Estimation of aboveground biomass of senescence grassland in China's arid region using multi-source data.

Aboveground Biomass (AGB) in the grassland senescence period is a key indicator for assessing grassland fire risk and autumnal pasture carrying capacity. Despite the advancement of remote sensing in rapid monitoring of AGB on a regional scale, accurately monitoring AGB during the senescence period in vast arid areas remains a major challenge. Using remote sensing, environmental data, and 356 samples of grassland senescence period AGB data, this study utilizes the Gram-Schmidt Pan Sharpening (GS) method, multivariate selection methods, and machine learning algorithms (RF, SVM, and BP_ANN) to construct a model for AGB during senescence grassland, and applies the optimal model to analyze spatio-temporal pattern changes in AGB from 2000 to 2021 in arid regions. The results indicate that the GS method effectively enhances the correlation between measured AGB and vegetation indices, reducing model error to some extent; The accuracy of grassland AGB inversion models based on a single vegetation index is low (0.03 ≤ |R| ≤ 0.63), while the RF model constructed with multiple variables selected by the Boruta algorithm is the optimal model for estimating AGB in arid regions during the senescence period (R2 = 0.71, RMSE = 519.74 kg/ha); In the span of 22 years, the annual average AGB in the senescence period of arid regions was 1413.85 kg/ha, with regions of higher AGB primarily located in the northeast and southwest of the study area. The area experiencing an increase in AGB during the senescence period (79.97 %) was significantly larger than that with decreased AGB (20.03 %).

LC-QTOF/MS-based non-targeted metabolomics to explore the toxic effects of di(2-ethylhexyl) phthalate (DEHP) on Brassica chinensis L.

Di(2-ethylhexyl) phthalate (DEHP) is a widely used plasticizer known to pose health risks to humans upon exposure. Recognizing the toxic nature of DEHP, our study aimed to elucidate the response mechanisms in Brassica chinensis L. (Shanghai Qing) when subjected to varying concentrations of DEHP (2 mg kg-1, 20 mg kg-1, and 50 mg kg-1), particularly under tissue stress. The findings underscored the substantial impact of DEHP treatment on the growth of Brassica chinensis L., with increased DEHP concentration leading to a notable decrease in chlorophyll levels and alterations in the content of antioxidant enzyme activities, particularly superoxide dismutase (SOD) and peroxidase (POD). Moreover, elevated DEHP concentrations correlated with increased malondialdehyde (MDA) levels. Our analysis detected a total of 507 metabolites in Brassica chinensis L., with 331 in shoots and 176 in roots, following DEHP exposure. There was a significant difference in the number of metabolites in shoots and roots, with 79 and 64 identified, respectively (VIP > 1, p < 0.05). Metabolic pathway enrichment in Brassica chinensis L. shoots revealed significant perturbations in valine, leucine, and isoleucine biosynthesis and degradation, aminoacyl-tRNA, and glucosinolate biosynthesis. In the roots of Brassica chinensis L., varying DEHP levels exerted a substantial impact on the biosynthesis of zeatin, ubiquinone terpenoids, propane, piperidine, and pyridine alkaloids, as well as glutathione metabolic pathways. Notably, DEHP's influence was more pronounced in the roots than in the shoots, with higher DEHP concentrations affecting a greater number of metabolic pathways. This experimental study provides valuable insights into the molecular mechanisms underlying DEHP-induced stress in Brassica chinensis L., with potential implications for human health and food safety.

Giant enhancement of second-harmonic generation of indium selenide on planar Au.

Two-dimensional (2D) van der Waals layered γ-type indium selenide (γ-InSe) holds great promise for the development of ultrathin and low-energy-consumption nonlinear optical devices due to its broken inversion symmetry regardless of layer number. Nevertheless, the 2D InSe thin flakes still exhibit short light-matter interaction lengths, thus resulting in low efficiencies of nonlinear optical processes. In this work, we provide a facile 2D semiconductor-metal structure consisting of InSe thin flakes (thickness: 11-54 nm) on planar Au film, which exhibits great second-harmonic generation (SHG) enhancement by a factor of up to 1182. The SHG enhancement is attributed to the interference effect-induced strong electric field in highly absorbing InSe; meanwhile, the increase in reflectivity by Au film also plays an important role. Furthermore, the InSe thickness and excitation wavelength dependences of enhancement factors are revealed. This work provides a convenient approach to developing high-efficiency 2D nonlinear optical devices with ultrathin form.

Polar Nitride Perovskite LaWN3-δ with Orthorhombic Structure.

Nitride perovskite LaWN3 has been predicted to be a promising ferroelectric material with unique properties for diverse applications. However, due to the challenging sample preparation at ambient pressure, the crystal structure of this nitride remains unsolved, which results in many ambiguities in its properties. Here, the authors report a comprehensive study of LaWN3 based on high-quality samples synthesized by a high-pressure method, leading to a definitive resolution of its crystal structure involving nitrogen deficiency. Combined with theoretical calculations, these results show that LaWN3 adopts an orthorhombic Pna21 structure with a polar symmetry, possessing a unique atomic polarization along the c-axis. The associated atomic polar distortions in LaWN3 are driven by covalent hybridization of W: 5d and N: 2p orbitals, opening a direct bandgap that explains its semiconducting behaviors. The structural stability and electronic properties of this nitride are also revealed to be closely associated with its nitrogen deficiency. The success in unraveling the structural and electronic ambiguities of LaWN3 would provide important insights into the structures and properties of the family of nitride perovskites.

Potentially toxic elements in lake sediments in China: Spatial distribution, ecological risks, and influencing factors.

Potentially toxic elements (PTEs) pollution in lake sediments is a serious threat to the ecological safety of lake water and human health, owing to anthropogenic activities. Studies on the distribution of pollution, the differences in lake types, and the influencing factors in China as a whole are lacking. This study collected data on PTEs (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) in Chinese lake sediments published from 2005 to 2021, and aimed to evaluate pollution levels and spatial distribution characteristics of PTEs in lake sediments, differences in pollution in different types of lakes, and influencing factors. The results showed that (1) All metals in the lake sediments accumulated to different degrees, when compared to the background values. (2) The lake type pollution levels were ranked: urban lakes > reservoirs > plateau lakes > natural lakes. (3) The geoaccumulation and potential ecological risk indexes both indicated that Cd and Hg are the main pollutants, and that the overall ecological risk level of lake sediments in China is high. (4) The degree of economic and population growth is highly correlated with the concentrations of eight PTEs; the amount of fertilizer and pesticide used in agricultural activities are the main factors affecting As and Hg; industrial activities and traffic pollution emissions are the predominant factors affecting Cu and Ni. (5) In the interaction detection analysis, the Cr content was mainly influenced by natural factors; Cd, Pb, and Zn contents were affected more by human activities. This study provides a reference for understanding the current status and influencing factors of PTE pollution in Chinese lakes.

General Synthesis of 2D Magnetic Transition Metal Dihalides via Trihalide Reduction.

Two-dimensional (2D) transition metal dihalides (TMDHs) have been receiving extensive attention due to their diversified magnetic properties and promising applications in spintronics. However, controlled growth of 2D TMDHs remains challenging owing to their extreme sensitivity to atmospheric moisture. Herein, using a home-built nitrogen-filled interconnected glovebox system, a universal chemical vapor deposition synthesis route of high-quality 2D TMDH flakes (1T-FeCl2, FeBr2, VCl2, and VBr2) by reduction of their trihalide counterparts is developed. Representatively, ultrathin (∼8.6 nm) FeCl2 flakes are synthesized on SiO2/Si, while on graphene/Cu foil the thickness can be down to monolayer (1L). Reflective magnetic circular dichroism spectroscopy shows an interlayer antiferromagnetic ordering of FeCl2 with a Neel temperature at ∼17 K. Scanning tunneling microscopy and spectroscopy further identify the atomic-scale structures and band features of 1L and bilayer FeCl2 on graphene/Cu foil.

Strain Tunability of Perpendicular Magnetic Anisotropy in van der Waals Ferromagnets VI3.

Layered ferromagnets with strong magnetic anisotropy energy (MAE) have special applications in nanoscale memory elements in electronic circuits. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements. For an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis loop with a large coercivity (1.775 T at 10 K) and remanent magnetization. Furthermore, the coercivity can be enhanced to a maximum of 2.6 T under a 3.8% external in-plane tensile strain. Our DFT calculations show that the increased MAE under strain contributes to the enhancement of coercivity. Meanwhile, the strain tunability on the coercivity of CrI3, with a similar crystal structure, is limited. The main reason is the strong spin-orbit coupling in V3+ in VI6 octahedra in comparison with that in Cr3+. The strain tunability of coercivity in VI3 flakes highlights its potential for integration into vdW heterostructures.

Continuously tunable ferroelectric domain width down to the single-atomic limit in bismuth tellurite.

Emerging functionalities in two-dimensional materials, such as ferromagnetism, superconductivity and ferroelectricity, open new avenues for promising nanoelectronic applications. Here, we report the discovery of intrinsic in-plane room-temperature ferroelectricity in two-dimensional Bi2TeO5 grown by chemical vapor deposition, where spontaneous polarization originates from Bi column displacements. We found an intercalated buffer layer consist of mixed Bi/Te column as 180° domain wall which enables facile polarized domain engineering, including continuously tunable domain width by pinning different concentration of buffer layers, and even ferroelectric-antiferroelectric phase transition when the polarization unit is pinned down to single atomic column. More interestingly, the intercalated Bi/Te buffer layer can interconvert to polarized Bi columns which end up with series terraced domain walls and unusual fan-shaped ferroelectric domain. The buffer layer induced size and shape tunable ferroelectric domain in two-dimensional Bi2TeO5 offer insights into the manipulation of functionalities in van der Waals materials for future nanoelectronics.

Hard ferromagnetic behavior in atomically thin CrSiTe3 flakes.

The research on two-dimensional (2D) van der Waals (vdW) magnets has promoted the development of ultrahigh-density data storage and nanoscale spintronic devices. However, the soft ferromagnetic behavior in most 2D magnets, which means the absence of remanent magnetization, severely limits their applications in realistic devices. Here, we report a layer-controlled ferromagnetic behavior in atomically thin CrSiTe3 flakes, where a transition from the soft to the hard ferromagnetic state occurs as the thickness of samples decreases down to several nanometers. Phenomenally, in contrast to the negligible hysteresis loop in the bulk counterparts, atomically thin CrSiTe3 shows a rectangular loop with finite magnetization and coercivity as the thickness decreases down to ∼8 nm, indicative of a single-domain and out-of-plane ferromagnetic order. We find that the stray field is weakened with decreasing thickness, which suppresses the formation of the domain wall. In addition, thickness-dependent ferromagnetic properties also reveal a crossover from 3 dimensional to 2 dimensional Ising ferromagnets, accompanied by a drop of the Curie temperature from 33 K for bulk to ∼17 K for the 4 nm sample. Our study paves the way towards exploring and learning much more about atomically thin and layered intrinsic ferromagnets.

Structure-property relationships of photofunctional diiridium(II) complexes with tetracationic charge and an unsupported Ir-Ir bond.

In contrast to the extensively studied dirhodium(II) complexes and iridium(III) complexes, neutral or dicationic dinuclear iridium(II) complexes with an unsupported ligand are underdeveloped. Here, a series of tetracationic dinuclear iridium(II) complexes, featuring the unsupported Ir(II)-Ir(II) single bond with long bond distances (2.8942(4)-2.9731(4) Å), are synthesized and structurally characterized. Interestingly, compared to the previous unsupported neutral or dicationic diiridium(II) complexes, our DFT and high-level DLPNO-CCSD(T) results found the largest binding energy in these tetracationic complexes even with the long Ir(II)-Ir(II) bond. Our study further reveals that London dispersion interactions enhance the stability cooperatively and significantly to overcome the strong electrostatic repulsion between two half dicationic metal fragments. This class of complexes also exhibit photoluminescence in solution and solid states, which, to our knowledge, represents the first example of this unsupported dinuclear iridium(II) system. In addition, their photoreactivity involving the generation of iridium(II) radical monomer from homolytic cleavage was also explored. The experimental results of photophysical and photochemical behaviours were also correlated with computational studies.

Probing Ultrafast Dynamics of Ferroelectrics by Time-Resolved Pump-Probe Spectroscopy.

Ferroelectric materials have been a key research topic owing to their wide variety of modern electronic and photonic applications. For the quick exploration of higher operating speed, smaller size, and superior efficiencies of novel ferroelectric devices, the ultrafast dynamics of ferroelectrics that directly reflect their respond time and lifetimes have drawn considerable attention. Driven by time-resolved pump-probe spectroscopy that allows for probing, controlling, and modulating dynamic processes of ferroelectrics in real-time, much research efforts have been made to understand and exploit the ultrafast dynamics of ferroelectric. Herein, the current state of ultrafast dynamic features of ferroelectrics tracked by time-resolved pump-probe spectroscopy is reviewed, which includes ferroelectrics order parameters of polarization, lattice, spin, electronic excitation, and their coupling. Several potential perspectives and possible further applications combining ultrafast pump-probe spectroscopy and ferroelectrics are also presented. This review offers a clear guidance of ultrafast dynamics of ferroelectric orders, which may promote the rapid development of next-generation devices.

Pressure-Enhanced Ferromagnetism in Layered CrSiTe3 Flakes.

Despite recent advances in layered ferromagnets, ferromagnetic interactions in these materials are rather weak. Here, we report pressure-enhanced ferromagnetism in layered CrSiTe3 flakes revealed by high-pressure magnetic circular dichroism measurements. Below ∼3 GPa, CrSiTe3 undergoes a paramagnetic-to-ferromagnetic phase transition at ∼32 K, and the field-induced spin-flip in the ferromagnetic phase produces nearly zero hysteresis loops, demonstrating soft ferromagnetism. Above ∼4 GPa, a soft-to-hard ferromagnetic transition occurs, signaled by rectangular-shaped hysteresis loops with finite coercivity and remanent magnetization. Interestingly, as pressure increases, the Curie temperature and coercivity dramatically increase up to ∼138 K and 0.17 T at 7.8 GPa, respectively, in contrast to ∼36 K and 0.02 T at 4.6 GPa. It indicates a remarkable influence of pressure on exchange interactions, which is consistent with DFT calculations. The effective interaction between magnetic couplings and external pressure offers new opportunities in pursuit of high-temperature layered ferromagnets.

Dynamic fingerprint of fractionalized excitations in single-crystalline Cu3Zn(OH)6FBr.

Beyond the absence of long-range magnetic orders, the most prominent feature of the elusive quantum spin liquid (QSL) state is the existence of fractionalized spin excitations, i.e., spinons. When the system orders, the spin-wave excitation appears as the bound state of the spinon-antispinon pair. Although scarcely reported, a direct comparison between similar compounds illustrates the evolution from spinon to magnon. Here, we perform the Raman scattering on single crystals of two quantum kagome antiferromagnets, of which one is the kagome QSL candidate Cu3Zn(OH)6FBr, and another is an antiferromagnetically ordered compound EuCu3(OH)6Cl3. In Cu3Zn(OH)6FBr, we identify a unique one spinon-antispinon pair component in the E2g magnetic Raman continuum, providing strong evidence for deconfined spinon excitations. In contrast, a sharp magnon peak emerges from the one-pair spinon continuum in the Eg magnetic Raman response once EuCu3(OH)6Cl3 undergoes the antiferromagnetic order transition. From the comparative Raman studies, we can regard the magnon mode as the spinon-antispinon bound state, and the spinon confinement drives the magnetic ordering.

Epitaxial growth of large-grain-size ferromagnetic monolayer CrI3 for valley Zeeman splitting enhancement.

Two-dimensional (2D) magnetic CrI3 has received considerable research attention because of its intrinsic features, including insulation, Ising ferromagnetism, and stacking-order-dependent magnetism, as well as potential in spintronic applications. However, the current strategy for the production of ambient-unstable CrI3 thin layer is limited to mechanical exfoliation, which normally suffers from uncontrollable layer thickness, small size, and low yet unpredictable yield. Here, via a confined vapor epitaxy (CVE) method, we demonstrate the mass production of flower-like CrI3 monolayers on mica. Interestingly, we discovered the crucial role of K ions on the mica surface in determining the morphology of monolayer CrI3, reacting with precursors to form a KIx buffer layer. Meanwhile, the transport agent affects the thickness and size of the as-grown CrI3. Moreover, the Curie temperature of CrI3 is greatly affected by the interaction between CrI3 and the substrate. The monolayer CrI3 on mica could act as a magnetic substrate for valley Zeeman splitting enhancement of WSe2. We reckon our work represents a major advancement in the mass production of monolayer 2D CrI3 and anticipate that our growth strategy may be extended to other transition metal halides.

Pressure-driven switching of magnetism in layered CrCl3.

Layered transition-metal compounds with controllable magnetic behaviors provide many fascinating opportunities for the fabrication of high-performance magneto-electric and spintronic devices. The tuning of their electronic and magnetic properties is usually limited to the change of layer thickness, electrostatic doping, and the control of electric and magnetic fields. However, pressure has been rarely exploited as a control parameter for tailoring their magneto-electric properties. Here, we report a unique pressure-driven isostructural phase transition in layered CrCl3 accompanied by a simultaneous switching of magnetism from a ferromagnetic to an antiferromagnetic ordering. Our experiments, in combination with ab initio calculations, demonstrate that such a magnetic transition hinders the bandgap collapse under pressure, leading to an anomalous semiconductor-to-semiconductor transition. Our findings not only reveal the potential applications of this material in electronic and spintronic devices but also establish the basis for exploring unusual phase transitions in layered transition-metal compounds.

Pressure-Dependent Intermediate Magnetic Phase in Thin Fe3GeTe2 Flakes.

We investigated the evolution of ferromagnetism in layered Fe3GeTe2 flakes under different pressures and temperatures using in situ magnetic circular dichroism (MCD) spectroscopy. We found that the rectangular shape of the hysteresis loop under an out-of-plane magnetic field sweep can be sustained below 7 GPa. Above that pressure, an intermediate state appears in the low-temperature region signaled by an 8-shaped skewed hysteresis loop. Meanwhile, the coercive field and Curie temperature decrease with increasing pressures, implying the decrease of the exchange interaction and the magneto-crystalline anisotropy under pressures. The intermediate phase has a labyrinthine domain structure, which is attributed to the increase of the ratio of exchange interaction to magneto-crystalline anisotropy based on Jagla's theory. Moreover, our calculations reveal a weak structural transition around 6 GPa that corresponds to a significant change in the FeI-FeI bond length, which has strong influences on magnetic interaction. Detailed analysis on exchange interaction and magneto-crystalline anisotropy with pressure shows a consistent trend with experiments.

Influence of a substrate on ultrafast interfacial charge transfer and dynamical interlayer excitons in monolayer WSe2/graphene heterostructures.

Efficient interfacial light-electric interconversion in van der Waals heterostructures is critical for their optoelectronic applications. Using time-resolved terahertz spectroscopy and transient absorption spectroscopy, the charge transfer and the dynamical interlayer excitons were investigated in the heterostructures comprising monolayer WSe2 and monolayer graphene with varying stacking order on a sapphire substrate. Herein, a more comprehensive understanding of ultrafast charge transfer and exciton dynamics in two-dimensional heterostructures is shown. Owing to the effective electric field induced by the sapphire substrate, the WSe2/graphene heterostructure exhibits positive terahertz photoconductivity after photoexcitation, while negative terahertz photoconductivity is observed in the graphene/WSe2 heterostructure. The transient absorption spectra indicate that the exciton lifetimes also exhibit a considerable difference, where WSe2/graphene exhibits the longest exciton lifetime, followed by monolayer WSe2, while graphene/WSe2 exhibits the shortest lifetime. These observations provide a new idea for using van der Waals heterostructures in electronic and photonic devices.

Ecological and human health risk of sulfonamides in surface water and groundwater of Huixian karst wetland in Guilin, China.

Sulfonamide antibiotics are contaminants of emerging concern (CEC). These CECs raise considerable alarm because they are commonly present in water environments. Studies on the environmental existence of CECs in karst areas of Guilin (Southern China) have yet to be reported. Thus, this study aims to investigate the presence, temporal and spatial distributions of sulfonamides in surface water and groundwater of four major aquatic environments (i.e., aquafarm water, ditch water, wetland water, and groundwater) in the Huixian karst wetland system of Guilin. Furthermore, this study aims to determine the ecological and human health risks of individual sulfonamides and their mixtures. Ten sulfonamides (i.e., sulfadiazine, sulfapyridine, sulfamerazine, trimethoprim, sulfamethazine, sulfamethoxypyridazine, sulfachloropyridazine, sulfamethoxazole, sulfadimethoxine, and sulfaquinoxaline) were observed in the study area. The highest average concentrations of aquafarm water, ditch water, wetland water, and groundwater were those of sulfadiazine (48.24 µg/L), sulfamethoxypyridazine (1281.50 µg/L), sulfamethoxazole (51.14 µg/L), and sulfamethazine (20.06 µg/L), respectively. The potential ecological risks of the detected compounds were much higher in ditch water than in aquafarm water, wetland water, and groundwater. The most ecological risks were observed for sulfachloropyridazine with a risk quotient (RQ) reaching 335.5 to green algae and 152 to Daphnia magna in ditch water. Similarly, sulfachloropyridazine posed the highest ecological risks to green algae among the ten sulfonamides in aquafarm water (RQ = 3.39), wetland water (RQ = 2.98), and groundwater (RQ = 3.6). Human health risk for age groups<12 months was observed from sulfonamide in drinking groundwater. Ecological and human health risks caused by sulfonamide mixtures were larger than the individual risks. Overall, ecological and human health risks caused by sulfonamides were observed in the study area.

Phase Identification and Strong Second Harmonic Generation in Pure ε-InSe and Its Alloys.

Two-dimensional material indium selenide (InSe) has offered a new platform for fundamental research in virtue of its emerging fascinating properties. Unlike 2H-phase transition-metal dichalcogenides (TMDs), ε phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers, and predicted to have a strong second harmonic generation (SHG) effect. In this work, we find that the as-prepared pure InSe, alloyed InSe1- xTe x and InSe1- xS x ( x = 0.1 and 0.2) are ε phase structures and exhibit excellent SHG performance from few-layer to bulk-like dimension. This high SHG efficiency is attributed to the noncentrosymmetric crystal structure of the ε-InSe system, which has been clearly verified by aberration-corrected scanning transmission electron microscopy (STEM) images. The experimental results show that the SHG intensities from multilayer pure ε-InSe and alloyed InSe0.9Te0.1 and InSe1- xS x ( x = 0.1 and 0.2) are around 1-2 orders of magnitude higher than that of the monolayer TMD systems and even superior to that of GaSe with the same thickness. The estimated nonlinear susceptibility χ(2) of ε-InSe is larger than that of ε-GaSe and monolayer TMDs. Our study provides first-hand information about the phase identification of ε-InSe and indicates an excellent candidate for nonlinear optical (NLO) applications as well as the possibility of engineering SHG response by alloying.

Defect Engineering in Single-Layer MoS2 Using Heavy Ion Irradiation.

Transition metal dichalcogenides (TMDs) have attracted much attention due to their promising optical, electronic, magnetic, and catalytic properties. Engineering the defects in TMDs represents an effective way to achieve novel functionalities and superior performance of TMDs devices. However, it remains a significant challenge to create defects in TMDs in a controllable manner or to correlate the nature of defects with their functionalities. In this work, taking single-layer MoS2 as a model system, defects with controlled densities are generated by 500 keV Au irradiation with different ion fluences, and the generated defects are mostly S vacancies. We further show that the defects introduced by ion irradiation can significantly affect the properties of the single-layer MoS2, leading to considerable changes in its photoluminescence characteristics and electrocatalytic behavior. As the defect density increases, the characteristic photoluminescence peak of MoS2 first blueshifts and then redshifts, which is likely due to the electron transfer from MoS2 to the adsorbed O2 at the defect sites. The generation of the defects can also strongly improve the hydrogen evolution reaction activity of MoS2, attributed to the modified adsorption of atomic hydrogen at the defects.