A Generic Hybrid Model for Bulk Elastodynamics, With Application to Ultrasonic Nondestructive Evaluation

Monolayer two-dimensional transitional metal dichalcogenides, such as MoS₂, WS₂, and WSe₂, are direct band gap semiconductors with large exciton binding energy. They attract growing attentions for optoelectronic applications including solar cells, photodetectors, light-emitting diodes and phototransistors, capacitive energy storage, photodynamic cancer therapy, and sensing on flexible platforms. While light-induced luminescence has been widely studied, luminescence induced by injection of free electrons could promise another important applications of these new materials. However, cathodoluminescence is inefficient due to the low cross-section of the electron–hole creating process in the monolayers. Here for the first time we show that cathodoluminescence of monolayer chalcogenide semiconductors can be evidently observed in a van der Waals heterostructure when the monolayer semiconductor is sandwiched between layers of hexagonal boron nitride (hBN) with higher energy gap. The emission intensity shows a strong dependence on the thicknesses of surrounding layers and the enhancement factor is more than 500-fold. Strain-induced exciton peak shift in the suspended heterostructure is also investigated by the cathodoluminescence spectroscopy. Our results demonstrate that MoS₂, WS₂, and WSe₂ could be promising cathodoluminescent materials for applications in single-photon emitters, high-energy particle detectors, transmission electron microscope displays, surface-conduction electron-emitter, and field emission display technologies

Atomically Dispersed Fe–N₄ and Ni–N₄ Independent Sites Enable Bidirectional Sulfur Redox Electrocatalysis

Single-atom catalysts (SACs) with high atom utilization and outstanding catalytic selectivity are useful for improving battery performance. Herein, atomically dispersed Ni–N4 and Fe–N4 dual sites coanchored on porous hollow carbon nanocages (Ni–Fe–NC) are fabricated and deployed as the sulfur host for Li–S battery. The hollow and conductive carbon matrix promotes electron transfer and also accommodates volume fluctuation during cycling. Notably, the high d band center of Fe in Fe–N4 site demonstrates strong polysulfide affinity, leading to an accelerated sulfur reduction reaction. Meanwhile, Li2S on the Ni–N4 site delivers a metallic property with high S 2p electron density of states around the Femi energy level, enabling a low sulfur evolution reaction barrier. The dual catalytic effect on Ni–Fe–NC endows sulfur cathode high energy density, prolonged lifespan, and low polarization

A New Type of Porous Graphite Foams and Their Integrated Composites with Oxide/Polymer Core/Shell Nanowires for Supercapacitors: Structural Design, Fabrication, and Full Supercapacitor Demonstrations

We attempt to meet the general design requirements for high-performance supercapacitor electrodes by combining the strategies of lightweight substrate, porous nanostructure design, and conductivity modification. We fabricate a new type of 3D porous and thin graphite foams (GF) and use as the light and conductive substrates for the growth of metal oxide core/shell nanowire arrays to form integrated electrodes. The nanowire core is Co₃O₄, and the shell is a composite of conducting polymer (poly­(3,4-ethylenedioxythiophene), PEDOT) and metal oxide (MnO₂). To show the advantage of this integrated electrode design (viz., GF + Co₃O₄/PEDOT–MnO₂ core/shell nanowire arrays), three other different less-integrated electrodes are also prepared for comparison. Full supercapacitor devices based on the GF + Co₃O₄/PEDOT–MnO₂ as positive electrodes exhibit the best performance compared to other three counterparts due to an optimal design of structure and a synergistic effect

Tailorable and Wearable Textile Devices for Solar Energy Harvesting and Simultaneous Storage

The pursuit of harmonic combination of technology and fashion intrinsically points to the development of smart garments. Herein, we present an all-solid tailorable energy textile possessing integrated function of simultaneous solar energy harvesting and storage, and we call it tailorable textile device. Our technique makes it possible to tailor the multifunctional textile into any designed shape without impairing its performance and produce stylish smart energy garments for wearable self-powering system with enhanced user experience and more room for fashion design. The “threads” (fiber electrodes) featuring tailorability and knittability can be large-scale fabricated and then woven into energy textiles. The fiber supercapacitor with merits of tailorability, ultrafast charging capability, and ultrahigh bending-resistance is used as the energy storage module, while an all-solid dye-sensitized solar cell textile is used as the solar energy harvesting module. Our textile sample can be fully charged to 1.2 V in 17 s by self-harvesting solar energy and fully discharged in 78 s at a discharge current density of 0.1 mA

Oxidation State Engineering in Octahedral Ni by Anchored Sulfate to Boost Intrinsic Oxygen Evolution Activity

Promoting the electron occupancy of active sites to unity is an effective method to enhance the oxygen evolution reaction (OER) performance of spinel oxides, but it remains a great challenge. Here, an in situ approach is developed to modify the valence state of octahedral Ni cations in NiFe2O4 inverse spinel via surface sulfates (SO42–). Different from previous studies, SO42– is directly anchored on the spinel surface instead of forming from uncontrolled conversion or surface reconstruction. Experiment and theoretical calculations reveal the precise adsorption sites and spatial arrangement for SO42– species. As a main promoting factor, surface SO42– effectively converts the crystal field stable Ni state (t2g6eg2) to the near-unity eg electron state (t2g6eg1). Moreover, the inevitable oxygen vacancies (Vo) further optimize the energy barrier of the potential-determining step (from OH* to O*). This co-modification strategy enhances turnover frequency-based electrocatalytic activity about two orders higher than the control sample without surface sulfates. This work may provide insight into the OER activity enhancement mechanism by the oxyanion groups

Robust, High-Density Zinc Oxide Nanoarrays by Nanoimprint Lithography-Assisted Area-Selective Atomic Layer Deposition

Polymer templates realized through a combination of block copolymer lithography (BCL) and nanoimprint lithography (NIL) are used to direct atomic layer deposition (ALD) to obtain high-quality ZnO nanopatterns. These patterns present a uniform array of ZnO nanostructures with sub-100 nm feature and spatial resolutions, exhibiting narrow distributions in size and separation, and enhanced mechanical stability. The process benefits from the high lateral resolutions determined by the copolymer pattern, controlled growth rates, material quality and enhanced mechanical stability from ALD and repeatability and throughput from NIL. The protocol is generic and readily extendible to a range of other materials that can be grown through ALD. By virtue of their high feature density and material quality, the electrical characteristics of the arrays incorporated within MOS capacitors display high hole-storage density of 7.39 × 10¹⁸ cm^–3, excellent retention of ∼97% (for 1000 s of discharging), despite low tunneling oxide thickness of 3 nm. These attributes favor potential application of these ZnO arrays as charge-storage centers in nonvolatile flash memory devices

High-Quality Metal Oxide Core/Shell Nanowire Arrays on Conductive Substrates for Electrochemical Energy Storage

The high performance of a pseudocapacitor electrode relies largely on a scrupulous design of nanoarchitectures and smart hybridization of bespoke active materials. We present a powerful two-step solution-based method for the fabrication of transition metal oxide core/shell nanostructure arrays on various conductive substrates. Demonstrated examples include Co₃O₄ or ZnO nanowire core and NiO nanoflake shells with a hierarchical and porous morphology. The “oriented attachment” and “self-assembly” crystal growth mechanisms are proposed to explain the formation of the NiO nanoflake shell. Supercapacitor electrodes based on the Co₃O₄/NiO nanowire arrays on 3D macroporous nickel foam are thoroughly characterized. The electrodes exhibit a high specific capacitance of 853 F/g at 2 A/g after 6000 cycles and an excellent cycling stability, owing to the unique porous core/shell nanowire array architecture, and a rational combination of two electrochemically active materials. Our growth approach offers a new technique for the design and synthesis of transition metal oxide or hydroxide hierarchical nanoarrays that are promising for electrochemical energy storage, catalysis, and gas sensing applications

Solution Transformation of Cu₂O into CuInS₂ for Solar Water Splitting

Though Cu₂O has demonstrated high performance as a photocathode for solar water splitting, its band gap is too large for efficient use as the bottom cell in tandem configurations. Accordingly, copper chalcopyrites have recently attracted much attention for solar water splitting due to their smaller and tunable band gaps. However, their fabrication is mainly based on vacuum evaporation, which is an expensive and energy consuming process. Here, we have developed a novel and low-cost solution fabrication method, and CuInS₂ was chosen as a model material due to its smaller band gap compared to Cu₂O and relatively simple composition. The nanostructured CuInS₂ electrodes were synthesized at low temperature in crystalline form by solvothermal treatment of electrochemically deposited Cu₂O films. Following the coating of overlayers and decoration with Pt catalyst, the as-fabricated CuInS₂ electrode demonstrated water splitting photocurrents of 3.5 mA cm^–2 under simulated solar illumination. To the best of our knowledge, this is the highest performance yet reported for a solution-processed copper chalcopyrite electrode for solar water splitting. Furthermore, the electrode showed good stability and had a broad incident photon-to-current efficiency (IPCE) response to wavelengths beyond 800 nm, consistent with the smaller bandgap of this material

Plasmonic Nanoclocks

Plasmonic spectra of “nanoclock” metamaterials can be topologically mapped on a torus. We manufactured arrays of such a metamaterial with different “time” shown on the clocks and demonstrated that the near-infrared spectra of the nanostructures can be predictably tuned exhibiting a rich series of high-order plasmon modes, from the electric dipole to exotic electric triakontadipole that could be engaged in chemo/biosensor applications

Three-Dimensional Graphene Foam Supported Fe₃O₄ Lithium Battery Anodes with Long Cycle Life and High Rate Capability

Fe₃O₄ has long been regarded as a promising anode material for lithium ion battery due to its high theoretical capacity, earth abundance, low cost, and nontoxic properties. However, up to now no effective and scalable method has been realized to overcome the bottleneck of poor cyclability and low rate capability. In this article, we report a bottom-up strategy assisted by atomic layer deposition to graft bicontinuous mesoporous nanostructure Fe₃O₄ onto three-dimensional graphene foams and directly use the composite as the lithium ion battery anode. This electrode exhibits high reversible capacity and fast charging and discharging capability. A high capacity of 785 mAh/g is achieved at 1C rate and is maintained without decay up to 500 cycles. Moreover, the rate of up to 60C is also demonstrated, rendering a fast discharge potential. To our knowledge, this is the best reported rate performance for Fe₃O₄ in lithium ion battery to date