Partial Oxidized Arsenene: Emerging Tunable Direct Bandgap Semiconductor

Enhancing the anti-oxidation stability of vapor-crystallized arsenic crystals via introducing iodine

The oxidation of arsenic restricts its application in high-performance electronic devices and functional materials. Herein, a removable iodine-regulation method was proposed for the first time to enhance the anti-oxidation behavior of arsenic. In a gradient of 500-650 ℃, the introduction of 0.6-5.0 at% iodine into arsenic vapor could regulate an arsenic crystal. The oxygen content on the regulated arsenic crystal surface was lowered below 2.5 at% after exposure to ambient conditions for 96 h, reducing over 90% compared with the control group. The residual iodine barrier, which was mainly in the As-I₂ state, suppressed the long-term oxidation of arsenic. First-principles calculation suggested that the adsorbed I₂ weakened the delocalization of lone-pair electrons and inhibited charge transfer from the arsenic surface. Iodine regulation stabilized arsenic surface, which preferred (003) or (012) facets. Their surface energies were 22.4 meV and 47.6 meV, respectively. The synergistic effect of surface stabilization and I₂ passivation lowered the surface energy and continuously slowed the oxidation of arsenic. Therefore, iodine regulation comprehensively enhanced the anti-oxidation properties of arsenic. Moreover, heating at 200 ℃ left the arsenic surface iodine content below 0.1 at% with little variation in structure. The improved anti-oxidation property of arsenic preserves resources for further advanced applications.

A DFT-based finite element approach for studying elastic properties, buckling and vibration of the arsenene

A finite element model is developed to modeli the arsenene nanosheet. To obtain the element properties, which are used to represent As-As bonds in the structure of the arsenene, first principle calculation is used. The developed model is then used to compute Young's modulus, critical compressive force and the fundamental frequency of the arsenene nanosheet with different geometrical parameters. It is seen that the employed finite element model can be efficiently used to predict surface Young's modulus of the arsenene. Furthermore, larger arsenene nanosheets have larger surface Young's modulus. In the next step, the critical compressive forces of the arsenene nanosheet under different boundary conditions are computed. It is seen that the influence of the boundary conditions has higher impact on the bunking force of the smaller arsenenes nanosheets. Finally, investigating the vibrational characteristics of the arsenene nanosheets revealed that increasing the horizontal side length at a constant vertical side length leads to a reduction in the fundamental natural frequency.

Tailorable Indirect to Direct Band-Gap Double Perovskites with Bright White-Light Emission: Decoding Chemical Structure Using Solid-State NMR

Efficient white-light-emitting single-material sources are ideal for sustainable lighting applications. Though layered hybrid lead-halide perovskite materials have demonstrated attractive broad-band white-light emission properties, they pose a serious long-term environmental and health risk as they contain lead (Pb²⁺) and are readily soluble in water. Recently, lead-free halide double perovskite (HDP) materials with a generic formula A(I)₂B'(III)B″(I)X₆ (where A and B are cations and X is a halide ion) have demonstrated white-light emission with improved photoluminescence quantum yields (PLQYs). Here, we present a series of Bi³⁺/In³⁺ mixed-cationic Cs₂Bi_1-xIn_xAgCl₆ HDP solid solutions that span the indirect to direct band-gap modification which exhibit tailorable optical properties. Density functional theory (DFT) calculations indicate an indirect-direct band-gap crossover composition when x > 0.50. These HDP materials emit over the entire visible light spectrum, centered at 600 ± 30 nm with full-width at half maxima of ca. 200 nm upon ultraviolet light excitation and a maximum PLQY of 34 ± 4% for Cs₂Bi_0.085In_0.915AgCl₆. Short-range structural insight for these materials is crucial to unravel the unique atomic-level structural properties which are difficult to distinguish by diffraction-based techniques. Hence, we demonstrate the advantage of using solid-state nuclear magnetic resonance (NMR) spectroscopy to deconvolute the local structural environments of these mixed-cationic HDPs. Using ultrahigh-field (21.14 T) NMR spectroscopy of quadrupolar nuclei (¹¹⁵In, ¹³³Cs, and ²⁰⁹Bi), we show that there is a high degree of atomic-level B'(III)/B″(I) site ordering (i.e., no evidence of antisite defects). Furthermore, a combination of XRD, NMR, and DFT calculations was used to unravel the complete atomic-level random Bi³⁺/In³⁺ cationic mixing in Cs₂Bi_1-xIn_xAgCl₆ HDPs. Briefly, this work provides an advance in understanding the photophysical properties that correlate long- to short-range structural elucidation of these newly developed solid-state white-light emitting HDP materials.

Impact of oxygen defects on a ferromagnetic CrI3 monolayer

Natural oxygen defects play a vital role in the integrity, functional properties, and performance of well-known two-dimensional (2D) materials. The recently discovered chromium triiodide (CrI₃) monolayer is the first real 2D magnet. However, its interaction with oxygen remains an open fundamental question, an understanding of which is essential for further exploration of its application potentials. Employing the quantum first-principles calculation method, we investigated the influence of oxygen defects on the structural, electronic, and magnetic properties of the CrI₃ monolayer at the atomic level. We considered two oxygen-defective CrI₃ monolayers with either a single O-attached or single O-doped structure, comparing them with an un-defective pristine monolayer. The two different oxygen defects significantly affect the original architecture of the CrI₃ monolayer, being energetically favorable and increasing the stability of the CrI₃ monolayer. Moreover, these point defects introduce either deep band lines or middle gap states in the band structure. As a result, the bandgap of oxygen-defective monolayers is reduced by up to 58%, compared with the pristine sheet. Moreover, the magnetic property of the CrI₃ monolayer is drastically induced by oxygen defects. Importantly, O-defective CrI₃ monolayers possess robust exchange coupling parameters, suggesting relatively higher Curie temperature compared with the un-defective sheet. Our findings reveal that the natural oxygen defects in the CrI₃ monolayer enrich its structural, electronic, and magnetic properties. Thus, the controlled oxidation can be an effective way to tune properties and functionalities of the CrI₃ monolayer and other ultrathin magnetic materials.

This work shows that the natural oxygen defects in the CrI₃ monolayer, a first 2D magnet, enrich its structural, electronic, and magnetic properties, offering an effective way of tuning the functionality of CrI₃ monolayer and other ultrathin magnets.

The sp2 character of new two-dimensional AsB with tunable electronic properties predicted by theoretical studies

As a competitive candidate for replacing graphene that possesses an appropriate fundamental bandgap, structural stability and tunable electronic properties, the recently synthesized honeycomb arsenene has rekindled much enthusiasm in the area of two-dimensional materials. By using first-principles calculations and acoustic phonon limited deformation potential theory, we identify a compelling two-dimensional electronic material, single-layer AsB, which is a direct-gap semiconductor with a bandgap (E_g) of 1.18 eV, almost the same as that of bulk silicon. The orbital projected band structure and electron density as well as partial density of states demonstrate that the frontier state of the planar atomic structural AsB is sp² orbital hybridization, which is distinct from that of buckled arsenene monolayers. Layer thickness, stacking order and strain are effective ways to tune the frontier states, and thus the band structure and bandgap of AsB. Moreover, thicker AsB exhibits one-layer localized states in the AB-stacking structure, which is in sharp contrast to other layered materials such as MoS₂ and phosphorene. Benefiting from the non-localized p_z orbital and larger elastic modulus, the carrier mobility of AsB is in the range of 10³-10⁴ cm² V^-1 s^-1, which is much higher than that of pristine arsenene and some other analogues. Our work provides an effective way to tailor the electronic properties of 2D arsenene, which may open up new avenues for applying it in future nano-optoelectronics and electronics.

Excellent Electrolyte Wettability and High Energy Density of B2S as a Two-Dimensional Dirac Anode for Non-Lithium-Ion Batteries

Two-dimensional (2D) Dirac materials with ultrahigh electronic conductivity exhibit great promise for application as an anode material in non-lithium-ion batteries (NLIBs) with a reduced conductive additive and a binder as a nonactive material additive. Graphene is one of the most prominent 2D Dirac materials with high electrolyte wettability; however, it cannot be used as an anode material in NLIBs owing to its poor affinity toward metal ions such as Na, K, Ca, Mg, and Al. In this work, we investigated the use of recently developed boron sulfide (B₂S) as a new lightweight 2D Dirac anode for NLIBs on the basis of first-principles calculations. We demonstrate that B₂S delivers excellent electronic conductivity and has a unique "self-doping" effect by forming S or B vacancies. The ultrahigh energy densities of 2245 and 1167 mWh/g, a product of capacity and open-circuit voltage referenced by standard hydrogen electrode potential ( Cao Nat. Nanotechnology . 2019 , 14 , 200 - 207 ), could be achieved for the B₂S anode in Na- and K-ion batteries, respectively, significantly larger than those of graphene. More importantly, the B₂S presents graphene-like wettability toward commonly used electrolytes in Na- and K-ion batteries, i.e., the solvent molecules and metal salt, indicating excellent compatibility. Moreover, the minimum energy path for Na- and K-ion diffusion on the B₂S surface shows energy barriers of 0.19 and 0.04 eV, which indicates high ionic conductivity. Furthermore, a small contraction of the B₂S lattice upon ion intercalation has been observed due to the adsorption-induced corrugation of the electrode, which offsets the lattice expansion. The results suggest that the B₂S electrode can be used as a lightweight 2D Dirac anode material with excellent energy density, desirable rate performance, and robust wettability toward the electrolytes.

Topologically nontrivial phase and tunable Rashba effect in half-oxidized bismuthene

Bismuth based (Bi-based) materials exhibit promising potential for the study of two-dimensional (2D) topological insulators or quantum spin Hall (QSH) insulators due to their intrinsic strong spin-orbit coupling (SOC). Herein, we theoretically propose a new inversion-asymmetry topological phase with tunable Rashba effect in a 2D bismuthene monolayer, which is driven by the sublattices half-oxidation (SHO). The nontrivial topology is identified by the SHO induced p-p band inversion at the Γ point, the Z2 topological number, and the metallic edge states. Interestingly, the SOC opens a band gap as large as 0.26 eV at Γ, which is twice as large as that of the freestanding bismuthene monolayer, revealing a predominant contribution of the orbital filtering effect. Inversion-symmetry breaking leads to a substantial Rashba constant of 11.5 eV Å near the valence band top, which is about twice as large as that of the freestanding bismuthene monolayer due to the SHO effect. In particular, the topological insulator-to-topological semimetal phase-transition and the tunable Rashba effect were achieved by exerting a moderate strain. We demonstrate that 3% stretching is the most desirable strain to obtain superior properties. Hexagonal boron nitrogen (h-BN) is proposed to serve as a suitable substrate for SHO-Bi in practical applications. Our findings not only provide a new route to engineering a 2D inversion-asymmetry topological insulator but also represent a significant advance in the exploration of 2D Bi-based topological materials.

Rao CN. Arsenene nanosheets and nanodots

Liquid exfoliation of grey arsenic results in few-layer arsenene nanosheets and nanodots.

Emerging novel electronic structure in hydrogen-Arsenene-halogen nanosheets: A computational study

Based on first-principles calculations including spin-orbit coupling, we investigated the stability and electronic structure of unexplored double-side decorated arsenenes. It has been found that these new double-side decorated arsenenes, which we call "hydrogen-arsenene-halogen (H-As-X, X is halogen)", are dynamically stable via the phonon dispersion calculations except H-As-F sheets. In particular, all of H-As-X nanosheets are direct band gap semiconductors with a strong dispersion near the Fermi level, which is substantially different from the previous works of double-side decorated arsenenes with zero band gaps. Our results reveal a new route to change the band gap of arsenene from indirect to direct. Furthermore, we also studied bilayer, trilayer, and multilayer H-As-Cl sheets to explore the effects of the layer number. The results indicate that bilayer, trilayer, and multilayer H-As-Cl sheets display novel electronic structure, namely multi-Dirac cones character, and the Dirac character depends sensitively on the layer number. It is noted that the frontier states near the Fermi level are dominantly controlled by the top and bottom layers in trilayer and multilayer H-As-Cl sheets. Our findings may provide the valuable information about the new double-side decorated arsenene sheets in various practical applications in the future.

Intrinsic point defects in buckled and puckered arsenene: a first-principles study

Using first-principles calculations, we study the impact of various point defects on the structural, energetic, and electronic properties of arsenene.