A new type of stable borophene with flat-band-induced magnetism

Based on first-principles calculations, we propose a new type of thermally and dynamically stable magnetic borophene (B11) with a tetragonal lattice. The magnetism is found coming from spin polarization of one bonding flat band located at the Fermi level. Despite of the 'anti-molecular' behavior in the monolayer, the interactions between thepzorbitals of the B atoms in the double-octahedron structural unit lead to the formation of the flat bands with localization behaviors. One tight binding model is built to comprehend the magnetic mechanism, which can guide us to tune other nonmagnetic borophene becoming magnetic. Biaxial tensile strain (>2.1%) is found triggering a phase transition from a semimetal to a semiconductor in the B11monolayer. The mechanism is analyzed based on the orbital-resolved crystal field effect. Our work provides a new route for designing and achieving two-dimensional magnetic materials with light elements.

δ-SnS: An Emerging Bidirectional Auxetic Direct Semiconductor with Desirable Carrier Mobility and High-Performance Catalytic Behavior toward the Water-Splitting Reaction

We propose a novel two-dimensional SnS allotrope (monolayer δ-SnS) based on an auxetic δ-phosphorene configuration using first-principles calculations. This monolayer appears to have outstanding stability as revealed by its energetic, kinetic, thermodynamic, and mechanic calculations, and it can withstand temperatures as high as 900 K. Monolayer δ-SnS is a wide direct-bandgap (2.354 eV) semiconductor, and its electron mobility is as high as ∼1.25 × 10³ cm² V^-1 s ^-1, higher than that of monolayer KTlO (∼450 cm² V^-1 s^-1) and MoS₂ (∼200 cm² V^-1 s^-1). Optical absorption spectra, reaching up to the order of ∼10⁵ cm^-1, are obviously excellent in the visible-light region, suggesting efficient harvesting of solar radiation. Because of its unique atomic motif, monolayer δ-SnS presents an unusual bidirectional auxetic effect: a high negative in-plane Poisson's ratio (-0.048 and -0.068), which is larger than those for many recently reported two-dimensional auxetic materials, e.g., black phosphorene (-0.027), borophene (-0.04), and monolayer penta-B₂N₄ (-0.02). The bandgap and band edge can be substantially manipulated under strain to meet the requirement of the water-splitting reaction. Particularly, when pH = 7, suitable band-edge alignments and small overpotentials of the photocatalytic OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) appear, endowing monolayer δ-SnS with great potential as an efficient visible-light-driven bifunctional photocatalyst for water splitting.

Vibrational properties and Raman spectra of pristine and fluorinated blue phosphorene

Using density functional theory, we investigated the vibrational properties and Raman spectra of pristine blue phosphorene and fluorinated blue phosphorene. The fluorinated blue phosphorene possesses a Dirac cone at the K point (about 310 cm-1). The shape of the Dirac cone remains unchanged under different tensile strains. The Raman tensor and thus angle-dependent Raman intensities of all Raman active modes are calculated for the polarizations of scattered light parallel and perpendicular to that of the incident light. The characteristics of angle-dependent Raman intensities are discussed. Moreover, the polarization direction averaged non-resonant Raman spectra of pristine blue phosphorene and fluorinated blue phosphorene are compared with that of germanene and black phosphorene.

Stability and carrier transport properties of phosphorene-based polymorphic nanoribbons

Few-layer black phosphorene has recently attracted significant interest in the scientific community. In this paper, we consider several polymorphs of phosphorene nanoribbons (PNRs) and employ deformation potential theory within the effective mass approximation, together with density functional theory, to investigate their structural, mechanical and electronic properties. The results show that the stability of a PNR strongly depends on the direction along which it can be cut from its 2D counterpart. PNRs also exhibit a wide range of line stiffnesses ranging from 6 × 10¹⁰ eV m^-1 to 18 × 10¹¹ eV m^-1, which has little dependence on the edge passivation. Likewise, the calculated electronic properties of PNRs show them to be either a narrow-gap semiconductor (E _g < 1 eV) or a wide-gap semiconductor (E _g > 1 eV). The carrier mobility of PNRs is found to be comparable to that of black phosphorene. Some of the PNRs show an n-type (p-type) semiconducting character owing to their higher electron (hole) mobility. Passivation of the edges leads to n-type ↔ p-type transition in many of the PNRs considered. The predicted novel characteristics of PNRs, with a wide range of mechanical and electronic properties, make them potentially suitable for use in nanoscale devices.

Tunnelling characteristics of Stone-Wales defects in monolayers of Sn and group-V elements

Topological defects in ultrathin layers are often formed during synthesis and processing, thereby strongly influencing the electronic properties of layered systems. For the monolayers of Sn and group-V elements, we report the results based on density functional theory determining the role of Stone-Wales (SW) defects in modifying their electronic properties. The calculated results find the electronic properties of the Sn monolayer to be strongly dependent on the concentration of SW defects, e.g. defective stanene has nearly zero band gap (≈0.03 eV) for the defect concentration of 2.2  ×  10¹³ cm^-2 which opens up to 0.2 eV for the defect concentration of 3.7  ×  10¹³ cm^-2. In contrast, SW defects appear to induce conduction states in the semiconducting monolayers of group-V elements. These conduction states act as channels for electron tunnelling, and the calculated tunnelling characteristics show the highest differential conductance for the negative bias with the asymmetric current-voltage characteristics. On the other hand, the highest differential conductance was found for the positive bias in stanene. Simulated STM topographical images of stanene and group-V monolayers show distinctly different features in terms of their cross-sectional views and distance-height profiles. These distinctive features can serve as fingerprints to identify the topological defects in experiments for the monolayers of group-IV and group-V elements.

van der Waals heterostructures based on allotropes of phosphorene and MoSe2

The van der Waals heterostructures of allotropes of phosphorene (α- and β-P) with MoSe₂ (H-, T-, ZT- and SO-MoSe₂) are investigated in the framework of state-of-the-art density functional theory. The semiconducting heterostructures, β-P/H-MoSe₂ and α-P/H-MoSe₂, form anti-type structures with type I and type II band alignments, respectively, whose bands are tunable with an external electric field. α-P/ZT-MoSe₂ and α-P/SO-MoSe₂ form ohmic semiconductor-metal contacts while the Schottky barrier in β-P/T-MoSe₂ can be reduced to zero by an external electric field to form ohmic contacts which is useful to realize high-performance devices. Simulated STM images of the given heterostructures reveal that α-P can be used as a capping layer to differentiate between various allotropes of underlying MoSe₂. The dielectric response of the considered heterostructures is highly anisotropic in terms of lateral and vertical polarization. The tunable electronic and dielectric response of van der Waals phosphorene/MoSe₂ heterostructures may find potential applications in the fabrication of optoelectronic devices.

Size and strain tunable band alignment of black-blue phosphorene lateral heterostructures

Single-element lateral heterostructures composed of black and blue phosphorene are not only free from lattice mismatch but also exhibit rich physical properties related to the seamlessly stitched interfaces, providing the building blocks for designing atomically thin devices. Using first-principles calculations, we investigate the influence of interface structure, size effect and strain engineering on the electronic structure, effective masses and band alignment of black-blue phosphorene lateral heterostructures. The lateral heterostructure with an octatomic-ring interface presents a strong metallic feature due to the interface states, while a metal-semiconductor transition takes place in the system with a hexatomic-ring interface upon hydrogen passivation. Following a reciprocal scaling law, the band gap is tuned in a wide energy range by synchronously increasing the widths of black and blue phosphorene or by only widening that of black phosphorene. Moreover, type-II band alignment is observed in the width ranges of 2.0-3.1 nm and 3.7-4.2 nm, out of which it is type-I. However, the band gap and effective masses show small changes if only the width of blue phosphorene is altered. When the lateral heterostructure is tensile loaded, the effective mass ratio of hole to electron is enlarged by an order of magnitude at a strain of 4% along the zigzag direction. Meanwhile, the band alignment undergoes a crossover from type-I to type-II at a strain of 2%, facilitating efficient electron-hole separation for light detection and harvesting.

Ultra-narrow blue phosphorene nanoribbons for tunable optoelectronics

We report optoelectronic properties of ultra-narrow blue phosphorene nanoribbons (BPNRs) within the state-of-the-art density functional theory framework.

Electronic properties of layered phosphorus heterostructures

By using ab initio approaches, the electronic properties of vertical heterostructured compounds of different structural phases of layered phosphorus have been studied.

Structural, elastic, electronic, and optical properties of the tricycle-like phosphorene

Tricycle-like phosphorenes with good structural stability, indirect band gaps, flexible properties, and good visible light absorption properties hold great promise for applications in the field of visible light harvesting and flexible nanoelectronic devices.