Recent Advances in Phosphorene: Structure, Synthesis, and Properties

Phosphorene is a 2D phosphorus atomic layer arranged in a honeycomb lattice like graphene but with a buckled structure. Since its exfoliation from black phosphorus in 2014, phosphorene has attracted tremendous research interest both in terms of synthesis and fundamental research, as well as in potential applications. Recently, significant attention in phosphorene is motivated not only by research on its fundamental physical properties as a novel 2D semiconductor material, such as tunable bandgap, strong in-plane anisotropy, and high carrier mobility, but also by the study of its wide range of potential applications, such as electronic, optoelectronic, and spintronic devices, energy conversion and storage devices. However, a lot of avenues remain to be explored including the fundamental properties of phosphorene and its device applications. This review recalls the current state of the art of phosphorene and its derivatives, touching upon topics on structure, synthesis, characterization, properties, stability, and applications. The current needs and future opportunities for phosphorene are also discussed.

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.

mech2d: An Efficient Tool for High-Throughput Calculation of Mechanical Properties for Two-Dimensional Materials

Two-dimensional (2D) materials have been a research hot topic in the passed decades due to their unique and fascinating properties. Among them, mechanical properties play an important role in their application. However, there lacks an effective tool for high-throughput calculating, analyzing and visualizing the mechanical properties of 2D materials. In this work, we present the mech2d package, a highly automated toolkit for calculating and analyzing the second-order elastic constants (SOECs) tensor and relevant properties of 2D materials by considering their symmetry. In the mech2d, the SOECs can be fitted by both the strain-energy and stress-strain approaches, where the energy or strain can be calculated by a first-principles engine, such as VASP. As a key feature, the mech2d package can automatically submit and collect the tasks from a local or remote machine with robust fault-tolerant ability, making it suitable for high-throughput calculation. The present code has been validated by several common 2D materials, including graphene, black phosphorene, GeSe2 and so on.

Two-dimensional IV-VA3 monolayers with enhanced charge mobility for high-performance solar cells

High-performance photovoltaics (PVs) constitute a subject of extensive research efforts, in which silicon (Si)-based solar cells (SCs) have been widely commercialized. However, the low carrier mobility of Si-based SCs can limit the effective charge separation, thereby negatively impacting the device performance. Here, via calculating the physicochemical and PV performance based on density functional theory, we demonstrate SCs based on two-dimensional (2D) group IV and V compounds with an AX₃ configuration. Firstly, the cleavage energies of AX₃ (A = Si, Ge; X = P, As, and Sb) are calculated to be less than 1 J m^-2, providing an experimental feasibility to be exfoliated from the corresponding bulk. Secondly, electronic and optical properties have been systematically investigated. To be specific, the band gap of monolayer AX₃ falls in the range of 1.11-1.27 eV, which is comparable with that of Si. Significantly, the electron mobility of monolayer AX₃ can reach as high as ∼30 000 cm² V^-1 s^-1, which is one order of magnitude higher than that of Si. Furthermore, the optical absorbance of monolayer SiAs₃, SiP₃ and GeAs₃ exhibits high coefficients in visible light. Therefore, we believe that our designed AX₃-based PV systems with power conversion efficiency of 20% can offer great potential in the application of high-performance two-dimension-based PVs.

Stability, optoelectronic and thermal properties of two-dimensional Janusα-Te2S

Motivated by recent progress in the two-dimensional (2D) materials of group VI elements and their experimental fabrication, we have investigated the stability, optoelectronic and thermal properties of Janusα-Te₂S monolayer using first-principles calculations. The phonon dispersion and MD simulations confirm its dynamical and thermal stability. The moderate band gap (∼1.5 eV), ultrahigh carrier mobility (∼10³cm²V^-1s^-1), small exciton binding energy (0.26 eV), broad optical absorption range and charge carrier separation ability due to potential difference (ΔV = 1.07 eV) on two surfaces of Janusα-Te₂S monolayer makes it a promising candidate for solar energy conversion. We propose various type-II heterostructures consisting of Janusα-Te₂S and other transition metal dichalcogenides for solar cell applications. The calculated power conversion efficiencies of the proposed heterostructures, i.e.α-Te₂S/T-PdS₂,α-Te₂S/BP andα-Te₂S/H-MoS₂are ∼21%, ∼19% and 18%, respectively. Also, the ultralow value of lattice thermal conductivity (1.16 W m^-1K^-1) of Janusα-Te₂S makes it a promising material for the fabrication of next-generation thermal energy conversion devices.

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...

New graphane: inspiration from the structure correlation with phosphorene

The application of phosphorene and graphane in different photoelectric devices and energy reserves has attracted wide attention. Here, we investigated the Raman spectra, phonon dispersion and vibration modes of four phosphorene monolayer polymorphs and four graphane allotropes with the corresponding crystal structures to analyze the structure correlation between them. Based on the "three identical, one divergent" pattern found in the sp3 hybrid atomic orbitals of phosphorene and graphane, four new graphane conformers with different hydrogenation modes named γδ-G, αγ-G, βγ-G and αδ-G are successfully predicted. Among these four new graphane conformers, βγ-G has the lowest binding energy, which is only 0.02 eV per atom higher than β-G, the most stable one among all graphane theoretically predicted. This means that βγ-G may co-exist with β-G during the experimental synthesis of graphane, which can be distinguished from the side views with threefold structures for βγ-G and twofold structures for β-G. All the new graphane conformers are direct-band-gap semiconductors with band gaps more than 3 eV, which indicate their great potential in optoelectronic devices. Furthermore, three of them exhibit in-plane negative Poisson's ratios under tensile deformation.

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.

Novel phosphorus-based 2D allotropes with ultra-high mobility

Electronic structure calculations based on density functional theory were performed to investigate structural, mechanical, and electronic properties of phosphorene-based large honeycomb dumbbell (LHD) hybrid structures and a new phosphorene allotrope, referred to as ψ″-P. The LHD hybrids (i.e., X₆P₄; X being C or Si or Ge or Sn) and ψ″-P have significantly higher bandgaps than the corresponding pristine LHD structures, except the case of C₆P₄, which is metallic. ψ″-P is found to be a highly flexible p-type material which shows strain-engineered photocatalytic activity in a highly alkaline medium. The carrier mobility of the considered systems is as high as 10⁵ cm² V^-1 s^-1 (specifically the electron mobility of LHD structures). The calculated STM images display the surface morphologies of the LHD hybrids and ψ″-P. The predicted phosphorus-based 2D structures with novel electronic properties may be candidate materials for nanoscale devices.

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.

Epitaxial Growth of Flat, Metallic Monolayer Phosphorene on Metal Oxide

In recent years, two-dimensional (2D) group VA elemental materials have attracted considerable interest from physics/chemistry and materials science communities, with particular attention paid to honeycomb blue phosphorene. To date, phosphorene is limited to its α-phase and small sizes because it can only be produced by exfoliating black phosphorus crystals. Here, we report the direct synthesis of high-quality phosphorene on a nonmetallic copper oxide substrate by molecular beam epitaxy. By combining scanning tunneling microscopy/spectroscopy, X-ray photoelectron spectroscopy, and first-principles calculations, we demonstrate the growth intermediates and electronic structures of phosphorene on Cu₃O₂/Cu(111). Surprisingly, the grown phosphorene has a flat honeycomb lattice, similar to graphene, which exhibits a metallic nature. We reveal that the growth mechanism and morphology of phosphorene are strongly correlated with the surface structures of prepared copper oxide, and the resulting phosphorene can be stabilized after high-temperature annealing above 600 K even in oxygen gas. The high stability is closely related to the irregular Moiré pattern and structural corrugations of phosphorene on Cu₃O₂/Cu(111) that efficiently relieve the surface strain. These results shed light on future fabrication of large-scale, versatile 2D structures for interconnect and device integration.

Strain-tunable electronic and optical properties of novel anisotropic green phosphorene: a first-principles study

The effect of in-layer strain on the optical and electrical properties of monolayer green phosphorene, a new anisotropic two-dimensional (2D) material, has been systematically studied. The studied strain includes in-layer uniaxial strain and biaxial strain. Green phosphorene can be viewed as a combination of black and blue phosphorene segments in regular order. We adopt the HSE06 method to correct the calculated results. The results reveal that, firstly, strain-free monolayer green phosphorene is a stable direct band gap 2D semiconductor with the anisotropic optical property. The transmittance of infrared and visible light along the zigzag direction is better than that along the armchair direction. However, the transmittance of the UV-light along the armchair direction is better than that along the zigzag direction. Secondly, the optical properties, such as the absorption coefficient and reflectivity, along armchair and zigzag direction respond very differently to the various applied strains. As for the electronic properties, the band gap exhibits different changing trends by applying either in-layer biaxial strain or uniaxial strain in different directions. Besides, the near-band-edge electronic orbitals exhibit different bond nature in different directions. These results suggest that green phosphorene shows strong anisotropy in electronic and optical properties. By calculating and comparing the energies of near-band-edge states after applying different strains on green phosphorene, the reason for the anisotropy of the new 2D material is analyzed. This study implies that the electronic and optical properties of green phosphorene, a stable direct band gap anisotropic semiconductor, could be efficiently tuned by in-layer biaxial or uniaxial strain. Therefore, green phosphorene can be used in linear polarizers and other anisotropic photoelectric devices.

Reversible Oxidation of Blue Phosphorus Monolayer on Au(111)

Practical applications of two-dimensional (2D) black phosphorus (BP) are limited by its fast degradation under ambient conditions, for which many different mechanisms have been proposed; however, an atomic level understanding of the degradation process is still hindered by the absence of bottom-up methods for the growth of large-scale few-layer black phosphorus. Recent experimental success in the fabrication of single-layer blue phosphorus provides a model system to probe the oxidation mechanism of two-dimensional (2D) phosphorene down to single-layer thicknesses. Here, we report an atomic-scale investigation of the interaction between molecular oxygen and blue phosphorus. The atomic structure of blue phosphorus and the local binding sites of oxygen have been precisely identified using qPlus-based noncontact atomic force microscopy. A combination of low-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements reveal a thermally reversible oxidation process of blue phosphorus in a pure oxygen atmosphere. Our study clearly demonstrates the essential role of oxygen in the initial oxidation process, and it sheds further light on the fundamental pathways of the degradation mechanism.

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.

Vibrational properties and Raman spectra of pristine and fluorinated blue phosphorene

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

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.

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.

Stacking sequences of black phosphorous allotropes and the corresponding few-layer phosphorenes

Possible bulk black phosphorus (BP) allotropes are constructed based on single-layer BP with various stacking sequences. Our stacking algorithm shows that there are eight possible allotropes with two stacking layers in their unit cells possessing relatively high symmetries, and six of them are retained after structural relaxation using a van der Waals correction of optB88-vdW. The AF, AG, and AH bulk structures are presented for the first time. The structural relationship of these configurations has been explained via an interlayer slipping process. The total energy of the AF allotrope is closest to the most stable bulk BP structure (AB stacking) among all explored 2-layer stacked bulk structures. The calculated band structure of the AF allotrope using HSE06 shows a direct band gap of 0.48 eV with anisotropic electronic structures. We also presented six possible BP allotropes with three stacking layers in their unit cells. The newly reported AAF and ABC stacked structures show semiconducting and metallic features, respectively. After the bulk structures were explored, we further built the corresponding few-layer phosphorene structures and investigated their electronic properties. The results show that all the few-layer phosphorenes show semiconducting features. The AE, AAE, and AEA phosphorenes have indirect band gaps while the other explored phosphorenes possess direct band gaps located at the Γ point.

Stability and carrier transport properties of phosphorene-based polymorphic nanoribbons

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

All-phosphorus flexible devices with non-collinear electrodes: a first principles study

With the continuous expansion of the family of two-dimensional (2D) materials, flexible electronics based on 2D materials have quickly emerged. Theoretically, predicting the transport properties of the flexible devices made up of 2D materials using first principles is of great importance. Using density functional theory combined with the non-equilibrium Green's function formalism, we calculated the transport properties of all-phosphorus flexible devices with non-collinear electrodes, and the results predicted that the device with compressed metallic phosphorene electrodes sandwiching a P-type semiconducting phosphorene shows a better and robust conducting behavior against the bending of the semiconducting region when the angle between the two electrodes is less than 45°, which indicates that this system is very promising for flexible electronics. The calculation of a quantum transport system with non-collinear electrodes demonstrated in this work will provide more interesting information on mesoscopic material systems and related devices.

Data-driven learning and prediction of inorganic crystal structures

Machine learning-based interatomic potentials, fitting energy landscapes “on the fly”, are emerging and promising tools for crystal structure prediction.

Two dimensional allotropes of arsenene with a wide range of high and anisotropic carrier mobility

Considering the rapid development of experimental techniques for fabricating 2D materials in recent years, various monolayers are expected to be experimentally realized in the near future.

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.

Assembling phosphorene flexagons for 2D electron-density-guided nanopatterning and nanofabrication

To build upon the rich structural diversity in the ever-increasing polymorphic phases of two-dimensional phosphorene, we propose different assembly methods (namely, the "bottom-up" and "top-down" approaches) that involve four commonly reported parent phases (i.e. the α-, β-, γ-, and δ-phosphorene) in combination with the lately reported remarkably low-energy one-dimensional defects in α-phosphorene. In doing so, we generate various periodically repeated phosphorene patterns in these so-called phosphorene flexagons and present their local electron density (via simulated scanning tunneling microscopy (STM) images). These interesting electron density patterns seen in the flexagons (mimicking symmetry patterns that one may typically see in a kaleidoscope) may assist as potential 2D templates where electron-density-guided nanopatterning and nanofabrication in complex organized nanoarchitectures are important.