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On‐chip coherent light source has always been fascinating and intriguing due to its various potential applications. In the past decades, there has been some progress in the development of chip coherent light (e.g., nanolaser, Bose‐Einstein condensation and nonlinear optical effects). However, these methods strictly depend on materials and extreme experimental conditions, and are usually not tunable. Here, a hybrid structure is designed which combines a chirped surface plasmon metasurface with a monolayer transition metal dichalcogenide (TMDC) to achieve a coherent second harmonic generation (SHG) covering the entire visible light spectrum. Using only finite number of metallic grooves, a continuous resonance tuning is obtained. By translating the metasurface in space, a space‐frequency locking SHG is demonstrated. Although the Q factor of the surface plasmon cavity is low, its near‐field enhancements of both fundamental and SH waves are still obvious. Significantly, the broad linewidth of plasmonic cavity leads to a large degree of overlap between adjacent localized modes, that enables the tuning of the output wavelength continuously at room temperature. Meanwhile, the exciton resonance also plays an important role. This monolithic tunable device demonstrates the potentials of 2D material‐plasmon hybrid metasurface and to construct an efficient broadband tunable on‐chip coherent light source. [ABSTRACT FROM AUTHOR]

Unreacted lead iodide is commonly believed to be beneficial to the efficiency of methylammonium lead iodide perovskite based solar cells, since it has been proposed to passivate the defects in perovskite grain boundaries. However, it is shown here that the presence of unreacted PbI2 results in an intrinsic instability of the film under illumination, leading to the film degradation under inert atmosphere and faster degradation upon exposure to illumination and humidity. The perovskite films without lead iodide have improved stability, but lower efficiency due to inferior film morphology (smaller grain size, the presence of pinholes). Optimization of the deposition process resulted in PbI2 -free perovskite films giving comparable efficiency to those with excess PbI2 (14.2 ± 1.3% compared to 15.1 ± 0.9%) Thus, optimization of the deposition process for PbI2 -free films leads to dense, pinhole-free, large grain size perovskite films which result in cells with high efficiency without detrimental effects on the film photostability caused by excess PbI2 . However, it should be noted that for encapsulated devices illuminated through the substrate (fluorine-doped tin oxide glass, TiO2 film), film photostability is not a key factor in the device degradation. [ABSTRACT FROM AUTHOR]

Structural symmetry of crystals plays important roles regarding the physical properties of two‐dimensional (2D) materials, particularly in the nonlinear optics regime. There has been a long‐term exploration of the physical properties in 2D materials with various stacking structures, which correspond to different structural symmetries. Usually, the manipulation of rotational alignment between layers in 2D heterostructures is realized at the synthetic stage through artificial stacking like assembling building blocks. However, the reconfigurable control of translational symmetry of crystalline structure is still challenging. High pressure, as a powerful external control knob, provides a very promising route to circumvent this constraint. Here, a pressure‐controlled symmetry transition in layered InSe is experimentally demonstrated. The continuous and reversible evolution of structural symmetries can be in situ monitored by using polarization‐resolved second‐harmonic‐generation (SHG) spectroscopy. As pressure changes, the reconfigurable symmetry transition of the SHG pattern from threefold rotational symmetry to mirror symmetry is experimentally observed in layered InSe samples and successfully explained by the proposed interlayer‐translation model. This opens new routes toward potential applications of manipulating crystal symmetry of 2D materials. [ABSTRACT FROM AUTHOR]

Cuprous oxide (Cu2O) is a kind of functional and promising energy material, which has been used as solar cells, adsorbents, and lithium ion battery electrode materials. And its mainly application is toward photocatalytic reactivity of organics' decomposition under visible light irradiation. However, the real role of Cu2O remains debated, for these reports don't give a powerful evidence to prove its significant photocatalytic properties. Here, we describe a one-pot synthesis method to construct Cu2O mesoporous spheres with large specific surface area at room temperature. This mesoporous microstructure is controlled through an Ostwald ripening-based dissolved partly process. And Cu2O mesoporous spheres are used to study the property of removing organic pollutants. We distinguish the real role of Cu2O by carefully measure the reduced portion of photocatalyst or adsorbent, while it is reacting with organic pollutants. At last, we find out the reason that has caused the concentration of organic pollutants to decrease is mainly based on the adsorption property of Cu2O, rather than its photocatalytic effect. However, the photocatalytic effect of Cu2O is very low under visible light irradiation.

We probe the polarization of the "A" exciton photoluminescence from monolayer and multilayer WS 2 at 10 K and 295 K under near-resonant and off-resonant conditions. The monolayer WS 2 exhibits relatively low valley polarization, around 24% at 10 K and 8% at 295 K, while all multilayer WS 2 samples show very high valley polarization, which is a more or less constant value of around 80% at 10 K under near-resonant excitation. At room temperature, it is observed experimentally that valley polarization in multilayer WS 2 monotonously increases with shrinking of the indirect bandgap energy. The phonon-assisted intervalley scattering via the K-Γ-K(K') valleys is identified as the primary valley polarization relaxation channel, which could be gradually suppressed as the thickness increases, leading to a valley polarization of up to 70% in multilayer WS 2 (>3 unit layers).

Mechanical exfoliation from bulk layered crystal is widely used for preparing two-dimensional (2D) layered materials, which involves not only out-of-plane interlayer cleavage but also in-plane fracture. Through a statistical analysis on the exfoliated 2D flakes, we reveal the in-plane cleavage behaviors of six representative layered materials, including graphene, h-BN, 2H phase MoS2, 1T phase PtS2, FePS3, and black phosphorus. In addition to the well-known interlayer cleavage, these 2D layered materials show a distinctive tendency to fracture along certain in-plane crystallography orientations. With theoretical modeling and analysis, these distinct in-plane cleavage behaviors can be understood as a result of the competition between the release of the elastic energy and the increase of the surface energy during the fracture process. More importantly, these in-plane cleavage behaviors provide a fast and noninvasive method using optical microscopy to identify the lattice direction of mechanical exfoliated 2D layered materials.

Iron diselenium (FeSe2) is a promising semiconductor for thin-film solar cells because it has a suitable band gap (E(g) = 1.0 eV) and high absorption coefficient. Despite these prospects, the controllable synthesis of FeSe2 nanostructures and the diversity of their geometries has hardly been studied previously. Here, we described a successful synthesis of phase-pure, high-quality, and stable orthorhombic FeSe2 nanocrystals (NCs) in aqueous solvents. A variety of morphologies of the FeSe2 NCs were achieved by adjusting synthetic methods. FeSe2 nanoparticles with diameters of 30-100 nm were synthesized in the presence of ethylenediamine (en). Moreover, the synthetic approach developed for nanoparticles proved to be quite universal and could be modified to produce nanowires and octahedrons, with which structure the material could display high crystallinity. The diameter of the FeSe2 nanowires was 300-500 nm with a length exceeding 2 μm. The octahedrons displayed lateral dimensions of 1 μm. Meanwhile, the probable growth mechanism and fabrication process of the NCs were proposed. Polycrystalline FeSe2 thin films were fabricated by modifying the sedimentation method. The obvious photoconductivity of FeSe2 has already been observed, and it was considered to be one candidate of solar cell for the very first time.