The propagation of fast electron currents in near solid-density media was investigated via proton probing. Fast currents were generated inside dielectric foams via irradiation with a short (∼0.6 ps) ...laser pulse focused at relativistic intensities (Iλ^{2}∼4×10^{19} W cm^{-2} μm^{2}). Proton probing provided a spatially and temporally resolved characterization of the evolution of the electromagnetic fields and of the associated net currents directly inside the target. The progressive growth of beam filamentation was temporally resolved and information on the divergence of the fast electron beam was obtained. Hybrid simulations of electron propagation in dense media indicate that resistive effects provide a major contribution to field generation and explain well the topology, magnitude, and temporal growth of the fields observed in the experiment. Estimations of the growth rates for different types of instabilities pinpoints the resistive instability as the most likely dominant mechanism of beam filamentation.
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► Prepared highly concentrated WS2 suspensions. ► Exfoliated WS2 nanoparticles show emergent photoluminescence. ► Exfoliated WS2 has an increased bandgap in comparison to the bulk ...material.
Single and few layered tungsten disulphide (WS2) nanoparticles were prepared using a surfactant assisted ultrasonication exfoliation technique with concentrations of up to 0.4mg/mL. The lateral dimension of the particles was in the range of 100–250nm. The exfoliated WS2 was stabilised against re-aggregation through adsorption of a tri-block non-ionic polymeric surfactant (PEO–PPO–PEO). These nanoparticles were characterised by absorption, Raman and photoluminescence spectroscopy (PL). Broadening of the E2g peak in the Raman spectrum was observed due to phonon confinement within a single layer of WS2. The exfoliated particles have significantly different properties than the bulk WS2 material, in particular, the emergence of strong photoluminescence at 1.97eV in energy coincidental with the excitonic peak in the UV–Vis spectrum. The emergent PL emission suggests that the monolayer WS2 is a direct gap material analogous to other dichalcogenides such as MoS2.
Conventional liquid-phase graphite exfoliation (LPE) dramatically reduce the lateral dimension of graphene sheets to submicrometer levels due to bond cleavage induced by high shearing force or long ...processing time, resulting in highly degraded properties of graphene materials. Herein, a modified high-yielding LPE for producing graphene in the cosolvents of ethanol and water is demonstrated, via the prior use of an electrochemical expansion process on graphite. The electrochemically expanded graphite allows the use of significantly lower sonication power and shorter sonication times. Therefore graphene platelets with largely increased lateral dimension were achieved compared to conventional LPE (the size can reach up to 10 μm). The electrical and mechanical properties of graphene film are significantly enhanced as a result, with the electrical conductivity doubled and the modulus increased by a factor of 4 as well as a considerably higher areal capacitance for the assembled solid supercapacitor. Furthermore, a type of multifunctional benzoxazine surfactant was used to stabilize graphene sheets, which can also facilitate to transfer graphene sheets into organic solvents from aqueous dispersions. On this basis, polymer-graphene nano-composites have been easily prepared for both water soluble poly(vinyl alcohol) (PVA) and organic soluble poly(methyl methacrylate) (PMMA) with improved mechanical properties and thermal diffusivity.
A way to highly effectively produce graphene sheets with large lateral dimension in aqueous solution is demonstrated by combining electrochemical and ultrasonic methods. Electrical and mechanical properties of graphene film materials fabricated with these graphene sheets are largely improved. As well, polymer-graphene composites can be prepared conveniently both for water soluble polymers and for organic soluble polymers. Display omitted
Single and few layer molybdenum disulfide (MoS2) was exfoliated from the bulk form through a liquid phase exfoliation procedure. Highly concentrated suspensions were prepared that were stabilized ...against reaggregation through adsorption of nonionic polymers to the sheet surface. These exfoliated particles showed strong photoluminescence at an energy of 1.97 eV which is in the visible-light region. These exfoliated MoS2 sheets were then used to catalyze the degradation of a model dye upon exposure to visible light.
Highly concentrated suspensions of graphene stabilized with surfactant were prepared using ultrasonic exfoliation. Concentrations of up to 1.5% w/w (15 mg/mL) were achieved through the continuous ...addition of the surfactant during the exfoliation process. Previous methods typically add the surfactant only once, prior to the commencement of sonication. The vast increase in the available solid–liquid interfacial area through delamination results in the rapid depletion of the surfactant from solution through adsorption. This leads to a change in the liquid–vapor surface tension outside of the optimum range for the efficient production of graphene sheets. By continuously replacing the surfactant to lower the surface tension during sonication and the production of the graphene surface area, the concentration of particles was significantly increased. Cationic, anionic, and nonionic surfactants were studied and all showed significant increases in the concentration of graphene produced using this continuous addition method.
One of the grand challenges of contemporary physics is understanding strongly interacting quantum systems comprising such diverse examples as ultracold atoms in traps, electrons in high-temperature ...superconductors and nuclear matter. Warm dense matter, defined by temperatures of a few electron volts and densities comparable with solids, is a complex state of such interacting matter. Moreover, the study of warm dense matter states has practical applications for controlled thermonuclear fusion, where it is encountered during the implosion phase, and it also represents laboratory analogues of astrophysical environments found in the core of planets and the crusts of old stars. Here we demonstrate how warm dense matter states can be diagnosed and structural properties can be obtained by inelastic X-ray scattering measurements on a compressed lithium sample. Combining experiments and ab initio simulations enables us to determine its microscopic state and to evaluate more approximate theoretical models for the ionic structure.
We report on the temporally and spatially resolved detection of the precursory stages that lead to the formation of an unmagnetized, supercritical collisionless shock in a laser-driven laboratory ...experiment. The measured evolution of the electrostatic potential associated with the shock unveils the transition from a current free double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. Supported by a matching particle-in-cell simulation and theoretical considerations, we suggest that this process is analogous to ion reflection at supercritical collisionless shocks in supernova remnants.
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► Polymer stabilised graphene nanoparticles used in multilayers. ► Layer-by-layer build-up dependent on the pH of the solution. ► Complete coverage was achieved after five bi-layers ...of graphene and polyelectrolyte. ► Kinetics of the adsorption process monitored using QCM.
Exfoliated graphene particles stabilised by the cationic polyelectrolyte polyethyleneimine (PEI) were used in conjunction with an anionic polyelectrolyte, poly(acrylic acid), to construct multilayers using the layer-by-layer technique on a silica substrate. In the first adsorption step, the surface excess of the cationic graphene was dependent on the overall charge on the nanoparticle which in turn can be tuned through modifying solution pH as PEI has weakly ionisable charged amine groups. The adsorbed amount onto the silica surface increased as the solution pH increased. Subsequently, a layer of PAA was adsorbed on top of the cationic graphene through electrostatic interaction. The multilayer could be assembled through this alternate deposition, with the influence of solution conditions investigated. The pH of the adsorbing solutions was the chief determinant of the overall adsorbed amounts, with more mass added at the elevated pH of 9 in comparison with pH 4. Atomic force microscopy confirmed that the graphene particles were adsorbed to the silica interface and that the surface coverage of the disc-like nanoparticles was complete after the deposition of five graphene-polyelectrolyte bi-layers. Furthermore, the graphene nanoparticles themselves could be modified through the consecutive addition of the oppositely charged polymers. A multilayered assembly of negatively charged graphene sheets modified with a bi-layer of PEI and PAA was also deposited on a silica surface with adsorbed PEI.
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