High Stability of Faceted Nanotubes and Fullerenes of Multiphase Layered Phosphorus: A Computational Study

Super-high carrier mobilities and excellent thermoelectric performances of Tri-Tri group-VA monolayers

High-performance thermoelectric materials in theoretical and experimental research are mostly composed of expensive, scarce, heavy elements and rarely of single light elements, which severely limit their application and development. Based on density functional and semiclassical Boltzmann transport theory, we determine that a stable phosphorene allotrope, named Tri-Tri phosphorene, has super-high electron mobility (23845.29 cm2 V-1 s-1) much higher than those of most two-dimension materials. Moreover, its optimized maximum ZT can reach up to 3.43 at room temperature (4.83 at 500 K and 5.92 at 700 K), exhibiting highly favorable prospects in practical thermoelectric systems. Motivated by the excellent properties of Tri-Tri phosphorene, we further demonstrate the structural stability of Tri-Tri arsenene and Tri-Tri antimonene and predict that the two Tri-Tri structures also have high Seebeck coefficients and electron mobilities. Their lattice thermal conductivities are dramatically decreased compared with Tri-Tri phosphorene. Thus, their predicted thermoelectric performances are also excellent, with maximum ZT values of 4.12 (Tri-Tri arsenene) and 3.54 (Tri-Tri antimonene) at room temperature. The low layer moduli of the three Tri-Tri structures indicate that they have high mechanical flexibility and suitability for current device assemblies. All these desirable properties make Tri-Tri group-VA materials promising for future applications in thermoelectric devices.

Electronic structures, transport properties, and optical absorption of bilayer blue phosphorene nanoribbons

Based on first-principles density functional theory and nonequilibrium Green's function, we study the electronic band structures, the electronic transport properties, and the optical absorption of bilayer blue phosphorene nanoribbons (BPNRs). Both bilayer armchair BPNRs (a-BPNRs) and zigzag BPNRs (z-BPNRs) behave as semiconductors in the narrow nanoribbon case and metals in the wide nanoribbon case, sharply different from their monolayer counterparts where the monolayer a-BPNRs (z-BPNRs) are always semiconducting (metallic). This indicates that interlayer couplings or the increasing layer number may induce the switching of the conductivity of the monolayer BPNRs, which is absent in graphene and phosphorene nanoribbons. Furthermore, we explore the edge states of the energy bands near Fermi energy, and find that there are almost no pure edge-state band branches in the bilayer BPNRs, which can be attributed to the interlayer couplings between the edge-states in one layer and the bulk-states in the other. Consequently, the resulting complex band structures cannot be directly analyzed any more in the framework of the two-body coupling picture just according to the simple band structures of the monolayer BPNRs. Finally, we present the current-voltage characteristics and the optical absorption of the bilayer a-BPNRs and z-BPNRs. The influences of the nanoribbon width and the interlayer couplings on the current and the anisotropic optical absorption can be understood based on the complex energy band structures. This research should be an important reference of extending the field of BPNRs from the monolayer to the bilayer case, and deepen the understanding of the difference between the monolayer and bilayer nanoribbons in different materials.

Renaissance of elemental phosphorus materials: properties, synthesis, and applications in sustainable energy and environment

The polymorphism of phosphorus-based materials has garnered much research interest, and the variable chemical bonding structures give rise to a variety of micro and nanostructures. Among the different types of materials containing phosphorus, elemental phosphorus materials (EPMs) constitute the foundation for the synthesis of related compounds. EPMs are experiencing a renaissance in the post-graphene era, thanks to recent advancements in the scaling-down of black phosphorus, amorphous red phosphorus, violet phosphorus, and fibrous phosphorus and consequently, diverse classes of low-dimensional sheets, ribbons, and dots of EPMs with intriguing properties have been produced. The nanostructured EPMs featuring tunable bandgaps, moderate carrier mobility, and excellent optical absorption have shown great potential in energy conversion, energy storage, and environmental remediation. It is thus important to have a good understanding of the differences and interrelationships among diverse EPMs, their intrinsic physical and chemical properties, the synthesis of specific structures, and the selection of suitable nanostructures of EPMs for particular applications. In this comprehensive review, we aim to provide an in-depth analysis and discussion of the fundamental physicochemical properties, synthesis, and applications of EPMs in the areas of energy conversion, energy storage, and environmental remediation. Our evaluations are based on recent literature on well-established phosphorus allotropes and theoretical predictions of new EPMs. The objective of this review is to enhance our comprehension of the characteristics of EPMs, keep abreast of recent advances, and provide guidance for future research of EPMs in the fields of chemistry and materials science.

Magnetic ε-Phosphorene for Sensing Greenhouse Gas Molecules

It is critical for gas sensors that sense greenhouse gas molecules to have both good sensitivity and selectivity for water molecules in the ambient environment. Here, we study the charge transfer, IV curves, and electric field tuning of vanadium-doped monolayer ϵ-phosphorene as a sensor for NO, NO₂, and H₂O gas molecules via first-principle and transport calculations. We find that the paramagnetic toxic molecules of NO and NO₂ have a high adsorption energy on V-ϵ-phosphorene, which originates from a large amount of charge transfer driven by the hybridisation of the localised spin states of the host with the molecular frontier orbital. Using the non-equilibrium Green's function, we investigate the IV responses with respect to the adsorption of different molecules to study the performance of gas molecule sensors. Our IV curves show a larger amount of changes in resistance of the paramagnetic NO and NO₂ than nonmagnetic H₂O gas molecules, suggesting both sensitivity and selectivity. Moreover, our calculations show that an applied external electric field (gate voltage) can effectively tune the amount of charge transfer. More charge transfer makes the sensor more sensitive to the molecule, while less charge transfer can reduce the adsorption energy and remove the adsorbed molecules, allowing for the repeated use of the sensor.

First-principles study on the structural properties of 2D MXene SnSiGeN4 and its electronic properties under the effects of strain and an external electric field

The MXene SnSiGeN4 monolayer as a new member of the MoSi2N4 family was proposed for the first time, and its structural and electronic properties were explored by applying first-principles calculations with both PBE and hybrid HSE06 approaches. The layered hexagonal honeycomb structure of SnSiGeN4 was determined to be stable under dynamical effects or at room temperature of 300 K, with a rather high cohesive energy of 7.0 eV. The layered SnSiGeN4 has a Young's modulus of 365.699 N m-1 and a Poisson's ratio of 0.295. The HSE06 approach predicted an indirect band gap of around 2.4 eV for the layered SnSiGeN4. While the major donation from the N-p orbitals to the band structure makes SnSiGeN4's band gap close to those of similar 2D MXenes, the smaller distributions from the other orbitals of Sn, Si, and Ge slightly vary this band gap. The work functions of the GeN and SiN surfaces are 6.367 eV and 5.903 eV, respectively. The band gap of the layered SnSiGeN4 can be easily tuned by strain and an external electric field. A semiconductor-metal transition can occur at certain values of strain, and with an electric field higher than 5 V nm-1. The electron mobility of the layered SnSiGeN4 can reach up to 677.4 cm2 V-1 s-1, which is much higher than the hole mobility of about 52 cm2 V-1 s-1. The mentioned characteristics make the layered SnSiGeN4 a very promising material for use in electronic and photoelectronic devices, and for solar energy conversion.

A new phosphorene allotrope: the assembly of phosphorene nanoribbon and chain

Phosphorene allotrope monolayers such as blue and red phosphorus are attempted to be designed and synthesized to be used in the optoelectronics field due to their tunable bandgap and high...

An alternative route towards the fabrication of 2D blue phosphorene

Blue phosphorene (BlueP) is a novel two-dimensional material that shares properties with black phosphorene and is potentially even more interesting for opto-electronic applications because of its layer dependent wide band gap of ≈ 2 to 3 eV and superior charge carrier mobility. It was first fabricated on Au(111), where, however, a network consisting of BlueP subunits and Au-linker atoms is formed. The physical properties of such an arrangement strongly differ from a freestanding BlueP monolayer. Here, we report on the growth of epitaxial BlueP on the Au(100) surface, which is an interesting alternative when aiming at quasi-freestanding BlueP domains. We find two different phosphorus phases by means of scanning tunneling microscopy and distortion-corrected low-energy electron diffraction. In the low coverage regime, we observe a commensurate (2 × 2) phase, whereas for higher coverage, a nearly hexagonal structure is formed. For the latter, the lattice parameters measured via atomically resolved scanning tunneling hydrogen microscopy closely resemble those of freestanding BlueP, and the typical height modulation of the phosphorus atoms is verified in our layers by means of x-ray photoelectron diffraction. We further analyze the chemical and electronic properties of these films by means of x-ray and (angle resolved) ultraviolet photoelectron spectroscopy.

Topological phase transition induced by band structure modulation in a Chern insulator

We study a systematic evolution of the topological properties of a Chern insulator upon smooth variation of a hopping parameter (t₁) of the electrons among a pair of nearest neighbour sites on a honeycomb lattice, while keeping the other two hopping terms (t) fixed. In the absence of a Haldane flux, the tuning oft₁results in gradual shifting of the Dirac cones which eventually merge into one at theMpoint in the Brillouin zone (BZ) att₁= 2twith a gapless semi-Dirac dispersion at low energies. In the presence of a Haldane flux, the system becomes a Chern insulator fort₁< 2t, but turns gapless att₁= 2twith the semi-Dirac dispersion being transformed to an anisotropic Dirac one. The spectrum eventually gaps out and transforms into a trivial insulator fort₁> 2t. The Chern number phase diagram obtained via integrating the Berry curvature over the BZ shows a gradual shrinking of the 'topological' lobes, and vanishes just beyondt₁= 2t, where a small but a finite Berry curvature still exists. Thus, there is a phase transition from a topological phase to a trivial phase across the semi-Dirac point (t₁= 2t). The vanishing of the anomalous Hall conductivity plateau and the merger of the chiral edge states with the bulk bands near theMpoint provide robust support of the observed phase transition.

The spin-polarized edge states of blue phosphorene nanoribbons induced by electric field and electron doping

Edge states of various two-dimensional materials such as graphene are intrinsically spin-polarized. In other materials, electric field and charge doping are required for introducing magnetism to their edges. In this work, by using first-principles calculations, we studied the effects of transverse electric field on the edge states of the armchair blue phosphorene nanoribbon (ABPNR), and found that a transverse electric field drives the edge electronic state occupied and at the same time spin-polarized. We also doped electrons to the ABPNR and found that these additional electrons occupy and spin-polarize the electronic states of both edges of the nanoribbon.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Hierarchically Structured Allotropes of Phosphorus from Data-Driven Exploration

The discovery of materials is increasingly guided by quantum-mechanical crystal-structure prediction, but the structural complexity in bulk and nanoscale materials remains a bottleneck. Here we demonstrate how data-driven approaches can vastly accelerate the search for complex structures, combining a machine-learning (ML) model for the potential-energy surface with efficient, fragment-based searching. We use the characteristic building units observed in Hittorf's and fibrous phosphorus to seed stochastic ("random") structure searches over hundreds of thousands of runs. Our study identifies a family of hierarchically structured allotropes based on a P8 cage as principal building unit, including one-dimensional (1D) single and double helix structures, nanowires, and two-dimensional (2D) phosphorene allotropes with square-lattice and kagome topologies. These findings yield new insight into the intriguingly diverse structural chemistry of phosphorus, and they provide an example for how ML methods may, in the long run, be expected to accelerate the discovery of hierarchical nanostructures.

Quantum confinement and edge effects on electronic properties of zigzag green phosphorene nanoribbons

First-principles density-functional theory calculations were performed to investigate quantum confinement and edge effects on electronic properties of zigzag green phosphorene nanoribbons (ZGPNRs) with edge chemical species including H, OH, F, Cl, O, and S for the ribbons width in the range of 0.5-3.7 nm. The ZGPNRs were obtained from relaxed two-dimensional green phosphorene monolayer with different cutting strategies and the most energetically favorable ribbon configuration was selected for further exploration of size and edge effects. It was found that the electronic properties of the ZGPNRs are strongly associated with the ribbon width and edge chemical species. They show either semiconducting or metallic features depending on the edge functionalization species. The ZGPNRs show semiconducting behavior with the edge species of H, OH, F, or Cl (Group I), while they exhibit metallic characteristics with pristine or O, S edges (Group II). The conduction band minimum and valence band maximum of the ZGPNRs with the Group I edge are primarily located at the inner P atoms and the edge P and functionalization atoms have little contribution. However, for the Group II edge, the electronic bands crossing the Fermi level are dominantly contributed by the edge atoms. It was also found that the band gap and work function of the ZGPNRs are sensitively tunable by varying ribbon width and edge functionalization species.

Quasi-bonding driven abnormal isotropic thermal transport in intrinsically anisotropic nanostructure: a case of study of a phosphorus nanotube array

Thermal anisotropy/isotropy is one of the fundamental characteristics of the thermal properties of a material, playing a significant role in the high-performance thermal management in micro-/nanoelectronics. It has been well documented in the literature that the symmetry of geometric structures governs the anisotropy/isotropy of thermal transport. However, the fundamental correlation and the underlying mechanism remain unclear. In this paper, using a new two-dimensional (2D) van der Waals (vdW) phosphorus nanotube array as a case study, we show that the lattice thermal conductivity can be abnormally almost isotropic although the geometric structure presents remarkable anisotropy, which contradicts the previous consensus. The key factor for the abnormal isotropic thermal conductivity is mainly the essentially analogous group velocities along the intratube and intertube directions. Compared with a carbon-nanotube array, a traditional vdW system, a microscopic picture is established to underpin the underlying mechanism. The quasi-bond (non-covalent bonding, but far stronger than the vdW interatomic interaction) between the phosphorus nanotubes is found to be responsible for such diverse isotropic transport phenomena. The findings in this paper are expected to deepen our understanding of the anisotropy/isotropy thermal transport of materials and are also helpful for future thermal management technology.

Changing the Phosphorus Allotrope from a Square Columnar Structure to a Planar Zigzag Nanoribbon by Increasing the Diameter of Carbon Nanotube Nanoreactors

Elemental phosphorus nanostructures are notorious for a large number of allotropes, which limits their usefulness as semiconductors. To limit this structural diversity, we synthesize selectively quasi-1D phosphorus nanostructures inside carbon nanotubes (CNTs) that act both as stable templates and nanoreactors. Whereas zigzag phosphorus nanoribbons form preferably in CNTs with an inner diameter exceeding 1.4 nm, a previously unknown square columnar structure of phosphorus is observed to form inside narrower nanotubes. Our findings are supported by electron microscopy and Raman spectroscopy observations as well as ab initio density functional theory calculations. Our computational results suggest that square columnar structures form preferably in CNTs with an inner diameter around 1.0 nm, whereas black phosphorus nanoribbons form preferably inside CNTs with a 4.1 nm inner diameter, with zigzag nanoribbons energetically favored over armchair nanoribbons. Our theoretical predictions agree with the experimental findings.

Two-dimensional Blue-AsP monolayers with tunable direct band gap and ultrahigh carrier mobility show promising high-performance photovoltaic properties

The successful fabrication of black phosphorene (Black-P) in 2014 and subsequent synthesis of layered black As1-xPx alloys have inspired research into two-dimensional (2D) binary As-P compounds. The very recent success in growing blue phosphorene (Blue-P) further motivated exploration of 2D Blue-AsP materials. Here, using ab initio swarm-intelligence global minimum structure-searching methods, we have obtained a series of novel and energetically favored 2D Blue-AsP (denoted x-AsP, x = I, II, III, IV, V) compounds with As : P = 1 : 1 stoichiometry. They display similar honeycomb structures to Blue-P. Remarkably, the lowest-energy AsP monolayer, namely I-AsP, not only possesses a quasi-direct band gap (2.41 eV), which can be tuned to a direct and optimal gap for photovoltaic applications by in-plane strain, but also has an ultrahigh electronic mobility up to ∼7.4 × 104 cm2 V-1 s-1, far surpassing that of Blue-P, and also exhibits high absorption coefficients (×105 cm-1). Our simulations also show that 30 nm-thick I-AsP sheet-based cells have photovoltaic efficiency as high as ∼12%, and the I-AsP/CdSe heterostructure solar cells possess a power conversion efficiency as high as ∼13%. All these outstanding characteristics suggest the I-AsP sheet as a promising material for high-efficiency solar cells.

Electronic and magnetic properties of the one-dimensional interfaces of two-dimensional lateral GeC/BP heterostructures

We studied the electronic and magnetic properties of one-dimensional (1D) interfaces of two dimensional (2D) GeC/BP lateral heterostructures by first-principles calculations. We showed that an inner electric field exists in the heterostructures, and the 1D interfaces exhibit metallic properties when the ribbon width is larger than a critical value. In other words, the 1DEG/1DHG (one dimensional electron gas/one dimensional hole gas) with π bonding character appear at the interfaces. We verified that the existence of the polar discontinuity at the interfaces is the pre-requisite for this insulator-to-metal transition, which can be understood by the topology classification of the formal polarization of the honeycomb structures with C3 symmetry and the corresponding charge compensation mechanism was also discussed. We predicted the emergence of magnetism at the interfaces, with the width-dependent spin polarization being attributed to the Stoner instability. Increasing DOS at the interfaces leads to a paramagnetic to ferromagnetic transition.

Spin-dependent carrier mobility and its gate-voltage modifying effects for functionalized single walled black phosphorus tubes

Phosphorene and its derivatives so far have attracted substantial research interest due to its promising properties for developing nanoscale electronic devices. Here, we present a theoretical investigation on the functionalized features, such as the improved electronic structure and carrier mobility, for armchair-edged single walled black phosphorus nanotubes (PNTs) with the substitutional doping of low-concentration transition-metal atoms (Ti, Mn, Fe, and Ni). They are predicted to be exceptional magnetic semiconductors (MSCs), such as half-semiconductor or bipolar MSC. Their spin-resolved carrier mobility at room temperature holds doping element- dependence as well as carrier and spin polarity. Particularly, the difference by two orders of magnitude for carrier mobility emerges due to different TM doping. More interestingly, the carrier mobility in armchair PNTs serving as the channel material of a spin field effect transistor is predicted to be modified strongly by a gate voltage. The enhanced carrier mobility and its gate voltage direction-dependent behavior, as well as the more obvious carrier and spin polarity of mobility, can be observed clearly under gate voltage, which further facilitates the separation of different carriers and spin states and also suggests that realistic carrier mobility is gate voltage-dependent in a field effect transistor.

2D Phosphorene: Epitaxial Growth and Interface Engineering for Electronic Devices

Black phosphorus (BP), first synthesized in 1914 and rediscovered as a new member of the family of 2D materials in 2014, combines many extraordinary properties of graphene and transition-metal dichalcogenides, such as high charge-carrier mobility, and a tunable direct bandgap. In addition, it displays other distinguishing properties, e.g., ambipolar transport and highly anisotropic properties. The successful application of BP in electronic and optoelectronic devices has stimulated significant research interest in other allotropes and alloys of 2D phosphorene, a class of 2D materials consisting of elemental phosphorus. As an atomically thin sheet, the various interfaces presented in 2D phosphorene (substrate/phosphorene, electrode/phosphorene, dielectric/phosphorene, atmosphere/phosphorene) play dominant roles in its bottom-up synthesis, and determine several key characteristics for the devices, such as carrier injection, carrier transport, carrier concentration, and device stability. The rational design/engineering of interfaces provides an effective way to manipulate the growth of 2D phosphorene, and modulate its electronic and optoelectronic properties to realize high-performance multifunctional devices. Here, recent progress of the interface engineering of 2D phosphorene is highlighted, including the epitaxial growth of single-layer blue phosphorus on different substrates, surface functionalization of BP for high-performance complementary devices, and the investigation of the BP degradation mechanism in ambient air.

Intermixing and periodic self-assembly of borophene line defects

Two-dimensional (2D) boron (that is, borophene) was recently synthesized following theoretical predictions^1-5. Its metallic nature and high in-plane anisotropy combine many of the desirable attributes of graphene⁶ and monolayer black phosphorus⁷. As a synthetic 2D material, its structural properties cannot be deduced from bulk boron, which implies that the intrinsic defects of borophene remain unexplored. Here we investigate borophene line defects at the atomic scale with ultrahigh vacuum (UHV) scanning tunnelling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Under suitable growth conditions, borophene phases that correspond to the v_1/6 and v_1/5 models are found to intermix and accommodate line defects in each other with structures that match the constituent units of the other phase. These line defects energetically favour spatially periodic self-assembly that gives rise to new borophene phases, which ultimately blurs the distinction between borophene crystals and defects. This phenomenon is unique to borophene as a result of its high in-plane anisotropy and energetically and structurally similar polymorphs. Low-temperature measurements further reveal subtle electronic features that are consistent with a charge density wave (CDW), which are modulated by line defects. This atomic-level understanding is likely to inform ongoing efforts to devise and realize applications based on borophene.

Dirac fermions induced in strained zigzag phosphorus nanotubes and their applications in field effect transistors

In this work, Dirac fermions have been obtained and engineered in one-dimensional (1D) zigzag phosphorus nanotubes (ZPNTs). We have performed a comprehensive first-principles computational study of the electronic properties of ZPNTs with various diameters. The results indicate that as the lattice parameter (L_c) along the axial direction increases, ZPNTs undergo transitions from metal to semimetal and semimetal to semiconductor, whereas Dirac fermions appear at L_c ranging from 3.90 Å to 4.10 Å. In particular, a field effect transistor (FET) based on 12-ZPNT (with 12 unit cells in the transverse direction) exhibits semiconductor behaviors with efficient gate-effect modulation at L_c = 4.60 Å. However, only weak gate modulation is demonstrated when the nanotube becomes a semimetal at L_c = 4.10 Å. This study indicates that ZPNTs are profoundly appealing for applications in strain sensors. Our findings pave the way for the development of high-performance strain-engineered electronics based on Dirac fermions in 1D materials.

Self-assembly of a parallelogram black phosphorus ribbon into a nanotube

A nanotube from single-layer black phosphorus (BP) has never been discovered in experiments. The present study proposed a method for the fabrication of a BP nanotube (BPNT) from a parallelogram nanoribbon self-assembled on a carbon nanotube (CNT). The nanoribbon has a pair of opposite sides along the third principal direction. According to the numerical simulation via molecular dynamics approach, we discover that a wider BP nanoribbon can form into a series of chiral nanotube by self-assembly upon CNTs with different radii. The radius of a BPNT from the same ribbon has a wide range, and depends on both geometry of the ribbon and the CNT. One can obtain a BPNT with the specified radius by placing the ribbon nearby a given CNT. The method provides a clue for potential fabrication of BPNTs.

Prediction of Green Phosphorus with Tunable Direct Band Gap and High Mobility

Black phosphorus is an emerging material in nanoelectronics and nanophotonics due to its high carrier mobility and anisotropic in-plane properties. In addition, the polymorphism of phosphorus leads to numerous searches for new allotropes that are more attractive than black phosphorus in a variety of applications. On the basis of ab initio evolutionary crystal structure search computation, we report the prediction of a phosphorus allotrope called green phosphorus (λ-P), which exhibits direct band gaps ranging from 0.7 to 2.4 eV and strong anisotropy in optical and transport properties. Free-energy calculations show that a single-layer form, termed green phosphorene, is energetically more stable than blue phosphorene, and a phase transition from black to green phosphorene can occur at temperatures above 87 K. We suggest that green phosphorene can be synthesized on corrugated metal surfaces rather than clean surfaces due to its buckled structure, providing guidance to achieving epitaxial growth.

Development of a Transferable Reactive Force Field of P/H Systems: Application to the Chemical and Mechanical Properties of Phosphorene

We developed ReaxFF parameters for phosphorus and hydrogen to give a good description of the chemical and mechanical properties of pristine and defected black phosphorene. ReaxFF for P/H is transferable to a wide range of phosphorus- and hydrogen-containing systems including bulk black phosphorus, blue phosphorene, edge-hydrogenated phosphorene, phosphorus clusters, and phosphorus hydride molecules. The potential parameters were obtained by conducting global optimization with respect to a set of reference data generated by extensive ab initio calculations. We extended ReaxFF by adding a 60° correction term, which significantly improved the description of phosphorus clusters. Emphasis was placed on the mechanical response of black phosphorene with different types of defects. Compared to the nonreactive SW potential ( Jiang , J.-W. Nanotechnology 2015 , 26 , 315706 ), ReaxFF for P/H systems provides a significant improvement in describing the mechanical properties of the pristine and defected black phosphorene, as well as the thermal stability of phosphorene nanotubes. A counterintuitive phenomenon is observed that single vacancies weaken the black phosphorene more than double vacancies with higher formation energy. Our results also showed that the mechanical response of black phosphorene is more sensitive to defects in the zigzag direction than that in the armchair direction. In addition, we developed a preliminary set of ReaxFF parameters for P/H/O/C to demonstrate that the ReaxFF parameters developed in this work could be generalized to oxidized phosphorene and P-containing 2D van der Waals heterostructures. That is, the proposed ReaxFF parameters for P/H systems establish a solid foundation for modeling of a wide range of P-containing materials.

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.

Element allotropes and polyanion compounds of pnicogenes and chalcogenes: stability, mechanisms of formation, controlled synthesis and characterization

The application of the EnPhaSyn (theoretical Energy diagrams, experimental Phase formation, Synthesis and characterisation) concept is reviewed with respect to prediction of structures and stability of element allotropes and compound polymorphs, their phase formation and transition processes, and their directed synthesis, respectively. Therein, the relative energetical stability (En) of target compounds and possible decomposition are determined from quantum chemical DFT calculations. Phase formation and transition (Pha) is probed by a gas balance method, developed as high temperature gas balance concept. It helped to study the synthesis and stability range of several compounds experimentally. Applications of the concept and synthesis principles (Syn) of non-equilibrium phases are presented for allotropes of P, As, P1-xAsx, as well as binary and ternary compounds including the Zintl and Laves like phases IrPTe, NiP2, CoSbS, NiBiSe, Li0.2CdP2, Cu3CdCuP10, and Cd4Cu7As.

Wedge energy bands of monolayer black phosphorus: a first-principles study

On the basis of first-principles calculations, we present intriguing electronic properties of halogen-striped functionalized monolayer black phosphorus. The halogen-striped monolayer black phosphorus is found to have a wedge energy band with the energy-momentum relation of [Formula: see text] when the stripe-stripe distance is smaller than ~40 Å. Our tight-binding study shows that the wedge energy band occurs when 2-atom basis 1D lattices are periodically repeated aligned with each other in a 2D lattice. We also discuss the possible applications of this wedge energy band in electron supercollimation with high mobility or severely anisotropic electronic transport, which can be used for the development of optics-like nano-electronics.

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.

High applicability of two-dimensional phosphorous in Kagome lattice predicted from first-principles calculations

A new semiconducting phase of two-dimensional phosphorous in the Kagome lattice is proposed from first-principles calculations. The band gaps of the monolayer (ML) and bulk Kagome phosphorous (Kagome-P) are 2.00 and 1.11 eV, respectively. The magnitude of the band gap is tunable by applying the in-plane strain and/or changing the number of stacking layers. High optical absorption coefficients at the visible light region are predicted for multilayer Kagome-P, indicating potential applications for solar cell devices. The nearly dispersionless top valence band of the ML Kagome-P with high density of states at the Fermi level leads to superconductivity with T_c of ~9 K under the optimal hole doping concentration. We also propose that the Kagome-P can be fabricated through the manipulation of the substrate-induced strain during the process of the sample growth. Our work demonstrates the high applicability of the Kagome-P in the fields of electronics, photovoltaics, and superconductivity.

Adding a new dimension to the chemistry of phosphorus and arsenic

We predict two novel highly stable free-standing 2D monolayers of P and As alloyed with Cu (Cu₂X) that have exotic hypercoordination motifs. These are the first predicted hexacoordinate P and planar hexacoordinate As extended alloy sheets.

Chiral phosphorus nanotubes: structure, bonding, and electronic properties

A systematic study of bonding and bands structures of armchair, zigzag and chiral black phosphorus nanotubes is reported.

Phosphorene: Synthesis, Scale-Up, and Quantitative Optical Spectroscopy

Phosphorene, a two-dimensional (2D) monolayer of black phosphorus, has attracted considerable theoretical interest, although the experimental realization of monolayer, bilayer, and few-layer flakes has been a significant challenge. Here, we systematically survey conditions for liquid exfoliation to achieve the first large-scale production of monolayer, bilayer, and few-layer phosphorus, with exfoliation demonstrated at the 10 g scale. We describe a rapid approach for quantifying the thickness of 2D phosphorus and show that monolayer and few-layer flakes produced by our approach are crystalline and unoxidized, while air exposure leads to rapid oxidation and the production of acid. With large quantities of 2D phosphorus now available, we perform the first quantitative measurements of the material's absorption edge-which is nearly identical to the material's band gap under our experimental conditions-as a function of flake thickness. Our interpretation of the absorbance spectrum relies on an analytical method introduced in this work, allowing the accurate determination of the absorption edge in polydisperse samples of quantum-confined semiconductors. Using this method, we found that the band gap of black phosphorus increased from 0.33 ± 0.02 eV in bulk to 1.88 ± 0.24 eV in bilayers, a range that is larger than that of any other 2D material. In addition, we quantified a higher-energy optical transition (VB-1 to CB), which changes from 2.0 eV in bulk to 3.23 eV in bilayers. This work describes several methods for producing and analyzing 2D phosphorus while also yielding a class of 2D materials with unprecedented optoelectronic properties.

Anisotropic Particle-Hole Excitations in Black Phosphorus

We report on the energy- and momentum-resolved optical response of black phosphorus (BP) in its bulk form. Along the armchair direction of the puckered layers, we find a highly dispersive mode that is strongly suppressed in the perpendicular (zigzag) direction. This mode emerges out of the single-particle continuum for finite values of momentum and is therefore interpreted as an exciton. We argue that this exciton, which has already been predicted theoretically for phosphorene-the monolayer form of BP-can be detected by conventional optical spectroscopy in the two-dimensional case and might pave the way for optoelectronic applications of this emerging material.

A new phase of phosphorus: the missed tricycle type red phosphorene

We predict a new two-dimensional allotrope of phosphorus, which we call red phosphorene, by restructuring the segments of the previously proposed blue and black phosphorenes. Its atomic and electronic structures as well as the thermodynamic and dynamic stabilities are systematically studied by first-principles calculations. The results indicate that the red phosphorene is dynamically stable and possesses remarkably thermodynamical stability comparable to that of the black one. Because of the sp(3)-hybridization and the formation of a localized lone pair, red phosphorene is a semiconductor with an indirect band gap of about 1.96 eV, which can be effectively modulated by in-plane strains due to its wave-like configuration. We find that the red, black and blue phosphorenes show evident distinction in their layer thicknesses, surface work functions, and possible colors, based on which one can distinguish them in future experiments.

Two-dimensional Kagome phosphorus and its edge magnetism: a density functional theory study

By means of density functional theory calculations, we predict a new two-dimensional phosphorus allotrope with the Kagome-like lattice(Kagome-P). It is an indirect gap semiconductor with a band gap of 1.64 eV. The gap decreases sensitively with the compressive strain. In particular, shrinking the lattice beyond 13% can drive it into metallic state. In addition, both the AA and AB stacked Kagome-P multi-layer structures exhibit a bandgap much smaller than 1.64 eV. Edges in the Kagome-P monolayer probably suffer from the edge reconstruction. An isolated zigzag edge can induce antiferromagnetic (AF) ordering with a magnetic transition temperature of 23 K. More importantly, when applying a stretching strain beyond 4%, such an edge turns to possess a ferromagnetic ground state. A very narrow zigzag-edged Kagome-P ribbon displays the spin moment distribution similar to the zigzag-edged graphene nanoribbon because of the coupling between the opposites edges. But the inter-edge coupling in the Kagome-P ribbon vanishes more rapidly as the ribbon width increases. These properties make it a promising material in spintronics.

Interfacing graphene and related 2D materials with the 3D world

An important prerequisite to translating the exceptional intrinsic performance of 2D materials such as graphene and transition metal dichalcogenides into useful devices precludes their successful integration within the current 3D technology. This review provides theoretical insight into nontrivial issues arising from interfacing 2D materials with 3D systems including epitaxy and ways to accommodate lattice mismatch, the key role of contact resistance and the effect of defects in electrical and thermal transport.