1. Introduction

Two-dimensional (2D) materials have attracted much attention due to their ultrathin atomic layer thickness, tunable band structures, super large specific surface area, good mechanical flexibility, and rich physical and chemical properties. In the last decade, great progress has been made in the application of 2D materials in the next generation of electronics, optoelectronics^[1–3], sensors^[4–6], energy storage device^[7], and flexible electronics^[8]. As an allotrope of carbon, graphene is the earliest-found and most popular 2D material (Figs. 1(a)–1(c)). It has excellent electrical, thermal, optical, and mechanical properties, and therefore has wide application in the fields of information, energy, medical treatment, military, and so on^[9]. However, as a leading member of the 2D material family, single-layer graphene has a zero-optical bandgap, which greatly limits its applications in semiconductor logic devices, storage, and photocatalysis. Moreover, the monolayer graphene only absorbs 2.3% of incident light power. Therefore, researchers have turned their attention to other 2D nanomaterials, such as black phosphorus^[10–12] and transition metal chalcogenides (TMDCs)^[13], and so on. Black phosphorus (Fig. 1(d)) has high carrier mobility (~1000 cm²/(V·s) at room temperature) and anisotropic mechanical and electrical transport properties; however, its environmental instability is challenging for its practical application in air. TMDCs (Fig. 1(e)) represent a large family with a chemical formula of MX₂, where M is a transition metal atom and X is a chalcogen atom. Most TMDCs with atomically thickness exhibit direct bandgap, strong spin-orbit coupling, fairly high mobility, and favorable electronic and optical properties. Following the research upsurge of graphene, black phosphorous, and transition metal dichalcogenides, multiple-element layered 2D materials have become a new research focus due to their more abundant structure and more diversified characteristics. Therefore, it is interesting and significant to find ternary or even multi-element 2D materials and explore their possibility in future applications.

Recently, Bi₂O₂Se, as a ternary layered Bi-based oxychalcogenide material semiconductor material, has attracted much attention. Bi₂O₂Se has a modest bandgap of 0.8 eV, high mobility (1.9 K, 29 000 cm²/(V·s), room temperature, 450 cm²/(V·s)) and good thermal and chemical stability^[11]. Bi₂O₂Se possesses a tetragonal crystal structure, with $[{\rm{Bi}}_2{\rm{O}}_2]_n^{2n+}$ and $[{\rm{Se}}]_n^{2n-}$ layer stacking alternatively. Due to its interlayered zipper-like structure, it exhibits unique characteristics^[16]. In contrast to other van der Waals 2D layered materials, the layers of Bi₂O₂Se are connected by relatively strong electrostatic forces. Therefore, special exfoliation method or growth strategy is developed to fabricate large-area and atomically thick Bi₂O₂Se thin films. In view of these stable, unique, and excellent properties, Bi₂O₂Se can be used for promising applications in integrated circuits^[17–19], optoelectronics^[20–22], thermoelectrics^[23], neuromorphic computing^{[24, 25]}, and so on.

In this review, we will briefly summarize the recent research progress of Bi₂O₂Se. First, the preparation methods, optical, mechanical, and thermoelectric properties are summarized. Then the applications as photodetectors (transistors), energy storage devices, memristors, optical switches, and biomedical devices are then elaborated. Finally, the existing open questions and prospects of Bi₂O₂Se are presented.

2. Preparation methods

A controllable preparation process of large-area and high-quality 2D Bi₂O₂Se thin films is the prerequisite for the application of electronic devices. Generally, the preparation methods of 2D materials are mainly divided into two categories: the top-down method and the bottom-up method. The top-down method is to obtain 2D thin films by peeling from bulk materials by chemical or mechanical means, including mechanical cleavage method, electrochemical Li-intercalation and exfoliation, Li-intercalation and exfoliation with n-butyllithium, and so on^[26–30]. The production of TMDCs monolayers is usually achieved by micromechanical exfoliation of large crystals from top to bottom^[9]. In theory, the mechanical exfoliation method can obtain thin films with high-quality, high-purity, and uniform thickness, which can be used to prepare electronic devices. However, this method has low repeatability and cannot effectively control the thickness and size of thin films. To improve reproducibility and controllability, a bottom-up method is needed to synthesize high-quality and large-area 2D single-crystal thin films. So far, many methods have been used to prepare more stable and high-quality Bi₂O₂Se single crystal, such as chemical vapor deposition (CVD)^{[11, 31–36]}, modified Bridgman method^{[22, 37]}, wet chemical synthesis, hydrothermal reactions^{[27, 38]}, and so on. The bottom-up method can be used to grow Bi₂O₂Se single crystal directly on the target substrate by precisely controlling the growth conditions. Therefore it has more freedom to optimize the growth and improve the film quality. Table 1 summarizes the preparation methods, growth conditions, and basic film characteristics for Bi₂O₂Se reported in the literature.

2.1 CVD growth

Chemical vapor deposition (CVD) is a common bottom-up method to fabricate 2D materials, in which a variety of growth parameters, such as heating temperature, gas pressure, precursor concentration, gas flow rate and so on, are needed to be optimized to realize the growth of atomically thin 2D film^[39]. In this process, gaseous compounds are formed first and transported by inert gas to the target substrate, as shown in Fig. 2(a). High-quality Bi₂O₂Se single crystal thin films with adjustable size, controllable thickness, and excellent electronic properties can be prepared by CVD method^{[11, 20, 40–42]}. In the early stages, Wu et al. and Li et al. synthesized 2D Bi₂O₂Se crystals with large area, high mobility, and thin film on mica ([KMg₃(AlSi₃O₁₀)F₂]) using Bi₂O₃ and Bi₂Se₃ as precursors as shown in Fig. 2(b)^{[31, 41]}. It has been confirmed that the nucleation sites, thickness, domain sizes, and crystal phase transition of Bi₂O₂Se thin films can be well controlled by adjusting growth conditions (Figs. 2(c)–2(f)). For example, by controlling the temperature, planar and vertical Bi₂O₂Se nanosheets can be synthesized as shown in Figs. 2(g) and 2(h). At lower temperatures, a kinetics-dominated growth was met. Precursors tend to get rid of the restriction of the substrate and grow into the vertical direction. At higher growth temperatures, only Bi₂O₂Se with the lateral layout was grown, which is a thermodynamics-controlled growth process due to higher migration rate of adatoms^{[41, 43]}.

Carrier concentration is one of the most important parameters for semiconductor materials. In field effect transistors, the low carrier concentration of the channel can induce good gating, which is very important for reducing the operating voltage to manufacture low-power digital devices. However, when using Bi₂O₃ and Bi₂Se₃ as precursors, Bi₂Se₃ is firstly decomposed according to Bi₂Se₃(s) = 2BiSe(g) + $\frac{1}{2}$ Se₂(g), and the decomposition may change greatly after prolonged heating, which will adversely affect the synthesis of 2D Bi₂O₂Se crystals, resulting in higher residual carrier concentration.

To eliminate the possible side reactions, defects, or vacancies, Wu et al. synthesized Bi₂O₂Se crystals using Se element and Bi₂O₃ powder as precursors through a two-temperature zone heating system. The fabricated Bi₂O₂Se film has an ultralow residual carrier concentration of 10¹⁶ cm^–3 and high Hall carrier mobility up to 410 cm²/(V·s) at room temperature (Fig. 2(i))^[32]. Compared with the complex and changeable decomposition reactions of Bi₂Se₃, Se elements mainly volatilize into single Se₂ molecules. The dual heating zone system can change the relative partial pressure of Se and Bi precursors by controlling the heating temperature of Se and Bi₂O₃ sources separately. Finally, by optimizing the growth conditions, the defects or vacancies that lead to the n-type conductivity of Bi₂O₂Se can be greatly reduced, thus reducing the carrier concentration by 2–3 orders of magnitude. In previous reports, the size of the synthesized Bi₂O₂Se flakes is mostly at the micron scale. Wu et al. developed a new self-limiting vapor-solid (VS) deposition method to achieve the growth of millimeter 2D Bi₂O₂Se thin films on a mica substrate, in which Bi₂O₂Se powder as the sole growth source was put in a tubular furnace under normal pressure as shown in Figs. 2(j) and 2(k), and the Bi₂O₂Se powder can be synthesized by hydrothermal method or CVD method^[20].

Because the Bi-O layers in Bi₂O₂Se are structurally compatible with many perovskite oxides, and it exhibits rich and interesting physical properties (e.g, ferroelectricity, magnetism, thermoelectricity, etc.). Tan et al. synthesized Bi₂O₂Se on perovskite oxide by the CVD method and studied the growth process as shown in Fig. 2(l), which provides an alternative platform for studying new physical phenomena of oxide heterostructures^[19].

Due to the good lattice matching and good thermal stability, mica substrate is commonly used in the CVD growth of 2D Bi₂O₂Se. However, thanks to the strong binding force between mica substrate and Bi₂O₂Se thin films, it is difficult to transfer Bi₂O₂Se thin films to the new substrate to prepare the devices for the following measurement. In the application of 2D Bi₂O₂Se to electronic devices, SiO₂/Si substrate has usually been used as a support for 2D materials. In transmission electron microscopy (TEM) characterization, Bi₂O₂Se needs to be transferred to a copper grid. It also requires a safety transfer method that could minimize the damage to Bi₂O₂Se as small as possible and at the same time is convenient to manipulate. Fu et al. used Raman spectroscopy to demonstrate the inevitable damage to Bi₂O₂Se when using hydrofluoric acid (HF) in the wet transferring. The authors developed a polystyrene (PS)-assisted noncorrosive transfer method. PS was firstly spin-coated onto the surface of f-mica and then baked. Subsequently, the PS film together with Bi₂O₂Se was peeled away from the f-mica with the assistance of deionized (DI) water. After this unique transfer method, the performance of Bi₂O₂Se devices is greatly improved^[34]. Khan et al. used a polydimethylsiloxane (PDMS) and poly(methyl methacrylate) (PMMA)-assisted method to transfer Bi₂O₂Se flakes grown on mica substrate synthesized by VS deposition. Nevertheless this method is only effective in detaching thick Bi₂O₂Se flakes from mica^{[20, 44]}. Chen et al. developed a method to transfer Bi₂O₂Se sheets using PDMS only, which is proved to be a more convenient and effective method to transfer thinner Bi₂O₂Se flakes^[44].

Wu et al. introduced Bi₂O₃ as a seed layer and realized the vertical growth of 2D Bi₂O₂Se films on mica substrate by the CVD method, as shown in Fig. 2(m)^[43]. These vertically grown Bi₂O₂Se thin films can be easily and cleanly transferred to the target substrate. Zhang et al. proposed a simple, rapid, and extensible solution-assisted method to synthesize high-quality Bi₂O₂Se thin films on flexible muscovite substrates through the decomposition of Bi(NO₃)₃·5H₂O precursor and the following selenization. By changing the rotation speed of precursor solution, the thickness of the Bi₂O₂Se thin films can be accurately controlled to a few atomic layers^[18]. However, due to the strain caused by limited growth temperature and the softness of the substrate, the electronic property of flexible device performance is usually poorer than that on rigid substrates.

2.2 The modified Bridgman method

Bridgman invented the crucible descent method in 1925, which was called the Bridgman method. Later, Stockbarger developed this method and named it as the B-S method, which is one of the most commonly used methods for preparing large-size single crystals. The brief preparation process is as follows: the precursor materials needed for crystals growth are placed in a cylindrical crucible and slowly lowered. The furnace temperature is controlled slightly above the melting point of the material through a heating furnace with a certain temperature gradient. When the crucible passes through the heating zone, the materials in the crucible are melted. When the crucible continues to fall, the temperature at the bottom of the crucible first drops below the melting point and starts to crystallize, and the crystals continue to grow as the crucible falls down. This method has many advantages. First, the shape of the crystals grown depends on the crucible, and the shape of the crucible can be designed according to the needs. Second, it is suitable for growing large-size single crystals and multiple crystals. Finally, the growth method is simple and easy to operate, which is convenient for automation and industrialization. Xu et al. and Chen et al. successfully synthesized uniform and high-quality Bi₂O₂Se single crystals in a vacuum quartz tube using Bi₂O₃, Se, and Bi powders as precursors by a modified Bridgman method^{[22, 37]}.

2.3 Hydrothermal method

The hydrothermal method is also known as a high-pressure solution method. This method uses high-temperature and high-pressure aqueous solutions to dissolve or react substances that are insoluble or difficult to dissolve in water under atmospheric conditions to form a dissolved product of the substance and then crystallize and grow after reaching a certain degree of supersaturation. The hydrothermal method has the characteristics of mild reaction conditions, convenient and simple operation, the synthesized crystals have few defects, high uniformity, and high purity^[45]. Tian et al. synthesized Bi₂O₂Se crystals by hydrothermal method using deionized water, hydrazine hydrate (N₂H₄·H₂O), NaOH, Se powder, Bi(NO₃)₃·5H₂O, LiNO₃ powder, and KNO₃ powder as raw materials^[46]. Chen^[21] and Khan et al.^[20] also synthesized bulk Bi₂O₂Se by hydrothermal method using deionized water, LiNO₃, KNO₃, Bi(NO₃)₃·5H₂O, Se, and N₂H₄·H₂O as precursors.

3. Properties and characterizations

3.1 Crystal structure

2D layered material Bi₂O₂Se is a typical bismuth-based oxychalcogenide material with a layered structure. As shown in Fig. 3(a)^[46], the 4-fold symmetric Bi₂O₂Se has a tetragonal crystal structure and belongs to I4/mmm space group ($a$ = b = 3.8 Å, c = 12.16 Å), in which there are 10 atoms in the unit cell^{[37, 50]}, and the monolayer thickness of Bi₂O₂Se is about 0.61 nm^{[32, 33]}. Wu et al. confirmed the single crystal structure and monolayer thickness of Bi₂O₂Se by high-resolution transmission electron microscopy (HRTEM)^[31]. As shown in Figs. 3(b) and 3(c), the obtained lattice spacing of 0.19, 0.28 nm, and the layer spacing along the [001] direction of 0.61 nm is consistent with the theoretical values. Bi, O and Se atoms located at Wyckoff positions of 4e (0,0,z), 4d (0,1/2,1/4) and 2a (0,0,0), respectively. Therefore, the only free atomic coordinate in the structure is the z coordinate of the Bi atom. In the crystal structure, Bi atoms and O atoms form Bi₂O₂ layers perpendicular to [001] direction, and Se atoms form atomic layers located between the Bi₂O₂ layers. Bi₂O₂Se has a repeating sequence of ...-(Bi₂O₂)₁-Se₁-(Bi₂O₂)₂-Se₂-... layers^[37]. For the Bi₂O₂Se nanosheet, the distance between Bi and Se (3.18 Å) is much larger than the sum of the effective ion radius of Bi³⁺ (~1.03 Å) and Se²⁻ (~1.98 Å) but is shorter than that in bulk Bi₂O₂Se (3.27 Å). However, the Bi–O bond length (2.37 Å) is longer than that in bulk Bi₂O₂Se (2.31 Å). The results indicate that in the Bi₂O₂Se nanosheet the Bi–O bond is weaker while the interaction between Bi and Se layers is stronger than that in the bulk, which is conducive to the stability of the Bi₂O₂Se nanosheet^{[46, 51, 52]}. In addition, the 2D square lattice formed by Bi-Bi with a distance of 3.8 Å in the $[{\rm{Bi}}_2{\rm{O}}_2]_n^{2n+}$ layer is structurally compatible with many perovskite oxides and their heterostructure shows rich and interesting physical characteristics (ferroelectricity, magnetism, and high-T_c superconductivity)^[9]. Bi₂O₂Se had very high stability and no phase transition occurred below 30 GPa^[53].

3.2 Electronic structure and optical property

Unlike other 2D semiconductor materials, layered Bi₂O₂Se lacks the standard van der Waals gap^[16]. Consequently, it can be cleaved along the Se plane and the atomic structure may be rearranged at the surface, indicating the existence of nonequilibrium electrons distributed between $[{\rm{Bi}}_2{\rm{O}}_2]_n^{2n+}$ and $[{\rm{Se}}]_n^{2n-}$. Wu et al. combined the first-principle calculations and angle-resolved photoemission spectroscopy (ARPES) measurement to study the band structure and state density of bulk Bi₂O₂Se (Fig. 4). The minimum value of the conduction band (CBM) and the maximum value of the valence band (VBM) are located at points Γ and X, respectively, revealing its indirect band gap. The bands near CBM are very steep, while those near VBM are relatively flat^[54]. By fitting the conduction band, a very low in-plane electron effective mass m* = 0.14 ± 0.02m₀ was obtained (m₀ is free electron mass), which is conducive to achieving ultrahigh electron mobility^[11]. Since the CBM and VBM of Bi and Se elements are mainly composed of p-orbitals, layered Bi₂O₂Se crystals are considered to have strong spin-orbit interaction^[46].

Bi₂O₂Se is a semiconductor with a narrow bandgap (0.8 eV), which is particularly valuable for infrared optoelectronic devices. Chen et al. also conducted systematic theoretical and experimental studies on the electronic structures of Bi₂O₂Se and plotted the integral band structure of Bi₂O₂Se by combining scanning tunneling microscope (STM) and angle-resolved photoelectron spectroscopy (ARPES), the bandgap from both STM and ARPES exhibited excellent spatial uniformity and robustness^[37]. However, although the crystal structure of Bi₂O₂Se did not change under the influence of high pressure 4 GPa, the electronic change occurred including the crossing and anti-crossing behaviors of the top and the second top valence bands at different locations of the Brillouin zone due to the gradual shortening and hardening of the long and weak Bi-Se bonds between layers^[53].

Raman spectroscopy is a non-destructive characterization tool, which can provide important information about structure, crystallinity, strain and defects by probing specific molecular vibration modes. According to group theory, there are 10 vibration modes based on the I4/mmm space group of Bi₂O₂Se, among which A_1g, B_1g, and E_g modes are Raman active. Khan et al. conducted Raman characterization for Bi₂O₂Se films with different thicknesses (Fig. 5(a)), showing that A_1g characteristic peak was located at ~159 cm⁻¹, and its strength decreased with the decrease of the thickness of Bi₂O₂Se films^[20]. No obvious Raman characteristic peaks were observed for the monolayer Bi₂O₂Se films. Cheng et al. combined the theory and experiment to study the Raman spectrum of Bi₂O₂Se. Fig. 5(b) demonstrates the four Raman vibration modes of Bi₂O₂Se at the Γ-point are 159.89 (A_1g), 364.02 (B_1g), 67.99 (E_g¹), and 428.68 cm^–1 (E_g²). It can be seen in Fig. 5(c) that the A_1g and B_1g modes correspond to the movement of Bi and O atoms along the Z axis of crystallography. The vibration of Bi and O atoms in the XY-plane can cause two sets of degenerate E_g modes^[55]. Whether or not the Raman vibration modes in crystals can be observed in the experiment depends on their experimental structures and Raman tensors. For Bi₂O₂Se, it is difficult to measure 2D thin film on XZ or YZ plane, so only two intrinsic Raman peaks (A_1g and B_1g) can be observed. However, in the experiment, Cheng et al. and Pereira et al. did not observe the characteristic peak of Bi₂O₂Se near 364 cm⁻¹ (B_1g) predicted by group theory and only observed the A_1g model (about 159.2 cm⁻¹). As shown in Fig. 5(d), Pereira et al. explained the missing vibration mode of B_1g based on the plasmon-phonon coupling L^– or L⁺ bands of B_1g modes due to the high carrier concentration of n-type semiconductor^{[53, 55]}, just like many other highly doped semiconductors. Cheng et al. found that all Raman modes show redshifts under tensile strains and blueshifts under compressive strains. The E_g mode exhibits to be the most sensitive mode affected by the uniaxial strain^[53]. Pereira et al. observed the pressure evolution of the experimental low-frequency Raman-active A_1g and E_g modes. They ascribed the occurrence of several vibrational modes especially above 11.3 GPa could be related to the creation of defects in the sample^[55]. Therefore, Raman spectroscopy affords a convenient and rapid means to identify the stain or defects in atomically thin film.

Light absorption is related to the band structure of semiconductors. It is helpful to study the evolution of the band structure and to extract the optical bandgap. Wu et al. made optical measurements on 2D Bi₂O₂Se crystals with different thicknesses grown by CVD. The blue shift of the optical absorption edges occurred as the thickness decreased (Fig. 6(a)), which indicates that the bandgap increased due to the quantum size effect. By fitting the bandgap, it was found that the optical band gap can be adjusted from 1.37 to 1.90 eV as the thickness of 2D Bi₂O₂Se crystals gradually changed to monolayer, as shown in Fig. 6(b)^[31]. Liu et al. compared the results of photoluminescence, light transmission, and transient absorption spectroscopy with the electronic structure calculated by first-principles calculations. Due to the transition between the conduction band and the valence band state in the Γ valley, the multilayer Bi₂O₂Se (13 nm) has a direct optical transition near 720 nm (1.7 eV), which is almost the same as the result of monolayer Bi₂O₂Se. Figs. 6(c) and 6(d) indicate that the electronic structure of the Γ valley does not change significantly with the change in thickness^[56].

3.3 Mechanical property

Graphene has been proven to be one of the strongest materials ever made. The mechanical strength (stiffness) of Bi₂O₂Se is also especially important for its application in various flexible functional devices. Zhang et al. calculated the mechanical flexibility of monolayer Bi₂O₂Se with the first-principles method, the monolayer Bi₂O₂Se has a greater Poisson's ratio and lower in-plane stiffness than other 2D materials (such as MoS₂ and graphene)^[57]. In the process of characterizing the crystal structure and chemical composition for the synthesized Bi₂O₂Se crystal, the Bi₂O₂Se can be transferred by a polydimethylsiloxane (PDMS) and poly(methyl methacrylate) (PMMA)-assisted method onto a Cu grid for transmission electron microscope (TEM) examination, as shown in Fig. 7(a)^[20]. Li et al. and Zhang et al. repeatedly measured the photoelectric properties of the Bi₂O₂Se devices on a flexible substrate, and the photoelectric properties showed excellent stability^{[18, 41]}. In addition, Yin et al. prepared 2D flexible Bi₂O₂Se photodetectors and photodetector arrays on mica, Bi₂O₂Se photodetector arrays exhibited excellent photoelectric performance and stability on a substrate with strain bending up to 1% (Figs. 7(b) and 7(d)). They showed a very stable light response within 5 weeks in the environment, proving that the 2D Bi₂O₂Se photodetectors can work on a flexible substrate^[40]. Chen et al. experimentally obtained the mechanical properties of 2D Bi₂O₂Se using the nanoindentation method. Few-layer Bi₂O₂Se exhibits a large intrinsic stiffness of 18–23 GPa, Young’s modulus of 88.7 ± 14.4 GPa, and can withstand a high radial strain of more than 3%, demonstrating excellent flexibility^[44]. The presence of strain in 2D materials can change the band structure, carrier mobility, and so on. The strain effect is very important to understand the performance of flexible electronics. In the Raman spectrum, the stretching strain causes the Raman mode softening and red shift, while the compression strain causes the mode hardening and blue shift. The degeneracy E_g modes of Bi₂O₂Se split under uniaxial strain and shear strain, and the frequency variation of the degeneracy modes are anisotropic under rotating uniaxial strain^[55]. In short, the excellent flexibility and stability of Bi₂O₂Se thin films make it an ideal semiconductor material for flexible and wearable or printable electronic devices.

3.4 Thermoelectric property

Bi₂O₂Se was initially investigated as a potential n-type thermoelectric material in its bulk (ceramic) form^[58]. Zhang et al. measured the electrical conductivity, thermal conductivity, and Seebeck coefficient of Bi₂O₂Se (65 μm) in the temperature range of 300–470 K as shown in Fig. 8. The thermal conductivity is only 0.346 W/(m·K) at 300 K^[59]. The thermoelectric properties of materials are determined by their dimensionless thermoelectric ZT (ZT = S²σT/κ, S is Seebeck coefficient, σ is electrical conductivity, T is temperature, and κ is thermal conductivity). Generally, a material with ZT > 1 is considered to be excellent thermoelectric material. According to the investigation, the ZT value of Bi₂O₂Se can reach 0.2 at 800 K^[58]. However, theoretical studies have shown that the dimensionless thermoelectric ZT of p-doped Bi₂O₂Se can reach 1.42 (800 K) with the in-plane strain. This can be compared with Bi₂Te₃, which is one of the most widely used and best thermoelectric materials. Recently, Yu et al. reported that the ZT value of n-doped Bi₂O₂Se is as high as 3.35 at 800 K^[60]. This is much higher than the ZT value (2.6) of SnSe at 923 K, which is known to be the most effective thermoelectric material^[61].

The thermal transport of 2D materials is a key factor in thermal management, nanometer electronic devices, and thermoelectric devices. Yang et al. studied the in-plane and interfacial heat transfer and energy dissipation of 2D Bi₂O₂Se by Raman spectroscopy. Due to the low phonon group velocity, large surface scattering, and strong anharmonicity of Bi₂O₂Se phonons, the in-plane thermal conductivity of Bi₂O₂Se thin films decreases with the decrease in thickness. When the thickness of Bi₂O₂Se thin film is 8 nm, the in-plane thermal conductivity of Bi₂O₂Se is as low as 0.926–0.18 W/(m·K), which is far lower than that of other 2D materials such as black phosphorus and MoS₂. However, contrary to the in-plane thermal conductivity coefficient, Bi₂O₂Se thin films have larger interface binding energy, the thinner Bi₂O₂Se has a stronger heat dissipation ability to the substrate, so its interface thermal conductivity increases as the thin films thickness decrease, reaching 21 MW/(m·K) when the thickness of Bi₂O₂Se thin film is 8 nm^[62]. Yang et al. exhibited the photo-bolometric effect in Bi₂O₂Se photodetectors, which is based on temperature-induced hot carrier generation by light heating^[63].

4. Applications

As a new 2D layered material, Bi₂O₂Se has a unique crystal structure and novel electron transport properties. Studies have shown that Bi₂O₂Se has ultrahigh carrier mobility, tunable bandgap, excellent thermoelectric property, perfect chemical and thermal stability, and controllable doping concentration, which are very attractive characteristics for electronic and optical applications. In the following, we present a brief review on the recent progress of Bi₂O₂Se in photodetectors, energy storage, memristors, optical switches, and biomedicine (Fig. 9).

4.1 Photodetectors (transistors)

The mobility of semiconductors is one of the most important parameters, which determines some key performance of electronic devices. Wu et al. measured the Hall mobility of Bi₂O₂Se thin films at a low temperature (1.9 K) as high as 29 000 cm²/(V·s), which is comparable to graphene^[11]. The top-gated Bi₂O₂Se (6.2 nm) field effect transistor (Fig. 10(a)) exhibited excellent performance at room temperature, including ultrahigh Hall mobility (up to 450 cm²/(V·s)), a high current on/off ratio (>10⁶), and near-ideal subthreshold swing value (~65 mV/dec)^[11]. In addition, the current on/off ratio and the Hall mobility of Bi₂O₂Se both changed with the thickness of thin films. As the channel thickness decreased, the value of the on/off ratio increased from ~10³ to ~10⁶. For thicker Bi₂O₂Se, the Hall mobility at room temperature remains almost constant. However, thanks to the Bi₂O₂Se thin films with a thickness of less than 6 nm, their Hall mobility would suddenly drop due to severe surface/interface scattering, as shown in Fig. 10(b)^[11]. Later, Tong et al. reported the hall mobility of Bi₂O₂Se thin film as 40 000 cm²/(V·s) at 2 K and attributed its ultrahigh mobility to the suppressed backscattering of electrons^[33]. Since the mobility is closely related to the device performance such as photoconductive gain, response speed, and so on^{[64, 65]}, it is significant to improve the mobility further by growth control, gate tuning, avoiding various scattering from the surface/interface, and surface encapsulation with a dielectric layer^[66].

The preparation of Bi₂O₂Se array is a prerequisite for fully exerting its potential in integrated optoelectronics and multi-pixel readout digital circuits. To this end, Wu et al. used diluted H₂O₂ and protonic mixed acid as an effective etching agent to accurately pattern the 2D semiconductor Bi₂O₂Se crystal with high mobility on mica and obtained a centimeter order 2D Bi₂O₂Se array. The etched 2D Bi₂O₂Se crystals still retain high carrier mobility of 209 cm²/(V·s) (room temperature), and the integrated photodetector of the prepared 2D Bi₂O₂Se arrays exhibited good air stability (After being exposed to the air for about 6 months, the performance of the device hardly changed, as shown in Fig. 10(d)) and had a photoresponsivity up to 2000 A/W at 532 nm^[67].

In recent decades, the near/medium infrared (IR) photodetectors have been widely used in military, academic, and commercial fields, and the semiconductor Bi₂O₂Se with a narrow bandgap of about 0.8 eV is particularly valuable for IR optoelectronic detection. Up to now, many people have reported on near/medium IR optoelectronic devices. First, Li et al. systematically studied the near-infrared photoelectric detection performance of Bi₂O₂Se thin films on mica substrate through variable temperature measurement. At 808 nm, the responsivity, detectivity, and response time reached 6.5 A/W, 8.3 × 10¹¹ Jones and 2.8 ms, respectively^[41]. When the temperature changed from 300 to 80 K, due to the thermal radiation of carriers at low temperatures being suppressed, the carrier density was reduced, resulting in a decrease of dark current from 152 to 1.5 nA. However, the photocurrent only changed slightly, and the photoelectric performance remained basically unchanged, as shown in Figs. 10(e)–10(g), indicating that the grown Bi₂O₂Se thin films had no surface trap states and shallow defect levels. In addition, there was no significant change in device performance when exposed to air for more than three months, demonstrating good air stability of the Bi₂O₂Se.

Most 2D layered materials, such as graphene, TMDCs, and so on, have not yet demonstrated both high sensitivity and rapid photoelectric response in infrared detection. However, the Bi₂O₂Se infrared photodetector on mica substrate reported by Yin et al. showed a high responsivity of 65 A/W at 1200 nm and an ultrafast response time of about 1 ps at room temperature^[40]. At the same time, it exhibited a broadband optical response from visible light to 1700 nm. The photoelectric response reached 5800 A/W at 532 nm and 0.1 A/W at 1550 nm, which is comparable to other 2D materials such as graphene and TMDCs. In addition, 2D Bi₂O₂Se photodetectors can be integrated on flexible substrates and have a good imaging capability^[40]. Due to the inherent indirect optical bandgap of Bi₂O₂Se, the photoresponse performance drops sharply at λ = 1550 nm (<0.1 A/W)^[40]. To expand the Bi₂O₂Se response spectrum to the wider infrared, Luo et al.^[68] used PbSe colloidal quantum dots (CQDs) to decorate Bi₂O₂Se to form a type II band structure, which facilitated the separation of photocarriers and improved the performance of Bi₂O₂Se photodetectors. Schematic diagrams of the hybrid photodetector under light excitation and band structure are shown in Figs. 11(a) and 11(b), resulting in an optical response time of less than 4 ms and an infrared response greater than 10³ A/W at 2 μm, as shown in Figs. 11(c) and 11(d).

Compared with the monolayer or few-layer film, the multilayer film has higher state density and higher absolute light absorption, which can generate higher density photocurrent. Moreover, multilayer Bi₂O₂Se has a wider spectral response than few-layer Bi₂O₂Se due to the narrower bandgap. Yang et al. systematically studied the near-infrared photoelectric properties of multilayer Bi₂O₂Se thin films with a thickness of 30 nm, which had an ultra-sensitive optical response in the range of 850–1550 nm. The responsivity, detectivity, and external quantum efficiency reach 101 A/W, 1.9 × 10¹⁰ Jones, and 20 300% respectively at 1000 nm, and the response time is 30 ms (1500 nm)^[69]. The results show that the multilayer Bi₂O₂Se has higher responsivity and external quantum efficiency than the few-layer Bi₂O₂Se reported in the literature^[40], while maintaining a higher detectivity and a faster response time. Fu^[34], Khan^[20], and Tong et al.^[33] investigated the performance of Bi₂O₂Se phototransistors in the UV–Visible–NIR spectrum, the maximum value of responsivity and detectivity reached 10⁵ A/W and 10¹⁵ Jones, respectively. Yang et al.^[70] fabricated Bi₂O₂Se-MoSe₂ photodetector as shown in Fig. 11(e). The heterostructure showed a detection range from visible light to near-infrared (405–808 nm) with response and detection efficiency of 413.1 mA/W and 3.79 × 10¹¹ Jones at 780 nm, respectively.

To avoid defects or contaminants generated during traditional electrode deposition or sample transfer on devices, Hong et al.^[71] and Liu et al.^[72] adopted different methods to improve the device structures. Hong et al. synthesized Bi₂O₂Se thin films with different thicknesses on the steps of mica substrate to form in-plane homojunction. The device’s optical image is shown in Fig. 11(f), and it exhibited a diode-like rectifying behavior with an on/off ratio of 10² (Fig. 11(g)). Maximum optical response of 2.5 A/W and a response time of 4.8 μs are achieved. Liu et al. transferred the pre-deposited gold electrodes onto the Bi₂O₂Se thin films with a probe and prepared Au/Bi₂O₂Se/Au MSM structures on mica substrates to form metal/semiconductor contacts (Fig. 11(h)). Under optimized annealing temperature, the maximum responsivity and response time of the device reached 9.1 A/W and 36 μs with a broadband spectral response ranging from UV to NIR (360–1090 nm). In the ultra-short channel, Yang et al. used the ab initio quantum transport simulation to predict the performance of Bi₂O₂Se FETs. The optimized n-type and p-type Bi₂O₂Se FET can meet or approach the high-performance requirements of the International Technology Roadmap for Semiconductors (ITRS)^[17]. Table 2 shows the comparisons of device performance of Bi₂O₂Se and other 2D materials.

4.2 Energy storage devices

Polymer solar cells (PSCs) have attracted much attention because of their outstanding advantages such as simple structure and preparation process, low cost, light weight, and their ability to be made into flexible devices. Recently, some 2D materials have been applied to PSC. Huang et al. applied high-mobility Bi₂O₂Se thin films as active layers in PSCs to promote charge transfer^[79]. The results show that the performance of the device has been significantly improved, the PCE of PBDB-T:ITIC-based device has increased from 10.09% (0% by weight) to 12.22% (2 wt%). The PCE of the PM6:Y6-based device reached 16.28% when 2 wt% Bi₂O₂Se is introduced. The optimized ternary device shows good air stability, indicating that the Bi₂O₂Se material has a good application prospect in photovoltaic devices (Fig. 12).

4.3 Memristors

With the rapid expansion of data information, modern computers based on the von Neumann architecture are facing severe challenges. Intelligent computers that can learn, memorize and process information flexibly like the human brain are the direction and goal of future computer development. Memory resistors can remember their resistance history, which can be used on many occasions, such as nonvolatile storage devices, energy-efficient computers, neuromorphic calculating, and so on. Synapse refers to the part where neurons and neurons are connected which includes three parts: presynaptic membrane, synaptic cleft, and postsynaptic membrane^{[25, 80–82]}. The use of memristors to simulate synapses is based on the basic idea that the electrical properties can be altered under external stimuli and then memorized similar to synaptic plasticity, which makes it possible for memristors to build brain-like large-scale integrated circuits in the future.

Recently, Zhang et al. used the newly emerging 2D layered semiconductor material Bi₂O₂Se to realize a three-terminal memristor that simulates brain functions. Schematic diagram of the cross-sectional structure and the optical image are shown in Fig. 13^[25]. Zhang et al. demonstrated for the first time the coexistence of long-term plasticity (LTP) and short-term plasticity (STP) by decoupling the sites where the physical LTP and STP processes occurred. The concerted action of STP and LTP can make the transient synaptic efficacy from depression to facilitation be comprehensively adjusted through stimulus frequency or intensity. Through the heuristic recurrent neural circuitry model, the complex neural process of "sleep-wake cycle autoregulation" was simulated to show the complex computing power of memristors and the well-designed LTP and STP^[25]. This work indicates that Bi₂O₂Se has great potential for complex neuromorphic functional devices for high dynamic neuromorphic computing. Furthermore, Yang et al.^[24] developed a bidirectional all-optical synapse based on a 2D Bi₂O₂Se/Graphene hybrid structure. The hybrid structure presents both positive and negative photoresponsibility, which was used to realize all optically stimulated potentiation and depression. Recently, Yan et al. fabricate Bi₂O₂Se/PMN-PT 2D-FeFETs through growing high-quality Bi₂O₂Se epitaxial films on ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) single-crystal substrates. Thus, nonvolatile electric field modulation is achieved by changing the polarization direction of the ferroelectric layer, enabling the application of Bi₂O₂Se in nonvolatile memory^[83].

4.4 Optical switches

Pulsed lasers working in the mid-infrared (3–25 µm) range have broad application prospects in the fields of national defense, military, biomedical research, environmental monitoring, sensing, atmospheric communications, and imaging. So far, nonlinear optical devices with the ability to generate pulsed lasers, called passive saturable absorbers or optical switches, are one of the key factors limiting the development of mid-infrared pulsed lasers. In particular, optical switches with broadband response can produce short pulse output by rapidly switching absorption^[84]. Although graphene can be used as a saturable absorber near 3 μm due to its zero bandgap, graphene has a low absorption coefficient (~2.3%) which limits its application in producing short-pulse lasers in the mid-infrared range. Therefore, finding new functional materials that can be used for mid-infrared short pulse generation is very important for the wide application of mid-infrared light sources.

Since layered Bi₂O₂Se can be split along its Se plane, the surface atomic structure may be rearranged. There are nonequilibrium electrons distribution between $[{\rm{Bi}}_2{\rm{O}}_2]_n^{2n+}$ and $[{\rm{Se}}]_n^{2n-}$ layer. A planar topology system can be constructed through the Majorana bound state. Nonequilibrium electrons and potential energy topological states can excite optical nonlinearities. Tian et al. proved that Bi₂O₂Se as a saturable absorber has an ultra-wideband saturable nonlinear optical response in the wavelength range of 0.80 to 5.0 μm through Z-scan technology and pump probe technology. At 5.0 μm, the response time reaches the order of picoseconds and the response amplitude is as high as ~330.1%. An optical modulator based on 2D Bi₂O₂Se semiconductor can provide a simple, low-cost, effective, and scalable nonlinear absorption material for a 3 µm Q-switched fiber laser. Fig. 14 shows a 3 µm compact fiber laser that generates laser pulses through a Bi₂O₂Se-based optical switch^[46]. Li et al. have applied optical switches to the field of terahertz systems. They used a PS-assisted transfer method to transfer Bi₂O₂Se to the silicon substrate to configure a Bi₂O₂Se/Si structured terahertz wave switch. The strong absorption of terahertz waves is caused by the accumulation of carriers at the interface. This device achieves an extinction ratio of 17.7 dB at an external laser irradiance of 1.3 W/cm² at a broadband (0.25–1.5 THz) power density and also has a switching speed of 2 MHz. This shows that Bi₂O₂Se has potential for terahertz sensing, security, imaging, spectroscopy, communications, and so on^[85].

4.5 Biomedical applications

Photothermal technology has always attracted much attention in the field of cancer photothermal therapy (PTT). To improve the efficiency of treatment, researchers are committed to finding suitable materials with good photothermal properties and high tissue penetration ability under near-infrared (NIR) light irradiation^[86–88]. In the past decade, 2D layered materials have been developed rapidly, which promoted the development of photothermal agents such as graphene oxide (GO), WS₂, and MoS₂. It is well known that Se is a low-toxicity therapeutic agent, and Bi is beneficial to the preparation of X-ray contrast agents with good biological tolerance.

Recently, Xie et al.^[23] synthesized Bi₂O₂Se quantum dots (QDs) from bulk Bi₂O₂Se crystals by a simple solution method. TEM, AFM image, and cross-sectional analysis of the synthesized Bi₂O₂Se QDs are shown in Fig. 15. Bi₂O₂Se QDs were used as photoacoustic (PA) imaging agents and photothermal therapy (PTT) reagents. It was confirmed that PA signal intensity increases with the increase of the concentration of QDs, and the smaller Bi₂O₂Se QDs had higher photothermal conversion efficiency. The photothermal conversion coefficient of Bi₂O₂Se QDs with size and thickness of 3.8 and 1.9 nm respectively is as high as 35.7%. Moreover, Bi₂O₂Se had a good photothermal stability. After four cycles of near-infrared laser irradiation, the temperature dropped only slightly (~1 °C). Experiments show that Bi₂O₂Se QDs have excellent PA performance and PTT efficiency. After the injection of the drug, QDs accumulated at the tumor site, making the PA imaging of the entire tumor clearer and stronger, which is conducive to imaging-guided PTT. The drug has no obvious toxicity. More importantly, Bi₂O₂Se QDs have an appropriate degradation rate in an aqueous solution, which can be almost completely degraded within 2 months. In contrast, the other two widely used inorganic PTT reagents, Au nanorods and GO nanosheets, both show high stability in aqueous solutions, indicating that they have poor degradability. Therefore, Bi₂O₂Se QDs have enough stability in the body to complete the treatment and will be discharged harmlessly from the body after treatment. The results show that the biodegradable Bi₂O₂Se QDs have promising application potential in imaging-guided PTT^[23].

5. Conclusion and outlook

In summary, we reviewed the recent research progress on Bi₂O₂Se. The review starts from the preparation method, the preparation conditions and growth characteristics. Then, the crystal structure, electronic structure, optical properties, mechanical properties, and thermoelectric performance of Bi₂O₂Se are introduced in detail. Subsequently, Bi₂O₂Se applications in optoelectronics, energy storage, neuromorphic computing, nonvolatile memory, terahertz, and biomedicine were presented.

2D Bi₂O₂Se has the characteristics of atomic layer thickness, ultrahigh mobility, moderate and tunable band structure, high chemical and thermal stability, and excellent mechanical flexibility. Therefore, it has quickly become a research focus since its first report by the Peng group. Its rich physical and chemical properties make Bi₂O₂Se a great promising application prospect in solar cells, energy storage, environmental catalysis, sensors, memory, and so on. Although Bi₂O₂Se is a new 2D material, we have seen rapid growth in research interest on it in recent years. However, there are still many challenges for its large-scale application in the future.

At present, 2D Bi₂O₂Se thin films have been synthesized generally by using a bottom-up method or top-down method. Nonetheless, the controllable preparation of 2D Bi₂O₂Se single crystals with large size and atomic thickness still needs further systematic study. Moreover, large-area transferring Bi₂O₂Se thin films without introducing extrinsic contamination or intrinsic defects still face tough challenges^[34]. In photoelectric device applications, although QDs were used to improve the photoelectric sensitivity of Bi₂O₂Se photodetector, expensive production costs, complicated sensitization process, and even the heavy metal ions contained in QDs are great obstacles against putting Bi₂O₂Se into practical applications. In addition, the dark current of the device after sensitization becomes substantially higher, which is unfavorable for improving the device detectivity and the signal to noise ratio^[68]. In an electronic Bi₂O₂Se thin film device, the heat generated in electric measurement is not easy to be dissipated in time due to its low in-plane thermal conductivity^[62]. Therefore, overheating in electrical measurement burns the device. So the fine metal-semiconductor contact and heat management need more consideration.

The photodetectors (transistors), flexible electronic devices, energy storage devices, and so on that are described in this article are only part of the Bi₂O₂Se applications. The less studied and meaningful properties in 2D Bi₂O₂Se such as ferroelectricity and ferroelasticity, have great promise for future research in phase change engineering, piezoelectricity, electrostriction, and so on^{[89, 90]}. 2D Bi₂O₂Se has exhibited many excellent properties and has achieved some important applications in the fields of industrial production, environment monitoring, energy harvesting and medical treatment. However, the properties and application of Bi₂O₂Se still have a big room to be explored and more wonderful performance is expected in future research work.

Acknowledgements

This work is supported by China University of Geosciences (Beijing) College Students' Innovative Entrepreneurial Training Plan Program(No. 202211415026). National Natural Science Foundation of China (No. 11974318). China University of Geosciences (Beijing) 2021 Undergraduate Education Quality Improvement Plan Construction Project (No. XNFZ202106).