Search

In this work, we present the results of electrochemical, Raman, FE-SEM/EDS, and XPS studies of graphene coatings deposited by chemical vapour deposition at atmospheric pressure on Mo foil and Pt-coated Mo foil as catalysts. The results showed that the graphene coating synthesized on Pt-coated molybdenum foil was bi-layer and had fewer defects, while the graphene coating on Mo foil was few-layer with more defects. The corrosion studies indicated that both graphene coatings on bare Mo and Pt-coated Mo exhibited good protective properties and can act as a barrier against corrosion in 0.1 M NaCl. [ABSTRACT FROM AUTHOR]

We investigate the effect of the chemical and physical defects on the humidity sensitivity of graphene. For this we apply reactive ion etching for physical defects and the poly(methyl methacrylate) (PMMA) coating for chemical defects to the CVD graphene. The tendency of humidity sensing is hardly found by the physical defects while the distinct changes are observed with chemical defects by control of the thickness and the coverage area of the PMMA on the graphene surface. The graphenes covered with thinner or smaller area of PMMA show an enhanced humidity sensitivity, indicating the possibility of H 2 O sensing. [ABSTRACT FROM AUTHOR]

Direct growth of a monolayer graphene film on a quartz substrate by a hot filament chemical vapor deposition technique is reported. The monolayer graphene film prepared was characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), selected area electron diffraction (SAED), and atomic force microscopy (AFM). The optical properties were studied by spectroscopic elliposmetry. The experimental data were fitted by the Forouhi-Bloomer model to estimate the extinction coefficient and the refractive index of the monolayer graphene film. The refractive index spectrum in the visible region was studied based on the harmonic oscillator model. The lattice dielectric constant, real and imaginary dielectric constants and the ratio of the charge carrier number to the effective mass were determined. The surface and volume energy loss parameters were also found and showed that the value of the surface energy loss is greater than the volume energy loss. The determination of these optical constants will open new avenue for novel applications of graphene films in the field of wave plates, light modulators, ultrahigh-frequency signal processing and LCDs. [ABSTRACT FROM AUTHOR]

Three dimensional (3D) graphene foams have potential applications in high performance energy storage devices such as lithium-ion batteries and super capacitors. In this study, microcellular 3D graphene foams were fabricated via ambient pressure chemical vapor deposition (APCVD) with electroless plated nickel (Ni) foam templates. The microcellular Ni foam templates exhibit sufficient thermal stability for few layer graphene (FLG) growth in the APCVD process, yielding graphene foams with a pore size less than 100 µm, density of 0.0020 g cm −3 , and pore wall thickness of about 5 nm. The 3D graphene foam has a cellular morphology with a surface-to-volume ratio of 2.5×10 5 m 2 m −3 . [ABSTRACT FROM AUTHOR]

Background: Conductive biomaterials are an ideal biosubstrate for modifying cellular behaviors by conducting either internal or external electrical signals. In this study, based on a simple-preparation graphite exfoliation method in organic reagent, a nonfunctionalized graphene nanosheet film (NGNF) with high conductivity and large size was simply fabricated through spraying coating. The biocompatibility of the NGNF was carefully tested with primary cortical neuron cells, and its biocompatibility properties were compared with a chemical vapor deposition (CVD) graphene film. Methods: Nonfunctionalized graphene nanosheet (NGN) was first exfoliated from graphite with a flat-tip ultrasonicator probe, and then spray-coated onto glass slide substrate to form the film. The morphology of NGNF was observed with light microscopy and SEM. The morphology and neuronal network formation of primary cortical neuron cells onto NGNF, as shown by DAPI and Alexa Fluor® 488 staining, were observed with fluorescent microscopy. Cell viability and proliferation were measured with MTT. Results: NGNF had better cell biocompatibility than CVD graphene film. MTT test showed that NGNF exhibited no cytotoxicity. According to neuronal network formation at 7 days of cell culture, primary neuron cells aggregated into 50-µm "nuclei"; the average neurite number and length were 3 and 100 µm, respectively. However, these values were almost doubled after 14 days of cell culture. Conclusions: These results may improve the use of NGNF as a conductive scaffold for nerve regeneration. [ABSTRACT FROM AUTHOR]

We demonstrate high-performance field-effect transistors consisting of hybrid graphene/fluoropolymer films by investigating the molecular interaction with strong carbon–fluorine dipoles and hydrophobic surface characteristics. A simple and reproducible solution-coating method on a chemical vapor deposition grown monolayer graphene was employed with fluoropolymer, amorphous CYTOP, and polycrystalline poly(vinylidene fluoride-co-trifluoroethylene) as an interacting layer. All of the key device metrics, including field-effect mobility, Dirac voltage, residual carrier concentration, ON–OFF current ratio, and electron–hole transport symmetry, were substantially improved. Significantly, the Dirac voltage shifted in both directions toward zero, regardless of its initial positions of the Dirac voltage after molecular encapsulation with fluoropolymers. We also demonstrate the improved stability of hybrid graphene/fluoropolymer films that possesses a hydrophobic surface through repelling hydroxyl (–OH) functional groups from the graphene surface. [ABSTRACT FROM PUBLISHER]

Low-energy ion scattering (LEIS)is known for its extreme surfacesensitivity, as it yields a quantitative analysis of the outermostsurface as well as highly resolved in-depth information for ultrathinsurface layers. Hence, it could have been generally considered tobe a suitable technique for the analysis of graphene samples. However,due to the low scattering cross section for light elements such ascarbon, LEIS has not become a common technique for the characterizationof graphene. In the present study we use a high-sensitivity LEIS instrumentwith parallel energy analysis for the characterization of CVD graphenetransferred to thermal silica/silicon substrates. Thanks to its highsensitivity and the exceptional depth resolution typical of LEIS,the graphene layer closure was verified, and different kinds of contaminantswere detected, quantified, and localized within the graphene structure.Utilizing the extraordinarily strong neutralization of helium by carbonatoms in graphene, LEIS experiments performed at several primary ionenergies permit us to distinguish carbon in graphene from that innongraphitic forms (e.g., the remains of a resist). Furthermore, metalimpurities such as Fe, Sn, and Na located at the graphene–silicainterface (intercalated) are detected, and the coverages of Fe andSn are determined. Hence, high-resolution LEIS is capable of bothchecking the purity of graphene surfaces and detecting impuritiesincorporated into graphene layers or their interfaces. Thus, it isa suitable method for monitoring the quality of the whole fabricationprocess of graphene, including its transfer on various substrates. [ABSTRACT FROM AUTHOR]

During chemical vapor deposition (CVD) growth of graphene on Ru(0001) surface, a specific C cluster, which has 13 C atoms arranged as three connected hexagons (so called 3-C), was supposed to be extremely stable. To verify this, we systemically explored the stabilities of carbon clusters C (N = 12, 13, 14) on the Ru(0001) surface by using first principle approach. While, it is surprising that neither 3-C is the ground state of C nor the C cluster shows exceptional stability. So, based on the experimental STM image with C symmetry and a dimmer of 1 nm, the magic cluster was proposed as C or C-3C, which have the same core composed of six hexagons and three pentagons, as that studied before. This study is helpful to resolve the debate on the dominating clusters observed during graphene CVD growth and is helpful for understanding the growth mechanism of graphene CVD growth. [ABSTRACT FROM AUTHOR]

This work presents an improved thin film approximation to extract the optical conductivity from infrared transmittance in a simple yet accurate way. This approximation takes into account the incoherent reflections from the backside of the substrate. These reflections are shown to have a significant effect on the extracted optical conductivity and hence on derived parameters as carrier mobility and density. By excluding the backside reflections, the error for these parameters for typical chemical vapor deposited (CVD) graphene on a silicon substrate can be as high as 17% and 45% for the carrier mobility and density, respectively. For the mid- and near-infrared, the approximation can be simplified such that the real part of the optical conductivity is extracted without the need for a parameterization of the optical conductivity. This direct extraction is shown for Fourier transform infrared (FTIR) transmittance measurements of CVD graphene on silicon in the photon energy range of 370-7000 cm-1. From the real part of the optical conductivity, the carrier density, mobility, and number of graphene layers are determined but also residue, originating from the graphene transfer, is detected. FTIR transmittance analyzed with the improved thin film approximation is shown to be a non-invasive, easy, and accurate measurement and analysis method for assessing the quality of graphene and can be used for other 2-D materials. [ABSTRACT FROM AUTHOR]

A hybrid anode system for lithium (Li) ion battery applications based on pulsed laser deposited silicon films on chemical vapor deposited multilayer graphene (MLG) layers on a nickel foam substrate was electrochemically characterized. The as-grown material was directly fabricated into an anode without a binder, and tested in a half-cell configuration. There is evidence of the participation of both the multilayer graphene and the Si in the transport of Li ions. Even when cycled under stressful voltage limits that accelerate degradation, the MLG-Si films displayed higher stability than Si-only anodes, especially at higher cycling rates. Unlike the Si cells that display capacity fade even within the first few cycles, the MLG-Si cells show a very narrow spread in capacity, indicative of the role of the graphene layers in improving adhesion of the Si and acting as a compliant buffer for its volume expansion. Stable average specific capacities of ~1,200 mAh/g per total weight of MLG + Si, over 80 cycles at C/5 rate, were obtained for the MLG-Si anode. Pre- and post-cycling characterization of the anode materials revealed the differences between the two systems. [ABSTRACT FROM AUTHOR]

Direct deposition of graphene on a dielectric substrate is demonstrated using a chemical vapor deposition system with a two-temperature reactor. The two-temperature reactor is utilized to offer sufficient, well-proportioned floating Cu atoms and to provide a temperature gradient for facile synthesis of graphene on dielectric surfaces. The evaporated Cu atoms catalyze the reaction in the presented method. C atoms and Cu atoms respectively act as the nuclei for forming graphene film in the low-temperature zone and the zones close to the high-temperature zones. A uniform and high-quality graphene film is formed in an atmosphere of sufficient and well-proportioned floating Cu atoms. Raman spectroscopy, scanning electron microscopy and atomic force microscopy confirm the presence of uniform and high-quality graphene. [ABSTRACT FROM AUTHOR]

Highlights: [•] Single 5ns laser shots ablate CVD graphene with threshold fluence ~200mJ/cm2. [•] Damage threshold for UV irradiation depends strongly on sample. [•] Raman spectra indicate formation of oxidized graphene in ablated areas. [•] Oxidized graphene residues in ablated areas produce strong fluorescence. [ABSTRACT FROM AUTHOR]

We investigated the use of the shrinkage of SU-8 resist caused by thermal annealing to apply strain into graphene grown by the chemical-vapor-deposition (CVD) method. We demonstrate that the shrinkage of resist deposited on top of graphene on a substrate induces a local tensile strain within a distance of 1–2 μm from the edge of the resist. The thermal shrinkage of SU-8 will allow us to design the local strain in graphene on substrates. We also show that the shrinkage induces a large tensile strain in graphene suspended between two bars of SU-8. We expect that a much larger strain can be induced by suppressing defects in CVD-grown graphene. [ABSTRACT FROM AUTHOR]