Emergence of ferroelectricity in a nonferroelectric monolayer.

Ferroelectricity in ultrathin two-dimensional (2D) materials has attracted broad interest due to potential applications in nonvolatile memory, nanoelectronics and optoelectronics. However, ferroelectricity is barely explored in materials with native centro or mirror symmetry, especially in the 2D limit. Here, we report the first experimental realization of room-temperature ferroelectricity in van der Waals layered GaSe down to monolayer with mirror symmetric structures, which exhibits strong intercorrelated out-of-plane and in-plane electric polarization. The origin of ferroelectricity in GaSe comes from intralayer sliding of the Se atomic sublayers, which breaks the local structural mirror symmetry and forms dipole moment alignment. Ferroelectric switching is demonstrated in nano devices fabricated with GaSe nanoflakes, which exhibit exotic nonvolatile memory behavior with a high channel current on/off ratio. Our work reveals that intralayer sliding is a new approach to generate ferroelectricity within mirror symmetric monolayer, and offers great opportunity for novel nonvolatile memory devices and optoelectronics applications.

Spin-Lattice Coupled Metamagnetism in Frustrated van der Waals Magnet CrOCl.

The long-range magnetic ordering in frustrated magnetic systems is stabilized by coupling magnetic moments to various degrees of freedom, for example, by enhancing magnetic anisotropy via lattice distortion. Here, the unconventional spin-lattice coupled metamagnetic properties of atomically-thin CrOCl, a van der Waals antiferromagnet with inherent magnetic frustration rooted in the staggered square lattice, are reported. Using temperature- and angle-dependent tunneling magnetoconductance (TMC), in complementary with magnetic torque and first-principles calculations, the antiferromagnetic (AFM)-to-ferrimagnetic (FiM) metamagnetic transitions (MTs) of few-layer CrOCl are revealed to be triggered by collective magnetic moment flipping rather than the established spin-flop mechanism, when external magnetic field (H) enforces a lattice reconstruction interlocked with the five-fold periodicity of the FiM phase. The spin-lattice coupled MTs are manifested by drastic jumps in TMC, which show anomalous upshifts at the transition thresholds and persist much higher above the AFM Néel temperature. While the MTs exhibit distinctive triaxial anisotropy, reflecting divergent magnetocrystalline anisotropy of the c-axis AFM ground state, the resulting FiM phase has an a-c easy plane in which the magnetization axis is freely rotated by H. At the 2D limit, such a field-tunable FiM phase may provide unique opportunities to explore exotic emergent phenomena and novel spintronics devices.

Two-dimensional ferroelectricity in a single-element bismuth monolayer.

Ferroelectric materials are fascinating for their non-volatile switchable electric polarizations induced by the spontaneous inversion-symmetry breaking. However, in all of the conventional ferroelectric compounds, at least two constituent ions are required to support the polarization switching1,2. Here, we report the observation of a single-element ferroelectric state in a black phosphorus-like bismuth layer3, in which the ordered charge transfer and the regular atom distortion between sublattices happen simultaneously. Instead of a homogenous orbital configuration that ordinarily occurs in elementary substances, we found the Bi atoms in a black phosphorous-like Bi monolayer maintain a weak and anisotropic sp orbital hybridization, giving rise to the inversion-symmetry-broken buckled structure accompanied with charge redistribution in the unit cell. As a result, the in-plane electric polarization emerges in the Bi monolayer. Using the in-plane electric field produced by scanning probe microscopy, ferroelectric switching is further visualized experimentally. Owing to the conjugative locking between the charge transfer and atom displacement, we also observe the anomalous electric potential profile at the 180° tail-to-tail domain wall induced by competition between the electronic structure and electric polarization. This emergent single-element ferroelectricity broadens the mechanism of ferroelectrics and may enrich the applications of ferroelectronics in the future.

Shear stress and plaque microenvironment induce heterogeneity: A multiscale microenvironment evolution model.

BACKGROUND: Both clinical images and in vivo observations have demonstrated the heterogeneity in atherosclerotic plaque composition. However, the quantitative mechanisms that contribute to the heterogeneity, such as the wall shear stress (WSS) and the interplays among microenvironmental factors are still unclear. METHODS: We develop a multiscale model coupling computational fluid dynamics, interactions of microenvironmental factors and evolutions of cellular behaviors to investigate the formation of plaque heterogeneity in a three-dimensional vessel segment. The model involves WSS, lipid deposition and inflammatory response to reveal the dynamic balance existed between the lipid metabolism and the phagocytosis of macrophages. RESULTS: The dynamic balance in microenvironment is influenced by both the WSS and the interactions with microenvironmental factors, and consequently results in the longitudinal heterogeneity observed in plaque pathology. In addition, plaque heterogeneity can be reduced by decreasing low WSS area at downstream, as well as by altering the phagocytic abilities of macrophage on lipoproteins, which may be used to develop future plaque regression strategies. CONCLUSIONS: This multiscale modeling provides a framework to understand the mechanisms in dynamics of plaque composition and also provides quantitative information to better risk stratification of plaque vulnerability in future clinical practice.

Solution epitaxy of polarization-gradient ferroelectric oxide films with colossal photovoltaic current.

Solution growth of single-crystal ferroelectric oxide films has long been pursued for the low-cost development of high-performance electronic and optoelectronic devices. However, the established principles of vapor-phase epitaxy cannot be directly applied to solution epitaxy, as the interactions between the substrates and the grown materials in solution are quite different. Here, we report the successful epitaxy of single-domain ferroelectric oxide films on Nb-doped SrTiO3 single-crystal substrates by solution reaction at a low temperature of ~200 oC. The epitaxy is mainly driven by an electronic polarization screening effect at the interface between the substrates and the as-grown ferroelectric oxide films, which is realized by the electrons from the doped substrates. Atomic-level characterization reveals a nontrivial polarization gradient throughout the films in a long range up to ~500 nm because of a possible structural transition from the monoclinic phase to the tetragonal phase. This polarization gradient generates an extremely high photovoltaic short-circuit current density of ~2.153 mA/cm2 and open-circuit voltage of ~1.15 V under 375 nm light illumination with power intensity of 500 mW/cm2, corresponding to the highest photoresponsivity of ~4.306×10-3 A/W among all known ferroelectrics. Our results establish a general low-temperature solution route to produce single-crystal gradient films of ferroelectric oxides and thus open the avenue for their broad applications in self-powered photo-detectors, photovoltaic and optoelectronic devices.

Comprehensive investigation on volatile and non-volatile metabolites in low-salt doubanjiang with different fermentation methods.

Doubanjiang is a well-known fermented condiment in China, but the high-salt concentration in its traditional manufacture process greatly lengthens the fermentation time, and leads to potential health risks. Here, the effects of salt reduction and co-inoculated starters (Tetragenococcus halophilus and Zygosaccharomyces rouxii) on the volatile metabolites (VMs) and non-volatile metabolites (NVMs) of doubanjiang were investigated using metabolomics technology and chemometrics analysis. Results showed that 75 VMs were identified, and 12 of them had significant aroma contribution (ROVAs ≥ 1). In addition, 106 NVMs were defined as significantly different metabolites (p < 0.05; VIP ≥ 1). Salt reduction could significantly increase the concentrations of VMs, but this strategy also promoted some undesirable odors like 2-phetylfuran and hexanoic acid, which could be totally suppressed by inoculation of starter. Moreover, the two starters improved amino acid, ester, and acid metabolites. This study provides a deeper insight into the development of low-salt fermented foods.

CRISPR-Cas based molecular diagnostics for foodborne pathogens.

Foodborne pathogenic infection has brought multifaceted issues to human life, leading to an urgent demand for advanced detection technologies. CRISPR/Cas-based biosensors have the potential to address various challenges that exist in conventional assays such as insensitivity, long turnaround time and complex pretreatments. In this perspective, we review the relevant strategies of CRISPR/Cas-assisted diagnostics on foodborne pathogens, focusing on biosensing platforms for foodborne pathogens based on fluorescence, colorimetric, (electro)chemiluminescence, electrochemical, and surface-enhanced Raman scattering detection. It summarizes their detection principles by the clarification of foodborne pathogenic bacteria, fungi, and viruses. Finally, we discuss the current challenges or technical barriers of these methods against broad application, and put forward alternative solutions to improve CRISPR/Cas potential for food safety.

Direct Observation of the Topological Surface State in the Topological Superconductor 2M-WS2.

The quantum spin Hall (QSH) effect has attracted extensive research interest because of the potential applications in spintronics and quantum computing, which is attributable to two conducting edge channels with opposite spin polarization and the quantized electronic conductance of 2e2/h. Recently, 2M-WS2, a new stable phase of transition metal dichalcogenides with a 2M structure showing a layer configuration identical to that of the monolayer 1T' TMDs, was suggested to be a QSH insulator as well as a superconductor with a critical transition temperature of around 8 K. Here, high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES are applied to investigate the electronic and spin structure of the topological surface states (TSS) in the superconducting 2M-WS2. The TSS exhibit characteristic spin-momentum-locking behavior, suggesting the existence of long-sought nontrivial Z2 topological states therein. We expect that 2M-WS2 with coexisting superconductivity and TSS might host the promising Majorana bound states.

DNA-mediated assembly of a gold-nanoparticle film with controllable sonic behavior detected by novel electric-induced ultrasound.

Two-dimensional gold-assembled nanomaterials have garnered considerable interest in biomedical applications such as wearable sensors and flexible devices. The assembly can be accomplished via attractive interactions between gold nanoparticles (GNPs) and a standard polymer. It is highly desirable and a great challenge to develop a new assembly method that can provide formed materials with novel properties. In this work, we explored complementary DNA-functionalized gold nanoparticles (DNA-GNPs) as building blocks to establish multifunctional two-dimensional gold nanomaterials via layer-by-layer (LBL) assembly. We found that the DNA-mediated assembly endows GNP films with a superlattice and their sonic behavior could be regulated in a controllable manner through altering the sequence of SH-DNA and linker DNA, which was detected by the electric-induced ultrasound method. Our observation suggested that this DNA-mediated assembly of GNP films with controllable sonic behaviors could greatly promote the biomedical application of two-dimensional assembled-gold nanomaterials.

Rapid screening of antimicrobial probiotics using CRISPR cascade.

Animal bacterial infection is increasingly threatening human health. Here we report a nucleic acid amplification-free CRISPR genetic assay that allows to rapidly screen potential food-origin antimicrobial probiotics. The assay (termed CRISPRzyme assay) is based on a CRISPR-DNAzyme cascade, where the target gene sequentially activated Cas12a protein and DNAzyme, yielding a limit of detection of 62 CFU Vibrio parahaemolyticus, 86 CFU Salmonella Typhimurium, and 82 CFU Listeria monocytogenes. The elimination of nucleic acid amplification shortens processing time and operational complexity. The assay was used to rapidly screen antimicrobial probiotics by end-measurement of fluorescence of pathogenic bacteria. Particularly, it can estimate the in vivo antimicrobial effect due to its capacity for pathogen quantification in complex samples. We found that isolates of Bacillus and lactic acid bacteria separated from fermented food exhibited strong antimicrobial activity for fish pathogen, Vibrio parahaemolyticus, and identified surfactin as the key antimicrobial component. The CRISPRzyme assay could ease antimicrobial probiotics screening, and constitutes a new tool for combatting pathogenic bacterial contamination and infection.

Giant Bandgap Engineering in Two-Dimensional Ferroelectric α-In2Se3.

Bandgap engineering is an efficient strategy for controlling the physical properties of semiconductor materials. For flexible two-dimensional (2D) materials, strain provides a nondestructive and adjustable method for bandgap adjustment. Here, we propose that, in 2D materials with out-of-plane ferroelectricity, the antibonding nature of the valence band maximum and conduction band minimum and polarized charge distribution induced by ferroelectricity give rise to giant changes of the bandgap under curvature strain field. This hypothesis was proven by scanning tunneling microscopy/spectroscopy measurements on monolayer α-In2Se3 that revealed that the bandgap of α-In2Se3 increases significantly due to bending. Both experiments and theoretical calculations indicated that the bandgap increases monotonically with the degree of bending of the α-In2Se3 layer. Our work suggests that bending is an effective method for tuning the gaps of 2D ferroelectric materials, providing a new platform for bandgap engineering under the combination of ferroelectricity and strain field.

M4 B6 X6 as a New Family of High-Efficient Electrocatalysts: The Role of Surface Reconstruction in Water Oxidization.

Searching for highly-efficient electrocatalysts for water splitting has been greatly endowed due to the huge demand for green energy sources. Two-dimensional (2D) materials are widely explored for the purpose because of their unique physical and chemical properties, abundant active sites, and easy fabrication. Here, we present a new family of 2D M4 B6 X6 (2D Boridenes) and investigate their physical and chemical properties for their potential applications into electrocatalysis based on first-principles calculations. We demonstrate that 2D M4 B6 X6 (M=Cr, Mo, and W; X=O and F) are dynamically, thermodynamically, and mechanically stable, and show intriguing electronic and catalytic properties. Importantly, we find that M4 B6 O6 are intrinsically active for oxygen evolution reaction (OER). Our results demonstrate that: (1) the adsorbate-escape mechanism dominates the OER process with a low overpotential of 0.652 V on Cr4 B6 O6 ; (2) the partial surface-oxidization can improve the catalytic performance of M4 B6 F6 dramatically; and (3) the surface reconstruction greatly affects the OER performance of M4 B6 X6 . Our findings illustrate that the surface reconstruction is critical to the OER activity, which may provide a new strategy on the design of 2D materials for electrocatalysis and offer theoretical insight into the catalytic mechanism.

Designing Ultra-flat Bands in Twisted Bilayer Materials at Large Twist Angles: Theory and Application to Two-Dimensional Indium Selenide.

Intertwisted bilayers of two-dimensional (2D) materials can host low-energy flat bands, which offer opportunity to investigate many intriguing physics associated with strong electron correlations. In the existing systems, ultra-flat bands only emerge at very small twist angles less than a few degrees, which poses a challenge for experimental studies and practical applications. Here, we propose a new design principle to achieve low-energy ultra-flat bands with increased twist angles. The key condition is to have a 2D semiconducting material with a large energy difference of band edges controlled by stacking. We show that the interlayer interaction leads to defect-like states under twisting, which forms a flat band in the semiconducting band gap with dispersion strongly suppressed by the large energy barriers in the moiré superlattice even for large twist angles. We explicitly demonstrate our idea in bilayer α-In2Se3 and bilayer InSe. For bilayer α-In2Se3, we show that a twist angle of ∼13.2° is sufficient to achieve the band flatness comparable to that of twist bilayer graphene at the magic angle ∼1.1°. In addition, the appearance of ultra-flat bands here is not sensitive to the twist angle as in bilayer graphene, and it can be further controlled by external gate fields. Our finding provides a new route to achieve ultra-flat bands other than reducing the twist angles and paves the way toward engineering such flat bands in a large family of 2D materials.

Van der Waals Antiferroelectric Magnetic Tunnel Junction: A First-Principles Study of a CrSe2/CuInP2S6/CrSe2 Junction.

Magnetic tunnel junctions (MTJs), ferroelectric/antiferroelectric tunnel junctions (FTJs/AFTJs), and multiferroic tunnel junctions (MFTJs) have recently attracted significant interest for technological applications of nanoscale memory devices. Until now, most of them are based on perovskite oxide heterostructures with a relatively high resistance-area (RA) product and low resistance difference unfavorable for practical applications. The recent discovery of the two-dimensional (2D) van der Waals (vdW) ferroelectric (FE) and magnetic materials has opened a new route to realize tunnel junctions with high performance and atomic-scale dimensions. Here, using first-principles calculations, we propose a new type of 2D tunnel junction: an antiferroelectric magnetic tunnel junction (AFMTJ), which inherits the features of both MTJ and AFTJ. This AFMTJ is composed of monolayer CuInP2S6 (CIPS) sandwiched between 2D magnetic electrodes of CrSe2. The AFTJ with nonmagnetic electrodes of TiSe2 on both sides of CIPS and the asymmetric AFTJ with both CrSe2 and TiSe2 electrodes are also investigated. Based on quantum-mechanical modeling of the electronic transport, sizeable tunneling electroresistance effects and multiple nonvolatile resistance states are demonstrated. More importantly, a remarkably low RA product (less than 0.1 Ω·µm2) makes the proposed vdW AFMTJs superior to the conventional MFTJs in terms of their promising nonvolatile memory applications. Our calculations provide new guidance for the experiment and application of nanoscale memory devices.

Direct Detection of Foodborne Pathogens via a Proximal DNA Probe-Based CRISPR-Cas12 Assay.

Foodborne pathogens can cause illnesses. Existing tools for detecting foodborne pathogens are typically time-consuming or require complex protocols. Here, we report an assay to directly analyze pathogenic genes based on CRISPR-Cas12. This new test, termed proximal DNA probe-based CRISPR-Cas12 (PPCas12), facilitates the detection of foodborne pathogens without amplification steps. The elimination of the nucleic acid amplification process dramatically reduced the processing time, complexity, and costs in the analysis of foodborne pathogens. The substitution of the frequently used dually labeled DNA reporter with a proximal DNA probe in the PPCas12 assay led to a 4-fold sensitivity enhancement. PPCas12 offered a limit of detection of 619 colony-forming units in the detection of Salmonella enterica (S. enterica) without the nucleic acid amplification process. The specific recognition of genes via PPCas12 allowed distinguishing S. enterica from other foodborne pathogens. The PPCas12 assay was applied in the screening of S. enterica contamination on fresh eggs with high precision. Hence, the new PPCas12 assay will be a valuable tool for on-site monitoring of foodborne pathogens.

G-Quadruplex-Probing CRISPR-Cas12 Assay for Label-Free Analysis of Foodborne Pathogens and Their Colonization In Vivo.

Foodborne pathogen infection is a key issue of food safety. Herein, we developed a label-free assay for Salmonella enterica (S. enterica) detection based on the G-quadruplex-probing CRISPR-Cas12 system (termed G-CRISPR-Cas), allowing highly sensitive detection of S. enterica and investigation of their colonization in chickens. The introduction of the G-quadruplex probe serving as the substrate of Cas 12a realized a label-free analysis for foodborne pathogens. Due to the amplification process induced by loop-mediated isothermal amplification (LAMP), G-CRISPR-Cas assay can detect S. enterica as low as 20 CFU. Specificity for pathogenic gene detection was guaranteed by the dual recognition process via LAMP primers and Cas 12a-guided RNA binding. The G-CRISPR-Cas assay was applied to explore S. enterica colonization in the intestinal tract and organs of chickens and showed the risk of S. enterica infection outside of the intestinal tract. The G-CRISPR-Cas assay is promising for on-site diagnosis of the infection or contamination of foodborne pathogens outside the laboratories, such as abattoirs and markets.

Digital Subtraction Angiography Contrast Material Transport as a Direct Assessment for Blood Perfusion of Middle Cerebral Artery Stenosis.

Digital subtraction angiography (DSA) is a fluoroscopic technique used extensively in interventional radiology for visualizing blood vessels. It has also been used to evaluate blood perfusion. However, the perfusion obtained in previous techniques was extracted from signal intensity rather than by the transport of contrast material (CM) through blood flow. The main aim of this study is to evaluate the morphological effects on the hemodynamics and the CM concentration in the middle cerebral artery (MCA) stenosis. We proposed a quantitative parameter, i.e., contrast material remaining time (CMRT), to describe the variation in the transport of CM over time. Computational fluid dynamics simulations were performed on both reconstructive synthetic and patient-derived models. In the synthetic models, we evaluated the variation of flow patterns and the transport of CM with different degrees of stenosis and the location of the lesion. It was found that an increase in the degree of stenosis (from 30 to 80%) resulted in a significant increase in CMRT at the anterior cerebral artery (ACA) outlet (p = 0.0238) and a significant decrease in CMRT at the MCA outlet (p = 0.012). The patient-derived models were reconstructed from the pre- and post-interventional DSA images of a patient with MCA stenosis. Both blood flow velocity and CMRT increased at the ACA outlet but decreased at the MCA outlet. The perfusion analysis demonstrated that the perfusion function was improved after interventional surgery. In conclusion, changes in stenotic degree at MCA may lead to apparent differences in the hemodynamic distribution and the transport of CM. CMRT could be a quantitative indicator to evaluate the changes in blood perfusion after the intervention for MCA stenosis.

Sodium Reduction in Traditional Fermented Foods: Challenges, Strategies, and Perspectives.

Sodium salt is a pivotal ingredient in traditional fermented foods, but its excessive consumption adversely affects human health, product quality, and production efficiency. Therefore, reducing sodium salt content in traditional fermented foods and developing low-sodium fermented foods have attracted increasing attention. Given the essential role of sodium salt in the safety and quality of fermented foods, appropriate approaches should be applied in the production of low-sodium fermented foods. In this review, the challenges of sodium reduction in traditional fermented foods are presented, including the possible growth of pathogenic bacteria, the formation of hazardous chemicals, flavor deficiency, and texture deterioration. Physical, chemical, and biological strategies are also discussed. This review provides references for improving the quality and safety of low-sodium fermented foods.

Bio-augmented effect of Bacillus amyloliquefaciens and Candida versatilis on microbial community and flavor metabolites during Chinese horse bean-chili-paste fermentation.

Chinese horse bean-chili-paste (CHCP), a fermented condiment in China, is traditionally manufactured through naturally spontaneous semi-solid fermentation procedures without intentionally inoculated microorganisms. The aim of this study was to investigate the effect on microbiota and quality variations during CHCP fermentation by inoculation of selected autochthonous microorganisms Bacillus amyloliquefaciens and Candida versatilis. The results showed that relative abundance of Bacillus in the samples inoculated with B. amyloliquefaciens were increased from about 0.6% to almost 25%, and the batches bio-augmented with C. versatilis exhibited clearly 0.7 Lg copies/g higher biomass than that of the other samples. By bio-augmentation, six enzyme activities, namely acid protease, leucine aminopeptidase, α-amylase, cellulose, ß-glucosidase and esterase, were considerably enhanced. As a result, inoculation of these two strains exhibited significant effect on the volatile profiles of CHCP. B. amyloliquefaciens herein was found to contribute mainly to the accumulation of acids, sulfur-containing compounds and pyrazines, whereas C. versatilis was considerably associated with the formation of alcohols, esters and phenols. This study proved that combination of B. amyloliquefaciens and C. versatilis could obtain more extensive aroma profiles, especially for the enrichment of miso-like and fruity flavors, which could provide a guideline for the tailored control of CHCP fermentation process.

Lanthanide-doping enables kinetically controlled growth of deep-blue two-monolayer halide perovskite nanoplatelets.

Impurity doping has been widely applied in nanomaterial synthesis for modulating the crystallographic phase, morphology, and size of nanocrystalline materials, but mostly by altering thermodynamic equilibria of final products. Here, we report the use of lanthanide dopants to manipulate the growing kinetics of halide perovskite nanocrystals to enable the preparation of highly anisotropic two-dimensional (2D) CsPbBr3-based nanoplatelets with precisely controlled thickness. We demonstrate that the incorporation of trivalent lanthanides increases the energy barrier in growing three-monolayer (3 ML) CsPbBr3 from a 2 ML intermediate. It enables the growth of thermodynamically unfavorable 2 ML CsPbBr3 products through kinetic control. This finding provides a novel approach for dimensional control of perovskite nanocrystals with strong quantum confinement. It offers opportunities to generate deep-blue emitting (at 430 nm) CsPbBr3:Lu3+ nanoplatelets with good structural- and photo-stabilities potentially useful for many applications including light-emitting, lasers, and photocatalysis.