Dirac State in the FeB2 Monolayer with Graphene-Like Boron Sheet

First-Principles Calculations of TiB4 and TiB5 as Anodes with High Capacity for Na-Ion Batteries

The electrochemical properties of TiB4 and TiB5 monolayers in Na-ion batteries (NIBs) were studied by using the first-principles calculation method based on density functional theory. The TiB4/TiB5 monolayer showed excellent Na storage capacity, capable of adsorbing two layers of Na with theoretical capacities of 1176.77 and 1052.05 mA g-1, respectively. The average operating voltages of the TiB4 and TiB5 monolayers are 0.073 and 0.042 eV, respectively, indicating that they can be used as anode materials for NIBs. More interestingly, the exposed B surface not only brings a high theoretical capacity but also provides a relatively small diffusion barrier of 0.16 (for TiB4) and 0.33 eV (for TiB5), enhancing their rate capability in NIBs.

Unveiling the Roles of Alloyed Boron in Hexavalent Chromium Removal Using Borohydride-Synthesized Nanoscale Zerovalent Iron: Electron Donor and Antipassivator

Nanoscale zerovalent iron synthesized using borohydride (B-NZVI) has been widely applied in environmental remediation in recent decades. However, the contribution of boron in enhancing the inherent reactivity of B-NZVI and its effectiveness in removing hexavalent chromium [Cr(VI)] have not been well recognized and quantified. To the best of our knowledge, herein, a core-shell structure of B-NZVI featuring an Fe-B alloy shell beneath the iron oxide shell is demonstrated for the first time. Alloyed boron can reduce H+, contributing to more than 35.6% of H2 generation during acid digestion of B-NZVIs. In addition, alloyed B provides electrons for Fe3+ reduction during Cr(VI) removal, preventing in situ passivation of the reactive particle surface. Meanwhile, the amorphous oxide shell of B-NZVI exhibits an increased defect density, promoting the release of Fe2+ outside the shell to reduce Cr(VI), forming layer-structured precipitates and intense Fe-O bonds. Consequently, the surface-area-normalized capacity and surface reaction rate of B-NZVI are 6.5 and 6.9 times higher than those of crystalline NZVI, respectively. This study reveals the importance of alloyed B in Cr(VI) removal using B-NZVI and presents a comprehensive approach for investigating electron pathways and mechanisms involved in B-NZVIs for contaminant removal.

Ultralow lattice thermal conductivity in type-I Dirac MBene TiB2

MBenes, the emergent novel two-dimensional family of transition metal borides have recently attracted remarkable attention. Transport studies of such two-dimensional structures are very rare and are of sparking interest. In this paper Using Boltzmann transport theory with ab-initio inputs from density functional theory, we examined the transport in TiB2MBene system, which is highly dependent on number of layers. We have shown that the addition of an extra layer (as in bilayer BL) destroys the formation of type-I Dirac state by introducing the positional change and tilt to the Dirac cones, thereby imparting the type-II Weyl metallic character in contrast to Dirac-semimetallic character in monolayer ML. Such non-trivial electronic ordering significantly impacts the transport behavior. We further show that the anisotropic room temperature lattice thermal conductivityκLfor ML (BL) is observed to be 0.41 (0.52) and 2.00 (2.04) W m-1 K-1forxandydirections, respectively, while the high temperatureκL(ML 0.13 W m-1 K-1and BL 0.21 W m-1 K-1at 900 K inxdirection) achieves ultralow values. Our analysis reveals that such values are attributed to enhanced anharmonic phonon scattering, enhanced weighted phase space and co-existence of electronic and phononic Dirac states. We have further calculated the electronic transport coefficients for TiB2MBene, where the layer dependent competing behavior is observed at lower temperatures. Our results further unravels the layer dependent thermoelectric performance, where ML is shown to have promising room-temperature thermoelectric figure of merit (ZT) as 1.71 compared to 0.38 for BL.

Janus Cr2BN Monolayer with Ferroelectricity and Room-Temperature Ferromagnetism

Janus monolayers, a special kind of two-dimensional materials, offer an exciting platform for the development of novel electronic/spintronic devices because of their out-of-plane asymmetry. Herein, we propose a sandwich liked Janus tetragonal Cr2BN monolayer with ferroelectricity and ferromagnetism through first-principles calculations. The predicted magnetic moment is up to ~3.0 μB/Cr originating from the distorted square crystal field induced by out-of-plane asymmetry. The Cr2BN monolayer possesses an intrinsic ferromagnetism with a high Curie temperature of 383 K and a sizeable magnetic anisotropy energy of 171 μeV/Cr. Its robust ferromagnetism, dominating by the multi-anion mediated super-exchange interactions, can even resist -5 % ~5 % biaxial strain. Its large cohesive energy and high dynamical/thermal stability provide a strong feasibility for experimental synthesis. These intriguing properties render the Cr2BN monolayer a promising material for nanoscale spintronic devices.

Two-Dimensional Transition Metal Boride TMB12 (TM = V, Cr, Mn, and Fe) Monolayers: Robust Antiferromagnetic Semiconductors with Large Magnetic Anisotropy

Currently, two-dimensional (2D) materials with intrinsic antiferromagnetism have stimulated research interest due to their insensitivity to external magnetic fields and absence of stray fields. Here, we predict a family of stable transition metal (TM) borides, TMB12 (TM = V, Cr, Mn, Fe) monolayers, by combining TM atoms and B12 icosahedra based on first-principles calculations. Our results show that the four TMB12 monolayers have stable antiferromagnetic (AFM) ground states with large magnetic anisotropic energy. Among them, three TMB12 (TM=V, Cr, Mn) monolayers display an in-plane easy magnetization axis, while the FeB12 monolayer has an out-of-plane easy magnetization axis. Among them, the CrB12 and the FeB12 monolayers are AFM semiconductors with band gaps of 0.13 eV and 0.35 eV, respectively. In particular, the AFM FeB12 monolayer is a spin-polarized AFM material with a Néel temperature of 125 K. Moreover, the electronic and magnetic properties of the CrB12 and the FeB12 monolayers can be modulated by imposing external biaxial strains. Our findings show that the TMB12 monolayers are candidates for designing 2D AFM materials, with potential applications in electronic devices.

Two-dimensional boron nanosheets for selective enrichment and detection of cis-diol compounds by surface-assisted laser desorption/ionization time-of-flight mass spectrometry

Surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) is an effective method for detecting of low-mass molecules. In this study, two-dimensional boron nanosheets (2DBs) were fabricated through thermal oxidation etching and coupling liquid exfoliation technologies, and applied as a matrix and selective sorbent for detecting cis-diol compounds by SALDI-TOF MS. The outstanding nanostructure and boric acid active sites of 2DBs endow them with sensitivity for cis-diol compound detection, excellent selectivity, and low background interference for complex samples. The specific in-situ enrichment faculty of the 2DBs as a matrix was investigated by SALDI-TOF MS using glucose, arabinose, and lactose as model analytes. In the presence of 100 -fold more interfering substances, the 2DBs showed high selectivity against cis-diol compounds, and exhibited a better sensitivity and a reduced limit of detection through enrichment treatment than graphene oxide matrices. The linearity, limit of detection (LOD), reproducibility, and accuracy of the method were evaluated under optimized conditions. The results showed that the linear relationships of six saccharides remained in the range of 0.05-0.6 mM with a correlation coefficient r ≥0.98. The LODs of six saccharides were 1 nM (glucose, lactose, mannose, fructose) and 10 nM (galactose, arabinose). Sample-to-sample (n = 6) with relative standard deviations (RSDs) of 3.2% to 8.1% were observed. Recoveries (n = 5) of 87.9-104.6% were obtained at three spiked levels in the milk samples. The proposed strategy promoted the development of a matrix for use with SALDI-TOF MS detection, in which the UV absorption properties and enrichment capabilities of 2DBs were combined.

Novel Piezoelectricity in Two-Dimensional Metallic/Semimetallic Materials with Out-of-Plane Polarization

Normally, the good conductivity of metals and semimetals is incompatible with the piezoelectricity since the internal electric current will dismiss any polarization. However, here, we reveal that the out-of-plane piezoelectric effect can exist in two-dimensional (2D) metallic/semimetallic materials due to their giant anisotropy. A method is developed to calculate the out-of-plane polarization in 2D systems, where the modern theory of polarization based on a Berry-phase approach is not applicable. Detailed calculation and analysis on a Dirac material, the FeB2 monolayer, show that it has an out-of-plane polarization of 8.3 pC/m and the piezoelectric coefficient of e31 = -59.3 pC/m and d31 = -0.25 pm/V. This work provides a formalism to discover more piezoelectric materials within the vast 2D metallic/semimetallic materials.

Dirac cones in bipartite square-octagon lattice: A theoretical approach

Dirac cones are difficult to achieve in a square lattice with full symmetry. Here, we have theoretically investigated a bipartite tetragonal lattice composed of tetragons and octagons using both Tight-Binding (TB) model and density functional theory (DFT) calculations. The TB model predicts that the system exhibits nodal line semi-metallic properties when the on-site energies of all atoms are identical. When the on-site energies differ, the formation of an elliptical Dirac cone is predicted. Its physical properties (anisotropy, tilting, merging, and emerging) can be regulated by the hopping energies. An exact analytical formula is derived to determine the position of the Dirac point by the TB parameters, and a criterion for the existence of Dirac cones is obtained. The "divide-and-coupling" method is applied to understand the origin of the Dirac cone, which involves dividing the bands into several groups and examining the couplings among inter-groups and intra-groups. Various practical systems computed by DFT methods, e.g., t-BN, t-Si, 4,12,2-graphyne, and t-SiC, are also examined, and they all possess nodal lines or Dirac cones as predicted by the TB model. The results provide theoretical foundation for designing novel Dirac materials with tetragonal symmetry.

Two-dimensional Be2Al and Be2Ga monolayer: anti-van't Hoff/Le Bel planar hexacoordinate bonding and superconductivity

Because of the electron deficiency of boron, a triangular network with planar hexacoordination is the most common structural and bonding property for isolated boron clusters and two-dimensional (2D) boron sheets. However, this network is a rule-breaking structure and bonding case for all other main-group elements. Herein, the Be₂M (M = Al and Ga) 2D monolayer with P6/mmm space group was found to be the lowest-energy structure with planar hexacoordinate Be/Al/Ga motifs. More interestingly, Be₂Al and Be₂Ga were observed to be intrinsic phonon-mediated superconductors with a superconducting critical temperature (T_c) of 5.9 and 3.6 K, respectively, where compressive strain could further enhance their T_c. The high thermochemical and kinetic stability of Be₂M make a promising candidate for experimental realization, considering its high cohesive energy, absence of soft phonon modes, and good resistance to high temperature. Moreover, the feasibility of directly growing Be₂M on the electride Ca₂N substrate was further demonstrated, where its intriguing electronic and superconducting properties were well maintained in comparison with the freestanding monolayer. The Be₂M monolayer with rule-breaking planar hexacoordinate motifs firmly pushes the ultimate connection of the "anti-van't Hoff/Le Bel" structure with promising physical properties.

2D Ladder Polyborane: An Ideal Dirac Semimetal with a Multi-Field-Tunable Band Gap

Hydrogen, a simple and magic element, has attracted increasing attention for its effective incorporation within solids and powerful manipulation of electronic states. Here, we show that hydrogenation tackles common problems in two-dimensional borophene, e.g., stability and applicability. As a prominent example, a ladder-like boron hydride sheet, named as 2D ladder polyborane, achieves the desired outcome, enjoying the cleanest scenario with an anisotropic and tilted Dirac cone, that can be fully depicted by a minimal two-band tight-binding model. Introducing external fields, such as an electric field or a circularly polarized light field, can effectively induce distinctive massive Dirac fermions, whereupon four types of multi-field-driven topological domain walls hosting tunable chirality and valley indexes are further established. Moreover, the 2D ladder polyborane is thermodynamically stable at room temperature and supports highly switchable Dirac fermions, providing an ideal platform for realizing and exploring the various multi-field-tunable electronic states.

Particle Swarm Predictions of a SrB8 Monolayer with 12-Fold Metal Coordination

Materials containing planar hypercoordinate motifs greatly enriched the fundamental understanding of chemical bonding. Herein, by means of first-principles calculations combined with global minimum search, we discovered the two-dimensional (2D) SrB₈ monolayer, which has the highest planar coordination number (12) reported so far in extended periodic materials. In the SrB₈ monolayer, bridged B₈ units are forming the boron monolayer consisting of B₁₂ rings, and the Sr atoms are embedded at the center of these B₁₂ rings, leading to the Sr@B₁₂ motifs. The SrB₈ monolayer has good thermodynamic, kinetic, and thermal stabilities, which is attributed to the geometry fit between the size of the Sr atom and cavity of the B₁₂ rings, as well as the electron transfer from Sr atoms to electron-deficient boron network. Placing the SrB₈ monolayer on the Ag(001) surface shows good commensurability of the lattices and small vertical structure undulations, suggesting the feasibility of its experimental realization by epitaxial growth. Potential applications of the SrB₈ monolayer on metal ions storage (for Li, Na, and K) are explored.

Dirac Fermions in the Boron Nitride Monolayer with a Tetragon

Two-dimensional (2D) boron nitride (BN) is a promising candidate for aerospace materials due to its excellent mechanical and thermal stability properties. However, its unusually prominent band gap limits its application prospects. In this work, we report a gapless monolayer BN, t-BN, which has four anisotropic Dirac cones in the first Brillouin zone exactly at the Fermi level. To further confirm the semimetallic character, the nontrivial topological properties are proven through the topologically protected edge states and the invariant non-zero Z₂. Additionally, the Young's modulus and Poisson ratio characterize the strong mechanical strength of t-BN. Our theoretical predictions provide more possibilities for exploring the Dirac cone in BN, which will enhance the 2D boron derivative materials.

Prediction of the Be2B2 monolayer: an ultrahigh capacity anode material for Li-ion and Na-ion batteries

In the recent past, beryllium-boron compounds have been the focus of attention as a consequence of their extremely rich structures and potential applications. Nevertheless, their complex phase structures greatly hinder further exploration of the physical and chemical properties. To this end, we propose a novel beryllium-boron compound based on a two-dimensional (2D) structure, namely a Be₂B₂ monolayer. Using first-principles methods, we first prove that the structure of the 2D Be₂B₂ monolayer is energetically and thermodynamically stable. Subsequently, we systematically explored its potential as a high performance anode material for LIBs and NIBs. The results reveal that the Be₂B₂ monolayer exhibits an excellent, comprehensive performance with a strong adsorption energy, low diffusion barrier, suitable average open circuit voltage and outstanding electronic conductivity. More importantly, the extremely high theoretical specific capacity (1352 mA h g^-1 for both LIBs and NIBs) well meets our expectations, which is quite ideal for an anode material. Meanwhile, we hope and believe that this work can provide some new insights into the study of beryllium-boron compounds.

Construction of the Largest Metal-Centered Double-Ring Tubular Boron Clusters Based on Actinide Metal Doping

Metal doping has been considered to be an effective approach to stabilize various boron clusters. In this work, we constructed a series of largest metal-centered double-ring tubular boron clusters An@B₂₄ (An = Th, Pa, Pu, and Am). Extensive global minimum structural searches combined with density functional theory predicted that the global minima of An@B₂₄ (An = Th, Pu, and Am) are double-ring tubular structures. Formation energy analysis indicates that these boron clusters are highly stable, especially for Th@B₂₄ and Pa@B₂₄. Detailed bonding analysis shows that the significant stability of An@B₂₄ is determined by the covalent character of the An-B bonding, which stems from the interactions of An 5f and 6d orbitals and B 2p orbitals. These results show that actinide metal doping is a feasible route to construct stable large metal-centered double-ring tubular boron clusters, offering the possibility to design boron nanomaterials with special physiochemical properties.

Tuning electronic, magnetic and catalytic behaviors of biphenylene network by atomic doping

Recently, a new two-dimensional allotrope of carbon named biphenylene has been experimentally synthesized. First-principles calculations are preformed to investigate the electronic properties of biphenylene and the doping effect is also considered to tune its electronic, magnetic, and catalytic properties. The metallic nature with an n-type Dirac cone is observed in the biphenylene. The magnetism can be induced by Fe, Cl, Cr, and Mn doping. More importantly, the doping position dependence of hydrogen evolution reaction (HER) performance of biphenylene is addressed, which can be significantly improved by atomic doping. In particular, the barrier for HER of Fe doping case is only -0.03 eV, denoting its great potential in HER catalysis.

Anisotropic nodal loop in NiB2 monolayer with nonsymmorphic configuration

Two-dimensional (2D) materials featuring a nodal-loop (NL) state have been drawing considerable attention in condensed matter physics and materials science. Owing to their structural polymorphism, recent high-profile metal-boride films have great advantages and the potential to realize a NL. Herein, a 2D NiB₂ monolayer with an anisotropic NL nature is proposed and investigated using first-principles calculations. We show that the NiB₂ monolayer has excellent thermal dynamics stability, suggesting the possibility of its synthesis in experiments. Remarkably, the NL with a considerable Fermi velocity is demonstrated to be protected by nonsymmorphic glide mirror symmetry, instead of the widely known horizontal mirror symmetry. Accompanied by the proper preservation of the NL, strain engineering can not only regulate the anisotropy of the NL but also give rise to a self-doping phenomenon characterized by effective modulation of the carrier type and concentration. Moreover, this NL state is robust against the correlation effect. These findings pave the way for exploring nonsymmorphic-symmetry-enabled NL nature in 2D metal-borides.

FeSi2: a two-dimensional ferromagnet containing planar hexacoordinate Fe atoms

As an unconventional bonding pattern different from conventional chemistry, the concept of planar hypercoordinate atoms was first proposed in the molecular system, and it has been recently extended to 2D periodic systems. Using first-principles calculations, herein we predict a stable FeSi₂ monolayer with planar hexacoordinate Fe atoms. Due to its abundant multicenter bonds, the FeSi₂ monolayer shows excellent thermal and kinetic stability, anisotropic mechanical properties and room-temperature ferromagnetism (T _C ∼360 K). Furthermore, we have demonstrated the feasibility of directly growing an FeSi₂ monolayer on a Si (110) substrate while maintaining the novel electronic and magnetic properties of the freestanding monolayer. The FeSi₂ monolayer synthesized in this way would be compatible with the mature silicon semiconductor technology and could be utilized for spintronic devices.

SiCP4 Monolayer with a Direct Band Gap and High Carrier Mobility for Photocatalytic Water Splitting

Photocatalytic water splitting is a promising method that uses sunlight to generate hydrogen from water to provide clean and renewable energy resources. Two-dimensional materials with abundant active sites are ideal candidates for achieving this goal; however, few of the known ones can meet the rigorous requirement of photocatalytic water splitting. By using first-principles swarm-intelligence search calculations, we have successfully identified two new semiconducting SiCP₂ and SiCP₄ monolayers. Their band-edge heights evidently straddle the redox potentials of water. For the more prominent SiCP₄ monolayer, additional external biases of 0.32 V for water oxidation and 0.03 V for the hydrogen reduction half-reaction would be enough to drive its reaction sequences at pH 0, and it can spontaneously proceed to the water oxidation half-reaction in a neutral solution. Interestingly, the excellent optical absorbance ability (∼10⁴ cm^-1) and high carrier mobility (∼10⁵ cm² V^-1 s^-1) of SiCP₂ and SiCP₄ facilitate the utilization of sunlight and the fast transportation of photogenerated carriers. All of these properties make SiCP₂ and SiCP₄ monolayers promising candidates for applications in photocatalytic water splitting.

An electron counting formula to explain and to predict hydrogenated and metallated borophenes

A generalized electron counting rule for borophene with varying hole density is formulated based on the graphitic electron count. The electronic requirement explains the preference of hydrogenation in β12 borophene...

From a Möbius-aromatic interlocked Mn2B10H10 wheel to the metal-doped boranaphthalenes M2@B10H8 and M2B5 2D-sheets (M = Mn and Fe): a molecules to materials continuum using DFT studies

The design of (1) Möbius aromatic interlocked boron wheel Mn2B10H10, (2) Hückel aromatic boron analogs of naphthalene (M2@B10H8; M = Mn and Fe), and (3) metal boride monolayers (FeB5 and Fe2B5), creating a molecules to materials continuum.

Theoretical Prediction of Graphene-like 2D Uranyl Material with p-Orbital Antiferromagnetism

Versatile graphene-like two-dimensional materials with s-, p- and d-block elements have aroused significant interests because of their extensive applications while there is a lack of f-block one. Herein we report...

The Thermal Stability of Janus Monolayers SnXY (X, Y = O, S, Se): Ab-Initio Molecular Dynamics and Beyond

In recent years, the Janus monolayers have attracted tremendous attention due to their unique asymmetric structures and intriguing physical properties. However, the thermal stability of such two-dimensional systems is less known. Using the Janus monolayers SnXY (X, Y = O, S, Se) as a prototypical class of examples, we investigate their structure evolutions by performing ab-initio molecular dynamics (AIMD) simulations at a series of temperatures. It is found that the system with higher thermal stability exhibits a smaller difference in the bond length of Sn-X and Sn-Y, which is consistent with the orders obtained by comparing their electron localization functions (ELFs) and atomic displacement parameters (ADPs). In principle, the different thermal stability of these Janus structures is governed by their distinct anharmonicity. On top of these results, we propose a simple rule to quickly predict the maximum temperature up to which the Janus monolayer can stably exist, where the only input is the ADP calculated by the second-order interatomic force constants rather than time-consuming AIMD simulations at various temperatures. Furthermore, our rule can be generalized to predict the thermal stability of other Janus monolayers and similar structures.

Electrical control of topological spin textures in two-dimensional multiferroics

The electrical control of topological magnetism is an intensive topic in spintronic devices. Here, using first-principles calculations and micromagnetic simulations, we show the potential of two-dimensional (2D) magnetoelectric multiferroics in the area of topological magnetism. Intrinsic Dzyaloshinskii-Moriya interactions (DMIs) can promote the stabilization of sub-10 nm skyrmions or bimerons in 2D multiferroics. In addition, the electric polarization in 2D multiferroics provides an opportunity for the electrical control of interfacial DMI chirality and thereby the topological magnetism. These results provide a promising route for the modulation of topological magnetism in 2D spintronic devices.

Theoretical probing of twenty-coordinate actinide-centered boron molecular drums

The exploration of metal-doped boron clusters has a great significance in the design of high coordination number (CN) compounds. Actinide-doped boron clusters are probable candidates for achieving high CNs. In this work, we systematically explored a series of actinide metal atom (U, Np, and Pu) doped B₂₀ boron clusters An@B₂₀ (An = U, Np, and Pu) by global minimum structural searches and density functional theory (DFT). Each An@B₂₀ cluster is confirmed to be a twenty-coordinate complex, which is the highest CN obtained in the chemistry of actinide-doped boron clusters so far. The predicted global minima of An@B₂₀ are tubular structures with actinide atoms as centers, which can be considered as boron molecular drums. In An@B₂₀, U@B₂₀ has a relatively high symmetry of C₂, while both Np@B₂₀ and Pu@B₂₀ exhibit C₁ symmetry. Extensive bonding analysis demonstrates that An@B₂₀ has σ and π delocalized bonding, and the U-B bonds possess a relatively higher covalency than the Np-B and Pu-B bonds, resulting in the higher formation energy of U@B₂₀. Therefore, the covalent character of An-B bonding may be crucial for the formation of these high CN actinide-centered boron clusters. These results deepen our understanding of actinide metal doped boron clusters and provide new clues for developing stable high CN boron-based nanomaterials.

Two-Dimensional TeB Structures with Anisotropic Carrier Mobility and Tunable Bandgap

Two-dimensional (2D) semiconductors with desirable bandgaps and high carrier mobility have great potential in electronic and optoelectronic applications. In this work, we proposed α-TeB and β-TeB monolayers using density functional theory (DFT) combined with the particle swarm-intelligent global structure search method. The high dynamical and thermal stabilities of two TeB structures indicate high feasibility for experimental synthesis. The electronic structure calculations show that the two structures are indirect bandgap semiconductors with bandgaps of 2.3 and 2.1 eV, respectively. The hole mobility of the β-TeB sheet is up to 6.90 × 10² cm² V^-1 s^-1. By reconstructing the two structures, we identified two new horizontal and lateral heterostructures, and the lateral heterostructure presents a direct band gap, indicating more probable applications could be further explored for TeB sheets.

Electronic and optical properties of two-dimensional heterostructures based on Janus XSSe (X = Mo, W) and Mg(OH)2: a first principles investigation

Two-dimensional (2D) materials have attracted numerous investigations after the discovery of graphene. 2D van der Waals (vdW) heterostructures are a new generation of layered materials, which can provide more desirable applications. In this study, the first principles calculation was implemented to study the heterostructures based on Janus TMDs (MoSSe and WSSe) and Mg(OH)₂ monolayers, which were constructed by vdW interactions. Both MoSSe/Mg(OH)₂ and WSSe/Mg(OH)₂ vdW heterostructures have thermal and dynamic stability. Besides, XSSe/Mg(OH)₂ (X = Mo, W) possesses a direct bandgap with a type-I band alignment, which provides promising applications for light-emitting devices. The charge density difference was investigated, and 0.003 (or 0.0042) |e| were transferred from MoSSe (or WSSe) layer to Mg(OH)₂ layer, and the potential drops were calculated to be 11.59 and 11.44 eV across the interface of the MoSSe/Mg(OH)₂ and WSSe/Mg(OH)₂ vdW heterostructures, respectively. Furthermore, the MoSSe/Mg(OH)₂ and WSSe/Mg(OH)₂ vdW heterostructures have excellent optical absorption wave. Our studies exhibit an effective method to construct new heterostructures based on Janus TMDs and develop their applications for future light emitting devices.

Two-dimensional (2D) materials have attracted numerous investigations after the discovery of graphene.

Ferromagnetic Dirac half-metallicity in transition metal embedded honeycomb borophene

Exploring two-dimensional (2D) ferromagnetic materials with intrinsic Dirac half-metallicity is crucial for the development of next-generation spintronic devices. Based on first-principles calculations, here we propose a simple valence electron-counting rule to design such materials and endow them with good stability and desirable magnetic properties. Taking honeycomb borophene as a prototype, we demonstrate that embedding open-shell transition metal (like Cr) atoms in the hexagonal ring of boron atoms can provide two valence electrons to fully occupy the in-plane σ and out-of-plane π bands of B atoms. The remaining four valence electrons reside in d orbitals that split under C_6v symmetry, yielding a magnetic moment of ∼2 μ_B per Cr atom. The resulting CrB₂ monolayer exhibits a Dirac half-metal band structure, a high Curie temperature of 175 K, and a large out-of-plane magnetic anisotropy energy of 4 meV per Cr simultaneously. Our work establishes a feasible route for the experimental realization of ferromagnetic Dirac half-metallicity in 2D materials and provides new opportunity to realize high-speed devices with low consumption.

Electronic and optical properties of boron-based hybrid monolayers

Anisotropic 2D Dirac cone materials are important for the fabrication of nanodevices having direction-dependent characteristics since the anisotropic Dirac cones lead to different values of Fermi velocities yielding variable carrier concentrations. In this work, the feasibility of the B-based hybrid monolayers BX (X = As, Sb, and Bi), as anisotropic Dirac cone materials is investigated. Calculations based on density functional theory and molecular dynamics method find the stability of these monolayers exhibiting unique electronic properties. For example, the BAs monolayer possesses a robust self-doping feature, whereas the BSb monolayer carries the intrinsic charge carrier concentration of the order of 10¹²cm^-2which is comparable to that of graphene. Moreover, the direction-dependent optical response is predicted in these B-based monolayers; a high IR response in thex-direction is accompanied with that in the visible region along they-direction. The results are, therefore, expected to help in realizing the B-based devices for nanoscale applications.

Oxygen-substituted borophene as a potential anode material for Li/Na-ion batteries: a first principles study

Two-dimensional graphene-like hexagonal borophene sheets (HBSs) have a thermodynamically unsteady configuration since boron has one electron less than the carbon in graphene. To overcome this problem, we proposed a novel 2D graphene-like HBS oxide (h-B3O) in theory, which is designed by substituting partial boron atoms in a HBS with oxygen atoms. Molecular dynamics simulations indicate that h-B3O has good thermal stability. Besides, we also explored the potential of h-B3O monolayers as anodes for Li-ion batteries (LIBs) and Na-ion batteries (NIBs) by using first-principles calculations. The results indicated that the h-B3O monolayer has high adsorption energies (-2.33/-1.70 eV for Li/Na), low diffusion barriers (0.67/0.42 eV for Li/Na) and suitable average open-circuit voltages (0.36/0.32 V for LIBs/NIBs). Particularly, h-B3O has a large theoretical specific capacity of 1161 mA h g-1 for LIBs. Thus, benefiting from these characteristics, the h-B3O monolayer is considered as a promising candidate for an anode material for LIBs/NIBs.

Semiconducting MnB5monolayer as a potential photovoltaic material

Exploring new two-dimensional (2D) materials is of great significance for both basic research and practical applications. Although boron can form various 3D and 2D allotropes due to its ease of forming multi-center bonds, the coexistence of honeycomb and kagome boron structures has never been observed in any 2D material yet. In this article we apply first-principle swarm structural searches to predict the existence of a stable MnB₅structure, consisting of a sandwich of honeycomb and kagome borophenes. More interestingly, a MnB₅nanosheet is a semiconductor with a band gap of 1.07 eV and a high optical absorption in a broad band, which satisfies the requirements of a very good photovoltaic material. Upon moderate strain, MnB₅undergoes a conversion from an indirect to a direct band gap semiconductor. The power conversion efficiency of a heterostructure solar cell made of MnB₅is up to 18%. The MnB₅nanosheet shows a robust dynamical and thermal stability, stemming from the presence of intra- and interlayer multi-center σ and π bonds. These characteristics make MnB₅a promising photovoltaic material.

Half-Auxeticity and Anisotropic Transport in Pd Decorated Two-Dimensional Boron Sheets

Upon strain, most materials shrink normal to the direction of applied strain. Similarly, if a material is compressed, it will expand in the direction orthogonal to the pressure. Few materials, those of negative Poisson ratio, show the opposite behavior. Here, we show an unprecedented feature, a material that expands normal to the direction of stress, regardless if it is strained or compressed. Such behavior, namely, half-auxeticity, is demonstrated for a borophene sheet stabilized by decorating Pd atoms. We explore Pd-decorated borophene, identify three stable phases of which one has this peculiar property of half auxeticity. After carefully analyzing stability and mechanical and electronic properties we explore the origin of this very uncommon behavior and identify it as a structural feature that may also be employed to design further 2D nanomaterials.

Novel topological states of nodal points and nodal rings in 2D planar octagon TiB4

Topological states of matter in two-dimensional (2D) materials have received increasing attention due to their potential applications in nanoscale spintronics. Here, we report the presence of unique topological electronic properties in a 2D planar octagon TiB₄ compound. Particularly, without considering the spin-orbit coupling (SOC), we found that the material showed a coexistence of novel quadratic node (QN), and two different types of nodal rings (NRs), namely type-I and type-II. The protection mechanism of fermions has been fully clarified in this study. Furthermore, these fermions showed clear edge states. It is worth noting that QN had a topological charge of 2 since it is different from linear nodes and exhibit clear Fermi arc edge states. Under lattice strain, we found that the system could further exhibit rich topological phase transition. When SOC was included, we determined that these crossing points open very tiny energy gaps, which were smaller than previously reported 3D and 2D examples. These results show that monolayer TiB₄ is an excellent nodal point and nodal ring semimetal, which also provides a feasible member for studying potential entanglements among multiple fermions.

Nanoscale Probing of Image-Potential States and Electron Transfer Doping in Borophene Polymorphs

Because synthetic 2D materials are generally stabilized by interfacial coupling to growth substrates, direct probing of interfacial phenomena is critical for understanding their nanoscale structure and properties. Using field-emission resonance spectroscopy with an ultrahigh vacuum scanning tunneling microscope, we reveal Stark-shifted image-potential states of the v_1/6 and v_1/5 borophene polymorphs on Ag(111) with long lifetimes, suggesting high borophene lattice and interface quality. These image-potential states allow the local work function and interfacial charge transfer of borophene to be probed at the nanoscale and test the widely employed self-doping model of borophene. Supported by apparent barrier height measurements and density functional theory calculations, electron transfer doping occurs for both borophene phases from the Ag(111) substrate. In contradiction with the self-doping model, a higher electron transfer doping level occurs for denser v_1/6 borophene compared to v_1/5 borophene, thus revealing the importance of substrate effects on borophene electron transfer.

Prediction of room-temperature ferromagnetism and large perpendicular magnetic anisotropy in a planar hypercoordinate FeB3 monolayer

Two-dimensional (2D) magnets simultaneously possessing a high transition temperature and large perpendicular magnetic anisotropy are extremely rare, but essential for highly efficient spintronic applications. By using ab initio and global minimization approaches, we for the first time report a completely planar hypercoordinate metalloborophene (α-FeB3) with high stability, unusual stoichiometry and exceptional magnetoelectronic properties. The α-FeB3 monolayer exhibits room-temperature ferromagnetism (Tc = 480 K), whose origin is first revealed by the B-mediated RKKY interaction in the 2D regime. Its perpendicular magnetic anisotropy is almost six times larger than that of the experimentally realized 2D CrI3 and Fe3GeTe2. Moreover, metallic α-FeB3 shows n- and p-type Dirac transport with a high Fermi velocity in both spin channels. Our results not only highlight a promising 2D ferromagnet for advanced spintronics, but also pave the way for exploring novel 2D magnetism in boron-based magnetic allotropes.

RuN2 Monolayer: A Highly Efficient Electrocatalyst for Oxygen Reduction Reaction

The transition metal-based nitride (TMN) holds great promise as catalysts with high efficiency for energy-related technologies. Herein, on the basis of global structure search and density functional theory calculations, a novel two-dimensional (2D) TMN was identified: RuN₂ monolayer with tetracoordinated Ru atoms and isolated N═N dimers, which is revealed to possess high thermal, dynamic, and chemical stabilities as well as metallic nature, thus providing great feasibility for its practical application in electrochemical reactions. Remarkably, we found that the predicted RuN₂ monolayer exhibits superior catalytic performance for the oxygen reduction reaction (ORR) with a rather high limiting potential (0.99 V) and an overwhelming four-electron reduction pathway selectivity. Thus, our results suggested the robust applicability of RuN₂ monolayer as a novel non-Pt catalyst due to its excellent catalytic efficiency and outstanding selectivity for ORR, which not only proposes a new member to the hypercoordinate 2D TMN with novel properties, but also provides a feasible strategy to further develop novel TMN-based nanomaterials for electrocatalytic energy conversion.

Highly Stable Two-Dimensional Iron Monocarbide with Planar Hypercoordinate Moiety and Superior Li-Ion Storage Performance

Stable planar hypercoordinate motifs have been recently demonstrated in two-dimensional (2D) confinement systems, while perfectly planar hypercoordinate motifs in 2D carbon-transition metal systems are rarely reported. Here, by using comprehensive ab initio computations, we discover two new iron monocarbide (FeC) binary sheets stabilized at 2D confined space, labeled as tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), which are energetically more favorable compared with the previously reported square and honeycomb lattices. The proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with four ambient iron atoms, considered as the quasi-planar tetragonal lattice. Strikingly, the o-FeC monolayer is an orthorhombic phase with a perfectly planar pentacoordinate carbon moiety and a planar seven-coordinate iron moiety. These monolayers are the first example of a simultaneously pentacoordinate carbon and planar seven-coordinate Fe-containing material. State-of-the-art theoretical calculations confirm that all these monolayers have significantly dynamic, mechanical, and thermal stabilities. Among these two monolayers, the t-FeC monolayer shows a higher theoretical capacity (395 mAh g^-1) and can stably adsorb Li up to t-FeCLi₄ (1579 mAh g^-1). The low migration energy barrier is predicted as small as 0.26 eV for Li, which results in the fast diffusion of Li atoms on this monolayer, making it a promising candidate for lithium-ion battery material.

Rational Prediction of Single Metal Atom Supported on Two-Dimensional Metal Diborides for Electrocatalytic N2 Reduction Reaction with Integrated Descriptor

Nitrogen reduction reaction (NRR) plays an important role in chemical industry, so it is significant to develop low-cost and efficient electrocatalysts for nitrogen fixation instead of the traditional Haber-Bosch process. In this paper, the electrocatalytic performance of various single atoms doped on two-dimensional metal diborides with a B vacancy for N₂ reduction to ammonia is calculated and predicted. By screening numerous catalysts, we find that Ti@VB₂ is the most active catalyst for NRR, and the limiting potential of Ti@VB₂ for NRR is -0.61 V. Through high-throughput search and LASSO regression, an integrated descriptor combining the intrinsic properties of the single transition metal atom (TM) and the substrate (MB₂) is proposed, which can fit the relationship between intrinsic properties of catalysts and NRR activity well. Therefore, this study not only discovers a promising electrocatalyst for nitrogen fixation but also provides a strategy for predicting the activity of catalysts.

Two-dimensional stable Mn based half metal and antiferromagnets promising for spintronics

In this paper, we predict that the tetragonal MnSi and MnC0.5Si0.5 monolayers are mechanically stable metallic ferromagnetic materials. The thermal stability of the MnC0.5Si0.5 monolayer is verified by our ab initio molecular dynamics (AIMD) result at 300 K. Both MnSi and MnC0.5Si0.5 monolayers exhibit room temperature half-metallic properties, which is very promising for spintronic applications. Both monolayers exhibit large perpendicular magnetic anisotropy, which is desirable for maintaining magnetic order and for high density storage spintronics. A bilayer of the MnSi nanosheet has obviously enhanced thermal stability and exhibits antiferromagnetic metal properties. The Néel temperature could be effectively manipulated and improved by surface functionalization. In addition, monolayer and bilayer MnSi nanosheets exhibit nodal lines in the reciprocal space, and the nodal lines are robust against spin orbit coupling.

Stability, electronic, and optical properties of two-dimensional phosphoborane

The structure and properties of two-dimensional phosphoborane sheets were computationally investigated using Density Functional Theory calculations. The calculated phonon spectrum and band structure point to dynamic stability and allowed characterization of the predicted two-dimensional material as a direct-gap semiconductor with a band gap of ~1.5 eV. The calculation of the optical properties showed that the two-dimensional material has a relatively small absorptivity coefficient. The parameters of the mechanical properties characterize the two-dimensional phosphoborane as a relatively soft material, similar to the monolayer of MoS₂ . Assessment of thermal stability by the method of molecular dynamics indicates sufficient stability of the predicted material, which makes it possible to observe it experimentally.

Planar Hypercoordinate Motifs in Two-Dimensional Materials

ConspectusAs one of the most important and versatile elements, carbon renders itself as one of the most fundamental and cutting-edge topics in chemistry, physics, and materials science. Many carbon-based chemical rules were established accordingly. While the tetrahedral predilection of tetracoordinate carbon has been a cornerstone of organic chemistry since 1874, almost a century later tetracoordinate carbon was found to be able to adopt planar structures known as planar tetracoordinate carbon (ptC), which are stabilized electronically by good π-acceptor (delocalization of a lone electron pair of ptC) or σ-donor (promoting electron transfer to electron-deficient bonding) substituents or mechanically by appropriate steric enforcement. The experimental and theoretical achievements for the rule-breaking ptC species totally refreshed our understanding of chemical bonding and triggered exploration of peculiar molecules featuring planar pentacoordinate carbon (ppC) and planar hexacoordinate carbon (phC) as well as other outlandish species such as planar hypercoordinate silicon.While the planar hypercoordinate carbon chemistry has been gradually established for molecules in the past five decades, there is growing interest in pursuing their extension systems, especially in two-dimensional (2D) space as a result of the recent extensive studies of graphene and its analogues. Though the natural 2D layered crystals do not contain any planar hypercoordinate carbon or silicon, several 2D nanosheets featuring planar or quasi-planar hypercoordinate ones have been theoretically suggested. Encouragingly, these unique planar configurations possess decent stabilities, and some of them are even the global minimum structure, exhibiting great potential for experimental realization. As the nature of a material is mainly determined by its structural characteristics (e.g., dimensionality, crystallography, and bonding), the combination of planar hypercoordinate chemistry and 2D nanoscience not only endows these rule-breaking systems with the merits of 2D materials but also may offer various promising properties and applications. For example, an unusual negative Poisson's ratio can be found in ppC-containing Be₅C₂ and planar pentacoordinate silicon (ppSi)-containing CaSi monolayers, of which the former has an anisotropic Dirac cone and the latter is a semiconductor with a desirable band gap for the semiconductor industry. Specially, shortly after the theoretical prediction, a planar hexacoordinate silicon (phSi)-containing Cu₂Si monolayer was experimentally synthesized and characterized with the 2D Dirac nodal line fermion, which offers a platform to achieve high-speed, low-dissipation nanodevices.In this Account, we review the recent progress, mostly by density functional theory (DFT) computations, in designing 2D materials with planar hypercoordinate motifs. We describe the key achievements in this field, paying special attention to the "bottom-up" and "isoelectronic substitution" design strategies. In addition, the fundamental stabilization mechanisms of planar hypercoordinate motifs in an infinite layer are discussed. We hope that this Account will inspire more experimental and theoretical efforts to explore nanomaterials with such unconventional chemical bonding.

Boron-based ternary Rb6Be2B6 cluster featuring unique sandwich geometry and a naked hexagonal boron ring

Boron-based ternary Rb₆Be₂B₆ cluster features a naked hexagonal boron ring and unique “Big Mac” sandwich shape, being stabilized collectively by four-fold 2σ/6π/6σ/2σ aromaticity.

A new single-layer structure of MBene family: Ti2B

In this manuscript, we have carried out a combined study of density functional theory and Monte Carlo (MC) simulations for a thorough examination of a single-layer (SL) Ti₂B structure. On the basis of first-principles, spin-polarized density functional calculations, we showed that a free standing SL-Ti₂B structure is dynamically and thermally stable. The atomic structure, phonon spectrum, electronic and magnetic properties of the SL-Ti₂B structure are analyzed. In order to determine ground state, the structure of Ti₂B is optimized for four types of spin oriented configurations, namely ferromagnetic (FM), antiferromagnetic Néel, antiferromagnetic Zigzag and antiferromagnetic Stripy and non-magnetic states. We found that the spin configuration FM corresponds to the ground state for SL-Ti₂B. We also found that the Raman-active modes are softening in the antiferromagnetic cases. On the basis of these results, MC simulations show that the magnetic susceptibility, thermal variations of magnetization, and specific heat curves of Ti₂B exhibit a phase transition between paramagnetic and FM phases at the Curie temperature of 39.06 K. While SL-Ti₂B possess a little out-of-plane magnetic anisotropy, it has not any in plane magnetic anisotropy energy.

Two novel triangular borophenes B3H and B6O: first-principles prediction

By means of density functional theory calculations, we successfully predict two stable 2D triangular borophenes, namely B₃H and B₆O. Our results indicate that B₃H is a Dirac material and its cone point is located at the K point of the Brillouin zone (BZ). B₆O is identified as having a node-line ring and Dirac cones together. Its node-line ring formed by the intersection of the extended energy band from the two Dirac cones located on K point. This modified 2D borophene has great thermal and dynamic stability due to the electron transfer from the triangular boron lattice to the O atoms. The electronic structure of B₆O nanofilm demonstrates novel properties such as two Dirac cones, more than 1.3 eV linear dispersion bands at some points of the BZ, as well as excellent transport properties for the extremely high mobility brought by the combination of the node-line semimetal and Dirac cones. Our study may motivate potential applications of 2D materials in nanoelectronics.

Two-dimensional ZrB2C2 with multiple tunable Dirac states

In this paper, we designed a two-dimensional honeycomb monolayer ZrB₂C₂, which is predicted to be a stable nanosheet and exhibits favorable mechanical and thermal properties. The Young's modulus of ZrB₂C₂ is 122.3 N m^-1, which is about one third of that of graphene. The ab initio molecular dynamics (AIMD) results show that ZrB₂C₂ can sustain up to 500 K. In addition, ZrB₂C₂ is semi-metallic and it exhibits twelve Dirac cones in the first Brillouin zone; the six pairs of Dirac cones form compensated electron-hole pockets, and the maximum Fermi velocity is about 6.3 × 10⁵ m s^-1, which is about 60% of that of graphene. Compared with other transition metal borides which possess Dirac states, the twelve Dirac cones are all robust under different types of external strains, and the Dirac states could be shifted along the opposite direction crossing the Fermi energy barrier by external strains and the gap of the two Dirac cones could be opened around the Fermi energy level with the effect of spin-orbit coupling.

Monolayer MBenes: prediction of anode materials for high-performance lithium/sodium ion batteries

The design and fabrication of new high-performance electrode materials are critical for driving the development of next-generation energy conversion and energy storage devices. Here, we report a series of orthogonal two-dimensional transition metal borides (MBenes) based on first-principles density functional theory, which can be obtained by mechanically stripping an MBene from a large MAB phase, including V₂B₂, Cr₂B₂, Mn₂B₂, Ti₂B₂, Zr₂B₂, and Nb₂B₂. The thermodynamic and kinetic stability of monolayer MBenes at room temperature was confirmed by AIMD simulations and phonon spectra. We investigated the potential of two-dimensional MBenes (V, Cr, Mn) as lithium/sodium ion anode electrode materials. Research shows that MBenes have inherent metallic properties, and their mechanical properties indicate that MBenes have high Young's modulus and anisotropy, low diffusion potential and low open-circuit voltage, with outstanding rate performance and good chemical stability in practical applications. In addition, functionalization tends to strongly reduce electrochemical performance and should be avoided as much as possible in experiments. These interesting findings fully demonstrate that the predicted monolayer MBenes are attractive anode materials for lithium/sodium ion batteries.