Recent progress in black phosphorus and black-phosphorus-analogue materials: properties, synthesis and applications

Yijun Xu a, Zhe Shi a, Xinyao Shi bc, Kai Zhang *b and Han Zhang *a
aShenzhen Engineering Laboratory of Phosphorene and Optoelectronics, Collaborative Innovation Center for Optoelectronic Science and Technology and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen 518060, China. E-mail: hzhang@szu.edu.cn
bCAS Key Laboratory of Nano-Bio Interface & Key Laboratory of Nanodevices and Applications, i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China. E-mail: kzhang2015@sinano.ac.cn
cSchool of Nano Technology and Nano Bionics, University of Science and Technology of China, Hefei 230026, China

Received 21st May 2019 , Accepted 1st July 2019

First published on 17th July 2019


Abstract

Black phosphorus (BP), a novel two-dimensional (2D) layered semiconductor material, has attracted tremendous attention since 2014 due to its prominent carrier mobility, thickness-dependent direct bandgap and in-plane anisotropic physical properties. BP has been considered as a promising material for many applications, such as in transistors, photonics, optoelectronics, sensors, batteries and catalysis. However, the development of BP was hampered by its instability under ambient conditions, as well as by the lack of methods to synthesize large-area and high quality 2D nanofilms. Recently, some BP-analogue materials such as binary phosphides (MPx), transition metal phosphorus trichalcogenides (MPX3), and 2D group V (pnictogens) and 2D group VI materials have attracted increasing interest for their unique and stable structures, and excellent physical and chemical properties. This article, which focuses on BP and BP-analogue materials, will present their crystal structure, properties, synthesis methods and applications. Also the similarity and difference between BP and BP-analogue materials will be discussed, and the presentation of the future opportunities and challenges of the materials are included at the end.


1 Introduction

2D materials have been triggering intensive research interest. Much progress has been made in semiconductor technology.1–30 Although the success in the wafer scale growth of TMDC films31 and the realization of 1 nm gate-length transistors by utilizing MoS2 as a channel material32 are considered to be a milestone, applications of these materials in integrated electronics and optoelectronics are restricted by the zero bandgap of graphene and low current mobility of TMDCs. Recently, black phosphorus (BP), a novel emerging layered semiconductor material that possesses a thickness-dependent tunable direct bandgap from 0.3 to 1.5 eV (bulk to monolayer), bridges the energy gap between graphene and TMDCs.33,34 Field-effect transistors (FETs) based on BP have presented a high carrier mobility over 1000 cm2 V−1 s−1,33,35–46 together with a high on/off ratio of 105. Additionally, BP exhibits much stronger photon absorption than other 2D materials and possesses in-plane anisotropic properties, which is beneficial for applications in high performance optoelectronics,47–76 such as broadband and chirality-sensitive photodetection, as well as other applications in batteries,77–93 catalysts,94–100 sensors,101–110 biomedicine,111–119etc. Though BP holds the above-mentioned great advantages, the synthesis of high-quality thin BP films on a large-scale is still a great challenge. Until now, only top-down methods have been reported to obtain few-layer BP, such as mechanical exfoliation, liquid-phase exfoliation, and plasma etching. These methods exhibit low efficiency, and cannot be used to realize large scale industrialization. Furthermore, BP is unstable under ambient conditions, which will hinder its applications in many fields.120–124 Recently, it was found that some BP-analogue elementary, binary and ternary phosphide materials such as binary phosphides (MPx),125–128 transition metal phosphorus trichalcogenides (MPX3),129–133 and 2D group V (pnictogens)134–150 and 2D group VI materials151–157 may be promising candidates to overcome the limitations of BP and hence they received increasing interest. BP-analogue materials share a similar honeycomb and layered crystal structure to BP. The structure induces some unique and excellent physical properties, such as broad and tunable bandgaps,128,129,158–162 anisotropic properties,134,159,163–168 and high carrier mobility.128,133,169–173 Similar to BP, 2D MPx (M = group III/IV/V elements) features layered hexagonal configurations, anisotropic properties and tunable bandgaps. Furthermore, 2D MPx displays better stability and higher carrier mobility than BP. 2D MPx is considered to be a promising candidate for various applications like nano-electronics, optoelectronics, light-emitting diodes and lasers, photo-catalysis and so on. Transition metal phosphorus trichalcogenides MPX3 (M = Fe, Co, Ni, Zn, etc.; X = S, Se or Te) are ternary compounds arranged as a honeycomb crystal structure, which induce fascinating electric and magnetic properties. MPX3 are layered structures with a large van der Waals gap between layers, which enables the material to be easily exfoliated into mono or few layers, and are much stable under ambient conditions compared with BP. MPX3 have attracted great attention for tremendous applications not only due to their extraordinary magnetic and ferroelectric properties, but also their excellent performances in hydrogen storage and lithium batteries because of the layered structures. The elements of group V or pnictogens include phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi). The heavy pnictogens (As, Sb, and Bi) crystallize with an orthorhombic (or the α phase) or rhombohedral (or the β phase) layered honeycomb structure. The α phase heavy pnictogens share a similar puckered crystal structure to black phosphorus, while the β phase elements are constructed with a buckled layered structure, which is similar to blue phosphorus. With their tunable bandgap, high carrier mobility, and excellent stability at room temperature, 2D Sb and Bi materials were explored as promising candidates for electronics and optoelectronics. More importantly, 2D As, Sb and Bi exhibit unique topological insulator properties which make heavy pnictogens promising candidates for topological phenomena and new quantum devices operating in nanoelectronics. Chalcogens are group VI materials, especially selenium (Se) and tellurium (Te), which are p-type narrow bandgap elemental semiconductors. Similar to BP, 2D group VI materials have a layered hexagonal crystal structure through weak van der Waals interactions, tunable bandgaps and high anisotropic properties. However, 2D group VI materials exhibit some properties superior to BP, such as better environmental stability, higher carrier mobility, better hydration and oxidation catalytic activity, higher piezoelectric, improved thermoelectric and nonlinear optical responses, etc. Unlike BP which lacks facile synthesis methods to obtain high-quality thin nanofilms, few-layer high-quality 2D BP-analogue materials can be synthesized through various reliable and feasible methods, like molecular beam epitaxy (MBE), van der Waals (vdW) epitaxy, physical vapor deposition (PVD), chemical vapor deposition (CVD), etc.

Therefore, 2D BP-analogue materials might be advantageous over BP according to their excellent physical and chemical properties, good stability under various ambient conditions,127,128,138,174–178 highly efficient synthesis methods etc. In this review, we summarize recent progress on BP and BP-analogue materials, focusing on similarities and differences in structure and properties between BP and BP-analogue materials. The synthesis methods, applications and the significance of the development of these fascinating materials are also exhibited. Finally, a prospective on future research and various applications of these materials is also proposed.

2. Black phosphorus

BP is the most stable among all kinds of phosphorus allotropes, with prior properties including high carrier mobility, direct tunable energy band-gap and so on, which renders it as one of the most outstanding 2D layered semiconductors for next-generation optoelectronics.

2.1 Crystal structure

Phosphorus exists in three allotropes: white, red and black phosphorus. In contrast to white and red phosphorus, BP is the most stable among all phosphorus allotropes. Fig. 1 shows the orthorhombic crystalline structure of black phosphorus, similar to graphene, where the phosphorus atoms are arranged in a honeycomb lattice. Different from graphene and other 2D nanomaterials which are atomically flat, BP is formed by puckered double layers.179 In a monolayer of BP, each sp3 P atom is covalently bonded to three other adjacent P atoms (bond length is 2.18 Å) with one lone pair of electrons, constructing a quadrangular pyramid structure. Two of the three adjacent phosphorus atoms with an angle of 98° are in the plane of the layer, and the third P atom lies at an angle of 103° between the layers, resulting in various anisotropic physical properties such as optical, mechanical, thermoelectric, and electrical conductance. Adjacent layers interact by weak van der Waals interactions along the z direction and stack with an ABA stacking order. The distance between two layers is about 5 Å. This layered structure promises the feasibility of employing mechanical or liquid exfoliation to obtain mono or few layer BP films from a bulk crystal.
image file: c9nr04348a-f1.tif
Fig. 1 Crystal structure of few layer BP.179 (a) 3D representation. (b) Lateral view. (c) Top view.

2.2 Prosperities of black phosphorus

2.2.1 Optical prosperities. Different from the in-plane symmetry crystal structure of graphene and TMDCs, the unique puckered structure of BP results in anisotropic in-plane optical conductivities and absorption. The optical properties of BP display a strong doping and thickness dependence, which has been demonstrated in many theoretical calculations and experimental measurements.47,180–182Fig. 2a shows the calculated optical conductivity along the x-direction (σxx) of BP films with various thicknesses ranging from 4–20 nm. The absorption edge exhibits a thickness dependence between 0.3 and 0.6 eV with decreasing thickness due to the increasing energy gap.47 The absorption spectra of few-layer and bulk BP were simulated by Yang et al.; the first absorption peak, which represents the excitonic bandgap, displays a red-shift behavior by increasing the thickness along the armchair (AC) direction (Fig. 2b). However, the absorption is clearly visible along the zigzag (ZZ) direction (Fig. 2c), making BP a potential candidate for a viable linear polarizer. The strong anisotropic optical properties of BP result from the symmetry-forbidden selection rule, and the crystal structure of BP possesses mirror reflection symmetry and inversion symmetry along the zigzag direction.182 Ji et al. revealed the linear dichroism of the optical absorption of BP: along the armchair direction, the band edge of the first absorption peak falls rapidly with the thickness of BP, while the position falls slightly with thickness when along the zigzag direction.40 In the visible regime, under the white light illumination of linear polarization, the optical intensities of the RGB channels of few-layer BP on 300 nm SiO2 change periodically when the sample is rotated, which indicates the optical anisotropy of BP in the visible regime,181 as shown in Fig. 2d. The optical anisotropy of BP results from the difference between the real and imaginary refractive indexes along AC and ZZ directions. The results not only provide an easy way to identify the crystalline orientation, but also make BP a promising candidate for light modulators with atomic thickness. In the infrared regime, it is obvious that the optical extinction depends on the various polarization angles (Fig. 2e), and the extinction reaches maximum when the incident light is parallel to the AC-direction of the BP thin film, while extinction decreases to minimum when the light is polarized parallel to the ZZ direction. The anisotropy of extinction is caused by the directional dependence of the interband transition matrix elements in anisotropic BP.40 Yan183 and Wang184 presented the layer dependent infrared spectra of few-layer BP, in accordance with ab initio calculations, and with an increased layer number, the optical bandgap of few layer BP decreases monotonously (Fig. 2f), which not only reveals the evolution of the electronic structure but also serves as its infrared fingerprints. All the optical absorption spectra in various layers exhibit strong anisotropy in AC- and ZZ-polarized directions, which is caused by the unique crystal structure of BP. It can be observed that there exist weak absorption peaks between main peaks (indicated by (*)), which are assigned to the hybrid transitions from the valence subband to the conduction subband with different quantum numbers. Lu et al. observed strong PL peaks in 2, 3, 4, and 5-layered phosphorene which correspond to the energy peaks at 1.29, 0.98, 0.88, and 0.80 eV, respectively (Fig. 2g).185 The PL peaks are attributed to the nature of excitons, which represent lower bounds on the fundamental bandgap values in few-layer phosphorene. The PL peak intensity exhibits a strong layer-sensitiveness in 2 to 5-layer phosphorene, with a dramatic increase with decreasing layer number. Wang et al. reported a strong PL close to the absorption bandgap in single-layer, bilayer and trilayer phosphorene (Fig. 2h), which reveals that phosphorene with different thicknesses has a direct bandgap. Furthermore, due to the quantum confinement along the thickness direction, optical absorption above the bandgap indicates existing additional resonances in few-layer phosphorene.184 Therefore, BP is also a natural optical linear polarizer, which can be used in three-dimensional visualization techniques, liquid-crystal displays, and optical quantum computers.
image file: c9nr04348a-f2.tif
Fig. 2 (a) Re (σxx) for intrinsic BP of different thicknesses as indicated.47 This figure has been reproduced from ref. 47 with permission from the American Physical Society. (b) and (c) Optical absorption spectra of phosphorene.180 These figures have been reproduced from ref. 47 with permission from the American Physical Society. (d) Optical images of BP sample rotation angle dependence.181 This figure has been reproduced from ref. 181 with permission from the American Chemical Society. (e) Infrared polarization-resolved extinction spectra.39 This figure has been reproduced from ref. 39 with permission from the Nature Publishing Group. (f) Layer-dependent infrared spectrum. Extinction spectra (1 − T/T0) for few-layer BP on quartz substrates with a layer number of 2–15 and bulk BP. The labels E11, E22, E33 and E44 denote the first, second, third and fourth subband transitions and asterisks (*) denote hybrid transitions, respectively. The black and red curves represent spectra for two different light polarizations.183 This figure has been reproduced from ref. 183 with permission from the Nature Publishing Group. (g) Photoluminescence spectra of phosphorene.185 This figure has been reproduced from ref. 185 with permission from the American Chemical Society. (h) Layer-dependent PL spectra.184 This figure has been reproduced from ref. 184 with permission from the Nature Publishing Group.

The Cmca space group of bulk BP permits 12 phonon vibration modes,186–188 including three in-plane transverse acoustic modes (TA), longitudinal acoustic modes (LA), and out-of-plane acoustic modes (ZA) as well as nine optical phonon modes including two infrared-active modes (B1u and B2u), and six Raman active modes (Ag1, Ag2, B1g, B2g, B3g1, and B3g2 and one silent mode Au). The atomic motions of monolayer phosphorene and the corresponding Raman spectra are shown in Fig. 3a and b, respectively.186,189 Raman active modes and inactive modes were marked with red and black, respectively. The atomic motions were associated with B2g mostly along the ZZ direction, and therefore, the variation of the intensity of this peak can be used to confirm the BP crystalline orientation quickly and precisely.185,188 The dependence of Ag2 and B2g intensities on the incident laser polarization angle is shown in Fig. 3c. The maximum intensity of the Ag2 mode can be achieved in the case of the laser polarized along the AC direction, and in contrast, the B2g mode intensity will reach the minimum at the same time. The low-frequency (LF) part corresponding to the interlayer vibrational modes, A3g, exhibits a large blue shift by decreasing the thickness of BP flakes (Fig. 3d).190 Therefore, LF Raman could be used to determine the thickness of the BP flake.


image file: c9nr04348a-f3.tif
Fig. 3 (a) Atomic motions of lattice vibrational modes.185 This figure has been reproduced from ref. 185 with permission from the American Chemical Society. (b) Raman spectrum of BP flakes.189 This figure has been reproduced from ref. 189 with permission from the American Chemical Society. (c) Polarization dependence of Ag1, Ag2, and B2g modes.185 This figure has been reproduced from ref. 185 with permission from the American Chemical Society. (d) Raman spectra calculated by DFT-LDA of different layer BP in the ultralow frequency range.190 This figure has been reproduced from ref. 190 with permission from the American Chemical Society.
2.2.2 Mechanical properties. Few-layer BP exhibits a superior and anisotropic mechanical flexibility. According to the DFT calculations, for the monolayer phosphorene shown in Fig. 4a, the ideal strengths are 8 and 18 GPa along the AC and ZZ directions, respectively.191 The corresponding tensile strains are measured to be up to 27% (ZZ) and 30% (AC). The enormous strain of phosphorene is caused by its unique puckered crystal structure as well as coupled hinge-like bonding structure. When uniaxial stress is applied along the zigzag direction, monolayer BP film has been shown to be auxetic. The calculated Young's modulus and strain of BP are highly anisotropic and nonlinear due to its quasi-2D puckered structure. The in-plane Young's modulus is 106.4 and 41.3 GPa in the parallel and perpendicular direction to the pucker, respectively. The ideal strain is 0.11 and 0.48 in the parallel and perpendicular directions, respectively. Zhang et al. systematically investigated the anisotropic mechanical properties of few-layer BP by nano-indentation in AFM.192 The measured Young's modulus of few-layer BP is about 27.2 ± 4.1 in the armchair direction and 58.6 ± 11.7 GPa in the zigzag direction (Fig. 4b). These results reveal the highly anisotropic mechanical properties of BP. Feng et al. presented a new method to investigate the mechanical anisotropy of the BP thin layers by spatially resolved multimode nano-mechanical resonances.193 The mapping results shown in Fig. 4c indicate the mechanical anisotropy of BP crystals.193 Rabczuk et al. studied the mechanical strain effects on monolayer BP nano-resonators through molecular dynamics simulations (Fig. 4d). The resonant frequency and quality factor are highly anisotropic in BP due to its intrinsic puckered structure. The quality factors of BP are intrinsically higher than those of other 2D materials.194 According to the first-principles calculations monolayer BP exhibits a negative Poisson's ratio.195Fig. 4e shows the expands (contracts) of monolayer BP along the z-direction when it is stretched (compressed) in the y-direction; the negative linear Poisson's ratio along the out-of-plane direction is calculated to be ν = −0.027. The negative linear Poisson's ratio results from the puckered structure of BP. Ye et al. experimentally proves the negative linear Poisson's ratio of BP. Fig. 4f exhibits the Raman spectra evolution of BP with compressive strains and uniaxial tensile. The Raman frequency of the Ag1 peak displays a red-shift when BP is strained along the armchair direction, which reveals the negative Poisson's ratio.196
image file: c9nr04348a-f4.tif
Fig. 4 (a) The strain–stress relationship of monolayer phosphorene structures.191 This figure has been reproduced from ref. 191 with permission from AIP Publishing. (b) Young's modulus of four pairs of suspended BP strips in the zigzag and armchair directions.192 This figure has been reproduced from ref. 192 with permission from the American Chemical Society. (c) Mechanical anisotropy of BP.193 This figure has been reproduced from ref. 193 with permission from the American Chemical Society. (d) Temperature dependence for the Q-factors of the armchair and zigzag SLBPRs.194 This figure has been reproduced from ref. 194 with permission from the Royal Society of Chemistry. (e) εzversus εy. Data are fitted by function y = −ν1x + ν3x3, with ν = ν1 = −0.027.195 This figure has been reproduced from ref. 195 with permission from the Nature Publishing Group. (f) Compressive and tensile armchair strains in the Raman spectra of BP.196 This figure has been reproduced from ref. 196 with permission from the American Chemical Society.
2.2.3 Electrical properties. As a semiconductor, electrical properties of bulk BP have been studied for several decades. In 2014, research on the electrical transport properties of few layer BP films was reported by Zhang and Ye.33,197 The band structure of bulk BP was investigated through angle-resolved photoemission spectroscopy (ARPES) together with calculations.33 As shown in Fig. 5a, the solid and dashed lines represent empty and filled bands, respectively. At the Z point, it is obvious that the bulk BP exhibits a direct bandgap of ∼0.3 eV. When peeled into mono or few layers, BP exhibits a thickness dependent energy bandgap from 1.6 to 0.46 eV, corresponding to 1 to 4 layers, respectively, as shown in Fig. 5b.198,199 Different from 2D MoS2 which changes to an indirect bandgap when decreased to a monolayer, BP maintains a direct bandgap regardless of the layer number. Fig. 5c shows the evolution of the energy bandgap of BP by different theoretical calculations.180 The calculated bandgap displays a change from 0.8 to 2 eV, which depends on the different calculation modes. Interestingly, the phosphorene bandgap experienced a direct–indirect–direct transition process when a moderate 2% compression was added along the ZZ direction (Fig. 5d).200 The mechanism was explained by the near-band-edge energy shifts due to the bond nature of BP electron orbitals. Due to the orthorhombic puckered structure, both the bottom of conduction bands and the top of valence bands are almost flat along the Γ–X direction. Consequently, the effective mass of electrons and holes is highly anisotropic along the AC and ZZ directions. For intrinsic monolayer phosphorene, the anisotropic effective mass of electrons is shown in Fig. 5e, which can differ by an order of magnitude along different directions.201 Another calculated effective mass of relaxed phosphorene of the electron and hole are 1.24me (ZZ), 0.16me (AC) and 4.92me (ZZ), 0.15me (AC), respectively. The prominently smaller effective masses suggest that charge carriers prefer transport in the AC direction.200 As mentioned in this review previously, BP is sensitive to strain, which is the same as the electrical conductivity, as shown in Fig. 5f, and the spatial conductance can be rotated by 90° in plane through either biaxial strain or uniaxial strain.201 The electrical conductance anisotropy was confirmed experimentally by Xia et al.,39 and according to Fig. 5g, a ratio of the conductivity along the AC to ZZ directions is 1.5, corresponding to the mobility ratio along the AC and ZZ directions. Along the armchair direction, the Hall mobility is about 1.8 fold higher than that in the zigzag direction (Fig. 5h), indicating a smaller anisotropy than the bulk BP but higher than that from the measurement.
image file: c9nr04348a-f5.tif
Fig. 5 (a) Measurement of the bulk BP band structure by ARPES.33 This figure has been reproduced from ref. 33 with permission from the Nature Publishing Group. (b) Calculated band structures for BP.198 This figure has been reproduced from ref. 198 with permission from the American Physical Society. (c) The evolution of the band gap and optical absorption peak of few-layer phosphorene.180 This figure has been reproduced from ref. 180 with permission from the American Physical Society. (d) The 2D phosphorene band gap dependence of strain εx applied in the zigzag direction.200 This figure has been reproduced from ref. 200 with permission from the American Physical Society. (e) Intrinsic phosphorene effective electron mass dependence in the spatial direction.201 This figure has been reproduced from ref. 201 with permission from the American Chemical Society. (f) Schematic of strain-induced electrical conductance rotation in single-layer phosphorene.201 This figure has been reproduced from ref. 201 with permission from the American Chemical Society. (g) IR relative extinction and DC conductivity of BP films.122 This figure has been reproduced from ref. 122 with permission from the Royal Society of Chemistry. (h) Angle-resolved Hall mobility.39 This figure has been reproduced from ref. 39 with permission from the Nature Publishing Group.

Although BP possesses many extraordinary properties, the environmental instability of BP has become a major obstacle for applications.202 It can be observed that after placing under ambient conditions, bubbles were formed on the surface of the few-layer BP film.122 After exposure in air for two weeks, no BP related Raman peaks can be observed. The carrier mobility drops dramatically after exposure of BP under ambient conditions for more than 50 hours.122 The mechanism of BP degradation was revealed through calculation and includes three steps under ambient conditions: generation of a superoxide under light, dissociation of the superoxide, and breakdown with the action of water.203

2.3 Synthesis methods

In 1914, BP was first synthesized by Bridgman, and white phosphorus (WP) was transformed to BP under a high pressure of 1.2 GPa with a temperature of about 200 °C.204 Unlike WP and red phosphorus (RP), BP shows high chemical stability. Besides, impurity and harsh reaction conditions are two other shortcomings of this synthesis process. These methods only yield small area crystals and ingots. Later, Bridgman explored another synthesis method by transforming RP into BP directly at room temperature, but this method requires a much higher pressure (8.0 GPa). In the 1980s, Japanese scientists explored other high pressure and temperature methods to synthesize high quality BP, but all these methods show low efficiency and are difficult to be carried out in most laboratories.205,206 In the 1950s to 1960s, some new methods were invented by utilizing mercury and bismuth-flux under low pressure. In 1955 Krebs and co-workers utilized the catalytic action of Hg on WP under high temperature conditions.206 By using this method, WP was reacted with Hg at a temperature of 370–410 °C for several days, and finally BP was obtained. Brown, Rundqvist and Maruyama presented the preparation of BP from a phosphorus–bismuth melt.205,207,208 In these methods, the bismuth and WP were heated to 400 °C for 20 h. After that, needle- or rod-like BP single crystals with a size of 5 × 0.1 × 0.07 mm3 were obtained.208 Wu et al. reported a new safe method of growing BP where RP was used instead of toxic WP, and BP nanobelts and microbelts were synthesized with a solid–liquid–solid one-step reaction process under ambient pressure, where RP and bismuth are used as the precursors.209 The starting materials were mixed using ball milling, and after heating, the micro- and nano-scaled BP belts were obtained at the surface and within the Bi substrate. Stacked along the b-axis, the BP nanobelts were observed to have a typical layered structure. However, these methods still suffer from high cost, low yield and efficiency and not being environmentally friendly.

Recently, Nilges et al. have reported a mineralizer assisted gas-phase transformation (AGPH) method to further improve the yield and crystallinity at low pressure and temperature without the use of toxic catalysts.210–212 BP can be synthesized from RP by using Sn and SnI4 as mineralization additives in a short way transport reaction. Precursors including RP, Sn, and SnI4 were placed in evacuated silica glass tubes and heated to 650 °C. Finally, BP single crystals were obtained at the cool end of the tubes, as shown in Fig. 6a and b. To further understand the growth mechanism of the AGPH method, Zhao and co-workers studied the growth process of BP at different stages.213 As shown in Fig. 5b, during the temperature increase, RP gas was reacted with tin iodide forming a P–Sn–I compound, which served as the nucleation site for the growth of BP microribbons. When the temperature was increased, the gas was condensed and deposited at the bottom of the glass tube, with the help of SnI2, which plays a role of a mineralizer to improve the stability or metastable phase of crystallization via phosphorus gas. Zhao also presented a number of experiments to completely understand the growth of BP crystals and explore the roles of metals (or alloys) and iodine elements during their growth process.214 It is found that I2 is necessary for BP growth. Besides Sn, BP can also be synthesized with the help of other metals, like Pb, In, Cd and Bi (Fig. 6c). They concluded that the role of I2 or its compounds in the BP crystal growth process is to act as a mineralizer. The function of a mineralizer in crystal growth is to improve the solubility and facilitate the transport of phosphorus. Under high temperature conditions, the specific metals (or alloys) provide a liquid solution to effectively dissolve phosphorus and then phosphorus precipitates and crystallizes to form BP crystals in an extremely slow cooling stage. Additionally, a new method to grow large area BP crystals by utilizing Sn24P22−xI8, under low pressure and temperature, was reported.215 Firstly, RP, Sn and I2 were sealed under vacuum and heated for the synthesis of ternary clathrate Sn24P22−xI8. In the second step, Sn24P22−xI8 powder and red phosphorus were sealed in the vacuum tube and heated for BP crystal growth. Sn24P22−xI8 served as the nucleation site for BP growth, and layered BP crystals were found at the exact place of the original ternary clathrate (Fig. 6d). The P vacancies in the structure of Sn24P22−xI8 play a key role in the growth of solid BP from phosphorus vapors. The mechanism of the synthesis of BP crystals was revealed through systematic investigation of every growth stage.216 It is found that the BP synthesis process is a stage-by-stage phase-induction process; firstly, without the assistance of mineralizers, RP transformed into Hittorf's phosphorus (HP). Then HP transformed into BP at the cooling stage subsequently. Importantly, Sn24P19.3I8 worked as the sole nucleation site, which leads to BP crystals nucleating and growing large in the heat preservation stage.


image file: c9nr04348a-f6.tif
Fig. 6 (a) A silica glass tube after the synthesis of BP.211 This figure has been reproduced from ref. 211 with permission from Elsevier. (b) Schematic of the microribbon growth stages.213 This figure has been reproduced from ref. 213 with permission from the American Chemical Society. (c) The silica ampoule photos of the grown BP crystals by using RP@Sn@I2, RP@Pb@I2, RP@In@SnI4, RP@Cd@PbI2, and RP@Bi@SnI4 groups, respectively.214 This figure has been reproduced from ref. 214 with permission from the Royal Society of Chemistry. (d) Photograph of BP crystals on a mm scale and the proposed growth mechanism.245 This figure has been reproduced from ref. 245 with permission from the American Chemical Society. (e) BP flakes synthesized by a deposition route.217 This figure has been reproduced from ref. 217 with permission from Wiley. (f) Large-scale BP synthesis scheme.218 This figure has been reproduced from ref. 218 with permission from Wiley. (g) Images of RP flakes on the PET substrate and RP/PET plate (left to right), respectively.219 This figure has been reproduced from ref. 219 with permission from IOP Publishing.

To realize the possible applications of BP in nanodevices, it is vitally important to develop facile, highly efficient and environment-friendly methods to synthesize wafer-scale crystalline BP nanofilms. A considerable amount of effort has been made to grow BP nanofilms directly on the substrate. Lau et al. first grew 2 nm thick BP films on various substrates by using a pulsed laser deposition method (Fig. 6e).217 The BP crystal was used as a target and ablated by using a KrF laser. Like BP exfoliated from the crystal, the thickness dependence of the energy bandgap has been observed in ultrathin BP flakes. However, the obtained film is amorphous, and the fabricated devices exhibit very low electronic performance, where Ion/Ioff is 102, and the mobility is only 14 cm2 V−1 s−1. Highly crystalline thin-flake BP was synthesized by transforming RP into BP at 1.5 GPa and 700 °C on sapphire substrates, as shown in Fig. 6f.218 The synthesized BP thin film is about 50 nm thick and features a polycrystalline structure with a crystal length of 40 to 70 μm. Along the armchair direction, at room temperature, the mobility of the thin BP flake is ∼160 cm2 V−1 s−1 and reaches up to about 200 cm2 V−1 s−1 at 90 K. Xia's group synthesized a large-sized thin BP flake on a flexible polyester substrate through converting the RP film into the BP film by pressurization in an anvil cell (Fig. 6g).219 The RP powders were evaporated and then deposited onto a flexible substrate by adjusting the time to form a thin flake of controllable thickness. The thickness of the film is about 40 nm, but the grain size of the grown BP film is only 10 nm, leading to a very low mobility of only ∼0.5 cm2 V−1 s−1, which is far from practical applications. Ji et al. reported an in situ chemical vapor deposition (CVD) method to grow large-sized BP with average areas beyond 3 μm2. Typical BP flakes were about four layers and thicker with average areas >100 μm2. However, growing high quality BP nanofilms on substrates still suffers from some difficulties and the challenges come from the absence of nucleation sites on substrates and the difficulty in controlling the phosphorous concentration during the growing processes. Fortunately, the success of growing wafer-scale few layer graphene and TMDCs by CVD may provide useful information on the fabrication of layer-controllable BP films with large-scale and high-efficiency in the future.220

2.4 Applications

2.4.1 Field-effect transistors. Transistors are the elementary “building blocks” of modern electronic devices and integrated circuits. The discovery of graphene triggered tremendous interest to fabricate transistors with 2D materials. Graphene possesses extremely high carrier mobility up to ∼20[thin space (1/6-em)]000 cm2 V−1 s−1 at room temperature. However, graphene suffers from a zero energy bandgap, which prevents its application in logical devices. 2D TMDC materials possess an intrinsic bandgap which ranges from 0.3 to 2.3 eV, while the relatively lower carrier mobility (less than 1000 cm2 V−1 s−1) restricts the device performance. BP is a typical p type semiconductor with a high carrier mobility and Ion/Ioff ratio. In 2014, Zhang33 and Ye196 first fabricated BP based FETs with different thicknesses; a field-effect hole mobility around 1000 cm2 V−1 s−1 is achieved on a 10 nm sample, which is superior to those of silicon and TMDCs, as shown in Fig. 7a, and shows an obvious dependence on the sample thickness.33 Compared with graphene, the carrier mobility of BP is much lower, however, the current on–off ratio (more than 105) is almost four orders of magnitude higher than that of graphene. Therefore, BP provides a bridge between graphene and TMDCs. Hall mobility shows temperature dependence, it decreases when the temperature is higher than 100 K, and shows saturation behavior at lower temperature. This phenomenon can be related to the electron–phonon scattering which dominates in the range of 100 to 300 K. After encapsulating the BP film into h-BN, the hole mobility can reach up to 45[thin space (1/6-em)]000 and 5000 cm2 V−1 s−1 at 2 K and room temperature, respectively.221 Due to the protection of h-BN, in conventional magnetic fields, BP devices exhibit Shubnikov–de Haas oscillations and a quantum Hall effect. Upon simulation, BP was found to exhibit anisotropic carrier mobility; however, the electron mobility was more anisotropic than that of the hole mobility (μxx/μyy ∼ 6.2).222 By using angle-resolved conductance measurements, the carrier mobility anisotropy was verified by Xia experimentally.39 In Haruyama's work, the Hall mobility along the x direction is ∼320 cm2 V−1 s−1 at T = 300 K, which is much higher than that along the y direction (∼120 cm2 V−1 s−1).223 The anisotropic mobility is due to the different scattering directions, which is related to the anisotropic effective mass. Appenzeller et al. demonstrated complementary tunneling field-effect transistors (TFETs) based on few-layer black phosphorus (Fig. 7b). By electrically tuning the doping types and levels in the source and drain regions, the device can be reconfigured to allow for TFET or metal–oxide–semiconductor field-effect transistor (MOSFET) operation and can be tuned to be n-type or p-type. Record-high tunneling-current densities of 2 × 104 A cm−2 can be achieved in the TFETs. These findings provide a possible method for fabricating energy-efficient tunneling devices based on BP.224 Flexible BP FETs were first realized by fabricating BP FETs on flexible polyimide (Fig. 7c).225 The device displays a highest carrier mobility of ∼310 cm2 V−1 s−1 as well as current modulation over 3 orders of magnitude. The flexible BP transistors show robust performance even after sustaining mechanical 5000 bending cycles and bending up to 2% tensile strain.
image file: c9nr04348a-f7.tif
Fig. 7 (a) The measured sheet conductivity versus different thicknesses of channel materials and gate voltage.33 This figure has been reproduced from ref. 33 with permission from the Nature Publishing Group. (b) Schematic of the BP RED-TFET.161 This figure has been reproduced from ref. 161 with permission from Elsevier. (c) Schematic of few-layer BP devices on a flexible substrate.225 This figure has been reproduced from ref. 225 with permission from the American Chemical Society. (d) 4-Terminal measurements of Rs (points) and μFE (lines) at 300 K.41 This figure has been reproduced from ref. 41 with permission from the Nature Publishing Group. (e) The Schottky barrier height profiles of monolayer phosphorene and metal contact interfaces.229 This figure has been reproduced from ref. 229 with permission from the American Chemical Society. (f) High hole mobility is sustained semi-permanently.238 This figure has been reproduced from ref. 238 with permission from the Royal Society of Chemistry. (g) The saturation current density of the C8-BTBT encapsulated BP FETs.239 This figure has been reproduced from ref. 239 with permission from the American Chemical Society.

For different contact metals, the metal/BP interface has different Schottky barrier heights; the proper work-function of metals can match the VBM/CBM to lower the Schottky barrier height, decrease the contact resistance, and therefore, increase the current.226,227 The work function difference of 0.6 eV between Ni and palladium (Pd) leads to a much lower contact resistance (Rc) on BP by using Pd; the Rc decreased from ∼7.10 to ∼1.05 Ω mm, therefore, resulting in an improved current and carrier mobility in Pd contacted FETs.228 The Schottky barrier height of Ti/Au BP FET was 200 meV, which could be further reduced to 50 meV when using TiO2/Cu contacts. The BP FETs exhibit ambipolar behavior with the hole carrier current and mobility being much higher than those of the electron. If an appropriate work function contacted metal was used, the desired types of carriers could be injected into the respective bands. If a larger work function metal was employed, then it can induce p-type dominant FETs, while metals with a smaller work function can produce n-type FETs. Lee et al. fabricated unipolar n-type BP FETs with aluminium (Al) contacts to BP; the transition process can change unipolar behaviour to ambipolar behaviour as the thickness of the flake increased from 3 to 13 nm.41 The 13 nm thickness Al/BP contacted metal flake exhibits graphene-like symmetric electron and hole mobilities up to 950 cm2 V−1 s−1 at 300 K, while a 3 nm thickness flake exhibits unipolar n-type switching with on/off ratios larger than 105 (107) and electron mobility up to 275 (630) cm2 V−1 s−1 at 300 K (80 K). The thickness dependence of dominant carrier types was attributed to the Schottky barrier existing at the Al/BP contact interface. For Pd contacts, p-type behaviour is dominated in the thick Pd flakes, while flakes with thickness ranging from 2.5 to 7 mm have symmetric ambipolar transport (Fig. 7d).41 In BP FETs with scandium (Sc) contacts, a record current density of 580 μA−1 μm is achieved, and all devices exhibit p-type performances with different thicknesses. Regardless of the flake thickness, the outcomes indicate that the Fermi level is pinned close to the valence band.38 The SBH between monolayer BP and a variety of metal contacts is investigated theoretically and shown in Fig. 7e.229

As mentioned before, BP shows instability under ambient conditions;37,230 after 50 hours of ambient exposure, the hole mobility and current on/off ratio decreased by 4 and 3 orders of magnitude, respectively, and therefore, many passivation methods were investigated.231–235 Dielectrics were usually used as passivation materials to protect the 2D materials, for example, Al2O3 was mostly used to prevent BP degradation.123,131,234,235 Compared with the unencapsulated device, the carrier mobility and Ion/Ioff of the encapsulated devices drop only slightly after 150 hours (Fig. 7f). Several groups reported the fabrication of BP FETs integrated with high-k gate dielectric hafnium dioxide (HfO2).232,235,236 The device exhibits an almost ideal subthreshold swing of 66 mV−1 dec and hole mobility over 500 cm2 V−1 s−1 at room temperature. These advantageous properties were caused by the passivation phosphorus dangling bonds of HF adatoms; consequently, a more stable chemical interface can be produced, which was predicted by the significant interface state density reduction. SiO2 was also used to enhance the environmental stability of BP flakes.237 The unpassivated BP devices showed a tenfold Ion/Ioff decrease after one week of ambient exposure, while the passivated BP devices exhibited a highly retained on/off current ratio and exceeded 600 after 7 days of ambient exposure. Besides dielectrics, organics were also explored to protect the BP from degradation under ambient conditions. Yoo et al. presented the air-passivated ambipolar BP transistor by applying organic benzyl viologen.238 Under an ambient atmosphere and vacuum conditions, the passivated BP devices show excellent stability and the performance could be maintained over a long time. Different from their intrinsic p-type properties, the fabricated passivated BP devices exhibit advantageous ambipolar properties (Fig. 7g). Duan et al. discovered that BP can sustain oxidation under ambient conditions over 20 days through dioctyl benzothieno benzothiophene (C8-BTBT) thin films. The noncovalent vdWs between the C8-BTBT and BP interface can maintain the intrinsic properties of BP effectively, and BP FETs exhibit a current density as high as 920 μA μm−1, hole drift velocity exceeding 1 × 107 cm s−1, and on/off ratio between 1 × 104 and 1 × 107 at room temperature.239 Park et al. presented another technical route of surface doping by self-assembled layers (SALs).240 They investigated n- and p-type doping effects in photodetectors and FETs with BP films of different thicknesses. The functional groups in OTS (CH3–) and APTES (NH3–) have positive and negative charges, which enable the tuning of the carrier density. The transistors formed on BP films with a thickness of 2 nm, and the n-doping process shifted the threshold voltage from 28.3 down to 19.5 V. However, by p-doping with OTS, the threshold voltage shifted from 20.6 up to 23.7 V, and this positive VTH decreased as the BP flake thickness increased (ΔVTH: 3.9 → 3.1 V). Capping the BP with a layer of PMMA can cause the conduction process to change from hole- to electron-dominated conduction, and this can be attributed to the introduction of interfacial charges caused by the capping process. This method not only avoids complicated fabrication techniques but also ignores the damage caused to the lattice structure of the doped 2D materials.241 Surface doping could not only protect the BP from degradation, but also modify the electric conductance of BP. Doped with tellurium (Te), the devices based on BP display a mobility of 1850 cm2 V−1 s−1 at room temperature, which was nearly 2 times larger than that of bulk BP. More importantly, the devices show a remarkable ability to resist degradation under ambient conditions, with a retained Ion/Ioff of ≈500 and mobility over 200 cm2 V−1 s−1 after 21 days of ambient exposure.242 Chen et al. demonstrated the surface transfer doping process by Cs2CO3 and MoO3 in BP devices. After doping with Cs2CO3 the electron mobility of BP enhanced to ∼27 cm2 V−1 s−1, suggesting a significant improvement of electron transport behaviour.243

2.4.2 Photodetectors. Due to the thickness dependent direct bandgap, which ranges from 0.35 to 2.0 eV (bulk to monolayer), BP is considered to be a promising candidate for wide spectra photodetection ranging from the visible to infrared region.48,49,53,244–247 The first reported BP photodetector shows a photoresponse covering from the visible band to 940 nm and an about 1 ms rise time (Fig. 8a).49 However, the photo-responsivity is only 4.8 mA W−1. To further improve the performance of the photodetector based on BP, a variety of studies have been carried out. The short channel (sub 100 nm) BP photo-transistors demonstrate a high-performance with broadband response ranging from 400 to 900 nm.53 The photo-responsivity reaches as high as 7 × 106 A W−1 at 20 K and 4.3 × 106 A W−1 at 300 K, respectively (Fig. 8b). This remarkable improvement is attributed to the interface and bulk trap freezing, which have the ability to capture the electron–hole pairs and introduce extra scattering; meanwhile, this process can reduce phonon scattering which affects the mobility effectively. Mid-infrared (MIR) wavelength photodetectors based on BP demonstrate a high internal gain and responsivity of 82 A W−1 at 3.39 μm (Fig. 8c).245 Due to limitations such as surface defects, thickness dependent absorption efficiency, and poor air stability, metal–semiconductor–metal (MSM) structure BP photodetectors were strictly restricted to achieve fast photoresponse and high photoresponsivity. Shi et al. demonstrated multilayer BP infrared photodetectors with a typical sandwiched structure of the top boron nitride protective layer and the bottom Au electrodes.244 The photodetectors display a fast photoresponse time of ∼16 ms, a near infrared response up to ∼2 μm, a record-high light on/off ratio of ∼103, and a photoresponsivity of ∼1.55 A W−1 under a wavelength of 1550 nm at room temperature. More importantly, noise measurements showed that such photodetectors based on BP are capable of sensing MIR light in the picowatt range. As shown in Fig. 8d, a 3D architecture of metallic nanoplasmonics and silicon photonics structures was applied to improve the performance of the BP photodetector.246 This method combines the advantages of the narrow bandgap of BP, the high-field confinement of a plasmonic nanogap and low loss during the propagation of silicon waveguides to obtain high responsivity for detection in the wavelength around the telecom-band and near-infrared (NIR) band. The photodetector shows an intrinsic responsivity up to 10 A W−1 which is caused by the internal gain mechanisms, as shown in Fig. 8e. A gated BP photodetector integrated on a silicon photonic waveguide was fabricated and worked in the NIR band.247 At room temperature, BP photodetectors can operate with a low dark current and achieve an intrinsic responsivity of 135 mA W−1 and 657 mA W−1 in 11.5 and 100 nm thick devices under bias, respectively. The intrinsic maximum detection range of BP is around 4 μm, and the photoresponse can dynamically extend beyond 7.7 μm through a vertical electric field (Fig. 8f), leveraging the Stark effect. At 77 K, the BP based photodetectors display a peak extrinsic photo-responsivity at 3.4, 5, and 7.7 μm of 518, 30, and 2.2 mA W−1, respectively.51 The extent of the detection range results from the dual-gate structure, which provides an additional gate variable and permits the device to attain the charge-neutrality position of BP under various biasing fields, and the light absorption edge shows a significant redshift behavior. Besides infrared detection, BP also exhibits an excellent UV response with a specific detectivity of ∼3 × 1013 Jones. On applying a VSD of 3 V, the photoresponsivity can be improved to ∼9 × 104 A W−1 at the UV band, which is attributed to band nesting and corresponding singularity in the joint density of states.248 The van der Waals heterostructure is particularly important for infrared detectors. BP/MoS2 heterojunction photodiodes were investigated as MIR detectors with improved performance: an external quantum efficiency (EQE) of 35% and a specific detectivity (D*) of 1.1 × 1010 cm Hz1/2 W−1 in the MIR region were achieved at room-temperature. By utilizing the anisotropic optical properties of BP, a bias-selectable polarization-resolved monolithic photodetector was developed. Under a bias of ±250 mV, one device is reverse-biased and the other forward-biased.48 As can be seen in Fig. 8g, only 0° or 90° linearly polarized MIR light under negative or positive biasing was collected. Similar to other BP based nanodevices, the BP photodetectors also exhibit an anisotropic photoresponse. As shown in Fig. 8h, under the illumination ranging from 400 to 1700 nm, the photocurrent with 90° light polarization is much smaller than that with 0° (AC direction) light polarization, indicating that the intrinsic polarization dependent photoresponse originates from the BP itself. The photoresponsivity at 1200 nm is 0.35 mA W−1 along 0° light polarization, and there is a large contrast ratio of 3.5 between the photoresponsivities along the two perpendicular polarizations.249 To improve the photoresponse of BP, various methods were investigated, such as surface doping, substitutional doping, constructing p–n junctions and so on. An effective performance modulation of few-layer BP photodetector was realized through in situ surface functionalization with molybdenum trioxide (MoO3) and caesium carbonate (Cs2CO3). The responsivity of the BP photodetector decorated with MoO3 increased from 0.6 to 2.56 A W−1, while the responsivity of the BP photodetector which was modified by Cs2CO3 improved from 0.58 to 1.88 A W−1. The enhanced photoresponse of the BP photodetector by applying the surface doping process is proposed to be the modulate effect of the Schottky junction formed at the metal/BP interface.243 Wang et al. reported the incorporation of n-type doping of few-layer BP by employing benzyl viologen (BV) as the surface charge transfer donor.187 Under illumination, a high performance device with a rise time of 15 ms, a high responsivity of 180 mA W−1 and an EQE of 0.75% was obtained. In addition, by adjusting the doping time, the barrier height and the efficient separation of the photogenerated electron/hole pairs can be tuned through the p–n junction. Doping BP by introducing Ag nanoclusters can induce an abnormal absorption at a wavelength of 746 nm, leading to about a 35 fold enhancement in 808 nm.252 This enhancement can be attributed to the strong coupling between the Ag and BP interface. In addition, the photoresponsivity and EQE of the fabricated AgNC doped BP photodetector were also largely enhanced. On doping BP crystals with selenium (Se), the Fermi level of BP can be tuned, which could efficiently lower the Schottky barrier height between the contact and BP; therefore, the responsivity and EQE of the BP photodetector have been significantly improved to 15.33 A W−1 and 2993%, which is almost 20 times improved compared to the pristine BP.250 Coating with dielectrics, transistors based on flexible BP radio frequency afforded an intrinsic maximum oscillation frequency of 14.5 GHz.253 The high-frequency can sustain up to ∼1.5% tensile strain. A multilayer BP photodetector is used to record high-contrast images of microscopic patterns in the visible (λVIS = 532 nm) (Fig. 8i, down) and NIR spectral regime (λIR = 1550 nm) (Fig. 8i, top). The results suggest BP as a promising material for novel multispectral photodetection and imaging.52
image file: c9nr04348a-f8.tif
Fig. 8 (a) Measurement of photoresponse time in one period of modulation.49 This figure has been reproduced from ref. 49 with permission from the American Chemical Society. (b) EQE for different incident wavelength lasers.53 This figure has been reproduced from ref. 53 with permission from the Nature Publishing Group. (c) The photoresponsivity versus power dependence at VDS = 100 and 500 mV, respectively.245 This figure has been reproduced from ref. 245 with permission from the American Chemical Society. (d) The illustration of the device configuration.246 This figure has been reproduced from ref. 246 with permission from the Nature Publishing Group. (e) Measurement of BP photoresponse of the photodetector versus different doping concentrations.247 This figure has been reproduced from ref. 247 with permission from the American Chemical Society. (f) Measurement of peak photocurrents for different incident wavelength laser beams.51 This figure has been reproduced from ref. 51 with permission from the Nature Publishing Group. (g) Measurement of the device spectrally resolved photoresponse.48 This figure has been reproduced from ref. 48 with permission from the Nature Publishing Group. (h) Photoresponsivity versus polarization in the incident laser wavelength range from 400 to 1700 nm.249 This figure has been reproduced from ref. 249 with permission from Wiley. (i) Measured image excited at λIR = 1550 nm (top) and at λIR = 532 nm (down).52 This figure has been reproduced from ref. 52 with permission from the American Chemical Society.
2.4.3 Logic inverters. Logical inverters can be constructed as in-plane homostructures or vertical heterostructures. The in-plane inverter was always achieved in a single piece of channel with half part doped into n- or p-type. For example, the addition of Cu adatoms is utilized to control n-doped few-layer BP (Fig. 9a), and consequently, the n-type conduction threshold voltage can be lowered without any degrading of the transport properties.254 By capping a cross-linked poly(methyl methacrylate) layer, the potential in the BP layer was modified, therefore the resulting conduction was dominated by electrons without an external electric field.242 Based on n- and p-type channels, logic inverters based on BP can be fabricated. The outcomes indicated the dependence of the gain and output voltage on the input voltage, and a maximum gain of 0.75 was achieved (Fig. 9b). The BP logic inverter could be fabricated in a single layer BP with a SixNy coated n-channel and an untreated p-channel, and exhibited a gain of 9.8 ranging from 100 Hz to 10 kHz with little output voltage loss (Fig. 9c).255 An all-BP complementary logic inverter can be fabricated by an in situ Al deposition process. It exhibits the invert output voltage transfers from 5 to near 0 V via Vin changing from 0 to 5 V of the inverter, and the gain is around 8.256 The vertical heterostructure inverter can be realized through stacking n- and p-type materials together. For example, fabricating an inverter by combining few-layer MoS2 n-MOSFET with a BP p-MOSFET. At a supply voltage of VDD = 2.5 V, a voltage gain of 3.5 was achieved, and large-signal operation was exhibited at higher frequency (Fig. 9d).257 Zeng et al. demonstrated a BP/SnSeS heterostructure-based logic inverter.258 When VDD = 1 V, the output voltage exhibits a low and high logic value of 0 and 1 at Vin > 2 V and Vin < −1 V, respectively. A unique middle logic state appeared at Vin = 0 V when the channel length was increased (Fig. 9e). A binary inverter was fabricated based on a heterojunction, which consisted of an ReS2 n-channel and a BP p-channel. The VDD values of 1, 2, 3, and 4 V and VIN values from −5.0 to 2.5 V were measured (Fig. 9f). VOUT approximated to 0 V when VIN exceeded −1.5 V, and for VIN < −2.5 V conditions, VOUT was close to VDD.259 Lee et al. found the BP work function (ΦBP) modulated with thickness which induces a drastic variation in current-transport across the BP/ReS2 heterojunction, and therefore, they fabricated two FETs with BP thicknesses of 5.8 and 38 nm and they shared one same ReS2 (13 nm) flake (Fig. 9g).260 The inverter-1 (BP thickness of 5.8 nm) shows a binary inverter signature with a voltage gain of 11.6 for VDD = 3 V (Fig. 9h). The inverter-2 (BP thickness of 38 nm) exhibits ternary inverter transfer characteristics with a tunable middle logic width for VDD > 1.5 V and acts as a binary inverter for VDD < 1.5 V (Fig. 9i).
image file: c9nr04348a-f9.tif
Fig. 9 (a) Plot of Voutversus Vin of the inverter.254 This figure has been reproduced from ref. 254 with permission from the American Chemical Society. (b) Vout and gain versus Vin, exhibiting a highest gain of 0.75.262 This figure has been reproduced from ref. 262 with permission from the Royal Society of Chemistry. (c) Voutversus Vin, VDD (supply voltages) = 2–4 V. Inset: The gain of the inverter.255 This figure has been reproduced from ref. 255 with permission from Wiley. (d) Measurement of the digital and small-signal AC performance of the inverter.257 This figure has been reproduced from ref. 257 with permission from IOP Publishing. (e) Inverter with different length channels of BP with a 1 V forward bias with a 2.5 μm channel length of SnSeS.258 This figure has been reproduced from ref. 258 with permission from Wiley. (f) Vout as a function of Vin for VDD = 1–4 V.259 This figure has been reproduced from ref. 259 with permission from the American Chemical Society. (g) Schematic image of the BP/ReS2 heterojunction.260 This figure has been reproduced from ref. 260 with permission from Wiley. (h) The fabricated BP based FET and the corresponding BP/ReS2 heterojunction transfer characteristics of inverter-1.260 This figure has been reproduced from ref. 260 with permission from Wiley. (i) The fabricated BP based FET and the corresponding BP/ReS2 NDR device transfer characteristics of inverter-2.260 This figure has been reproduced from ref. 260 with permission from Wiley.

3. 2D layered binary compound (MPx)

The 2D layered binary compound MPx is mainly classified into three categories: a group III element (B/In) and P, a group IV element (Si/Ge/Sn) and P, and a group V element (As/Sb) and P. Configurations of 2D MPx are similar to graphene, but possess an intrinsic energy bandgap. Similar to BP, MPx exhibits anisotropic properties and a tunable bandgap, but an excellent stability and higher carrier mobility than BP. 2D MPx is considered to be a promising contender for various applications like nanoelectronics, optoelectronics, batteries, light-emitting diodes and lasers, photocatalysis and so on. In this part, we will introduce the structure, physical and chemical properties, and synthesis methods of 2D MPx materials.

3.1 Structure

Although MPx is mainly classified into three categories, different main group elements (IIIA, IVA and VA) and P may form different structures at different atomic ratios. For example, as shown in Fig. 10, there are two-dimensional BP/BP3/InP3 crystals in which the P atoms in phosphorene are substituted by group III boron atoms or indium atoms.158,169,261,262 They feature a hexagonal and layered geometrical structure which is analogous to BP.
image file: c9nr04348a-f10.tif
Fig. 10 Crystal structure of Group III-P 2D layered binary compound MPx (M = B, In; x = 1, 3). (a) Top and side views of the monolayer BP.261 This figure has been reproduced from ref. 261 with permission from the Royal Society of Chemistry. (b) Three stacking structures of trilayer BP.221 This figure has been reproduced from ref. 221 with permission from the American Chemical Society. (c) Chemical structures of the two most stable phases of the BP3 monolayer.232 This figure has been reproduced from ref. 232 with permission from the Royal Society of Chemistry. (d) Top and side views of the InP3 monolayer.251 This figure has been reproduced from ref. 251 with permission from Elsevier.

However, when MPx was formed by the group IV element and P, a variety of stable crystal structures could be obtained.126,263,264Fig. 11 shows different crystal structures of SiPx (x = 1, 2, 3),64 GePx (x = 1, 2, 3, 5)226,265–267 and SnP3.268 The crystal structures of the monolayer SiPx are shown in Fig. 11a. Interestingly, when x ≥1, a large number of SiPx monolayers with negative formation enthalpies ΔH emerge and most of these satisfy the classical electron counting rule. For the most stable structure of SiP and GeP that have been experimentally synthesized, every P (presented as B in Fig. 11b) atom is bonded to group IV element atoms, and each group IV element atom is bonded to three neighbouring B atoms and one other group IV element atom. In addition, the buckling distance δ is also increased as the atomic number is increased. As shown in Fig. 12,126 the as-synthesized SiP and GeP exhibit an obvious layered structure and good crystallinity. Fig. 11c shows the chemical structures of the three most stable phases of GeP2, namely, tetragonal (T), orthorhombic (O), and cubic (C). The T and O phases are 2D layered structures (Fig. 11c(I) and (II)), while the C phase features a non-layered structure that is covalently bonded in three dimensions (Fig. 11c(III)). It should be noted that the T phase is predicted to be the most stable structure. As depicted in Fig. 11d and e, in each layer of GeP3, SnP3 and GeP5, the atoms are located in two hexagonal planes in close proximity, forming a buckled monolayered structure analogous to that of blue phosphorene and germanene. In general, this structure is similar to that of graphene. Such a 2D layered structure promises application for its excellent structural flexibility for bonding with carbon and high electrical conductivity.269


image file: c9nr04348a-f11.tif
Fig. 11 Crystal structure of Group IV-P 2D layered binary compound MPx (M = Si, Ge,Sn; x = 1,2,3,5). (a) Schematic of 2D SixPy structures. The top side view (upper and lower) of P[6 with combining macron]m2 Si1P1, P[3 with combining macron]m1 Si1P1, P21/m Si1P1, Pm Si1P2, P[4 with combining macron]21m Si1P2, and C2/m Si1P3. Yellow and violet spheres denote Si and P atoms, respectively.126 This figure has been reproduced from ref. 126 with permission from Wiley. (b) Structures of SiP and GeP. Green and purple colours denote phosphorus (P) atoms and silicon (Si)/germanium (Ge), respectively.269 This figure has been reproduced from ref. 269 with permission from the American Chemical Society. (c) Chemical structures of the three most stable bulk phases of GeP2.266 This figure has been reproduced from ref. 266 with permission from the Royal Society of Chemistry. (d) Top and side views of the atomic structure of single-layer GeP3 or SnP3. Blue and purple colours denote phosphorus (P) atoms and germanium (Ge)/tin (Sn), respectively.267 This figure has been reproduced from ref. 267 with permission from the Royal Society of Chemistry. (e) Crystal structure of rhombohedral GeP5.266 This figure has been reproduced from ref. 266 with permission from the Royal Society of Chemistry.

image file: c9nr04348a-f12.tif
Fig. 12 Structures of SiP crystals (a–d) and GeP crystals (e–h).35 These figures have been reproduced from ref. 35 with permission from AIP Publishing.

Unlike the structures of MPx formed by group III or group IV elements with P, the monolayer of AsxP1−x270,271 or SbxP1−x171 is configurated on a single-layer BP super-cell doped with different concentrations of As atoms or Sb atoms. As shown in Fig. 13, in contrast to BP the monolayer of AsxP1−x or SbxP1−x consists of two unparallel atomic planes. This can be attributed to the substitution of As or Sb atoms with P atoms in pristine BP, and the As or Sb atoms are slightly displaced outwards and the rectangular primitive cell lattice constants are increased to further increase the stability. Furthermore, akin to other 2D layered materials, these BP-like materials possess the weak van der Waals interlayer interactions and strong in-plane covalent bonding.


image file: c9nr04348a-f13.tif
Fig. 13 Crystal structure of group V-P 2D layered binary compound MPx (M = As, Sb). (a) The most stable b-AsP system configurates with different compositions, special points and Brillouin zones for the b-AsP supercell electronic bands.271 This figure has been reproduced from ref. 271 with permission from the Royal Society of Chemistry. (b) Top and two side views of relaxed Sb0.75P0.25. Purple and blue colours denote antimonide (Sb) and phosphorus (P) atoms, respectively.171 This figure has been reproduced from ref. 171 with permission from Springer.

3.2 Synthesis methods

Although a variety of stable structures of the layered MPx have been obtained through theoretical calculations, there are only three materials that have been experimentally synthesized: SiP,263,272 GeP,126,266,272 and AsxP1−x.273,274 Nowadays, there are three main methods to grow SiP, GeP or AsxP1−x, single crystals: (1) flux zone method, (2) high-pressure (HP) melt growth method and (3) chemical vapor transport method (CVT).

The flux zone method and the HP method were mainly used to grow SiP and GeP single crystals. In the flux zone method, silicon (5 N)/Ge (5 N) and RP (6 N) were used as precursors and Sn/Bi was used as the flux for the synthesis of o-SiP or o-GeP. The starting reactants were sealed under 5 × 10−4 Pa pressure in a quartz ampoule tube and then located in a furnace. For SiP, the furnace was slowly heated to 500 °C and the temperature was maintained for 36 hours. Then the temperature was increased again up to 1150 °C and maintained for 12 hours. Finally, o-SiP crystals were achieved after the cooling process. The Sn flux was diluted at 600 °C, and then the Sn flux was separated from the products by using a centrifuge. After removing the residual Sn on the surface, the bulk o-SiP single crystals were achieved, as shown in Fig. 14a. For GeP, the furnace was heated to 950 °C slowly and maintained for 24 hours, and then cooled down to 600 °C eventually. The produced crystals are of strip-like shape and shiny with typical dimensions of 6 × 3 × 0.2 mm3.


image file: c9nr04348a-f14.tif
Fig. 14 (a) o-SiP crystals synthesized by the Sn flux method.263 This figure has been reproduced from ref. 263 with permission from the Royal Society of Chemistry. (b) Crystals of GeP from HP melt growth.229 This figure has been reproduced from ref. 229 with permission from the American Chemical Society. (c) Crystals of As0.83P0.17 (size up to 0.5 cm).274 This figure has been reproduced from ref. 274 with permission from Wiley.

The CVT method is a conventional universal method to synthesize single crystals with high quality. For SiP and GeP, the pure elements were mixed first. Then the mixture was placed in an ampoule and sealed in a vacuum environment. Then the ampoule was placed in a two-zone furnace and heated to Thot at the hot end. After a few days, the furnace was turned off and the temperature was reduced to room temperature. For AsxP1−x, a mixture of gray arsenic and red phosphorus with molar ratios ranging from 9[thin space (1/6-em)]:[thin space (1/6-em)]1 to 2[thin space (1/6-em)]:[thin space (1/6-em)]8 was used as the precursor. Lead iodide (PbI2) and tin iodide (SnI4) were added to the starting materials and acted as mineralizer additives. All chemicals were enclosed in evacuated silica glass ampoules during the reaction. The mixture was heated up to 550 °C and held at this temperature for 20 to 80 hours, and then slowly cooled to room temperature. Fig. 14c shows large-sized crystals of As0.83P0.17.274

3.3 Properties and applications

These 2D layered binary compound MPx are considered to have unique electrical and optical properties. Through theoretical calculations, it is found that MPx have high carrier mobility, large capacity, excellent stability and high absorbance, and can be considered as a potential candidate for an excellent anode material, providing high power and energy density for alkali metal-based batteries (Li-ion batteries and K-ion batteries), and is also suitable as a nanoscale photocatalyst for photo-electrochemical water splitting or photocatalyzing CO2 splitting to CO under acidic conditions.

However, for the three MPx single crystals that have been experimentally synthesized (SiP, GeP and AsxP1−x), the current research remains focused on their semiconductor properties, such as strong anisotropic physical properties, conductance and photoresponsivity. Similar to BP, the energy bandgap of SiP or GeP decreases with increasing layer numbers.264 The energy bandgap and mobility of bulk SiP are found to be 1.71 eV and 2.034 × 103 cm2 V−1 s−1 which are two times higher than those of BP while the indirect bandgap of GeP can be tuned from 1.68 to 0.51 eV (monolayer to bulk). The calculated mobility of GeP is as low as 0.35 cm2 V−1 s−1 with a high on/off ratio of about 1 × 104. In order to improve the dielectric environment and the contact to realize the highest field effect mobility, extensive investigation is in process. The mobility of GeP increased while the on/off ratio decreased with increasing layer numbers, respectively. Compared with other two-dimensional semiconductors, SiP and GeP have highly anisotropic dispersions of band structures, providing potential for applications in electronic and optoelectronic devices. As shown in Fig. 15, the on/off switching ratio of the SiP-based device along the b-axis is 11; however, the on/off switching ratio of the same device along the a-axis is 3.5. The anisotropic factor of the on/off switching ratio is about 3.14. For GeP, the angle-resolved electronic and optoelectronic measurements showed high in-plane anisotropic conductance.126


image file: c9nr04348a-f15.tif
Fig. 15 The anisotropic, electronic and photoelectronic properties of SiP (a–d)264 and GeP (e–h).126 These figures have been reproduced from ref. 264 and 126 with permission from the Royal Society of Chemistry and Wiley, respectively.

Because of sharing a similar crystal structure to BP, AsxP1−x is also called black arsenic phosphorus, b-AsxP1−x in short. With different and tunable compositions (b-AsxP1−x, with x in the range of 0–0.83), layered b-AsP materials have many unique properties compared with BP.273 As shown in Fig. 16a, polarization-resolved infrared absorption revealed the in-plane anisotropic properties of these materials. Fig. 16b shows the collection of b-AsP infrared absorption spectra with different compositions. By increasing x in b-AsxP1−x, an obvious blueshift of the absorption edges is observed, confirming that the b-AsxP1−x bandgaps change with the chemical composition concentration of the material. These outcomes make b-AsP a promising material for various applications. It can be obviously seen from Fig. 16c that the bandgaps increase with decreasing arsenic concentration. More importantly, the bandgaps of b-AsxP1−x can be tuned from 0.3 to 0.15 eV (b-As0P1 to b-As0.83P0.17), which indicates a broader detection range than that of BP. Fig. 16d–f show back-gated FETs utilizing exfoliated b-As0.83P0.17 flakes as channel materials. The FETs exhibited a hole mobility of 110 cm2 V−1 s−1 and ambipolar transport behavior with an Ion/Ioff of 1.9 × 103. The measured results of this transport behavior revealed that b-AsP is a semiconductor material. Fig. 16g–i show the photo-response of b-AsP in the MIR band.275Fig. 16g shows the photocurrent of the same device along the x- and y-directions (armchair and zigzag edge) (inset of Fig. 16g) at room temperature. Without light illumination, the conductivity along the y-direction is nearly 1.73 times smaller than that along the x-direction. This anisotropic factor value of 1.73 is larger than the value of 1.6 in BP. Under the illumination with a 4.034 μm incident MIR beam, IPx/IPy is about 3.51 at Vds = 1 V. Similar to the observation in BP, the photocurrent was minimum when the light polarization was parallel to the y-direction and maximum when the light was parallel to the x-direction. The photocurrent anisotropy ratio was approximately 0.59, which is larger than the value of 0.3 in BP. The photoresponse of the detectors based on b-AsP under zero-biased conditions was measured in the wavelength range from 2.4 to 8.05 μm, as shown in Fig. 16h. Although the responsivity decreased slightly as the incident laser wavelength increased, which is mainly due to the decrease in optical absorption efficiency with the increase in incident laser wavelength, the device still exhibited a high responsivity (15 to 30 mA W−1) across the whole tested MIR band. The 0.54 ms rise time and 0.52 ms fall time were achieved, as shown in Fig. 16i.


image file: c9nr04348a-f16.tif
Fig. 16 The anisotropic, electronic and photoelectronic properties of b-AsxP1−x. (a) Polarization-resolved b-As0.83P0.17 infrared absorption spectra with different polarization angles. (b) Different b-AsxP1−x infrared absorption spectra. x denotes the corresponding 0, 0.25, 0.40, and 0.83 concentrations of arsenic. (c) The x-dependent band gaps of b-AsxP1−x. The b-AsxP1−x flake thickness is >30 nm. (d) AFM measurement of b-AsP FETs. (e) Measurement of thick b-As0.83P0.17 flake transfer curves.273 These figures in (a) to (e) have been reproduced from ref. 273 with permission from Wiley. (f) Measurement of a thin b-As0.83P0.17 flake transfer curve. (g) The device IdsVds curves with and without the wavelength of 4.034 μm and the power of 21.5 W cm−2 of the incident laser beam. (h) Photoresponsivity and EQE of the b-As0.83P0.17 based device. (i) Measurement of device photoresponse.275 These figures in (f) to (i) have been reproduced from ref. 275 with permission from the American Association for the Advancement of Science.

The b-As0.83P0.17 based MIR heterostructure photodetectors were also fabricated,276 as shown in Fig. 17. As shown in Fig. 17a and b, the electron and hole mobilities of hBN/b-As0.83P0.17/hBN photodetectors are about 83 and 79 cm2 V−1 s−1, respectively. Noticeably, the hBN encapsulation leads to an excellent air stability and elimination of trap states at the interfaces. b-As0.83P0.17 is a typical p-type semiconductor, while MoS2 is a typical n-type semiconductor. The rectification curves are shown in Fig. 17c, confirming that the van der Waals p–n junction is formed. The current at the reverse bias is more than two orders of magnitude smaller than that at the forward bias. This is mainly due to the energy barrier in the b-As0.83P0.17/MoS2 heterostructure,232 and the dark current is dramatically inhibited. As shown in Fig. 17d, the noise figure of the b-As0.83P0.17/MoS2 heterostructure was significantly enhanced compared to the b-As0.83P0.17 FETs. These outcomes further indicate that the energy barrier at the junction depressed the photogenerated carrier random transport efficiently and suppressed the 1/f noise. Consequently, by utilizing the b-As0.83P0.17/MoS2 junction, the total noise was successfully suppressed. In general, these findings not only provide a promising photodetector for advanced MIR detection but also for space communication, infrared imaging, and biomedical sensing.


image file: c9nr04348a-f17.tif
Fig. 17 Device structure and electrical characterization of b-As0.83P0.17 based MIR photodetectors. (a and b) hBN/b-As0.83P0.17/hBN heterostructure photodetector. (c and d) b-As0.83P0.17/MoS2 heterostructure detector.275 These figures have been reproduced from ref. 275 with permission from the American Association for the Advancement of Science.

4. Transition metal phosphorus trichalcogenides (MPX3)

Transition metal phosphorus trichalcogenides (MPX3) are ternary compounds, and the MPX3 family extends to a wider region of the periodic table than TMDCs, where M mostly includes IVB, VB and VIB (M = Fe, Co, Ni, Zn, etc.; X = S, Se or Te). The larger chemical diversity and structural complexity would lead to a variety of fascinating electric and magnetic properties. MPX3 materials have been investigated since the 19th century, but lack comprehensive investigation at the atomically thin level. MPX3 is stable under ambient conditions, and displays great potential applications for short wavelength optoelectronics, hydrogen storage and lithium batteries, as well as magnetic and ferroelectric devices.

4.1 Crystal structure

As shown in Fig. 18a, in MPX3 each M atom is coordinated with six X atoms, and the P atoms are tetrahedrally coordinated with three X atoms to form a [P2X6]4− unit, and each unit is located at the centre of the neighboring six M atoms, which form a hexagonal structure similar to BP. However, proved by the phonon spectra, MPX3 exhibits much stronger structural stability than BP, which induces the stable magnetic phases of MPX3. The MPX3 materials have two types of similar plane structure packing modes (Fig. 18a–c).130 The atomic layers of S are arranged along the c axis in an ABC stacking and correspond to the C2/m space group (Fig. 18b), while the layers of Se are stacked along the c axis in an AB sequence which corresponds to the space group of R3 (Fig. 18c). Similar to other 2D materials like BP, MPX3 is a layered structure with weak van der Waals interactions, which causes the easy exfoliation of the material into mono or few layers. According to the calculation of the lattice constants of MPX3, it is found that the in-plane lattice constant and monolayer thicknesses are increased with larger chalcogens, while the dependence on the metal atom number is not straightforward.277 The Raman spectra of FePS3 and FePSe3 are shown in Fig. 18f and g, and the peaks at 160 cm−1, 229 cm−1, 281 cm−1 and 381 cm−1 are attributed to the Eu, Eg, Eg, and A1g Raman modes, respectively. These peaks can be observed in both bulk and single-layer FePS3. In the bulk and three layer FePSe3, the peaks at 149 cm−1, 170.8 cm−1, and 217 cm−1 are attributed to Eg, A1g and A1g modes, respectively (Fig. 18g). After exposure to an ambient environment for 24 hours, the Raman peaks still remain sharp, indicating that ultrathin FePS3 and FePSe3 possess excellent stability in the ambient environment.
image file: c9nr04348a-f18.tif
Fig. 18 (a) Structure of MPS3. (b and c) Packing structure of MPS3 and MPSe3, respectively; (d and e) TEM and SAED measurement of FePS3 and FePSe3, respectively.130 (f and g) Raman spectra of FePS3 and FePSe3.130 These figures have been reproduced from ref. 130 with permission from the American Chemical Society.

4.2 Properties

4.2.1 Magnetic properties. Magnetism possesses great significance for fundamental research and is considered as a promising candidate for a variety of applications. However, magnetism in most 2D materials is induced from extrinsic effects, like vacancies, edges, defects, or chemical dopants. Fascinatingly, the magnetism of 2D MPX3 is intrinsic due to the metal elements. It is found that the magnetic properties of MPX3 depend systematically on the transition-metal/chalcogen element combination. Calculations show that it is the bond length rather than the metal atomic number that correlates more strongly with the magnetic state. In general, the ferromagnetic (FM) phase has the largest lattice constants, and the lattice constants of antiferromagnets (AFM) are intermediate. Generally, the AFM phases are the most common. Xiong et al. studied the spin-phonon coupling magnetic properties of FePS3 through Raman spectra.278 The Raman peak around 88 cm−1 is induced by the spin order, and the disappearance of this peak can be identified as a signature of the transition from the antiferromagnetic to paramagnetic phase around the Néel temperature (Fig. 19a). Therefore, according to the peak at 88 cm−1, it can be concluded that FePS3 exhibits the same magnetic transition from the antiferromagnetic to the ferromagnetic phase even down to a single layer. The Néel temperature shows obvious layer dependency (Fig. 19b), suggesting that magnetic ordering also exists in-plane at the monolayer. According to the Raman measurement, it is found that FePS3 displays an Ising-type antiferromagnetic ordering down to the single layer.131 It can be seen from Fig. 19c that the P1a and P2 peak intensities significantly increase at low temperature, which is an obvious feature of the antiferromagnetic phase. Sekizawa et al. investigated the heat and magnetic properties of MPS3 (M = Mn, Fe and Zn).279 This reveals that ZnPS3 is nonmagnetic, while MnPS3 and FePS3 are antiferromagnets at 78 and 118 K, respectively. The temperature dependence of the magnetic specific heat CM of MnPS3 exhibits a broad peak which is typical of the 2D magnetic material, while the CM of FePS3 exhibits a sharp peak at TN. Electric field is a widely used method to control the magnetic order in a number of materials. According to the study of the dependence of magnetic properties on carrier density, the magnetic state of AFM will transfer to FM states by applying a sufficient amount of electron or hole carrier densities. Because the magnetic properties of MPX3 are related to the structure, there exists the possibility of modifying magnetic properties through straining the lattice. It is found that the magnetic phase transitions can be triggered by c-axis pressure or by in-plane compression/expansion of the lattice constants. The magnitude of the required strains varies greatly for different MPX3 compounds.
image file: c9nr04348a-f19.tif
Fig. 19 (a) Temperature-dependent Raman spectra of mono-, tri- and five-layer FePS3. (b) The sample thickness dependence of the Néel temperature and the outcome is compared with the bulk sample measured by PPMS.278 These figures in (a) and (b) have been reproduced from ref. 278 with permission from IOP Publishing. (c) The temperature dependence of 1L FePS3 Raman spectra. * represents the Si substrate signal.131 This figure has been reproduced from ref. 131 with permission from the American Chemical Society.
4.2.2 Electrical properties. Studying the electronic properties of 2D MPX3 materials has great significance to realize the application of electronic nanodevices. It is found that AFMs are almost semiconductors, FM are metallic, and NM phases can be either metallic or semiconducting.280 The electronic structure of MPX3 is strongly sensitive to the electron–electron interaction model, and for the AFM, the s and p orbitals of the P atom are significant on the conduction-band while the chalcogen atom orbitals have an important effect on the valence-band edges. The MPS3 (M = Fe, Mn, Ni, Cd, Zn) and MPSe3 (M = Fe, Mn) single-layer band structures are shown in Fig. 20a–f.133 The bandgaps of MPS3 and MPSe3 single-layers range from 1.90 to 3.44 eV.133 MnPS3 and MnPSe3 single layers feature a direct bandgap with values of 3.14 and 2.32 eV, respectively,41 while FePS3, NiPS3, CdPS3, ZnPS3 and FePSe3 exhibit an indirect bandgap. The electron mobility of MnPSe3 is 625.9 cm2 V−1 S−1 while the hole mobility is 34.7 cm2 V−1 S−1.281 The monolayer MnPSe3 exhibits a carrier mobility superior to many other 2D semiconductors, including MoS2, BN, and so on. Additionally, the huge distinction of mobility suggests the effective separation of electron–hole pairs and the small possibility of recombination for photogenerated carriers. FePSe3 exhibits superconductivity (SC), as shown in Fig. 20h and i, and a sharp drop of resistance (R) at an initiating temperature of ∼2.5 K can be observed near the critical pressure Pc ≈ 9.0 GPa, indicating the appearance of SC in the high pressure (HP) phase. The SC state can remain under pressure up to 41.4 GPa, and zero resistance can be observed for most of the pressure points. All these pieces of evidence prove that the SC state shows an intrinsic behavior of FePSe3 in the HP phase.132
image file: c9nr04348a-f20.tif
Fig. 20 (a to f) Band structures of monolayer FePS3, MnPS3, NiPS3, CdPS3, ZnPS3 and FePSe3, respectively.133 (g) The MPS3 and MPSe3 monolayers at VBM and CBM locations simulated by the HSE06 functional.133 These figures in (a) to (g) have been reproduced from ref. 133 with permission from Wiley. (h and i) The FePSe3 single crystal in-plane electrical resistivity as a function of temperature.132 These figures have been reproduced from ref. 132 with permission from the Nature Publishing Group.
4.2.3 Electrochemical properties. Hydrogen is regarded as a promising green energy, and water splitting is the most eco-friendly method for the production of H2. The electrochemical H2 evolution reaction (HER) is the most fundamental reaction in electrochemistry. Both S and P are expected to be excellent catalysts due to their H adsorption and desorption activities, and therefore, MPX3 materials have attracted enormous attention for the HER. The ability of reduction/oxidation could be evaluated by aligning the CBM and VBM with respect to the water redox potential levels. Fig. 20g shows the band edge position of single-layer MPS3 and MPSe3 for photocatalytic water splitting. Besides FePSe3, for all the remaining compounds, the CBM potentials are enough higher than the hydrogen reduction potential, and the VBM potentials are much lower than the O2/H2O oxidation potential. Single-layer FePSe3, FePS3, MnPSe3, and NiPS3 show obvious optical absorption in the visible band (Fig. 21a), which means that more than 40% of the solar spectrum could be captured.41 The effective masses of carriers of MPX3 are smaller than other common photocatalysts, which promise an easier carrier transfer to the reactive sites during the photocatalytic process. These results indicate that MPX3 are potential materials for water-splitting photocatalysts. To evaluate the electrocatalytic performance of the MPSx materials, the catalytic activities in the HER, oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) were tested.282 From Fig. 21b and c, NiPS3 and CoPS3 exhibit high HER activity which can be attributed to the crystal structure preferential orientation in the (001) plane. CoPS3 exhibits the best OER performance with an initial potential of 0.84 at 10 mA−1 cm2 and the current density reaches as high as 30 mA−1 cm2. CoPS3 also exhibits stable performance after being tested by 100 CV cycles. The high OER activity of CoPS3 is probably attributed to cobalt. MnPS3 displays the best ORR performance with a distinct reduction peak around −0.28 V versus RHE (Fig. 21d–g).282
image file: c9nr04348a-f21.tif
Fig. 21 (a) Measured MPS3 and MPSe3 optical absorption coefficients.282 (b) The average overpotential at a current density of −1 mA cm−2.282 (c and d) LSV of the OER and ORR, respectively. (f) The average overpotential at a current density of 10 mA cm−2. (e) The peak ORR reduction potentials. (g) Error bars correspond to standard deviations.282 These figures have been reproduced from ref. 282 with permission from the American Chemical Society.

4.3 Synthesis methods

The bulk crystals of MPX3 could be obtained by the chemical vapor transport (CVT) method in a two-zone furnace (Fig. 22).130 High-purity elements (mole ratio metal[thin space (1/6-em)]:[thin space (1/6-em)]RP[thin space (1/6-em)]:[thin space (1/6-em)]chalcogen = 1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]3) and transport agents of iodine were sealed into a quartz ampoule and heated in a furnace (650–600 °C). Then the bulk crystals were obtained by cooling down the ampoule to room temperature after 1 week of heating. Due to the weak vdW interaction, MPX3 can be easily mechanically exfoliated into mono or few layers. The SEM image of the MPSx crystals is shown in Fig. 23, and an obvious layered structure can be observed in all compounds.282 2D MnPX3 (X = S or Se) nanosheets can be synthesized through the CVD method on a flexible carbon fiber substrate. MnPX3 nanosheets were synthesized in a two zone furnace. The MnPS3 and MnPSe3 nanosheet thickness is ≈6 nm and ≈28 nm with a lateral size of 1.5 and 0.45 mm, respectively (Fig. 24a and b). Both MnPS3 and MnPSe3 are high quality crystals. As shown in Fig. 24c and d, the Raman modes originating from P2S6 units are identified as A1g and Eg. Prominent modes due to the P–S bond symmetric stretching vibration are also corroborated from the peaks of the Raman spectrum at 245.9 and 491 cm−1. Additionally, the peaks appearing at 274 and 581 cm−1 correspond to the in-plane vibration Eg mode. The strongest peak in MnPS3 at 381 cm−1 shifts to 216 cm−1 in MnPSe3.281 The successful synthesis methods to obtain high quality 2D MPX3 provide a significant step towards future applications.
image file: c9nr04348a-f22.tif
Fig. 22 Schematic diagram of the CVT method and photographs of the bulk crystals.130 This figure has been reproduced from ref. 130 with permission from the American Chemical Society.

image file: c9nr04348a-f23.tif
Fig. 23 SEM images of metal thiophosphite materials.282 This figure has been reproduced from ref. 282 with permission from the American Chemical Society.

image file: c9nr04348a-f24.tif
Fig. 24 (a and b) AFM images of MnPS3 and MnPSe3, respectively. (c and d) Raman spectra of MnPS3 and MnPSe3, respectively.281 These figures have been reproduced from ref. 281 with permission from Wiley.

4.4 Applications

Few-layer FePS3 and few-layer rGO-FePS3 were usually used as catalysts for electrochemical hydrogen evolution and investigated over a wide pH range, as shown in Fig. 25a.283 The initial potential of FePS3 nanosheets is about −95 mV, while that of rGO-FePS3 in acidic solutions is about −50 mV. The improved HER activity may result from the enhanced conductivity caused by the presence of rGO. The stabilities of FePS3 and rGO-FePS3 are shown in Fig. 25b. The catalytic activities of FePS3 are measured from 0.3 to 0.4 V (compared to RHE) for 1000 cycles, and neither structural nor compositional change during the HER after 1000 cycles proves that the material is very stable. Sunlight driven catalysis of water splitting was carried out by using MnPX3 nanosheets.281 As can be seen from Fig. 25e, both MnPSe3 and MnPS3 display excellent photocatalytic abilities with a H2 production rate of 6.5 and 3.1 μmol h−1, respectively. Furthermore, both MnPSe3 and MnPS3 show promising stability in photocatalysis after three cycles. MPX3 can also be used as an electrode material; however, the Li2S–FePS3 composite was prepared and used for electrode materials. The Li2S-rich Li2S–FePS3 composite cells demonstrated an initial discharge capacity of about 780 mA h g−1, which is much higher than that of the Li2S–FeS composite cells (Fig. 25f). The incorporation of P ions into low-crystallinity Li2S–FePS3 significantly improves the structural reversibility, thus resulting in an enhanced electrochemical performance.284
image file: c9nr04348a-f25.tif
Fig. 25 (a) iR-Corrected voltammograms on different catalysts in H2SO4. (b) rGO-FePS3 electrochemical stability.283 Copyright 2016, American Chemical Society. (c) HER activity of FePS3. (d) Measurement of rGO-few layer FePS3 electrochemical durability.283 These figures in (a) and (b) have been reproduced from ref. 283 with permission from the American Chemical Society. (e) Measurement of photocatalytic H2 evolution rates, red: MnPS3, blue: MnPSe3.281 This figure has been reproduced from ref. 281 with permission from Wiley. (f) Charge and discharge profiles of Li2S–FePS3 and Li2S–FeS.284 This figure has been reproduced from ref. 284 with permission from Elsevier.

5. Pnictogens

The elements of group V or pnictogens include phosphorus (P), arsenic (As),134,136 antimony (Sb)139–141 and bismuth (Bi).285,286 As, Sb and Bi have similar honeycomb and layered crystal structures to BP, and are expected to have some unique and excellent physical properties. In this part, the crystal structure and physical properties of 2D As, Sb and Bi will be discussed firstly, and then we will focus on the preparation methods and potential applications of these materials.

5.1 Crystal structure

The heavy pnictogens (As, Sb, and Bi) feature an orthorhombic (termed also the α phase) or rhombohedral (termed also the β phase) layered structure.287,288 The α phase As, Sb, and Bi share similar honeycomb and puckered crystal structures to black phosphorus, while the β phase elements, which are usually termed gray As, gray Sb, and metallic Bi, respectively, are constructed with a buckled layered structure, which is similar to blue phosphorus. The β phase is the most stable structure among these allotropic elements, which is different from the lowest-energy configuration of α phase BP. The crystal structure parameters of the α and β phase pnictogens are summarized in Table 1 and Scheme 1. The α phase As, Sb, and Bi share the space group Cmca with BP. In contrast, the β phase As, Sb, and Bi has the same space group R3m. Although both the α and β phase pnictogens are layered structures, different from the α phase, the β phase is not formed with van der Waals interactions. The difference between α and β phase As, Sb, and Bi is caused by the interaction of atomic orbitals between two layers. The differences between in- and out-of-plane interatomic distances of rhombohedral As, Sb, and Bi are shown in Table 2,287 also see Scheme 2. The differences are high enough to result in the anisotropical physical properties of the β phase As, Bi, and Sb. Until now, α phase P, As, Sb, Bi and β phase P, As, Bi experimentally existed.
image file: c9nr04348a-s1.tif
Scheme 1 The orthorhombic and rhombohedral atomic structures of pnictogens.

image file: c9nr04348a-s2.tif
Scheme 2 Schematics of the rhombohedral structure of As, Sb or Bi.287 This figure has been reproduced from ref. 287 with permission from Wiley.
Table 1 Parameters of crystal constants
Pnictogens Lattice constants (Å) Bond length (Å) Angle (°)     Ref.
a 1 (ZZ) a 2 (AC) d 1 d 2 θ 1 θ 2 Δ (Å) c (Å)
Black As 3.64, 3.67, 3.69 4.46, 4.765, 4.769 2.5 2.48,2.5 100.5, 100.8 94.62, 94.64 2.39 5.46, 5.66 33, 42 and 43
Gray As 3.63., 607 2.503, 2.51 91.91, 92.22 1.39, 1.4 4.5, 4.67 42 and 44
α-Sb 4.74 4.36 2.87 2.94 102.4 95.3 45
β-Sb 4.12,4.28 2.89,2.93 90.8,94 1.5, 1.57, 1.65 3.73, 3.8 45–47 and 290
α-Bi 4.54 4.75 48
β-Bi 4.54, 4.64 2.9, 3.3 1.61, 1.63 3.22,3.9 48, 49 and 292


Table 2 The structural parameters of rhombohedral As, Sb, and Bi.287 Copyright: 2017, Wiley
Material In-plane distance [nm] In-plane distance [nm] Anisotropy [%] Bond angle [°]
Arsenic 0.251 0.315 25.5 97
Antimony 0.287 0.337 17.4 96
Bismuth 0.31 0.347 11.9 94


5.2 Properties

5.2.1 Electronic properties.
Arsenic. Bulk As is a semimetal, while monolayer As is an indirect semiconductor with a 2.49 eV bandgap for both buckled and puckered structures (Fig. 26a).136 Like BP, the energy bandgap of As can be modified through the thickness of the material, and the bandgap of monolayer As is reduced by ∼1/2 in the bilayer and ∼1/6 in the trilayer. In addition, the puckered and buckled arsenene bandgap can be adjusted by adding strain. The puckered As can be transformed from an indirect into a direct-gap semiconductor by applying only 1% strain, Fig. 26b.135 However, under tensile strains, the bandgap of monolayer arsenene decreased monotonously with the increase in strains,50 as shown in Fig. 26c. Similar to BP, black arsenene displays anisotropic electric transport in the armchair and zigzag directions. The α-As conductance along the armchair direction is more than six times higher than that along the zigzag direction. The hall electron and hole mobility display an excellent anisotropy as μZZe = 376.7 cm2 V−1 s−1, μACe = 1.5 cm2 V−1 s−1, μZZh = 60.7 cm2 V−1 s−1, and μACh = 10[thin space (1/6-em)]606 cm2 V−1 s−1, respectively (Fig. 26d).134 Zhong et al. discovered that the electronic properties of few layer b-As are thickness and temperature dependent. A sample with 5.7 nm thickness exhibits the highest carrier mobility of about 59 cm2 V−1 s−1, and the largest current on/off ratio over 105 is measured from the sample with 4.6 nm thickness. The peak value of carrier mobility is obtained at 230 K; while below 230 K, due to impurity scattering, the carrier mobility is strictly limited, under high temperature conditions, lattice scattering becomes dominant. Noticeably, after exposure to an ambient environment over a month, b-As FETs still showed an excellent performance, which means that α-As possesses an excellent ambient stability compared to BP, as shown in Fig. 26e.137 The 2D arsenene also exhibits topological insulator properties. When the tensile strain is higher than 11.14%, the s–p band inversion appears at the Γ point, and results in a nontrivially topological state. The spin–orbit coupling (SOC) provides a quantum spin Hall (QSH) gap at the Dirac point and with a value of 193 meV, and for the edge of arsenene, a single pair of topologically protected helical edge states is established, and their QSH states are proved with nontrivial topological invariant Z2 = 1. These interesting findings provide a promising candidate for new quantum devices and topological phenomena for nanoelectronics operation.
image file: c9nr04348a-f26.tif
Fig. 26 (a) Electronic band structures of arsenic.136 This figure has been reproduced from ref. 136 with permission from Wiley. (b) The puckered arsenene band structure by applying strain along lattice vector a.135 This figure has been reproduced from ref. 135 with permission from the American Physical Society. (c) The dependence of the monolayer arsenene band gap on the in-plane biaxial strain.293 This figure has been reproduced from ref. 293 with permission from Elsevier. (d) The b-As magnetic field-dependence of longitudinal/transverse conductivity (σxx/σxy) along the armchair and zigzag directions.134 This figure has been reproduced from ref. 134 with permission from Wiley. (e) The dependence of transistor carrier mobility and Ion/Ioff ratios versus air-exposure time.137 This figure has been reproduced from ref. 137 with permission from Wiley.

Antimony. According to the first-principles study, the Sb single bilayer (BL) is a semiconductor with a bandgap of 0.75 eV. The bandgap of antimonene could transform from indirect- to direct-bandgap through applying a moderate strain, as shown in Fig. 27a and b.294 When a small strain is applied, the Sb BL is a topologically trivial band insulator with Z2 = 0, and by increasing the applied strain, it transformed to a nontrivial QSH state with Z2 = 1, indicating the probability of realizing the QSH state for thin antinomy flakes. The buckling structure of Sb provides a tunable bandgap by applying an out-of-plane external electric field, Fig. 27c and d. By applying biaxial strain larger than 14.5%, Sb can be tuned to a topological insulator, as shown in Fig. 27e. The antimonene buckled structure can sustain tensile strain as large as 18%, which induced a 270 meV SOC gap. These findings make antimonene a promising 2D material for realizing the QSH effect which satisfied the demand of low power consumption electronic devices at high temperatures. Based on the first-principles calculations, Zhang et al. reported that due to an interesting interaction between the surface effect and quantum confinement, the antinomy films exhibit topological and electronic (top o-electronic) properties experiencing a series of transitions with the reduction of thickness, as shown in Fig. 27f, transforming from a topological semimetal to a topological insulator at 7.8 nm, then to a QSH phase at 2.7 nm, and eventually to be a semiconductor at 1.0 nm. The electron and hole mobilities of Sb are 630 and 1737 cm2 V−1 s−1, respectively.291
image file: c9nr04348a-f27.tif
Fig. 27 (a) Schematic to illustrate the transition of the trivial state to non-trivial state.294 (b) The band gap dependence on strain at the Γ point.294 (c) Topological phase transition caused by an additional electric field.294 (d) Evolution of band gaps.294 (e) The electronic band structures of antimonene.294 These figures in (a) to (e) have been reproduced from ref. 294 with permission from AIP Publishing. (f) The direct and indirect bulk gap and surface splitting versus the flake thickness.291 This figure has been reproduced from ref. 291 with permission from the American Physical Society.

Bismuth. Using first-principles calculations, for strains and electric fields up to ±6% and 0.8 eV Å−1, respectively, the bi-layer Bi can maintain a bandgap and a nontrivial band topology (Fig. 28a and b).295 This indicates that the BL-Bi is a 2D topological insulator (TI) against strain and electrical field on a substrate to be a QSH insulator. Liu et al. revealed that all the ultrathin Bi (111) flakes are independent of the flake thickness and characterized by a nontrivial Z2 number, without the topological triviality odd–even oscillation, as is well known, as shown in Fig. 28c and d.145 Bi is a topological insulator when the flake is a number less than 4 BLs, while when the flake is over 4 BLs, the band structure consists of surface bands superimposed onto a 2D projected bulk band. With the increasing film thickness, the Bi (111) film maintains the nontrivial topology with a bandgap, as shown in Fig. 28e. PBE calculations confirm a direct bandgap of Eg = 0.55, 0.16 and 0.31 eV for SL b-Bi, w- and aw-Bi, respectively, in contrast to the 3D Bi crystal, which is a narrow band gap, nonmagnetic semiconductor. An interesting feature is provided by the strong spin-orbit effects in Bi and the resulting Rashba-type spin splitting of the surface states.145
image file: c9nr04348a-f28.tif
Fig. 28 (a) Band gap of 2D bi-layer Bi versus strain.295 (b) Band structures of BL-Bi (111) with an external electric field of 0.8 eV Å.295 These figures have been reproduced from ref. 295 with permission from the American Physical Society. (c and d) The band structure of 2 and 3 BLs.145 (e) The direct and indirect band gap versus the thickness of flakes.145 These figures have been reproduced from ref. 145 with permission from the American Physical Society.
5.2.2 Optical properties. The optical absorption of monolayer and bilayer arsenene commonly concentrates in the ultraviolet to visible band (Fig. 29a).289 Additionally, the absorbance of arsenene increased as the layer number increased (Fig. 29b). The biaxial and uniaxial in-plane strains always induce a red-shifted behavior of the fundamental frequency, as can be seen in Fig. 29c. Furthermore, directional anisotropy, changing the layer number, and biaxial or uniaxial in-plane strain were discovered to modify the optical properties of 2D arsenene, which could be available for optoelectronic and photovoltaic applications. For a given polarization angle, three strong peaks of the α-As Raman spectrum are located at 223.6, 230.2, and 257.9 cm−1, and could be ascribed to the Ag1, B2g and Ag2 modes, respectively. As shown in Fig. 29d and e, the intensity of Raman peaks mentioned above varies by changing the polarization angle of the laser, resulting in a two- and four-fold symmetry for Ag1 and B2g, respectively.134 The refractive index values of α-Sb and β-Sb are 2.3 and 1.5, respectively, and increase to 3.6 in the ultraviolet band, as can be seen in Fig. 29f. The reflection enhanced to 86% at the UV energy band (Fig. 29g). The calculations confirm that Sb is promising for nano-electronic, solar cell and optoelectronic nanodevice applications, as well as for some other applications such as detection, modulation and manipulation functions.296
image file: c9nr04348a-f29.tif
Fig. 29 (a) Absorption spectra of SL b-As optical.289 (b) Absorbance of SL and BL w-As.289 (c) The optical absorption spectra under tensile strain.289 These figures have been reproduced from ref. 289 with permission from the American Physical Society. (d and e) Raman intensity of A1g and B2g modes.134 These figures have been reproduced from ref. 134 with permission from Wiley. (f) The real and imaginary parts of the complex dielectric function of monolayers a-Sb and b-Sb (left to right).299 (g) The absorption coefficient I(ω), energy loss spectrum L(ω) and reflectivity R(ω) of monolayer a-Sb and b-Sb (left to right).299 These figures have been reproduced from ref. 296 with permission from Royal Society of Chemistry.
5.2.3 Mechanical properties. One of the disadvantages of BP is vulnerable degradation under ambient conditions. In contrast, at temperatures T = 1000 K, the structures of As maintained their form for 2 ps, and no clustering or bond breaking occurred (except thermally induced deformation).297 The MD results cannot indicate that SL of arsenene can sustain at temperatures as high as 1000 K (Fig. 30a); they rather imply the stability at least slightly above room temperature since the statistics is speeded up at high temperature. Elastic properties are important which can provide useful information on strength, stability and elastic behaviors. The b-As in-plane stiffness value is C = 58 N m−1 and is homogeneous, while the w-As in-plane stiffness shows directionality caused by its structure and consequently Cx = 20 N m−1 and Cy = 55 N m−1. The b-As homogeneous Poisson's ratio was found to be νx = 0.21. For bismuthene, MD calculations started at 300 K and ended at 1000 K.298 These structures maintained excellent stability at high temperature, but the instability caused by thermal excitation took place at 700 K (Fig. 30b). The b-Bi in-plane stiffness is homogeneous and Cx = Cy = 23.9 N m−1, for w-Bi, Cx = 8.0 N m−1 and Cy = 22.6 N m−1, and for aw-Bi, Cx = 10.0 N m−1 and Cy = 25.5 N m−1 and are larger than those of w-Bi. The SL bismuthene in-plane stiffness simulation values are smaller than graphene (330 N m−1), h-BN (240 N m−1), silicene (65 N m−1), and MoS2 (138 N m−1), and all have the same hexagonal structure.
image file: c9nr04348a-f30.tif
Fig. 30 (a) The ab initio molecular dynamics calculations of the SL w-As atomic structure at different temperatures.135 This figure has been reproduced from ref. 135 with permission from the American Physical Society. (b) The calculated atomic structure at different temperatures.298 This figure has been reproduced from ref. 298 with permission from the American Physical Society. (c) Thermal conductivity of arsenene versus temperature.163 (d) Calculated ZT value of a monolayer Bi versus temperature.163 These figures have been reproduced from ref. 163 with permission from the American Physical Society.
5.2.4 Thermal properties. Thermal conductivity of arsenene has been investigated using ab initio calculations. An excellent anisotropic thermal conductivity of 30.4 W m−1 K−1 along the ZZ direction and 7.8 W m−1 K−1 along the AC direction is predicted, as shown in Fig. 30c.163 The mean free paths of phonons ranging from 20 to 1 μm contribute to the thermal conductivity in the ZZ direction, and those ranging from 20 to 100 nm contribute in the AC directions. Compared to phosphorene, the thermal conductivity of arsenene is 3 times smaller, but is more strongly anisotropic. The ZT value of single-layer bismuth is estimated to be 2.1 for n-type doping and 2.4 for p-type doping at room temperature (Fig. 30d).163 Additionally, a maximum value of temperature dependence of ZT is 4.1, which can be obtained at 500 K.

5.3 Synthesis methods

In this part we summarized three commonly used synthesis methods of group V elements, including molecular beam epitaxy (MBE), liquid-phase exfoliation (LPE) and van der Waals (vdW) epitaxy.

As shown in Fig. 31, through the MBE method, a monolayer Sb film can grow on Sb2Te3 (111), Bi2Te3 (111) and PdTe2 substrates due to the small lattice mismatch between Sb and the substrate.143,299 The Sb film has a layered structure along the (111) direction where two such layers form one BL unit, and the interaction between BL and substrates is vdW force. The Bi film was successfully grown on the Si (111)-7 × 7 surface with a (012) oriented phase.144


image file: c9nr04348a-f31.tif
Fig. 31 (a) Up and down panels, the Sb2Te3 and 1BL Sb/Sb2Te3 LEED patterns, respectively.299 (b) The Sb XPS spectra derived from different substrates.299 These figures have been reproduced from ref. 299 with permission from AIP Publishing. (c) STM image of Sb on PdTe2.51 (d) STM image of single-layer Sb.51 These figures have been reproduced from ref. 51 with permission from the Nature Publication Group. (e and f) STM image of the Bi film on the Si (111) substrate.56 These figures have been reproduced from ref. 56 with permission from Wiley. (g) STM images of occupied and empty states.144 This figure has been reproduced from ref. 144 with permission from Wiley.

Another facile method to obtain few-layer Sb and Bi films could be through liquid-phase exfoliation. By sonication of crystal materials and centrifugation of dispersion, Sb nanosheets larger than 1–3 mm2 are obtained, and the mono/bilayer thickness is about 4 nm, as shown in Fig. 32a and b. A new strategy combines acid-interaction and liquid exfoliation to successfully transform metal bulk Bi into a few-layer semiconductor, a typical sample with a height of 6.7 nm is comprised of 17 bi-layers.141,300


image file: c9nr04348a-f32.tif
Fig. 32 (a) AFM image of FL Sb flakes.300 (b) Height profile of the samples.300 These figures have been reproduced from ref. 300 with permission from Wiley. (c) Schematic of the sample synthesis process.141 (d) AFM image of a typical triangular Sb sheet. (e and f) AFM images of antimonene and bismuth nanosheets.141 These figures have been reproduced from ref. 141 with permission from the Nature Publication Group.

Using van der Waals epitaxy could synthesize few-layer antimonene polygons directly on a dielectric substrate. In this method, fluorophlogopite mica was employed as the substrate, and after heating and cooling processes in a two-zone tube furnace at separate temperatures, single-layer antimonene with thickness around 1 nm and lateral size around 100 nm was obtained (Fig. 32e).141

5.4 Applications

2D Bi presented advantageous saturable absorption properties compared to that of the MoS2 benchmark in the near infrared band. Additionally, 2D Bi is usually used in mode-locking fiber lasers to obtain a short duration 2 μm pulse (Fig. 33a–c). By utilizing its superior nonlinear properties, optical switching based on spatial cross-phase modulation was achieved, as can be seen in Fig. 33d–i. Therefore, bismuthene is a promising candidate for all optical switching materials. Chai et al. reported a Yb-doped mode-locking fiber laser at 1 μm by utilizing Bi as a saturable absorber (SA) (Fig. 33j). The nonlinear optical absorption of Bi microfiber SA has been demonstrated by utilizing an ultrafast fiber laser. The dissipative solitons were generated with a center wavelength of 1034.4 nm and a frequency of 21.74 MHz. The generated pulse bandwidth is 30.25 ps pulse. The bismuthene SA property is demonstrated at the telecommunication band with a 2.03% optical modulation depth and a 30 MW cm−2 saturable intensity experimentally (Fig. 33k). A sub-200 fs soliton mode-locked erbium-doped fiber laser was realized by utilizing a bismuthene SA. The generated soliton pulses with a centre wavelength of 1561 nm (Fig. 33l) and a shortest pulse width of 193 fs (Fig. 35m).146,147,150,301,302
image file: c9nr04348a-f33.tif
Fig. 33 (a) Schematic of the experimental setup.150 (b and c) The generated chain and single pulse of the mode-locked Tm-doped fiber laser.150 These figures have been reproduced from ref. 150 with permission from the Optical Society of America. (d to i) The formation process of 532 and 633 nm lasers.301 These figures have been reproduced from ref. 301 with permission from the American Chemical Society. (j) Spectra of the dissipative solitons with a centre wavelength of 1034.4 nm.302 This figure has been reproduced from ref. 302 with permission from the Royal Society of Chemistry. (k–m) Spectrum of mode-locking and the output optical spectrum and its corresponding oscilloscope trace, respectively.146 These figures have been reproduced from ref. 146 with permission from Wiley.

image file: c9nr04348a-f34.tif
Fig. 34 Geometry and stability of Te allotropes. Crystal structures of bulk (a) and bilayer α-Te in the top- (b) and side-views ((c) and (d)). Crystal structures of bilayer β-Te in the top- (e) and side-views (f). Crystal structures of bilayer γ-Te in the top- (g) and side-views (h).

image file: c9nr04348a-f35.tif
Fig. 35 Electrical and optical properties of Te. (a and b) The Brillouin zones of bulk and few-layer α-Te.303 (c and d) Band structures of bulk and bilayer α-Te.303 These figures have been reproduced from ref. 303 with permission from Elsevier. (e to g) Band structure and partial density of states of bulk, monolayer and multilayer β-Te with PBE-GGA.304 These figures have been reproduced from ref. 304 with permission from IOP Publishing. (h and i) Absorptance of 2L- and 6L-α-Te.303 These figures have been reproduced from ref. 303 with permission from Elsevier. (j) Simulated absorption coefficients of bulk, few-layer and monolayer β-Te.304 This figure has been reproduced from ref. 304 with permission from IOP Publishing.

6. 2D group VI materials

Chalcogens are group VI materials, in particular selenium (Se) and tellurium (Te), which are p-type semiconductors with a narrow bandgap.151–153,156,157,303 Similar to BP, 2D group VI materials feature a honeycomb structure through vdW forces, a tunable bandgap, and high anisotropic properties. However, 2D group VI materials possess many physical and chemical properties superior to BP, like better environmental stability and higher carrier mobility, and could be synthesized through various facile methods (such as liquid exfoliation, MBE, PVD). 2D group VI materials have attracted tremendous attention due to their fascinating properties including excellent photoconductivity, anisotropic thermal conductivity, and high piezoelectric, thermoelectric and catalytic activities. In this part, we will highlight some representative investigations on 2D chalcogens.

6.1 Structure

Bulk Te possesses three phases, α (monoclinic), β (monoclinic) and γ.303 The crystal structure of the α-phase is presented in Fig. 34 (space group P3121 (no. 152)), it is composed of parallel Te helical chains, and each repeating unit includes three Te atoms. When the α-phase Te thickness decreased to the single-layer, the β-phase (Fig. 34e and f) emerged. Similar to BP and other 2D materials, β-Te has a hexagonal and layered structure held together by weak vdW force. The cohesive energy of monolayer β-Te is 2.567 eV per atom, and no soft phonon modes were found, which suggests that the monolayer β-Te is kinetically stable.304 γ-Te is a variation of α-Te in the xz plane, which is similar to the 1T phase of TMDCs. Stability is critical to semiconductor materials and the energy barrier ranges from 0.85 to 0.94 eV, which enables α-Te to remain stable in ambient environments.303

6.2 Properties

6.2.1 Electrical properties. Similar to BP, few-layer (FL) α-Te features a tunable bandgap.303 Bulk α-Te possesses a direct bandgap of 0.31 eV near the H point in the Brillouin zone (Fig. 35a and c). Fig. 35d shows the band structure of 2L α-Te and exhibits an indirect bandgap of 1.17 eV. FL-α-Te exhibits larger complicity and stronger anisotropy of electrons and holes, which indicate better thermoelectrics performance than Te nanowires. FL-α-Te is partially one-dimensional in nature, and the Te chains undergo covalent-like quasi-bonding (CLQB) rather than vdW interactions. Compared with the covalent bond, the CLQB interaction is weaker, and therefore, leads to a small in-plane deformation potential along the y direction. These excellent properties result in large hole mobilities along the y direction, with a high mobility of 104–106 cm2 V−1 s−1 in 5L and 6L, which is 1–3 orders of magnitude larger than the hole mobility of few-layer BP and TMDCs. Such a large mobility also indicates that 2D Te is a promising candidate in high speed and energy-efficient electronics. The mobilities along the z direction are generally smaller than those along the y direction, indicating that the electronic properties of FL-α-Te are highly anisotropic.

To determine the electronic properties of the bulk and few-layer β-Te, the band structures were computed and are shown in Fig. 35e–g. Bulk β-Te has an indirect bandgap value of 0.325 eV, multilayer β-Te are also semiconductors, as shown in Fig. 35g, the bandgap increase with decreasing layer numbers, and the monolayer has an indirect bandgap of 1.265 eV. Interestingly, the bandgap of β-Te can be controlled by external strain. The electron and hole masses also show a strong direction and layer-dependent evolution along Γ–X and Γ–Y, respectively.304

6.2.2 Optical properties. The absorptance is around 2%–3% and 6%–9% per layer at 1.6 and 3.2 eV (Fig. 35h and i), respectively, and these values are almost two to three fold higher than that of BP. This excellent absorptance indicates promising potential of FL-α-Te for optical applications in the visible and infrared light regions.132 The few-layer optical absorption intensity of β-Te increased with increasing layer numbers (Fig. 35j). The maximum absorption intensity of bulk β-Te is obtained in the ultraviolet region with a wavelength of 140 nm. For few-layer β-Te, the absorptance is higher than 5 × 104 cm−1 at 175 nm, and gradually extends to the visible band. The peak absorptance was observed in the UV-visible band. These outcomes suggest that the few-layer β-Te is promising for UV-visible and acousto-optic devices.304
6.2.3 Environmental stability. Although BP and TMDCs possess many advantageous properties, such as thickness dependent bandgaps, strong and wide optical absorptance, and high carrier mobilities, the terrible environmental stability of these materials has severely restricted their further application in electronic and photoelectronic fields. In contrast, 2D Te exhibited an excellent environmental stability and was proved by experiment.306 As mentioned before, this superior environmental stability is mainly due to an energy barrier, which can prevent the oxidation process effectively. This excellent environmental stability provides a wide range of potential applications of 2D Te in fundamental and practical application fields.

6.3 Synthesis

Different from BP which lacks facile methods to synthesize high quality 2D nanofilms directly on the substrate, 2D group VI materials can be synthesized through a variety of methods. In this part we summarized four commonly used synthesis methods of 2D group VI materials, in particular, selenium (Se) and tellurium (Te), including physical vapor deposition (PVD), molecular beam epitaxy (MBE), hydrothermal synthesis and liquid exfoliation.
6.3.1 Physical vapor deposition (PVD). 2D Se nanosheets can be synthesized by the PVD method;305 Se powder was used as the precursor and placed in a multizone furnace, and after the heating and cooling process, a black-colored needlelike Se nanosheet was obtained and coated on the Si substrate (Fig. 36a–d). Under thermodynamic equilibrium conditions, Se tends to grow into a 1D structure along the [0001] direction. But the products in these experiments exhibit a 2D structure which is different from the previous product. The nanorods with high chemical activity play a key role in the 2D nanosheet growth. During the synthesis process, the growth along the c axis is partly hindered, and therefore the nanorods grow along the (1210) direction to form 2D thin nanosheets. Raman spectroscopy of the Se nanosheet displays in-plane anisotropy, as shown in Fig. 36e, and a clear peak intensity change could be observed as the sample is rotated in steps of 30° from −90° to 90°. 2D Te nanoplates were successfully obtained on a mica substrate by using the PVD method.152 A chemically inert mica surface is crucial for the lateral growth of 2D hexagonal tellurium nanoplates due to two reasons: (1) it facilitates the migration of tellurium adatoms along the mica surface and (2) it allows a large lattice mismatch. The typical shape of 2D Te is a regular hexagonal structure with a lateral size of about 6–10 μm and a thickness of 32 nm (Fig. 36f and g). Due to its highly anisotropic crystal structure, Te has a strong tendency to become a 1D nanostructure. The mica substrate plays a crucial role in 2D Te growth. Due to the absence of surface dangling bonds, mica supplies ideal vdWE substrates, which profoundly enhances the large migration rate of Te adatoms along the mica surface, thus greatly improving the lateral growth rate of 2D Te hexagonal nanoplates.
image file: c9nr04348a-f36.tif
Fig. 36 (a to d) 2D Se nanosheets synthesized by the PVD method.305 (e) The Raman peak intensity as a function of the sample rotated angle.305 These figures have been reproduced from ref. 305 with permission from the American Chemical Society. (f and g) 2D Te nanosheets synthesized by the PVD method.152 These figures have been reproduced from ref. 152 with permission from Wiley.
6.3.2 Molecular beam epitaxy (MBE). Two-dimensional Te films with monolayer and few-layer thickness were achieved by molecular beam epitaxy on a graphene/6H-SiC (0001) substrate. Graphene was prepared on a 6H-SiC substrate by thermal treatment to achieve the (6 × 6) reconstruction. High-purity Te (99.999%) was heated while the graphene/SiC substrate was kept at room temperature. After growth, large-scale, atomically flat Te films are obtained, which can cover two adjacent terraces of the substrate continuously across the step (Fig. 37a and b). The lowest step height between the Te film and graphene is ∼1.5 Å (Fig. 37b and c), indicating that the Te film is a monolayer. The STM fast Fourier transform image (Fig. 37d and e) proves the vdW epitaxy of the Te film on graphene. The bandgap of Te increases monotonically with decreasing thickness, up to 0.92 eV for the monolayer Te.306
image file: c9nr04348a-f37.tif
Fig. 37 (a to c) 2D Te flakes synthesized by the MBE method.306 (d and e) STM fast Fourier transform image of Te flakes.306 These figures have been reproduced from ref. 306 with permission from the American Chemical Society.
6.3.3 Hydrothermal synthesis. Te nanoflakes can also be obtained by the hydrothermal synthesis method. In this method, PVP and Na2TeO3 were dissolved firstly, and after that, ammonium hydroxide solution and hydrazine monohydrate were dissolved in the solution as mentioned before. After heating, washing and purification processes, the product was obtained finally.307 Te nanoflakes exhibit an irregular shape with a length of tens of micrometers, width of a few micrometers, and thickness of 10–30 nm (Fig. 38a). A typical Te nanoflake with a thickness of 16.1 nm is shown in Fig. 38b. A Te nanoflake nanogenerator (TFNG) device was fabricated by using the Au layer on the textile as a bottom electrode, and a 50 nm thick Au layer on the surface of the PDMS as the top electrode (Fig. 38c and d).
image file: c9nr04348a-f38.tif
Fig. 38 (a and b) Te flakes synthesized by the hydrothermal synthesis method.261,307 These figures have been reproduced from ref. 261 and 307 with permission from the Royal Society of Chemistry and the American Chemical Society, respectively. (c and d) Ultra-thin Te flakes synthesized by the hydrothermal method under low-temperature conditions.308 These figures have been reproduced from ref. 308 with permission from Elsevier.
6.3.4 Liquid exfoliation. Due to weak vdW forces in interchains, Te and Se nanosheets could be synthesized through the liquid exfoliation method. The Te powders were firstly mixed with IPA solvent, and after that, by using a sonication technique in an Ar environment, the Te nanosheet was obtained finally, as shown in Fig. 39. As shown in Fig. 39a, the 2D Te nanosheets have lateral dimensions in the range from 41.5 to 177.5 nm. The crystal lattice spacing of ≈3.2 Å in Fig. 39b is assigned to the (101) plane of Te. The ultrathin 2D Te nanosheets exhibit a thickness varying from 3.4 ± 0.3 to 6.4 ± 0.2 nm. The Raman spectra of 2D Te remain almost unchanged after two weeks, which proves the stability of 2D Te under ambient conditions (Fig. 39d).156 As mentioned before, the liquid exfoliation method could be utilized to synthesize Se nanosheets.157 Similar to the Te nanosheet synthesis method, the Se powders were firstly mixed with IPA solvent, and then by using the ultrasonication technique, the Se nanosheet was obtained eventually. The lateral size of the nanosheets of 2D Se falls into 40–124 nm with an average thickness of 3–6 nm with the centrifugation speed of 4000 rpm, as can be seen in Fig. 39e and f. The TEM images in Fig. 39g and h show the lattice fringe values of Se to be ≈0.38, 0.22, and 0.29 nm, corresponding to the (100), (110), and (101) planes, respectively.
image file: c9nr04348a-f39.tif
Fig. 39 Characterization of ultrathin 2D Te and Se nanosheets synthesised by the liquid exfoliation method: (a) TEM image of 2D Te.156 (b) HRTEM image of the crystalline lattices of Te. Inset: The corresponding SED pattern (right-top) and FTT photograph (right-bottom).156 (c) AFM image of 2D Te thickness. (d) Raman spectra of the obtained Te flakes.156 These figures have been reproduced from ref. 156 with permission from Wiley. (e) Morphology of the as-prepared 2D Se nanosheets obtained at a centrifugation speed of 4000 rpm.157 (f) AFM of the 2D non-layered Se nanosheets.157 (g) High-resolution TEM and (h) HR-TEM images of the lattice fringes of the 2D Se nanosheets.157 These figures have been reproduced from ref. 157 with permission from Wiley.

6.4 Applications

Due to the helical chain structure, high environmental stability and carrier mobility of 2D Te and Se, it has been considered as a promising material in high performance electronic and photoelectronic devices. In this section, we highlight some recent progress in 2D Te and Se.
6.4.1 Field-effect transistors. Solution synthesized Te nanoflake based FETs were fabricated. The IdVd measurement of a quasi-2D Te FET is shown in Fig. 40a. The thickness-dependent transport properties of 2D Te nanoflakes were also investigated, and as shown in Fig. 40b, hole mobilities of 1430 and 450 cm2 V−1 s−1 were achieved at 77 and 300 K, respectively.307 Wu et al. also investigated the solution synthesized Te based FETs.309 The device exhibits p-type characteristics with slight ambipolar transport behaviour due to its narrow bandgap, with a large drain current over 300 mA mm−1 and a high on/off ratio on the order of ∼1 × 105 (Fig. 40c). The p-type behaviour originates from the high level of the Te valence band edge. The field-effect mobilities of 2D Te transistors peak at ∼700 cm2 V−1 s−1 at room temperature with a 16 nm thick Te flake and decrease gradually with a further increase in thickness (Fig. 40d). Se nanosheet FETs exhibit a p-type behavior with Ion/Ioff exceeding 106 (Fig. 40e and f). The hole mobility was measured to be 0.26 cm2 V−1 s−1 at 300 K, which is much lower than that of other p-type 2D materials. However, the special band structure and low mobility make Se possess the highest Seebeck coefficient (+1250 μV K−1) than all other elements and is promising for thermoelectric applications. The Se nanosheet FETs exhibit a good stability even after 15 days’ exposure to ambient conditions.305
image file: c9nr04348a-f40.tif
Fig. 40 (a) Measurement of back-gated IdVg curves.307 (b) Thickness dependence of the on/off ratio and mobility of Te nanoflakes of different thicknesses ranging from 12 to 36 nm.307 These figures have been reproduced from ref. 307 with permission from the American Chemical Society. (c) Transfer curve of a typical 7.5 nm thick long-channel 2D tellurene transistor.305 (d) Thickness dependent on/off ratio and field-effect mobility for 2D tellurene transistors.305 These figures have been reproduced from ref. 305 with permission from the American Chemical Society. (e) Transfer curves of a typical 16 nm thick Se nanosheet FET.305 (f) Output characteristic of the same Se FET.305 These figures have been reproduced from ref. 305 with permission from the American Chemical Society.
6.4.2 Photodetector. A 2D Te nanoplate based photodetector is shown in Fig. 41a and b.139Fig. 41c shows the IV curves of a Te based device with and without illumination, and an obvious photoconducting response can be observed. With and without illumination, the current showed the same level for both noise and photocurrent, indicating repeatability and high stability properties of the photodetector (Fig. 41d). The linear relationship (Fig. 41e) between ΔI and laser intensity indicates that the Te based device was a typical photon-dependent resistor. The photoresponse time of the device is shown in Fig. 41f, and the response and recovery times are 4.4 and 2.8 s, respectively. The slow response suggests that Te nanoplates may be imperfect and have numerous trap states. The photoresponse of the solution synthesized Te nanoflake was investigated,307 and the highest responsivity of 27 A W−1 was achieved at 78 K. The device responsivity of the whole spectra is shown in Fig. 42a. For Vd = 5 V, a highest responsivity of 27 and 16 A W−1 at λ = 1.7 μm and at 78 and 297 K was recorded, respectively. The wavelength dependence of detectivity is shown in Fig. 42b at 78 and 297 K. These devices exhibit the highest D* of 2.9 × 109 cm Hz1/2 W−1 at 297 K. This value is further increased as the temperature decreased to 78 K. Due to the anisotropic structure of the Te nanosheets, the devices exhibit a polarization-sensitive photoresponse, as shown in Fig. 42c and d.261 The liquid exfoliated 2D Te nanosheets exhibited an excellent photoresponse under illumination (Fig. 42e). The responsivity was in the range of 11–13, 10–11 and 8–16 μA W−1 for different wavelength laser beams (Fig. 42f). These interesting findings indicate that the Te nanosheet is a promising candidate for further photoelectric applications.156
image file: c9nr04348a-f41.tif
Fig. 41 (a and b) Image of the device based on 2D Te flakes. (c) IV curves of a single Te flake photodetector with and without illumination.139 (d) Measurement of the response and recovery time of the Te based device.139 (e) The photocurrent measurement of the Te based device.139 (f) The responsivity of the Te based device of different wavelength incident lasers.139 These figures have been reproduced from ref. 139 with permission from the American Physical Society.

image file: c9nr04348a-f42.tif
Fig. 42 (a) Measurement of responsivity of a Te based photoconductor.261 (b) The wavelength dependence of detectivity.261 (c) Polarization-resolved photoresponse of a quasi-2D Te flake. (d) The polarization dependence of the device response.261 These figures have been reproduced from ref. 261 with permission from the Royal Society of Chemistry. (e) Measurement of Iph∼t with various wavelength incident lasers.156 (f) Rph curves as a function of Pλ.156 These figures have been reproduced from ref. 156 with permission from Wiley. (g) The photocurrent measurement of the Se based device at different powers.305 (h) Photocurrent of the device with and without illumination.305 These figures have been reproduced from ref. 305 with permission from the American Chemical Society.

Se is well known as a candidate for high-sensitivity photodetectors. The Se nanosheet phototransistor presents an excellent ability of gate tunable photocurrent response (Fig. 42g). The Se based device displays a pronounced photoresponse even at an illumination power as low as 0.21 mW cm−2 and the photoresponsivity was measured to be 263 A W−1, which is four orders of magnitude larger than that of Se flakes grown by the PVD method. The photoresponse speed is characterized by a typical rise and decay time of 0.10 and 0.12 s (Fig. 42h), respectively. The Se nanosheet phototransistor exhibits stable and repeatable performance, and in addition, the device still presents a stable and repeatable photoresponse after four illumination cycles.305

6.4.3 Mode-locking laser. As mentioned before, the 2D Se flake was considered to be an excellent SA, and Zhang et al. presented the mode-locking laser by utilizing Se-SA. In this investigation, Se-SA was located in a fiber ring cavity (Fig. 43a).157 The simulated frequency was confirmed by a 14.6 m cavity. As presented in Fig. 43b, the pulse width was 3.1 ps, assuming a sech2 profile. Therefore, the time-bandwidth product was ≈2.032, implying that the amplified mode-locked pulses have a large chirp. The radio frequency (RF) spectrum of the mode-locked pulses indicates a high signal-to-noise ratio of 65 dB and a fundamental peak located at the cavity repetition rate of 13.68 MHz, which proved that the fiber laser operated in the stable soliton regime. These outcomes further confirmed that the Se-SA based fiber laser was operated in the stable soliton regime. Moreover, the repeatability measurement showed that the Se-SA based fiber laser possesses an excellent long time stability and repeatability.142
image file: c9nr04348a-f43.tif
Fig. 43 (a) Schematic of the experimental setup.50 (b) The corresponding autocorrelation trace.50 These figures have been reproduced from ref. 50 with permission from the American Chemical Society. (c) Measurement of the TFNG device outputs.308 (d) Photographs of the device attached to a human arm.308 These figures have been reproduced from ref. 308 with permission from Elsevier.
6.4.4 Piezoelectric device. The outputs of the Te nanoflake nanogenerator (TFNG) device were measured through periodic bending and unbending motions. During periodic bending tests, the TFNG device achieved a 3 V open-circuit voltage and a 290 nA closed-circuit current (Fig. 43c). A TFNG device attached to a human arm is shown in Fig. 43d. The generated output current and voltage of periodic bending and unbending were 650 nA and 2.5 V, respectively. Additionally, the output power density was measured to be 2.07 mW cm−2, which was enough to light up 10 LEDs at least.308

7. Summary and prospective

This review summarized recent progress in BP and BP-analogue materials, including the crystal structure, chemical and physical properties, synthesis methods, and various applications. BP is a layered semiconductor material with in-plane anisotropic physical properties. The unique direct tunable energy band (0.3–1.5 eV) of BP provides a bridge between gapless graphene and TMDCs. Though its carrier mobility is lower than graphene, BP exhibits electrical transportation properties much superior to TMDCs. Therefore, BP is considered as a potential material in transistors, photodetectors, logical devices, batteries, photovoltaic cells and gas sensors. Although there have been tremendous opportunities, BP still faces challenges. The first challenge of BP is the degradation under ambient conditions, which hinders more extensive applications. Another challenge is the lack of reliable and reproducible methods to produce high-quality BP thin films with wafer-scale size. Although some efforts have been made, such as PLD and converting RP films, the quality of BP is still far from application. Although bottom-up synthesis techniques like CVD have been successful in producing other 2D materials like graphene and TMDCs, they are not suitable for growing high-quality BP thin films. Liquid exfoliation is a highly efficient and low-cost method to produce few-layer BP films, but both the quality and purities are not good enough, and the size is uncontrollable. Therefore, a facile method to synthesize high quality BP films is very necessary to realize the commercial and industrial applications of BP. The successful synthesis of wafer-scale few-layer graphene and TMDCs by CVD may provide a possible solution to synthesize layer-controllable BP films on a large scale and with high efficiency in the future.

The 2D layered binary compound MPx has a layered hexagonal crystal structure which is similar to BP. These 2D MPx are considered to have unique electrical and optical properties. Through theoretical calculations, MPx are found to possess high carrier mobility, large capacity, excellent stability, direct tunable band, strong anisotropic physical properties, conductance and photoresponsivity, and high absorbance, owing to which they can be considered as potential candidates for promising anode materials, for alkali metal-based batteries (Li-ion batteries and K-ion batteries), and are also suitable as nanoscale photocatalysts for photo-electrochemical water splitting or photocatalyzing CO2 splitting to CO under acidic conditions. Moreover, the environmental stability of 2D MPx displays a probable perspective superior to BP. However, there are only three materials that have been experimentally synthesized: SiP, GeP and AsxP1−x, respectively. Therefore, there is plenty of room to explore the 2D MPx materials as well as their related applications.

Layered transition metal phosphorus trichalcogenides MPX3 are honeycomb and layered structures with weak van der Waals interactions between the layers, which is similar to BP. These materials have attracted tremendous attention due to their intrinsic magnetic and ferroelectric properties which are absent in other 2D materials. Furthermore, thanks to the layered structures, MPX3 exhibit excellent properties in lithium batteries and hydrogen storage.

P, As, Sb and Bi are elements of group V or pnictogens. As, Sb and Bi have a similar honeycomb and layered crystal structure to BP. The anisotropic mechanical, optical, thermal and electrical properties result from the differences between the in- and out-of-plane interatomic distances of pnictogens. Similar to BP, As, Sb and Bi exhibit a tunable bandgap as well as high carrier mobility, but a much better environmental stability than BP. Compared with other BP-analogue and two-dimensional materials, As, Sb and Bi display unique topological properties, and are considered to be promising candidates for topological insulators and novel quantum nanoelectronics.

2D group VI materials, especially Se and tellurium Te, are p-type semiconductors with a narrow bandgap, which are of wide interest for their excellent physical and chemical properties, such as high photoresponsivity, anisotropic thermal conductivity, and high piezoelectric and nonlinear optical responses. Compared with BP and other 2D materials, 2D chalcogens are environmentally stable. Moreover, Te nanosheets can be reliable and highly efficiently synthesized by various methods like PVD and solution-synthesis, which is a great advantage for industrialization.

In conclusion, BP is a very fascinating 2D material due to its excellent chemical and physical properties and exhibits great potential in many applications, such as transistors, photodetectors, logical devices, batteries, photovoltaic cells and sensors. But BP also faces some severe challenges, for example, vulnerable degradation under ambient conditions and lack of reliable and reproducible synthesis methods. Other 2D BP-analogue materials such as 2D layered binary compound MPx, transition metal phosphorus trichalcogenides, 2D pnictogens and 2D group VI materials share similar crystal structures and chemical and physical properties to BP; however, they exhibit better environmental stability than BP. Additionally, 2D BP-analogue materials can be synthesized through reliable and highly efficient methods, and they possess some unique and superior properties to BP, which may be a substitutional choice for BP in the future as shown in Fig. 44 and Table 3.


image file: c9nr04348a-f44.tif
Fig. 44 BP and BP-analogue materials: their structure and main applications. Copyright 2014, Nature Publishing Group. Copyright 2016, American Chemical Society. Copyright 2018, Royal Society of Chemistry. Copyright 2017, American Chemical Society.
Table 3 Summary of the structure, properties and applications of BP and BP analogue materials
  Crystal structure Energy band structure Mechanical properties Electrical conductance Photoresponse
BP Honeycomb, puckered-layered Direct tunable, 0.3–1.5 eV Anisotropic, flexibility, Young's modulus: 100 GPa Anisotropic, high hole mobility: 5000 cm2 V−1 s−1 Visible-near infrared, responsivity 82 A W−1 (1.55 μm)
MPx Honeycomb buckled (SiPx, GeP, SnPx) or puckered (AsPx, SbPx) layered Direct tunable (AsPx 0.15–0.3 eV) or indirect (SiPx, GeP, SnPx) Anisotropic Anisotropic high carrier mobility, SiP: 2 × 103 cm2 V−1 s−1 Visible-mid infrared, responsivity 30 mA W−1 (8 μm)
MPX3 Layered honeycomb Direct tunable 1.9–3.4 eV Anisotropic MnPSe3: 626 cm2 V−1 S−1 No report
Pnictogens Honeycomb puckered or buckled layered Indirect tunable Anisotropic, stiffness: 58 N m−1 (As), 25 N m−1 (Bi) Anisotropic (μZZh = 60 cm2 V−1 s−1, μACh = 1 × 105 cm2 V−1 s−1) No report
2D VI Hexagonal layered Indirect: 1.2 eV Anisotropic, flexibility High carrier mobility, 700 cm2 V−1 s−1 UV-visible, responsivity 16 A W−1 (1.7 μm)
Graphene Hexagonal flat-layered Zero bandgap Isotropic flexibility, Young's modulus: 1 TPa Isotropic high mobility 20[thin space (1/6-em)]000 cm2 V−1 s−1 Visible-mid infrared, responsivity 6 mA W−1 (1.55 μm)
TMDCs Hexagonal flat-layered Direct (monolayer 1.9 eV) to indirect (multilayer 1.2 eV) Isotropic Isotropic low mobility < 1000 cm2 V−1 s−1 Visible, responsivity 110 mA W−1 (633 nm)


Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (51802199, 61875223), the Postdoctoral Science Foundation of China (2017M612728), and the Natural Science Foundation of Guangdong Province (2018A030310545).

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