The counterfeiting of goods is growing worldwide, affecting practically any marketable item ranging from consumer goods to human health. Anticounterfeiting is essential for authentication, currency, and security. Anticounterfeiting tags based on structural color materials have enjoyed worldwide and long‐term commercial success due to their inexpensive production and exceptional ease of percept. However, conventional anticounterfeiting tags of holographic gratings can be readily copied or imitated. Much progress has been made recently to overcome this limitation by employing sufficient complexity and stimuli‐responsive ability into the structural color materials. Moreover, traditional processing methods of structural color tags are mainly based on photolithography and nanoimprinting, while new processing methods such as the inkless printing and additive manufacturing have been developed, enabling massive scale up fabrication of novel structural color security engineering. This review presents recent breakthroughs in structural color materials, and their applications in optical encryption and anticounterfeiting are discussed in detail. Special attention is given to the unique structures for optical anticounterfeiting techniques and their optical aspects for encryption. Finally, emerging research directions and current challenges in optical encryption technologies using structural color materials is presented.

Metal halide perovskites have received much attention for their application in light-emitting diodes (LEDs) in the past several years. Rapid progress has been made in efficient green, red, and near-infrared perovskite LEDs. However, the development of blue perovskite LEDs is still lagging far behind. Here, we report efficient sky-blue perovskite LEDs by rearranging low-dimensional phase distribution in quasi-2D perovskites. We incorporated sodium ions into the mixed-Cl/Br quasi-2D perovskites with phenylethylammonium as the organic spacer and cesium lead halide as the inorganic framework. The inclusion of the sodium ion was found to significantly reduce the formation of the n = 1 phase, which was dominated by nonradiative transition, and increase the formation of other small-n phases for efficient exciton energy transfer. By managing the phase distribution, a maximum external quantum efficiency (EQE) of 11.7% was achieved in the sky-blue perovskite LED, with a stable emission peak at 488 nm. Further optimizing the phase distribution and film morphology with Pb content, we demonstrated the sky-blue devices with the average EQE approaching 10%. This strategy of engineering phase distribution of quasi-2D perovskites with a sodium ion could provide a useful way for the fabrication of high-performance blue perovskite LEDs.

Lead-free perovskite infrared light-emitting diodes are achieved by using a halide perovskite CsSnI as an emissive layer. The film shows compact micrometer-sized grains with only a few pinholes and cracks at the grain boundaries. The device exhibits maximum radiance of 40 W sr m at a current density of 364.3 mA cm and maximum external quantum efficiency of 3.8% at 4.5 V.

Photonic crystals, which are materials with periodic dielectric constants on the submicroscale, have been the focus of research for an extended period. Photonic soft materials have been extensively developed for use as colorimetric indicators and mechanochromic sensors, but their limited mechanical properties and molding characteristics only suitable for films restrict their practical implementation. Herein we report an approach to synthesize highly stretchable photonic soft materials based on a hydrogel system that is cross-linked by a crystalline colloidal array. The intrinsic inhomogeneous submicroscale structure is exploited for effective reinforcement in the multiphase system of the photonic crystals. The photonic hydrogels exhibit a high deformation capacity, with a stretching deformation above 2800% and compression above 98%. The gel has a full-color tunable range and shows 460 nm photonic shifts that can be reversibly actuated by a small compressive stress (kPa level) and can be ink-written to form patterns and freestanding structures. Mechanochromic sensors are constructed based on the three-dimensional and two-dimensional Bragg diffraction of the gel. Owing to its mechanical strength, formability, and tunable colors, the gel can be used in wearable optical devices, colorimetric tactile sensors, and full-color displays.

High-quality homogeneous junctions are of great significance for developing transition metal dichalcogenides (TMDs) based electronic and optoelectronic devices. Here, we demonstrate a lateral p-type/intrinsic/n-type (p-i-n) homojunction based multilayer WSe2 diode. The photodiode is formed through selective doping, more specifically by utilizing self-aligning surface plasma treatment at the contact regions, while keeping the WSe2 channel intrinsic. Electrical measurements of such a diode reveal an ideal rectifying behavior with a current on/off ratio as high as 1.2 × 106 and an ideality factor of 1.14. While operating in the photovoltaic mode, the diode presents an excellent photodetecting performance under 450 nm light illumination, including an open-circuit voltage of 340 mV, a responsivity of 0.1 A W–1, and a specific detectivity of 2.2 × 1013 Jones. Furthermore, benefiting from the lateral p-i-n configuration, the slow photoresponse dynamics including the photocarrier diffusion in undepleted regions and photocarrier trapping/detrapping due to dopants or doping process induced defect states are significantly suppressed. Consequently, a record-breaking response time of 264 ns and a 3 dB bandwidth of 1.9 MHz are realized, compared with the previously reported TMDs based photodetectors. The above-mentioned desirable properties, together with CMOS compatible processes, make this WSe2 p-i-n junction diode promising for future applications in self-powered high-frequency weak signal photodetection.

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TianQin is a planned space-based gravitational wave (GW) observatory consisting of three Earth-orbiting satellites with an orbital radius of about $10^5 \, {\rm km}$. The satellites will form an equilateral triangle constellation the plane of which is nearly perpendicular to the ecliptic plane. TianQin aims to detect GWs between $10^{-4} \, {\rm Hz}$ and $1 \, {\rm Hz}$ that can be generated by a wide variety of important astrophysical and cosmological sources, including the inspiral of Galactic ultra-compact binaries, the inspiral of stellar-mass black hole binaries, extreme mass ratio inspirals, the merger of massive black hole binaries, and possibly the energetic processes in the very early universe and exotic sources such as cosmic strings. In order to start science operations around 2035, a roadmap called the 0123 plan is being used to bring the key technologies of TianQin to maturity, supported by the construction of a series of research facilities on the ground. Two major projects of the 0123 plan are being carried out. In this process, the team has created a new-generation $17 \, {\rm cm}$ single-body hollow corner-cube retro-reflector which was launched with the QueQiao satellite on 21 May 2018; a new laser-ranging station equipped with a $1.2 \, {\rm m}$ telescope has been constructed and the station has successfully ranged to all five retro-reflectors on the Moon; and the TianQin-1 experimental satellite was launched on 20 December 2019—the first-round result shows that the satellite has exceeded all of its mission requirements.

1wileyonlinelibrary.com on crystalline and molecular engineering could turn the band gap of g-C 3 N 4 by element doping, [4] using different precursors, [5][6][7][8][9] introducing crystal defects or amorphous structure, [3,10,11] forming carbon vacancies, [12] fabricating heterojunction composites, [13][14][15] and so on. Structural regulation mainly focused on increasing specific surface area, [16][17][18][19][20][21] typically making mesoporous by templates with high specific surface area. [16,17,20,21] However, most of studies reported focused on g-C 3 N 4 in the powder form due to the contradiction between the highly open framework and the insufficient mechanical strength. Some recent progresses have been made to achieve 3D macroform g-C 3 N 4 based on robust substrate, [19] but major challenges remain to develop a substrate-free g-C 3 N 4 film with high photocatalytic activity. Photonic crystals (PCs) are materials with spatially periodic variation of the dielectric permittivity on the order of the wavelength of light. The propagation of light within a certain frequency range is forbidden in a certain crystal direction within a certain spectrum regime, namely photonic stopband. [22,23] Special properties of PCs, such as inhibition of spontaneous emission, [24][25][26] slow light, and amplified photon absorption/emission, [27][28][29][30] provide numerous possibilities for "photon management" applications. Thus introducing PC structure is a promising strategy for promoting photocatalytic activity of photocatalyst and the performance in optoelectronic devices. [27][28][29][30][31][32][33] Consequently, if g-C 3 N 4 is fabricated in a PC form, the spontaneous emission by the recombination of photogenerated carriers could be inhibited and the photocatalytic activity could benefit from the bicontinous network with increased visible light absorption via slow photon effects.Inspired by considerations above, herein we demonstrate a general solution approach to tackle the challenge via a freestanding colloidal crystal templating method (see Scheme 1). Freestanding, highly ordered, crack-free silica PCs are generated by evaporative-vertical deposition. Subsequently, the calcination of the precursor dicyandiamide (DCDA) followed by removal of silica PCs generates highly uniform macroscopic g-C 3 N 4 PC films. The resultant freestanding g-C 3 N 4 PCs possesses a 3D interconnected network with tunable photonic stop band, exhibiting significantly improved photocatalytic activity in photodegrading and water splitting under visible light. Freestanding Graphitic Carbon Nitride Photonic Crystals for Enhanced PhotocatalysisLu Sun, Meijia Yang, Jianfeng Huang, Dingshan Yu, Wei Hong,* and Xudong Chen* Graphitic carbon nitride (g-C 3 N 4 ) has attracted tremendous attention in photocatalysis due to its extraordinary features, such as good thermal and chemical stability, metal-free composition, and easy preparation. However, the photocatalytic performance of g-C 3 N 4 is still restricted by the limited surface area, inefficient v...