Two-dimensional group-VA nanomaterials beyond black phosphorus: synthetic methods, properties, functional nanostructures and applications

Rijun Gui† *, Hui Jin† , Yujiao Sun , Xiaowen Jiang and Zejun Sun
College of Chemistry and Chemical Engineering, Intellectual Property Research Institute, Qingdao University, Shandong 266071, P. R. China. E-mail: guirijun@163.com; guirijun@qdu.edu.cn; Fax: +86 532 85953320; Tel: +86 532 85953981

First published on 23rd October 2019

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Rijun Gui

Rijun Gui received his BS degree in 2005 and completed his MS degree in Applied Chemistry from Shanghai Ocean University in 2009. He then studied at the East China University of Science and Technology to pursue his PhD degree (2012) in Physical Chemistry. Afterwards, he worked as a Postdoctoral Fellow at Shanghai Jiao Tong University. In 2014, he joined Qingdao University, where he is currently an Associate Professor. His research interests focus on fluorescent and electronic active nanomaterials.

Hui Jin

Hui Jin received her BS degree in 2005 and completed her MS degree in Applied Chemistry from Shanghai Ocean University in 2008. Then, she studied at Tongji University to purse her PhD degree in Organic Chemistry. After completing her PhD degree in 2012, she worked as a research assistant at Shanghai Jiao Tong University. In 2015–2016, she joined Qingdao University as a Postdoctoral Fellow. Currently, she is an Associate Professor in Qingdao University. Her research interests focus on the synthesis and applications of nanomaterials with luminescence properties and electronic activities.

Yujiao Sun

Yujiao Sun was born in 1995. Since 2017, she has been a Master's candidate in the College of Chemistry and Chemical Engineering, Qingdao University. Her research interests focus on the preparation and applications of fluorescent and electronic active nanomaterials.

Xiaowen Jiang

Xiaowen Jiang was born in 1994. Since 2017, she has been a Master's candidate in the College of Chemistry and Chemical Engineering, Qingdao University. Her research interests focus on the preparation and applications of fluorescent and electronic active nanomaterials.

Zejun Sun

Zejun Sun was born in 1996. Since 2018, she has been a Master's candidate in the College of Chemistry and Chemical Engineering, Qingdao University. Her research interests focus on the preparation and applications of fluorescent and electronic active nanomaterials.

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1. Introduction

In comparison with their bulk counterparts, two-dimensional (2D) layered nanomaterials have unique physiochemical and structural properties because of their high aspect ratios, quantum-size effects and unusual surface chemistry.^1–3 Since the discovery of graphene,^4–7 2D layered nanomaterials have received much attention. Especially, the vast library of 2D layered nanomaterials has attracted tremendous interest. The past few decades have witnessed an explosive growth in the scientific research on various 2D layered nanomaterials, including transition-metal dichalcogenides (TMDs),⁸ graphitic carbon nitride,^9–11 hexagonal boron nitride,¹² layered double hydroxides,¹³ silicone,¹⁴ germanene,¹⁵ 2D metals,¹⁶ 2D metal oxides and sulphides,^17,18 transition metal carbides, nitrides and carbonitrides (MXenes),^19–22 2D polymers,²³ 2D metal–organic frameworks (MOFs),^24–26 2D covalent-organic frameworks,^27,28 2D perovskites,^29,30 and others.^31,32 A principal reason for the exploration of 2D nanomaterials is that monolayer (ML) materials have increased bandgaps and tunable electronic, optical, catalytic and electrochemical properties. 2D nanomaterials have great prospects for modern nanoscience and nanotechnology, showing wide applications in the optical, electronic, optoelectronic and biomedical fields. The majority of 2D layered nanomaterials are rediscovered and contemporary studies are an extension of the early research completed a few decades ago.^33–36 Currently, the 2D layered nanomaterial field is among the most active research fields in materials science and nanoscience. Except for graphene and its counterparts, abundant binary, ternary or complex materials with 2D layered structures are exfoliated to ML or few layers. However, the number of anisotropic layered materials is very limited.

In recent years, scientific research interests have turned to 2D monoelemental structures, including layered black phosphorus (BP) and phosphorene and its cousins (arsenene, antimonene, and bismuthene). As a new member of the 2D layered nanomaterial family, group-VA (P, As, Sb, and Bi) nanomaterials with 2D layered monoelemental structures have emerged with increasing interest, strong momentum in their development and great potential applications.^37–40 Unlike semimetallic group-IVA (graphene, silicene, germanene, and stanine) and metallic group-IIIA borophene materials, 2D layered group-VA monoelemental nanomaterials are semiconductors with marked and fundamental band gaps, which endow them with a great potential as promising candidates for future nanodevices. The transition from metallic conductors to semiconductors can be regulated by reducing the layer number of materials, accompanied by optical, electronic and electrocatalytic properties different from those of their bulk counterparts. This is an important aspect of the materials, which deserves further investigations. As an emerging star of post-graphene 2D layered nanomaterials, phosphorene and 2D layered BP materials have been largely studied in recent years.^41–64 Their tunable direct bandgap, charge-carrier mobility and unique in-plane anisotropic structures render them significant in a broad range of research fields.

Since graphene was isolated by Novoselov et al.,⁴ graphene studies have achieved extraordinary scientific success,⁵ opening the door to new groups of 2D materials with complementary physical properties to graphene.^65,66 Graphene is a semimetal without a band gap, which restrains its practical use in the electronic and optoelectronic fields. In the research of novel 2D materials, generally TMDs are dominant since most of them have a band gap in the range of 1.5–2.5 eV,⁶⁷ which can be tuned by adjusting the layer number, stress level and chemical functionalization. However, this band gap range is not appropriate for optoelectronic devices, where a lower range of 0.1–1 eV is usually demanded.⁸ In contrast, the direct band gap of ML phosphorene is ∼1.5 eV,^8,68 and thus it is interesting for applications in electronics and ultrafast optoelectronics. Phosphorene (2D allotrope of BP) suffers from strong reactivity under environment conditions. The exfoliated flakes of phosphorene are very oxophilic and can form a hydrophilic BP surface. This process facilitates moisture uptake from air to form phosphoric acid and related species, leading to the degradation of BP flakes.⁶⁹ Hence, the discovery and production of novel 2D materials with proper bandgaps and stability under atmospheric conditions are challenging. Recently, the field of monoelemental nanomaterials related to 2D layered As, Sb and Bi (few-layer or ML arsenene, antimonene, and bismuthene) has become one of the most developing and popular research fields.^70–112 Building on early studies,^113–117 currently, there are considerable studies on 2D layered As, Sb, and Bi nanomaterials in the literature, which explore their various synthetic methods, fundamental properties and functional nanostructures for promising applications.

The crystallization of heavy pnictogens (As, Sb, and Bi) results in a rhombohedral (β-form) layered structure,¹¹⁸ which is the most stable allotropic of pnictogen elements. Anisotropy is visible on the cleaved crystals. As and Sb have the highest anisotropies of physical properties.³⁹ Rhombohedral structures of As and other pnictogens are related to the structure of BP consisting of puckered, six-membered rings of atoms, but the individual layers are held together by stronger interactions. Different from BP, 2D (As, Sb and Bi) materials do not generate true van der Waals-bonded layered structures. The difference between BP and products with rhombohedral modifications (gray As, Sb and metallic Bi) comes from the interactions of the atomic orbitals between individual double layers. The difference between the in-plane and out-of-plane interatomic distance is substantially high, resulting in layered structures with anisotropy.³⁹ The interlayer interactions cause semimetal behavior. The (As, Sb, and Bi) elements with metallic sheen serve as superior conductors. With the most thermodynamically stable rhombohedral structures, pnictogen materials are brittle and easily pulverized. This unique property is suitable for the top-down preparation of few-layer and ML materials through mechanical exfoliation.

Before proceeding with this review, here we investigate the recent reviews relative to 2D group-VA nanomaterials. Pumera et al. reported a short review on the structures and properties of 2D monoelemental arsenene, antimonene and bismuthene.³⁹ Ares et al. reported the recent progress on antimonene as a new bidimensional material, focusing on theoretical work and experimental realizations.⁴⁰ Gablech et al. discussed the development of field-effect transistor-based sensors using 2D arsenene and antimonene.¹¹⁹ Wang et al. reported a mini-review to summarize the experimental preparation and practical applications of antimonene.¹²⁰ Zhang et al. explored the theoretical and experimental progress in 2D group-VA semiconductors.³⁷ Ye et al. reviewed bismuth-based photocatalysts for solar photocatalytic carbon dioxide conversion.¹²¹ Xu et al. summarized 2D bismuth-based layered materials for energy-related applications.¹²² These reviews partially mentioned 2D group-VA materials, but they did not systematically summarize 2D layered As, Sb, and Bi (few-layer or ML arsenene, antimonene, and bismuthene) nanomaterials and pnictogens-containing 2D materials. With respect to 2D-layer structural As, Sb and Bi materials, there are a rapidly increasing number of publications involving in the emerging and popular research field of 2D layered group-VA nanomaterials in recent years (Scheme 1), which provide relevant researchers strong motivation and a high necessity for this timely and comprehensive review.

Herein, this present review comprehensively introduces the state-of-the-art current research in 2D layered group-VA monoelemental nanomaterials beyond BP. This review covers 2D layered (As, Sb, and Bi) nanomaterials, few-layer, their ML counterparts (arsenene, antimonene, and bismuthene) and functionalized nanostructures (hybridization, doping, and functionalization). This review is divided into four sections, mainly including synthetic methods, properties, functional nanostructures and applications. We systematically summarize each section based on both theoretical predictions and experimental studies. We summarize the different synthetic methods for these 2D materials, including mechanical, sonication, electrochemical exfoliation, hydrothermal, solvothermal synthesis, and epitaxial growth. Then, we highlight their unique band structures, carrier transport, mechanical, optical, electronic, thermal and magnetic properties. Moreover, various functional nanostructures are discussed, including different heterostructures, doping, adsorption, hybrid and functionalized nanostructures based on these 2D layered materials. Finally, the broad range of promising applications of these 2D materials is elaborated, including various significant fields such as catalysis, energy storage, field-effect transistors, topological spintronic devices, electronic devices, nonlinear photonics, light-emitting devices, gas sensors, thermoelectric materials and biomedicine. The current research advances, potential challenges and future perspectives are discussed rationally (Scheme 2). This timely and overall review presents new and exciting latest advances on 2D layered group-VA monoelemental nanomaterials, which appeal to international research communities from a wide range of scientific disciplines, mainly including materials science, chemistry, physics, engineering, biology, and medicine. This review is beneficial for the further development of layered materials, mono-elemental materials, hybrid materials and functionalized low-dimensional materials.

2. Synthetic methods

2.1. Top-down methods

2.2. Bottom-up methods

3. Properties

3.1. Band structures

Bulk arsenic (As) has three common allotropes including metallic gray, yellow and black As. Gray As is the most common and stable phase with layered rhombohedral (β-form) structures (Fig. 12a).¹¹⁸ Layered gray As crystals exist naturally. Gray As is a semimetal material with bands. When heated to 370 K, a layered orthorhombic α-phase of As arises with a structure similar to that of BP. Orthorhombic (α-phase) As is a narrow-gap semiconductor with a band gap of 0.3 eV. Bulk antimony (Sb) possesses three known allotropes under normal conditions, including gray, black and explosive Sb. Bulk Sb has the same rhombohedral structure as gray As. Among the allotropes, gray Sb is the most stable. Gray Sb and As have typical semimetal characteristics in layered bulk forms. Gray Sb is a layered material. Black Sb is produced upon rapid cooling of Sb vapor, showing an identical structure to red phosphorus. Black Sb has high chemical activity in atmospheric environment. Under vacuum, black Sb can transform easily into stable crystalline gray Sb at 373 K. Explosive Sb often transforms fiercely into gray Sb under mechanical stress or heating. Explosive Sb is probably not an allotrope, but is a mixed form. Bulk bismuth (Bi) has only one stable format. Bulk Bi has a natural layered structure, which is a rhombohedral A7-type structure similar to gray As and Sb. In addition, layered Bi has a feature of metallicity.

Fig. 12 illustrates the stable phases and natural layered crystals. Two stable phases were isolated for P and As, namely the α and β phases. There is only one stable phase for Sb and Bi, namely the β phase. The natural layered crystals of As, Sb and Bi possess the β phase. Derived from bulk phosphorus, As, Sb and Bi, few-layer or ML 2D group-VA nanomaterials are termed phosphorene, arsenene, antimonene and bismuthene. In the case of layered As, Sb, and Bi crystals, arsenene has puckered and buckled ML structures, while antimonene and bismuthene are likely to exhibit buckled forms. The ML structure of 2D group-VA materials is the most stable. Zhang et al. predicted group-VA monolayers with five typical honeycombs and four non-honeycomb structures (Fig. 12a).¹¹⁸ The average binding energies for all possible group-VA ML configurations are shown in Fig. 12b. Based on the calculations, α-phosphorene with a puckered form is the most stable. In the case of arsenene, antimonene and bismuthene ML allotropes, all their β phases with buckled forms have the lowest energies. The three phases of bismuthene (α, β, and ζ) have very close average binding energies. The counterpart bulk material of α-phosphorene ML is BP, which is the most stable form for allotropic bulk crystals under standard conditions. Their counterpart bulk materials (β-arsenene, β-antimonene and β-bismuthene) are β phases (rhombohedral layered As, gray Sb and Bi). Only α and β layered phases were experimentally proved in group-VA bulk crystals. The phonon spectra of free-standing group-VA monolayers with puckered and buckled forms were studied by FPC, acting as a criterion to judge structure stability (Fig. 12c). No obvious imaginary phonon mode was found, demonstrating the kinetic stability of the free-standing group-VA monolayers. Currently, α-/β-phosphorene, β-arsenene, β-antimonene and α-/β-bismuthene have been synthesized and characterized experimentally.

FPC was used to study the band structures and electronic properties of 2D group-VA materials.¹⁸³ Especially, the electronic bandgap is an important feature of emerging 2D materials, which guides electronic and optoelectronic applications. Band gaps at different levels of group-VA monolayers for α and β phases can be calculated by theoretical predictions, such as the Perdew–Burke–Ernzerhof (PBE), Heyd–Scuseria–Ernzerhof (HSE) hybrid function, Green's function and screened Coulomb interaction (GW) methods. HSE and GW are reliable in the determination of bandgaps. PBE offers a correct physical picture, consistent with the results from the hybrid density functional theory (DFT) and GW methods. BP belongs to a typical direct-band-gap semiconductor. When BP is thinned to few-layer and ML, it retains a direct band gap.¹⁸⁴ The band gap of phosphorene sensitively depends on the layer number (Fig. 13a and b). The fundamental band gaps of semiconductors are dominated by electron–electron interactions. The optical gap of semiconductors can be predicted using the GW method with the Bethe–Salpeter equation (BSE) based on considerable excitonic effects. Group-VA elemental atoms are heavier than the carbon atom. Spin-orbital coupling (SOC) calculations of phosphorene, arsenene, antimonene and bismuthene can be used to evaluate their potential effects on electronic band structures. Zhang et al. predicted an abrupt transition from semimetallic (or metallic) As and Sb bulk crystals to wide-band-gap semiconductor arsenene and antimonene MLs.¹⁵⁴ Arsenene and antimonene suffer from an indirect-to-direct band-gap transition under tensile strain. As predicted, the β phase with a buckled form is the most stable structure among the arsenene, antimonene and bismuthene allotropes.¹¹⁸ Based on FPC, the β phases of arsenene and antimonene monolayers possess 1.76 and 1.65 eV indirect band-gaps at the PBE level. Using the GW method, the calculated results revealed more reliable fundamental bandgaps of arsenene and antimonene, which were predicted to be 2.47 and 2.38 eV, respectively (Fig. 13c).¹⁸⁵ The difference between their electronic and optical bandgaps (exciton binding energies) was found to be 0.9 eV for arsenene and 0.8 eV for antimonene. In addition, β-antimonene has a strong SOC effect. The band gap of β-antimonene with SOC is 1.04 eV at the PBE level (1.55 eV at HSE level) in the presence of indirect bandgaps. For the calculated imaginary part of dielectric functions of arsenene and antimonene MLs, optical absorption occurred at 1.6 eV for arsenene and at 1.5 eV for antimonene (Fig. 13d).¹⁸⁵

Layered β-Bi crystals are characterized by small density of states around the Fermi level and had semi-metallic properties. The ML structure meant a narrow bandgap semiconductor.^186,187 Bi is heavier than other group-VA elements. A stronger SOC affects the band structures of bismuthene. HSE correction applied to PBE states can increase the direct bandgap to 0.80 eV. When the SOC was included, the fundamental bandgap reduced to 0.32 eV. The band structure transformed from direct bandgap into indirect bandgap.¹⁸⁶ Besides the β phase, the α phases of arsenene, antimonene and bismuthene have high thermodynamic stability, regardless of their metastable phases. After PBE (HSE) calculations, their band gaps were determined to be 0.77, 0.37 and 0.16 eV (1.66, 1.18 and 0.99 eV), respectively.¹¹⁸ Besides, α-bismuthene had a direct bandgap and transformed into an indirect bandgap semiconductor with the inclusion of SOC, similar to β-bismuthene.^186,187 By contrast, the α phases of arsenene, antimonene or bismuthene had smaller bandgaps than the β phases with the most stable states.^39,188–199

3.2. Carrier transport

2D group-VA semiconductor nanomaterials have superior carrier transport properties, showing practical uses in electronic or optoelectronic devices. Carrier transport properties can be studied by mobility calculations. Theoretically, the carrier mobility of 2D materials relies on their intrinsic properties, including effective mass, elastic modulus and deformation potential constant.¹⁸⁴ Based on HSE06 calculations, Qiao et al. predicted the mobility of BP, which was hole-dominated, anisotropic and rather high.¹⁸⁴ The electron mobility was 1100–1140 cm² V⁻¹ s⁻¹ along the armchair direction and 80 cm² V⁻¹ s⁻¹ along the zig-zag direction. The hole mobility along the armchair direction was 640–700 cm² V⁻¹ s⁻¹ and that along the zig-zag direction reached up to 10⁴ cm² V⁻¹ s⁻¹. The hole mobility of BP was higher than that of MoS₂ (200 cm² V⁻¹ s⁻¹),²⁰⁰ benefiting from the extremely small deformation potential (0.15 eV). The carrier mobility of BP has strong anisotropic characteristics due to its high anisotropic atomic structures, which result in big differences in effective mass, elastic modulus and deformation potential constant along the armchair and zig-zag directions (Fig. 14a). The high in-plane transport anisotropy of BP distinguishes it from other typical 2D materials, including graphene, silicone and hexagonal boron nitride. The carrier transport properties of arsenene, antimonene and bismuthene were evaluated by deformation potential methods.^{39,124,185,201–203} The carrier mobilities of α-/β-arsenene, β-antimonene and β-bismuthene reach several thousand cm² V⁻¹ s⁻¹. The electron and hole mobilities were calculated to be 635 and 1700 cm² V⁻¹ s⁻¹ for β-arsenene (630 and 1737 cm² V⁻¹ s⁻¹ for β-antimonene).¹²⁴ The SOC effects hardly affected the conduction bands of As and Bi, but led to changes in the topmost valence band, effective mass and deformation potential. Hole mobility was increased by 25% for the As MLs and 84% for the Bi MLs.

3.3. Mechanical properties

2D group-VA materials possess unique mechanical properties due to their puckered and buckled structures. BP presents special and anisotropic mechanical properties.^43,204,205 Based on FPC, the maximum Young's modulus of 166 GPa is along the zig-zag direction,⁴³ and the minimum value of 44 GPa is along the armchair direction (Fig. 14b). Along the armchair direction, BP suffers from strain up to 30%. During deformation of phosphorene in the zig-zag direction, a negative Poisson's ratio was determined in the out-of-plane direction, resulting from its puckered structures.²⁰⁴ The fantastic properties of phosphorene imply its potential as an auxetic material. As testified by theoretical studies,^186,206,207 α-arsenene and α-bismuthene have directional elastic behavior. The in-plane stiffness C_x–C_y values are 20–55 N m⁻¹ for As (8.04–22.6 N m⁻¹ for Bi). Clear anisotropic Poisson's ratios for As and Bi were determined, with v_xy–v_yx values of 0.33–0.91 for As (0.33–0.93 for Bi). Along the zig-zag direction, the v_yx values are very high for As and Bi. Significantly, the in-plane stiffness affected the robustness of the structures. The Poisson's ratio indicated a variation in electronic structures and possible metal-insulator transitions under tensile strain. The mechanical properties of 2D layered group-VA pnictogen materials were studied by theoretical calculations. The in-plane stiffness values of β-arsenene and β-bismuthene are isotropic, and their C values are 58 and 23.9 N m⁻¹, respectively.^186,206,207 The isotropic Poisson's ratio for As (Bi) has a C value of 0.21 (0.33). The in-plane stiffness values for 2D group-VA materials are much smaller than that of other typical honeycomb materials, such as graphene and hexagonal boron nitride with C values of 330 and 240 N m⁻¹, respectively.

3.4. Thermal properties

The thermal properties of 2D group-VA materials were theoretically and experimentally studied.^208–224 Anisotropic phonon properties in the α phases of phosphorene, arsenene, antimonene and bismuthene facilitated the achievement of asymmetrical thermal conductivity and thermoelectric efficiency.^210–214 Based on FPC, Aierken et al. calculated the thermal properties of black and blue phosphorene.²¹⁰ The linear thermal expansion coefficients of BP along the zig-zag and armchair directions are anisotropic, reaching up to 20%. The blue phase is isotropic in thermal expansion. The thermal conductivity in BP was found to be anisotropic due to its orientation-dependent group velocities and phonon relaxation times.²¹⁴ In terms of theoretical calculations, the thermal conductivities at 300 K were 83.5 and 24.3 W m⁻¹ K⁻¹ along the zig-zag and armchair directions, respectively. Notably, the electronic conductivity along the armchair direction is much larger than that along the zig-zag direction.²²⁵ The thermoelectric figure of merit (ZT) depends on both thermal conductivity and electronic conductivity. This feature is highly anisotropic for BP because its ZT value along the armchair direction is larger than that along the zig-zag direction (Fig. 14c). The intrinsic ZT value in phosphorene is rather low, but it increases by means of different improved methods.

Zeraati et al. evaluated the phonon dispersion and lattice thermal conductivity of α-arsenene through first-principles calculations for anharmonic lattice dynamics and the Boltzmann transport equation for phonons.²⁰⁸ Arsenene has a smaller and more anisotropic thermal conductivity than phosphorene. The room-temperature thermal conductivities of arsenene along zig-zag and armchair directions are 30.4 and 7.8 W m⁻¹ K⁻¹, respectively. Due to the puckered structures of arsenene, its thermal conductivity is mainly provided by longitudinal acoustic phonon modes at a temperature over 100 K. Generally, the buckled structures of group-VA materials suggest the isotropic characteristics of their thermal properties. Antimonene was predicted to hold a low lattice thermal conductivity of ∼15.1 W m⁻¹ K⁻¹ at 300 K,²¹⁵ implying a small group velocity, low Debye temperature and large buckling height. By minimizing the sample size and chemical functionalization, thermal conductivity was properly tuned to be smaller.^215,216 Nevertheless, the intrinsic thermoelectric figures of merit for antimonene and bismuthene are not high enough, limiting their potential applications. Besides, their intrinsic ZT values can be effectively increased by n- or p-doping.^209,217

3.5. Optical properties

2D group-VA nanomaterials have unique optical properties, which expand their potential applications. Through FPC, the optical adsorption spectra of BP were predicted.^184,226 The calculated absorption spectra are anisotropic. Light is linearly polarized in the armchair and zig-zag directions. At the bandgap, the band edge of the first absorption peak was found to slightly decrease with an increase in layer thickness in the armchair direction. The optical adsorption peak was detected at 3.14 eV in the ML, which quickly decreased with an increase in thickness in the zig-zag direction. The optical detection of the crystalline orientation and optical activation of anisotropic transport properties were performed by in-plane linear dichroism. As calculated, phosphorene ML absorbed light between 1.1 and 2.8 eV along the armchair direction.²²⁶ In the same energy range along the zig-zag direction, phosphorene ML is transparent to light, covering the infrared and partial visible-light regime. By tuning the number of stacking layers, the polarization energy window was tuned in a wide range, showing potential to fabricate smart optical devices. The optical conductivity tensor of BP was studied using the Kubo formula within a low-energy Hamiltonian.²²⁷ This method was used to express physical quantities observed in optical experiments. In terms of the calculated results, the optical conductivity of BP was similar to the optical adsorption spectrum and was sensitive to layer number (Fig. 14d).

The optical properties of arsenene, antimonene and bismuthene were studied by calculating their dielectric functions, electron energy loss spectra, absorption coefficients, refractive indices and optical reflectivity in a broad energy range.^{56,79,87,89,92,98,108,228–242} As confirmed, the optical properties of antimonene are suitable for its use in ultraviolet optical nanodevices, micro-electronic devices and solar cells.²²⁸ Based on the calculation results, the dielectric functions were negative, namely 5.1–9.0 eV (6.9–8.4 eV) for α-antimonene (β-antimonene). The results illustrated its metallic characters in the UV part of electro-magnetic spectra. The electron energy loss spectra indicated the plasmon energy of ∼9 eV, revealing its metallic behavior based on light refection. The refractive indices were close to 2.3 for α-antimonene and 1.5 for β-antimonene at zero energy limit and scaled up to 3.6 in the ultraviolet region. When the magnitude and nature of bandgaps are required to be correctly reproduced, DFT band structure calculations only give superficial information on the optical properties of materials. Before the optical applications of 2D group-VA materials, theoretical predictions are required and often arise from calculations of dielectric functions based on random phase approximation. Currently, the complex dielectric functions, concomitant refractive indexes, absorption coefficients, electron loss spectra and optical reflectivity were reported in the energy range of 0–21 eV.²²⁸ The absorption process is expected to start in the infrared part of the spectrum and reach the maximum in the UV region. In the case of β-Sb, when the polarization direction of incident light is out-of-plane, the reflectivity in the visible region is high, but absorption is nearly negligible. Based on calculations, β-Sb is a polarization transparent material.²³⁰ Antimonene exhibits promising uses in opto-electronic devices, such as smart solar cells, with the combined advantages of light emission, modulation and detection.

3.6. Magnetic properties

The magnetic properties of layered nanomaterials depend on the volume, surface and step atoms of 2D nanostructures.²⁴³ Anisotropic magnetic properties are attractive for memory device applications, where nanomaterials are used as ferromagnets. Isotropic nanostructures of magnetic metals (Fe, Co and Ni) are generally superparamagnetic at room temperature due to nanosized confinement effects. An effective strategy to improve magnetic anisotropy is to form magnetic nanocrystals with structural anisotropy. For 2D layered group-VA nanomaterials, the doping of transition metal atoms (Ti, V, Cr, Mn, Fe, and Co) can generate magnetism in α-phosphorene.^244–248 Other atoms (H, B, C, O, Si, S, Ge, and Sn) and gas molecules (NO and NO₂) can produce magnetic states in arsenene.^249–257 Through FPC, the adsorption of NO and NO₂ on phosphorene had higher adsorption energies compared with other gas molecules (CO, CO₂, NH₃, and N₂).²⁵⁸ NO and NO₂ adsorption led to a magnetic moment of 1 μ_B. Among the adsorption of different gases on buckled arsenene ML, NO_x adsorbents had the largest charge transfer, showing a greater change in conductivity.²⁵⁷ Only the adsorption of NO_x produced a magnetic moment of 1 μ_B. Theoretical calculations indicated that defects affect the electronic properties of 2D layered group-VA materials and produce magnetism in nonmagnetic pristine phosphorene, arsenene and antimonene.^259–264 Typical point defects in arsenene and antimonene were studied.²⁶³ Most of the defective configurations retained indirect bandgaps with reduced bandgap values, and most of the single vacancy defects possessed magnetic moments due to their dangling bonds.

Abid et al. reported the magnetic properties of zig-zag arsenene nanoribbons (ZAsNRs).²⁶⁵ The edge magnetism for different magnetic configurations of ZAsNRs was investigated to remove instabilities. A transition was observed from the nonmagnetic to magnetic edge states. An intra-edge antiferromagnetic semiconducting ground state was found. To tune the edge states, strain engineering was applied on the magnetic ground states. At a critical value of compressive strain (−6%), a transition from magnetic edge states to nonmagnetic ones occurred (Fig. 15a). The geometrical structures and chemical termination of arsenene nanoribbons had obvious effects on their magnetic properties.²⁶⁶ Using the DFT method, the normal or one-atom terminated zig-zag nanoribbon was a weak antiferromagnetic semiconductor due to the magnetic interactions between the edge states. In the case of bare ZAsNRs, tensile strain stabilized the antiferromagnetic states with enhanced magnetic moments. Liu et al. evaluated the magnetic properties of arsenene that was functionalized by 3d transition-metal (TM) atoms.²⁶⁷ The pristine arsenene is a nonmagnetic material, but its dilute magnetism can be produced upon chemisorption of TM atoms. Magnetism is mainly due to TM adatoms and magnetic properties can be tuned through moderate external strain. TM-adsorbed arsenene is a superior candidate for promising applications in nanoelectronic and spintronic devices.

Min et al. evaluated the magnetic properties of adsorbed arsenene ML with nonmagnetic metal atoms based on FPC.²⁶⁸ Magnetism was found for Al and Ga adatoms. By studying the magnetic interactions between moments induced by Al and Ga adatoms, p–d exchange-like p–p hybridization in the ferromagnetic states was found. When the distance of Ga–Ga or Al–Al increased, the ferromagnetic interactions were extremely depressed. This phenomenon was explained using the Heisenberg model. Al- or Ga-adsorbed arsenene is a superior candidate for application in spintronic devices. Xu et al. explored the influence of vacancies and nonmetallic atoms on the magnetic properties of buckled arsenene.²⁶⁹ Because of the formation of one nonbonding p-electron (from dopant C, Si) or a neighboring As atom (around O, Si and vacancies), a doping (C, Si, O, and S) atom and a vacancy induced a magnetic moment of 1.0 μ_B in buckled arsenene. The magnetic coupling between moments caused by two (C, Si, O, and S) atoms is long-range anti-ferromagnetic. Based on the calculated density of states and spin density distribution, p–p hybridization interactions involve polarized electrons and are responsible for magnetic coupling. Magnetism in buckled arsenene can be engineered via vacancies and substitutional doping of nonmetallic atoms. Kadioglu et al. revealed that point defects modified the magnetic structure of 2D ML structural Bi.²⁷⁰ The interactions between foreign adatoms and bismuthene structures comprised magnetic structures. Localized states in diverse locations of bandgaps and resonant states in the band continua of bismuthene were induced upon the adsorption of different adatoms, which modified its magnetic properties. Dai et al. investigated the magnetic properties of Fe-doped and defect-tuned antimonene systems.²⁷¹ A large magnetic moment was obtained in defect systems. Stable ferromagnetism was obtained in the Fe-doped system. Due to the presence of intrinsic vacancies, Fe-doped antimonene with anti-ferromagnetism order is not suitable for practical applications in spintronics nanodevices.

3.7. Electronic properties

2D layered group-VA materials have functional nanostructures and tunable electronic properties under different conditions of strains, electric fields and defects. When an externally compressive strain was applied along the zig-zag direction of phosphorene, its band gap increased and then decreased with an increase in strain (Fig. 15b), suggesting that the bandgap of phosphorene is sensitive to strain.²⁷² The bandgap reached the maximum with a critical tensile strain of 4% in the zig-zag direction. A similar bandgap change was determined under an external strain applied along the armchair direction. Under external strain, the effective mass of phosphorene causes dramatic transformation. The favorable direction of electron transport was switched with applied biaxial strain.²⁷³ Electron mobility along the zig-zag direction is much larger than that along the armchair direction, but hole mobility is not sensitive to the applied strain. An applied electric field tuned the electronic properties of phosphorene. Under an external electric field, the bandgap of few-layer phosphorene decreased (0.22 eV) with an increase in the electric field.²⁷⁴ ML phosphorene had a slight decrease (0.08 eV). By applying an external electric field, a specific band inversion was induced to acquire topological states.^275–279 Arsenene and antimonene had an indirect-to-direct bandgap transition under small biaxial strain.¹⁵⁴ Under 0–3% biaxial strain, the minimal conduction band of buckled arsenene was retained in the Brillouin zone halfway between the G and M high-symmetry points. Upon tensile strain up to 4%, the indirect-bandgap buckled arsenene was changed into a direct-bandgap semiconductor. Under tensile strain of 4–12%, arsenene maintained a direct-bandgap characteristic of semiconductors (Fig. 15c).

The electronic properties of few-layer or ML allotropes of arsenene, antimonene and bismuthene were studied under different conditions of strain, electric field and defects by means of theoretical calculations.^{186,206,230,263,266,280–290} Antimonene has electronic band characteristics similar to that of arsenene under biaxial tensile strain.²⁸¹ Both arsenene and antimonene have a changing trend from an indirect to a direct bandgap semiconductor. Both of them are strained and present direct band structures. Electronic excitation becomes feasible and has lower phonon energies. The merits of arsenene and antimonene endow them promising applications in optical devices. The electronic band structures of arsenene and antimonene were modulated by strain and strain, resulting in a topological insulator transition.^194,291 Theoretical studies suggested that strain drives normal insulators of arsenene and antimonene to novel topological nontrivial 2D materials (Fig. 15e).²⁷⁵ According to theoretical calculations, defects impact the electronic properties of 2D layered group-VA materials to yield magnetism in nonmagnetic pristine phosphorene, arsenene and antimonene.^259–264 Scientists have studied the electronic band structures and bias-dependent transport properties of defective phosphorene systems. The bandgap is closed in single-vacancy phosphorene, but it reappears in the divacancy system. Vacancy defects greatly increase the current of phosphorene-based devices. Most defective configurations retain indirect bandgaps with reduced bandgap values. The defects in antimonene cause a transition from an indirect bandgap to a direct bandgap. Most single vacancy defects carry magnetic moments because of their dangling bonds. A single vacancy in antimonene endows it with metallic character. Four divacancies retained the semiconducting characters of antimonene with reduced bandgaps.^263,283,292

4. Functional nanostructures

4.1. Heterostructures

2D group-VA nanomaterials can be modified to construct functional nanostructures. These functional nanostructures involve heterostructures, doping, adsorption and surface functionalization of 2D group-VA materials and pnictogen-containing 2D hybrids (Table 2). van der Waals heterostructures consist of different MLs free of dangling bonds. Previous studies reported van der Waals hybrid heterostructures based on 2D group-VA materials, such as arsenene/graphene,^293,294 antimonene/graphene,^295–297 antimonene/GaAs,²³⁷ arsenene/GaS,²⁹⁸ arsenene/WSe₂,²⁹⁹ arsenene/MoS₂,^236,300 β-As/MX₂ (TMDs),³⁰¹ arsenene/silicone,²³⁵ antimonene/germanene,²³⁸ arsenene/C₃N,²³⁴ arsenene/Cd(OH)₂,³⁰² arsenene/Ca(OH)₂,³⁰³ O-arsenene/Cs₂CO₃–antimonene,³⁰⁴ and arsenene/FeCl₂.³⁰⁵ After theoretical and experimental studies, van der Waals heterostructures show superior electronic and optical properties compared to individual components. The formation of van der Waals heterostructures integrates the characteristics of individual components with enhanced and novel properties to explore electronic and optoelectronic devices. The phosphorene/TMD hetero-bilayer has a semiconductor characteristic, and its bandgap was reduced under vertical electric field.³⁰⁶ In the case of the phosphorene/MoS₂ heterobilayer, its power conversion efficiency reached up to 17.5%. Wang et al. performed theoretical studies on graphene/phosphorene/graphene van der Waals heterostructures.³⁰⁷ The thermionic transport barriers were tuned by adjusting the layer number. There is weak thermal conductance across noncovalent structures. Layered van der Waals structures are potential candidate for applications in solid-state energy-conversion devices. The GeSe/phosphorene van der Waals p–n heterostructure has type-II band alignment and an indirect bandgap.³⁰⁸ Under external strain, there are indirect or direct insulator-metal transitions and spontaneous electron–hole charge separation. GeSe/phosphorene heterostructures have great prospect in optoelectronic devices.

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Except for phosphorene,^37,38 2D group-VA materials (arsenene/antimonene/bismuthene) act as building blocks for van der Waals heterostructures.^{293–305,309–313} The structural and electronic properties of arsenene-based hetero-structures were studied. A linear Dirac-like dispersion relation exists in arsenene/graphene heterostructures. By tuning their interlayer distance, there is a transition from a p-type Schottky barrier to an n-type Schottky barrier, which facilitates the integration of arsenene and graphene with tunable functions.^293,294 In arsenene/silience heterostructures, the p-type Schottky barriers were retained with small bandgaps, which were opened with a linear changing trend by tuning the interlayer distance.²³⁵ After the interfacial coupling of arsenene with FeCl₂, spin splitting appeared at the minimum conduction band of arsenene, with a maximum splitting energy of 123 meV in the arsenene/FeCl₂ van der Waals heterostructure configuration.³⁰⁵ Various types of van der Waals heterostructures, such as antimonene/graphene,^295–297 arsenene/antimonene,^124,185 antimonene/germanene,²³⁸ arsenene/BP,³¹³ and antimonene/hexagonal boron nitride,³¹⁰ were theoretically predicted (Fig. 16a). Antimonene/silience had a p-type Schottky barrier with sizeable bandgaps opened at the Dirac points. Antimonene/graphene had a Schottky-to-Ohmic contact transition induced by compressive strain. Different from the indirect bandgap of antimonene and gapless characteristic of germanene, antimonene/germanene van der Waals heterostructures had a remarkable bandgap opened with a feature of direct bandgaps. The fabrication of stacking van der Waals heterostructures can act as a promising strategy to tune the electronic properties of 2D layered group-VA nanomaterials.

Different from the vertically stacked van der Waals heterostructures, the lateral heterostructures of 2D group-VA nanomaterials are connected in-plane by covalent bonds with atomically clean and sharp interfaces. The lateral heterostructures have potential to regulate the ultimate thickness of heterostructures for electronic and optical device applications. Arsenene and antimonene can be used as building blocks to assemble lateral heterostructures. Compared with pristine As and Sb monolayers, freestanding As/Sb lateral heterostructures with zig-zag interlines and the stitching of As/Sb monolayers in-plane have high stability and intrinsic direct energy gaps without modulations.³¹⁴ Lateral heterostructures (arsenene/BP) were linked by covalent bonds to fabricate a type-II band alignment, which enhanced charge carrier separation spatially.³¹³ High charge carrier mobility often depends on the width of the building blocks (arsenene and antimonene). 2D group-VA nanomaterials with lateral heterostructures have high electronic and optoelectronic properties, superior to other types of heterostructures. 2D group-VA pnictogen materials can form various hybrid heterostructures by combination with metals,³¹¹ semiconductors,^237,298 TMDs,^{236,299–301} 2D inorganic nanomaterials,^302–305etc. ML bismuthene underwent metallization upon contact with metal electrodes due to strong interactions.³¹¹ ML bismuthene formed an n-type (p-type) Schottky contact with Ir/Ag/Ti (Pt/Al/Au) electrodes. The tunable interfacial properties of ML bismuthene–metal contacts render them promising application as metal electrodes in ML bismuthene devices.

Arsenene (β-As) can form heterostructures with TMDs (WSe₂, WS₂, MoS₂, and MoSe₂), showing tunable electronic properties through FPC.^{236,299–301} Arsenene/MoS₂ and arsenene/WSe₂ van der Waals heterostructures underwent a transition from type-II to type-I (then from type-I to type-II) under an external electric field. Engineered arsenene/MoS₂ heterostructures consisting of two MLs MoS₂ and arsenene had novel electronic and optical conductivity. Arsenene-based heterostructures paired with MoS₂ and tetracyano/tetracyano naphtho quinodimethane formed type-II band alignments, which were used as catalysts for photocatalytic water splitting and photovoltaics with high power conversion efficiency (∼20%).³⁰⁹ Wang et al. studied the Sb/GaAs 2D/3D van der Waals heterostructure.²³⁷ This unique heterostructure without constraint of lattice matching is suitable for achieving improved electronic and optoelectronic properties. van der Waals interactions are crucial for the stability of Sb/GaAs heterointerfaces. The interfacial coupling strength and band structural characteristics of the heterostructures were affected by interface structures (Fig. 16b). The stable Sb/GaAs heterostructures had a type-II band alignment with small bandgaps of 0.71–1.39 eV compared with the independent Sb ML and GaAs substrates. Similar to arsenene/GaS van der Waals heterostructures,²⁹⁸ the Sb/GaAs van der Waals heterostructures had carrier separation and high optical absorption coefficient in the visible-light range. Thus, the Sb/GaAs heterostructure may be a superior candidate for application in optoelectronic devices, especially solar cells and photocatalysts for water splitting.

The suitable electronic structures of α-AsP enabled the generation of perfect type-II semiconductor hetero-junctions with GaN (Fig. 16c), which boosted the separation and transport of photo-generated carriers with the assistance of built-in field and high mobility.³¹² α-phase few-layer As–phosphorus alloys were prepared, paving the way to realize black As–phosphorus ML donors. Arsenene/Ca(OH)₂ van der Waals heterostructures showed strain-tunable electronic and photocatalytic properties (Fig. 16d).³⁰² Electric field modulations of band alignments in arsenene/Ca(OH)₂ heterobilayers were used for multifunctional device applications.³⁰³ The electronic and optical properties of arsenene/C₃N van der Waals heterostructures were regulated by an applied vertical strain and external electric field, showing superior light-harvesting performance.²³⁴ By surface charge transfer doping of arsenene/antimonene heterostructures, atomically thin p–n/p–n nanodevices were explored.³⁰⁴ A type-II energy band alignment was produced in O-arsenene/Cs₂CO₃–antimonene heterostructures, which extended light absorption into the near-infrared region and promoted the spatial separation of photo-generated electron–hole pairs.

4.2. Doping

Besides heterostructures, doping with specific species is another way to develop functional nanostructures of 2D group-VA nanomaterials. The electronic structures and properties of 2D group-VA nanomaterials can be altered by doping metallic or nonmetallic agents. Compared to traditional substitutional and interstitial doping in a lattice, the deposition of a specific atomic or molecular layer on the surface of 2D group-VA nanomaterials is an efficient strategy, which tunes or optimizes the electronic and magnetic properties for specific applications (Table 2). The physicochemical properties of the doped counterparts depend on the dopant compositions and concentrations. Alkali metals exhibit small electron affinities and transfer electrons to BP as dopants. K was deposited on few-layer BP via an in situ surface doping technique.³¹⁵ Based on FPC, the bandgap of phosphorene was tuned by this type of electron doping. With an increase in K doping, the valence band was shifted down and the phosphorene bandgap decreased. Few-layer BP was transformed from a semiconductor to a band-inverted semimetal by K doping. To tune the type of dominant charge carriers in BP, Cu was deposited on the surface of BP or intercalated between BP layers.³¹⁶ The Cu adatoms deposited on BP by sputtering and integrated in field effect transistors (FETs) induced a shift in threshold voltage. After Cu deposition, the BP channel was shifted from p-type to n-type at room temperature.

According to FPC, Fe-doped antimonene had tunable electronic and magnetic properties.²⁷¹ In the Fe-doped system, a large magnetic moment and stable room-temperature ferromagnetism were detected. The simultaneous strong orbital hybridization (p–d) and spin–orbit interaction induced significant spin splitting around the Fermi level. There was a transformation from narrow band-gap semiconductor to semimetallic material. When two intrinsic vacancies are introduced into Fe-doped antimonene, anti-ferromagnetism order appears, which limits its use in spintronic devices. DFT was used to study the mechanical properties of pristine, Fe-, Ti- and V-doped arsenene.²⁰⁷ By applying uniaxial and biaxial strains on the pristine and doped arsenene, the Young's and bulk moduli were studied. The elastic modulus of the doped arsenene was smaller than that of the pristine arsenene, but the inharmonic region of the pristine arsenene was larger than the doped arsenene. The plastic properties of pristine and doped arsenene were studied. Upon an increase in applied strain, the second critical strain as the beginning of plastic behavior decreased due to doping. The coexistence of Co-doping and strain could control the spin states of arsenene and antimonene structures (Fig. 17).³¹⁷ The unstrained Co-doped arsenene or antimonene structure was nonmagnetic. Under strain, the magnetic moment abruptly increased to about 2 μ_B. The emergence of magnetism was reflected by the reduction in the interactions between Co and neighboring atoms by strain, causing the spin-splitting of the Co-3d states. Upon magnetism transition, modification of the electronic properties of arsenene and antimonene occurred under strain, showing novel half-metallic behaviors. Considering the remarkable charge transfer from adatoms to arsenene, metallic states were found in Li-, Na-, Al-, Co-doped systems, where the Co-doped arsenene had a half-metal property.³¹⁸

Substitutional doping of 3d transition metal (TM) atoms on arsenene or antimonene produced tunable structural, electronic and magnetic properties.²⁵⁶ According to the calculated binding energies, TM-substituted arsenene was robust. Magnetic states were found because of the doping of Ti, V, Cr, Mn, Fe and Ni. The Sc-/Co-doped arsenene nanosheets had nonmagnetic semiconducting properties. The substitution of As with Ti, Cr and Cu atoms led to a dilute magnetic semiconductor phase (Fig. 17).²⁵² The magnetism of Fe-/Ni-doped arsenene was tuned by adjusting the doping concentration.²⁵⁵ Half-metallic states were found in Ti-, V-, Mn-, Fe- and Ni-doped arsenene and antimonene.³¹⁹ Spin-polarized semiconducting states occurred with V, Cr, and Fe doping. Ti-, V-, Mn- and Fe-doped arsenene nanosheets exhibited ferromagnetic coupling. Cr substitutional doping induced antiferromagnetic coupling under the most stable configuration. TM-substituted arsenene shows potential applications for spintronics and magnetic storage devices. Other types of metallic doping of arsenene involved Sn, Ga, Ge, Li and metal compounds (Cs₂CO₃),³⁰⁴ presenting tunable structural, electronic and magnetic properties.^251,253,320 The high-temperature superconducting, magnetism and magneto-optical effects of hole-doped gray, ML or buckled arsenene were proven under strain conditions.^240,321,322 In the case of n- or p-type doping of antimonene, tetrathiafulvalene and tetracyano-quinodimethane served as electron and hole dopants to obtain n- and p-type antimonene semiconductors, respectively, which widen the applications of 2D semiconductors in electronics and optoelectronics.^239,323

The nonmetallic doping of 2D group-VA nanomaterials was explored. Through FPC investigations on arsenene doped with non-magnetic elements, dopants from groups III, V and VII with odd numbers of valence electrons maintained the semiconducting character of the pristine system, while that (groups IV and VI) with an odd number of valence electrons caused a change in metallic character (Fig. 17).^320,324 C-/O-doped systems were spin-polarized and modulated into half-metals by external electric fields. Buckled arsenene with doping of vacancies and nonmetal atoms was thermodynamically stable at room temperature. The substitutional doping of H, F, B, N and P did not produce magnetism in buckled arsenene. Saturation or pairing of valence electrons from the dopants and neighboring As atoms occurred.²⁶⁹ Vacancy and doping of C, Si, O and S induced a magnetic moment of 1.0 μ_B in buckled arsenene, resulting from one nonbonding valence electron of C and Si or neighboring As atom around O, S and vacancy. The magnetic coupling between magnetic moments induced by two elements of C, Si, O and S was long-range anti-ferromagnetic due to the p–p hybridization interactions involving polarized electrons. Arsenene and antimonene had tunable electronic structures and magnetic properties through impurity doping of H, B, C, N, O, S, Si, P, F, Cl, Br, I, Sb and Se.^{251,253,325,326} Engineered nanostructures of arsenene and antimonene with metallic or nonmetallic doping have a broad range of applications in electronic, optoelectronic and magnetic devices.

4.3. Absorption

Through interstitial deposition and stacking interactions, atoms or molecules can be absorbed onto 2D group-VA nanomaterials to fabricate functional nanostructures. Based on FPC, Chen et al. found that Li-intercalated bilayer arsenene with AB stacking is dynamically stable and is different from the pristine bilayer with AA stacking. Electron–phonon coupling of stable Li-intercalated bilayer arsenene was dominated by low frequency vibrational modes and produced a superconductivity of T_c (8.68 K) with isotropic Eliashberg functions.³²¹ Metal adatoms on arsenene have binding energies larger than the cohesive energies of bulk metals, implying the formation of stable adsorbates. Due to the localized states from adatoms, adsorption systems have various electronic properties, such as metal, half-metal, semiconducting and spin-semiconducting behaviors. The adsorption of Cu, Ag and Au turned semiconducting arsenene into a narrow gap spin-semiconductor (Fig. 18a).³¹⁸ Arsenene functionalized with metal adatoms acted as spintronics and dilute magnetic semiconductor materials. Yang et al. studied the interface between Co(0001) and ML or bi/tri-layer antimonene with different stacking models.³²⁷ After contact, a high Schottky barrier was formed. The barrier height was tuned by different Co/antimonene stacking patterns. The Co d and Sb sp orbitals had strong hybridization near E_F. Strong chemical bonds formed between Co and antimonene at the interface. The results implied applications in spin diodes and spin FETs based on antimonene. Nonmagnetic metal atoms (Li, Na, Al, Ga, Mg, Ga, Ge, and In) and TM atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) were absorbed onto ML, few-layer arsenene, antimonene, free-standing As and Sb nanosheets.^{250,267,268,328} Magnetism was observed for nonmagnetic Al and Ga adatoms adsorbed onto arsenene ML. TM-adsorbed arsenene or antimonene systems are promising candidates for electronic and spintronic devices.

Adsorption of nonmetal atoms onto 2D group-VA nanomaterial involved adatoms of H, B, C, N, O, F, Si, P, Cl, As, Se and Sb.^250,329 Adatoms produce chemisorption bonds and modify atomic structures and physical properties locally. Some adatoms cause obvious local reconstruction of atomic structures. A majority of adsorbed atoms cause localized states in fundamental bandgaps. Light atom (B, C, N, O, and F) absorbed arsenene nanosheets were studied by FPC.²⁵⁴ Most adatoms prefer to occupy bridge sites on arsenene nanosheets except for C adatom (valley sites). Defect states were detected in the middle gap of the F-adsorbed system. N adatoms caused n-type doping. O adatoms had negligible effects on its electronic structures. B, C, N and F adatoms induced magnetism in arsenene nanosheets. For As-adsorbed ML hexagonal arsenene (hAs) (Fig. 18b),²⁵¹ its Fermi level crossed the spin-up states, yielding metallic behaviour. Organic molecules can be adsorbed on 2D group-VA materials. Using FPC, Gao et al. proved selective organic molecular adsorption control of carrier types in arsenene.³³⁰ Tetracyano-quinodimethane (TCNQ) and tetrathiafulvalene (TTF) with electron-withdrawing and donating abilities, respectively, were selected as organic molecules. The donors were adsorbed on the surface of arsenene via a physisorption process. There was a considerable electron transfer from arsenene to TCNQ, yielding p-doped arsenene. However, the electrons transferred from TTF to arsenene are not enough to make effective n-doped arsenene. An additional tensile strain assisted this process. Due to the precise control of carrier types and combination of the p-/n-doped components, p–n junctions can be formed in arsenene-based nanodevices and exhibit applications in transistors and photodetectors.^239,323

The adsorption of gas molecules onto 2D group-VA nanomaterials refers to various small molecules, including H₂, N₂, CO₂, CO, O₂, H₂O, NH₃, SO₂, NO and NO₂.^{250,329,331–335} By first-principles spin-polarized density functional calculations, the chemisorption of selected adatoms and physisorption of molecules on two antimonene (or ML arsenene) phases with buckled honeycomb (b-Sb, b-As) and symmetric washboard (w-Sb, w-As) structures were studied.^250,329 Molecules such as H₂, O₂ and H₂O neither formed strong chemical bonds nor dissociated. They were physisorbed with weak binding energies without affecting the properties of antimonene (Fig. 18c). The interactions of molecules with nanomaterials are crucial for hydrogen storage, evaluation reaction and oxidation–deoxidation. Molecules weakly interact with antimonene. The binding energies are weak, showing a predominant van der Waals character. Selected metal and nonmetal adatoms formed strong bonds with antimonene by exchanging electronic charges, causing local reconstructions and defects. Electronic states from adatoms led to a diversity of electronic states, together with high carrier mobility, magnetism (spin-polarized) and half-metal characters. Some molecules were dissociated at the edges of arsenene flake structures. Constituents were adsorbed to the edge atoms to cause local reconstructions.²⁵⁰

CO adsorption on pristine antimonene is physical adsorption that is then converted to chemical adsorption after doping.³³² An external electric field improved the CO gas sensitivity on antimonene to realize CO sensing at room temperature. Adsorption and desorption of CO can be controlled by applying an external electric field, which is helpful to collect and store CO gas. Gas adsorption of pristine antimonene was studied by FPC, which promoted the exploration of its high-performance gas sensing.³³⁵ Atmospheric gas molecules (N₂, CO₂, O₂, and H₂O) were weakly adsorbed on antimonene. However, pollutant gas adsorbates (NH₃, SO₂, NO, and NO₂) were physically adsorbed on antimonene, showing stronger adsorption energies and elevated charge transfer due to the contributions of the frontier orbitals of the molecules being closer to the Fermi level and more apparent orbital hybridizations. Due to the moderate physical and chemical adsorption of their atom-doped analogues, appreciable charge transfer and susceptibility of electronic properties, ML antimonene and arsenene may be potential sensing materials for the sensitive detection of pollutant gases.^{257,331,333,335,336}

4.4. Pnictogen-containing hybrids

2D layered group-VA materials, such as ML or few-layer BP, As, Sb and Bi (phosphorene, arsenene, antimonene, bismuthene), can be modified by forming heterostructures, doping and absorption of atoms or molecules to develop functional nanostructures. In addition, pnictogen-containing 2D hybrids have been developed for particular applications. Sun et al. designed seamless lateral heterostructures (As_mSb_n LHS) that had excellent stabilities within pristine arsenene and antimonene.³¹⁴ Based on theoretical predictions, As_mSb_n LHS had direct and reduced energy gaps without modulations. A coveted type-II alignment and high carrier mobility were identified and had enhanced quantum efficiency. The tensile strain led to efficient bandgap engineering. Antimonene FETs based on ML-trilayer LHS can act as candidates for low-power device applications.^336–338 Besides traditional α/β allotropes, other 2D allotropes of arsenene, antimonene and bismuthene were studied, such as square–octagon (S/O) and tricycle types, nanoribbons, nanotubes, van der Waals bilayer heterostructures, functional ultrathin films, halogenated and decorated 2D films.^{123,228,291,326,339–346} Pnictogen-containing 2D hybrids with functional nanostructures have unique, tunable and enhanced properties for particular applications.

Hydrogenated arsenene (AsH) decorated with TM atoms (Cr, Mo, and Cu) was studied by DFT.³⁴⁷ A unique quantum anomalous Hall (QAH) effect in TM@AsH was predicted. The quantum states of Mo@AsH were tuned by external strain. Under 5.0% tensile strain, its topological gap was ∼35 meV, which is large enough to realize the QAH effect at room temperature. There is a quantum valley Hall effect in Cu@AsH due to the inequality of its AB sublattices. The QAH effect is superior to the quantum spin Hall (QSH) effect because it can avoid inelastic scattering of two edge electrons located on one side of topological nontrivial materials. This superiority is desirable for electronics and spintronics. Candidates of 2D topological insulators (TIs) with large bandgaps were predicted, such as arsenene functionalized with F, OH and CH₃ groups (Fig. 19a).³⁴⁸Ab initio molecular dynamic simulations implied the thermal stabilities of AsX monolayers at 500 K. The nontrivial topological phase was proven by the topological invariant Z₂ and edge states. The topological electronic bandgaps of the AsF ML were modulated by biaxial tensile strain and vertical external electric field. Pronounced light absorption in the near-infrared and visible range of the solar spectrum was expected for AsH or AsF monolayers, which are attractive for light harvesting. Nontrivial QSH insulators AsX are promising candidates for applications in dissipationless transport devices and photovoltaics. Robust 2D TIs in methyl-functionalized Bi and Sb bilayer films were predicted by FPC. Me–Bi and Me–Sb had protected Dirac-type topological helical edge states, which are suitable for QSH systems.³⁴⁹ 2D TIs with large topological energy gaps are superior platforms for topological phenomena and applications at high temperature.

Eremeev et al. reported a giant Rashba-type spin splitting in 2D electron systems that resided at the Te-terminated surface of Bi tellurohalides.³⁵⁰ BiTeCl semiconductor had an isotropic metallic surface-state band lying in deep inside bulk bandgaps. The giant spin splitting of this band ensured substantial spin asymmetry of inelastic mean free path of quasiparticles with different spin orientations. Amsler et al. developed quasi-2D CuBi nanosheets from ab initio calculations.³⁵¹ Through predictions, single layers of CuBi were isolated from high-pressure bulk CuBi materials. The nanosheets exhibited superior electronic and electrochemical properties. When used as a superconductor, there was a moderate electron–phonon coupling (λ = 0.5 and T_c ≈ 1 K). The CuBi nanosheets were readily intercalated with lithium with high diffusibility and applied potential to boost the rate capacity of current electrodes in lithium-ion batteries. Li et al. studied the electronic structures of atomically thin layers of Bi₂Ti₃ quasi-2D crystals.³⁵² Quintuple layers of Bi₂Te₃ indicated semiconductors with localized shallow bands. Weak covalent Bi–Te₂ interactions in the quintuple layers allowed them to be exfoliated to bi/tri-layer nanosheets. The bi/tri-layer nanosheets of Bi₂Te₃ are metallic because valence electrons cannot fully occupy valence bands. Arsenene oxide transited its bandgap from an indirect band to a direct one.³⁵³ The transition is due to a new (conduction) bottom band, consisting of 4s-orbitals of partial oxidized arsenic (layered As_xO_y), p-orbitals of oxygen and unoxidized arsenic. The direct bandgap width was narrowed from the near-infra to infra region in proportion to the oxygen content. Arsenene oxide (AsO) is a good candidate 2D material with QSH effects. Through ab initio calculations, AsO had high stability, flexibility and tunable SOC gaps (Fig. 19b).³⁵⁴ The maximum nontrivial bandgap of AsO reached 89 meV, which was enhanced to 130 meV under biaxial strain. AsO with oxidized surfaces was stable against surface oxidization and degradation, which is suitable for designing topological quantum devices.

2D β-phase group-VA binary monolayers (PAs, PSb, PBi, AsSb, AsBi, and SbBi) were explored by means of DFT calculations.³⁵⁵ They were verified to be stable free-standing materials with versatile electronic structures, and had direct or indirect band gaps of 0.90–2.39 eV, as predicted at the HSE06 level with spin-orbital coupling corrections. A linear correlation was explored between the cohesive energy and band gaps of different composites with average ionization energies (AIEs), showing the potential to engineer desirable properties of these 2D materials from the AIEs of component atoms. These 2D binary compounds had extremely small effective masses of carriers and high electron mobility. They exhibited considerable absorption of solar energy and suitable band alignments for photocatalytic water splitting. The in-plane heterostructure could be gained by combining the elemental 2D group-VA monolayers (Sb/Bi, As/P, and As/Sb) or other 2D materials (graphene, WSe₂, MoS₂, and silicene).³⁵⁵ Binary 2D compounds with various compositions and structures are expected to hold exciting properties, such as semiconductor-topological insulator transition, magnetism and extraordinary sunlight absorbance. After experimental fabrication, novel 2D materials based on group-VA binary compounds enable applications in nanoelectronics and photocatalysts.

4.5. Surface functionalization

Surface functionalization is an efficient strategy to improve the performance of 2D group-VA materials. However, despite the tremendous progress on studies of BP-based nanostructures, a key barrier for BP applications is its degradation upon long-term exposure under ambient conditions. The origin of BP degradation is due to PO_x species generated by reaction between oxygen and lone-pair electrons perpendicular to the BP surface. 2D group-VA pnictogen materials beyond BP such as ML and few-layer As, Sb, and Bi (arsenene, antimonene and bismuthene, respectively) are promising candidates of BP or phosphorene, and have superior physical and chemical properties. Their high charge-carrier mobility, tunable direct bandgap and unique in-plane anisotropic structure give them remarkable applications in a broad range of fields. Before practical applications, various functional methods are used to modify 2D group-VA nanomaterials, including covalent, van der Waals and electrostatic functionalization. Covalent functionalization is mediated by bonding. Lone-pairs of nanomaterial surfaces interact with Lewis acids, such as TM ions, organic molecules and alkali ions. van der Waals functionalization depends on the interactions between nanomaterials and capping layers, such as graphene, hexagonal boron nitride or organic molecules. In electrostatic functionalization, the positively charged adsorbates spontaneously interact with nanomaterials modified with the negatively charged species. Surface functionalization enhances the stability of nanomaterials, and retains and promotes their specificity for long-term use. For 2D group-VA pnictogen nanomaterials, surface functionalization mainly involves physical and chemical steps. Physical steps are ball-mill mixing, thermal vaporization, stacking, impregnation, vacuum deposition, solvent evaporation, spin coating, etc. Chemical steps refer to self-assembly, chemical reaction, photoreduction, focused laser-induced oxidation, phase transformation, etc. These steps are widely used to develop a variety of electronic, optical, thermal, optoelectronic and magnetic devices, such as Li/Na-ion batteries, electrocatalysis for oxygen evolution reaction, visible-light photocatalysis, p–n diodes, junction FETs, photodetectors, memory devices, etc.

5. Applications

5.1. Catalysis

Due to their unique structural and electronic properties, 2D layered group-VA nanomaterials have attracted much attention in photo-/electrocatalysis.⁴⁸ The high surface-to-volume ratios in 2D nanostructures endow them with abundant catalytically active sites exposed on their surface. The engineering of 3D bulk materials into a 2D format enhances the exposure of active edge sites, which is the origin of high catalytic activity. 2D few-layer Sb nanosheets (SbNSs) prepared from cathodic exfoliation acted as active 2D electrocatalysts for the reduction of CO₂ to formate with high efficiency.¹⁰⁴ High activity is due to the exposure of numerous catalytically active edge sites. The cathodic exfoliation was coupled with anodic exfoliation of graphite in a single-compartment cell for the in situ production of few-layer SbNSs and graphene composites. The increased activity of SbNS–graphene arose from the strong electronic coupling between graphene and SbNSs. As proven by Raman spectroscopy, a red shift in the E_g and A_1g peaks, especially the A_1g peak (7.8 cm⁻¹) was observed in SbNS–graphene. This red shift in the Raman spectrum was observed in other 2D composites,^356–359 which is attributed to n-type doping from graphene to other materials. In SbNS–graphene, the electrons from a lower work function of graphene (4.43 eV) tend to migrate across the Sb–graphene boundary to a higher work function of SbNS (4.55–4.70 eV),^104,360 which increases the electron density at the surface of SbNS. SbNS–graphene had a higher selectivity to formate at a lower overpotential compared with bulk Sb and SbNSs (Fig. 20a). At a potential of −0.96 V (over-potential of 0.87 V), SbNS–graphene delivered the maximal efficiency for its formation (88.5%). SbNS had the maximal efficiency of 84% at −1.06 V. The partial current density for formate from SbNS–graphene was larger than of bulk Sb and SbNSs (Fig. 20b). The current density of SbNS–graphene at −1.07 V was 1.5 (or 16) times higher than that of SbNSs (or bulk Sb). After 2D engineering of nanostructures and compositing with graphene, SbNSs were transferred from catalytically inactive materials into an active form. 2D mosaic Bi nanosheets were utilized for selective ambient electrocatalytic N₂ reduction.³⁶¹ The high electrocatalytic activity of Bi nanosheets is due to sufficient exposure of its edge sites coupled with effective p-orbital electron delocalization in mosaic Bi nanosheets. Their semiconducting feature limits surface electron accessibility and effectively enhances the faradaic efficiency.

Photocatalysis of 2D group-VA nanomaterials was reported. 2D square-like BiOI nanosheets with a thickness of ∼10 nm and exposed {001} facets were prepared by a hydrothermal route without surfactants and special solvents.¹¹⁰ By evaluating the photodegradation of rhodamine B, methyl orange and phenol under visible-light irradiation, the BiOI nanosheets had high photocatalytic performance, photostability and recyclability. Thin 2D square-shape nanosheets with exposed {001} facets were responsible for their visible-light driven photocatalytic activity. The unique nanostructure offered a suitable diffusion length and self-induced internal static electric field direction of BiOI, improving the separation efficiency of photoinduced electron–hole pairs in the BiOI nanosheets. Thin 2D nanosheets have a greater percentage of {001} facet exposure. A stronger internal static electric field is induced, with improved photocatalytic activity.^362,363 2D BiOBr was coupled with co-catalyst MoS₂via a hydrothermal process.¹⁷⁸ The photoactivity of the hybrid photocatalyst MoS₂/BiOBr was studied under irradiation of a 15 W energy-saving light bulb under ambient conditions using Reactive Black 5 (RB5) as the model dye solution (Fig. 20d–f). After 3 h of irradiation, the photodegradation of RB5 by BiOBr loaded with 0.2 wt% MoS₂ (MoBi-2) was 1.4- and 5.0-fold higher than that of the pristine BiOBr and TiO₂. This high photocatalytic performance resulted from the effective migration of the excited electrons from BiOBr to MoS₂, which could prolong the recombination rate of electron–hole pairs.

A layered architecture consisting of 2D corrugated [Sb₂O₂(OH)]⁺ layers with linear α,ω-alkanedisulfonate anions residing in the interlamellar space was used as a Lewis acid catalysst.¹⁰¹ The cationic material had chemical robustness under high acidic aqueous conditions (pH = 1). Upon combination of the robust nature and high density of Sb^III sites on the exposed crystal facets, this cationic layered-material was shown to be an efficient and recyclable catalyst for the cyanosilylation of benzaldehyde derivatives with trimethylsilyl cyanide. The Lewis acidity of the Sb^III sites catalyzed the ketalization of carbonyl groups under green solvent-free conditions. Based on the superior electronic properties of 2D group-VA nanomaterials, they are predicted to have high photocatalytic capability. Organic–inorganic bismuth halides have tunable electronic structures with potential as the enhanced 2D light-harvesting materials.¹⁰⁷ By pairing with MoS₂ or a quinodimethane complex, arsenene-based heterostructures formed type-II band alignments, satisfying the requirements of photocatalysts for photocatalytic water splitting.³⁰⁹ Photocatalysis of BP/arsenene LHSs with superior electronic properties was predicted.³¹³ By comparing band-edge positions with redox potentials, β-stacking arsenene/Ca(OH)₂ van der Waals heterostructure with strain-tunable electronic and photocatalytic properties is an excellent photocatalyst for water splitting.³⁰² Also, by comparing band-edge positions with the redox potentials of water, the arsenene/GaS van der Waals heterostructure with tunable electronic properties is a good photocatalyst for water splitting.²⁹⁸ Bi₄Ti₃O₁₂/BiOCl 2D/0D composites exhibited improved photocatalytic capacity for the degradation of the antibiotic tetracycline hydrochloride.¹⁸⁰ Their high photocatalytic capacity is due to the matched crystal structures, suitable energy band structures, and intimate contact interfaces among Bi₄Ti₃O₁₂ nanosheets, ultrafine BiOCl NPs and 2D/0D composite nanostructures. Z-scheme photocatalytic water splitting was conducted based on the 2D heterostructure of BP/bismuth vanadate (BiVO₄) using visible light.³⁶⁴ Their respective band structures with staggered alignment were used for effective charge separation, allowing the reduction and oxidation of water on BP and BiVO₄. Heterojunctions of 0D Bi nanodots/2D Bi₃NbO₇ nanosheets were used for the efficient visible light photocatalytic degradation of antibiotics.³⁶⁵ Semimetal Bi increased the visible light absorption of photocatalysts and promoted the molecular oxygen activation of Bi₃NbO₇, improving its photocatalytic performance for the degradation of ciprofloxacin.

5.2. Energy storage

2D group-VA nanocrystals have higher theoretical capacities than graphite, implying their potential as electrode candidates for Li- and Na-ion batteries.³⁶⁶ Compared to 3D crystals, 2D group-VA materials possess superior structural characteristics, high surface area and specific capacity. According to theoretical calculations, BP has ultrafast Li-ion diffusion and large capacity for applications in Li-ion batteries.^367–370 Li atoms formed strong bonding with P atoms, staying in the cationic state.³⁶⁷ At a high concentration of Li atoms, the phosphorene–Li complex becomes metallic and exhibits high electrical conductivity. The diffusion of Li atoms on phosphorene with a puckered honeycomb nanosheet is extremely anisotropic. Li-diffusion along the armchair direction is almost prohibited, but its diffusion along the zig-zag direction is energetically favorable. The diffusion barrier of Li atoms along the zig-zag direction was ∼0.08 eV lower in energy than other Li-battery anode materials, such as graphene (0.3 eV) and MoS₂ (0.28 eV).^371–374 This low energy barrier led to diffusivity of 10⁴ (10²) times faster than that in graphene (MoS₂). The average voltage of adsorbed Li-atoms is ∼2.9 V, which is suitable for phosphorene-based Li-ion batteries. Na-diffusion was anisotropic. The diffusion barrier along the zig-zag direction was predicted to be ∼0.04 eV.^375,376 Phosphorene was studied as an anode material for Li-/Na-ion batteries.^377–384 The few-layer phosphorene sandwiched between graphene layers was used as a high-capacity anode for Na-ion batteries.³⁷⁸ Hybrid phosphorene–graphene materials had a specific capacity of 2440 mA h g⁻¹ at a current density of 0.05 A g⁻¹. There was 83% capacity retention after 100 cycles at an operating potential of 0–1.5 V. The large capacity of this anode material is due to the intercalation of Na ions along the zig-zag direction of phosphorene and the formation of Na₃P alloys. Owing to their sandwiched structures, phosphorene–graphene hybrids with superior electronic properties are good anode materials for Na-ion batteries. BP-based hybrids were widely applied for energy storage, including Li-/Na-ion batteries, perovskite solar cells and supercapacitors.^{48,49,51,55,57,59}

A metallic buckled SbNS–graphene film exhibited a high rate capability, high volumetric capacity and good cycle performance for Na storage.⁷¹ At a current density of 0.1 mA cm⁻², the reversible volumetric capacity in the initial cycle reached a high value of 1226 mA h cm⁻³ for the film with a mass loading of 1.6 mg cm⁻², but it was almost stable at 650 mA h cm⁻³ after 50 cycles. The high flexibility of graphene relieved the stress of the notorious volume changes of metallic Sb. Metallic Sb nanosheets are applied in Na-ion batteries. Due to their puckered and buckled structural properties, 2D group-VA materials with low atomic packing factors are favorable for the accommodation of Li and Na atoms. The rich alloy phases of Li₃X and Na₃X (X: P, As, Sb, and Bi) give high capacities for Li-/Na-ion batteries. The high surface areas of 2D group-VA nanosheets endow a higher capacity and faster ion diffusion as anodes in Li-/Na-ion batteries. Wu et al. explored interconnected 2D carbon/Sb hybrids as advanced anodes for Na storage.¹⁰³ Multi-dimensional and multi-scale hybrid nanostructures promoted the electron-ion transport kinetics for electrode materials, and ensured the integrity of electrode structures upon cycling. The Sb-NDs ⊂ CNs electrode had high electronic performance for Na storage through reversibility, rate capability and cycle life studies. The reversible capacity of Sb-NDs ⊂ CNs did not perceptibly decay after 100 cycles at 0.1 A g⁻¹, and had a capacity retention of 94 wt% compared with the second cycle after 100 cycles.

Antimonene is an excellent anode material in Na-ion batteries due to its high theoretical capacity of 660 mA h g⁻¹ and enlarged surface active sites. The Na storage and sodiation/desodiation mechanisms of 2D few-layer antimonene (FLA) were explored.⁸⁵ FLA had anisotropic volume expansion along the a/b planes and reversible crystalline phase evolution (Sb NaSb Na₃Sb) during cycling (Fig. 21). Based on FPC, FLA had a small Na-ion diffusion barrier of 0.14 eV. FLA delivered a large capacity of 642 mA h g⁻¹ at 0.1C (1C = 660 mA g⁻¹) and a high rate capability of 429 mA h g⁻¹ at 5C. It retained a stable capacity of 620 mA g⁻¹ at 0.5C with 99.7% capacity retention from 10 to 150 cycles. Based on the theoretical capacity of 660 mA h g⁻¹ for Sb, the electronic use of Sb atoms for FLA reached up to 93.9% at a rate of 0.5C for over 150 cycles. The investigation of the Na storage mechanism boosted the applications of 2D FLA as an advanced large-capacity and long-life Na-ion battery material. Smooth and large 2D antimonene was produced with uniform and specific number of layers.⁷⁸ In experiments, the bandgap depending on antimonene thickness was finely tuned to 0.8–1.44 eV. Antimonene acted as a hole transport layer in perovskite solar cells (Fig. 21a–e), which resulted in a significant improvement in hole extraction and current (∼30%). This work paved the way for the widespread applications of the emerging 2D group-VA nanomaterials with superior electronic properties in optoelectronics. In addition to Na-ion batteries and perovskite solar cells, antimonene was also explored as a new 2D nanomaterial for supercapacitors.⁸¹ Antimonene served as an electrode material of supercapacitors. Antimonene improved the energy storage capabilities of carbon electrodes in cyclic voltammetry and galvanostatic charging. Antimonene presented a capacitance of 1578 F g⁻¹ and a high charging current density of 14 A g⁻¹. Antimonene is an excellent material for energy storage, and due to its competitive energy and power densities of 20 mW h kg⁻¹ and 4.8 kW kg⁻¹, antimonene-based systems have excellent charge storing and cycling capabilities.

According to theoretical predictions, 2D layered group-VA pnictogen materials and pnictogen-containing 2D hybrids have applications in photovoltaics. Arsenene-based heterostructures paired with MoS₂ or quinodimethane complex can form type-II band alignments and have a high power conversion efficiency of 20% for photovoltaic solar cells.³⁰⁹ Sb/GaAs van der Waals heterostructures led to the separation of carriers and a high optical absorption coefficient in the visible-light range, implying these heterostructures are candidates for application in optoelectronic devices (solar cells).²³⁷ Black arsenic–phosphorus (α-AsP) ML has a direct bandgap (1.54 eV) and a mobility of over 1.4 × 10⁴ cm² V⁻¹ s⁻¹, indicating that ML α-AsP is a superior donor for application in 2D solar cells. ML α-AsP may be the next material with broad applications in photovoltaic devices.³¹² An arsenene/C₃N van der Waals heterostructure, with a broad absorption range, remarkable visible light absorption and high light harvesting, was expected and functionalized as a photovoltaic component in solar cells.²³⁴ 2D Sb as a superior candidate as an anode material in Na-ion batteries has a specific capacity of 320 mA h g⁻¹, an open circuit voltage of 1.22 V and a small diffusion barrier of 0.114 eV. Its high capacity and superior Na diffusion properties demonstrate its promise for application in Na–air batteries and supercapacitors.³²⁸ Functionalized arsenene (AsH and AsF) MLs had obvious light absorption in the near-infrared and visible range of the solar spectrum, showing adsorption peaks in the range of 0.45–1.6 eV. This feature is attractive for light harvesting. Nontrivial QSH insulators (AsF, AsOH, and AsCH₃) are promising candidates for photovoltaics.³⁴⁸ Partially oxidized arsenene with a tunable direct bandgap has potential in photovoltaic devices.³⁵³ Quasi-2D CuBi nanosheet superconductors have tunable electronic properties and moderate electron–phonon coupling (λ = 0.5 and T_c ≈ 1 K). CuBi nanosheet intercalated with Li shows high ion diffusivity, which can act as a candidate material to boost the rate capacity of current electrodes in Li-ion batteries.³⁵¹

5.3. Field-effect transistors

2D group-VA semiconductor materials are used as the body terminals of field-effect transistors (FETs). FETs are indispensable building blocks of modern integrated circuits. Compared with graphene or MoS₂ as channel materials of FETs, BP has two main merits, high on/off ratio for effective switching and charge carrier mobility for fast operation. Phosphorene FETs were fabricated based on few-layer phosphorene.^44,385 Drain current modulation of FETs is in the order of 10⁵ at room temperature and is four orders of magnitude larger than that in graphene. Their charge carrier mobility reached 1000 cm² V⁻¹ s⁻¹, which is superior to that of silicon-based devices. Low on-state current and high sub-threshold swing were enhanced by optimizing gate dielectrics.³⁸⁵ Through the encapsulation of few-layer phosphorene between hexagonal boron nitride layers, a sandwiched heterostructure was prepared and used in FETs with ultra-clean interfaces.³⁸⁶ The device exhibited a mobility of 1350 cm² V⁻¹ s⁻¹ at room temperature and an on–off ratio exceeding 10⁵. The carrier mobility of BP is highly dependent on direction due to its structural anisotropy. A lower effective mass along the armchair direction induces a higher drive current at the same bias. A higher degree of anisotropy enhances the performance of p-type devices. The transport feature of devices depends on transport direction. The transport anisotropy is conducive to the on-state current improvement. There are some limitations in phosphorene FETs, which are attributed to their strong asymmetric ambipolar characteristics, high Schottky barrier and severe surface degradation.

To simplify circuit design and save layout area, ambipolar channel materials with both n- and p-type transports are used in complementary metal oxide semiconductor transistor logic circuits. Phosphorene FETs have ambipolar character, but ambipolar behavior is strongly asymmetric and is unfavorable for complementary logic devices. To balance ambipolar behavior, slow hole transport is improved through optimizing device structures, fabrication conditions and flake thickness. By in situ surface functionalization with Cs₂CO₃, a phosphorene FET was designed to achieve effective modulation of ambipolar behaviour.³⁸⁷ After coverage of 0.5 nm Cs₂CO₃, the on-current in electron regime was close to that in the hole regime, revealing a very symmetric and balanced ambipolar characteristic. With a thickness of Cs₂CO₃ larger than 10 nm, the electron mobility was distinctly improved, reaching 27 cm² V⁻¹ s⁻¹, indicating improved electron transport behavior. Differently from BP or phosphorene, other 2D group-VA materials such as buckled arsenene or antimonene are highly stable when exposed to ambient conditions. ML arsenene or antimonene with a wide range of bandgaps has potential for FET applications. The performance of sub-10 nm ML arsenene/antimonene metal oxide semiconductor FETs (MOSFETs) was predicted.^124,185 The electron mobility and hole mobility were 635 and 1700 cm² V⁻¹ s⁻¹ for As (630 and 1737 cm² V⁻¹ s⁻¹ for Sb), respectively.¹²⁴ Excellent performance is applicable for ultra-scaled devices in the sub-10 nm scale.

Chen et al. reported an ML-trilayer lateral heterostructure-based FET (Fig. 22a–d).³³⁷ A low tunneling barrier and Schottky barrier were obtained with trilayer antimonene electrodes compared with the promising 2D contact materials, graphene and metal aluminum. According to theoretical calculations to evaluate the device performance, the on/off ratio is 4.87 × 10⁸ (1.06 × 10⁶) with a gate length of 10 (5) nm. The lowest power supply voltage (V_dd = 0.76 V) at V_ds = 0.6 V to switch “on” and “off” is close to the requirement of 0.72 V. The on-current is enhanced and the on/off ratio is simultaneously increased via hydrogen passivation, offering a way to optimize device behaviors. High air stability, low off-current and high on/off current ratio make antimonene FETs (based on ML-trilayer lateral heterostructure) a superior candidate for low-power device applications. Co d and Sb sp orbitals have strong hybridization, resulting in chemical bonding between Co and antimonene at the interface.³²⁷ A high Schottky barrier was formed after contacting. Barrier height can be tuned by different Co/antimonene stacking patterns. Barrier height decreases with an increase in the layer number of antimonene. The results imply potential applications in spin diodes and FETs based on antimonene. 2D Bi₂Se₃ crystals with a high aspect ratio of 3500 were prepared via EDTA and Cl⁻ dual-assisted and seed-mediated growth at low temperature under reflux conditions.¹⁷⁷ The 2D Bi₂Se₃ crystals had a thickness of ∼10 nm and an edge length of ∼50 mm. Due to the large aspect ratio of the crystals and their decent room-temperature charge carrier mobility, an FET device was easily fabricated based on 2D Bi₂Se₃ crystals. Arsenene and antimonene MOSFETs based on ab initio quantum transport exhibited excellent device performance.¹⁸⁵ The low power and high performance of ML arsenene MOSFETs surpassed the Schottky barrier of ML MoS₂ FETs on the sub-10 nm scale, satisfying the requirements for high-quality FETs.³⁸⁸

5.4. Topological spintronic devices

Topological insulators (TIs), as new quantum states of matters, attract much interest due to bulk insulating gaps and topologically protected boundary states. 2D TIs are promising materials compared to 3D TIs for spin transport applications because the edge states of 2D TIs are more robust against back scattering than the surface states of 3D TIs. Due to weak SOC, graphene as the first predicted 2D TI with QSH effects suffers from deficiencies of small bulk gap and low-temperature operation. 2D group-VA monoelemental MLs (phosphorene, arsenene, antimonene and bismuthene) have strong SOC effects over graphene and hold TIs phases under suitable conditions. For few-layer BP, a normal-to-topological phase transition was detected under an applied electric field along the stacking direction.²⁷⁵ Four-layer phosphorene transformed from a normal insulator into a topological insulator at an electric field of 0.3 V Å⁻¹. Tunable topological behavior under an electric field induces QSH effects. By using an in-plane time-periodic laser field, phosphorene ML may possess Lifshitz transitions compared to topological insulating phases.³⁸⁹ Through FPC, arsenene and antimonene MLs become TIs under appropriate biaxial tensile strain larger than 11.7% (14.5%).^194,291 The buckled configuration of arsenene (antimonene) had the capability of enduring large tensile strain of up to 18.4% (18%). The maximum bulk gaps were gained at maximum strain. Arsenene and antimonene are promising candidate materials of 2D TIs to achieve QSH effects.

Flat honeycomb Sb or Bi ML grown on a ferromagnetic MnO₂ layer was predicted, combining large intrinsic QSH and anomalous Hall conductivity.³⁹⁰ Due to proximity effects, h-Sb and h-Bi sheets were magnetized. The Dirac points were split into different spin channels. There were intrinsic QSH states with large bandgaps of 228 meV for h-Sb and 941 meV for h-Bi. There were nearly quantized anomalous Hall states with a bandgap of 10 meV for h-Sb or h-Bi. This alternative practical method is efficient to obtain quantized intrinsic spin Hall states and anomalous Hall conductance states in a single material. TIs are obtained by chemical functionalization of arsenene, antimonene and bismuthene. 2D TIs BiX/SbX (X = H, F, Cl, and Br) MLs were predicted.³⁹¹ Their large bulk bandgaps (0.32–1.08 eV) are due to the strong spin–orbit interactions of Bi/Sb atoms. BiX MLs with honeycomb structures are stable at high temperature. Thus, 2D TIs BiX/SbX MLs with intriguing features are candidates to develop new quantum devices operating at room temperature. 2D TIs Bi/Sb/Pb bilayers with methyl-functionalization were predicted.³⁴⁹ Owing to the protected Dirac-type topological helical edge states, 2D TIs have suitable quantum spin Hall properties and a large nontrivial bulk gap of 0.9 eV for room-temperature applications. Antimonene oxide was used as a 2D TI with a bandgap of 117 meV.¹⁹² Upon decoration with H and doping of magnetic atoms, Sb(111) MLs had topological properties of quantum spin-quantum anomalous Hall insulators with a bandgap of 53 meV.³⁹² According to tight-binding models, hydrogenated Sb₂H ML coated on an LaFeO₃ substrate was used to study the topological properties of Sb₂H/LaFeO₃,³⁹³ and exotic quantum spin-quantum anomalous Hall states were observed. The bandgap opened up to 35 meV, which was enlarged by strain and electric fields. The freedom degree of carriers in heterostructures-based 2D TIs is tunable. Thus, 2D TIs have promising applications in electronics and spintronics.

Bismuthene with a bandgap of 0.8 eV is a good candidate as a high-temperature quantum spin Hall materials.⁹⁹ Bismuthene grown on an SiC (0001) substrate was used as the stabilizer of quasi-2D TIs to form large gaps. Due to the strong on-site SOC, the on-current and intrinsic switching speed were improved in bismuthene/SiC 2D TIs. Ultrathin Sb islands grown on Bi₂Te₂Se were fabricated to develop Sb few-layer films as 2D TI materials.¹⁶⁴ As predicted for 3–4 bilayer films, topological edge states emerged by 2D topological phase transition. Non-trivial phase transition and edge states were proven in epitaxial films based on FPC. The evolution of the topological surface states in Sb(111) ultra-thin films with 4–30 bilayers was studied.¹⁶⁰ With a decrease in thickness, inter-surface coupling degraded the spin polarization of TSS and opened new wavevector-dependent scattering channels to cause spin degenerate states. Märkl et al. reported the successful realization of α-antimonene and predicted engineering multiple topological phases in van der Waals nano-heterostructures.⁸⁶ Both the hexagonal β-form and rectangular α-form of antimonene were used to build islands on top of TI α-bismuthene. 2D TIs are topologically non-trivial materials in the quantum spin Hall class. Ning et al. provided unambiguous transport evidence for the topological 2D metallic surface states in thinner Bi nanoribbons.¹⁰⁹ The free-standing quasi-2D layers of Te₂Bi₃ crystals were researched as new TIs in spintronic applications.¹¹¹ Based on ab initio calculations, Wang et al. studied arsenene oxide (AsO) with high stability, flexibility and tunable SOC gaps (Fig. 22e–g).³⁵⁴ Compared with pristine and functionalized arsenene, the maximum nontrivial bandgap of AsO was 89 meV, which became 130 meV under biaxial strain. A quantum well was designed by sandwiching 2D AsO between BN sheets. The band topology of AsO was retained with a sizeable bandgap. AsO possesses fully oxidized surfaces that are stable against surface oxidization and degradation. Group-VA 2D sheets are superior 2D TIs with large bulk gaps and have potential applications in 2D quantum spin Hall devices.

5.5. Electronic devices

Unlike unstable BP, other 2D layered group-VA (As, Sb, and Bi) nanomaterials were predicted to show high stability and excellent physical properties. These unique merits were experimentally proven and have great prospects for use in advanced electronic devices. Ji et al. reported the van der Waals epitaxy of few-layer antimonene monocrystalline polygons prepared on various substrates.⁹³ The antimonene polygons had high electrical conductivity of up to 10⁴ S m⁻¹ and superior optical transparency in the visible-light range, which are suitable for transparent conductive electrode applications. Ultrathin and flat Bi nanosheets are beneficial for electronics and act as building blocks in catalysts, solar cells, batteries and electronic devices.¹⁷⁵ Superconductivity of ultrathin free-standing Bi films was detected on different layers of Bi film (Fig. 23a–d).⁷³ In the case of Bi(111) ultrathin films grown on the superconducting substrate NbSe₂, the pairing potential had an exponential decay with layer thickness. Topological edge states may coexist with superconductivity, making the Bi(111)/NbSe₂ heterostructure a superior platform for exploring the Majorana fermions. 2D group-VA nanosheets can serve as semiconductor materials due to their high charge-carrier mobility, tunable direct bandgaps and in-plane anisotropic structures. Liu et al. explored a new family of layered semiconducting materials, namely black As–phosphorus (b-AsP).⁷⁶ Electron transport measurements implied the semiconductor nature of b-AsP. Through infrared absorption studies and tuning chemical compositions (x) during synthesis, b-As_xP_1−x, as a layered anisotropic infrared semiconductor, exhibited tunable electronic and optical properties.

Chung et al. reported direct evidence of 2D high-temperature superconductivity in a single crystalline nanohybrid of organic-Bi cuprate.¹¹² The coordination compound of HgI₂(pyridine)₂ was intercalated into a single crystalline Bi₂–Sr₂CaCu₂O_y high-T_c super-conductor via an interlayer complexation reaction between pyridine molecules and Bi cuprate pre-intercalated with mercuric iodide. The superconductivity in organic intercalates of the Bi (2201) phase implied the 2D superconductivity of layered copper oxides. A distinct decrease in shielding fraction and broadening of the superconducting transition were observed after intercalation of HgI₂(pyridine)₂, which can be understood based on the enhanced fluctuation of 2D superconductors. Through FPC, the superconductivity in Li-intercalated bilayer arsenene and hole-doped ML arsenene was studied, which exhibited a T_c of 8.68 K with an isotropic Eliashberg function.³²¹ A small biaxial tensile strain (2%) improved the T_c to 11.22 K due to the increase in the DOS and phonon softening. The almost flat top valence band of arsenene is suitable for 2D high-temperature superconductivity. The strain is crucial to enhance the transition temperature (T_c). Buckled honeycomb and symmetric washboard structures of ML arsenene are stable in the freestanding form.²⁵⁰ ML arsenene as a nonmagnetic semiconductor in energy is suitable for electronic applications. Based on calculations, ML arsenene has two phases and semiconducting behavior. When ML arsenene is functionalized with two types of organic molecules (electrophilic acceptors and nucleophilic donors), the interfacial charge transfer between ML arsenene and the acceptor–donor molecules reduces the bandgap of arsenene and leads to p- and n-type semiconducting behaviors.²³⁹ These n- and p-type arsenene semiconductors show promising applications in electronic and optoelectronic devices, such as photodiodes.

Quintuple layers of Bi₂Te₃ are semiconductors with the localized shallow bands. Bilayer and trilayer nanosheets are metallic, because valence electrons cannot fully occupy valence bands.³⁵² A mixture of different nanosheets is responsible for the high electric conductivity of atomic thin films. Besides semiconductor and superconductivity materials, 2D group-VA materials can be used as other advanced electronic materials, such as infrared detectors,³¹⁰ metal electrodes,³¹¹ magnetic storage devices,²⁵⁶ and dissipationless transport devices.³⁴⁸ Lu et al. found the biaxial strain-tunable electronic properties of 2D As/Sb and h-BN/Sb van der Waals heterostructures with an indirect-to-direct gap transition.³¹⁰ The tunable bandgaps from 1 to 0 eV correspond to a spectrum range from near-infrared to mid-infrared wavelengths, implying the potential applications of antimonene-based heterostructures in infrared detectors and photoelectric devices. The sensitivity of the bandgaps of van der Waals heterojunctions to external strain shows applications in microelectronics, piezoelectric and biomaterials, such as flexible, wearable monitors of human body health.³⁹⁴ Theoretical studies revealed the superior interfacial properties of ML bismuthene–metal (Al, Ag, Au, Ir, Ti, and Pt) contacts, which provide guidance for the selection of metal electrodes in ML bismuthene devices.³¹¹ Through FPC, magnetic states were achieved for Ti, V, Cr, Mn and Fe doped-ML arsenene, showing uses in spintronics and magnetic storage devices.²⁵⁶ Nontrivial QSH insulators AsX (X = F, OH, and CH₃) MLs with obvious light absorption are candidates for room-temperature applications in dissipationless transport devices and photovoltaics.³⁴⁸ Similar to BP, other 2D layered group-VA (As, Sb, and Bi) nanomaterials will receive much attention in forthcoming studies and show significant uses in transparent electrodes, advanced electronic and optoelectronic devices.^{50,52,62,63,108}

5.6. Nonlinear photonics

2D group-VA monoelemental nanomaterials present tunable broadband nonlinear optical properties under laser pulses and show applications in nonlinear photonic devices.³⁹⁵ Few-layer antimonene (FLA) was prepared by LPE and decorated on a microfiber to design optical devices.^79,94 Considering the strong Kerr nonlinearity of FLA at 1550 nm, a nonlinear photonic device was fabricated via an evanescent field interaction scheme. The FLA-coated microfiber device operated as an all-optical Kerr switcher with an extinction ratio up to 12 dB. This device operated as a four-wave mixing-based wavelength converter with a conversion efficiency of 63 dB. The modulated high-speed signal at a radio frequency of 18 GHz could be converted with the wavelength conversion device. FLA had a high nonlinear refractive index of up to 10⁻⁵ cm² W⁻¹.⁸⁹ Because of their long-term stability in ambient conditions and high durability, 2D FLA-based nonlinear photonic devices (optical switchers, Kerr shutters, and beam shapers) can be used in next-generation, high-speed optical communication. Antimonene, as a new 2D group-VA material beyond phosphorene, was predicted to exhibit significant electronics and optical properties with improved stability. FLA has broadband nonlinear optical responses and potential as superior optical Kerr media with high stability.^89,92 The performance of FLA-based devices is superior to its unstable counterpart phosphorene. FLA serves as a nonlinear optical medium for high-speed optical switching and wavelength conversion in optical communication.^396,397

To employ the saturable absorber (SA) properties of FLA, an FLA-decorated microfiber was designed as an optical SA, allowing passive mode-locking and Q-switching operations at the telecommunication band.⁸⁷ The microfiber had ultra-short pulse generation and all-optical thresholding with long-term stability. According to open-aperture Z-scan laser measurements, FLA is stable and has broadband nonlinear optical responses. In the mode-locking regime, pulses centered at 1.55 μm were detected with a pulse width of ∼550 fs. The intracavity pulse energy was 60 picojoules. In the Q-switching regime, a tunable pulse repetition rate of 20–50 kHz was obtained. Thus, this microfiber can serve as an effective optical thresholder to suppress the noise of pulses in transmission systems. The signal-to-noise ratio of the transmitted signals was improved up to ∼10 dB. These results provide guidance for the applications of 2D group-V monoelemental materials in ultra-short pulse generation and all-optical thresholding with long-term stability. Zhang et al. verified the superior nonlinear absorption properties of β-antimonene sheets, which were studied based on Z-scan methods.⁸² Both saturated absorption and optical limiting were observed. Antimonene had intense saturated absorption and two-photon absorption properties.³⁹⁸ The nonlinear absorption at 1064 nm excitation is slightly better than that at 532 nm excitation. Antimonene is a promising candidate as an SA and optical limiting material, such as invisible infrared laser. Based on the saturable absorption feature of 2D nanomaterials, a passive Q-switched Nd³⁺ solid-state laser with antimonene as the SA was realized (Fig. 23g).⁸⁴ Upon 946 and 1064 nm laser emissions of Nd:YAG crystals, the Q-switched pulse widths were 209 and 129 ns and the peak powers were 1.48 and 1.77 W, respectively. Upon the 1342 nm laser emission of Nd:YVO₄ crystals, the Q-switched pulse width was 48 ns and the peak power was 28.17 W. Antimonene can be employed as a stable broadband optical modulating device for solid-state lasers and exhibits effective long-wavelength operations.^396,397

Few-layer bismuthene was prepared via sonochemical exfoliation, and its nonlinear optical response in the visible region was studied.⁹⁷ The nonlinear refractive index of bismuthene was ∼10⁻⁶ cm² W⁻¹, which was measured based on spatial self-phase modulation. Bismuthene has a direct energy bandgap at 1550 nm. Its saturable absorption properties were studied at the telecommunication band, showing an optical modulation depth of 2.03% and a saturable intensity of 30 MW cm⁻². Under the optimal laser parameters, a 652 nm femtosecond (fs) optical pulse centered at 1559 nm was generated. The results implied that bismuthene-based SA is an excellent material for application in ultrafast SA devices (Fig. 23e and f). Owing to the great potential of bismuthene in ultrafast photonics, future studies should focus on the exploration of efficient bismuthene-based photonic devices, such as optical modulators, optical switchers and detectors. Few-layer bismuthene with a thickness of 3 nm and a lateral size of 0.2 μm had a thickness-dependent energy gap from almost zero to 0.55 eV.⁹⁸ Considering its strong nonlinear refraction effects, all-optical switching of two laser beams was realized in the few-layer bismuthene based on spatial cross-phase modulation. The high all-optical switching implied that bismuthene-based 2D materials are candidates as all-optical switchers.^399,400 The semi-metallic and long-term stable properties of bismuthene make it a new nonlinear optical material for applications in infrared and mid-infrared optoelectronics, such as broadband detectors, nonlinear optical switchers and modulators.

5.7. Light-emitting devices

2D group-IVA materials (graphene, silicene, germanene, and stanene) are semimetallic owing to their lack of suitable bandgaps, restraining their use in light-emitting devices. 2D group-VA materials (phosphorene, arsenene, antimonene, and bismuthene) are semiconductors and have fundamental bandgaps, making them candidates as new materials in light-emitting devices. Phosphorene acts as an atomically thin 2D optical nanomaterial and has direct exciton emission. The emission wavelength of ML or few-layer phosphorene is tunable by controlling the number of stacking layers. Few-layer phosphorene exfoliated on a silicon substrate had intense layer-dependent photoluminescence (PL).⁴⁰¹ By measuring PL spectra, the strong PL emission peaks of phosphorene with 2–5 layers were located at 961, 1268, 1413 and 1558 nm, corresponding to the energy peaks of 1.29, 0.98, 0.88 and 0.80 eV, respectively. The PL emission peaks of few-layer phosphorene mainly originated from excitons, implying the lower bounds on its fundamental bandgaps. The key information provided in PL spectra promotes studies on the exciton features and electronic structures in phosphorene. The PL spectra of ML and bi- and tri-layer phosphorene were measured at 77 K under the unpolarized photoexcitation of 2.33 eV.⁴⁰² The PL peak energy matched well with the absorption spectra, verifying the direct bandgap of phosphorene. Defect state-derived near-infrared PL emission was detected in phosphorene.⁴⁰³ With a linear increase in excitation intensity, sublinear increase in PL intensity at 1240 nm was obtained, which verified the defect-based emission of phosphorene. The defect emission was much brighter than the exciton emission at low temperature, and the brightness of the defect emission was retained at room temperature. Bright room-temperature PL emission can serve as a light source for near-infrared optoelectronics.

The PL emission of 2D group-VA materials beyond phosphorene was demonstrated. Multilayer arsenene nanoribbons were prepared and had a PL emission peak of 540 nm at room temperature.⁷⁷ According to the green PL emission, the bandgap of the multilayer arsenene nanoribbons was calculated to be ∼2.3 eV. There are two main factors that cause bandgap opening. One is the quantum confinement effect caused by dimensionality reduction, and the other is turbostratic stacking. Multilayer antimonene nanoribbons uniformly distributed on InSb were prepared via a plasma-assisted process and had room-temperature orange PL (Fig. 24).⁹⁶ Notably, the bandgap opening was induced by the quantum confinement effect of the nanoribbon structures and turbostratic stacking of the antimonene layers, including various types of stacking, such as AA stacking.⁷⁷ Based on the orange PL emission at ∼610 nm, the bandgap of the multilayer antimonene nanoribbons was ∼2.03 eV. PL measurements suggested that 2D multilayer arsenene and antimonene nanoribbons with proper band structures have potential use in transistors and light-emitting diodes. Hussain et al. prepared free-standing ultrathin Bi nanosheets (BiNSs) with superior PL (Fig. 24a–d).⁷² The PL spectra of the BiNSs were measured at room temperature upon excitation of 325 nm and exhibited a sharp peak at ∼661 nm. This peak vanished upon 532 nm excitation since it came from the second harmonic generation of the 325 nm laser. The PL responses of large-area and high crystalline BiNSs may be due to the carrier confinement effect. The process-dependent crystal defect and dislocation caused the trapping of electrons and excitons in ultrathin BiNSs. Further theoretical and experimental studies are needed to study the PL emission of 2D group-VA elemental materials for their significant applications, especially in optical sensors, bioimaging, transistors, light-emitting diodes and optoelectronic devices.^{50,62,89,395,404–408}

5.8. Gas sensors

2D materials often have large specific surface areas, high surface activities and electrical conductivity because of quantum size effects. The absence of bandgaps for typical 2D materials, such as 2D group-IVA materials, restrains their use in semiconductor devices (gas sensors). 2D group-VA materials with moderate bandgaps of 0.36–2.62 eV have emerging and fascinating physical properties, rendering them superior candidates for gas detection.^47,409–411 The adsorption of CO, CO₂, NH₃, NO and NO₂ gas molecules on ML phosphorene was studied by FPC coupled with non-equilibrium Green's functions.²⁵⁸ Compared with graphene and MoS₂, phosphorene exhibits stronger adsorption of gas molecules with higher sensitivity and selectivity, implying its potential as an efficient gas sensor. Phosphorene is sensitive to N-containing gas molecules (NO and NO₂). There is a dramatic I–V relation change before and after gas adsorption on phosphorene. The adsorption of gas molecules on phosphorene has significant transport features and current/resistance changes. By theoretical calculations, the sensing of NO₂ gas occurred on few-layer phosphorene-based FETs.⁴⁰⁹ Through Raman spectroscopy measurements, there was no obvious change in the characteristic peaks before and after target gas adsorption, implying stable NO₂ adsorption on multilayer phosphorene. The limit of detection of the phosphorene-based gas sensor was estimated to be ∼5 ppb of NO₂, showing high sensitivity for gas detection. The conductance measurements revealed that the sensor had reversible adsorption and desorption of NO₂ gas and high recovery after flushing the sensor device with Ar.

Cho et al. studied the gas-sensing performance of phosphorene, graphene and MoS₂.⁴¹⁰ Through electrical sensing measurements, the sensitivity of phosphorene was determined to be ∼20 times higher than that of graphene and MoS₂. The excellent performance in response/recovery time, selectivity, molar response factor and adsorption verified that phosphorene is a superior gas sensing material. Besides NO₂, phosphorene was sensitive to methanol vapor.⁴¹¹ In the presence of other vapors, the phosphorene-based sensing device was selective for methanol and had long-term stability. Other 2D group-VA nanomaterials were predicted to be gas sensing materials, such as the typical buckled arsenene and antimonene.^329,333 Arsenene had high sensitivity to gas molecules.^257,331 Liu et al. studied the adsorption of CO, CO₂, N₂, NH₃, NO and NO₂ on ML arsenene.²⁵⁷ NO_x adsorbents had the largest charge transfer and greater changes in conductivity. The adsorption of NO_x led to a magnetic moment of 1 μ_B. Thus, arsenene is a candidate for NO and NO₂ gas sensing. Through FPC, some gases (N₂, CO₂, O₂, H₂O, and CO) were weakly adsorbed on antimonene. Other toxic air-pollution gases (NH₃, SO₂, and NO) can be strongly adsorbed on antimonene, with considerable adsorption energies and elevated charge transfers.³³⁵ This is ascribed to the contribution of the frontier orbitals of molecules closer to the Fermi level and apparent orbital hybridization. The moderate adsorption energies, appreciable charge transfer and physical adsorption features of antimonene make a potential pollutant gas sensor to detect NH₃, SO₂ and NO gases. Antimonene enabled the adsorption and desorption of these gases easily. Antimonene with NO₂ activated and chemisorbed can serve as a disposable gas sensor or metal-free catalyst to detect and catalyze NO₂ gas.

The sensitivity of CO to antimonene was evaluated by FPC.³³² CO adsorption on pristine antimonene is physical adsorption and it is converted to chemical adsorption after its doping with atoms. An external electric field varying from −0.5 to 0.21 eV Å⁻¹ improves the sensitivity of CO on doped antimonene, which is helpful to realize CO gas sensing at room temperature. The adsorption and desorption of CO molecules on doped antimonene can be controlled via an external electric field, which is more suitable for gas collection, storage and sensing compared to pristine antimonene. Different types of doped antimonene (Si–Sb, Al–Sb and Co–Sb nanosheets) were explored as potential sensing materials for CO detection. NO₂ or SO₂ gas had chemisorption on B-doped arsenene and had physisorption on pristine or N-doped arsenene with moderate adsorption energies (Fig. 25a–e).³³³ When two gas molecules were adsorbed on pristine or doped arsenene, a positive change in electronic properties was detected through the density of states (DOS) analysis. According to the I–V characteristic curves, the conductivity of pristine (N-doped) arsenene was enhanced obviously after NO₂ (SO₂) adsorption due to the increase in hole-carriers. N-doped arsenene was applied as a new material for SO₂ gas sensing. Pristine arsenene has potential application in NO₂ gas sensing, showing high sensitivity.

Kistanov et al. studied the interactions of antimonene with small molecules (CO, NO, NO₂, H₂O, O₂, NH₃, and H₂).³³⁴ NO, NO₂, H₂O, O₂ and NH₃ served as charge acceptors, and CO had negligible charge transfer. H₂ acted as the charge donor to antimonene, with 10 times higher charge transfer than H₂ on phosphorene. The interaction of O₂ with antimonene was much stronger than that with phosphorene. Pristine antimonene may suffer from oxidation in ambient conditions especially at elevated temperature. The kinetic barrier for the splitting of O₂ molecules on antimonene is low (∼0.4 eV). Different from the donor role of H₂O in phosphorene, the acceptor role of H₂O on antimonene suppresses the structure degradation of oxidized antimonene by preventing proton transfer between the H₂O molecule and O₂ species to yield acids. To achieve high environmental stability, the acceptor role of H₂O can avoid the structural decomposition of 2D layered materials. The surface oxidation layer of antimonene acts as an efficient passivation layer from the degradation of underlying layers, similar to other 2D layered materials, such as BP and graphene.^{41–63,412–414} Antimonene layers are separated and protected by noncovalent modification with O₂ and environmental molecules. Surface functionalized antimonene with high stability and antioxidant ability demonstrates promising applications in catalysis, storage and gas sensors.

5.9. Thermoelectric materials

As functional materials, thermoelectric materials convert waste heat directly into electrical energy. The performances of these materials are quantified by a dimensionless figure of merit, ZT = S²σT/k, where S is the Seebeck coefficient, σ is electrical conductivity, T is temperature and k = k_e + k_p is total thermal conductance, which can be split into electron (k_e) and phonon (k_p) contributions.⁴¹⁵ Materials with a ZT larger than 3 are suitable for thermoelectric generation.⁴¹⁶ In contrast to bulk crystals, low-dimensional materials possess potential to gain a higher ZT via different engineering approaches.^417,418 Based on theoretical studies, 2D group-VA materials present great potential in thermoelectric applications.^{216,218,299,419–421} Owing to their anisotropic puckered structures, group-VA crystals have strong anisotropy in electrical and thermal conductivities. The ZT value along the armchair direction is much larger than that along the zig-zag direction.^{420,422–424} By p-/n-doping and orthogonal electric fields, the thermoelectric performance of phosphorene was predicted and improved. The figure of merit ZT reached up to 1.5 along the armchair direction at room temperature.²²⁵ The influence of applied strain on the thermoelectric effect of BP was studied.^419,421,425 Upon 5% applied strain along the zig-zag direction of phosphorene, the Seebeck coefficient and electrical conductivity were improved dramatically. Upon 8% applied strain along the armchair direction of phosphorene, the ZT was 2.12 at room temperature.⁴²¹ High thermoelectric properties of phosphorene were achieved by cutting phosphorene along the armchair and zig-zag directions to form engineered nanostructures.⁴²⁶ The ZT of phosphorene nanoribbons with armchair edges was optimized to be 6.4 at room temperature, indicating their excellent thermoelectric use.

Sandonas et al. reported the thermoelectric properties of puckered phosphorene and arsenene.⁴²⁰ Puckered arsenene had stronger anisotropic thermoelectric responses than phosphorene. Arsenene had moderate n-type doping at 300 K. The ZT of arsenene reached up to 1.0 along the armchair direction. Buckled antimonene had a relatively low thermal conductivity, which was further lowed by chemical functionalization.²¹⁶ Antimonene may be a potential and excellent thermoelectric material. Cheng et al. investigated the figure of merit, ZT, of buckled and puckered bismuthene at different temperatures.^209,217 Based on FPC combined with the Boltzmann transport equation, buckled bismuthene had much larger ZT than its bulk structure.²⁰⁹ Large power factors (S²σ) and low thermal conductivity are responsible for the high thermoelectric performance of buckled bismuthene. The ZT of buckled bismuthene reached 2.4 at room temperature and 4.1 at 500 K. Based on theoretical predictions, puckered bismuthene had a high ZT of 6.4 for n-type systems at room temperature.²¹⁷ZT distinctly exceeded that of buckled bismuthene with a ZT of 2.4.²⁰⁹ The ZT of the buckled bismuthene was higher than 2.0 in a broad region of temperature and carrier concentration. Puckered bismuthene with low deformation potential constants and excellent thermoelectric features is inherently relative to weak electron-phonon coupling strength.

Sun et al. investigated the thermoelectric transport properties of arsenene featuring puckered and buckled structures.²²¹ The two types of arsenene as indirect bandgap semiconductors had high charge carrier mobilities in the range of 40–800 cm² V⁻¹ s⁻¹. The puckered arsenene had low and anisotropic lattice thermal conductivities of 9.6 W m⁻¹ K⁻¹ in the armchair direction and 30.7 W m⁻¹ K⁻¹ in the zig-zag direction. The preferential thermal transport direction is orthogonal to electrical transport direction, which enhances the thermoelectric figure of merit in the armchair direction to 0.7 for p-doping and 1.6 for n-doping at room temperature. According to FPC, puckered arsenene is a good thermoelectric material. Sharma et al. studied the thermoelectric properties of arsenene and antimonene (Fig. 26a and b).²²⁰ Both materials had large bandgaps and low lattice thermal conductivities, resulting in large Seebeck coefficients. Compared with arsenene, antimonene has smaller phonon frequencies and group velocities at room temperature, causing sensitive thermoelectric responses. A ZT of up to 0.58 was achieved through moderate n-type doping of ∼1013 cm⁻². The room-temperature ZT of ML bismuth was calculated to be ∼2.1 for n-type doping and ∼2.4 for p-type doping (Fig. 26c).²⁰⁹ The maximal ZT of 4.1 was achieved at 500 K. For the distorted bismuth (110) layer, the maximum ZT of 6.4 was achieved for n-type systems, stemming from the weak scattering of electrons.²¹⁷ The distorted Bi layer maintained a high ZT in a wide temperature and carrier concentration range. Deformation potential constant with electron–phonon scattering strength is the paradigm in the search for high-performance thermoelectric materials. The buckled antimonene had a ZT of 2.15 at room temperature.²²³ After simple biaxial strain engineering, the ZT increased to 2.9 under 3% tensile strain (Fig. 26d), which is attributed to its tunable electronic structures and reduced thermal conductance. Recently, fabricated buckled antimonene was stable in ambient conditions, making it a new candidate for application in thermoelectric devices. After engineering modification, 2D group-VA materials exhibit great potential for promising thermoelectric applications.^59,219

5.10. Biomedical applications

2D layered group-VA nanomaterials have great prospects in biomedical applications, such as biosensing,^42,427,428 bioimaging,⁴²⁹ drug delivery and release,^430,431 photodynamic theranostics,⁴³² and photothermal therapy.⁴³³ The non-toxic BP is electrocatalytic for some reactions and can act as an ideal material for biosensing.⁴² An electrochemical myoglobin biosensor was designed via the functionalization of BP nanosheets with poly-L-lysine and a myoglobin-specific aptamer.⁴³⁴ The BP-based complex was immobilized on a screen-printed electrode and the detection of myoglobin was performed via Fe²⁺/Fe³⁺ chemistry reaction. A field-effect transistor was fabricated using few-sheet BP,⁴³⁵ which could detect IgG via anti-IgG linked to Au NPs placed on the top of BP nanosheets. BP has a wide electrochemical cathodic window because it is not highly electrocatalytic for hydrogen evolution. Nevertheless, its anodic window is somewhat limited due to the electrochemical oxidation of BP itself, ∼0.6 V (vs. Ag/AgCl) at neutral pH.⁴³⁶ BP shows significant anisotropic electrochemical properties, with much faster heterogeneous electron transfer at its edge sites compared with its basal plane, similar to that of graphite and MoS₂.⁴² This feature is conducive to the sensing of electroactive biomolecules at the edge plane sites of BP monocrystal electrodes.

Due to the intrinsic unique properties of BP, including negligible elemental cytotoxicity, high drug-loading potential, long blood circulation time and specific clearance pathways, in recent years BP has been considered as a promising nanoplatform for various biomedical applications, such as bioimaging, phototherapy, drug delivery, combination therapy and theranostics.⁴³⁷ Zhao et al. explored Nile blue dye-modified BP nanosheets for near-infrared imaging-guided photothermal therapy.⁴³³ Because the lack of air and water stability may hinder the biomedical applications of BP, the covalent functionalization strategy was expected to enhance its stability and biocompatibility, also resulting in near-infrared fluorescence. Under the irradiation of an 808 nm laser, the dye-modified BP showed strong photothermal therapy and near-infrared imaging in MCF7 breast tumor-bearing nude mice. Besides, the photodynamic theranostics and drug release of BP have been developed towards cancer therapy in vivo,^{429,430,432,435} similar to other 2D nanomaterials such as 2D boron nanosheets.⁴³⁸ Besides BP, other 2D layered group-VA nanomaterials have been explored for biomedical applications. Recently, Xue et al. reported a surface plasmon resonance (SPR) sensor based on 2D antimonene for the specific label-free detection of clinically relevant biomarkers miRNA-21 and miRNA-155.⁴²⁷ The high sensitivity of this SPR sensor depends on the strong interactions between antimonene and single-stranded DNA, and the enhanced coupling between localized-SPR of gold nanorods (AuNRs) and propagating-SPR of a gold film (Fig. 27a). This biosensor is the first report that uses antimonene for clinically relevant nucleic acid detection, constituting an extraordinary opportunity to develop lab-on-chip platforms.

Tao et al. developed a photonic drug-delivery platform based on 2D PEGylated antimonene nanosheets that had multiple advantages,⁴³⁹ including excellent photothermal, high drug-loading, spatiotemporally controllable drug release triggered by near-infrared light and moderate acidic pH, high accumulation at tumor sites, deep tumor penetration by both extrinsic NIR light and intrinsic pH stimulus, multimodal-imaging and inhibition of tumor growth without side effects and potential degradability, which addressed several key limitations of cancer nanomedicine (Fig. 27b). Deep insights on intracellular actions improve the cellular-level understanding of antimonene-based nanosheets and other emerging 2D nanomaterials. This work first reported 2D antimonene-based photonic drug delivery platforms, probably marking an exciting jumping-off point for studies into the applications of 2D antimonene nanomaterials in cancer theranostics. Currently, the biomedical applications of 2D layered group-VA pnictogen materials and pnictogen-containing 2D hybrids are still rarely reported, such as some relative studies on antimonene.^427,439 The forthcoming biomedical studies of 2D layered group-VA pnictogen materials are rationally driven by the explosive development of their cousin BP or other 2D materials such as graphene, TMDs, MXenes, and MOFs.

In recent years, the ultralow and non-toxic properties of BP have facilitated its explosive development in biomedical applications. Among the 2D layered group-VA (P, As, Sb, and Bi) monoelemental nanomaterials (phosphorene, arsenene, antimonene, and bismuthene) and their 2D hybrids, the non-toxic element (P, Sb, and Bi)-based phosphorene, antimonene and bismuthene demonstrate great prospects for biosensing, bioimaging, drug release, photodynamic theranostics and photothermal therapy.^427–439 Nevertheless, the elemental toxicity of As may hinder the experimental exploration of arsenene and its consequent biomedical applications.³⁷ A plasma-assisted process can be used to prepare multilayer or few-layer arsenene on solid substrates such as InAs. The prepared arsenene can be modified with low-toxic molecules and materials to construct various functionalized nanostructures, which endow arsenene with high stability, biocompatibility and multi-functions for biomedical applications. The long-term stability of 2D group-VA nanomaterials was evaluated in previous reports.^{430,431,433,439} BP nanosheets had almost no cytotoxicity to 4T1, HeLa, L929 and A549 cells even at a concentration of 200 μg mL⁻¹. After in vivo toxicity investigation on healthy Sprague-Dawley rats, no obvious change in liver and kidney functions was detected after the injection of BP for 7 d.⁴³⁰ Tao et al. conducted a long-term biodistribution study via inductively coupled plasma mass spectrometric analysis of antimony in different organs.⁴³⁹ The antimony levels in major organs showed a distinct trend of persistent decrease. Antimonene-based nanosheets were barely detectable after intravenous injection for 30 d. These results indicated the clearance of the nanosheets from the mouse body and may represent complete metabolic degradation. This potential degradability makes them quite promising for applications in cancer theranostics.

6. Conclusion and perspective

Different from semimetallic group-IVA and metallic group-IIIA materials, 2D layered group-VA monoelemental nanomaterials are semiconductors with a broad range of fundamental bandgaps that can be tuned from 0.36 to 2.62 eV, which give them great potential as promising candidates for application in advanced nanodevices. The recent significant advances in emerging 2D group-VA materials have aroused numerous scientists to accelerate the discovery of new 2D materials that have many intriguing physiochemical properties, functional structures and important applications. Compared with phosphorene and BP, 2D group-VA pnictogen monoelemental (As, Sb and Bi) nanomaterials possess superior stability, tunable direct bandgaps, charge-carrier mobility and unique in-plane anisotropic structures, making them superior candidates for efficient applications in extensive research areas. This present review performed a timely and comprehensive overview of the principal theoretical and experimental studies on 2D group-VA nanomaterials beyond BP reported in recent years, focusing on their general synthetic methods, fundamental properties, functionalized nanostructures and potential applications.

The synthetic methods for 2D group-VA materials were divided into top-down and bottom-up methods. The top-down methods include mechanical, ultrasonic, electrochemical exfoliation, plasma-assisted process, and hot-pressing method. The bottom-up methods mainly mention molecular beam epitaxy, van der Waals epitaxy, chemical vapor deposition, solvothermal, hydrothermal synthesis, high-temperature melting, etc. Different methods have particular merits towards various research objectives. The large-scale preparation of 2D group-VA materials with high stability against oxidation and degradation is especially crucial for enhancing their physiochemical properties and versatile practical applications. The combination of different methods is considered an effective strategy to prepare high-quality 2D group-VA materials. For the top-down methods, mechanical and electrochemical exfoliations can yield multilayer nanosheets from bulk crystals or powder. Then, ultrasonic exfoliation efficiently transfers multilayer nanosheets into few- or single-layer nanosheets. In the case of the bottom-up methods, molecular beam epitaxy, van der Waals epitaxy and chemical vapor deposition are suitable for the preparation of 2D group-VA pnictogen monoelemental materials, while pnictogen-containing hybrids are often prepared from solvothermal and hydrothermal synthesis and high-temperature melting. The combination of two bottom-up methods can promote the realization of new 2D group-VA pnictogen hybrids or new heterostructures with unique structures and enhanced properties. The assistance of organic ligands during the preparation process can improve the stability of the products. Thus, future studies should focus on the exploration and improvement of the synthetic methods, mainly considering low-cost, environmentally friendly raw materials, facile manipulation, low-energy-consuming process and high productivity.

According to theoretical predictions and experimental studies, 2D group-VA materials belong to semiconductors and have tunable direct bandgaps, unique in-plane anisotropy and superior carrier transport capability, facilitating their promising applications in advanced electronic and optoelectronic devices. Great efforts have been devoted to the investigations of the mechanical, thermal, optical, magnetic and electronic properties of 2D group-VA materials. These fundamental properties are mentioned, but some challenges still remain. Thus, forthcoming studies need to elucidate the electron structure-related mechanisms to improve the present properties of 2D group-VA materials and extend these properties to new research fields. The combination of theoretical models and practical experiments is an indispensable way to explore new 2D group-VA nanostructures, which are expected to gain novel and superior properties. In the case of 2D group-VA pnictogen materials, some significant properties are still hardly reported, such as photothermal effects and upconversion luminescence properties. Similar to phosphorene and BP,^53,54,58,440 these nanomaterials have a wide optical absorption covering the visible-light to infrared regions, implying their excellent thermoelectric properties. They are expected to exhibit photothermal properties for phototherapy. These nanomaterials possess tunable broadband nonlinear optical absorption for application in nonlinear photonic devices and down-conversion PL for light-emission devices.^56,395,398 These results imply the potential upconversion luminescence of 2D group-VA pnictogen nanomaterials for bioimaging applications, which need to be further verified in future studies. The forthcoming studies will be rationally driven by the explosive development of other 2D materials, such as BP, graphene, TMDs, MXenes, and MOFs.

2D group-VA nanomaterials can be further modified to fabricate various functional nanostructures for a wide range of applications. Functional nanostructures mainly involve hybrid heterostructures, doping of atoms and molecules, surface functionalization of 2D group-VA nanomaterials and pnictogen-containing 2D hybrids. Notably, the synergistic interactions between nanomaterials and other 2D or low-dimensional nanomaterials can result in novel and improved physiochemical properties in the resultant hybrid materials. Functional nanostructures consisting of 2D group-VA nanomaterials and zero-/one-dimensional (0D/1D) materials are barely explored. Based on the synergistic enhancement effects between typical 2D graphene (or counterparts) and zero-dimensional (0D) noble nanoclusters (or quantum dots), the electronic conductivity of 2D–0D hybrids can be improved efficiently, which have high capability for use in sensors and intelligent electronic devices.^441–446 In forthcoming studies, various novel and functionalized nanostructures, such as 2D–0D, 2D–1D heterostructures and hybrids, will be smartly designed and fabricated based on group-VA nanomaterials. These newly developed heterostructures and hybrids are expected to exhibit novel and synergistically enhanced physiochemical properties for use in advanced nanodevices and intelligent systems.

After functional modification, 2D group-VA pnictogen materials-based systems have a broad range of significant applications involving popular research fields, such as catalysis, energy storage, field-effect transistors, topological spintronic devices, electronic devices, nonlinear photonics, light emitting devices, gas sensors, thermoelectric materials and biomedicine. These applications are explored based on both theoretical calculations and experimental studies, but the practical performance stabilities and device efficiencies of these systems are still required to be further improved when compared with typical 2D nanomaterials-based systems and commercialized nanodevices. The superior properties of 2D group-VA nanomaterials can be realized by improving their synthetic methods and functional structures. Thus, future studies should focus on rational design schemes and optimized fabrication procedures for advanced functional systems with the use of 2D group-VA pnictogen nanomaterials. The improved systems and devices with high-performance stability and efficiency will receive promising applications in current fields. They will find new opportunities for applications in other extended and significant fields, including phototherapy, biomedical imaging, electrochemical sensing, flexible and wearable devices, intelligent electronics, and versatile optoelectronics.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Shandong Province, China (no. ZR2019MB026); and the Source Innovation Plan Application Basic Research Project of Qingdao, China (no. 18-2-2-26 jch, 17-1-1-72-jch).

Notes and references

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Footnote

† The two authors have the equivalent contribution to this work.

This journal is © The Royal Society of Chemistry 2019