: sheet resistance is locally as low as 500 Ω sq?1. Finally the samples equipped with printed contacts appear to be efficient humidity sensors.     

Low-temperature synthesis of graphene on nickel foil by microwave plasma chemical vapor deposition

Microwave plasma chemical vapor deposition (MPCVD) was employed to synthesize high quality centimeter scale graphene film at low temperatures. Monolayer graphene was obtained by varying the gas mixing ratio of hydrogen and methane to 80:1. Using advantages of MPCVD, the synthesis temperature was decreased from 750?°C down to 450?°C. Optical microscopy and Raman mapping images exhibited that a large area monolayer graphene was synthesized regardless of the temperatures. Since the overall transparency of 89% and low sheet resistances ranging from 590 to 1855 Ω∕sq of graphene films were achieved at considerably low synthesis temperatures, MPCVD can be adopted in manufacturing future large-area electronic devices based on graphene film.     

Determining the number of layers in graphene films synthesized by filtered cathodic vacuum arc technique

We have synthesized graphene film by the filtered cathodic vacuum arc (FCVA) technique and determined the number of layers in graphene films by various techniques. Amorphous carbon (a-C) films of different thicknesses (1, 2, 3, 6, 10 and 18 nm) were synthesized by the FCVA technique on Si/SiO₂/Ni substrate and then annealed in vacuum at 800°C and cooled down to room temperature naturally to obtain graphene. Prepared graphene films were transferred on different substrates and characterized by the Raman spectroscopy, UV-VIS-NIR spectroscopy, high-resolution transmission electron microscopy (HRTEM), optical microscopy, atomic force microscopy (AFM) and sheet resistance to determine the number of layers present in the graphene films. Raman spectra of the prepared graphene films exhibit that there is red shift in the position of D, G and 2 D peak. The value of I_2D/I_G varied from 0.18 to 0.51, I_D/I_G varied from 0.82 to 1.02 and full width at half maximum of 2 D peak varied from 101.2 to 128.0 cm^?1, for different thicknesses of graphene films, respectively. The value of transmittance decreases from 97 to 63.7% and that of sheet resistance increases from 460 to 1400 Ω/square with the increase in the thickness of the prepared graphene film. The HRTEM and AFM study revealed that the graphene synthesis from 1 nm thick a-C film possesses a single layer structure.     

Vapour-phase graphene epitaxy at low temperatures

We report an epitaxial growth of graphene, including homo- and hetero-epitaxy on graphite and SiC substrates, at a temperature as low as ∼540 °C. This vapour-phase epitaxial growth, carried out in a remote plasma-enhanced chemical vapor deposition (RPECVD) system using methane as the carbon source, can yield large-area high-quality graphene with the desired number of layers over the entire substrate surfaces following an AB-stacking layer-by-layer growth model. We also developed a facile transfer method to transfer a typical continuous one layer epitaxial graphene with second layer graphene islands on top of the first layer with the coverage of the second layer graphene islands being 20% (1.2 layer epitaxial graphene) from a SiC substrate onto SiO₂ and measured the resistivity, carrier density and mobility. Our work provides a new strategy toward the growth of graphene and broadens its prospects of application in future electronics.      

Superparamagnetic properties of carbon-encapsulated Ni nanoparticle assemblies

Carbon-encapsulated Ni nanoparticles [Ni(C)] were synthesized using a modified arc-discharge reactor under methane atmosphere. The average particle size was revealed to be typically 10.5 nm with a spherical shape. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers. Superparamagnetic property studies indicate that the blocking temperature (TB) is around 115 K at 1000 Oe applied field. Below TB, the temperature dependence of the coercivity is given by Hc = Hci[1 -(T/TB)1/2], with Hci approximately 500 Oe. Above TB, the magnetization M(H, T) can be described by the classical Langevin function L using the relationship M/Ms(T = 0) = coth(microH/kT)-kT/microH. The particle size can be inferred from the Langevin fit (particle moment mu) and the blocking temperature theory (TB), with values slightly larger than the high-resolution transmission electron microscopy observations. It is suggested that these assemblies of carbon-encapsulated Ni nanoparticles have typical single-domain, field-dependent superparamagnetic relaxation properties.     

Graphene films with large domain size by a two-step chemical vapor deposition process

The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square micrometers. In this paper, we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, domain size, and surface coverage of graphene as determined by Raman spectroscopy, and transmission and scanning electron microscopy. On the basis of the results, we developed a two-step CVD process to synthesize graphene films with domains having an area of hundreds of square micrometers. Scanning electron microscopy and Raman spectroscopy clearly show an increase in domain size by changing the growth parameters. Transmission electron microscopy further shows that the domains are crystallographically rotated with respect to each other with a range of angles from about 13 to nearly 30°. Electrical transport measurements performed on back-gated FETs show that overall films with larger domains tend to have higher carrier mobility up to about 16,000 cm(2) V(-1) s(-1) at room temperature.     

Effects of ambient conditions on the quality of graphene synthesized by chemical vapor deposition

In this work, we investigated the effects of methane concentration and gas flow rate ratio between hydrogen and methane on the quality of graphene synthesized by chemical vapor deposition. It is found that a critical concentration of methane is needed to grow continuous graphene films, while discontinuous graphene flakes are formed at low methane concentrations. Under the condition without hydrogen, a graphene film in which monolayer areas are predominant is grown, whereas a great proportion of hydrogen causes thick graphene, which reduces the transmittance of the film. Our results present an instructive reference to the large-area synthesis of graphene for the potential applications in electronics.     

Synthesis of large area, homogeneous, single layer graphene films by annealing amorphous carbon on Co and Ni

The synthesis of large area, homogenous, single layer graphene on cobalt (Co) and nickel (Ni) is reported. The process involves vacuum annealing of sputtered amorphous carbon (a-C) deposited on Co/sapphire or Ni/sapphire substrates. The improved crystallinity of the metal film, assisted by the sapphire substrate, proves to be the key to the quality of as-grown graphene film. The crystallinity of the Co and Ni metal films was improved by sputtering the metal at elevated temperature as was verified by X-ray diffraction (XRD). After sputtering of a-C and annealing, large area, single layer graphene that occupies almost the entire area of the substrate was produced. With this method, 100 mm²-area single layer graphene can be synthesized and is limited only by the substrate and vacuum chamber size. The homogeneity of the graphene film is not dependent on the cooling rate, in contrast to syntheses using polycrystalline metal films and conventional chemical vapor deposition (CVD) growth. Our facile method of producing single layer graphene on Co and Ni metal films should lead to large scale graphene-based applications.     

Synthesis and characterization of electrochemically-reduced graphene

Graphene has superior electrical conductivity than graphite and other allotropes of carbon because of its high surface area and chemical tolerance. Electrochemically processed graphene sheets were obtained through the reduction of graphene oxide from hydrazine hydrate. The prepared samples were heated to different temperatures such as 673 and 873 K. X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), transmission electron microscopy (TEM), Raman spectra and conductivity measurements were made for as-prepared and heat-treated graphene samples. XRD pattern of graphene shows a sharp and intensive peak centred at a diffraction angle (2θ) of 26·350. FTIR spectra of as-prepared and heated graphene were used to confirm the oxidation of graphite. TEM results indicated that the defect density and number of layers of graphene sheets were varied with heating temperature. The hexagonal sheet morphology and purity of as-prepared and heat treated samples were confirmed by SEM–EDX and Raman spectroscopy. The conductivity measurements revealed that the conductivity of graphene was decreased with an increase in heating temperature. The present study explains that graphene with enhanced functional properties can be achieved from the as-prepared sample.     

Plasma oxidation of thermally grown graphenes and their characterization

We report the plasma oxidation of thermally synthesized graphenes and their characterization using Raman spectroscopy and atomic force microscope (AFM). A graphene was synthesized by thermal chemical vapor deposition with methane and transferred onto trench substrate to make suspended configuration in order to exclude substrate effects. The air plasma treatment at 0.4 W for 5 min and property characterization were alternately performed to address the effect of oxidation. After the oxidation, a drastic change in Raman spectra was observed, which implies that considerable structural changes occurred in the graphene. Interestingly, we observed from the Raman and AFM analyses that the number of layers can be reduced by the controlled plasma oxidation treatment. The results may open the possibility of graphene formation from graphite sheets through the precise control of plasma treatment conditions.     

Yield and shape selection of graphene nanoislands grown on ni(111)

The catalytic decomposition of hydrocarbons on transition-metal surfaces has attracted increasing interest as a method to prepare high quality graphene layers. Here, we study the optimal reaction path for the preparation of graphene nanoislands of selected shape using controlled decomposition of propene on Ni(111). Scanning tunneling microscopy performed at different stages of the reaction provides insight into the temperature and dose-dependent growth of graphene islands, which precedes the formation of monolayer graphene. The effect of postreaction annealing on the morphology of the islands is studied. By adjusting the initial propene dose, reaction temperature, and postannealing procedure, islands with a triangular or hexagonal shape can be selectively obtained.     

Thermal oxidation of synthesized graphenes and their optical property characterization

The results of the thermal oxidation of synthesized graphenes and their optical property characterization using Raman spectroscopy are reported. Graphene was synthesized via thermal-chemical vapor deposition on Ni catalytic thin films deposited by electron beam deposition, and was successfully transferred onto three-dimensional trench substrates to obtain a suspended structure, which is the most appropriate template for use in probing the changes of physical properties of graphene by ignoring the substrate effects. The thermal oxidation was performed in a tube furnace at an elevated temperature of 500 degrees C under air, and Raman analysis was repeatedly carried out to investigate the oxidation effects. A drastic structural change of graphene was anticipated from the based on the dramatic changes in the Raman spectra. It is expected that controlled oxidation will help systematically decrease in the number of graphene layers, which will contribute to the successful development of graphene-based devices that are capable of operating under oxidative environments.     

Surface transformations of carbon (graphene, graphite, diamond, carbide), deposited on polycrystalline nickel by hot filaments chemical vapour deposition

The deposition of carbon has been studied at high temperature on polycrystalline nickel by hot filaments activated chemical vapor deposition (HFCVD). The sequences of carbon deposition are studied by surface analyses: Auger electron spectroscopy (AES), electron loss spectroscopy (ELS), X-ray photoelectron spectroscopy (XPS) in a chamber directly connected to the growth chamber. A general scale law of the (C/Ni) intensity lines is obtained with a reduced time. Both, shape analysis of the AES C KVV line and the C1s relative intensity suggest a three-step process: first formation of graphene and a highly graphitic layer, then multiphase formation with graphitic, carbidic and diamond-like carbon and finally at a critical temperature that strongly depends on the pretreatment of the polycrystalline nickel surface, a rapid transition to diamond island formation. Whatever the substrate diamond is always the final product and some graphene layers the initial product. Moreover it is possible to stabilize a few graphene layers at the initial sequences of carbon deposition. The duration of this stabilization step is strongly depending however on the pre-treatment of the Ni surface.