Carrier doping modulates the magnetoelectronic and magnetic anisotropic properties of two-dimensional MSi2N4 (M = Cr, Mn, Fe, and Co) monolayers.

Through extensive density functional theory (DFT) calculations, our investigation delves into the stability, electrical characteristics, and magnetic behavior of monolayers (MLs) of MSi2N4. Computational analyses indicate intrinsic antiferromagnetic (AFM) orders within the MSi2N4 MLs, as a result of direct exchange interactions among transition metal (M) atoms. We further find that CrSi2N4 and CoSi2N4 MLs with primitive cells (pcells) exhibit half-metallic properties, with respective spin-ß electron gaps of 3.661 and 2.021 eV. In contrast, MnSi2N4 and FeSi2N4 MLs with pcells act as semiconductors, having energy gaps of 0.427 and 0.282 eV, respectively. When the SOC is considered, the CrSi2N4, MnSi2N4 and FeSi2N4 MLs are metals, while the CoSi2N4 ML is a semiconductor. Our findings imply the dynamics and thermodynamic stability of MSi2N4 MLs. We have also explored the influence of carrier doping on the electromagnetic attributes of MSi2N4 MLs. Interestingly, charge doping could transform CrSi2N4, MnSi2N4, and CoSi2N4 MLs from their original AFM state into a ferromagnetic (FM) order. Moreover, carrier doping transformed CrSi2N4 and CoSi2N4 MLs from spin-polarized metals to half-metals (HMs). It is of particular note that doping of CrSi2N4 MLs with +0.9 e per pcell or more holes caused a switch in the easy axis (EA) to the [001] axis. The demonstrated intrinsic AFM order, excellent thermodynamic and kinetic stability, adjustable magnetism, and half-metallicity of the MSi2N4 family suggest its promising potential for applications in the realm of spintronics.

Molecular Recognition of the Self-Assembly Mechanism of Glycosyl Amino Acetate-Based Hydrogels.

The self-assembly of supramolecular hydrogels has attracted the attention of many researchers, and it also has a broad application prospect in biomedical fields. However, there are few studies on the intrinsic mechanism of molecular self-assembly of hydrogels. In this paper, the self-assembly process of glycolipid-based hydrogels is studied by combining quantum chemistry calculation and molecular dynamics simulation. Using quantum chemistry calculation, the stable stacking mode of gelator dimers was explored. Then, by varying the water content in the gelation system, three different morphologies of hydrogels after self-assembly were observed on the nanoscale. When the water content is low, the molecular chains were entangled with each other to form a three-dimensional network structure. When the water content is moderate, the system had obvious stratification, forming the typical structure of "gel-water-gel". The gelators can only form small micelle-like agglomerations when the water content is too high. According to the analysis of the interaction between gelators and that between gelators and water molecules, combined with the study of the radial distribution function and hydrogen bonding, it is determined that the hydrogen bonds formed between gel molecules are the main driving force of the gelation process. Our work is of guiding significance for further exploration of the formation mechanism of a hydrogel and developing its application in other fields.

An unusual high ozone event over the North and Northeast China during the record-breaking summer in 2018.

Under the background of global warming, the summer temperature of the North and Northeast China (NNEC) has significantly increased since 2017, which was accompanied by the aggravated ozone (O3) pollution. In 2018, the NNEC experienced a record-breaking summer of the past 40 years. Influenced by the abnormal high temperatures, a regional ozone event occurred on 2-3 August, over 63% of 79 selected cities in the NNEC were exposed to O3 pollution, and the maximum value of MDA8 O3 reached 268 µg/m3. Observations indicated that ozone concentrations agree well with the maximum temperature at 2 meters (MT2M) over NNEC with a correlation coefficient of 0.69. During the pollution episode, strong downdraft in the local high (35°N-42.5°N, 112.5°E-132.5°E; LH) over the NNEC created the favourable meteorological conditions for O3 formation. By analyzing the horizontal wind and wave activity fluxes (WAFs) at 200 hPa, we found that the LH formation was resulted from the Rossby wave propagation from upstream along the mid-latitude Asian jet. The split polar vortex intrusion further strengthened the amplitude of the Rossby wave and reinforced the LH. Moreover, a secondary circulation between Typhoon Jongdari and the LH contributed to the enhanced LH with strong subsidence. On the other hand, the stratospheric intrusions under the deep subsidence also contributed to the enhanced surface O3. In this study, the deep-seated meteorological dynamical mechanisms contributing to the abnormal high temperatures were investigated, which can lead to a better understanding of the regional O3 pollution over NNEC under the global-warming background.

Strain-Controllable High Curie Temperature, Large Valley Polarization, and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Janus VSeTe Monolayer.

Two-dimensional (2D) intrinsic ferromagnetic semiconductors are important for spintronics. A highly stable ML (monolayer) Janus 2H-VSeTe with intrinsic ferromagnetism is investigated by density functional theory. The biaxial strain could effectively tune the magnetic and electronic properties of Janus VSeTe. Specifically, the magnetic moment, band gap, Curie temperature (Tc), and valley splitting (Δ) could be modulated, as the states near the Fermi level are mainly contributed by the in-plane atomic orbitals. The VSeTe could be switched from ferromagnetic (FM) order to antiferromagnetic (AFM) ground state, under biaxial strains. And the corresponding Tc is tuned from 360 K (4%) to 0 K (-10.7%). However, VSeTe can be modulated from bipolar magnetic semiconductor (BMS) to half-semiconductor (HSC), spin gapless semiconductor (SGS), half-metal (HM), and even normal metal as the biaxial strain varies from -13 to 10%. Moreover, the easy and hard axes could be switched from each other, and the magnetocrystalline anisotropy (MCA) energy is also controlled by the strains. The Δ is also increased from 158 to 169 meV as the strain varies from 3.3 to -3.0%. The magnetic and electronic phase transitions in the strained VSeTe are observed, which could help researchers to investigate the controllable electronic and magnetic properties in electronics, spintronics, and valleytronics.

Predicted 2D ferromagnetic Janus VSeTe monolayer with high Curie temperature, large valley polarization and magnetic crystal anisotropy.

The development of two-dimensional (2D) intrinsic ferromagnetic semiconductors is urgent in the spintronic field. Motivated by the recent experiments on the successful synthetization of monolayer (ML) Janus transition-metal dichalcogenides (MoSSe) and ferromagnetic (FM) VSe2, a highly stable ML Janus 2H-VSeTe is fabricated by density functional theory and confirmed by a global minimum search. The Janus VSeTe shows a large valley polarization of 158 meV as the space- and time-reversal symmetry is broken. The VSeTe shows FM order with Curie temperature (Tc) of 350 K and a sizable magnetocrystalline anisotropy (MCA) of -8.54 erg cm-2. The high Tc and large valley polarization suggest the 2D Janus VSeTe is a promising magnetic material for potential applications in electronics, spintronics, and valleytronics.

A mask R-CNN model for reidentifying extratropical cyclones based on quasi-supervised thought.

The applications of machine learning/deep learning (ML/DL) methods in meteorology have developed considerably in recent years. Massive amounts of meteorological data are conducive to improving the training effect and model performance of ML/DL, but the establishment of training datasets is often time consuming, especially in the context of supervised learning. In this paper, to identify the two-dimensional (2D) structures of extratropical cyclones in the Northern Hemisphere, a quasi-supervised reidentification method for extratropical cyclones is proposed. This method first uses a traditional automatic cyclone identification method to construct a trainable labeled dataset and then reidentifies extratropical cyclones in a quasi-supervised fashion by using a (pre-trained) Mask region-based convolutional neural network (Mask R-CNN) model. In comparison, the new method increases the number of identified cyclones by 8.29%, effectively supplementing the traditional method. The newly recognized cyclones are mainly shallow or moderately deep subsynoptic-scale cyclones. However, a considerable portion of the new cyclones along the coastlines of the oceans are accompanied by strong winds. In addition, the Mask R-CNN model also shows good performance in identifying the horizontal structures of tropical cyclones. The quasi-supervised concept proposed in this paper may shed some light on accurate target identification in other research fields.

The alignment-dependent properties and applications of graphene moiré superstructures on the Ru(0001) surface.

The moiré superstructure of graphene on a lattice-mismatched metal substrate has profound effects on the electronic properties of graphene and can be used for many applications. Here, we propose to systematically tune the moiré superstructure of graphene on the Ru(0001) surface by rotating the graphene layer. Our study reveals two kinds of graphene moiré superstructures: (i) the ultra-flat graphene layers with height variations of less than 0.1 Å for rotation angles greater than 20° that have the same structural and electronic properties everywhere, and (ii) the highly corrugated graphene moiré superstructures with height variations from 0.4 to 1.6 Å for rotation angles less than 20°, whose electronic properties are highly modulated by the interaction with the substrate. Moreover, these rotated graphene moiré superstructures can serve as templates to produce matrices of size-tunable metal clusters from a few to ∼100 atoms. This study reveals the causes of the structural fluctuation of moiré superstructures of graphene on the transition metal surface and suggests a pathway to tune graphene's electronic properties for various applications.

Tuning the Properties of Graphdiyne by Introducing Electron-Withdrawing/Donating Groups.

The properties of graphdiyne (GDY), such as energy gap, morphology, and affinity to alkali metals, can be adjusted by including electron-withdrawing/donating groups. The push-pull electron ability and size differences of groups play a key role on the partial property adjusting of GDY derivatives MeGDY, HGDY, and CNGDY. Cyano groups (electron-withdrawing) and methyl groups (electron-donating) decrease the band gap and increase the conductivity of the GDY network. The cyano and methyl groups affects the aggregation of GDY, providing a higher number of micropores and specific surface area. These groups also endow the original GDY additional advantages: the stronger electronegativity of cyano groups increase the affinity of GDY frameworks to lithium atoms, and the larger atomic volume of methyl groups increases the interlayer distance and provides more storage space and diffusion tunnels.

First-Principles Study of 3d Transition-Metal-Atom Adsorption onto Graphene Embedded with the Extended Line Defect.

A type of line defect (LD) composed of alternate squares and octagons (4-8) as the basic unit is currently an experimentally available topological defect in the graphene lattice, which brings some interesting modifications to the magnetic and electronic properties of graphene. The transitional-metal (TM) atoms adsorb on graphene with a line defect (4-8), and they show interesting and attractive structural, magnetic, and electronic properties. For different TMs such as Fe, Co, Mn, Ni, and V, the complex systems show different magnetic and electronic properties. The TM atoms can spontaneously adsorb at quadrangular sites, forming a metallic atomic chain along LD on graphene. The most stable configuration is the hollow site of a regular tangle. The TMs (TM = Co, Fe, Mn, Ni, V) tend to form extended metal lines, showing a ferromagnetic (FM) ground state. For the Co, Fe, and V atoms, the system is half-metal. The spin-α electron is insulating, while the spin-ß electron is conductive. For the Mn and Ni atoms, Mn-LD and Ni-LD present a spin-polarized metal; for the Fe atom, Fe-LD shows a semimetal with Dirac cones. For Fe and V atoms, both Fe-LD and V-LD show spin-polarized half-metallic properties. And its spin-α electron is conducting, while the spin-ß electron is insulating. Different TMs adsorbing on a graphene nanoribbon forming the same stable configurations of metal lines show different electronic properties. The adsorption of TMs induces magnetism and spin polarization. These metal lines have potential applications in spintronic devices and work as a quasi-one-dimensional metallic wire, which may form building blocks for atomic-scale electrons with well-controlled contacts at the atomic level.

Tuning the Electronic and Magnetic Properties of Graphene Flake Embedded in Boron Nitride Nanoribbons with Transverse Electric Fields: First-Principles Calculations.

The electronic and magnetic properties of h-BN nanoribbions embedded with graphene nanoflakes (CBNNRs) are systematically studied by ab initio calculations. The CBNNRs with zigzag or armchair edges are all bipolar magnetic semiconductors (BMSs). The band gaps of zigzag CBNNRs (zCBNNRs) change linearly with the transverse electric field (E-field) for the first-order Stark effect, whereas for the armchair CBNNRs (aCBNNRs), the band gaps vary quadratically with the E-field for the second-order Stark effect. For zCBNNRs and aCBNNRs, they could transform from BMS to spin gapless semiconductor (SGS), metal, and half-metal (HM) under different transverse E-fields. The CBNNRs may transform into a semiconductor or HM, under the same E-fields, depending on the position of graphene flakes. The CBNNRs introduce local magnetic moment at carbon atoms, and the magnetic moment is determined by the size of the graphene flakes. These observations open the door to applications of CBNNRs in spintronic devices.

Fcc versus Non-fcc Structural Isomerism of Gold Nanoparticles with Kernel Atom Packing Dependent Photoluminescence.

Structural isomerism allows the correlation between structures and properties to be investigated. Unfortunately, the structural isomers of metal nanoparticles are rare and genuine structural isomerism with distinctly different kernel atom packing (e.g., face-centered cubic (fcc) vs. non-fcc) has not been reported until now. Herein we introduce a novel ion-induction method to synthesize a unique gold nanocluster with a twist mirror symmetry structure. The as-synthesized nanocluster has the same composition but different kernel atom packing to an existing gold nanocluster Au42 (TBBT)26 (TBBT=4-tert-butylbenzenethiolate). The fcc-structured Au42 (TBBT)26 nanocluster shows more enhanced photoluminescence than the non-fcc-structured Au42 (TBBT)26 nanocluster, indicating that the fcc-structure is more beneficial for emission than the non-fcc structure. This idea was supported by comparison of the emission intensity of another three pairs of gold nanoclusters with similar compositions and sizes but with different kernel atom packings (fcc vs. non-fcc).

Recent strengthening of the stratospheric Arctic vortex response to warming in the central North Pacific.

The stratospheric Arctic vortex (SAV) plays a critical role in forecasting cold winters in northern mid-latitudes. Its influence on the tropospheric mid- and high-latitudes has attracted growing attention in recent years. However, the trend in the SAV during the recent two decades is still unknown. Here, using three reanalysis datasets, we found that the SAV intensity during 1998-2016 has a strengthening trend, in contrast to the weakening trend before that period. Approximately 25% of this strengthening is contributed by the warming of sea-surface temperature (SST) over the central North Pacific (CNP). Observational analysis and model experiments show that the warmed CNP SST tends to weaken the Aleutian low, subsequently weakening the upward propagation of wavenumber-1 planetary wave flux, further strengthening the SAV. This strengthened SAV suggests important implications in understanding the Arctic warming amplification and in predicting the surface temperature changes over the northern continents.

Synthesis and Electronic Structure of Boron-Graphdiyne with an sp-Hybridized Carbon Skeleton and Its Application in Sodium Storage.

Boron-graphdiyne (BGDY), which has a unique π-conjugated structure comprising an sp-hybridized carbon skeleton and evenlydistributed boron heteroatoms in a well-organized 2D molecular plane, is prepared through a bottom-up synthetic strategy. Excellent conductivity, a relatively low band gap and a packing mode of the planar BGDY are observed. Notably, the unusual bonding environment of the all sp-carbon framework and the electron-deficient boron centers generates affinity to metal atoms, and thus provides extra binding sites. Furthermore, the expanded molecule pores of the BGDY molecular plane can also facilitate the transfer of metal ions in the perpendicular direction. The practical effect of the all sp-carbon structure and boron heteroatoms on the properties of BGDY are demonstrated in its performance as the anode in sodium-ion batteries.

First-principles study of line-defect-embedded zigzag graphene nanoribbons: electronic and magnetic properties.

Based on first-principles calculations, we present the electronic and magnetic properties of a class of line defect-embedded zigzag graphene nanoribbons, with one edge saturated by two hydrogen atoms per carbon atom and the other edge terminated by only one hydrogen atom. Such edge-modified nanoribbons without line defects are found to be typical bipolar magnetic semiconductors (BMS). In contrast, when the line defect is introduced into the ribbons, the ground state is ferromagnetic, and the resulting nanoribbons can be tuned to spin-polarized metal, metal with Dirac point, or half-metal by varying the position of the line defect, owing to the defect-induced self-doping of the BMS. Specifically, when the line defect is far away from the edges of the ribbon, the system shows half-metallicity. We further confirm the structural and magnetic stability at room temperature by first-principles molecular dynamics simulations. Our findings reveal the possibility of building metal-free electronic/spintronic devices with magnetic/half-metallic graphene nanoribbons.

An atmospheric origin of the multi-decadal bipolar seesaw.

A prominent feature of recent climatic change is the strong Arctic surface warming that is contemporaneous with broad cooling over much of Antarctica and the Southern Ocean. Longer global surface temperature observations suggest that this contrasting pole-to-pole change could be a manifestation of a multi-decadal interhemispheric or bipolar seesaw pattern, which is well correlated with the North Atlantic sea surface temperature variability, and thus generally hypothesized to originate from Atlantic meridional overturning circulation oscillations. Here, we show that there is an atmospheric origin for this seesaw pattern. The results indicate that the Southern Ocean surface cooling (warming) associated with the seesaw pattern is attributable to the strengthening (weakening) of the Southern Hemisphere westerlies, which can be traced to Northern Hemisphere and tropical tropospheric warming (cooling). Antarctic ozone depletion has been suggested to be an important driving force behind the recently observed increase in the Southern Hemisphere's summer westerly winds; our results imply that Northern Hemisphere and tropical warming may have played a triggering role at an stage earlier than the first detectable Antarctic ozone depletion, and enhanced Antarctic ozone depletion through decreasing the lower stratospheric temperature.

Electronic and optical properties of TiO2 nanotubes and arrays: a first-principles study.

Recently, the synthesis, properties, modifications, and applications of TiO2 nanomaterials have attracted much research attention. Here, based on extensive density functional theory calculations, we explored the stability, electronic structures and optical absorption properties of single-walled TiO2 nanotubes (SWTONTs) and TiO2 nanotube arrays (TONTAs), which are constructed from anatase TiO2(101) monolayers and bilayers, respectively. We obtained the stable Dnd (n = 3-5) and S2n(-n, n) (n = 3-9) SWTONTs, and found that SWTONTs energetically prefer S2n symmetry. Compared with S2n(-n, n) SWTONTs, the calculated Young's moduli of Dnd(-n, n) SWTONTs are more stiff due to their relatively large strain energies. The band gaps of hexagonal TONTAs are not sensitive to their apertures, which are less than that of TiO2 bilayers. The narrow band gaps of TONTAs originate from the edge states mainly contributed by the Ti and O atoms at the core region. The calculated optical absorptions of both SWTONTs and TONTAs display anisotropic features. These results clearly reveal that the electronic and optical properties of TiO2 nanostructures are strongly associated with their symmetry, dimensions and morphology, which provide useful insights into the understanding of the related experimental observations.