Fluid-Dynamics-Rectified Chemical Vapor Deposition (CVD) Preparing Graphene-Skinned Glass Fiber Fabric and Its Application in Natural Energy Harvest

Graphene chemical vapor deposition (CVD) growth directly on target using substrates presents a significant route toward graphene applications. However, the substrates are usually catalytic-inert and special-shaped; thus, large-scale, high-uniformity, and high-quality graphene growth is challenging. Herein, graphene-skinned glass fiber fabric (GGFF) was developed through graphene CVD growth on glass fiber fabric, a Widely used engineering material. A fluid dynamics rectification strategy was first proposed to synergistically regulate the distribution of carbon species in 3D space and their collisions with hierarchical-structured substrates, through which highly uniform deposition of high-quality graphene on fibers in large-scale 3D-woven fabric was realized. This strategy is universal and applicable to CVD systems using various carbon precursors. GGFF exhibits high electrical conductivity and photothermal conversion capability, based on which a natural energy harvester was first developed. It can harvest both solar and raindrop energy through solar heating and droplet-based electricity generating, presenting promising potentials to alleviate energy burdens.

Graphene Growth on Electroformed Copper Substrates by Atmospheric Pressure CVD

Chemical vapor deposition (CVD) is regarded as the most promising technique for the mass production of graphene. CVD synthesis under vacuum is the most employed process, because the slower kinetics give better control on the graphene quality, but the requirement for high-vacuum equipment heavily affects the overall energy cost. In this work, we explore the possibility of using electroformed Cu substrate as a catalyst for atmospheric-pressure graphene growth. Electrochemical processes can produce high purity, freestanding metallic films, avoiding the surface defects that characterize the rolled foils. It was found that the growth mode of graphene on the electroformed catalyst was related to the surface morphology, which, in turn, was affected by the preliminary treatment of the substrate material. Suitable conditions for growing single layer graphene were identified.

Chemical vapour deposition of graphene on copper-nickel alloys: the simulation of a thermodynamic and kinetic approach

Chemical vapour deposition (CVD) of graphene on transition metals is generally believed to be the fabrication route best suited for the production of high-quality large-area graphene sheets. The mechanism of CVD graphene growth is governed by interactions in both the gas phase and at the surface. Here we present a simulation of the CVD graphene growth mechanism which includes thermodynamics, gas phase kinetics and the surface reaction in a sequential manner. The thermodynamic simulation shows that the deposition driving force is the greatest for high carbon to hydrogen ratios and reaches a maximum at around 850 °C. No graphene growth is observed below this temperature. The surface kinetic model also shows that below this temperature, the carbon surface concentration is less than the solubility limit, thus no film can grow. The effect of the reaction chamber geometry on the product concentrations was clear from the gas phase decomposition reactions. The gas residence times studied here (around 0.07 s) show that the optimum gas phase composition is far from that expected at thermodynamic equilibrium. The surface kinetics of CH₄ reactions on Ni, Cu and Cu-Ni surfaces shows good agreement with the experimental results for different growth pressures (0.1 to 0.7 mbar), temperatures (600 to 1200 °C) and different Ni thicknesses (25-500 μm). Also, the model works well when substrates with various C solubilities are used. The thermodynamic and kinetic models described here can be used for the design of improved reactors to optimise the production of graphene with differing qualities, either single or multi-layer and sizes. More importantly, the transfer to a continuous process with a moving substrate should also be possible using the model if it is extended from 2D to 3D.

Graphene growth through a recrystallization process in plasma enhanced chemical vapor deposition

Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (I _D/I _G < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq.

Temperature effect on the nucleation of graphene on Cu (111)

Repeated thermal cycling by using an organic precursor is shown to be a successful technique for growing graphene on metal substrates. Having control on this process is of vital importance in producing large areas of high quality graphene with well-ordered surface characteristics, which leads us to investigate the effect of temperature on the microscopic mechanisms behind this process. Apart from being an important factor in the dissociation of the organic precursor and promoting the reactions taking place on the surface of the catalyst, temperature also plays a major role in the structure of the catalyst surface. First, we used eight thermal cycles to successfully grow graphene on the surface of Cu (111). Then, we employed Ab Initio Molecular Dynamics (AIMD) simulations to study graphene island alignment evolution at two temperatures. The results shed light on our experimental observations and those reported in the literature and point to the effectiveness of controlled thermal cycling in producing high quality graphene sheets on transition metal catalyst surfaces.

Repeated thermal cycling by using an organic precursor is shown to be a successful technique for growing graphene on metal substrates.

In-Plane Thermal Conductivity of Polycrystalline Chemical Vapor Deposition Graphene with Controlled Grain Sizes

Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH₄/H₂), allowed for synthesis of polycrystalline single-layered graphene with controlled grain sizes. The graphene samples were then suspended on 8 μm diameter patterned holes on a silicon-nitride (Si₃N₄) substrate, and the in-plane thermal conductivities k(T) for 320 K < T < 510 K were measured to be 2660-1230, 1890-1020, and 680-340 W/m·K for average grain sizes of 4.1, 2.2, and 0.5 μm, respectively, using an opto-thermal Raman technique. Fitting of these data by a simple linear chain model of polycrystalline thermal transport determined k = 5500-1980 W/m·K for single-crystal graphene for the same temperature range above; thus, significant reduction of k was achieved when the grain size was decreased from infinite down to 0.5 μm. Furthermore, detailed elaborations were performed to assess the measurement reliability of k by addressing the hole-edge boundary condition, and the air-convection/radiation losses from the graphene surface.

Mechanical Robustness of Graphene on Flexible Transparent Substrates

This study reports on a facile and widely applicable method of transferring chemical vapor deposited (CVD) graphene uniformly onto optically transparent and mechanically flexible substrates using commercially available, low-cost ultraviolet adhesive (UVA) and hot-press lamination (HPL). We report on the adhesion potential between the graphene and the substrate, and we compare these findings with those of the more commonly used cast polymer handler transfer processes. Graphene transferred with the two proposed methods showed lower surface energy and displayed a higher degree of adhesion (UVA: 4.40 ± 1.09 N/m, HPL: 0.60 ± 0.26 N/m) compared to equivalent CVD-graphene transferred using conventional poly(methyl methacrylate) (PMMA: 0.44 ± 0.06 N/m). The mechanical robustness of the transferred graphene was investigated by measuring the differential resistance as a function of bend angle and repeated bend-relax cycles across a range of bend radii. At a bend angle of 100° and a 2.5 mm bend radius, for both transfer techniques, the normalized resistance of graphene transferred on polyethylene terephthalate (PET) was around 80 times less than that of indium-tin oxide on PET. After 10(4) bend cycles, the resistance of the transferred graphene on PET using UVA and HPL was found to be, on average, around 25.5 and 8.1% higher than that of PMMA-transferred graphene, indicating that UVA- and HPL-transferred graphene are more strongly adhered compared to PMMA-transferred graphene. The robustness, in terms of maintained electrical performance upon mechanical fatigue, of the transferred graphene was around 60 times improved over ITO/PET upon many thousands of repeated bending stress cycles. On the basis of present production methods, the development of the next-generation of highly conformal, diverse form factor electronics, exploiting the emerging family of two-dimensional materials, necessitates the development of simple, low-cost, and mechanically robust transfer processes; the developed UVA and HPL approaches show significant potential and allow for large-area-compatible, near-room temperature transfer of graphene onto a diverse range of polymeric supports.

Realizing controllable graphene nucleation by regulating the competition of hydrogen and oxygen during chemical vapor deposition heating

Controllable graphene nucleation on a CuO/Cu substrate can be realised by adjusting the hydrogen introduction temperature.

Metamaterials and imaging

Resolution of the conventional lens is limited to half the wavelength of the light source by diffraction. In the conventional optical system, evanescent waves, which carry sub-diffraction spatial information, has exponentially decaying amplitude and therefore cannot reach to the image plane. New optical materials called metamaterials have provided new ways to overcome diffraction limit in imaging by controlling the evanescent waves. Such extraordinary electromagnetic properties can be achieved and controlled through arranging nanoscale building blocks appropriately. Here, we review metamaterial-based lenses which offer the new types of imaging components and functions. Perfect lens, superlenses, hyperlenses, metalenses, flat lenses based on metasurfaces, and non-optical lenses including acoustic hyperlens are described. Not all of them offer sub-diffraction imaging, but they provide new imaging mechanisms by controlling and manipulating the path of light. The underlying physics, design principles, recent advances, major limitations and challenges for the practical applications are discussed in this review.

Nitrogen segregation in nanocarbons

We explore the behaviour of nitrogen doping in carbon nanomaterials, notably graphene, nanotubes, and carbon thin films. This is initially via a brief review of the literature, followed by a series of atomistic density functional calculations. We show that at low concentrations, substitutional nitrogen doping in the sp(2)-C graphenic basal plane is favoured, however once the nitrogen concentration reaches a critical threshold there is a transition towards the formation of the more thermodynamically-favoured nitrogen terminated 'zigzag' type edges. These can occur either via formation of finite patches (polycyclic aromatic azacarbons), strips of sp(2) carbon with zigzag nitrogen edges, or internal nitrogen-terminated hole edges within graphenic planes. This transition to edge formation is especially favoured when the nitrogen can be partially functionalised with, e.g. hydrogen. By comparison with available literature results, notably from electron energy loss spectroscopy and X-ray spectroscopy, the current results suggest that much of the nitrogen believed to be incorporated into carbon nanoobjects is instead likely to be present terminating the edges of carbonaceous impurities attached to nanoobject's surface. By comparison to nitrogen-doped tetrahedrally amorphous carbon, we suggest that this transition at around 10-20% nitrogen concentration and above towards sp(2) coordination via internal nitrogen-terminated edge formation may be a general property of nitrogen-doped carbon materials.

Stripe distributions of graphene-coated Cu foils and their effects on the reduction of graphene wrinkles

The wrinkle distribution of graphene domain was obtained as trenches after hydrogen etching. Parallel stripes on graphene domains are always perpendicular to these trenches, suggesting the suppressed wrinkle formation along the stripes' direction.

Frictional behavior of atomically thin sheets: hexagonal-shaped graphene islands grown on copper by chemical vapor deposition

Single asperity friction experiments using atomic force microscopy (AFM) have been conducted on chemical vapor deposited (CVD) graphene grown on polycrystalline copper foils. Graphene substantially lowers the friction force experienced by the sliding asperity of a silicon AFM tip compared to the surrounding oxidized copper surface by a factor ranging from 1.5 to 7 over loads from the adhesive minimum up to 80 nN. No damage to the graphene was observed over this range, showing that friction force microscopy serves as a facile, high contrast probe for identifying the presence of graphene on Cu. Consistent with studies of epitaxially grown, thermally grown, and mechanically exfoliated graphene films, the friction force measured between the tip and these CVD-prepared films depends on the number of layers of graphene present on the surface and reduces friction in comparison to the substrate. Friction results on graphene indicate that the layer-dependent friction properties result from puckering of the graphene sheet around the sliding tip. Substantial hysteresis in the normal force dependence of friction is observed with repeated scanning without breaking contact with a graphene-covered region. Because of the hysteresis, friction measured on graphene changes with time and maximum applied force, unless the tip slides over the edge of the graphene island or contact with the surface is broken. These results also indicate that relatively weak binding forces exist between the copper foil and these CVD-grown graphene sheets.

Influence of gas phase equilibria on the chemical vapor deposition of graphene

We have investigated the influence of gas phase chemistry on the chemical vapor deposition of graphene in a hot wall reactor. A new extended parameter space for graphene growth was defined through literature review and experimentation at low pressures (≥0.001 mbar). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy, and dark field optical microscopy, with the latter showing promise as a rapid and nondestructive characterization technique for graphene films. The equilibrium gas compositions have been calculated across this parameter space. Correlations between the graphene films grown and prevalent species in the equilibrium gas phase revealed that deposition conditions associated with a high acetylene equilibrium concentration lead to good quality graphene deposition, and conditions that stabilize large hydrocarbon molecules in the gas phase result in films with multiple defects. The transition between lobed and hexagonal graphene islands was found to be linked to the concentration of the monatomic hydrogen radical, with low concentrations associated with hexagonal islands.