Two-Dimensional Black Phosphorus: An Emerging Anode Material for Lithium-Ion Batteries

Two-dimensional black phosphorus(2D BP),an emerging material,has aroused tremendous interest once discovered.This is due to the fact that it integrates unprecedented properties of other 2D materials,such as tunable bandgap structures,outstanding electrochemical properties,anisotropic mechanical,thermodynamic,and photoelectric properties,making it of great research value in many fields.The emergence of 2D BP has greatly promoted the development of electrochemical energy storage devices,especially lithium-ion batteries.However,in the application of 2D BP,there are still some problems to be solved urgently,such as the difficulty in the synthesis of large-scale high-quality phosphorene,poor environmental stability,and the volume expansion as electrode materials.Herein,according to the latest research progress of 2D BP in the field of energy storage,we systematically summarize and compare the preparation methods of phosphorene and discuss the basic structure and properties of BP,especially the environmental instability and passivation techniques.In particular,the practical application and challenges of 2D BP as anode material for lithium-ion batteries are analyzed in detail.Finally,some personal perspectives on the future development and challenges of BP are presented.

Strategies to improve electrocatalytic and photocatalytic performance of two-dimensional materials for hydrogen evolution reaction

Two-dimensional materials(2D)with unique physicochemical properties have been widely studied for their use in many applications,including as hydrogen evolution catalysts to improve the efficiency of water splitting.Recently,typical 2D materials MoS2,graphene,MXenes,and black phosphorus have been widely investigated for their application in the hydrogen evolution reaction(HER).In this review,we summarize three efficient strategies—defect engineering,heterostructure formation,and heteroatom doping—for improving the HER performance of 2D catalysts.The d-band theory,density of states,and Fermi energy level are discussed to provide guidance for the design and construction of novel 2D materials.The challenges and prospects of 2D materials in the HER are also considered.

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power.Moreover,the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades.Among these compounds,layered two-dimensional(2D)materials,such as graphene,black phosphorus,transition metal dichalcogenides,IVA–VIA compounds,and MXenes,have generated a large research attention as a group of potentially high-performance thermoelectric materials.Due to their unique electronic,mechanical,thermal,and optoelectronic properties,thermoelectric devices based on such materials can be applied in a variety of applications.Herein,a comprehensive review on the development of 2D materials for thermoelectric applications,as well as theoretical simulations and experimental preparation,is presented.In addition,nanodevice and new applications of 2D thermoelectric materials are also introduced.At last,current challenges are discussed and several prospects in this field are proposed.

Electronic, optical property and carrier mobility of graphene, black phosphorus, and molybdenum disulfide based on the first principles

The band structure, density of states, optical properties, carrier mobility, and loss function of graphene, black phosphorus（BP）, and molybdenum disulfide（MoS_2） were investigated by the first-principles method with the generalized-gradient approximation. The graphene was a zero-band-gap semiconductor. The band gaps of BP and MoS_2 were strongly dependent on the number of layers. The relationships between layers and band gap were built to predict the band gap of few-layer BP and MoS_2. The absorption showed an explicit anisotropy for light polarized in（1 0 0） and（0 0 1） directions of graphene, BP,and MoS_2. This behavior may be readily detected in spectroscopic measurements and exploited for optoelectronic applications. Moreover, graphene（5.27 × 10~4 cm^2·V^（-1）·s^（-1））, BP（1.5 × 10~4 cm^2·V^（-1）·s^（-1））, and MoS_2（2.57×102 cm2·V-1·s-1）have high carrier mobility. These results show that graphene, BP, and MoS_2 are promising candidates for future electronic applications.

A brief review of tribological properties for black phosphorus

Black phosphorus (BP) is a new class of two-dimensional (2D) layered material, which shows the unanticipated characteristics in many aspects including electronics, transistors, sensors, energy storage, batteries, photocatalysis, and other applications due to its high charge carrier mobility, tunable direct bandgap, and unique in-plane anisotropic structure. In addition, BP has drawn tremendous attention in the field of tribology due to the low shear strength, the layered structure, and the weak connected force between the layers by van der Waals interaction. In recent years, many significant progresses have been made in experimental studies on BP materials as solid lubricants or lubrication additives. This work offers a review of researching regarding the tribological properties of BP. Moreover, the lubrication mechanisms of BP as the lubrication additive including the formation of the tribo-film, micro-bearing effect, and self-repair performance are also summarized. Finally, the current challenges and prospects of BP material as lubricant are proposed.

Recent advances in anisotropic two-dimensional materials and device applications

Two-dimensional(2D)materials,such as transition metal dichalcogenides(TMDs),black phosphorus(BP),MXene and borophene,have aroused extensive attention since the discovery of graphene in 2004.They have wide range of applications in many research fields,such as optoelectronic devices,energy storage,catalysis,owing to their striking physical and chemical properties.Among them,anisotropic 2D material is one kind of 2D materials that possess different properties along different directions caused by the intrinsic anisotropic atoms5 arrangement of the 2D materials,mainly including BP,borophene,low-symmetry TMDs(ReSe2 and ReSa)and group IV monochalcogenides(SnS,SnSe,GeS,and GeSe).Recently,a series of new devices has been fabricated based on these anisotropic 2D materials.In this review,we start from a brief introduction of the classifications,crystal structures,preparation techniques,stability,as well as the strategy to discriminate the anisotropic characteristics of 2D materials.Then,the recent advanced applications including electronic devices,optoelectronic devices,thermoelectric devices and nanomechanical devices based on the anisotropic 2D materials both in experiment and theory have been summarized.Finally,the current challenges and prospects in device designs,integration,mechanical analysis,and micro-/nano-fabrication techniques related to anisotropic 2D materials have been discussed.This review is aimed to give a generalized knowledge of anisotropic 2D materials and their current devices applications,and thus inspiring the exploration and development of other kinds of new anisotropic 2D materials and various novel device applications.

Revealing of the ultrafast third-order nonlinear optical response and enabled photonic application in two-dimensional tin sulfide

Black phosphorus(BP), a typical mono-elemental and two-dimensional(2D) material, has gathered significant attention owing to its distinct optoelectronic properties and promising applications, despite its main obstacle of long-term stability. Consequently, BP-analog materials with long-term chemical stability show additional potential. In this contribution, tin sulfide(SnS), a novel two-elemental and 2D structural BP-analog monochalcogenide, has been demonstrated to show enhanced stability under ambient conditions. The broadband nonlinear optical properties and carrier dynamics have been systematically investigated via Z-scan and transient absorption approaches. The excellent nonlinear absorption coefficient of 50.5 × 10^-3 cm∕GW, 1 order of magnitude larger than that of BP, endows the promising application of SnS in ultrafast laser generation. Two different decay times of τ1~873 fs and τ2~96.9 ps allow the alteration between pure Q switching and continuous-wave(CW) mode locking in an identical laser resonator. Both mode-locked and Q-switched operations have been experimentally demonstrated using an SnS saturable absorber at the telecommunication window. Femtosecond laser pulses with tunable wavelength and high stability are easily obtained, suggesting the promising potential of SnS as an efficient optical modulator for ultrafast photonics. This primary investigation may be considered an important step towards stable and high-performance BP-analog material-based photonic devices.

Investigation of black phosphorus as a nano-optical polarization element by polarized Raman spectroscopy

Manipulating the polarization of light at the nanoscale is essential for the development of nano-optical devices. Owing to its corrugated honeycomb structure, two-dimensional （2D） layered black phosphorus （BP） exhibits outstanding in-plane optical anisotropy with distinct linear dichroism and optical birefringence in the visible region, which are superior characteristics for ultrathin polarizing optics. Herein, taking advantage of polarized Raman spectroscopy, we demonstrate that layered BP with a nanometer thickness can remarkably alter the polarization state of a linearly-polarized laser and behave as an ultrathin optical polarization element in a BP-Bi2Se3 stacking structure by inducing the exceptionally polarized Raman scattering of isotropic Bi2Se3. Our findings provide a promising alternative for designing novel polarization optics based on 2D anisotropic materials, which can be easily integrated in micro- sized all-optical and optoelectronic devices.

Nanoscopy reveals surface-metallic black phosphorus

Black phosphorus(BP)is an emerging two-dimensional material with intriguing physical properties.It is highly anisotropic and highly tunable by means of both the number of monolayers and surface doping.Here,we experimentally investigate and theoretically interpret the near-field properties of a-few-atomic-monolayer nanoflakes of BP.We discover near-field patterns of bright outside fringes and a high surface polarizability of nanofilm BP consistent with its surface-metallic,plasmonic behavior at mid-infrared frequencies o1176 cm−1.We conclude that these fringes are caused by the formation of a highly polarizable layer at the BP surface.This layer has a thickness of~1 nm and exhibits plasmonic behavior.We estimate that it contains free carriers in a concentration of n≈1.1×10^(20) cm^(−3).Surface plasmonic behavior is observed for 10–40 nm BP thicknesses but absent for a 4-nm BP thickness.This discovery opens up a new field of research and potential applications in nanoelectronics,plasmonics and optoelectronics.

Two-dimensional square transition metal dichalcogenides with lateral heterostructures

Fabrication of lateral heterostructures （LHS） is promising for a wide range of next-generation devices and could sufficiently unlock the potential of two-dimensional materials.Herein,we demonstrate the design of lateral heterostructures based on new building materials,namely 1S-MX2 LHS,using first-principles calculations.1S-MX2 LHS exhibits excellent stability,demonstrating high feasibility in the experiment.The desired bandgap opening can endure application at room temperature and was confirmed in 1S-MX2 LHS with spin-orbit coupling （SOC）.A strain strategy further resulted in efficient bandgap engineering and an intriguing phase transition.We also found that black phosphorus can serve as a competent substrate to support 1S-MX2 LHS with a coveted type-Ⅱ band alignment,allowing versatile functionalized bidirectional heterostructures with built-in device functions.Furthermore,the robust electronic features could be maintained in the 1S-MX2 LHS with larger components.Our findings will not only renew interest in LHS studies by enriching their categories and properties,but also highlight the promise of these lateral heterostructures as appealing materials for future integrated devices.

Interstitial copper-doped edge contact for n-type carrier transport in black phosphorus

Black phosphorus(BP)has been shown as a promising two-dimensional(2D)material for electronic devices owing to its high carrier mobility.To realize complementary electronic circuits with 2D materials,it is important to fabricate both n-type and p-type transistors with the same channel material.By engineering the contact region with copper(Cu)-doped BP,here we demonstrate an n-type carrier transport in BP field-effect transistors(FETs),which usually exhibit strongly p-type characteristics.Cu metal atoms are found to severely penetrate into the BP flakes,which forms interstitial Cu(Cuint)-doped edge contact and facilitates the electron transport in BP.Our BP FETs in backgated configuration exhibit n-type dominant characteristics with a high electron mobility of^138 cm^2 V^−1 s^−1 at room temperature.The Schottky barrier height for electrons is relatively low because of the edge contact between Cuint-doped BP and pristine BP channel.The contact doping of BP by highly mobile Cu atoms gives rise to n-type transport property of BP FETs.Furthermore,we demonstrate a p-n junction on the same BP flake with asymmetric contact.This strategy on contact engineering can be further extended to other 2D materials.

2020 Roadmap on two-dimensional nanomaterials for environmental catalysis

Environmental catalysis has drawn a great deal ofattention due to its clean ways to produce useful chemicals or carry out some chemical processes.Photocatalysis and electrocatalysis play important roles in these fields.They can decompose and remove organic pollutants from the aqueous environment,and prepare some fine chemicals.Moreover,they also can carry out some important reactions,such as 02 reduction reaction(ORR),O2 evolution reaction(OER),H2 evolution reaction(HER),CO2 reduction reaction(C02 RR),and N2 fixation(NRR).For catalytic reactions,it is the key to develop high-performance catalysts to meet the demand fortargeted reactions.In recentyears,two-dimensional(2 D) materials have attracted great interest in environmental catalysis due to their unique layered structures,which offer us to make use of their electronic and structural characteristics.Great progress has been made so far,including graphene,black phosphorus,oxides,layered double hydroxides(LDHs),chalcogenides,bismuth-based layered compounds,MXenes,metal organic frameworks(MOFs),covalent organic frameworks(COFs),and others.This content drives us to invite many famous groups in these fields to write the roadmap on two-dimensional nanomaterials for environmental catalysis.We hope that this roadmap can give the useful guidance to researchers in future researches,and provide the research directions.

Covalent carbene modification of 2D black phosphorus

Black phosphorus(BP)has attracted an ever-growing interest due to its unique anisotropic two-dimensional structure,impressive photoelectronic properties and attractive application potential.However,the tools for bandgap engineering and passivation via covalent modification of BP nanosheets remain limited to diazonium salt and nucleophilic addition methods,so that developing new modification strategies for BP nanosheets is crucial to explore its physical and chemical properties and enrich the toolbox for functionalization.Herein,we report the covalent modification of liquid-phase exfoliated BP nanosheets based on a rational analysis of BP structure.The modification of BP is achieved via carbene,a highly reactive organic mediate.The carbene modification improves the solubility and stability of BP nanosheets.Detailed microscopic and spectroscopic characterizations including infrared spectra,Raman spectra,X-ray photoelectron spectra,SEM and TEM were conducted to provide insights for the reaction.The proof of the existence of covalent bonds between BP nanosheets and organic moieties confirms the successful modification.Moreover,theoretical calculations were conducted to unveil the reaction mechanism of the two different types of bonds and the chemical property of two-dimensional BP.

2D layered black arsenic-phosphorus materials:Synthesis,properties,and device applications

Phosphorene,especially black phosphorus(BP),has attracted considerable attention due to the unique characteristics,such as tunable direct bandgap,high carrier mobility,and strong in-plane anisotropy.Recently,a new modification strategy for black phosphorus has been developed by alloying black phosphorus with the congener element arsenic.The elemental composition tuning of black phosphorus with arsenic can not only maintain its special crystal structure and high anisotropy but also modify its electrical and optical properties for the further applications of multifunctional devices.The achieved two-dimensional(2D)black arsenic-phosphorus materials exhibit outstanding optical,electrical,and photoelectric properties,such as very narrow band gap,anisotropic infrared absorption,and bipolar transfer characteristics,presenting great potential in infrared photodetectors and highperformance field effect transistors(FETs).In this review,we introduce the recent progress made in the synthesis and applications of black arsenic-phosphorus,and provide an outlook and perspectives on the current challenges and future opportunities in this field.We hope that this review can bring new insights and inspirations on the further development of 2D black arsenic-phosphorus based materials and devices.

Black-phosphorus-based junctions and their optoelectronic device applications

Black phosphorus(BP)has attracted significant attention owing to its unique structure and preeminent photoelectric properties,which can be utilized to create novel junctions.Based on different BP-based junctions,versatile optoelectronic devices have been fabricated and investigated in recent years,providing a fertile library for the characteristics of BP-based junctions and their optoelectronic applications.This review summarizes diverse BP-based junctions and their optoelectronic device applications.We firstly introduce the structure and properties of BP.Then,we emphatically describe the formation,properties,and optoelectronic device applications of the BP-based junctions including heterojunctions of BP and other two-dimensional(2D)semiconductors,BP p–n homojunctions,and BP/metal Schottky junctions.Finally,the challenge and prospect of the development and application of BP-based junctions are discussed.This timely review gives a snapshot of recent research breakthroughs in BP-based junctions and optoelectronic devices based on them,which is expected to provide a comprehensive vision for the potential of BP in the optoelectronic field.

Observation of robust anisotropy in WS_(2)/BP heterostructures

Two-dimensional(2D)anisotropic materials have garnered significant attention in the realm of anisotropic optoelectronic devices due to their remarkable electrical,optical,thermal,and mechanical properties.While extensive research has delved into the optical and electrical characteristics of these materials,there remains a need for further exploration to identify novel materials and structures capable of fulfilling device requirements under various conditions.Here,we employ heterojunction interface engineering with black phosphorus(BP)to disrupt the C_(3) rotational symmetry of monolayer WS_(2).The resulting WS_(2)/BP heterostructure exhibits pronounced anisotropy in exciton emissions,with a measured anisotropic ratio of 1.84 for neutral excitons.Through a comprehensive analysis of magnetic-field-dependent and temperature-evolution photoluminescence spectra,we discern varying trends in the polarization ratio,notably observing a substantial anisotropy ratio of 1.94 at a temperature of 1.6 K and a magnetic field of 9 T.This dynamic behavior is attributed to the susceptibility of the WS_(2)/BP heterostructure interface strain to fluctuations in magnetic fields and temperatures.These findings provide valuable insights into the design of anisotropic optoelectronic devices capable of adaptation to a range of magnetic fields and temperatures,thereby advancing the frontier of material-driven device engineering.

Lattice vibrations and Raman scattering in two- dimensional layered materials beyond graphene

We review lattice vibrational modes in atomically thin two-dimensional （2D） layered materials, focusing on 2D materials beyond graphene, such as group VI transition metal dichalcogenides, topological insulator bismuth chalcogenides, and black phosphorus. Although the composition and structure of those materials are remarkably different, they share a common and important feature, i.e., their bulk crystals are stacked via van der Waals interactions between ＂layers＂, while each layer is comprised of one or more atomic planes. First, we review the background of some 2D materials （MX2, M = Mo, W; X = S, Se, Te. Bi2X3, X = Se, Te. Black phosphorus）, including crystalline structures and stacking order. We then review the studies on vibrational modes of layered materials and nanostructures probed by the powerful yet nondestructive Raman spectroscopy technique. Based on studies conducted before 2010, recent investigations using more advanced techniques have pushed the studies of phonon modes in 2D layered materials to the atomically thin regime, down to monolayers. We will classify the recently reported general features into the following categories： phonon confinement effects and electron-phonon coupling, anomalous shifts in high-frequency intralayer vibrational modes and surface effects, reduced dimensionality and lower symmetry, the linear chain model and the substrate effect, stacking orders and interlayer shear modes, polarization dependence, and the resonance effect. Within the seven categories, both intralayer and interlayer vibrational modes will be discussed. The comparison between different materials will be provided as well.

New materials and designs for 2D-based infrared photodetectors

Infrared photodetectors have attracted much attention considering their wide civil and military applications.Two-dimensional(2D)materials offer new opportunities for the development of costless,high-level integration and high-performance infrared photodetectors.With the advent of a broad investigation of infrared photodetectors based on graphene and transition metal chalcogenides(TMDs)exhibiting unique properties in recent decades,research on the better performance of 2D-based infrared photodetectors has been extended to a larger scale,including explorations of new materials and artificial structure designs.In this review,after a brief background introduction,some major working mechanisms,including the photovoltaic effect,photoconductive effect,photogating effect,photothermoelectric effect and bolometric effect,are briefly offered.Then,the discussion mainly focuses on the recent progress of three categories of 2D materials beyond graphene and TMDs.Noble transition metal dichalcogenides,black phosphorus and arsenic black phosphorous and 2D ternary compounds are great examples of explorations of mid-wavelength or even long-wavelength 2D infrared photodetectors.Then,four types of rational structure designs,including type-II band alignments,photogating-enhanced designs,surface plasmon designs and ferroelectric-enhanced designs,are discussed to further enhance the performance via diverse mechanisms,which involve the narrower-bandgap-induced interlayer exciton transition,gate modulation by trapped carriers,surface plasmon polaritons and ferroelectric polarization in sequence.Furthermore,applications including imaging,flexible devices and on-chip integration for 2D-based infrared photodetectors are introduced.Finally,a summary of the state-of-the-art research status and personal discussion on the challenges are delivered.

Complementary doping of van der Waals materials through controlled intercalation for monolithically integrated electronics

Doping control has been a key challenge for electronic applications of van der Waals materials.Here,we demonstrate complementary doping of black phosphorus using controlled ionic intercalation to achieve monolithic building elements.We characterize the anisotropic electrical transport as a function of ion concentrations and report a widely tunable resistivity up to three orders of magnitude with characteristic concentration dependence corresponding to phase transitions during intercalation.As a further step,we develop both p-type and n-type field effect transistors as well as electrical diodes with high device stability and performance.In addition,enhanced charge mobility from 380 to 820 cm^2/(V·s)with the intercalation process is observed and explained as the suppressed neutral impurity scattering based on our ab initio calculations.Our study provides a unique approach to atomically control the electrical properties of van der Waals materials,and may open up new opportunities in developing advanced electronics and physics platforms.