Controlled Growth of Submillimeter-Scale Cr5Te8 Nanosheets and the Domain Wall Nucleation Governed Magnetization Reversal Process.

Two-dimensional (2D) ferromagnets have attracted widespread attention for promising applications in compact spintronic devices. However, the controlled synthesis of high-quality, large-sized, and ultrathin 2D magnets via facile, economical method remains challenging. Herein, we develop a hydrogen-tailored chemical vapor deposition approach to fabricating 2D Cr5Te8 ferromagnetic nanosheets. Interestingly, the time period of introducing hydrogen was found to be crucial for controlling the lateral size, and a Cr5Te8 single-crystalline nanosheet of lateral size up to ∼360 µm with single-unit-cell thickness has been obtained. These samples exhibit a leading role of domain wall nucleation in governing the magnetization reversal process, providing important references for optimizing the performances of associated devices. The nanosheets also show notable magnetotransport response, including nonmonotonous magnetic-field-dependent magnetoresistance and sizable anomalous Hall resistivity, demonstrating Cr5Te8 as a promising material for constructing high-performance magnetoelectronic devices. This study presents a breakthrough of large-sized CVD-grown 2D magnetic materials, which is indispensable for constructing 2D spintronic devices.

Discovery of Robust Ferroelectricity in 2D Defective Semiconductor α-Ga2 Se3.

2D ferroelectrics with robust polar order in the atomic-scale thickness at room temperature are needed to miniaturize ferroelectric devices and tackle challenges imposed by traditional ferroelectrics. These materials usually have polar point group structure regarding as a prerequisite of ferroelectricity. Yet, to introduce polar structure into otherwise nonpolar 2D materials for producing ferroelectricity remains a challenge. Here, by combining first-principles calculations and experimental studies, it is reported that the native Ga vacancy-defects located in the asymmetrical sites in cubic defective semiconductor α-Ga2 Se3 can induce polar structure. Meanwhile, the induced polarization can be switched in a moderate energy barrier. The switched polarization is observed in 2D α-Ga2 Se3 nanoflakes of ≈4 nm with a high switching temperature up to 450 K. Such polarization switching could arise from the displacement of Ga vacancy between neighboring asymmetrical sites by applying an electric field. This work removes the point group limit for ferroelectricity, expanding the range of 2D ferroelectrics into the native defective semiconductors.

Effect of layer and stacking sequence in simultaneously grown 2H and 3R WS2 atomic layers.

In two-dimensional layered materials, layer number and stacking order have strong effects on the optical and electronic properties. Tungsten disulfide (WS2) crystal, as one important member among transition metal dichalcogenides, has been usually prepared in a layered 2H prototype structure with space group P63/mmc ([Formula: see text]) in spite of many other expected ones such as 3R. Here, we report simultaneous growth of 2H and 3R stacked multilayer (ML) WS2 crystals in large scale by chemical vapor deposition and effects of layer number and stacking order on optical and electronic properties. As revealed in Raman and photoluminescence (PL) measurements, with an increase in layer number, 2H and 3R stacked ML WS2 crystals show similar variation of PL and Raman peaks in position and intensity. Compared to 2H stacked ML WS2, however, 3R stacked one always exhibits the larger red (blue) shift of Raman [Formula: see text] (A1g) peak and the appearance of PL A, B and I peaks at lower energies. Thereby, PL and Raman features depend on not only layer number but also stacking order. In addition, circularly polarized luminescence from two prototype WS2 crystals under circularly polarized excitation has also been investigated, showing obvious spin or valley polarization of these CVD-grown multilayer WS2 crystals.

Study of the Decomposition and Phase Transition of Uranium Nitride under UHV Conditions via TDS, XRD, SEM, and XPS.

Uranium nitrides are among the most promising fuels for Generation IV nuclear reactors, but until now, very little has been known about their thermal stability properties under nonequilibrium conditions. In this work, thermal decomposition of nitrogen-rich uranium nitride (denoted as UN2-x) under ultrahigh-vacuum (UHV) conditions was investigated by thermal desorption spectroscopy (TDS). It has been shown that the nitrogen TDS spectrum consists of two peaks at about 723 and 1038 K. The X-ray diffraction, scanning electron microscopy, and X-ray photoelectron microscopy results indicate that UN2-x (UN2 phase) decomposed into the α-U2N3 phase in the first step and the α-U2N3 phase decomposed into the UN phase in the second step.

Layer-dependent ferroelectricity in 2H-stacked few-layer α-In2Se3.

Atomically thin two-dimensional (2D) van der Waals materials have exhibited many exotic layer-dependent physical properties including electronic structure, magnetic order, etc. Here, we report a striking even-odd layer dependent oscillation in the ferroelectric polarization of 2H-stacked few-layer α-In2Se3 nanoflakes. As characterized by piezoresponse force microscopy (PFM), when the in-plane (IP) electric polarization of 2H-stacked α-In2Se3 films is electrically aligned, the out-of-plane (OOP) polarization of the odd-layer (OL) samples is obviously larger than that of the even-layer (EL) ones. Similarly, samples with electrically aligned OOP polarization also show even-odd layer dependent IP polarization. Such an even-odd oscillation, as confirmed by the density functional theory calculations, can be attributed to the strong intercorrelation of the IP and OOP electric polarization of the α-In2Se3 monolayers and the special 2H-stacking structure of a 180 degree IP rotation with respect to the adjacent layers. Moreover, a negative differential resistance, interestingly, is induced by the polarization flip with a small coercive field of ∼1.625 kV cm-1, and its peak-to-valley ratio can be tuned up to ∼7 by the gate. This work demonstrates that the delicate stacking geometry of multilayer α-In2Se3 can bring an interesting even-odd ferroelectric effect, enriching the layer-dependent physical properties of the 2D materials family.

Dysregulation of Neuronal Gαo Signaling by Graphene Oxide in Nematode Caenorhabditis elegans.

Exposure to graphene oxide (GO) induced some dysregulated microRNAs (miRNAs), such as the increase in mir-247, in nematode Caenorhabditis elegans. We here further identified goa-1 encoding a Gαo and pkc-1 encoding a serine/threonine protein kinase as the targets of neuronal mir-247 in the regulation of GO toxicity. GO exposure increased the expressions of both GOA-1 and PKC-1. Mutation of goa-1 or pkc-1 induced a susceptibility to GO toxicity, and suppressed the resistance of mir-247 mutant to GO toxicity. GOA-1 and PKC-1 could also act in the neurons to regulate the GO toxicity, and neuronal overexpression of mir-247 could not affect the resistance of nematodes overexpressing neuronal goa-1 or pkc-1 lacking 3'-UTR to GO toxicity. In the neurons, GOA-1 acted upstream of diacylglycerol kinase/DGK-1 and PKC-1 to regulate the GO toxicity. Moreover, DGK-1 and GOA-1 functioned synergistically in the regulation of GO toxicity. Our results highlight the crucial role of neuronal Gαo signaling in response to GO in nematodes.

One-Step Growth of Spatially Graded Mo1- xW xS2 Monolayers with a Wide Span in Composition (from x = 0 to 1) at a Large Scale.

Alloying is an effective way to modulate material's properties. In particular, graded alloying within a single domain of two-dimensional transition-metal chalcogenide (2D-TMC) is of great technological importance, for example, for achieving band gap modulations. Here, we report a facile method to grow gradient alloying of Mo1- xW xS2 monolayers with large domain sizes and high crystal qualities via the chemical vapor deposition technique. The as-grown Mo1- xW xS2 monolayers have a gradient composition of W from x = ∼0 to ∼1 in a single domain with a lateral dimension up to 300 µm, and the span in band gap can be readily tuned. Owing to the grading in band offsets, the compositionally graded Mo1- xW xS2 alloy monolayer demonstrates an excellent rectifying effect with the ratio of forward to reverse current up to ∼104. Moreover, phototransistors based on the compositionally graded Mo1- xW xS2 monolayers exhibit a high responsivity up to 298.4 A/W in the visible light regime, and particularly a decent responsivity of 28.7 A/W in the near-infrared regime. The control of band gap offset gradient and span in alloyed 2D-TMC semiconductors provides an additional degree of freedom in designing fascinating applications in achieving multifunctional optoelectronic devices on individual substrates.

Sulfur-Doped Black Phosphorus Field-Effect Transistors with Enhanced Stability.

Black phosphorus (BP) has drawn great attention owing to its tunable band gap depending on thickness, high mobility, and large Ion/ Ioff ratio, which makes BP attractive for using in future two-dimensional electronic and optoelectronic devices. However, its instability under ambient conditions poses challenge to the research and limits its practical applications. In this work, we present a feasible approach to suppress the degradation of BP by sulfur (S) doping. The fabricated S-doped BP few-layer field-effect transistors (FETs) show more stable transistor performance under ambient conditions. After exposing to air for 21 days, the charge-carrier mobility of a representative S-doped BP FETs device decreases from 607 to 470 cm2 V-1 s-1 (remained as high as 77.4%) under ambient conditions and a large Ion/ Ioff ratio of ∼103 is still retained. The atomic force microscopy analysis, including surface morphology, thickness, and roughness, also indicates the lower degradation rate of S-doped BP compared to BP. First-principles calculations show that the dopant S atom energetically prefers to chemisorb on the BP surface in a dangling form and the enhanced stability of S-doped BP can be ascribed to the downshift of the conduction band minimum of BP below the redox potential of O2/O2-. Our work suggests that S doping is an effective way to enhance the stability of black phosphorus.

Ultrahigh-Gain and Fast Photodetectors Built on Atomically Thin Bilayer Tungsten Disulfide Grown by Chemical Vapor Deposition.

The low responsivity observed in photodetectors based on monolayer transition-metal dichalcogenides has encouraged the pursuit of approaches that can efficiently enhance the external quantum efficiency, which relies predominantly on the light absorption, the lifetime of the excess carriers, and the charge collection efficiency. Here, we demonstrate that phototransistors fabricated on large-area bilayer tungsten disulfide (WS2) grown by chemical vapor deposition exhibit remarkable performance with photoresponsivity, photogain, and detectivity of up to ∼3 × 103 A/W, 1.4 × 104, and ∼5 × 1012 Jones, respectively. These figures of merit of bilayer WS2 provide a significant advantage over monolayer WS2 due to the greatly improved carrier mobility and significantly reduced contact resistance. The photoresponsivity of bilayer WS2 phototransistor can be further improved to up to 1 × 104 A/W upon biasing a gate voltage of 60 V, without evident reduction in detectivity. Moreover, the bilayer WS2 phototransistor exhibits a high response speed of less than 100 µs, large bandwidth of 4 kHz, high cycling reliability of over 105 cycles, and spatially homogeneous photoresponse. These outstanding figures of merit make WS2 bilayer a highly promising candidate for the design of high-performance optoelectronics in the visible regime.

Highly sensitive and fast monolayer WS2 phototransistors realized by SnS nanosheet decoration.

Two-dimensional chalcogenide monolayers are strong candidates for next-generation flexible and transparent optoelectronics. Due to the intrinsic ultrathin thickness and limited optical absorption, however, their responsivity is normally low. Here we develop a simple and low-cost method to fabricate high-performance hybrid phototransistors of monolayer WS2 with significantly enhanced responsivity and an extended spectral response range, by virtue of surface decoration with liquid-phase exfoliated SnS nanosheets (NSs). The hybrid phototransistors show a much enhanced responsivity of ∼2 A W-1 and an ultrahigh light/dark signal-to-noise ratio of 106 under 457 nm excitation, exhibiting a significant increase of 3 orders of magnitude in responsivity and a 100 fold increase in signal-to-noise ratio, compared with pure WS2 devices. Our hybrid photodetectors also exhibit a respectable response speed, with a rise and decay time of 51 µs and 98 µs, respectively. After optimal surface decoration with narrow bandgap SnS NSs atop a monolayer WS2 channel, an emergent optical responsivity in the near infrared region (1064 nm) is also observed.

Fabrication of NiCo2-Anchored Graphene Nanosheets by Liquid-Phase Exfoliation for Excellent Microwave Absorbers.

Graphene nanosheets (GNSs) were prepared by an efficient liquid-phase exfoliation method, and then the NiCo2/GNS nanohybrids were fabricated using the single-mode microwave-assisted hydrothermal technique. The NiCo2/GNS composites with different GNS proportions were investigated as microwave absorbers. Morphology investigation suggested that NiCo2 nanocrystals were uniformly anchored on the GNS without aggregation. The electromagnetic parameters of NiCo2/GNS nanohybrids could be artificially adjusted by changing the GNS proportion, which led to an exceptional microwave-absorbing performance. A reflection loss (RL) exceeding -20 dB was obtained in the frequency range of 5.3-16.4 GHz for the absorber thicknesses of 1.2-3.2 mm, while an optimal RL of -30 dB was achieved at 11.7 GHz for a thickness of 1.6 mm. The enhanced microwave-absorbing performance indicated that the NiCo2/10 wt % GNS composite has great potential for use as an excellent microwave absorber.

Strain Release Induced Novel Fluorescence Variation in CVD-Grown Monolayer WS2 Crystals.

Tensile strain is intrinsic to monolayer crystals of transition metal disulfides such as Mo(W)S2 grown on oxidized silicon substrates by chemical vapor deposition (CVD) owing to the much larger thermal expansion coefficient of Mo(W)S2 than that of silica. Here we report fascinating fluorescent variation in intensity with aging time in CVD-grown triangular monolayer WS2 crystals on SiO2 (300 nm)/Si substrates and formation of interesting concentric triangular fluorescence patterns in monolayer crystals of large size. The novel fluorescence aging behavior is recognized to be induced by the partial release of intrinsic tensile strain after CVD growth and the induced localized variations or gradients of strain in the monolayer crystals. The results demonstrate that strain has a dramatic impact on the fluorescence and photoluminescence of monolayer WS2 crystals and thus could potentially be utilized to tune electronic and optoelectronic properties of monolayer transition metal disulfides.

Intestinal Insulin Signaling Encodes Two Different Molecular Mechanisms for the Shortened Longevity Induced by Graphene Oxide in Caenorhabditis elegans.

Graphene oxide (GO) has been shown to cause multiple toxicities in various organisms. However, the underlying molecular mechanisms for GO-induced shortened longevity are still unclear. We employed Caenorhabditis elegans to investigate the possible involvement of insulin signaling pathway in the control of GO toxicity and its underlying molecular mechanisms. Mutation of daf-2, age-1, akt-1, or akt-2 gene induced a resistant property of nematodes to GO toxicity, while mutation of daf-16 gene led to a susceptible property of nematodes to GO toxicity, suggesting that GO may dysregulate the functions of DAF-2/IGF-1 receptor, AGE-1, AKT-1 and AKT-2-mediated kinase cascade, and DAF-16/FOXO transcription factor. Genetic interaction analysis suggested the involvement of signaling cascade of DAF-2-AGE-1-AKT-1/2-DAF-16 in the control of GO toxicity on longevity. Moreover, intestinal RNA interference (RNAi) analysis demonstrated that GO reduced longevity by affecting the functions of signaling cascade of DAF-2-AGE-1-AKT-1/2-DAF-16 in the intestine. DAF-16 could also regulate GO toxicity on longevity by functioning upstream of SOD-3, which encodes an antioxidation system that prevents the accumulation of oxidative stress. Therefore, intestinal insulin signaling may encode two different molecular mechanisms responsible for the GO toxicity in inducing the shortened longevity. Our results highlight the key role of insulin signaling pathway in the control of GO toxicity in organisms.

A mir-231-Regulated Protection Mechanism against the Toxicity of Graphene Oxide in Nematode Caenorhabditis elegans.

Recently, several dysregulated microRNAs (miRNAs) have been identified in organisms exposed to graphene oxide (GO). However, their biological functions and mechanisms of the action are still largely unknown. Here, we investigated the molecular mechanism of mir-231 in the regulation of GO toxicity using in vivo assay system of Caenorhabditis elegans. We found that GO exposure inhibited the expression of mir-231::GFP in multiple tissues, in particular in the intestine. mir-231 acted in intestine to regulate the GO toxicity, and overexpression of mir-231 in intestine caused a susceptible property of nematodes to GO toxicity. smk-1 encoding a homologue to mammalian SMEK functioned as a targeted gene for mir-231, and was also involved in the intestinal regulation of GO toxicity. Mutation of smk-1 gene induced a susceptible property to GO toxicity, whereas the intestinal overexpression of smk-1 resulted in a resistant property to GO toxicity. Moreover, mutation of smk-1 gene suppressed the resistant property of mir-231 mutant to GO toxicity. In nematodes, SMK-1 further acted upstream of the transcriptional factor DAF-16/FOXO in insulin signaling pathway to regulate GO toxicity. Therefore, mir-231 may encode a GO-responsive protection mechanism against the GO toxicity by suppressing the function of the SMK-1 - DAF-16 signaling cascade in nematodes.

Field-induced magnetic instability and quantum criticality in the antiferromagnet CeCu2Ge2.

The magnetic quantum criticality in strongly correlated electron systems has been considered to be closely related with the occurrence of unconventional superconductivity. Control parameters such as magnetic field, pressure or chemical doping are frequently used to externally tune the quantum phase transition for a deeper understanding. Here we report the research of a field-induced quantum phase transition using conventional bulk physical property measurements in the archetypal antiferromagnet CeCu2Ge2, which becomes superconductive under a pressure of about 10 GPa with Tc ~ 0.64 K. We offer strong evidence that short-range dynamic correlations start appearing above a magnetic field of about 5 T. Our demonstrations of the magnetic instability and the field-induced quantum phase transition are crucial for the quantum criticality, which may open a new route in experimental investigations of the quantum phase transition in heavy-fermion systems.

Enhanced Photoresponse of SnSe-Nanocrystals-Decorated WS2 Monolayer Phototransistor.

Single-layer WS2 has shown excellent photoresponse properties, but its promising applications in high-sensitivity photodetection suffer from the atomic-thickness-limited adsorption and band-gap-limited spectral selectivity. Here we have carried out investigations on WS2 monolayer based phototransistors with and without decoration of SnSe nanocrystals (NCs) for comparison. Compared to the solely WS2 monolayer, SnSe NCs decoration leads to not only huge enhancement of photoresponse in visible spectrum but also extension to near-infrared. Under excitation of visible light in a vacuum, the responsivity at zero gate bias can be enhanced by more than 45 times to ∼99 mA/W, and the response time is retained in millisecond level. Particularly, with extension of photoresponse to near-infrared (1064 nm), a responsivity of 6.6 mA/W can be still achieved. The excellent photoresponse from visible to near-infrared is considered to benefit from synergism of p-type SnSe NCs and n-type WS2 monolayer, or in other words, the formed p-n heterojunctions between p-type SnSe NCs and n-type WS2 monolayer.

Metallothioneins act downstream of insulin signaling to regulate toxicity of outdoor fine particulate matter (PM2.5) during Spring Festival in Beijing in nematode Caenorhabditis elegans.

In this study, we performed the toxicological assessment of outdoor PM2.5 collected from Beijing during Spring Festival using the in vivo assay system of Caenorhabditis elegans. Acute exposure to outdoor PM2.5 at a concentration of 10 mg L-1 and prolonged exposure to outdoor PM2.5 at concentrations of 0.1-10 mg L-1 decreased locomotion behavior and caused significant induction of intestinal ROS production. Meanwhile, outdoor PM2.5 exposure induced significant expression of gene (mtl-1 and mtl-2) encoded metallothioneins in the intestine. Mutation of the mtl-1 or mtl-2 gene resulted in a susceptible property of nematodes to outdoor PM2.5 toxicity. Genetic assays suggested that mtl-1 and mtl-2 genes acted downstream of the daf-16 gene encoding a FOXO transcriptional factor and daf-2 gene encoding an insulin receptor in the insulin signaling pathway to regulate outdoor PM2.5 toxicity. DAF-2 further acted upstream of DAF-16 and suppressed the function of DAF-16 to regulate outdoor PM2.5 toxicity. Therefore, we identified a signaling cascade of DAF-2-DAF-16-MTL-1/2 in the control of outdoor PM2.5 toxicity in nematodes. Our study provides an important molecular basis for the potential toxicity of outdoor PM2.5 during Spring Festival in Beijing in nematodes. Especially, our study will highlight the potential adverse effects of outdoor PM2.5 during Spring Festival on environmental organisms.

Lactic Acid Bacteria Protects Caenorhabditis elegans from Toxicity of Graphene Oxide by Maintaining Normal Intestinal Permeability under different Genetic Backgrounds.

Lactic acid bacteria (LAB) is safe and useful for food and feed fermentation. We employed Caenorhabditis elegans to investigate the possible beneficial effect of LAB (Lactobacillus bulgaricus) pretreatment against toxicity of graphene oxide (GO) and the underlying mechanisms. LAB prevented GO toxicity on the functions of both primary and secondary targeted organs in wild-type nematodes. LAB blocked translocation of GO into secondary targeted organs through intestinal barrier by maintaining normal intestinal permeability in wild-type nematodes. Moreover, LAB prevented GO damage on the functions of both primary and secondary targeted organs in exposed nematodes with mutations of susceptible genes (sod-2, sod-3, gas-1, and aak-2) to GO toxicity by sustaining normal intestinal permeability. LAB also sustained the normal defecation behavior in both wild-type nematodes and nematodes with mutations of susceptible genes. Therefore, the beneficial role of LAB against GO toxicity under different genetic backgrounds may be due to the combinational effects on intestinal permeability and defecation behavior. Moreover, the beneficial effects of LAB against GO toxicity was dependent on the function of ACS-22, homologous to mammalian FATP4 to mammalian FATP4. Our study provides highlight on establishment of pharmacological strategy to protect intestinal barrier from toxicity of GO.