1,721,066 research outputs found
Plasma enhanced hot filament CVD growth of thick carbon nanowall layers
No full textCarbon nanowalls are carbon nanostructures consisting of arrays of graphitic carbon plates which are mainly positioned perpendicularly to the growth surface. Carbon nanowalls have received considerable interest in recent years, since they are closely related to graphene from the structural point of view, while maintaining an open honeycomb lattice on the nanoscale. They are thus believed to be an interesting electrode material for many applications since they offer high chemical resistance, low electrical resistance and high surface area. In this paper we are presenting a method that allows the growth of thick layers of carbon nanowalls onto flat and porous substrates, both carbon and refractory metal based. Such methods are promising for making electrodes for use in electrochemical devices. © 2017 Author(s)
An ab initio study of hydroxylated graphane
No full textGraphene-based derivatives with covalent functionalization and well-defined stoichiometry are highly desirable in view of their application as functional surfaces. Here, we have evaluated by ab initio calculations the energy of formation and the phase diagram of hydroxylated graphane structures, i.e., fully functionalized graphene derivatives coordinated with -H and -OH groups. We compared these structures to different hydrogenated and non-hydrogenated graphene oxide derivatives, with high level of epoxide and hydroxyl groups functionalization. Based on our calculations, stable phases of hydroxylated graphane with low and high contents of hydrogen are demonstrated for high oxygen and hydrogen partial pressure, respectively. Stable phases of graphene oxide with a mixed carbon hybridization are also found. Notably, the synthesis of hydroxylated graphane has been recently reported in the literature. © 2017 Author(s)
Effects of the substrate on graphone magnetism: A density functional theory study
No full textThe magnetism of graphone, a single-side-hydrogenated graphene derivative, has been related to the localized and unpaired p-electrons associated with the unhydrogenated carbon atoms. In the present density functional theory study, the effects the adhesion to either Cu(111) or α-quartz (0001) surface on the magnetic properties of graphone have been investigated. The total magnetization of the graphone adsorbed to copper and quartz surface is reduced by four and two times, respectively, with respect to the isolated graphone. We have shown there is electronic charge transfer from surface towards three-fold coordinated C atoms of graphone, but the main role in the partial magnetism quenching is played by bond formation and the consequent electron pairing of p-electrons. The critical temperature has been investigated on the basis of the mean field theory to evaluate the stability of the magnetism at ordinary temperature. © 2015 Elsevier B.V. All rights reserved
Contamination-free graphene by chemical vapor deposition in quartz furnaces
No full textAlthough the growth of graphene by chemical vapor deposition is a production technique that guarantees high crystallinity and superior electronic properties on large areas, it is still a challenge for manufacturers to efficiently scale up the production to the industrial scale. In this context, issues related to the purity and reproducibility of the graphene batches exist and need to be tackled. When graphene is grown in quartz furnaces, in particular, it is common to end up with samples contaminated by heterogeneous particles, which alter the growth mechanism and affect graphene's properties. In this paper, we fully unveil the source of such contaminations and explain how they create during the growth process. We further propose a modification of the widely used quartz furnace configuration to fully suppress the sample contamination and obtain identical and clean graphene batches on large areas. © 2017 The Author(s)
Ethanol-CVD Growth of Sub-mm Single-Crystal Graphene on Flat Cu Surfaces
No full textHigh-quality graphene can be produced in large scale by chemical vapor deposition (CVD). Ethanol is emerging as a versatile carbon source alternative to methane for the growth of graphene on a copper (Cu) foil catalyst. To date, rigorous studies of the ethanol-based process still lack, especially concerning the first stages of the growth, which ultimately determines graphene's properties, such as defect density and crystal size, and performance, such as electrical conductance and mechanical strength. In particular, so far the growth of isolated graphene grains by ethanol-CVD has been achieved only on preoxidized Cu foils folded in enclosures, in an attempt to limit the partial pressure of the precursor, and thus the nucleation rate. We systematically explored the process parameters of ethanol-CVD to obtain full control over the nucleation rate, grain size, and crystallinity of graphene on flat Cu foils, which are of interest for any realistic production in large scale. To limit the nucleation density and increase the grain size, preoxidized Cu foils (250 °C in air) were used as substrates, and the process parameters were thoroughly investigated and tuned. Ultimately, at an ethanol vapor flow of 1.5 × 10-3 sccm the nucleation density reduced to less than 3 nuclei/mm2 and isolated single-crystal grains grew with a lateral size above 500 μm. When transferred onto Si/SiO2 substrates, these grains showed field-effect mobility beyond 1300 cm2/(V s). Our results provide a step closer towards an affordable commercialization of electronic-grade, large-area graphene. © 2018 American Chemical Society
Carbon dioxide valorisation with partial oxidation of methane in a water cooled DBD plasma
Full text linkThe valorisation of carbon dioxide in chemical plasmas implies as a principal reaction step its energy efficient dissociation into carbon monoxide. For hydrogen production, the reaction carbon monoxide with water (WGS) may lead to the generation of green hydrogen and reusable carbon dioxide. Beyond hydrogen, most valorisation processes require the removal of O2 to avoid its recombination with CO on any downstream hot catalytic surface. Moreover, if the oxygen scavenging is performed directly inside the plasma volume, it can also shift the dissociation equilibrium that is responsible for the well-known trade-off between energy efficiency and conversion, thus improving efficiency when larger specific energy densities are applied. In this paper we first report on the plasma dissociation of pure CO2 in a water cooled, high power(<2 kW), Dielectric Barrier Discharge with high gas flow regime (<3600sccm), and then we explore the synergistic oxygen removal by the partial oxidation of methane for syngas production. The presence of CH4, even in small amounts, removes oxygen from the outstream and from the discharge region, as confirmed by mass and optical emission spectroscopies, and enhances the process in two ways: it allows to feed the system with gas having a low CH4 to CO2 ratio (0.1–0.3) instead of pure CO2, where landfill and waste gases are undesired climate altering emissions with a similar composition that currently require flaring; it will allow to use directly the reactor outstream into a WGS reactor, or any other CO valorisation process without the necessity to remove downstream O2
Electronic and optical properties of metal decorated nitrogen-doped vacancy defects in graphene
No full textWe present a first principles study of the stability, and of the electronic and optical properties of graphene with nitrogen doped vacancies. Moreover, we use the vacancies as anchoring sites for Mg, Zn, Pd al Pt atoms and vary the concentration of defects. Decoration of the defects with metal atoms produces semi-metallic systems for any studied size of the cell, with linear bands crossing at the Fermi level. The peculiar electronic properties of massless Dirac fermions in graphene are hence kept, although with anisotropic Fermi velocities. New sharp peaks appear in the optical conductivity in the visible range, thus strongly enhancing the optical response of graphene
High frequency performance limits of nanointerconnects based on CVD-grown graphene films transferred on SiO2-substrate
No full textGraphene films are grown by chemical vapour deposition on copper layer and then transferred onto a silicon substrate, coated with silicon dioxide. The topological characterization of the produced film is performed by atomic force microscopy, and the sheet resistance is measured by applying the four-probe test method. The equivalent single conductor model is then used in order to analyze the signal propagation along a nanointerconnect made with multilayer graphene over silicon dioxide, in a wide frequency range, up to 100 GHz. The comparison of the radio-frequency performances of the nanointerconnect, modeled by using either the measured value of effective resistivity or a theoretical estimation of the p.u.l. resistance, suggests that graphene films grown by chemical vapor deposition are more suitable for application as low frequency electrical interconnections in flexible electronics, than in high-speed integrated circuits. © 2013 IEEE
Electrochemical synthesis of self-organized TiO2 crystalline nanotubes without annealing
No full textThis work demonstrates that upon anodic polarization in an aqueous fluoride-containing electrolyte, TiO2 nanotube array films can be formed with a well-defined crystalline phase, rather than an amorphous one. The crystalline phase was obtained avoiding any high temperature annealing. We studied the formation of nanotubes in an HF/H2O medium and the development of crystalline grains on the nanotube wall, and we found a facile way to achieve crystalline TiO2 nanotube arrays through a one-step anodization. The crystallinity of the film was influenced by the synthesis parameters, and the optimization of the electrolyte composition and anodization conditions (applied voltage and time) were carried out. For comparison purposes, crystalline anatase TiO2 nanotubes were also prepared by thermal treatment of amorphous nanotubes grown in an organic bath (ethylene glycol/NH4F/H2O). The morphology and the crystallinity of the nanotubes were studied by field emission gun-scanning electron microscopy (FEG-SEM) and Raman spectroscopy, whereas the electrochemical and semiconducting properties were analyzed by means of linear sweep voltammetry, impedance spectroscopy, and Mott-Schottky plots. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) allowed us to determine the surface composition and the electronic structure of the samples and to correlate them with the electrochemical data. The optimal conditions to achieve a crystalline phase with high donor concentration are defined. © 2018 IOP Publishing Ltd
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