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It is well known that monolayer transition-metal dichalcogenides (MX$_2$, M = Mo, W and X = S, Se, Te) could exist in three common structures, i.e. 1$T$, 1$T'$, and 1$H$ phases. In order to reveal their possible phase transitions driven by biaxial strain, we use first-principles calculations to determine the energy landscapes associated with these three phases. Due to its intrinsic dynamical instability, the centrosymmetric 1$T$ phase is known to be metastable and will transform into the 1$T'$ phase where pairs of metal atoms pull together toward each other. Moreover, controlling the metallic 1$T'$ and semiconducting 1$H$ phases is of particular interest as this can introduce novel opportunities in a series of applications. When a biaxial strain is simultaneously compressed along the zigzag direction and stretched along the armchair direction，phase transitions from 1$H$ to 1$T'$ have occurred in MSe$_2$ and MoTe$_2$, but for MSe$_2$ the biaxial strain is much difficult to realize in experiments. For WTe$_2$, the 1$T'$ structure will transform into the 1$H$ form when a biaxial strain is just compressed along the armchair direction. The transitions between 1$H$ and 1$T'$ phases could be attributed to the changes of metal-chalcogen bonds along the armchair direction by analyzing the Crystal Orbital Hamilton Population. Only half M-X bonds along the armchair direction is the main factor, because their lengths are robust in 1$T'$ phase and decrease in 1$H$ form with the tensile strain applied along the armchair direction. Our findings provide a guideline for the phase engineering of transition-metal dichalcogenides with biaxial strain.

Skyrmion bags are spin structures with arbitrary topological charges, which consist of a large skyrmion and multiple small skyrmions. In this work, by applying in-plane ac magnetic fields, we investigate the spin-wave modes of skyrmion bags, which behave differently from the clockwise (CW) and counterclockwise (CCW) rotation modes of skyrmions because of their complex spin topological structure. The in-plane excitation power spectral density shows that there are four resonance frequencies for each skyrmion bag. By further calculating the spin dynamics of a skyrmion bag at each resonant frequency, the four spin-wave modes appear as the composition modes of outer skyrmion-inner skyrmions, i.e., a CCW-CW mode, two CW-breathing modes with different resonance strength, and an inner CCW mode. Our results provide an understanding of the in-plane spin excitation of skyrmion bags, which may contribute to the characterization and detection of skyrmion bags, as well as the applications in logic devices.

Two-dimensional (2D) materials attract enormous attention and show promising applications in many fields of science and technologies (nanodevices, energy storage/harvest and catalytic processes, etc.). Pentagonal compounds emerge as a new family in 2D materials along with classic trigonal transition metal dichalcogenides and MXenes, which have been intensively investigated to date. Encouraged by the successful synthesis of pentagonal PdSe2 using CVD method, we explore nine pentagonal monolayers, MX2 (M = Ni, Pd & Pt, and X = S, Se & Te), based on the first-principles calculations. We find that all MX2 are dynamically and thermodynamically stable, and intrinsic semiconductors. Our results show that PdTe2 exhibits excellent potential application in solar water splitting due to optimal band gap and suitable band-edge positions matching with both the water reduction and oxidation potentials (0 and 1.23 V vs. NHE). We further find that the majority of MX2 monolayers (except NiTe2) are applicable in photocatalytic oxygen production. Our findings are expected to shed light on the possible synthesis of pentagonal MX2 and their application in photocatalytic water splitting.

Broadband near-infrared (NIR) phosphors are necessary materials for developing portable NIR light sources. Moreover, exploiting an NIR phosphor with a main peak located beyond a wavelength of 900 nm remains a challenge because this spectral range has great potential in biological nondestructive testing and solution testing. In this study, a range of Cr

Two-dimensional (2D) elemental metals have attracted considerable attention because of their fascinating physical properties and potential diverse applications. Information on the structures, mechanical properties, and dynamical stabilities of single atom thick phases of metallic elements in the periodic table has been previously reported. Here, the magnetic properties of 45 2D metals in three lattice structures (hexagonal, square, and honeycomb) were systematically explored using density functional theory calculations. Although it is well known that there are only five magnetic elements in their 3D counterparts, it is interesting to find that 18 of the 45 2D elemental metals are endowed with magnetism due to coordination number decreases and energy band narrowing of the out-of-plane orbitals. In particular, the honeycomb lattice is more likely to be magnetic than the square lattice. Most of the magnetic 2D metals are ferromagnetic and only a few are anti-ferromagnetic. It is further shown that the 2D metals with higher structural symmetry have smaller magnetic moments. These results provide a foundation for the further study of their magnetic/spintronic properties and their potential applications, such as in catalysis and gas sensing.

Transition metal oxychlorides (TMOCl) have attracted great attention as promising cathode materials for chloride ion batteries (CIBs). However, current research on TMOCl has been mainly focused on FeOCl and VOCl. On the other hand, the theoretical study of anionic rechargeable batteries faces the difficulty of predicting the discharge voltage of the electrode materials based on the half-cell system. Herein, a reliable theoretical voltage formula for CIBs is proposed based on a full-cell system with Li/LiCl as the reference anode. A high throughput screening method for TMOCl is applied among 16 transition metals. After the screening according to energetic and dynamic stability, Co is identified, which can form a new cathode material of CoOCl. Compared to FeOCl and VOCl, CoOCl has a higher discharge voltage, which is beneficial for achieving a larger energy density. The small crystal field splitting energy and exchange splitting energy of Co3+ result in higher electronic conductivity. In addition, the uniform Cl− binding environment leads to a low Cl− diffusion barrier of 0.37 eV, which is much smaller than that of VOCl (0.65 eV) and FeOCl (0.58 eV).

Due to the unique properties and diverse applications, two-dimensional (2D) nanomaterials have been widely investigated in these years. In this work, a group of new pentagonal 2D transitional-metal sulphide monolayers (MS, M = Fe, Mn, and V) are designed based on density-functional-theory (DFT) calculations. We show that all three monolayer structures are thermally, dynamically and mechanically stable. We find that FeS, MnS and VS are metallic, and FeS and MnS are also magnetic, in which the magnetic moments are contributed by the Fe and Mn atoms, respectively. We further find that these MS monolayers show high activities for hydrogen evolution reactions (HER) as the calculated Gibbs free energies are close to zero electron volt (eV), especially for FeS and VS. Our findings broaden the family of noble-metal-free two-dimensional materials, and also help to develop low cost commercial HER catalysts.

For zigzag nanoribbons of transition metal dichalcogenides (TMDCs), experiments show a band gap with different gap size at the Mo edges, while theoretical models often predict metallic edges. Such an inconsistency could be attributed to the inadequate understanding of the possible mechanisms of edge reconstructions. Here, we revisit the mechanisms of different edge reconstructions along the Mo edge of $\mathrm{Mo}{X}_{2}$ ($X=\mathrm{Se}$, S) zigzag nanoribbons by first-principles calculations. We demonstrate that the valency of $\frac{1}{3}$ edge Mo increases to 5+ with $X$ adatom reconstruction, or all edge Mo decreases to 3+ with a Mo trimer reconstruction. Based on these Mo valence reconstructions, thermodynamically stable reconstructions are proposed for the Mo edge in $6\ifmmode\times\else\texttimes\fi{}$ and $4\ifmmode\times\else\texttimes\fi{}$ periodicities, which are comparable with the recent experimental observation of a small energy gap (of 0.36 eV) at the Mo edge of zigzag $\mathrm{Mo}{\mathrm{Se}}_{2}$ nanoribbons. The edge Mo valency reconstruction leads to quasi-one-dimensional Peierls distortion and charge- and spin-density waves at the edge. These findings should be applicable to other TMDCs.

Multifunctionality, interference-free signal readout, and quantum effect are important considerations for flexible sensors equipped within a single unit towards further miniaturization. To address these criteria, we present the slotted carbon nanotube (CNT) junction features tunable Fano resonance driven by flexoelectricity, which could serve as an ideal multimodal sensory receptor. Based on extensive ab initio calculations, we find that the effective Fano factor can be used as a temperature-insensitive extrinsic variable for sensing the bending strain, and the Seebeck coefficient can be used as a strain-insensitive intrinsic variable for detecting temperature. Thus, this dual-parameter permits simultaneous sensing of temperature and strain without signal interference. We further demonstrate the applicability of this slotted junction to ultrasensitive chemical sensing which enables precise determination of donor-type, acceptor-type, and inert molecules. This is due to the enhancement or counterbalance between flexoelectric and chemical gating. Flexoelectric gating would preserve the electron–hole symmetry of the slotted junction whereas chemical gating would break it. As a proof-of-concept demonstration, the slotted CNT junction provides an excellent quantum platform for the development of multistimuli sensation in artificial intelligence at the molecular scale.

Ladder phenylenes (LPs) composed of alternating fused benzene and cyclobutadiene rings have been synthesized in solution with a maximum length no longer than five units. Longer polymeric LPs have not been obtained so far because of their poor stability and insolubility. Here, we report the synthesis of linear LP chains on the Au(111) surface via dehalogenative [2+2] cycloaddition, in which the steric hindrance of the methyl groups in the 1,2,4,5-tetrabromo-3,6-dimethylbenzene precursor improves the chemoselectivity as well as the orientation orderliness. By combining scanning tunneling microscopy and noncontact atomic force microscopy, we determined the atomic structure and the electronic properties of the LP chains on the metallic substrate and NaCl/Au(111). The tunneling spectroscopy measurements revealed the charged state of chains on the NaCl layer, and this finding is supported by density functional theory calculations, which predict an indirect bandgap and antiferromagnetism in the polymeric LP chains.

To solve the intractable problem of momentum-mismatch in heterojunctions, we propose a universal approach to obtain type II band alignment in two-dimensional (2D) semiconductor homojunctions with wide range momentum-space-match by a band-nesting effect. 2D homojunctions are implemented in van der Waals multilayered semiconductors through supporting them on a metal surface. There are two advantages in 2D multilayered semiconductor homojunctions (MSHs) compared to 2D heterojunctions: (1) momentum-matched band alignments are easy to achieve due to the inherent lattice-orientation-match between van der Waals layers of the same 2D material, and moreover, a wide-range momentum-space-match can be obtained by metal-induced Fermi-level movement to achieve ‘parallel’ band edges in degenerately doped MSHs; and (2) largely tunable band offsets make band alignment change from type II to type III to inversed type II due to the charge redistribution at the junction interfaces. In addition, we found that 2D MSHs are better supported on 2D metals (e.g. MXene) rather than 3D metals (e.g. Pt), with the advantages of being free of metal-induced-gap-states and easily obtained n- and p-type Schottky-barrier-free contacts. Thus, our proposed van der Waals homojunctions, with unexpected excellent properties compared to heterojunctions, stimulate studies on their various applications.

High-performance nanodevices require fast and reversible tunability of electronic and optical properties under external stimuli. In the current work, using first-principles simulations and non-equilibrium Green function transport calculations, we demonstrate that bending can effectively and asymmetrically modulate the optoelectronic properties of Janus transition-metal dichalcogenides (J-TMDCs), due to their out-of-plane flexoelectric gating. The dynamic correlation of the electronic and optical behaviors is revealed by the bending-induced interplay between the quantum confined giant Stark effect and deformation potential. The nonsymmetric directional-information encoded in the concave and convex bending motions and the intrinsic dipole of the atomically thin film renders J-TMDCs promising for wearable motion sensors and chemical sensors.

The thickness-dependent performances of metal–two-dimensional (2D) semiconductor junctions in electronics/optoelectronics have attracted increasing attention but, currently, little knowledge about the micro-mechanism of this thickness (or layer-number) dependence is available. Here, by first-principles calculations based on density functional theory, we show that the Fermi-level pinning (FLP) factor of a metal–2D multilayered semiconductor junction (MmSJ) has a sensitive dependence on the layer-number of the MmSJ for few-layer 2D semiconductors, in a proposed extension of FLP theory. Taking a MmSJ with MoS2 as a typical example, we find that strong pinning arises right at the metal–1st-layer semiconductor interface, while depinning occurs between the MoS2 layers. The depinning effect mainly contributes to the variation of the FLP factor as a function of the layer-number of the semiconductor, making p-type Schottky barrier contact more favorable in MmSJs than in metal–2D monolayer semiconductor junctions, especially for large work-function metals. Moreover, our results shed light on recent controversial experimental observations relating to MmSJs and metal–2D monolayer semiconductor junctions.

MoN2 monolayer is a newly discovered two-dimensional material with strong ferromagnetic ordering in its ground state. We found through the first-principle calculations that the ferromagnetic spin ordering within MoN2 varies dramatically due to moderate structural deformations. For example, absorption of a Fe atom on the top of the Mo atom results in structural deformation and the magnetic moment decreases significantly. Further exploration by manually moving the N atom horizontally towards the Mo atom or the hollow site reveals that the ferromagnetic spin ordering vanishes only if the displacement of the N atom from its equilibrium position is larger than 0.5 A. Based on these findings, we further propose and demonstrate a nanomechanical modulation by imposing redial deformation to sensitively recover or quench the spin polarization within a MoN2 nanotube. Our work suggests the great potential of the 2D MoN2 material in the nanoscale electronic and spintronic applications.

One of the major challenging issues in lithium-sulfur (Li-S) batteries is that the lithium polysulfides easily dissolve in the electrolyte and shuttle between anode and cathode. To overcome this problem, an ideal anchoring materials is urgent. In the current work, we explore the potential application of BC2N allotropes (approximated by Type-I and Type-II structures) as anchoring materials for Li-S batteries by using first-principles method with van der Waals interaction. Type-I is a direct-bandgap semiconductor, while type-II is semi-metallic due to inversion symmetry of its structure. They show remarkable but not too strong chemical interaction with S8 and Li2Sx clusters and enhanced electrical conductivity. In addition, the introduction of dopants and defects result in semiconductor-to-metal transition on type-I BC2N, and enhancement of binding energies and still keep intact Li2Sx clusters. Our findings demonstrate that defective and doped BC2N monolayers are promising anchoring materials for Li-S batteries.

The conventional layered-growth modes of atom/small molecule self-assembly on solid surfaces are usually obtained from the “mean field approximation” description of adsorbate–substrate interactions, which neglected the differences in kinetic parameters between different adsorption configurations. Here, through a multiscale computational method, we showed a novel mixed layered self-assembly growth of C60 molecules on an oxygen-passivated Fe(001)-p(1 × 1)O substrate through considering differences in kinetic parameters between adsorption configurations in detail. The simulated mixed growth is consistent with the experimental observation (Picone, et al. Appl. Mater. Interfaces 2016, 8, 26418) in which regions of large C60 islands that have been nucleated coexist with areas of well-separated C60 molecules. With a simple general model, the mixed layered-growth mode can be applied for other large molecules’ self-assembly, provided that the multidegrees of freedom in adsorption configurations give distinct growt...

Owning the merits of both lead-free and air-stable, the double-perovskite Cs2AgBiBr6 has attracted increasing attention, but suffers low visible-light absorption coefficient due to its large indirect bandgap. Moreover, the electronic structure of its synthesized films has not been explored clearly yet. In this work, we developed a general and promising method to fabricate continuous, uniform and highly orientated Cs2AgBiBr6 films in large scale on various substrates through capillary-assisted dip-coating method. Strikingly, those optimized films are single crystalline verified by φ-scan XRD. Its electronic structure was carefully studied independently by multi-photo-physical characterizations. Its bandgap can be tuned from 2.65 to 2.25 eV by changing the substrate temperature in growth from 40 to 160 °C. Essentially, their work-function (WF) was determined at −5.01 eV and WF-VBM is around 2 eV. This novel band structure with typical n-type characteristic, was further confirmed by DFT calculations, which reveals that the Cs interstitials and Br vacancies derived deep defect levels were fixed around its Fermi level, closer to the conduction band. This conclusion is different from its widely accepted p-type feature, but definitely deepens our understanding of this material and inspires us to find more valuable strategies of modulating its band structure and optoelectronic properties.

Two-dimensional (2D) materials with versatile properties are promising for diverse applications. In this work, a new family of 2D pentagonal NbX (penta-NbX, X = S, Se or Te) monolayers are designed to achieve the objectives. We demonstrate that these new materials are stable against mechanical strains, lattice dynamics and thermal fluctuations, because of the co-existence of ionic and covalent bonding between the Nb and X elements in these materials. We find that penta-NbX changes from metal to semiconductor as X changes from S/Se to Te. We show that penta-NbTe is a direct band-gap semiconductor with ultra-high carrier mobility (in the order of ~104 cm2 V−1 s−1), which is higher than or comparable to that of most 2D semiconductors and promising for ultra-fast electronics. We further show that metallic penta-NbS are catalytically active for hydrogen evolution reaction because of its low overpotential over a wide range of hydrogen coverages. We expect that the penta-NbX monolayers may find applications in electronics and electrocatalysis.

Developing highly efficient FeF3-based cathode materials for Na/K-ion batteries is greatly needed, which needs long cycling life and rate performance besides large voltage and capacity. Accordingly...

Due to Fermi-level pinning in metal-two-dimensional MoS

Two-dimensional transition metal dichalcogenides (TMDCs) are promising in spintronics due to their spin–orbit coupling, but their intrinsic non-magnetic properties limit their further development. Here, we focus on the energy landscapes of TMDC (MX2, M = Mo, W and X = S, Se, Te) monolayers by rhenium (Re) substitution doping under axial strains, which controllably drive 1H ↔ 1Td structural transformations. For both 1H and 1Td phases without strain, Re-doped TMDCs have an n-type character and are non-magnetic, but the tensile strain could effectively induce and modulate the magnetism. Specifically, 1H-Re0.5Mo0.5S2 gets a maximum magnetic moment of 0.69 μB at a 6% uniaxial tensile strain along the armchair direction; along the zigzag direction it exhibits a significant magnetic moment (0.49 μB) at a 2.04% uniaxial tensile strain but then exhibits no magnetism in the range of [5.10%, 7.14%]. By contrast, for 1Td-Re0.5Mo0.5S2 a critical uniaxial tensile strain along the zigzag direction reaches up to ∼9.18%, and a smaller uniaxial tensile strain (∼5.10%) along the zigzag direction is needed to induce the magnetism in 1Td-Re0.5M0.5Te2. The results reveal that the magnetism of Re-doped TMDCs could be effectively induced and modulated by the tensile strain, suggesting that strain engineering could have significant applications in doped TMDCs.

We report the resonance excitations and the spin-wave modes of a single bimeron in a confined nanodot by using micromagnetic simulations. Magnetic bimerons can be considered as in-plane topological spin textures of magnetic skyrmions, which means that the spin-wave modes of bimerons also rotate in-plane compared to skyrmions, for example, through the application of out-of-plane microwave magnetic fields, the spin-wave mode of bimerons is no longer a breathing mode but contains a counterclockwise mode at low frequencies and a clockwise mode at high frequencies. When in-plane microwave magnetic fields rotated at different angles are applied, the spin-wave mode of bimerons has an anisotropic property, i.e., the spin-wave mode presents as a breathing mode for the microwave magnetic field applied along the x-direction, and a couple of azimuthal modes for the microwave magnetic field applied along the y-direction. Moreover, we demonstrate that the breathing mode, the counterclockwise rotation mode, and the clockwise rotation mode can simultaneously appear together when the microwave magnetic field is applied at a specific angle in the plane. In addition to the three typical spin-wave modes, two high-phase counterclockwise rotation modes lead to the periodic deformation of bimerons due to the broken rotational symmetry of the spin texture. Our results reveal the rich spin-wave modes of bimerons, which may contribute to the applications in spintronics and magnonics.

It is widely accepted that quantum confinement and strain effect display opposite impacts on the band gap size of spherical or tubular transition metal dichalcogenide (TMDC) nanostructures. However, our extensive ab initio calculations and correlation of the band edge evolution of single-wall (SW) TMDC nanotubes (NTs) to their in-plane or out-of-plane orbital characters reveal that the previous interpretation of the band gap evolution behavior (which focused on strain energy) can be revised to the cooperation of deformation potential and flexoelectricity. Specifically, we scrutinize the band profile of multiwall (MW) TMDC NTs and assign the experimentally observed red/blue shift in excitonic transition energy to the decoupling effect arising from flexoelectric field rather than from the commonly expected quantum confinement effect. More importantly, we further apply these novel insights to nested Janus TMDC NTs, which offer an unprecedented platform to realize both negative electron and negative hole compressibilities without the electron correlation effect. Such compressibility gives rise to negative quantum capacitance. This in turn endows these 1D van der Waals heterostructures with emerging applications in hysteresis-free steep-slope transistors and multivalued logic devices.

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., Spinon excitations of a Kagome quantum spin liquid are expected to give rise to a magnetic continuum in Raman spectroscopy. Here, the authors report a magnetic Raman continuum in the Kagome spin liquid candidate Cu3Zn(OH)6FBr, in contrast to a sharp magnon Raman peak in the Kagome antiferromagnet EuCu3(OH)6Cl3.

Layer-number-dependent performance of metal-semiconductor junctions (MSJs) with multilayered two-dimensional (2D) semiconductors has attracted increasing attention for their potential in ultrathin electronics and optoelectronics. However, the mechanism of the interaction and the resulting charge transfer/redistribution at the two kinds of interfaces in MSJ with multilayered 2D semiconductors, namely, the metal-semiconductor (M-S) and the semiconductor-semiconductor (S-S) interfaces, have not been well understood until now, although that is important for the overall Schottky barrier height and the energy-band-offset between different layers of the 2D semiconductors. Here, based on state-of-the-art density functional theory calculations, the mechanisms of bonding and asymmetric electron redistribution at the M-S and S-S interfaces of metal-bilayer MoS

Searching photocatalysts for efficient hydrogen production has been a challenging issue for solar-energy harvesting. Using co-catalyst is proven to be an effective approach to improve the efficienc...

As for FeF2.2(OH)0.8, introducing appropriate vacancy defects has been recently discovered to be a new experimental method for the improvement of the lithium storage performance. However, owing to the limitation of experimental methods, for Li/Na rechargeable batteries, the vacancy formation mechanism of FeF2.2(OH)0.8 and its regulation mechanism on the electrochemical properties associated with rate-performance and cycling stability are still poorly understood. Therefore, we carried out first principles calculations to investigate the defect chemistry in FeF2.2(OH)0.8. Eleven representative vacancy defects, such as neutral iron vacancy (V0Fe), charged iron vacancy (VFe2− and VFe3−), neutral oxygen vacancy (V0O), charged oxygen vacancy (VO+ and VO2+), neutral hydrogen vacancy (V0H), charged hydrogen vacancy (VH−), neutral hydroxide vacancy (V0OH), charged fluorine vacancy (VF+) and neutral fluorine vacancy (V0F) are included. The vacancy formation energies clearly reveal that FeF2.2(OH)0.8 with neutral hydroxide vacancy (V0OH) (FeF2.2(OH)0.64O0.08□0.08) is most likely to form under hydrogen-poor (H-poor) conditions. With the introduction of V0OH, the band gap of FeF2.2(OH)0.8 reduces from 1.47 eV to 0.99 eV, which contributes to the improvement of electronic conductivity. Moreover, by analysis of the defect structure and Li/Na diffusion process, it was proposed that the ion diffusion channel of FeF2.2(OH)0.8 is broadened. Besides, the balance between Li/Na ions and surrounding anions also occurs at the saddle point, which induces higher ionic conductivity to appear in FeF2.2(OH)0.64O0.08□0.08 relative to FeF2.2(OH)0.8 (5.16 × 10−4 S cm−1vs. 1 × 10−6 S cm−1 for Li; 6.88 × 10−2 S cm−1vs. 2.03 × 10−2 S cm−1 for Na). Accordingly, FeF2.2(OH)0.64O0.08□0.08 possesses higher rate performance and more stable discharge voltage than FeF2.2(OH)0.8. As a whole, our theoretical results successfully clarify the origin of the favorable electrochemical properties of FeF2.2(OH)0.64O0.08□0.08 during the Li intercalation/deintercalation process in the experiment. Furthermore, they also clearly simulate the process of Na diffusion kinetics and electrochemistry in FeF2.2(OH)0.8 and FeF2.2(OH)0.64O0.08□0.08. Furthermore, it is verified that introducing V0OH is an effective strategy to design FeF2.2(OH)0.8-based cathode materials for ultra-high rate and long-life Li/Na-ion batteries.

Herein, we implement first-principles calculations to design Li7P3S11-xOx at an atomic scale, aiming to obtain stable Li7P3S11-xOx-type solid electrolyte materials with good Li+ conductivity. After searching for chemical potentials, Li2O2 is expected to be the potential raw material, and it can afford the most favorable growth environment for the synthesis of Li7P3S11-xOx (x = 0.25, 0.50, 0.75 and 1). Among these compounds, it is found that Li7P3S10.25O0.75 exhibits the most desirable Li+ conductivity of 109 mS cm-1 at 300 K, which is far higher than that of Li7P3S11 (50 mS cm-1 at 300 K). By structural analysis, it is demonstrated that the Li diffusion pathway in Li7P3S10.25O0.75 is significantly broadened relative to that in Li7P3S11 (71.38 A3vs. 69.48 A3), which breaks the bottleneck during Li diffusion. Moreover, the resistance of Li ion diffusion in Li7P3S10.25O0.75 decreases due to the balance of interactions between Li and its neighbouring atoms at the transition state, which induces a much lesser energy barrier of Li7P3S10.25O0.75 than that of Li7P3S11 (0.20 eV vs. 0.31 eV). Moreover, introducing Li vacancies is unlikely to alter the essence of the inherent superionic conductivity of Li7P3S10.25O0.75. Furthermore, Li7P3S10.25O0.75 can maintain good thermal stability and similar electrochemical stability to Li7P3S11. This study successfully clarifies the role of oxygen in enhancing the Li+ conductivity of Li7P3S11-xOx. Moreover, it affords a new strategy to design other solid-state electrolytes with good Li+ conductivity.

The strong interaction between transition metal (TM) atoms and semiconductor surface atoms may diminish the magnetic moments of the TM atoms and prevent them from being used as single atom spin-based devices. A carbon cage that can encapsulate TM atoms and isolate them from interacting with surface atoms is considered to protect the magnetic moments of the TM atoms. We have studied the magnetic moments of Fe, Co, and Ni atoms adsorbed inside the corner hole of Si(111)-(7 × 7) by using first-principles calculations based on the density functional theory. The results show that when Co and Ni atoms are directly adsorbed inside the corner hole, the magnetic moments are 1.353μB and 0, respectively. However when a C60 cage is used to encapsulate the atoms, the magnetic moments increase to 1.849μB and 0.884μB, respectively. The results show a clear protecting effect of a carbon cage. For Fe with and without C60, the magnetic moments are 2.909μB and 2.825μB, respectively. The presence of a C60 cage can also maintain their magnetic moments. Further analysis shows that the TM atoms possess magnetic moments when the conduction electrons are localized around them. All the results can be well understood in the framework of the Anderson impurity model. Our results demonstrate that a carbon cage may effectively protect the magnetic moments of TM atoms. This provides a new strategy for developing single atom spin-based devices on semiconductors.

Investigations of van der Waals (vdW) heterostructures based on distinct low-dimensional materials have attracted significant attention for higher performance devices. Here we use density functional theory computations to scrutinize the band alignment in one-dimensional (1D) vdW heterostructures. In particular, using nanoribbons (NRs) encapsulated inside nanotubes (NTs) based on ten binary-compounds of group IV–IV and group III–V elements, we identified both momentum-matched and -mismatched type II heterostructures with gaps varying from 0.56 eV to 4.37 eV. In addition, we demonstrate a substantial reduction (up to near 0.95 eV) in the staggered band gap of BN compounds by both transverse electric field and longitudinal tensile strain. These findings are favorable for enhancing light harvesting through a wide spectrum and reducing the carrier recombination; our designed heterostructures are expected to offer opportunities for photocatalytic water splitting with safe storage of H2 products inside the NTs.

The authors combined the Heyd-Scuseria-Ernzerhof hybrid density functional first-principles calculation and the cluster exact diagonalization to study the strongly correlated electronic structures of the nickelate oxides Nd1\ensuremath{-}xSrxNiO2 and derive the effective one-band Hamiltonian model for the superconductivity.

Magnetic skyrmions, whose shapes are ellipse due to the presence of anisotropic Dzyaloshinskii–Moriya interaction (DMI), have already been discovered in experiments recently. By using micromagnetic simulations, we discuss the ground state and the spin-wave modes of a single elliptical skyrmion in a confined nanodot. It is found that the shapes of skyrmion are stretched into a horizontal ellipse, vertical ellipse, or stripe shape under different strengths of anisotropic DMI. When elliptical skyrmions are excited by in-plane ac magnetic fields, the spin-wave mode contains a counterclockwise rotation mode at high frequencies and a clockwise (CW) rotation mode at low frequencies, and the CW mode depends on the strength of anisotropic DMI. When elliptical skyrmions are excited by out-of-plane ac magnetic fields, the spin-wave mode is split from a simple breathing mode into two complex breathing modes, including a mixed mode of CW rotation and breathing, and another anisotropic breathing mode. Our results provide an understanding of the rich spin-wave modes for skyrmions, which may contribute to the applications in magnonics.

Highly efficient, earth-abundant, and low-cost photocatalysts are widely pursued for solar-driven hydrogen generation from water. Herein, we first report vanadium disulfide (VS2) with high hydrogen evolution reaction (HER) activity both in basal and edges to be the co-catalyst of graphitic carbon nitride (g-C3N4) for ultrahigh solar-driven hydrogen production. VS2-decorated g-C3N4 shows an impressing photocatalytic hydrogen evolution with a rate of 87.4 μmol/h, 26 times higher than pristine g-C3N4. Our combined experimental and computational studies reveal that the excellent efficiency of the composite is attributed to: (1) effective electron-hole separation and electron transfer from g-C3N4 to VS2, resulting from the optimal band alignment between VS2 and g-C3N4 and metallic characteristic of VS2; (2) fast hydrogen generation on the surface due to the high surface area and excellent HER activity of VS2. Our findings demonstrate that VS2/g-C3N4 may be applicable in solar-driven water splitting, and the design principle can be applied to search for novel photocatalysts.

Searching for earth-abundant and efficient electrocatalysts for large-scale hydrogen generation from the electrolysis of water is of paramount importance. Incorporating nonmetal or metal elements into host catalysts is a possible way to tune the catalytic capability of an electrocatalyst on the hydrogen evolution reaction (HER). Here, a simple and facile way was presented to synthesize N and V coincorporated Ni nanosheets on self-supported conductive carbon paper (NV–Ni/CP) as noble-metal-free catalysts for the HER. Compared with Ni/CP, the NV–Ni/CP shows better HER property with low overpotential of 95 mV (10 mA cm–2), small Tafel slope (140 mV dec–1), and superior long-term stability. We further show that the outstanding HER performance of NV–Ni/CP is ascribed to the increased active sites and enhanced conductivity. It is expected that the coincorporation of nonmetal and metal elements will provide more chances to enhance the HER catalytic ability and extend the scope of cost-effective electrocatalysts.

Graphene has been widely studied as an electrocatalyst for hydrogen evolution reaction (HER). However, pure flat graphene is catalytically inert in HER. In this work, we investigate the effect of curvature on the improvement in the catalytic activity of pure and doped graphenes. We find that the HER performance can be dramatically improved on waved-graphene due to localized chemical potential and Pt-analogous activity can be achieved at suitable compression. (1) For pure graphene, the calculated HER performance increases more than 50% as tuned by curvature due to reduced calculated Gibbs free energies. (2) For B- and N-doped graphene, their optimal HER catalytic ability occurs at low curvature conditions. (3) For metal-doped graphene, Mo-doped graphene exhibits excellent catalytic ability in HER at certain compressions. (4) For nitrogen-metal co-doped graphene, N–Ni and N–V co-doped graphenes can be tuned by curvature to show outstanding performance in HER with their exothermal formation energies. Our cal...

The fast development of high-performance devices for diverse applications requires nanoscale materials with multifunctional properties, motivating theoretical exploration into novel two-dimensional (2D) materials. In this work, we propose a new family of 2D nanomaterials, Janus transition metal oxides and chalcogenides MXY (M = Ti, Zr, or Hf; X = S or Se; Y = O or S; X ≠ Y) monolayers, for their versatile applications. We find that the Janus MXY monolayers are semiconductors with a wide range of band gaps ranging from 0.739 to 2.884 eV. We show that TiSO, ZrSO, and HfSO monolayers are promising candidates for photocatalysis because of their suitable band gaps and optimal redox potentials for water splitting, and ZrSeS and HfSeS monolayers are suitable candidates for nanoscale electronics because of their high carrier mobility. We further show that TiSO, ZrSO, and ZrSeO monolayers possess large piezoelectric properties because of the broken inversion symmetry stemmed from the different atomic sizes and electronegativities of the X and Y elements, which are better or comparable to other 2D and bulk piezoelectric materials. Our study demonstrates that the 2D Janus MXYs may find versatile applications into photocatalysts, electronics, sensors, and energy harvesting/conversion.

Spin-tunable materials are very appealing for applications in multifunctional spintronics devices because of their ability to switch between different spin states with external stimuli such as strain, light or electric-field, etc. As a promising material for spintronics, transition-metal-nitrogen-carbon (TM-N-C) materials have achieved considerable attentions. Based on density functional calculations, we systematically investigate the magnetic properties of four-nitrogen coordinated transition-metal embedded graphene (TMN4-G, TM = all 3d-elements). The embedded configurations of TMN4-G show rich tunable spin states by external strains, exhibiting two kinds of spin-state transitions: 1) Spin crossover between low- to high-spin states (TMN4-G, TM = Mn, Fe and Co); 2) Spin polarization turned up or turned off in nonmagnetic systems (ScN4-G and NiN4-G) or in magnetic CuN4-G, respectively. The spin-state transitions occur abruptly for systems with magnetic TM elements, while continuously for those with nonmagnetic elements. The origin of the spin-state transitions is explored and found related with the TM valance electrons and charge transferred. Our findings could open an avenue for the efficient tuning of spin and magnetism in graphene based TMN4 structures for application in spintronics.

Organic–inorganic halide perovskites have attracted increasing interest for solar-energy harvesting because of the simple fabrication process, high efficiency, and low cost. In this work, we systematically investigate the structural and electronic properties, and stability of two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs) based on density-functional-theory calculations. We explore a general rule to predict the bandgap of the 2D HOIP: its bandgap decreases as the thickness increases and the size of metal atom decreases as well as that of the halide atom increases. We find that effective mass of hole increases as the thickness of 2D HOIP increases. Importantly, the 2D HOIPs exhibit high stability on the resistance of water and oxygen than bulk HOIPs due to high positive adsorption energy. Our results confirm that the 2D HOIPs may be excellent alternatives to the unstable bulk HOIPs in solar energy harvesting with improved performance due to suitable bandgap, small carrier effective mass, ...

Iron fluoride with high operating voltage and theoretical energy density has been proposed as a high-performance cathode material for Li-ion batteries. However, the inertness of pristine bulk FeF3 results in poor Li kinetics and cycling life. Developing nanosheet-based electrode materials is a feasible strategy to solve these problems. Herein, on the basis of first-principles calculations, first the stability of FeF3 (012) nanosheet with different atomic terminations under different environmental conditions was systematically studied, then the Li-ion adsorption and diffusion kinetics were thoroughly probed, and finally the voltages for different Li concentrations were given. We found that F-terminated nanosheet is energetically favorable in a wide range of chemical potential, which provide a vehicle for lithium ion diffusion. Our Li-ion adsorption and diffusion kinetics study revealed that (1) the formation of Li dimer is the most preferred, (2) the Li diffusion energy barrier of Li dimer is lower than is...

Two dimensional (2D) materials with hexagonal building blocks have received tremendous interest in recent years and show promise as nanoscale devices for versatile applications. Herein, we propose a new family of 2D pentagonal CrX (X = S, Se or Te) monolayers (penta-CrX) for applications in electronics, spintronics and photocatalysis. We find that the 2D penta-CrX monolayers are thermally, structurally and mechanically stable. The penta-CrX monolayers are antiferromagnetic and semiconducting. We show that the magnetism is attributed to the super-exchange induced by the ionic interactions between the Cr and X atoms and can be enhanced upon applying tension. We further show that the penta-CrS and penta-CrSe monolayers show good redox potentials versus a normal hydrogen electrode, and their band gaps are comparable to the energy of a photon in the visible light region, indicating their capability of maximal utilization of solar energy for water splitting. With intrinsic semiconducting and controllable magnetic properties, the proposed penta-CrX monolayers may hold promise as flexible spintronics and photocatalysts.

The development of two-dimensional (2D) materials with high conductivity and catalytic activity is important for the proposed hydrogen economy. Herein, we design a new family of 2D monolayers, pentagonal transition-metal borides/carbides (penta-TMBs and penta-TMCs), as electrocatalysts for the hydrogen evolution reaction (HER) on the basis of density functional theory (DFT). We find that all of the stable 2D penta-TMBs/TMCs are metallic, and 2D WB and HfC are ferromagnetic metals. We demonstrate that penta-TMBs and penta-TMCs show high catalytic performance for the HER. The Gibbs free energy for the adsorption of hydrogen atoms on catalyst surfaces (such as WB and ZrC) is close to the ideal value of zero electron volt (eV). Our findings highlight a new family of promising noble metal-free HER catalysts and provide new insight into the design of advanced 2D materials.

Active catalysts for nitrogen fixation (N2-fixation) have been widely pursued through constant efforts for industrial applications. Here, we report a family of catalysts, MXenes (M2X: M = Mo, Ta, Ti, and W; X = C and N), for application in N2-fixation based on density functional theory calculations. We find that the catalytic performance of MXenes strongly depends on the reaction energy in each reaction step. More exothermic steps lead to higher catalytic performance in the course of N2-fixation. We show that the reaction energy in N2-fixation is strongly affected by the charge transfer: (1) if N atoms gain more electrons in a step, the reaction is exothermic with a larger reaction energy; (2) if N atoms lose electrons in a step, the reaction is endothermic in general. We further show that Mo2C and W2C are highly active for N2-fixation due to their exothermic reactions and strong charge transfer, which may be applicable in the chemical-engineering industry.

Two dimensional (2D) nanomaterials have received increasing interest because of their unique properties for versatile applications. In this work, we present a first-principles study on a new family of 2D nanostructures, Janus transition metal chalcogenide MSX (M = Ti or V; and X = C, N, Si, or P) monolayers, for their multifunctional applications. In this work, we show that the Janus MSXs possess diverse electronic and magnetic properties, and can be semiconducting or metallic, and nonmagnetic or magnetic, depending on their composition. We find that a TiSC monolayer with a 1H phase (TiSC-1H) is suitable as a cathode for Li ion batteries and anode materials for Na and Mg ion batteries due to its high open circuit voltage (OCV) (2.121 eV) for Li, and low OCVs upon Na (0.676 eV) and Mg (1.044 eV) intercalation, respectively. Importantly, TiSC-1H shows fast charge/discharge rates, good cycling stability, and high storage density as electrode materials for rechargeable batteries because of low ion diffusion barriers, small volume expansion and high specific capacity. We further show that TiSP-1H has the best performance in the hydrogen evolution reaction due to both its catalytic activities on the surfaces and relatively low overpotentials upon hydrogenation. Our study demonstrates that the 2D Janus MSXs may find multifunctional applications in nanodevices, spintronics, catalysis, and electrochemical energy storage.

Two-dimensional (2D) nanomaterials with tunable electronic and magnetic properties are promising for their versatile applications in multifunctional devices. In this work, we present a first-principles study on the electronic and magnetic transitions of 2D Ti2NO2 monolayer achieved by applying tension. We show that 2D Ti2NO2 experiences a transition from ferromagnetic half-metal to antiferromagnetic semiconductor as tension increases up to a certain level. We find that charge redistribution occurs with the extension of bond length due to the suppressed crystal field splitting. We further show that the charge transfer between eg and t2g orbitals gives rise to the shift of the energy levels and opens a gap near the Fermi level, resulting in the electronic transition. The semiconducting nature favors the superexchange coupling, leading to the magnetic transition from the ferromagnetic state to the antiferromagnetic state. We expect that the 2D Ti2NO2 monolayer with tunable electronic and magnetic properties ...