Strain and orientation modulated bandgaps and effective masses of phosphorene nanoribbons

Gram-Scale Synthesis of Black Phosphorus through Shock-Induced Phase Transformation

This article presents the utilization of a cylindrical double-tube shock loading device in conjunction with amorphous red phosphorus as the precursor to investigate the complete phase transformation of red phosphorus into black phosphorus under the influence of shockwaves. Multiple experiments were conducted by varying the shock pressure and temperature parameters. The characterization of the recovered samples involved analysis of the phase composition and microstructure. The obtained experimental results demonstrate that within the cylindrical double-tube shock loading setup, a pressure of 9 GPa and a temperature of 800 K are optimal for achieving the complete phase transition of amorphous red phosphorus into orthorhombic black phosphorus. By precisely controlling these experimental conditions, a high-quality orthorhombic black phosphorus powder with excellent crystallinity was successfully prepared. This method offers several advantages, including simplicity, cost-effectiveness, and high yield. Consequently, this presents a promising pathway for the industrial-scale production of black phosphorus. The implementation of this approach not only reduces the cost involved in black phosphorus synthesis but also contributes to the broad range of applications for this material.

Comprehensive Insights into the Family of Atomically Thin 2D-Materials for Diverse Photocatalytic Applications

2D materials with their fascinating physiochemical, structural, and electronic properties have attracted researchers and have been used for a variety of applications such as electrocatalysis, photocatalysis, energy storage, magnetoresistance, and sensing. In recent times, 2D materials have gained great momentum in the spectrum of photocatalytic applications such as pollutant degradation, water splitting, CO2 reduction, NH3 production, microbial disinfection, and heavy metal reduction, thanks to their superior properties including visible light responsive band gap, improved charge separation and electron mobility, suppressed charge recombination and high surface reactive sites, and thus enhance the photocatalytic properties rationally as compared to 3D and other low-dimensional materials. In this context, this review spot-lights the family of various 2D materials, their properties and their 2D structure-induced photocatalytic mechanisms while giving an overview on their synthesis methods along with a detailed discussion on their diverse photocatalytic applications. Furthermore, the challenges and the future opportunities are also presented related to the future developments and advancements of 2D materials for the large-scale real-time photocatalytic applications.

A Theoretical Study of Armchair Antimonene Nanoribbons in the Presence of Uniaxial Strain Based on First-Principles Calculations

The optimized geometry and also the electronic and transport properties of passivated edge armchair antimonene nanoribbons (ASbNRs) are studied using ab initio calculations. Due to quantum confinement, the size of the bandgap can be modulated from 1.2 eV to 2.4 eV (indirect), when the width is reduced from 5 nm to 1 nm, respectively. This study focuses on nanoribbons with a width of 5 nm (5-ASbNR) due to its higher potential for fabrication and an acceptable bandgap for electronic applications. Applying uniaxial compressive and tensile strain results in a reduction of the bandgap of the 5-ASbNR film. The indirect to direct bandgap transition was observed, when introducing a tensile strain of more than +4%. Moreover, when a compressive strain above 9% is introduced, semi-metallic behavior can be observed. By applying compressive (tensile) strain, the hole (electron) effective mass is reduced, thereby increasing the mobility of charge carriers. The study demonstrates that the carrier mobility of ASbNR-based nanoelectronic devices can be modulated by applying tensile or compressive strain on the ribbons.

Effects of spin-orbit coupling on transmission and absorption of electromagnetic waves in strained armchair phosphorene nanoribbons

We compute the optical conductivity, both the imaginary and real parts of the dielectric constant, and the optical coefficients of armchair phosphorene nanoribbons under application of biaxial and uniaxial strains. The Kane-Mele model Hamiltonian has been applied to obtain the electronic band structure of phosphorene nanoribbons in the presence of a magnetic field. The effects of uniaxial and biaxial in-plane strain on the frequency behavior of the optical dielectric constant, and the frequency behavior of the optical absorption and refractive index of phosphorene nanoribbons have been studied, in terms of magnetic field, spin-orbit coupling and strain effects. Linear response theory and the Green's function approach have been exploited to obtain the frequency behavior of the optical properties of the structure. Moreover, the transmissivity and reflectivity of electromagnetic waves between two media separated by a phosphorene-nanoribbon layer are determined. Our numerical results indicate that the frequency dependence of the optical absorption includes a peak due to applying a magnetic field. Moreover, the effects of both in-plane uniaxial and biaxial strains on the refractive index of single-layer phosphorene have been addressed. Also, the frequency dependence of the transmissivity and reflectivity of electromagnetic waves between two media separated by armchair phosphorene nanoribbons for normal incidence has been investigated in terms of the effects of magnetic field and strain parameters. Both compressive and tensile strain have been considered for the armchair phosphorene nanoribbons in order to study the optical properties of the structure. In particular, the control of the optical properties of phosphorene nanoribbons could lead to extensive applications of phosphorene in the optoelectronics industry. Also, such a study of the optical properties of phosphorene nanoribbons has further applications in light sensors. Meanwhile, the effects of spin-orbit coupling on the optical absorption and transmissivity of electromagnetic waves in phosphorene nanoribbons could be a novel topic in condensed-matter physics.

Two dimensional Zr2CO2/H-FeCl2van der Waals heterostructures with tunable band gap, potential difference and magnetic anisotropy

Two dimensional (2D) van der Waals (vdW) heterostructures have potential applications in novel low dimensional spintronic devices due to their unique electronic and magnetic properties. Here, the electronic and magnetic properties of 2D Zr₂CO₂/H-FeCl₂heterostructures are calculated by first principles calculations. The 2D Zr₂CO₂/H-FeCl₂heterostructures are magnetic semiconductor. The electronic structure and magnetic anisotropy of Zr₂CO₂/H-FeCl₂heterostructure can be regulated by the biaxial strain and external electric field. The band gap and potential difference of Zr₂CO₂/H-FeCl₂heterostructure can be affected by in-plane biaxial strain. At a compressive strain of -8%, the Zr₂CO₂/H-FeCl₂heterostructure becomes metallic. All of the Zr₂CO₂/H-FeCl₂heterostructures are magnetic with in-plane magnetic anisotropy (IMA). The Zr₂CO₂/H-FeCl₂heterostructure is a semiconductor at the electric field from -0.5 V Å^-1to +0.5 V Å^-1. Furthermore, Zr₂CO₂/H-FeCl₂heterostructure shows IMA at the negative electric field, while it shows perpendicular magnetic anisotropy at the positive electric field. These results show that Zr₂CO₂/H-FeCl₂heterostructure has potential applications in multifunctionalnanoelectronic devices.

Ultratough Hydrogen-Bond-Bridged Phosphorene Films

The rapid development of flexible electronic devices, especially based on 2D materials, has triggered the demand for high-strength materials. Mono- or few-layer phosphorene with excellent electronic properties has attracted extensive attention. However, phosphorene is affected by its low Young's modulus when applied to flexible electronic devices. Here, a strategy via ion intercalation to significantly improve the mechanical properties of black phosphorus to generate hydrogen-bond-bridged phosphorene films with Young's modulus as high as 316 GPa is reported. This value is several times larger than the theoretical values of 166 GPa in the zigzag direction, 44 GPa in the armchair direction, and the averaged Young's modulus among all directions of 94 GPa. The impact of intercalation on mechanical properties is also explored. Experimental nanoindentation results obtained by atomic force microscopy indicate that the relationship between the ratio of intercalated ions to phosphorus atoms and the corresponding Young's modulus satisfies the formula E = e a e - [ ln ( x ) + b ] 2 c ( 0 < x ≤ 1.80 ) \[E{\bm{ = }}{e^{a{e^{\frac{{{\bm{ - }}{{[\ln (x){\bm{ + }}b]}^2}}}{c}}}}}(0{\bm{ &lt; }}x{\bm{ \le }}1.80)\] . Furthermore, a flexible NO₂ gas sensor device based on this ultratough material presents excellent performance, even after 10 000 bending cycles. The results provide new insight into the potential for practical applications of black phosphorus devices.

Enhanced Spin Thermopower in Phosphorene Nanoribbons via Edge-State Modifications

We investigated spin-dependent thermoelectric transport in zigzag phosphorene nanoribbons with a ferromagnetic stripe. We explored the possibility to enhance the spin thermopower via modifications of the edge states in zigzag ribbons. Two methods are proposed to modulate the edge transport: one is applying gate voltages on the edges; the other is including notches on the ribbon edges. The transport gap is enlarged by the edge-state modifications, which enhance the charge and spin Seebeck coefficients almost twofold. Our results suggest phosphorene to be a promising material for thermoelectric applications and open a possibility to design a tunable spin-thermoelectric device.

Strain Engineering: A Boosting Strategy for Photocatalysis

Whilst the photocatalytic technique is considered to be one of the most significant routes to address the energy crisis and global environmental challenges, the solar-to-chemical conversion efficiency is still far from satisfying practical industrial requirements, which can be traced to the suboptimal bandgap and electronic structure of photocatalysts. Strain engineering is a universal scheme that can finely tailor the bandgap and electronic structure of materials, hence supplying a novel avenue to boost their photocatalytic performance. Accordingly, to explore promising directions for certain breakthroughs in strained photocatalysts, an overview on the recent advances of strain engineering from the basics of strain effect, creations of strained materials, as well as characterizations and simulations of strain level is provided. Besides, the potential applications of strain engineering in photocatalysis are summarized, and a vision for the future controllable-electronic-structure photocatalysts by strain engineering is also given. Finally, perspectives on the challenges for future strain-promoted photocatalysis are discussed, placing emphasis on the creation and decoupling of strain effect, and the modification of theoretical frameworks.

Facile Production of Phosphorene Nanoribbons towards Application in Lithium Metal Battery

Like phosphorene, phosphorene nanoribbon (PNR) promises exotic properties but unzipping phosphorene into edge-defined PNR is non-trivial because of uncontrolled cutting of phosphorene along random directions. Here a facile electrochemical strategy to fabricate zigzag-edged PNRs in high yield (>80%) is reported. The presence of chemically active zigzag edges in PNR allows it to spontaneously react with Li to form a Li⁺ ion conducting Li₃ P phase, which can be used as a protective layer on Li metal anode in lithium metal batteries (LMBs). PNR protective layer prevents the parasitic reaction between lithium metal and electrolyte and promotes Li⁺ ion diffusion kinetics, enabling homogenous Li⁺ ion flux and long-time cycling stability up to 1100 h at a current density of 1 mA cm^-2 . LiFePO₄ |PNR-Li full-cell batteries with an areal capacity of 2 mAh cm^-2 , a lean electrolyte (20 µl mAh^-1 ) and a negative/positive (N/P) electrodes ratio of 3.5 can be stably cycled over 100 cycles.

Robust charge spatial separation and linearly tunable band gap of low-energy tube-edge phosphorene nanoribbon

Versatile applications have been proposed for phosphorene nanoribbons (PNRs), whose properties depend strongly on the edge structures. Recently, a unique tube-reconstruction at the zigzag edge (ZZ[Tube]) of PNRs was discovered to be the lowest configuration. Therefore, studies on PNRs should be reconsidered. In this paper, we systemically explore the width and strain effects on zigzag PNRs with different edge structures, including ZZ[Tube], ZZ and ZZ[ad] edges. ZZ PNRs always have small band gaps which are nearly independent of both width and strain. A remarkable band gap exists in ZZ[ad] PNRs which increases with a decrease in the ribbon width but is not sensitive to strain. In contrast, the band gaps of ZZ[Tube] PNRs change from 1.08 to 0.70 eV as the width increases from 12 to 65 Å. In addition, the band gaps of ZZ[Tube] PNRs show a linear response under a certain range of strain. In addition, the carrier effective masses (0.50 m ₀ for electrons and 0.94 m ₀ for holes) of ZZ[Tube] PNRs are much lower than for ZZ[ad], and the VBM and CBM charges are robustly spatially separated even under strains ranging from -5% to 5%. Their ease of formation, lowest energy, light effective mass, linear band gap response to strain and robust charge spatial separation provide ZZ[Tube] PNRs with potentially excellent performance in microelectronic and opto-electric applications.

Liquid-Exfoliated 2D Materials for Optoelectronic Applications

Two-dimensional (2D) materials have attracted tremendous research attention in recent days due to their extraordinary and unique properties upon exfoliation from the bulk form, which are useful for many applications such as electronics, optoelectronics, catalysis, etc. Liquid exfoliation method of 2D materials offers a facile and low-cost route to produce large quantities of mono- and few-layer 2D nanosheets in a commercially viable way. Optoelectronic devices such as photodetectors fabricated from percolating networks of liquid-exfoliated 2D materials offer advantages compared to conventional devices, including low cost, less complicated process, and higher flexibility, making them more suitable for the next generation wearable devices. This review summarizes the recent progress on metal-semiconductor-metal (MSM) photodetectors fabricated from percolating network of 2D nanosheets obtained from liquid exfoliation methods. In addition, hybrids and mixtures with other photosensitive materials, such as quantum dots, nanowires, nanorods, etc. are also discussed. First, the various methods of liquid exfoliation of 2D materials, size selection methods, and photodetection mechanisms that are responsible for light detection in networks of 2D nanosheets are briefly reviewed. At the end, some potential strategies to further improve the performance the devices are proposed.

Biaxial strain, electric field and interlayer distance-tailored electronic structure and magnetic properties of two-dimensional g-C3N4/Li-adsorbed Cr2Ge2Te6 van der Waals heterostructures

Recently, it has been proven that the biaxial strain (ε), electric field (E) and interlayer distance (d) can effectively modulate the electronic structure and magnetic properties of two-dimensional (2D) van der Waals (vdW) heterostructures, which have potential applications in spintronic devices. Here, the electronic structure and magnetic properties of 2D g-C3N4/Li-adsorbed Cr2Ge2Te6 vdW heterostructures are investigated using first-principles calculations. Their lattice structures are seriously affected by adsorption combination. With external stimulation, the band gap of the heterostructures changes. The heterostructures are metallic at ε = -6% and -4%, and others are n-type semiconductors, where the band gap is 23 meV at ε = 6%. In addition, the magnetic moments of g-C3N4 in the adsorption systems are in the range from 0.029 to 0.226 μB. The vdW heterostructures show in-plane magnetic anisotropy (IMA) at ε = -6%, -2% and 6% and perpendicular magnetic anisotropy (PMA) at ε = -4%, 0, 2% and 4%. On applying an electric field and changing the interlayer distance, the vdW heterostructures show PMA. These results are significant to the low-dimensional spintronic devices.

Edge reconstructions of black phosphorene: a global search

Despite reports of possible edge reconstructions of black phosphorene, the underlying mechanisms that determine the atomic configurations and appearance of black phosphorene edges have not been elucidated to date. In this study, the particle swarm optimization (PSO) algorithm is used to perform a global search of black phosphorene edge structures. In addition to the most stable edges, three databases of the typical black phosphorene zigzag edge, armchair edge, and skewed diagonal edge are constructed for the first time. The local phosphorus concentration plays an important role in determining the edge atomic configurations and the appearance of an edge. Variations in the local phosphorus concentration result in the rearrangement of sp3-hybrid bonds or the formation of double bonds that balance the dangling bonds at the edges and stabilize the black phosphorene edges. The black phosphorene edge databases provide a useful reference for edge studies of other 2D materials with puckered honeycomb structures.

Dual-Functional Phosphorene Nanocomposite Membranes for the Treatment of Perfluorinated Water: An Investigation of Perfluorooctanoic Acid Removal via Filtration Combined with Ultraviolet Irradiation or Oxygenation

Nanomaterials with tunable properties show promise because of their size-dependent electronic structure and controllable physical properties. The purpose of this research was to develop and validate environmentally safe nanomaterial-based approach for treatment of drinking water including removal and degradation of per- and polyfluorinated chemicals (PFAS). PFAS are surfactant chemicals with broad uses that are now recognized as contaminants with a significant risk to human health. They are commonly used in household and industrial products. They are extremely persistent in the environment because they possess both hydrophobic fluorine-saturated carbon chains and hydrophilic functional groups, along with being oleophobic. Traditional drinking water treatment technologies are usually ineffective for the removal of PFAS from contaminated waters, because they are normally present in exiguous concentrations and have unique properties that make them persistent. Therefore, there is a critical need for safe and efficient remediation methods for PFAS, particularly in drinking water. The proposed novel approach has also a potential application for decreasing PFAS background levels in analytical systems. In this study, nanocomposite membranes composed of sulfonated poly ether ether ketone (SPEEK) and two-dimensional phosphorene were fabricated, and they obtained on average 99% rejection of perfluorooctanoic acid (PFOA) alongside with a 99% removal from the PFOA that accumulated on surface of the membrane. The removal of PFOA accumulated on the membrane surface achieved 99% after the membranes were treated with ultraviolet (UV) photolysis and liquid aerobic oxidation.

Direct Measurement of Folding Angle and Strain Vector in Atomically Thin WS2 Using Second-Harmonic Generation

Structural engineering techniques such as local strain engineering and folding provide functional control over critical optoelectronic properties of 2D materials. Local strain engineering at the nanoscale level is practically achieved via permanently deformed wrinkled nanostructures, which are reported to show photoluminescence enhancement, bandgap modulation, and funneling effect. Folding in 2D materials is reported to tune optoelecronic properties via folding angle dependent interlayer coupling and symmetry variation. The accurate and efficient monitoring of local strain vector and folding angle is important to optimize the performance of optoelectronic devices. Conventionally, the accurate measurement of both strain amplitude and strain direction in wrinkled nanostructures requires the combined usage of multiple tools resulting in manufacturing lead time and cost. Here, we demonstrate the usage of a single tool, polarization-dependent second-harmonic generation (SHG), to determine the folding angle and strain vector accurately and efficiently in ultrathin WS₂. The folding angle in trilayer WS₂ folds exhibiting 1-9 times SHG enhancement is probed through variable approaches such as SHG enhancement factor, maxima and minima SHG phase difference, and linear dichroism. In compressive strain induced wrinkled nanostructures, strain-dependent SHG quenching and enhancement is observed parallel and perpendicular, respectively, to the direction of the compressive strain vector, allowing us to determine the local strain vector accurately using a photoelastic approach. We further demonstrate that SHG is highly sensitive to band-nesting-induced transition (C-peak), which can be significantly modulated by strain. Our results show SHG as a powerful probe to folding angle and strain vector.

Unzipping of black phosphorus to form zigzag-phosphorene nanobelts

Phosphorene, monolayer or few-layer black phosphorus, exhibits fascinating anisotropic properties and shows interesting semiconducting behavior. The synthesis of phosphorene nanosheets is still a hot topic, including the shaping of its two-dimensional structure into nanoribbons or nanobelts. Here we report electrochemical unzipping of single crystalline black phosphorus into zigzag-phosphorene nanobelts, as well as nanosheets and quantum dots, via an oxygen-driven mechanism. The experimental results agree well with our theoretical calculations. The calculation for the unzipping mechanism study suggests that interstitial oxygen-pairs are the critical intermediate species for generating zigzag-phosphorene nanobelts. Although phosphorene oxidation has been reported, lengthwise cutting is hitherto unreported. Our discovery of phosphorene cut upon oxidation represents a previously unknown mechanism for the formation of various dimensions of phosphorene nanostructures, especially zigzag-phosphorene nanobelts. It opens up a way for studying the quantum effects and electronic properties of zigzag-phosphorene nanobelts.

Here, the authors demonstrate the electrochemical unzipping of single crystalline black phosphorus into zigzag-phosphorene nanobelts, nanosheets, and quantum dots, via a top-down oxygen-driven mechanism.

Analytical study on strain tunable electronic structure and optical transitions in armchair black phosphorene nanoribbons

We theoretically investigate the electronic structure and optical absorption spectrum of armchair-edged black phosphorene nanoribbons (APNRs) with and without uniaxial strain based on the tight-binding Hamiltonian and Kubo formula. We analytically obtain the energy spectrum and wavefunction, and reveal the band gap scaling law as 1/(N + 1)² for APNRs in the presence and absence of uniaxial strain, where N is the number of armchair dimer across the ribbon. We find the band gap of APNRs linearly increases (decreases) with increasing in-plane uniaxial tensile (compressive) strain ɛ _x/y , but shows contrary dependence on the out-of-plane uniaxial strain ɛ _z . The effective mass versus strain exhibits the same behavior to that of band gap but with nonlinear dependence. Under an incident light linearly-polarized along the ribbon, we demonstrate that the inter-band optical transitions obey the selection rule Δn = n - n' = 0, but the intra-band transitions are forbidden for both pristine and strained APNRs originating from the orthogonality between the transverse wavefunctions of the sublattices belonging to different subbands. Importantly, the transverse electric field or impurities can release the optical selection rules by breaking the wavefunction orthogonality, which results in that the optical transitions between any subbands are all possible. Our findings provide further understanding on the electronic and optical properties of APNRs, which may pave the way for designing optoelectronic devices based on phosphorene.

Twistronics in tensile strained bilayer black phosphorus

In this work, by performing state-of-the-art first-principles methods combined with molecular dynamic (MD) simulation, we theoretically investigate the electronic and mechanical behaviours of small-angle twisted bilayer black phosphorus (tbBP) under uniaxial tensile deformation. Twistronics, namely the regulation of electronic properties by Moiré physics, is demonstrated as the gene - the most crucial factor dominating not only electronic behaviour but also mechanical behaviour of tensile deformed tbBP. Compared to untwisted few-layer black phosphorus (utBP) with strong electronic sensitivity to geometric deformation, the existence of Moiré patterns in tbBP leads to spatial electronic localization, giving rise to the conservation of direct band gaps and stability of phonon limited carrier mobility under tensile deformation along the armchair direction. Moreover, during the fracture failure process, the nucleation of micro-cracks is preferentially detected at the transitional pattern boundary areas in tbBP, which could be attributed to the intra-layer maldistribution of mechanical strengths in Moiré superlattices. The explorations of twistronics in tensile strained bilayer black phosphorus contribute to the better understanding of such Moiré superlattice structures and provide insights for the design of new 2D van der Waals heterostructures in flexible nano-electronic devices.

Stochastic many-body perturbation theory for Moiré states in twisted bilayer phosphorene

We implement stochastic many-body perturbation theory for systems with 2D periodic boundary conditions. The method is used to compute quasiparticle excitations in twisted bilayer phosphorene. Excitation energies are studied using stochastic [Formula: see text] and partially self-consistent [Formula: see text] approaches. The approach is inexpensive; it is used to study twisted systems with unit cells containing  >2700 atoms (>13 500 valence electrons), which corresponds to a minimum twisting angle of [Formula: see text] [Formula: see text]. Twisted bilayers exhibit band splitting, increased localization and formation of localized Moiré impurity states, as documented by band-structure unfolding. Structural changes in twisted structures lift band degeneracies. Energies of the impurity states vary with the twisting angle due to an interplay between non-local exchange and polarization effects. The mechanisms of quasiparticle energy (de)stabilization due to twisting are likely applicable to a wide range of low-dimensional Moiré superstructures.

Property-Activity Relationship of Black Phosphorus at the Nano-Bio Interface: From Molecules to Organisms

As a novel member of the two-dimensional nanomaterial family, mono- or few-layer black phosphorus (BP) with direct bandgap and high charge carrier mobility is promising in many applications such as microelectronic devices, photoelectronic devices, energy technologies, and catalysis agents. Due to its benign elemental composition (phosphorus), large surface area, electronic/photonic performances, and chemical/biological activities, BP has also demonstrated a great potential in biomedical applications including biosensing, photothermal/photodynamic therapies, controlled drug releases, and antibacterial uses. The nature of the BP-bio interface is comprised of dynamic contacts between nanomaterials (NMs) and biological systems, where BP and the biological system interact. The physicochemical interactions at the nano-bio interface play a critical role in the biological effects of NMs. In this review, we discuss the interface in the context of BP as a nanomaterial and its unique physicochemical properties that may affect its biological effects. Herein, we comprehensively reviewed the recent studies on the interactions between BP and biomolecules, cells, and animals and summarized various cellular responses, inflammatory/immunological effects, as well as other biological outcomes of BP depending on its own physical properties, exposure routes, and biodistribution. In addition, we also discussed the environmental behaviors and potential risks on environmental organisms of BP. Based on accumulating knowledge on the BP-bio interfaces, this review also summarizes various safer-by-design strategies to change the physicochemical properties including chemical stability and nano-bio interactions, which are critical in tuning the biological behaviors of BP. The better understanding of the biological activity of BP at BP-bio interfaces and corresponding methods to overcome the challenges would promote its future exploration in terms of bringing this new nanomaterial to practical applications.

Fabrication and Imaging of Monolayer Phosphorene with Preferred Edge Configurations via Graphene-Assisted Layer-by-Layer Thinning

Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plane properties. To fundamentally understand these various physical properties, it is critically important to conduct an atomic-scale structural investigation of phosphorene, particularly regarding various defects and preferred edge configurations. However, it has been challenging to investigate mono- and few-layer phosphorene because of technical difficulties arising in the preparation of a high-quality sample and damages induced during the characterization process. Here, we successfully fabricate high-quality monolayer phosphorene using a controlled thinning process with transmission electron microscopy and subsequently perform atomic-resolution imaging. Graphene protection suppresses the e-beam-induced damage to multilayer BP and one-side graphene protection facilitates the layer-by-layer thinning of the samples, rendering high-quality monolayer and bilayer regions. We also observe the formation of atomic-scale crystalline edges predominantly aligned along the zigzag and (101) terminations, which is originated from edge kinetics under e-beam-induced sputtering process. Our study demonstrates a new method to image and precisely manipulate the thickness and edge configurations of air-sensitive two-dimensional materials.

Transport characteristics of multi-terminal pristine and defective phosphorene systems

Atomic vacancies and nanopores act as local scattering centers and modify the transport properties of charge carriers in phosphorene nanoribbons (PNRs). We investigate the influence of such atomic defects on the electronic transport of multi-terminal PNR. We use the non-equilibrium Green's function approach within the tight-binding framework to calculate the transmission coefficient and the conductance. Terminals induce band mixing resulting in oscillations in the conductance. In the presence of atomic vacancies and nanopores the conductance between non-axial terminals exhibit constructive scattering, which is in contrast to mono-axial two-terminal systems where the conductance exhibits destructive scattering. This can be understood from the spatial local density of states of the transport modes in the system. Our results provide fundamental insights into the electronic transport in PNR-based multi-terminal systems and into the ability of atomic defects and nanopores through tuning the transport properties.

Strain engineering of the electronic and transport properties of monolayer tellurenyne

Two-dimensional (2D) materials exhibiting quality electronic properties such as suitable band gap, giant Rashba effect and high carrier mobility are essential for promising applications in electronics and spintronics. Strain engineering has been recognized as an effective strategy to engineer the atomic and electronic properties of 2D materials. Herein, based on density functional theory, we demonstrate that the electronic properties of tellurenyne can be tuned well by using uniaxial strain. We find that tellurenyne retains the unique noncovalent bond structure and exhibits good stability under the uniaxial strain. Meanwhile, the band gap of tellurenyne can be tuned to a large scale (0.33-1.18 eV and 0.73-1.27 eV under the uniaxial strain along and perpendicular to the chain direction, respectively). Under 10% tension strain along the chain direction, the Rashba constant reaches 2.96 eV Å, belonging to giant Rashba systems. More importantly, the hole mobility of tellurenyne along the chain direction reaches 1.1 × 105 cm2 V-1 s-1 under 10% tension strain along the chain direction, which is one order of magnitude larger than that of phosphorene. Therefore, these remarkable electronic properties of tellurenyne engineered by using strain indicate its potential applications in electronics and spintronics.

Edge-modulated dual spin-filter effect in zigzag-shaped buckling Ag2S nanoribbons

Unlike MoS2, single-layered Ag2S nanoribbons (Ag2SNRs) exhibit a nonmetal-shrouded and a zigzag-shaped buckling structure and possess two distinct edges, S- or Ag-terminated ones. By performing first principle calculations, the spin-dependent electron transport of Ag2SNRs in a ferromagnetic state has been investigated. It is found that the SS- and AgAg-terminated Ag2SNRs exhibit semi-metallic characteristics, but with opposite spin-polarized directions. And AgS-terminated ones show metallic characteristics, but with completely spin-unpolarized transmission. That is to say, all three states, i.e., spin up polarized, spin down polarized and spin unpolarized ones, could be achieved by modulating the edge geometry. Further analysis shows that, the spatial separation on edges of the energy states with different spins around EF is responsible for the switch in the three states. The system could operate as a dual spin-filter, and the direction of the spin polarization can be switched by the edge morphology. Furthermore, calculations show that such a phenomenon is robust to the width of the ribbon and strain, showing great application potential.

Optical interband transitions in strained phosphorene

In this paper, we have concentrated on the orbital and hybridization effects induced by applied triaxial strain on the interband optical conductivity (IOC) of phosphorene using a two-band Hamiltonian model, linear response theory and the Kubo formula. In particular, we study the dependence of the electronic band structure and of the IOC of a phosphorene single layer on the modulus and direction of the applied triaxial strain. The triaxial strain is included in a model through the introduction of strain-dependent hopping parameters using the Harrison rule. Among the various configurations for applying the triaxial strain, considerable findings are presented here in three classes: (i) uniform, (ii) in-plane uniform and (iii) non-uniform triaxial strain. The main consequence of applying triaxial strain is that of increasing and decreasing the band gap depending on the considered class of study, resulting in a blue shift and red shift of the interband optical transitions, respectively. Our results show that a pure blue shift independent of the strain modulus as well as strain sign (tensile or compressive) emerges when applying non-uniform triaxial strain. The overall feature of our outcomes is tailoring the edge-dependent optical responses of phosphorene in the presence of triaxial strain, which provides the required conditions of tuning the optical properties of phosphorene for future experimental research.

Recent progress in 2D group IV-IV monochalcogenides: synthesis, properties and applications

Coordination-related, 2D structural phase transitions are a fascinating facet of 2D materials with structural degeneracy. Phosphorene and its new phases, exhibiting unique electronic properties, have received considerable attention. The 2D group IV-IV monochalcogenides (i.e. GeS, GeSe, SnS and SnSe) like black phosphorous possess puckered layered orthorhombic structure. The 2D group IV-IV monochalcogenides with advantages of earth-abundance, less toxicity, environmental compatibility and chemical stability, can be widely used in optoelectronics, piezoelectrics, photodetectors, sensors, Li-batteries and thermoelectrics. In this review, we summarized recent research progress in theory and experiment, which studies the fundamental properties, applications and fabrication of 2D group IV-IV monochalcogenides and their new phases, and brings new perspectives and challenges for the future of this emerging field.

From a volatile molecular precursor to twin-free single crystals of bismuth

A highly volatile molecular precursor, dibismuth(ii) (tetra)trifluoroacetate, was successfully utilized for the gas phase growth of twin-free single crystals of bismuth. This allowed high-quality single-crystal X-ray diffraction data to be collected, providing accurate structural parameters for elemental bismuth under ambient conditions.

Electronic Properties of Armchair Black Phosphorene Nanoribbons Edge-Modified by Transition Elements V, Cr, and Mn

The structural, electrical, and magnetic properties of armchair black phosphorene nanoribbons (APNRs) edge-functionalized by transitional metal (TM) elements V, Cr, and Mn were studied by the density functional theory combined with the non-equilibrium Green's function. Spin-polarized edge states introduce great varieties to the electronic structures of TM-APNRs. For APNRs with Mn-stitched edge, their band structures exhibit half-semiconductor electrical properties in the ferromagnetic state. A transverse electric field can then make the Mn-APNRs metallic by shifting the conduction bands of edge states via the Stark effect. The Mn/Cr-APNR heterojunction may be used to fabricate spin p-n diode where strong rectification acts only on one spin.

Direct Observation of Raman Spectra in Black Phosphorus under Uniaxial Strain Conditions

In this paper, we systematically studied the Raman vibration of black phosphorus (BP) transferred onto a germanium (Ge)-coated polydimethylsiloxane (PDMS) substrate, which generates a much higher contrast in BP. This engineered flexible substrate allowed us to directly observe a much thinner BP layer on the flexible substrate at the desired location. Therefore, it enabled us to perform Raman spectroscopy immediately after exfoliation. The Raman spectra obtained from several BP layers with different thicknesses revealed that the clear peak shifting rates for the A_g¹, B_2g, and A_g² modes were 0.15, 0.11, and 0.11 cm^-1/nm, respectively. Using this value to identify a 2⁻3-layered BP, a study on the strain⁻Raman spectrum relationship was conducted, with a maximum uniaxial strain of 0.89%. The peak shifting of A_g¹, B_2g, and A_g² caused by this uniaxial strain were measured to be 0.86, 0.63, and 0.21 cm^-1/Δε, respectively.

Surface engineering of phosphorene nanoribbons by transition metal heteroatoms for spintronics

Modulating the electronic and magnetic properties of phosphorene is important for fabricating multi-functional electronic and spintronic devices. Employing density functional theory combined with the non-equilibrium Green's function, we systematically investigate the electronic, magnetic and transport properties of hydrogenated armchair phosphorene nanoribbons chemically modified by 3d transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co and Ni). With chemical adsorption of transition metal atoms, the phosphorene nanoribbons exhibit excellent spin-polarized transport properties. A giant magnetoresistance effect is found with Ti, Fe and Mn adsorption, in which ratios higher than 102 for the Ti and Mn cases, and 105 for the Fe case, are exhibited. Moreover, in the bias range of (-0.2 V, 0.2 V), the Ti, V, Mn and Fe-adsorbed nanoribbons with parallel spin configurations demonstrate a remarkable bias-independent spin filtering efficiency at about 100%, while the Fe and Mn-adsorbed nanoribbons with antiparallel spin configuration show a dual spin filtering effect. The spin-polarized electronic transport properties are closely related to the band structures. Remarkable spin-polarization of the current occurs when the dispersed and flat bands near the Fermi level originate from different spin orientations. The magnetic moments of transition metal adatoms on nanoribbons are reduced by 0.2-2 μB relative to the isolated atoms due to electron rearrangement and charge transfer, which results in various degrees of spin polarization. These results provide a fundamental understanding of the electronic, magnetic and transport properties of transition metal modified hydrogenated armchair phosphorene nanoribbons, and suggest a referential approach to manufacture spintronic devices based on phosphorene.

Electronic and magnetic properties of a black phosphorene/Tl2S heterostructure with transition metal atom intercalation: a first-principles study

Using density functional theory calculations, the structural, electronic and magnetic properties of a black phosphorene/Tl₂S heterostructure (BP/Tl₂S) and the BP/Tl₂S intercalated with transition metal atoms (TMs) have been detailed investigated. It is demonstrated that the BP/Tl₂S is a type-I van der Waals (vdW) heterostructure with an indirect band gap of approximately 0.79 eV. The BP/Tl₂S experiences a transition from type-I to type-II when various strains are applied. In addition, the BP/Tl₂S intercalated with TMs (TM-BP/Tl₂S) exhibits various kinds of meaningful electronic and magnetic properties. Several TM-BP/Tl₂S systems are still non-magnetic ground states and six TM-BP/Tl₂S (Ti-, V-, Cr-, Mn-, Fe-, Tc-) systems are ferromagnetic. Interestingly, three TM-BP/Tl₂S (V-, Cr-, Mn-) systems display half-metallic character. The Fe-BP/Tl₂S and Tc-BP/Tl₂S are dilute magnetic semiconductors (DMSs), while TM-BP/Tl₂S (Mo-, Pd-, Ni-) systems are semiconductors. The other TM-BP/Tl₂S systems become metals. These results may open a new avenue for application of the BP/Tl₂S in future spintronic and electronic devices.

Using density functional theory calculations, the structural, electronic and magnetic properties of a black phosphorene/Tl₂S heterostructure (BP/Tl₂S) and the BP/Tl₂S intercalated with transition metal atoms (TMs) have been detailed investigated.

Tuning the electronic and magnetic properties of antimonene nanosheets via point defects and external fields: first-principles calculations

Defects are inevitably present in materials, and their existence in a material strongly affects its fundamental physical properties.

Recent Advances in Synthesis and Applications of 2D Junctions

Recent progress in the methods of integration of 2D materials is reviewed. Integrated 2D circuits are one of the most promising candidates for advanced electronics and flexible devices. Specifically, methods such as mechanical transfer, chemical vapor deposition growth, high temperature conversion, phase engineering, surface doping, electrostatic doping, and so on to fabricate 2D heterostructures are discussed in detail. Applications of these integrated 2D heterostructures in p-n junctions, ohmic contact, high-performance transistors, and phototransistors are also highlighted. Finally, challenges and opportunities of methods to integrate 2D materials are proposed.

Highly Anisotropic GeSe Nanosheets for Phototransistors with Ultrahigh Photoresponsivity

2D GeSe possesses black phosphorous-analog-layered structure and shows excellent environmental stability, as well as highly anisotropic in-plane properties. Additionally, its high absorption efficiency in the visible range and high charge carrier mobility render it promising for applications in optoelectronics. However, most reported GeSe-based photodetectors show frustrating performance especially in photoresponsivity. Herein, a 2D GeSe-based phototransistor with an ultrahigh photoresponsivity is demonstrated. Its optimized photoresponsivity can be up to ≈1.6 × 105 A W-1. This high responsivity can be attributed to the highly efficient light absorption and the enhanced photoconductive gain due to the existence of trap states. The exfoliated GeSe nanosheet is confirmed to be along the [001] (armchair direction) and [010] (zigzag direction) using transmission electron microscopy and anisotropic Raman characterizations. The angle-dependent electric and photoresponsive performance is systematically explored. Notably, the GeSe-based phototransistor shows strong polarization-dependent photoresponse with a peak/valley ratio of 1.3. Furthermore, the charge carrier mobility along the armchair direction is measured to be 1.85 times larger than that along the zigzag direction.

Extracting the Energy Sensitivity of Charge Carrier Transport and Scattering

It is a challenge to extract the energy sensitivity of charge carriers’ transport and scattering from experimental data, although a theoretical estimation in which the existing scattering mechanism(s) are preliminarily assumed can be easily done. To tackle this problem, we have developed a method to experimentally determine the energy sensitivities, which can then serve as an important statistical measurement to further understand the collective behaviors of multi-carrier transport systems. This method is validated using a graphene system at different temperatures. Further, we demonstrate the application of this method to other two-dimensional (2D) materials as a guide for future experimental work on the optimization of materials performance for electronic components, Peltier coolers, thermoelectricity generators, thermocouples, thermopiles, electrical converters and other conductivity and/or Seebeck-effect-related sensors.

Low-bias negative differential resistance in junction of a benzene between zigzag-edged phosphorene nanoribbons

We study the electron transport properties through the junction of a benzene molecule in conjunction with two monolayer zigzag-edged phosphorene nanoribbon (ZPNR) electrodes by applying the nonequilibrium Green's functions in combination with the density functional theory. We find that the molecular junction with two phosphorus-carbon bonds exhibits an interesting low-bias negative differential resistance effect with a peak-to-valley ratio of 29, which originates from the edge states in ZPNR due to the anisotropic band structure of phosphorene. Importantly, the performance of the junction can be tuned via the molecule-ZPNR interface bonding. The findings may be useful in sensitive-device applications. Furthermore, the physical mechanisms are revealed and discussed in terms of the electronic transmission spectrum, the evolution of the frontier molecular orbitals, the local device density of states around the Fermi level, and the projected density of states.

Bandgap scaling and negative differential resistance behavior of zigzag phosphorene antidot nanoribbons (ZPANRs)

This work examines the prospect of phosphorene antidot nanoribbons (PANRs) using the density functional based tight binding (DFTB) method. Horizontally perforated PANRs with both armchair (A) and zigzag (Z) configurations were considered for electrical simulations. Our simulation results found that the APANRs cannot be scaled down with nanoribbon width, whereas ZPANRs can be scaled easily. Bandgap scaling in terms of ribbon width, length and antidot number was thoroughly analyzed for ZPANRs. In the end, a two-terminal device was constructed and transmission analysis was performed using the non-equilibrium Green's function (NEGF) methodology. A negative differential resistance (NDR) region appeared in the current-voltage characteristics of the ZPANRs, which paved a pathway for nano-device application.

Plasmonics with two-dimensional semiconductors: from basic research to technological applications

Herein, we explore the main features and the prospect of plasmonics with two-dimensional semiconductors. Plasmonic modes in each class of van der Waals semiconductors have their own peculiarities, along with potential technological capabilities. Plasmons of transition-metal dichalcogenides share features typical of graphene, due to their honeycomb structure, but with damping processes dominated by intraband rather than interband transitions, unlike graphene. Spin-orbit coupling strongly affects the plasmonic spectrum of buckled honeycomb lattices (silicene and germanene), while the anisotropic lattice of phosphorene determines different propagation of plasmons along the armchair and zigzag directions. Black phosphorus is also a suitable material for ultrafast plasmonics, for which the active plasmonic response can be initiated by photoexcitation with femtosecond pulses. We also review existing applications of plasmonics with two-dimensional materials in the fields of thermoplasmonics, biosensing, and plasma-wave Terahertz detection. Finally, we consider the capabilities of van der Waals heterostructures for innovative low-loss plasmonic devices.

Reactive molecular dynamics simulations of the mechanical properties of various phosphorene allotropes

Although various phosphorene allotropes have been theoretically predicted to be stable at 0 K, the mechanical properties and fracture mechanism at room temperature remain unclear for many of them. We investigate through reactive molecular dynamics simulations at room temperature the mechanical properties of phosphorene allotropes including: five sheets with hexagonal structures (β-, γ-, δ-, θ-, and α-phosphorene), one sheet with 4-8 membered rings (4-8-P), and two sheets with 5-7 membered rings. High, moderate and slight anisotropies in their mechanical properties are observed, depending on their crystal structures. Their Young's moduli and tensile strength are approximately in the range from 7.3% through 25%, and from 8.6% through 22% of those of graphene, respectively. At the early stage of fracture, eye-shaped cracks are formed by local bond breaking and perpendicular to the tensile direction in hexagonal and 4-8-P sheets. Complete fractures take place with straight cracks in these hexagonal sheets under tension along the zigzag direction and under tension along the square edge direction in the 4-8-P sheet. Crack meandering and branching are observed during the tension of α-, β-, and γ-phosphorene along the armchair direction; and along the square diagonal direction in the 4-8-P sheet. Under uniaxial tension of two phosphorene sheets with 5-7 atom rings, 12 and 10 membered rings are formed by merging two neighbor heptagons, and a heptagon and its neighbor pentagon, respectively. These 12 and 10 membered rings coalesce subsequently, causing the failure of these two sheets. The results are of great importance in the design of these novel phosphorene allotropes.

Robust ferroelectricity in two-dimensional SbN and BiP

Based on first-principles calculations, we discover two new two-dimensional (2D) ferroelectric materials SbN and BiP. Both of them are stable in a phosphorene-like structure and maintain their ferroelectricity above room temperature. Till date, SbN has the largest in-plane spontaneous polarization of about 7.81 × 10-10 C m-1 ever found in 2D ferroelectric materials, and it can retain its ferroelectricity until melting at about 1700 K. The spontaneous polarizations and switching barriers can easily be tuned by strains. Additionally, the ferroelectricity can still be maintained in their multilayers. These advantages make SbN and BiP promising candidate materials for future integrated ferroelectric devices.

Closed-edged bilayer phosphorene nanoribbons producing from collapsing armchair phosphorene nanotubes

A new phosphorous allotrope, closed-edged bilayer phosphorene nanoribbon, is proposed via radially deforming armchair phosphorene nanotubes. Using molecular dynamics simulations, the transformation pathway from round PNTs falls into two types of collapsed structures: arc-like and sigmoidal bilayer nanoribbons, dependent on the number of phosphorene unit cells. The fabricated nanoribbions are energetically more stable than their parent nanotubes. It is also found via ab initio calculations that the band structure along tube axis substantially changes with the structural transformation. The direct-to-indirect transition of band gap is highlighted when collapsing into the arc-like nanoribbons but not the sigmoidal ones. Furthermore, the band gaps of these two types of nanoribbons show significant size-dependence of the nanoribbon width, indicative of wider tunability of their electrical properties.