Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm(2) current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far.

Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma 1 can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread 2-5 , low emittance 6 and up to 1 GeV peak energy 7,8 . Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection 9 , contains a temporal feature that can be identified as a 15 pC, 1.4-1.8 fs electron bunch (root mean square) leading to a peak current of 3-4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources 10,11 , and will benefit the realization of laboratory-scale free-electron lasers 12-14 .The ponderomotive force generated by the focused laser pulse is proportional to the gradient of the laser intensity. It pushes electrons out of regions of high intensity and separates them from the ions, thus creating a plasma wave that propagates in the wake of the laser pulse with a phase velocity close to c, the speed of light in vacuum. The characteristic length of the accelerating cavity that forms behind the driving laser pulse is the plasma wavelength λ p . In a typical laser wakefield acceleration experiment, λ p = 10-30 µm for plasma densities n e = 10 18 -10 19 cm −3 . The electric field changes along the length of the plasma wave, therefore, to generate an electron beam with low energy spread and low divergence, the electron bunch should reside within the focusing and accelerating phase of the wave, which has a length λ p /4. These heuristic arguments indicate that one would expect the bunch duration to be ultrashort, τ < λ p /4c ≈ 10 fs (refs 9,15). However, to the best of our knowledge, such short durations have not previously been directly measured.Traditional techniques to measure the electron bunch duration, such as streak cameras and radio-frequency sweeping cavities, do not have the temporal resolution required for femtosecond bunches. Therefore, we have employed a method in the frequency domain and measured the coherent transition radiation (CTR) that is emitted by the electron bunch as it passes through a thin metallic foil. CTR is an established particle beam diagnostic and has been used to diagnose micro-structures of picosecond-bunches 16,17 and for benchmarking simulations of femtosecond-bunch dynamics

Assembly of graphene into functional macroscopic objects, such as fi lms, [ 1 ] sheets, [ 2 ] fi bers, [ 3 ] foams, [ 4,5 ] and other complex architectures, [ 6 ] is of enormous research interest. How to attain desired structures in a cost effective and manufacturable manner is crucial for energy harvest/storage, catalysis, sensors and so on. Unlike fullerene or carbon nanotubes, whose assembly generally relies on weak van der Walls force or chemical modifi cation, two-dimensional graphene may straightforwardly exploit strong interlayer π -π stacking. Unfortunately, such a strong and directional interaction frequently results in graphitic stacking with minimal surface area. [ 7,8 ] Gelation is a straightforward route to macroscopic functional materials from graphene. Taking advantage of high electrical conductivity, large surface area, and soft hydrated character, graphene gel possesses enormous potentials for supercapacitor electrode, [9][10][11][12] catalytic support, [ 13,14 ] cell growth scaffold [ 15 ] and so on. [ 16,17 ] To date, several graphene gelation principles have been developed, including reduction of graphene oxide (GO) dispersion, [ 9,10,18 ] fl ow directed interfacial assembly [ 19 ] and template assembly. [ 20 ] Nevertheless, arbitrary large scale production of optimized porous structures via minimal processing steps remains formidable technological challenge.We present a surprisingly simple and versatile graphene gelation principle capable of three-dimensional shape engineering of micrometer thick hydrogels without any practical size limit. Simple immersion of arbitrary shaped Zn objects in aqueous GO dispersion spontaneously generates graphene hydrogel fi lms at Zn surfaces. This site specifi c gelation enables a wide range controllability of three-dimensional gel structures in porous morphology as well as macroscopic object scale according to customized purposes. Signifi cantly, this gelation principle has been exploited for high rate, large capacity supercapacitor electrodes. In general, fast charging/discharging rate (or power density) is hardly compatible with large areal capacity [21][22][23] (or energy density) for energy devices. While thin supercapacitor electrodes with facile electrolyte transports are favorable for high rate capacity, thick electrodes are desired for large areal capacity. [24][25][26] In this work, three-dimensional controllability of graphene gel morphology optimized the aqueous electrolyte transport within suffi ciently thick gel structures. Consequently, fundamental challenge to attain large areal capacity without sacrifi cing rate capability is successfully addressed.Synthetic scheme of graphene hydrogel is presented in Figure 1 a. While Zn foils are immersed in mild acidic dispersion of GO, black graphene hydrogels spontaneously grow at Zn surface. The grown gel thickness is roughly tunable with immersion time. Typically, one hour deposition produced 78-μ m-thick gel fi lms in 10 −3 M hydrochlorid acid (HCl) containing GO dispersion (Supporting information, Fig...

Substitutional heteroatom doping is a promising route to modulate the outstanding material properties of carbon nanotubes and graphene for customized applications. Recently, (nitrogen-) N-doping has been introduced to ensure tunable work-function, enhanced n-type carrier concentration, diminished surface energy, and manageable polarization. Along with the promising assessment of N-doping effects, research on the N-doped carbon based composite structures is emerging for the synergistic integration with various functional materials. This invited feature article reviews the current research progress, emerging trends, and opening opportunities in N-doped carbon based composite structures. Underlying basic principles are introduced for the effective modulation of material properties of graphitic carbons by N-doping. Composite structures of N-doped graphitic carbons with various functional materials, including (i) polymers, (ii) transition metals, (iii) metal oxides, nitrides, sulphides, and (iv) semiconducting quantum dots are highlighted. Practical benefits of the synergistic composite structures are investigated in energy and catalytic applications, such as organic photovoltaics, photo/electro-catalysts, lithium ion batteries and supercapacitors, with a particular emphasis on the optimized interfacial structures and properties.

To take full advantage of a laser-plasma accelerator, stability and control of the electron beam parameters have to be achieved. The external injection scheme with two colliding laser pulses is a way to stabilize the injection of electrons into the plasma wave, and to easily tune the energy of the output beam by changing the longitudinal position of the injection. In this Letter, it is shown that by tuning the optical injection parameters, one is able to control the phase-space volume of the injected particles, and thus the charge and the energy spread of the beam. With this method, the production of a laser accelerated electron beam of 10 pC at the 200 MeV level with a 1% relative energy spread at full width half maximum (3.1% rms) is demonstrated. This unique tunability extends the capability of laser-plasma accelerators and their applications.

Substitutional N‐doping of carbon nanomaterials refers to the chemical functionalization method that replaces a part of the carbon atoms in fullerene, carbon nanotubes, or graphene by nitrogen. N‐doping has attracted a tremendous amount of research attention for their unique possibilities, spanning from its ability to engineer various physiochemical properties of carbon nanomaterials in a stable manner with different dopant configurations. Many viable configurations of N‐dopants are accompanied by typical structural defects, while still preserving the structural symmetry in the basal graphitic plane. Here, the physicochemical features are highlighted and the exciting challenges of N‐dopants in carbon nanomaterials identified, with particular emphasis on the broad tunability of the material properties and relevant emerging applications.

The refractive index of natural transparent materials is limited to 2–3 throughout the visible wavelength range. Wider controllability of the refractive index is desired for novel optical applications such as nanoimaging and integrated photonics. We report that metamaterials consisting of period and symmetry-tunable self-assembled nanopatterns can provide a controllable refractive index medium for a broad wavelength range, including the visible region. Our approach exploits the independent control of permeability and permittivity with nanoscale objects smaller than the skin depth. The precise manipulation of the interobject distance in block copolymer nanopatterns via pattern shrinkage increased the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1,000 nm wavelength bandwidth. Spatially graded and anisotropic refractive indices are also obtained with the design of transitional and rotational symmetry modification.

Graphene oxide (GO) is aqueous-dispersible oxygenated graphene, which shows colloidal discotic liquid crystallinity. Many properties of GO-based materials, including electrical conductivity and mechanical properties, are limited by the small flake size of GO. Unfortunately, typical sonochemical exfoliation of GO from graphite generally leads to a broad size and shape distribution. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction catalysis. In the biphasic GO dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously concentrated within the liquid crystalline phase. N-Doping and reduction of the size-selected GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO demonstrates quaternary dominant N-doping and the lowest onset potential (-0.08 V) for oxygen reduction catalysis, signifying that quaternary N-dopants serve as principal catalytic sites in N-doped graphene.