Three-Dimensional Metallic Boron Nitride

Unveiling a Family of Dimerized Quantum Magnets, Conventional Antiferromagnets, and Nonmagnets in Ternary Metal Borides

Dimerized quantum magnets are exotic crystalline materials where Bose-Einstein condensation of magnetic excitations can happen. However, known dimerized quantum magnets are limited to only a few oxides and halides. Here, we unveil 9 dimerized quantum magnets and 11 conventional antiferromagnets in ternary metal borides MTB4 (M = Sc, Y, La, Ce, Lu, Mg, Ca, and Al; T = V, Cr, Mn, Fe, Co, and Ni), where T atoms are arranged in structural dimers. Quantum magnetism in these compounds is dominated by strong antiferromagnetic (AFM) interactions between Cr (Cr and Mn for M = Mg and Ca) atoms within the dimers, with much weaker interactions between the dimers. These systems are proposed to be close to a quantum critical point between a disordered singlet spin-dimer phase, with a spin gap, and the ordered conventional Néel AFM phase. They greatly enrich the materials inventory that allows investigations of the spin-gap phase. Conventional antiferromagnetism in these compounds is dominated by ferromagnetic Mn (Fe for M = Mg and Ca) interactions within the dimers. The predicted stable and nonmagnetic (NM) YFeB4 phase is synthesized and characterized, providing a scarce candidate to study Fe dimers and Fe ladders in borides. The identified quantum, conventional, and NM systems provide a platform with abundant possibilities to tune the magnetic exchange coupling by doping and study the unconventional quantum phase transition and conventional magnetic transitions. This work opens new avenues for studying novel magnetism in borides arising from spin dimers and establishes a theoretical workflow for future searches for dimerized quantum magnets in other families of materials.

Two-dimensional monolayer C5-10-16: a metallic carbon allotrope as an anode material for high-performance sodium/potassium-ion batteries

Carbonaceous materials are promising candidates as anode materials for non-lithium-ion batteries (NLIBs) due to their appealing properties such as good electrical conductivity, low cost, and high safety. However, graphene, a classic two-dimensional (2D) carbon material, is chemically inert to most metal atoms, hindering its application as an electrode material for metal-ion batteries. Inspired by the unique geometry of a four-penta unit, we explore a metallic 2D carbon allotrope C5-10-16 composed of 5-10-16 carbon rings. The C5-10-16 monolayer is free from any imaginary frequencies in the whole Brillouin zone. Due to the introduction of a non-sp2 hybridization state into C5-10-16, the extended conjugation of π-electrons is disrupted, leading to the enhanced surface activity toward metal ions. We investigate the performance of C5-10-16 as the anode for sodium/potassium-ion batteries by using first-principles calculations. The C5-10-16 sheet has high theoretical specific capacities of Na (850.84 mA h g-1) and K (743.87 mA h g-1). Besides, C5-10-16 exhibits a moderate migration barrier of 0.63 (0.32) eV for Na (K), ensuring rapid charging/discharging processes. The average open-circuit voltages of Na and K are 0.33 and 0.62 V, respectively, which are within the voltage acceptance range of anode materials. The fully sodiated (potassiated) C5-10-16 shows tiny lattice expansions of 1.4% (1.3%), suggesting the good reversibility. Moreover, bilayer C5-10-16 significantly affects both the adsorption strength and the mobility of Na or K. All these results show that C5-10-16 could be used as a promising anode material for NLIBs.

Boosting Potassium Adsorption and Diffusion Performance of Carbon Anodes for Potassium-Ion Batteries via Topology and Curvature Engineering: From KT-Graphene to KT-CNTs

We propose a two-dimensional carbon allotrope (named KT-graphene) by incorporating kagome and tetragonal lattices consisting of trigonal, quadrilateral, octagonal, and dodecagonal rings. The introduction of non-hexagonal rings can give rise to the localized electronic states that improve the chemical reactivity toward potassium, making KT-graphene a high-performance anode material for potassium-ion batteries. It shows a high theoretical capacity (892 mAh g-1), a low diffusion barrier (0.33 eV), and a low average open-circuit voltage (0.51 V). The presence of electrolyte solvents is propitious to boost the K-ion adsorption and diffusion capabilities. Moreover, one-dimensional nanotubes (KT-CNTs), rolled up by the KT-graphene sheet, are metallic regardless of the tube diameter. As the curvature increases, KT-CNTs exhibit significantly increased surface activity, which can promote the electron-donating ability of K. Furthermore, the curvature effect greatly enhances the efficiency of K diffusion on the inner surface compared to that on the outer surface.

Electron confinement promoted the electric double layer effect of BiOI/β-Bi2O3 in photocatalytic water splitting

In the realm of photocatalysis, understanding the interface issues (solid/solid and solid/liquid) inherent in heterojunction at the atomic level is the ultimate for engineering an efficient photocatalyst. Herein, an electrophoretic deposition technique is adopted to synthesize BiOI/β-Bi2O3 heterojunction, exhibiting superior photocatalytic activity and stability in H2 evolution (91.5 μmol g-1 h-1) and H2O2 production (11.3 mg L-1 h-1). Combined with the experimental and computational results, a lower free energy of hydrogen evolution reaction (252.4 meV) has been observed contrast to BiOI and β-Bi2O3 samples. A carrier transfer process of like S-scheme heterojunction is proposed based on density of states (DOS) and carrier distribution calculations. The theoretical calculations illustrate the transition dipole moment, migration and accumulation of carrier in BiOI/β-Bi2O3 heterojunction. Subsequent ab initio molecular dynamics (AIMD) results of solid/liquid interface systems (BiOI/β-Bi2O3/H2O and β-Bi2O3/H2O) unravel the interface H2O (solvent) behaviors. The local aggregation of photo-generated electrons in BiOI/β-Bi2O3/H2O leads to a large potential drop, high proton migration rate and the steady electric double layer (EDL) structure compared to the β-Bi2O3/H2O, which facilitates the occurrence of photocatalytic reactions in solution. In addition to offering new insights into the hydrogen evolution and proton transfer in the EDL model and the association between the heterojunction effect and EDL structure, this work also introduces a novel design strategy for Bi-based heterojunctions.

Combined Analyses on Electronic Structure and Molecular Orbitals of d10 Bimetal Oxide In2Ge2O7 and Photocatalytic Performances for Overall Water Splitting and CO2 Reduction

Semiconducting photocatalytic overall water splitting and CO2 reduction are possible solutions to the emerging worldwide challenges of oil shortage and continual temperature increase, and the key is to develop an efficient photocatalyst. Most photocatalysts contain the d0, d10 or d10ns2 metals, and a guiding principle is desired to help to distinguish outstanding semiconductors. Here, the d10 bimetal oxide In2Ge2O7 was selected as the target. First, density functional theory (DFT) calculations point out that the nonbonding O 2p orbitals dominate the valence band maximum (VBM), and In 5s-O 2s and Ge 4s-O 2s antibonding orbitals are the major components of conduction band minimum (CBM). Moreover, the molecular orbitals were analyzed to consolidate the DFT calculations and make it more understandable for chemists. Due to the very small specific surface area (0.51 m2/g) and wide band gap (4.14 eV), as-prepared In2Ge2O7 did not exhibit any overall water splitting activity; nevertheless, when loading with 1 wt% cocatalyst (i.e., Pt, Pd), the surficial charge recombination can be greatly eliminated and the overall water splitting activity is significantly improved to 33.0(4) and 17.2(7) μmol/h for H2 and O2 generation, respectively. The apparent quantum yield (AQY) at 254 nm is 8.28%. This observation is proof that the inherent electronic structure of In2Ge2O7 is beneficial for the charge migration in bulk. Moreover, this catalyst also exhibits an observable CO2 reduction activity in pure water, which is a competition reaction with water splitting, anyway, the CH4 selectivity can be enhanced by loading Pd. This is a successful attempt to unravel the structure-property relationship by combining the analyses on electronic structure and molecular orbitals and is enlightening to further discover good candidates to photocatalysts.

Monolayer polar metals with large piezoelectricity derived from MoSi2N4

The advancement of two-dimensional polar metals tends to be limited by the incompatibility between electric polarity and metallicity as well as dimension reduction. Here, we report polar and metallic Janus monolayers of the MoSi2N4 family by breaking the out-of-plane structural symmetry through Z (P/As) substitution of N. Despite the semiconducting nature of MoSi2X4 (X = N/P/As), four Janus MoSi2NxZ4-x monolayers are found to be polar metals owing to the weak coupling between the conducting electrons and electric polarity. The metallicity is originated from the Z substitution induced delocalization of occupied electrons in Mo-d orbitals. The out-of-plane electric polarizations around 1.5-15.7 pC m-1 are determined by the asymmetric out-of-plane charge distribution due to the non-centrosymmetric Janus structure. The corresponding out-of-plane piezoelectricity is further revealed as high as 18.7-73.3 pC m-1 and 0.05-0.25 pm V-1 for the piezoelectric strain and stress coefficients, respectively. The results demonstrate polar metallicity and high out-of-plane piezoelectricity in Janus MoSi2NxZ4-x monolayers and open new vistas for exploiting unusual coexisting properties in monolayers derived from the MoSi2N4 family.

First-principles study of point defects in U3Si2: effects on the mechanical and electronic properties

In recent years, U₃Si₂ has been proposed as an alternative nuclear fuel material to uranium dioxide (UO₂) because of its intrinsically high uranium density and thermal conductivity. However, the operation environment in the nuclear reactor is complex and extreme, such as in-pile neutron irradiation, and thus it is necessary to explore the radiation response behavior of U₃Si₂ and the physical properties of its damaged states. In the present study, first-principles calculations based on density functional theory were carried out to investigate the mechanical and electronic properties of defective U₃Si₂. Our results showed that the defect stability in U₃Si₂, except its interstitial defects, is dependent on its chemical environment. When vacancy, antisite or interstitial defects are introduced into U₃Si₂, its elastic modulus are decreased and its ductility is enhanced. Although the presence of defects in U₃Si₂ does not change its metallic nature and the electron distribution in its Fermi level, their effect on the partial chemical bonding interaction is significant. This study suggests that under a radiation environment, the created defects in U₃Si₂ remarkably affect its mechanical and electronic properties.

Synergistic effects of B/S co-doped spongy-like hierarchically porous carbon for a high performance zinc-ion hybrid capacitor

Zinc-ion hybrid capacitors (ZIHCs) are regarded as a potential candidate for large-scale energy storage devices. However, the inadequate cathode and the inferior wettability between the electrode and electrolyte hinder the construction of high-performance ZIHCs. Herein, boron (B) and sulfur (S) co-doped spongy-like hierarchically porous carbon (B₂S₃C) is first proposed as a cathode material for ZIHCs. Here, B doping is favorable for improving the wettability, while S doping contributes to enhancing the electrical properties. In addition, the density functional theory (DFT) results uncover that B and S atoms contribute to reducing the energy barrier between Zn²⁺ and the cathode, leading to boosted chemical adsorption ability of Zn²⁺ on the cathode. As a result, the assembled ZIHC based on B₂S₃C exhibits a high specific capacity of 182.6 mA h g^-1 at 0.1 A g^-1, an excellent capacity retention of 96.2% after 10 000 cycles and a remarkable energy density of 292.2 W h kg^-1 at a power density of 62.2 W kg^-1, superior to the previously reported ZIHCs. Due to the flexibility of the assembled electrodes, the solid-state ZIHC can sustain various deformations. This work paves a feasible path for the development of cost-effective and high-performance porous carbon materials.

Subsurface-Carbon-Induced Local Charge of Copper for an On-Surface Displacement Reaction

Transition-metal carbides have sparked unprecedented enthusiasm as high-performance catalysts in recent years. Still, the catalytic properties of copper carbide remain unexplored. By introducing subsurface carbon to Cu(111), a displacement reaction of a proton in a carboxyl acid group with a single Cu atom is demonstrated at the atomic scale and room temperature. Its occurrence is attributed to the C-doping-induced local charge of surface Cu atoms (up to +0.30 e/atom), which accelerates the rate of on-surface deprotonation via reduction of the corresponding energy barrier, thus enabling the instant displacement of a proton with a Cu atom when the molecules adsorb on the surface. This well-defined and robust Cu^δ+ surface based on subsurface-carbon doping offers a novel catalytic platform for on-surface synthesis.

First-principles calculations on the deposition behavior of LixNay (x + y ≤ 5) clusters during the hybrid storage of Li and Na atoms on graphene

A new strategy of sodium ion batteries with the hybrid storage of Li and Na ions has attracted much attention in the field of large-scale energy storage. For revealing the mechanism of hybrid storage of Li and Na atoms in carbon materials, the lowest energy configuration, adsorption energy, differential charge density and density of states of Li_xNa_y clusters on graphene, as a structural unit of carbon materials, were calculated and investigated based on first principles density functional theory. The calculation results show that the deposition behavior of single Li or Na atoms on graphene is similar, and both are preferentially deposited at the hollow of graphene (H-site). The Li atom is deposited preferentially over the Na atom, and the deposition height of the Li atom is lower. When the total number of metal atoms x + y ≥ 3, Li_xNa_y clusters are deposited on graphene in the form of a stereotypical atomic cluster, in which the Li atom is usually at the bottom of the Li_xNa_y cluster, while the Na atom is usually at the top of the cluster. The electronic structure analysis shows that the electrons of the Li_xNa_y cluster are transferred to the anti-bonding π orbitals adjacent to graphene. The 2s orbitals of Li atoms and the 2s and 2p orbitals of Na atoms are hybridized with the 2p orbitals of C atoms. Therefore, the Li-C bonds or Na-C bonds formed between Li or Na atoms and C atoms of graphene are usually ionic bonds with partial covalent bond properties. Meanwhile, the Li-Li, Na-Na or Li-Na bonds formed inside Li_xNa_y clusters are usually multiple metal-metal bonds.

Electron Transport of the Nanojunctions of (BN) n (n = 1-4) Linear Chains: A First-Principles Study

We applied the density functional theory and nonequilibrium Green's function method (DFT + NEGF) to investigate the relationship between the conductance and chain length in the stretching process, the asymmetric coupling of contact points, and the influence of positive and negative biases on the electron transport properties of the nanojunctions formed by the coupling of (BN) _n (n = 1-4) linear chains and Au(100)-3 × 3 semi-infinite electrodes. We find that the BN junction has the lowest stability and the (BN)₂ junction has the highest stability. Under zero bias, the equilibrium conductance decreases as the chain length increases; p_x and p_y orbitals play a leading role in electron transport. In the bias range of -1.6 to 1.6 V, the current of the (BN) _n (n = 1-4) linear chains increases linearly with increasing voltage. Under the same bias voltage, (BN)₁ has the largest current, so its electron transport property is the best. The rectification effect reflects the asymmetry of the structure of BN linear chains themselves and the asymmetry of coupling with the Au electrode surfaces at both ends. With the chain length increasing, the transmission spectrum near E _f is suppressed, the tunneling current decreases, and the rectification ratio increases. (BN)₄ molecular junctions have the largest rectification ratio, reaching 13.32 when the bias voltage is 1.6 V. Additionally, the Au-N strong coupling is more conducive to the electron transport of the molecular chain than the Au-B weak coupling. Our calculations provide an important theoretical reference for the design and development of BN linear-chain nanodevices.

B5N3 and B7N5 Monolayers with High Carrier Mobility and Excellent Optical Performance

An ab initio evolutionary search algorithm was combined with density functional theory (DFT) calculations to predict a series of 2-D B_xN_y (1 < x/y ≤ 2). Particularly, B₅N₃ and B₇N₅ monolayers have sufficiently low formation enthalpy and excellent dynamic stability that make them promising for synthesis in experiments. Electronic structure calculations reveal that B₅N₃ and B₇N₅ monolayers possess an indirect band gap of 1.99 eV and a direct band gap of 2.40 eV, respectively. The calculated absorption coefficients for B₅N₃ and B₇N₅ monolayers are significantly improved in the low end of the visible region compared with that of 2-D h-BN. Moreover, our calculations reveal that both B₅N₃ and B₇N₅ monolayers have high electron carrier mobilities. The narrow band gaps, high carrier mobilities, strong near-ultraviolet absorption, and high synthesis possibility of B₅N₃ and B₇N₅ monolayers render them promising new materials for application in novel electronics and environmentally benign solar energy conversion.

Pentagonal B2N3-based 3D metallic boron nitride with high energy density

Different from conventional insulating or semiconducting boron nitride,metallicBN has received increasing attention in recent years as its intrinsic metallicity grants it great potential for broad applications. In this study, by assembling the experimentally synthesized pentagonal B₂N₃units, we have proposed the first pentagon-based three-dimensional (3D) metallic boron nitride, labeled penta-B₄N_7.First-principles calculations together with molecular dynamics simulations and convex hull diagram show that penta-B₄N₇is not only thermally, dynamically and mechanically stable, but also three dimensionally metallic. A detailed analysis of its electronic structure reveals that the intrinsic metallicity comes from the delocalized electrons in the partially occupied antibonding N-Nπorbitals. Equally important, the energy density of penta-B₄N₇is found to be 4.07 kJ g^-1, which is the highest among that of all the 3D boron nitrides reported so far.

Design of a Series of Metallic BxNx+1 with Tunable Mechanical Properties

Here a series of sp²-sp³ B_xN_x+1 (x = 1, 2, 3, 4, 5, 6) structures was constructed. These structures can be viewed as diamond-like BN blocks connected by single N-N bonds. Elastic constants and phonon dispersion curves confirm that all of the proposed structures are mechanically and dynamically stable. These structures all possess metallicity originating from the conductive channels formed by sp²-hybridized N atoms and adjacent sp³-hybridized B and N atoms. These structures exhibit tunable mechanical properties with a regular change in the sp²/sp³ ratio. The theoretical Vickers hardness increases and the ductility decreases as the number of diamond-like BN blocks increases, gradually approaching those of c-BN. Moreover, the convex hull at ambient pressure and 50 GPa indicates that high pressure is beneficial in the synthesis of these B-N phases. The simulated X-ray diffraction patterns of these structures were also calculated to provide more information for further experiments.

Effective CO Migration among Multiabsorbed Sites Achieves the Low-Barrier and High-Selective Conversion to C2 Products on the Ni2B5 Monolayer

For the electrochemical reduction of CO₂, CO is a crucial single-carbon product and a major intermediate to multicarbon products. Direct dimerization of CO is the most charming channel to C2 products, although the corresponding kinetic energy barrier causes a huge gap compared with other alternative pathways. The effective CO migration among multiple catalytic sites is predominant but has not been fully explored during the C-C bond formation and further protonation processes. Herein, the entirely planar global-minimum Ni₂B₅ monolayer with multikinds of catalytic sites is selected as an appropriate instance, on which CO can effectively migrate among different types of sites with the highest barrier of 0.64 eV. Most importantly, the computed ultralow barrier of direct *CO dimerization (0.17 eV), the limiting potentials for CH₂CH₂ (-0.13 V), and CH₃CH₂OH (-0.17 V) reach the optimal value until now, which all happen on the p-p type of dual-CO adsorption configurations after CO migration. Moreover, the hydrogen reduction side reaction is uncompetitive with the CO electrochemical reduction on all possible adsorption sites. This study demonstrates the significance of CO migration and opens a new avenue for CO reduction to high-density multicarbon products on the surface of catalysts possessing multikinds of catalytic sites.

Elucidating Stress-Strain Relations of ZrB12 from First-Principles Studies

Transition-metal boron-rich compounds exhibit favorable synthesis conditions and mechanical properties that hold great promise for wide-ranging applications. However, the complex bonding networks of these compounds produce diverse structural and mechanical behaviors that require in-depth studies. A notable case is ZrB₁₂, which has been reported to possess high Vickers hardness comparable to those of ReB₂ and WB₄. Surprisingly, first-principles calculations of stress-strain relations reveal unexpected low indentation strengths of ZrB₁₂ well below those of ReB₂ and WB₄. Such contrasting results are reconciled by noting that the additional presence of a boron-rich phase of ZrB₅₀ in the experimental synthesis process likely plays a key role in the extrinsic strengthening. These findings uncover mechanisms for the higher measured strength of ZrB₁₂ and offer insights for elucidating extrinsic hardening phenomena that may exist in other transition-metal compounds.

Graphene-Like Covalent Organic Framework with a Wide Band Gap Synthesized On Surface via Stepwise Reactions

Developing graphene-like two-dimensional materials naturally possessing a band gap has sparked enormous interest. Thanks to the inherent wide band gap and high mobility in the 2D plane, covalent organic frameworks containing triazine rings (t-COFs) hold great promise in this regard, whilst the synthesis of single-layer t-COFs remains highly challenging. Herein, we present the fabrication of a well-defined graphene-like t-COF on Au(111). Instead of single/multiple-step single-type reactions commonly applied for on-surface synthesis, distinct stepwise on-surface reactions, including alkynyl cyclotrimerization, C-O bond cleavage, and C-H bond activation, are triggered on demand, leading to product evolution in a controlled step-by-step manner. Aside from the precise control in sophisticated on-surface synthesis, this work proposes a single-atomic-layer organic semiconductor with a wide band gap of 3.41 eV.

Persistent Cohomology for Data With Multicomponent Heterogeneous Information

Persistent homology is a powerful tool for characterizing the topology of a data set at various geometric scales. When applied to the description of molecular structures, persistent homology can capture the multiscale geometric features and reveal certain interaction patterns in terms of topological invariants. However, in addition to the geometric information, there is a wide variety of nongeometric information of molecular structures, such as element types, atomic partial charges, atomic pairwise interactions, and electrostatic potential functions, that is not described by persistent homology. Although element-specific homology and electrostatic persistent homology can encode some nongeometric information into geometry based topological invariants, it is desirable to have a mathematical paradigm to systematically embed both geometric and nongeometric information, i.e., multicomponent heterogeneous information, into unified topological representations. To this end, we propose a persistent cohomology based framework for the enriched representation of data. In our framework, nongeometric information can either be distributed globally or reside locally on the datasets in the geometric sense and can be properly defined on topological spaces, i.e., simplicial complexes. Using the proposed persistent cohomology based framework, enriched barcodes are extracted from datasets to represent heterogeneous information. We consider a variety of datasets to validate the present formulation and illustrate the usefulness of the proposed method based on persistent cohomology. It is found that the proposed framework outperforms or at least matches the state-of-the-art methods in the protein-ligand binding affinity prediction from massive biomolecular datasets without resorting to any deep learning formulation.

Optical properties of a hexagonal C/BN framework with sp2 and sp3 hybridized bonds

We investigated the optical properties and roles of sp2- and sp3-hybridized bonds of a hexagonal C/BN family using first-principles calculations. The calculated phonon dispersions confirm the dynamic stability of Hex-(BN)6C12 and Hex-C12(BN)6. The complex dielectric function evolves from the infrared to the ultraviolet region and has a significant anisotropy for different polarizations. The reflectivity and refractive index spectra show that the sp2-hybridized C atoms are more sensitive to the light from infrared to visible region than B-N pairs while the C atoms and B-N pairs have a similar sensitivity to high frequencies. The sharp peaks of the energy-loss spectrum are all concentrated in the 23-30 eV energy region, which can be used to identify these hexagonal structures. The calculated band structures show Hex-C24 and Hex-(BN)6C12 are metals, but Hex-C12(BN)6 and Hex-(BN)12 are semiconductors with indirect band gaps of 3.47 and 3.25 eV, respectively. The electronic states near the Fermi level primarily originate from sp2-hybridized atoms. In addition, sp2-hybridized bonds are the main elements affecting the optical and electronic structure of C/BN materials with sp2- and sp3-hybridizations. We expect that the results presented will help understand the optical properties of C/BN materials containing sp2- and sp3-hybridized C atoms and B-N pairs.

Electro-Optical Properties of Monolayer and Bilayer Pentagonal BN: First Principles Study

Two-dimensional hexagonal boron nitride (hBN) is an insulator with polar covalent B-N bonds. Monolayer and bilayer pentagonal BN emerge as an optoelectronic material, which can be used in photo-based devices such as photodetectors and photocatalysis. Herein, we implement spin polarized electron density calculations to extract electronic/optical properties of mono- and bilayer pentagonal BN structures, labeled as B2N4, B3N3, and B4N2. Unlike the insulating hBN, the pentagonal BN exhibits metallic or semiconducting behavior, depending on the detailed pentagonal structures. The origin of the metallicity is attributed to the delocalized boron (B) 2p electrons, which has been verified by electron localized function and electronic band structure as well as density of states. Interestingly, all 3D networks of different bilayer pentagonal BN are dynamically stable unlike 2D structures, whose monolayer B4N2 is unstable. These 3D materials retain their metallic and semiconductor nature. Our findings of the optical properties indicate that pentagonal BN has a visible absorption peak that is suitable for photovoltaic application. Metallic behavior of pentagonal BN has a particular potential for thin-film based devices and nanomaterial engineering.

Three metallic BN polymorphs: 1D multi-threaded conduction in a 3D network

In this paper, three novel metallic sp2/sp3-hybridized boron nitride (BN) polymorphs are proposed by first-principles calculations. One of them, denoted as tP-BN, is predicted based on the evolutionary particle swarm structural search. tP-BN is composed of two interlocked rings forming a tube-like 3D network. The stability and band structure calculations show that tP-BN is metastable and metallic at zero pressure. Calculations for the density of states and electron orbitals confirm that the metallicity originates from the sp2-hybridized B and N atoms, forming 1D linear conductive channels in the 3D network. According to the relationship between the atomic structure and electronic properties, another two 3D metastable metallic sp2/sp3-hybridized BN structures are constructed manually. Electronic property calculations show that both of these structures have 1D conductive channels along different axes. The polymorphs predicted in this study enrich the structures and provide a different picture of the conductive mechanism of BN compounds.

Robustness of the electronic structure and charge transfer in topological insulator Bi2Te2Se and Bi2Se2Te thin films under an external electric field

The electronic structure and charge transfer of Bi₂Te₂Se and Bi₂Se₂Te thin films are robust to an external electrical field.

Electronic structure and transport properties of 2D RhTeCl: a NEGF-DFT study

2D materials are considered as excellent candidates for next-generation electronic and optoelectronic devices. However, the corresponding systems with both an appropriate direct band gap and high carrier mobility are urgently required. Here, a new 2D semiconductor, monolayer RhTeCl, is investigated based on first-principles calculations. Monolayer RhTeCl possesses a direct band gap of 2.16 eV, with a high electron mobility up to 1.5 × 10⁴ cm² V^-1 s^-1. Thus, monolayer RhTeCl double-gated metal-oxide-semiconductor field-effect transistors (MOSFETs) with a 6 nm gate length are simulated by quantum transport methods. The 6 nm monolayer RhTeCl n-MOSFET displays a steep sub-kT/q switching characteristic and a high on/off ratio (10⁶), which demonstrates a superior gate control. Therefore, these promising semiconductor characteristics and device performances of 2D RhTeCl provide new opportunities for novel low power ultra-scaled devices.

Graphene-like monolayer monoxides and monochlorides

Two-dimensional monolayer materials, with thicknesses of up to several atoms, can be obtained from almost every layer-structured material. It is believed that the catalogs of known 2D materials are almost complete, with fewer new graphene-like materials being discovered. Here, we report 2D graphene-like monolayers from monoxides such as BeO, MgO, CaO, SrO, BaO, and rock-salt structured monochlorides such as LiCl, and NaCl using first-principle calculations. Two-dimensional materials containing d-orbital atoms such as HfO, CdO, and AgCl are predicted. Adopting the same strategy, 2D graphene-like monolayers from mononitrides such as scandium nitride (ScN) and monoselenides such as cadmium selenide (CdSe) are discovered. Stress engineering is found to help stabilize 2D monolayers, through canceling the imaginary frequency of phonon dispersion relation. These 2D monolayers show high dynamic, thermal, kinetic, and mechanic stabilities due to atomic hybridization, and electronic delocalization.

Isoelectronic analogues of graphene: the BCN monolayers with visible-light absorption and high carrier mobility

By employing particle-swarm optimization (PSO) and first-principles computations, we theoretically predicted five stable phases of graphene-like borocarbonitrides (g-BCN) with the stoichiometric ratio of 1:1:1 and uniformly distributed B, C, N atoms, which are the isoelectronic analogues of graphene. These g-BCN monolayers are effectively stabilized by their relatively high proportion of robust C-C or B-N bonds and strong partial ionic-covalent B-C and C-N bonds within them, leading to pronounced thermal and kinetic stability. The visible-light absorption and high carrier mobility of the investigated g-BCN monolayers indicate their possible applications in high-efficiency photochemical processes and electronic devices. Our computations could provide some guidance for designing the graphene-like materials with earth-abundant elements, as well as some clues for the experimental synthesis and practical applications of ternary BCN nanosheets.

The role of sp2 and sp3 hybridized bonds on the structural, mechanical, and electronic properties in a hard BN framework

A first-principles approach is used to systematically investigate the role of sp² and sp³ hybridized bonds on the structural, mechanical, and electronic properties in a new BN phase (denoted Hex-(BN)₁₂). Hex-(BN)₁₂ has the same number of sp² and sp³ hybridized atoms. The calculated cohesion energy, phonon frequencies, and elastic constants unambiguously confirm the structural stability of this compound. Due to the different types of hybridization and B–N covalent bonds with ionic characteristics, Hex-(BN)₁₂ has unequal bond lengths and bond angles in these hybrid orbitals. These cause the relative energetic stability to be slightly lower than c-BN and w-BN. The hardness of Hex-(BN)₁₂ is estimated to range from 33 to 40 GPa. The bond-breaking order under stress is sp³–sp³, sp²–sp³, and sp²–sp². DFT calculations with the gradient approximation (GGA) and HSE06 functional indicate the electronic structure contains an indirect band gap at 3.21 and 4.42 eV, respectively. The electronic states in the region near the Fermi level primarily arise from the 2p orbitals in sp²-hybridized atoms. In general, sp³ bonded B and N atoms guarantee higher mechanical properties, and sp² bonded atoms ensure ductility and even conductivity, although all changes vary with spatial structure. Hex-(BN)₁₂ can be obtained from multilayer yne-BN, and BN nanosheets, nanotubes and nanoribbons under pressure.

A first-principles approach is used to systematically investigate the role of sp² and sp³ hybridized bonds on the structural, mechanical, and electronic properties in a new BN phase (denoted Hex-(BN)₁₂).

Theoretical Studies on the Electronic and Optical Properties of Honeycomb BC3 monolayer: A Promising Candidate for Metal-free Photocatalysts

By employing first-principles computations and particle-swarm optimization calculations, we theoretically confirmed the honeycomb geometry of experimentally realized BC3 sheet, which is constructed by the hexagonal carbon-ring fragments surrounded by six boron atoms and has pronounced thermodynamic stabilities. Remarkably, the computations also demonstrate the visible-light absorption, high carrier mobilities, and promising reduction and oxidation capacities of the BC3 monolayer, indicating its efficient absorption of solar radiation, fast migration of electron and holes, and excellent capabilities of photoinduced carriers in a photocatalytic process, respectively. Additionally, its indirect band gap, spatially separated charge distributions, and great difference in mobilities of electrons and holes should lead to the restricted recombination of photoactivated e--h+ pairs within BC3 monolayer. All above-mentioned characteristics suggest that the honeycomb BC3 monolayer should be a recommendable candidate for metal-free photocatalysts, which is worthy of further verifications and explorations in experimental studies.

First principles study of electronic transport properties in novel FeB2 flake-based nanodevices

First-principles calculations provide theoretical support for the promising applications of innovative two-probe devices based on FeB₂ flakes and reveal the superiority of devices with FeB₂ flakes at temperatures not above 1000 K in transport properties.

Multiporous sp2-hybridized boron nitride (d-BN): stability, mechanical properties, lattice thermal conductivity and promising application in energy storage

sp²-hybridized multiporous d-BN is used for solid electrolytes in lithium–sulfur batteries.

Computational Dissection of Two-Dimensional Rectangular Titanium Mononitride TiN: Auxetics and Promises for Photocatalysis

Recently, two-dimensional (2D) transition-metal nitrides have triggered an enormous interest for their tunable mechanical, optoelectronic, and magnetic properties, significantly enriching the family of 2D materials. Here, by using a broad range of first-principles calculations, we report a systematic study of 2D rectangular materials of titanium mononitride (TiN), exhibiting high energetic and thermal stability due to in-plane d-p orbital hybridization and synergetic out-of-plane electronic delocalization. The rectangular TiN monolayer also possesses enhanced auxeticity and ferroelasticity with an alternating order of Possion's Ratios, stemming from the competitive interactions of intra- and inter- Ti-N chains. Such TiN nanosystem is a n-type metallic conductor with specific tunable pseudogaps. Halogenation of TiN monolayer downshifts the Fermi level, achieving the optical energy gap up to 1.85 eV for TiNCl(Br) sheet. Overall, observed electronic features suggest that the two materials are potential photocatalysts for water splitting application. These results extend emerging phenomena in a rich family 2D transition-metal-based materials and hint for a new platform for the next-generation functional nanomaterials.

BN-schwarzite: novel boron nitride spongy crystals

Novel 3-D BN crystals with a negative curvature, intrinsic porosity and a large specific surface area are proposed for the first time by first-principles study, suggesting that the BN crystals hold great promise in the fields of energy storage, molecular sieving, and environmental remediation.

Pnma-BN: Another Boron Nitride Polymorph with Interesting Physical Properties

Structural, mechanical, electronic properties, and stability of boron nitride (BN) in Pnma structure were studied using first-principles calculations by Cambridge Serial Total Energy Package (CASTEP) plane-wave code, and the calculations were performed with the local density approximation and generalized gradient approximation in the form of Perdew-Burke-Ernzerhof. This BN, called Pnma-BN, contains four boron atoms and four nitrogen atoms buckled through sp³-hybridized bonds in an orthorhombic symmetry unit cell with Space group of Pnma. Pnma-BN is energetically stable, mechanically stable, and dynamically stable at ambient pressure and high pressure. The calculated Pugh ratio and Poisson's ratio revealed that Pnma-BN is brittle, and Pnma-BN is found to turn brittle to ductile (~94 GPa) in this pressure range. It shows a higher mechanical anisotropy in Poisson's ratio, shear modulus, Young's modulus, and the universal elastic anisotropy index A^U. Band structure calculations indicate that Pnma-BN is an insulator with indirect band gap of 7.18 eV. The most extraordinary thing is that the band gap increases first and then decreases with the increase of pressure from 0 to 60 GPa, and from 60 to 100 GPa, the band gap increases first and then decreases again.

Cubic C₃N: A New Superhard Phase of Carbon-Rich Nitride

Using the particle swarm optimization technique, we proposed a cubic superhard phase of C₃N (c-C₃N) with an estimated Vicker’s hardness of 65 GPa, which is more energetically favorable than the recently proposed o-C₃N. The c-C₃N is the most stable phase in a pressure range of 6.5–15.4 GPa. Above 15.4 GPa, the most energetic favorable high pressure phase R3m-C₃N is uncovered. Phonon dispersion and elastic constant calculations confirm the dynamical and mechanical stability of c-C₃N and R3m-C₃N at ambient pressure. The electronic structure calculations indicate that both c-C₃N and R3m-C₃N are indirect semiconductor.

Boron Nitride Nanostructures: Fabrication, Functionalization and Applications

Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two-dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN-involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One-dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.