The structure and elastic properties of phosphorene edges

DFT-based finite element analysis of compressive response in armchair phosphorene nanotubes

In this paper, the finite element method is utilized to evaluate the behavior of the armchair phosphorene nanotubes under the compressive loading. The energy equations of the molecular and structural mechanics are used to obtain the elemental properties. The critical compressive forces of various armchair phosphorene nanotubes are computed with clamped-clamped and clamped-free boundary conditions. Results show that the stability of armchair phosphorene nanotubes increases with increasing nanotube aspect ratio, particularly under clamped-clamped boundary conditions. Finally, the buckling mode shapes of armchair phosphorene nanotubes under different boundary conditions are compared. Our work offers valuable insights into how these nanotubes respond to mechanical stress, helps determine elemental properties, and investigates the effects of nanotube geometry and different boundary conditions on their stability. This knowledge has broad applications in fields like nanotechnology, materials science, and nanomechanics, advancing the understanding of nanoscale materials and their potential for various practical uses.

Edge-Confined Excitons in Monolayer Black Phosphorus

Quantum confinement of two-dimensional excitons in van der Waals materials via electrostatic trapping, lithographic patterning, Moiré potentials, and chemical implantation has enabled significant advances in tailoring light emission from nanostructures. While such approaches rely on complex preparation of materials, natural edges are a ubiquitous feature in layered materials and provide a different approach for investigating quantum-confined excitons. Here, we observe that certain edge sites of monolayer black phosphorus (BP) strongly localize the intrinsic quasi-one-dimensional excitons, yielding sharp spectral lines in photoluminescence, with nearly an order of magnitude line width reduction. Through structural characterization of BP edges using transmission electron microscopy and first-principles GW plus Bethe-Salpeter equation (GW-BSE) calculations of exemplary BP nanoribbons, we find that certain atomic reconstructions can strongly quantum-confine excitons resulting in distinct emission features, mediated by local strain and screening. We observe linearly polarized luminescence emission from edge reconstructions that preserve the mirror symmetry of the parent BP lattice, in agreement with calculations. Furthermore, we demonstrate efficient electrical switching of localized edge excitonic luminescence, whose sites act as excitonic transistors for emission. Localized emission from BP edges motivates exploration of nanoribbons and quantum dots as hosts for tunable narrowband light generation, with future potential to create atomic-like structures for quantum information processing applications as well as exploration of exotic phases that may reside in atomic edge structures.

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.

Self-passivation leads to semiconducting edges of black phosphorene

The edges of black phosphorene (BP) have been extensively explored. The previous experimental observations that all the BP edges are semiconducting implies that the as-cut edges of BP tend to be reconstructed. Here we present a global structural search of three typical BP edges, namely armchair, zigzag and zigzag-1 edges. It is found that all the three pristine edges are metastable, and all of them can be quickly self-passivated by (i) forming P[double bond, length as m-dash]P double bonds (one σ and one π bond), (ii) reconstructing new polygonal rings will all P atoms bonded with three sp³ bonds or (iii) forming a special P(2)-P(4) configuration with a two-coordinated P atom accommodating two lone pair electrons and one four-coordinated P atom without lone pair electrons. Highly different from the pristine edges, all these highly stable reconstructed edges are semiconducting. This study showed that the reconstruction of the edges of a 2D material, just like the surfaces of a 3D crystal, must be considered for both fundamental studies and practical applications. Besides BP, this study also sheds light on the structures and properties of the edges of many other 2D materials.

Effect of surface oxidation on the electronic transport properties of phosphorene gas sensors: a computational study

The potential for phosphorene-based devices has been compromised by the material's fast degradation under ambient conditions. Its tendency to fully oxidize under O₂-rich and humid environments, leads to the loss of its appealing semiconducting properties. However, partially-oxidized phosphorene (po-phosphorene), has been demonstrated to remain stable over significantly longer periods of time, thereby enabling its use in sensing applications. Here, we present a computational study of po-phosphorene-based gas sensors, using the Density-Functional-based Tight Binding (DFTB) method. We show that DFTB accurately predicts the bandgap for the pristine material and po-phosphorene, the electronic transport properties of po-phosphorene at different surface oxygen concentrations, and the appropriate trends in Density-of-States (DOS) contributions caused by adsorbed gas molecules, to demonstrate its potential application in the development of gas sensors. Results are compared against the more traditional and expensive Density Functional Theory (DFT) method using generalized gradient approximation (GGA) exchange–correlation functionals, which significantly underestimates the material's bandgap.

Computational study of surface oxidation effects on phosphorene-based gas sensors, and potential for nM L⁻¹ detection and measurement of nitrogen–oxygen moieties.

Initial Relative Position Influencing Self-Assembly of a Black Phosphorus Ribbon on a CNT

It is difficult to obtain a nanotube from phosphorus with a 3sp² electron configuration by chemical synthesis. However, a physical fabrication approach, such as self-assembly, is worth trying. In an experiment, when using a carbon nanotube (CNT) to trigger self-assembly of a black phosphorus (BP) ribbon, the final configuration of the BP component may be sensitive to the initial relative position of the CNT to the BP ribbon. For instance, using the same CNT with different initial relative positions to the BP ribbon, the BP ribbon may finally become a nanotube, or a scroll, or just wind upon the CNT, or escape from the CNT, etc. In this study, the sensitivity is investigated using molecular dynamics simulations. Numerical results illustrate some essentials for potential fabrication of a BP nanotube from ribbon.

Mechanical properties of pristine and defective carbon-phosphide monolayers: a density functional tight-binding study

Using density functional tight-binding theory, we investigated the elastic properties and deformation and failure behaviors of pristine and defective carbon-phosphide (CP) monolayers subjected to uniform uniaxial tensile strain along arm-chair (AC) and zig-zag (ZZ) directions. Two variants of CP (α-CP and β-CP) were studied and two types of carbon and phosphorous vacancies (single and double) were considered. It was found that carbon monovacancies have the lowest formation energy, while phosphorous divacancies have the highest one in both CP allotropes. A strong mechanical anisotropy for CP was found with the Young's modulus and the failure stress along ZZ direction being an order of magnitude larger than those along AC direction. In both allotropes, the Young's modulus, failure stress and strain are considerably affected by vacancies, especially along AC direction. Fracture of pristine CP monolayer occurred via the rupture of phosphorous-phosphorous bonds when CP monolayer is stretched along AC direction, while via the rupture of carbon-phosphorous bonds when stretched along ZZ direction. Defective α-CP and β-CP monolayers both undergo a brittle-like failure initiated around the hosted vacancies at a lower critical strain. The failure strain and stress along the AC direction are affected only by phosphorous vacancies, while along the ZZ direction, they are almost equally affected by both phosphorous and carbon vacancies. These understandings may provide useful guidelines for potential applications of CP monolayers in nanoelectromechanical systems.

Effect of vacancies on the mechanical properties of phosphorene nanotubes

Using density functional tight-binding method, we studied the mechanical properties, deformation and failure of armchair (AC) and zigzag (ZZ) phosphorene nanotubes (PNTs) with monovacancies and divacancies subjected to uniaxial tensile strain. We found that divacancies in AC PNTs and monovacancies in ZZ PNTs possess the lowest vacancy formation energy, which decreases with the tube diameter in AC PNTs and increases in ZZ PNTs. The Young's modulus is reduced, while the radial and thickness Poisson's ratios are increased by hosted vacancies. In defective AC PNTs, deformation involves fracture of the intra-pucker bonds and formation of the new inter-pucker bonds at a critical strain, and the most stretched bonds around the vacancy rupture first, triggering a sequence of the structural transformations terminated by the ultimate failure. The critical strain of AC PNTs is reduced significantly by hosted vacancies, whereas their effect on the critical stress is relatively weaker. Defective ZZ PNTs fail in a brittle-like manner once the most stretched bonds around a vacancy rupture, and vacancies are able to significantly reduce the failure strain but only moderately reduce the failure stress of ZZ PNTs. The understandings revealed here on the mechanical properties and the deformation and failure mechanisms of PNTs provide useful guidelines for their design and fabrication as building blocks in nanodevices.

Probing the shear modulus of two-dimensional multiplanar nanostructures and heterostructures

Generalized high-fidelity closed-form formulae have been developed to predict the shear modulus of hexagonal graphene-like monolayer nanostructures and nano-heterostructures based on a physically insightful analytical approach. Hexagonal nano-structural forms (top view) are common for nanomaterials with monoplanar (such as graphene and hBN) and multiplanar (such as stanene and MoS₂) configurations. However, a single-layer nanomaterial may not possess a particular property adequately, or multiple desired properties simultaneously. Recently, a new trend has emerged to develop nano-heterostructures by assembling multiple monolayers of different nanostructures to achieve various tunable desired properties simultaneously. Shear modulus assumes an important role in characterizing the applicability of different two-dimensional nanomaterials and heterostructures in various nanoelectromechanical systems such as determining the resonance frequency of vibration modes involving torsion, wrinkling and rippling behavior of two-dimensional materials. We have developed mechanics-based closed-form formulae for the shear modulus of monolayer nanostructures and multi-layer nano-heterostructures. New results of shear modulus are presented for different classes of nanostructures (graphene, hBN, stanene and MoS₂) and nano-heterostructures (graphene-hBN, graphene-MoS₂, graphene-stanene and stanene-MoS₂), which are categorized on the basis of fundamental structural configurations. The numerical values of shear modulus are compared with the results from the scientific literature (as available) and separate molecular dynamics simulations, wherein a good agreement is noticed. The proposed analytical expressions will enable the scientific community to efficiently evaluate shear modulus of a wide range of nanostructures and nanoheterostructures.

Thermal expansion producing easier formation of a black phosphorus nanotube from nanoribbon on carbon nanotube

As a novel one-dimensional material having excellent electrical properties, a black phosphorus (BP) nanotube has wide potential applications in nanodevices. A BP nanotube has not yet, however, been discovered in experiments or fabricated via chemical synthesis. In this study, the feasibility of forming a nanotube from a parallelogram nanoribbon upon a carbon nanotube (CNT) at different temperatures is discussed through the use of molecular dynamics simulations. Results obtained demonstrate that an ideal BP nanotube from the same nanoribbon can be obtained via self-assembly on a CNT at 50 K or lower temperature. At temperatures between 50-100 K, the BP nanotube formed from a single ribbon has defects at both ends. When the temperature is higher than 100 K, it is difficult to obtain a BP nanotube of high quality. It is discovered that when the ribbon can only wind upon the same CNT at low temperature, it may form into an ideal nanotube by increasing the temperature of the system. The reason is that the BP ribbon has a higher thermal expansion than the CNT under the same temperature difference.

Nonlinear modeling of crystal system transition of black phosphorus using continuum-DFT model

In this paper, the nonlinear behavior of black phosphorus crystals is investigated in tandem with dispersion-corrected density functional theory (DFT-D) analysis under uniaxial loadings. From the identified anisotropic behavior of black phosphorus due to its morphological anisotropy, a hyperelastic anisotropic (HA) model named continuum-DFT is established to predict the nonlinear behavior of the material. In this respect, uniaxial Cauchy stresses are employed on both the DFT-D and HA models along the zig-zag and armchair directions. Simultaneously, the transition of the crystal system is recognized at about 4.5 GPa of the applied uniaxial tensile stress along the zig-zag direction on the DFT-D simulation in the nonlinear region. In order to develop the nonlinear continuum model, unknown constants are surveyed with the optimized least square technique. In this regard, the continuum model is obtained to reproduce the Cauchy stress-stretch and density of strain-stretch results of the DFT-D simulation. Consequently, the modified HA model is introduced to characterize the nonlinear behavior of black phosphorus along the zig-zag direction. More importantly, the specific transition of the crystal system is successfully predicted in the new modified continuum-DFT model. The results reveal that the multiscale continuum-DFT model is well defined to replicate the nonlinear behavior of black phosphorus along the zig-zag and armchair directions.

Strain-Dependent Edge Structures in MoS2 Layers

Edge structures are low-dimensional defects unavoidable in layered materials of the transition metal dichalcogenides (TMD) family. Among the various types of such structures, the armchair (AC) and zigzag (ZZ) edge types are the most common. It has been predicted that the presence of intrinsic strain localized along these edges structures can have direct implications for the customization of their electronic properties. However, pinning down the relation between local structure and electronic properties at these edges is challenging. Here, we quantify the local strain field that arises at the edges of MoS₂ flakes by combining aberration-corrected transmission electron microscopy (TEM) with the geometrical-phase analysis (GPA) method. We also provide further insight on the possible effects of such edge strain on the resulting electronic behavior by means of electron energy loss spectroscopy (EELS) measurements. Our results reveal that the two-dominant edge structures, ZZ and AC, induce the formation of different amounts of localized strain fields. We also show that by varying the free edge curvature from concave to convex, compressive strain turns into tensile strain. These results pave the way toward the customization of edge structures in MoS₂, which can be used to engineer the properties of layered materials and thus contribute to the optimization of the next generation of atomic-scale electronic devices built upon them.

Fabrication of an ideal nanoring from a black phosphorus nanoribbon upon movable bundling carbon nanotubes

As a low dimensional material, black phosphorus (BP) continues to attract much attention from researchers due to its excellent electric properties. In particular, the one-dimensional material, in the form of a ring or tube formed from BP, has been extensively studied and found to be a perfect semiconductor. But the BP ring has never been reported in laboratories. To form an ideal ring from a rectangular BP ribbon, we choose a carbon nanotube (CNT) bundle to attract the ribbon and move one or more CNTs in the bundle to induce the unsaturated ends of the BP ribbon to become covalently bonded. Numerical experiments are applied to BP ribbons with lengths either equal to, shorter, or longer than the perimeter of the CNT bundle, to investigate the formation of a BP ring. Experiments show that if one end of the BP ribbon is attracted by a CNT, moving the other CNTs away endows the ribbon with high probability of forming an ideal ring. The conclusions drawn from these results will benefit future in situ experiments involving forming a ring from a BP ribbon.

Quantum confinement in black phosphorus-based nanostructures

The modification of an idealized infinite bulk system by dimensional reduction or structural distortion results in quantum confinement effects (QCEs). For example, dimensional reduction of a black phosphorus structure leads to the realization of few-layer systems, creation of edges and surfaces, nanoribbons, quantum dots, and antidot lattices while structural distortion involves simple bending (including nanotubes) and rippling. Black phosphorus ('phosphorene' in the single-layer limit) has been of recent interest due to its relatively large charge carrier mobility and moderate semiconducting band gap, which remains direct irrespective of the number of layers. In this review the state-of-the-art properties of black phosphorus in its dimensionally reduced and structurally distorted forms are discussed, with emphasis on how quantum confinement impacts the material's properties.

Mechanical properties and failure behavior of phosphorene with grain boundaries

Using the density-functional tight-binding method, we studied the effect of grain boundaries on the mechanical properties and failure behavior of phosphorene. We found that the high-angle tilt boundaries with a higher density of (5∣7) defect pairs (oriented along the armchair direction) are stronger than the low-angle tilt boundaries with a lower defect density, and similarly the high-angle boundaries with a higher density of (4∣8) defect pairs (oriented along the zigzag direction) are stronger than the low-angle boundaries with a lower defect density. The failure is due to the rupture of the most pre-strained bonds in the heptagons of the (5∣7) defect pair or octagons of the (4∣8) pairs. The high-angle grain boundaries are better at accommodating the pre-strained bonds in heptagon and octagon defects, leading to a higher failure stress and strain. The results cannot be described by a Griffith-type fracture mechanics criterion, since this does not take into account the bond pre-stretching. Interestingly, these anomalous mechanical and failure characteristics of tilt grain boundaries in phosphorene are also shared by graphene and hexagonal boron nitride, signifying that they may be universal for 2D materials. The findings revealed here may be useful in tuning the mechanical properties of phosphorene via defect engineering for specific applications.

Prediction of mechanical properties of 2D solids with related bonding configuration

A chemical reference model is introduced to estimate unknown mechanical properties of 2D solids for groups with related binding configuration using a minimal data base.

Phosphorene and Phosphorene-Based Materials - Prospects for Future Applications

Phosphorene, a single- or few-layered semiconductor material obtained from black phosphorus, has recently been introduced as a new member of the family of two-dimensional (2D) layered materials. Since its discovery, phosphorene has attracted significant attention, and due to its unique properties, is a promising material for many applications including transistors, batteries and photovoltaics (PV). However, based on the current progress in phosphorene production, it is clear that a lot remains to be explored before this material can be used for these applications. After providing a comprehensive overview of recent advancements in phosphorene synthesis, advantages and challenges of the currently available methods for phosphorene production are discussed. An overview of the research progress in the use of phosphorene for a wide range of applications is presented, with a focus on enabling important roles that phosphorene would play in next-generation PV cells. Roadmaps that have the potential to address some of the challenges in phosphorene research are examined because it is clear that the unprecedented chemical, physical and electronic properties of phosphorene and phosphorene-based materials are suitable for various applications, including photovoltaics.

Mechanical properties of phosphorene nanotubes: a density functional tight-binding study

Using the density functional tight-binding method, we studied the elastic properties, deformation and failure of armchair (AC) and zigzag (ZZ) phosphorene nanotubes (PNTs) under uniaxial tensile strain. We found that the deformation and failure of PNTs are very much anisotropic. For ZZ PNTs, three deformation phases are recognized: the primary linear elastic phase-which is associated with interactions between neighboring puckers, succeeded by the bond rotation phase-where the puckered configuration of phosphorene is smoothed via bond rotation, and lastly the bond elongation phase-where the P-P bonds are directly stretched up to the maximally allowed limit and failure is initiated by the rupture of the most stretched bonds. For AC PNTs, the applied strain stretches the bonds up to the maximally allowed limit, causing their ultimate failure. For both AC and ZZ PNTs, their failure strain and failure stress are sensitive- while the Young's modulus, flexural rigidity, radial Poisson's ratio and thickness Poisson's ratio are relatively insensitive-to the tube diameter. More specifically, for AC PNTs, the failure strain decreases from 0.40 to 0.25 and the failure stress increases from 13 GPa to 21 GPa when the tube diameter increases from 13.3 Å to 32.8 Å; while for ZZ PNTs, the failure strain decreases from 0.66 to 0.55 and the failure stress increases from 4 GPa to 9 GPa when the tube diameter increases from 13.2 Å to 31.1 Å. The Young's modulus, flexural rigidity, radial and thickness Poisson ratios are 114.2 GPa, 0.019 eV · nm(2), 0.47 and 0.11 for AC PNTs, and 49.2 GPa, 0.071 eV · nm(2), 0.07 and 0.21 for ZZ PNTs, respectively. The present findings provide valuable references for the design and application of PNTs as device elements.

Atomic vacancies significantly degrade the mechanical properties of phosphorene

Due to low formation energies, it is very easy to create atomic defects in phosphorene during its fabrication process. How these atomic defects affect its mechanical behavior, however, remain unknown. Here, we report on a systematic study of the effect of atomic vacancies on the mechanical properties and failure behavior of phosphorene using molecular dynamics simulations. It is found that atomic vacancies induce local stress concentration and cause early bond-breaking, leading to a significant degradation of the mechanical properties of the material. More specifically, a 2% concentration of randomly distributed mono-vacancies is able to reduce the fracture strength by ∼40%. An increase in temperature from 10 to 400 K can further deteriorate the fracture strength by ∼60%. The fracture strength of defective phosphorene is also found to be affected by defect distribution. When the defects are patterned in a line, the reduction in fracture strength greatly depends on the tilt angle and the loading direction. Furthermore, we find that di-vacancies cause an even larger reduction in fracture strength than mono-vacancies when the loading is in an armchair direction. These findings provide important guidelines for the structural design of phosphorene in future applications.

Edge phonons in black phosphorus

Black phosphorus has recently emerged as a new layered crystal that, due to its peculiar and anisotropic crystalline and electronic band structures, may have important applications in electronics, optoelectronics and photonics. Despite the fact that the edges of layered crystals host a range of singular properties whose characterization and exploitation are of utmost importance for device development, the edges of black phosphorus remain poorly characterized. In this work, the atomic structure and behaviour of phonons near different black phosphorus edges are experimentally and theoretically studied using Raman spectroscopy and density functional theory calculations. Polarized Raman results show the appearance of new modes at the edges of the sample, and their spectra depend on the atomic structure of the edges (zigzag or armchair). Theoretical simulations confirm that the new modes are due to edge phonon states that are forbidden in the bulk, and originated from the lattice termination rearrangements.

Black phosphorus is an allotrope of phosphorous that, like graphite, can be exfoliated to create two-dimensional materials. Here, the authors use Raman spectroscopy and density functional theory calculations to investigate the anomalous behaviour of phonons near different black phosphorus edges.

Strength and stability analysis of a single-walled black phosphorus tube under axial compression

Few-layered black phosphorus materials currently attract much attention due to their special electronic properties. As a consequence, a single-layer black phosphorus (SLBP) nanotube has been theoretically built. The corresponding electronic properties of such a black phosphorus nanotube (BPNT) were also evaluated numerically. However, unlike graphene formed with 2sp(2) covalent carbon atoms, SLBP is formed with 3sp(3) bonded atoms. It means that the structure from SLBP will possess lower Young's modulus and mechanical strength than those of carbon nanotubes. In this study, molecular dynamics simulation is performed to investigate the strength and stability of BPNTs affected by the factors of diameter, length, loading speed and temperature. Results are fundamental for investigating the other physical properties of a BPNT acting as a component in a nanodevice. For example, buckling of the BPNT happens earlier than fracture, before which the nanostructure has very small axial strain. For the same BPNT, a higher load speed results in lower critical axial strain and a nanotube with lower axial strain can still be stable at a higher temperature.

Thermal stability of a free nanotube from single-layer black phosphorus

Similar to a carbon nanotube fabricated from a graphene sheet, a black phosphorus nanotube (BPNT) can also be theoretically produced by curling rectangular single-layer black phosphorus (SLBP). In the present study, the effect of the thermal vibration of atoms on the failure of a BPNT is investigated using molecular dynamics simulations. Two types of double-shell BPNTs obtained by curling the SLBP along its armchair/pucker and zigzag directions respectively are involved in simulation. At finite temperature, a bond on the outer shell of the tube is under tension due to both the curvature of the tube and the serious thermal vibration of the atoms. As the length of a bond with such elongation approaches its critical value, i.e. 0.279 nm, or the smallest distance between two nonbonding phosphorus atoms is over 0.389 nm caused by a great variation of the bond angle, the tube fails quickly. The critical stable states of either an armchair or a zigzag BPNT at finite temperature are calculated and compared. To achieve a stable BPNT with high robustness, the tube should have a higher radius or should work at a lower temperature. Only when the BPNT has structural stability does it have the potential application as a nanowire in a future nano electro-mechanical system.

Phosphorene under electron beam: from monolayer to one-dimensional chains

Phosphorene, a single sheet of black phosphorus, is an elemental two-dimensional material with unique properties and potential applications in semiconductor technology. While few-layer flakes of the material have been characterized using transmission electron microscopy, very little is known about its response to electron irradiation, which may be particularly important in the context of top-down engineering of phosphorus nanostructures using a focused electron beam. Here, using first-principles simulations, we study the production of defects in a single phosphorene sheet under impacts of energetic electrons. By employing the McKinley-Feshbach formalism and accounting for the thermal motion of atoms, we assess the cross section for atom displacement as a function of electron energy. We further investigate the energetics and dynamics of point defects and the stability of ribbons and edges under an electron beam. Finally, we show that P atomic chains should be surprisingly stable, and their atomic structure is not linear giving rise to the absence of a gap in the electronic spectrum.

Substitutionally doped phosphorene: electronic properties and gas sensing

Phosphorene, a new elemental two-dimensional material, has attracted increasing attention owing to its intriguing electronic properties. In particular, pristine phospohorene, due to its ultrahigh surface-volume ratio and high chemical activity, has been shown to be promising for gas sensing (Abbas et al 2015 ACS Nano 9 5618). To further enhance its sensing ability, we perform first-principles calculations based on density functional theory to study substitutionally doped phosphorene with 17 different atoms, focusing on structures, energetics, electronic properties and gas sensing. Our calculations reveal that anionic X (X = O, C and S) dopants have a large binding energy and highly dispersive electronic states, signifying the formation of covalent X-P bonds and thus strong structural stability. Alkali atom (Li and Na) doping is found to donate most of the electrons in the outer s-orbital by forming ionic bonds with P, and the band gap decreases by pushing down the conduction band, suggesting that the optical and electronic properties of the doped phosphorene can be tailored. For doping with VIIIB-group (Fe, Co and Ni) elements, a strong affinity is predicted and the binding energy and charge transfer are correlated strongly with their electronegativity. By examining NO molecule adsorption, we find that these metal doped phosphorenes (MDPs) in general exhibit a significantly enhanced chemical activity compared with pristine phosphorene. Our study suggests that substitutionally doped phosphorene shows many intriguing electronic and optic properties different from pristine phosphorene and MDPs are promising in chemical applications involving molecular adsorption and desorption processes, such as materials growth, catalysis, gas sensing and storage.

Photogalvanic effect in monolayer black phosphorus

We report a first-principles theoretical approach for analyzing linear and circular photogalvanic effects (PGEs) based on density functional theory within the nonequilibrium Green's function formalism. Using this approach we investigate the PGE phenomena in monolayer black phosphorus (MBP) doped with sulfur atoms. The impurity doping breaks the space inversion symmetry of pristine MBP, leading to a C s symmetry with a mirror reflection plane normal to the zigzag direction of the MBP lattice. Governed by this symmetry, a linear PGE is induced in both zigzag and armchair directions, and a circular PGE is induced along the zigzag direction. A robust broadband photoresponse is found from the near-infrared to the visible range for the MBP device. There is a strong anisotropy in PGE: photoresponse in the zigzag direction can be larger by an order of magnitude than that in the armchair direction. We identify the origin of the observed PGE as the inter-band transitions from the impurity and valence bands to the conduction bands, which involves a transfer of angular momentum from photons to electrons.

Intrinsic Defects, Fluctuations of the Local Shape, and the Photo-Oxidation of Black Phosphorus

Black phosphorus is a monatomic semiconducting layered material that degrades exothermically in the presence of light and ambient contaminants. Its degradation dynamics remain largely unknown. Even before degradation, local-probe studies indicate non-negligible local curvature-through a nonconstant height distribution-due to the unavoidable presence of intrinsic defects. We establish that these intrinsic defects are photo-oxidation sites because they lower the chemisorption barrier of ideal black phosphorus (>10 eV and out of visible-range light excitations) right into the visible and ultraviolet range (1.6 to 6.8 eV), thus enabling photoinduced oxidation and dissociation of oxygen dimers. A full characterization of the material's shape and of its electronic properties at the early stages of the oxidation process is presented as well. This study thus provides fundamental insights into the degradation dynamics of this novel layered material.