We demonstrate experimentally that a photonic crystal made of Al_{2}O_{3} cylinders exhibits topological time-reversal symmetric electromagnetic propagation, similar to the quantum spin Hall effect in electronic systems. A pseudospin degree of freedom in the electromagnetic system representing different states of orbital angular momentum arises due to a deformation of the photonic crystal from the ideal honeycomb lattice. It serves as the photonic analogue to the electronic Kramers pair. We visualized qualitatively and measured quantitatively that microwaves of a specific pseudospin propagate only in one direction along the interface between a topological photonic crystal and a trivial one. As only a conventional dielectric material is used and only local real-space manipulations are required, our scheme can be extended to visible light to inspire many future applications in the field of photonics and beyond.

Exploring high‐efficiency, stable, and cost‐effective bifunctional electrocatalysts for overall water splitting is greatly desirable and challenging. Herein, a newly designed hybrid catalyst with Cr‐doped FeNi–P nanoparticles encapsulated into N‐doped carbon nanotubes (Cr‐doped FeNi–P/NCN) with unprecedented electrocatalytic activity is developed by a simple one‐step heating treatment. The as‐synthesized Cr‐doped FeNi–P/NCN with moderate Cr doping exhibits admirable oxygen evolution reaction and hydrogen evolution reaction activities with overpotentials of 240 and 190 mV to reach a current density of 10 mA cm−2 in 1 m KOH solution. When used in overall water splitting as a bifunctional catalyst, it needs only 1.50 V to give a current density of 10 mA cm−2, which is superior to its typically integrated Pt/C and RuO2 counterparts (1.54 V @ 10 mA cm−2). Density functional theory calculation confirms that Cr doping into a FeNi‐host can effectively alter the relative Gibbs adsorption energy and reduces the theoretical overpotential. Additionally, the synergetic effects between Cr‐doped FeNi–P nanoparticles and NCNs are regarded as significant contributors to accelerate charge transfer and promote electrocatalytic activity in hybrid catalysts.

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In order to realize the full potential of nanowires for optical applications, it is essential to synthesize nanowires that can emit predominantly via band to band or band edge (BE) transitions. However, many compound semiconductor nanowires, irrespective of the method of their growth, contain a high density of native defects; these result in competing deep defect (DD) related emission, limiting their utility for optoelectronic device applications. The concentration of these native defect states depends on the gas phase stoichiometry. In this work, we report on the influence of gas phase stoichiometry on the structural and optical properties of single crystal zinc selenide (ZnSe) nanowires. We find that nanowires grown under stoichiometric conditions contain such defect states with associated weak BE emission and strong DD emission. However, nanowires grown under Zn-rich conditions were characterized by photoluminescence spectra dominated by strong BE emission while those grown under Se-rich conditions showed strong DD related emission. Hence, it is necessary to develop a strategy for enhancing the BE emission while simultaneously quenching the DD emission. We demonstrate a technique of postgrowth treatment that can effectively perform this function, and using this strategy the ratio of the BE/DD emission can be increased by a factor of several thousands, at least an order of magnitude higher than previously reported values. This reveals BE dominated photoluminescence in these nanowires and makes these nanowires suitable for developing future optoelectronic devices.

Uniform CdS/ZnO core/shell nanowires are hydrothermally synthesized using a two‐step process and assembled into a photodetector and a NO2 optoelectronic sensor for the first time. The corresponding photodetector exhibits a fast, reversible, and stable optoelectronic response with a rise time of ∼26 ms, a decay time of ∼2.1 ms and a stability of over 5 months. The remarkable photosensitivity and fast photoresponse are attributed to the formation of a heterojunction structure between CdS and ZnO, which greatly inhibits the recombination of photoinduced electrons and holes. The CdS/ZnO core/shell nanowires also show an excellent visible‐light‐activated gas sensing performance towards ppb‐level NO2 at room temperature. The responses range from 6.7% to 337% toward NO2 concentrations of 5 to 1000 ppb. It is found that the sensitivity of the NO2 sensor is dependent on the illuminated light intensity with a maximum value at 0.68 mW/cm2. The sensing mechanisms of the CdS/ZnO nanowires under visible‐light irradiation and the influence of light intensity are also discussed. The present CdS/ZnO core/shell nanowire not only benefits the fabrication of efficient photodetectors, but also makes the instant, optically controlled sensing of ppb‐level NO2 gas possible.