Two-dimensional monolayer BiSnO3: a novel wide-band-gap semiconductor with high stability and strong ultraviolet absorption.

The exploration of two-dimensional (2D) wide-band-gap semiconductors (WBGSs) holds significant scientific and technological importance in the field of condensed matter physics and is actively being pursued in optoelectronic research. In this study, we present the discovery of a novel WBGS, namely monolayer BiSnO3, using first-principles calculations in conjunction with the quasi-particle G0W0approximation. Our calculations confirm that monolayer BiSnO3exhibits moderate cleavage energy, positive phonon modes, mechanical resilience, and high temperature resistance (up to 1000 K), which demonstrate its structural stability, flexibility, and potential for experimental realization. Furthermore, band-structure calculations reveal that monolayer BiSnO3is a typical WBGS material with a band-gap energy (Eg) of 3.61 eV and possesses a unique quasi-direct electronic feature due to its quasi-flat valence band. The highest occupied valence flat-band originates from the electronic hybridization between Bi-6pand O-2pstates, which are in close proximity to the Fermi level. Remarkably, monolayer BiSnO3exhibits a high absorption capacity for ultraviolet light spanning the UVA to UVC regions, displaying optical isotropy absorption and an unusual excitonic effect. These intriguing structural and electronic properties establish monolayer BiSnO3as a promising candidate for the development of new multi-function-integrated electronic and optoelectronic devices in the emerging field of 2D WBGSs.

Honeycomb Electron Lattice Induced Dirac Fermion with Trigonal Warping in Bilayer Electrides.

Emergent fermions arising from the excess electrons of electrides provide a new perspective for exploring semimetal states with unique Fermi surface geometries. In this study, a class of unique two-dimensional (2D) highly anisotropic Dirac fermions is designed using a sandwich structure. Based on the structural design and first-principles calculations, 2D electride MB (M = Ca/Sr, B = Cl/Br/I) is an ideal candidate material. The excess electrons of the bilayer MB could be stably localized in the interstitial cavities, constructing a natural zigzag honeycomb electron sublattice that further forms a Dirac fermion. Compared with traditional Dirac semimetals, 2D Dirac electrides exhibited rich physical properties: i) The Fermi surface shows trigonal warping in low-energy regions. In particular, the geometry of the Fermi surface determines the high anisotropy of the Fermi velocity. ii) A pair of Dirac fermions are protected by three-fold rotational symmetry and exhibit strong robustness. iii) Electride MB possesses a lower work function that strongly correlates with the surface area of the emission channel. Based on these properties, an electron-emitting device with multifunctional applications is fabricated. Therefore, this study provides an ideal platform for studying potential entanglement between structures, electrides, and topological states.

Two-dimensional ferromagnetic semiconductors of monolayer BiXO3 (X = Ru, Os) with direct band gaps, high Curie temperatures, and large magnetic anisotropy.

Two-dimensional (2D) ferromagnetic semiconductors are highly promising candidates for spintronics, but are rarely reported with direct band gaps, high Curie temperatures (Tc), and large magnetic anisotropy. Using first-principles calculations, we predict that two ferromagnetic monolayers, BiXO3 (X = Ru, Os), are such materials with a direct band gap of 2.64 and 1.69 eV, respectively. Monte Carlo simulations reveal that the monolayers show high Tc beyond 400 K. Interestingly, both BiXO3 monolayers exhibit out-of-plane magnetic anisotropy, with magnetic anisotropy energy (MAE) of 1.07 meV per Ru for BiRuO3 and 5.79 meV per Os for BiOsO3. The estimated MAE for the BiOsO3 sheet is one order of magnitude larger than that for the CrI3 monolayer (685 µeV per Cr). Based on the second-order perturbation theory, it is revealed that the large MAE of the monolayers BiRuO3 and BiOsO3 is mainly contributed by the matrix element differences between dxy and dx2-y2 and dyz and dz2 orbitals. Importantly, the ferromagnetism remains robust in 2D BiXO3 under compressive strain, while undergoing a ferromagnetic to antiferromagnetic transition under tensile strain. The intriguing electronic and magnetic properties make BiXO3 monolayers promising candidates for nanoscale electronics and spintronics.

Two-dimensional carbon materials with an anisotropic Dirac cone: high stability and tunable Fermi velocity.

Among two-dimensional (2D) materials, Dirac-cone materials have attracted much attention due to their extraordinary electrical properties. In this work, we propose a new 2D carbon allotrope, 2D Pmc21, which consists of 5-, 6-, 7- and 11-membered rings, and all carbon atoms are in one plane. Phonon dispersion curve calculations indicate that 2D Pmc21 is kinetically stable under ambient conditions. First-principles molecular dynamics simulations show that 2D Pmc21 has excellent thermal stability, even up to 3000 K. Of most interest are the electronic properties of the Pmc21 structure: it is a Dirac semimetal with highly anisotropic Dirac cones, and its Fermi velocity is of the same order of magnitude as that of graphene. Furthermore, the structure exhibits strain self-doping properties and an in-plane strain-tunable Fermi velocity. The structure of Pmc21 reported here shows great application potential in future electronic products.

A perfect match between borophene and aluminium in the AlB3 heterostructure with covalent Al-B bonds, multiple Dirac points and a high Fermi velocity.

By performing a swarm-intelligent global structure search combined with first-principles calculations, a stable two-dimensional (2D) AlB3 heterostructure with directed, covalent Al-B bonds forms due to a nearly perfect lattice match between 2D borophene and the Al(111) surface. The AlB3 heterosheet with the P6mm space group is composed of a planar Al(111) layer and a corrugated borophene layer, where the in-plane coordinates of Al covalently link with the corrugated B atoms. The resulting structure shows a similar interlayer interaction energy to that of the Al(111) surface layer to the bulk and high mechanical and thermal stability, possesses multiple Dirac points in the Brillouin zone with a remarkably high Fermi velocity of 1.09 × 106 m s-1, which is comparable to that of graphene. Detailed analysis of the electronic structure employing the electron localisation function and topological analysis of the electron density confirm the covalent Al-B bond with high electron localisation between the Al and B centres and with only little interatomic charge transfer. The combination of borophene with metal monolayers in 2D heterostructures opens the door to a rich chemistry with potentially unprecedented properties.

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.

Strain-controlled electronic and magnetic properties of tVS2/hVS2 van der Waals heterostructures.

The structural, electronic and magnetic properties of the T-phase and H-phase of the VS2 monolayer and their heterobilayers are studied by means of first-principles calculations. We find that the two phases of the VS2 monolayer are both ferromagnetic (FM) semiconductors and that these two monolayers form a typical van der Waals (vdW) heterostructure with a weak interlayer interaction. By comparing the energy of different magnetic configurations, the FM state of the tVS2/hVS2 heterostructure is found to be in the ground state under normal conditions or biaxial strains. Under compressive strains, the anti-FM (AFM) and FM states degenerate. Based on the band structure obtained and the work function, it is found that the T-phase and H-phase are capable of forming an efficient p-n heterostructure. Due to spontaneous charge transfer at the interface, a gapless semiconductor is formed in our HSE06 calculations. We also find that the twist angle between the monolayers has a negligible impact on the band structure of the heterostructure in its spin-down channel. Moreover, the tVS2/hVS2 heterostructure is found to switch from a gapless semiconductor to a metal or a half-metal under some given biaxial or uniaxial strains. Therefore, the heterostructure could be a half-metallic property with strains, realizing 100% polarization at the Fermi level. Our study provides the possibility of realizing 100% spin-polarization at the Fermi level in these FM vdW heterostructures, which is significant for further spin transport exploration.

Polymorphism of low dimensional boron nanomaterials driven by electrostatic gating: a computational discovery.

The successful synthesis of two-dimensional (2D) boron sheets typically relies on the utilization of a silver surface, which acts as a gated substrate compensating for the electron-deficiency of boron. However, how the structures of one-dimensional (1D) boron are affected by the gating effect remains unclear. By means of an unbiased global minimum structure search and density functional theory (DFT) computations, we discovered the coexistence of 2D boron sheets and 1D ribbons triggered by electrostatic gating. Specifically, at a low excess charge density level (<0.1 e per atom), 2D boron sheets dominate the low energy configurations. As the charge density increases (>0.3 e per atom), more 1D boron ribbons emerge, while the number of 2D layers is reduced. Additionally, a number of low-lying 1D boron ribbons were discovered, among which a flat borophene-like ribbon (FBR) was predicted to be stable and possess high mechanical strength. Moreover, the electride Ca2N was identified as an ideal substrate for the fabrication of the FBR because of its ability to supply a strong electrostatic field. This work bridges the gap between 2D and 1D boron structures, reveals the polymorphism of 1D boron ribbons under the electrostatic gating effect, and in general provides broad implications for future synthesis and applications of low-dimensional boron materials.

Room temperature ferromagnetism and antiferromagnetism in two-dimensional iron arsenides.

The discovery of two-dimensional (2D) magnetic materials with high critical temperature and intrinsic magnetic properties has attracted significant research interest. By using swarm-intelligence structure search and first-principles calculations, we predict three 2D iron arsenide monolayers (denoted as FeAs-I, II and III) with good energetic and dynamical stabilities. We find that FeAs-I and II are ferromagnets, while FeAs-III is an antiferromagnet. FeAs-I and III have sizable magnetic anisotropy comparable to the magnetic recording materials such as the FeCo alloy. Importantly, we show that FeAs-I and III have critical temperatures of 645 K and 350 K, respectively, which are above room temperature. In addition, FeAs-I and II are metallic, while FeAs-III is semiconducting with a gap comparable to Si. For FeAs-III, there exist two pairs of 2D antiferromagnetic Dirac points below the Fermi level, and it displays a giant magneto band-structure effect. The superior magnetic and electronic properties of the FeAs monolayers make them promising candidates for spintronics applications.

Rhombohedral Lanthanum Manganite: A New Class of Dirac Half-Metal with Promising Potential in Spintronics.

Dirac half-metals have drawn great scientific interests in spintronics because of their outstanding physical properties such as the large spin polarization and massless Dirac fermions. By using first-principles calculations, we investigate the perovskite-type lanthanum manganite (LaMnO3) as a novel Dirac half-metal. Specifically, LaMnO3 displays multiple linear band crossings in the spin-up direction, while it has a large band gap (∼5 eV) in the spin-down orientation. The intriguing linear band dispersions guarantee the ultrafast electron transport and the significant band differences between spin up and down directions promise the realization of 100% spin-polarized current and the extremely low energy consumption. Such a spin-polarized Dirac material is rare among perovskite-type compounds. By adopting the mean-field theory, the estimated Curie temperature Tc is 438.4 K. Importantly, the LaMnO3 crystal has been experimentally realized 2 decades ago, which facilitates the future experimental validation. With the novel spin-polarized electronic properties and the high possibility of experimental fabrication, LaMnO3 is ideal for the spintronic application.

Free-radical gases on two-dimensional transition-metal disulfides (XS2, X = Mo/W): robust half-metallicity for efficient nitrogen oxide sensors.

The detection of single gas molecules is a highly challenging work because it requires sensors with an ultra-high level of sensitivity. By using density functional theory, here we demonstrate that the adsorption of a paramagnetic unpaired free radical gas (NO) on a monolayer of XS2 (X = Mo, W) can trigger the transition from semiconductor to half metal. More precisely, the single-layer XS2 (X = Mo, W) with NO adsorbed on it would behave like a metal in one spin channel while acting as a semiconductor in the other spin orientation. The half-metallicity is robust and independent of the NO concentration. In contrast, no half-metallic feature can be observed after the adsorption of other free radical gases such as NO2. The unique change in electronic properties after the adsorption of NO on transition-metal sulfides highlights an effective strategy to distinguish NO from other gas species by experimentally measuring spin-resolved transmission. Our results also suggest XS2 (X = Mo, W) nanosheets can act as promising nanoscale NO sensors.

Two-dimensional GeP3 as a high capacity electrode material for Li-ion batteries.

Two-dimensional (2D) materials are promising for use in lithium (Li) electrodes due to their high surface ratio. By using density functional theory (DFT) calculations, we investigate the adsorption and diffusion of Li on a newly predicted 2D GeP3 material [Nano Lett., 2016, 17, 1833]. The most favourable adsorption sites for Li are identified, and a semiconducting to metallic transition induced by Li adsorption is found, which indicates excellent electrical conductivity. The GeP3 monolayer has an estimated capacity of 648 mA h g-1, which is almost twice that of commercially used graphite (375 mA h g-1). During full Li intercalation, the GeP3 layer undergoes only 1.2% lattice parameter reduction. Moreover, GeP3 possesses the advantages of a small diffusion barrier (∼0.5 eV) and low average open-circuit voltages (∼0.4 V). Our results highlight a new class of promising anode materials, i.e. 2D phosphide, as potential rechargeable lithium batteries with ultrahigh-capacity, superior ionic conductivity, and low average open-circuit voltage.

First-Principles Prediction of Spin-Polarized Multiple Dirac Rings in Manganese Fluoride.

Spin-polarized materials with Dirac features have sparked great scientific interest due to their potential applications in spintronics. But such a type of structure is very rare and none has been fabricated. Here, we investigate the already experimentally synthesized manganese fluoride (MnF_{3}) as a novel spin-polarized Dirac material by using first-principles calculations. MnF_{3} exhibits multiple Dirac cones in one spin orientation, while it behaves like a large gap semiconductor in the other spin channel. The estimated Fermi velocity for each cone is of the same order of magnitude as that in graphene. The 3D band structure further reveals that MnF_{3} possesses rings of Dirac nodes in the Brillouin zone. Such a spin-polarized multiple Dirac ring feature is reported for the first time in an experimentally realized material. Moreover, similar band dispersions can be also found in other transition metal fluorides (e.g., CoF_{3}, CrF_{3}, and FeF_{3}). Our results highlight a new interesting single-spin Dirac material with promising applications in spintronics and information technologies.

Predicting a graphene-like WB4 nanosheet with a double Dirac cone, an ultra-high Fermi velocity and significant gap opening by spin-orbit coupling.

The zero-band gap nature of graphene prevents it from performing as a semi-conductor in modern electronics. Although various graphene modification strategies have been developed to address this limitation, the very small band gap of these materials and the suppressed charge carrier mobility of the devices developed still significantly hinder graphene's applications. In this work, a two dimensional (2D) WB4 monolayer, which exhibits a double Dirac cone, was conceived and assessed using density functional theory (DFT) methods, which would provide a sizable band gap while maintaining higher charge mobility with a Fermi velocity of 1.099 × 106 m s-1. Strong spin-orbit-coupling can generate an observable band gap of up to 0.27 eV that primarily originates from the d-orbit of the heavy metal atom W; therefore a 2D WB4 nanosheet would be operable at room temperature (T = 300 K) and would be a promising candidate to fabricate nanoelectronics in the upcoming post-silicon era. The phonon-spectrum and ab initio molecular dynamics calculations further demonstrate the dynamic and thermal stability of such nanosheets, thus, suggesting a potentially synthesizable Dirac material.

Gas Protection of Two-Dimensional Nanomaterials from High-Energy Impacts.

Two-dimensional (2D) materials can be produced using ball milling with the help of liquid surfactants or solid exfoliation agents, as ball milling of bulk precursor materials usually produces nanosized particles because of high-energy impacts. Post-milling treatment is thus needed to purify the nanosheets. We show here that nanosheets of graphene, BN, and MoS2 can be produced by ball milling of their bulk crystals in the presence of ammonia or a hydrocarbon ethylene gas and the obtained nanosheets remain flat and maintain their single-crystalline structure with low defects density even after a long period of time; post-milling treatment is not needed. This study does not just demonstrate production of nanosheets using ball milling, but reveals surprising indestructible behaviour of 2D nanomaterials in ammonia or hydrocarbon gas under the high-energy impacts; in other milling atmospheres such as air, nitrogen or argon the same milling treatment produces nanosized particles. A systematic study reveals chemisorption of ammonia and hydrocarbon gases and chemical reactions occurring at defect sites, which heal the defects by saturating the dangling bonds. Density functional theory was used to understand the mechanism of mechanochemical reactions. Ball milling in ammonia or hydrocarbon is promising for mass-production of pure nanosheets.

Anomalous Enhancement of Mechanical Properties in the Ammonia Adsorbed Defective Graphene.

Pure graphene is known as the strongest material ever discovered. However, the unavoidable defect formation in the fabrication process renders the strength of defective graphene much lower (~14%) than that of its perfect counterpart. By means of density functional theory computations, we systematically explored the effect of gas molecules (H2, N2, NH3, CO, CO2 and O2) adsorption on the mechanical strength of perfect/defective graphene. The NH3 molecule is found to play a dominant role in enhancing the strength of defective graphene by up to ~15.6%, while other gas molecules decrease the strength of graphene with varying degrees. The remarkable strength enhancement can be interpreted by the decomposition of NH3, which saturates the dangling bond and leads to charge redistribution at the defect site. The present work provides basic information for the mechanical failure of gas-adsorbed graphene and guidance for manufacturing graphene-based electromechanical devices.

Substantial Band-Gap Tuning and a Strain-Controlled Semiconductor to Gapless/Band-Inverted Semimetal Transition in Rutile Lead/Stannic Dioxide.

By first-principle calculations, we have systematically studied the effect of strain/pressure on the electronic structure of rutile lead/stannic dioxide (PbO2/SnO2). We find that pressure/strain has a significant impact on the electronic structure of PbO2/SnO2. Not only can the band gap be substantially tuned by pressure/strain, but also a transition between a semiconductor and a gapless/band-inverted semimetal can be manipulated. Furthermore, the semimetallic state is robust under strain, indicating a bright perspective for electronics applications. In addition, a practical approach to realizing strain in SnO2 is then proposed by substituting tin (Sn) with lead (Pb), which also can trigger the transition from a large-band-gap to a moderate-gap semiconductor with enhanced electron mobility. This work is expected to provide guidance for full utilization of the flexible electronic properties in PbO2 and SnO2.

Tubular Shape Fullerenes Inside Single Wall Boron Nitride Nanotubes: A Theoretical Simulation.

The orientations of fullerene molecules filled in nanotubes have important influence on the electronic properties of the formed peapods and their transformations such as polymerization under certain conditions. Here we present a investigation on the preferable orientations of tubular C70, C80 and C90 fullerenes confined inside single-walled boron nitride nanotubes (SWBNNTs) by calculating the van der Waals energy between the encapsulated molecule and the hosting nanotube. The minimum entering radius and the energetically favorable radius for encapsulating C70, C80 and C90 have been determined by the reaction energy calculation. We also show that the three studied molecules filled in SWBNNTs exhibit a transition from lying (five-fold axis) orientation to tilted orientation and then to standing orientation (two-fold axis) with increasing the tube radius. The preferable orientations of the encapsulated fullerenes are irrelevant on the tube chirality, but are dependent on the radius.

Two-Dimensional Boron Hydride Sheets: High Stability, Massless Dirac Fermions, and Excellent Mechanical Properties.

Two-dimensional (2D) boron sheets have been successfully synthesized in recent experiments, however, some important issues remain, including the dynamical instability, high energy, and the active surface of the sheets. In an attempt to stabilize 2D boron layers, we have used density functional theory and global minimum search with the particle-swarm optimization method to predict four stable 2D boron hydride layers, namely the C2/m, Pbcm, Cmmm, and Pmmn sheets. The vibrational normal mode calculations reveal all these structures are dynamically stable, indicating potential for successful experimental synthesis. The calculated Young's modulus indicates a high mechanical strength for the C2/m and Pbcm phases. Most importantly, the C2/m, Pbcm, and Pmmn structures exhibit Dirac cones with massless Dirac fermions and the Fermi velocities for the Pbcm and Cmmm structures are even higher than that of graphene. The Cmmm phase is reported as the first discovery of Dirac ring material among boron-based 2D structures. The unique electronic structure of the 2D boron hydride sheets makes them ideal for nanoelectronics applications.

Graphene-like Two-Dimensional Ionic Boron with Double Dirac Cones at Ambient Condition.

Recently, partially ionic boron (γ-B28) has been predicted and observed in pure boron, in bulk phase and controlled by pressure [ Nature 2009 , 457 , 863 ]. By using ab initio evolutionary structure search, we report the prediction of ionic boron at a reduced dimension and ambient pressure, namely, the two-dimensional (2D) ionic boron. This 2D boron structure consists of graphene-like plane and B2 atom pairs with the P6/mmm space group and six atoms in the unit cell and has lower energy than the previously reported α-sheet structure and its analogues. Its dynamical and thermal stability are confirmed by the phonon-spectrum and ab initio molecular dynamics simulation. In addition, this phase exhibits double Dirac cones with massless Dirac Fermions due to the significant charge transfer between the graphene-like plane and B2 pair that enhances the energetic stability of the P6/mmm boron. A Fermi velocity (vf) as high as 2.3 × 10(6) m/s, which is even higher than that of graphene (0.82 × 10(6) m/s), is predicted for the P6/mmm boron. The present work is the first report of the 2D ionic boron at atmospheric pressure. The unique electronic structure renders the 2D ionic boron a promising 2D material for applications in nanoelectronics.