Two-dimensional (2D) materials are promising candidates for next-generation electronic devices. In this regime, insulating 2D ferromagnets, which remain rare, are of special importance due to their potential for enabling new device architectures. Here we report the discovery of ferromagnetism in a layered van der Waals semiconductor, VI 3 , which is based on honeycomb vanadium layers separated by an iodine-iodine van der Waals gap. It has a BiI 3 -type structure (R-3, No.148) at room temperature, and our experimental evidence suggests that it may undergo a subtle structural phase transition at 78 K. VI 3 becomes ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly shows ferromagnetic domains, which can be manipulated by the applied external magnetic field. The optical band gap determined by reflectance measurements is 0.6 eV, and the material is highly resistive.

We use solid-state methods to synthesize single crystals of perovskite-phase cesium lead iodide (γ-CsPbI3) that are kinetically stable at room temperature. Single crystal X-ray diffraction characterization shows that the compound is orthorhombic with the GdFeO3 structure at room temperature. Unlike conventional semiconductors, the optical absorption and the joint density-ofstates of bulk γ-CsPbI3 is greatest near the band edge and decreases beyond Eg for at least 1.9 eV.Bulk γ-CsPbI3 does not show an excitonic resonance and has an optical band gap of 1.63(3) eV, ~90 meV smaller than has been reported in thin films; these and other differences indicate that previously-measured thin film γ-CsPbI3 shows signatures of quantum confinement. By flowing gases over γ-CsPbI3 during in situ powder X-ray diffraction measurements, we confirm that γ-CsPbI3 is stable to oxygen but rapidly and catalytically converts to non-perovskite δ-CsPbI3 in the presence of moisture. Our results on bulk γ-CsPbI3 provide vital parameters for theoretical and experimental investigations into perovskite-phase CsPbI3 that will the guide the design and synthesis of atmospherically stable inorganic halide perovskites.

Despite the tremendous interest in halide perovskite solar cells, the structural reasons that cause the all‐inorganic perovskite CsPbI3 to be unstable at room temperature remain mysterious, especially since many tolerance‐factor‐based approaches predict CsPbI3 should be stable as a perovskite. Here single‐crystal X‐ray diffraction and X‐ray pair distribution function (PDF) measurements characterize bulk perovskite CsPbI3 from 100 to 295 K to elucidate its thermodynamic instability. While Cs occupies a single site from 100 to 150 K, it splits between two sites from 175 to 295 K with the second site having a lower effective coordination number, which, along with other structural parameters, suggests that Cs rattles in its coordination polyhedron. PDF measurements reveal that on the length scale of the unit cell, the PbI octahedra concurrently become greatly distorted, with one of the IPbI angles approaching 82° compared to the ideal 90°. The rattling of Cs, low number of CsI contacts, and high degree of octahedral distortion cause the instability of perovskite‐phase CsPbI3. These results reveal the limitations of tolerance factors in predicting perovskite stability and provide detailed structural information that suggests methods to engineer stable CsPbI3‐based solar cells.

Currently under active study in condensed matter physics, both theoretically and experimentally, are quantum spin liquid (QSL) states, in which no long-range magnetic ordering appears at low temperatures due to strong quantum fluctuations of the magnetic moments. The existing QSL candidates all have their intrinsic disadvantages, however, and solid evidence for quantum fluctuations is scarce. Here we report a new compound, Na 2 BaCo(PO 4 ) 2 , a geometrically frustrated system with effective spin-1/2 local moments for Co 2+ ions on an isotropic two-dimensional triangular lattice. Magnetic susceptibility and neutron scattering experiments show no magnetic ordering down to 0.05 K. Thermodynamic measurements show that there is a tremendous amount of magnetic entropy present below 1 K in zero applied magnetic field. The presence of localized low-energy spin fluctuations is revealed by inelastic neutron measurements. At low applied fields, these spin excitations are confined to low energy and contribute to the anomalously large specific heat. In larger applied fields, the system reverts to normal behavior as evident by both neutron and thermodynamic results. Our experimental characterization thus reveals that this new material is an excellent candidate for the experimental realization of a quantum spin liquid state. 1 arXiv:1905.02115v1 [cond-mat.str-el] 6 May 2019 SIGNIFICANCE STATEMENTThe experimental search and verification of theoretically predicted quantum spin liquids (QSLs) is challenging, especially because there is no universally accepted evidence to support such exotic quantum states. At present, broad continuous magnetic excitations, i.e. a spinon continuum, observed via inelastic neutron scattering, is considered as the most crucial evidence for a QSL, as described in many studies on the existing QSL candidates. However, as another potential origin of such broad spin excitations, disorder effects in those candidates cannot be neglected. Here we present a new compound with a Co-based triangular lattice that is structurally perfect and without intrinsic disorder. We present experimental results that suggest that this new compound is an ideal QSL candidate for future studies.

Water-soluble, fluorescent carbon dots (CDs) have been successfully synthesized from waste paper. The as-prepared CDs, with an amorphous structure and small particle size, exhibit good photostability, high photoluminescence quantum yields (PLQYs) and fairly low toxicity, which indicate potential applications in the field of biolabelling.Due to its important role and increasing consumption in civilization processes, especially in writing and painting, large quantities of paper is manufactured each year, leading to increased amounts of waste paper. To date, waste paper is mainly recycled for paper manufacturing. However, its potential to be applied in material science has been ignored. As is known to all, the main components of paper are cellulose, hemicelluloses and lignin, which means that paper could act as a new source of carbonaceous materials, such as CDs.Ever since their discovery from the purification of crude nanotubes in 2004, 1 CDs have been attracting more and more attention due to their chemical inertness, excitation wavelength dependent photoluminescence (PL) behavior, optical stability, biocompatibility and low toxicity. 2-4 So far, a variety of methods have been developed to prepare fluorescent CDs, such as arc-discharge, laser ablation, 5 electrochemical oxidation, 6 and microwave heating, 7,8 etc. However, these approaches often suffer from the involvement of complex post-treatment processes, expensive carbon sources and experimental equipment, which severely restricts the practical applications of CDs. Not long ago, a simple and effective synthetic route was reported for the preparation of fluorescent CDs from waste paper ash. 9However, this method involved open flame, uncontrollable reaction conditions and products. Recently, there have been several interesting demonstrations of routes to prepare CDs using renewable carbon sources by green hydrothermal methods, which exhibited potential applications in various fields, such as biological nanomaterials, energy nanomaterials and other functionalized materials. 10-13 The hydrothermal method is a rising technique for the synthesis of reactants, due to the easy control of the reaction reactants, and low energy consumption. 14 In this work, fluorescent CDs were prepared by a hydrothermal method using waste paper as the carbon source. It was proved that the as-prepared CDs exhibit fairly high PLQYs, good water solubility, good particle size distribution and low cytotoxicity. It is worth mentioning that the reaction conditions and products can be controlled by this method, compared with previous reports. 9 A definite mechanism of the hydrothermal synthesis of carbon dots has not been explained, and the plausible reasons for this is that the mechanism is the carbonization of the major constituents of waste paper such as cellulose, hemicellulose and lignin. This work would pave the way for the recycling of waste paper and support the potential applications of CDs in bio-imaging.Briefly, waste paper was shredded and dispersed in deionized (DI) water, and so...

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D eep-ultraviolet (DUV) coherent light sources (λ < 200 nm), which are always rare resources, have attracted much attention because of the significant role played in semiconductor photolithography, high density storage, laser micromachining, material processing, photochemical synthesis and scientific equipment. 1−4 As is well accepted, the most efficient way to produce practical DUV laser is using solid-state lasers with nonlinear optical (NLO) crystal through cascaded second-harmonic generation (SHG) conversion. Hence, an appropriate DUV NLO crystal that can produce SHG below 200 nm is the key to generating high quality DUV lasers.According to the anionic group theory proposed by Chen, 5−8 a good DUV NLO crystal should possess the following three optical properties: (1) wide transparent range in the UV region with the UV cutoff wavelength far below 200 nm; (2) a moderate birefringence (Δn ∼ 0.08) to ensure the phasematching ability; (3) a sufficient second-order nonlinear coefficient. Specifically, the first two conditions are indispensable for a DUV NLO material. Taking the famous LiB 3 O 5 (LBO) crystal as an example, 9 the transmittance spectrum is wide and the UV absorption edge is down to about 155 nm.

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