Quasi-Two-Dimensional Se-Terminated Bismuth Oxychalcogenide (Bi2O2Se)

Programmable Interfacial Band Configuration in WS2/Bi2O2Se Heterojunctions

van der Waals heterojunctions based on transition-metal dichalcogenides (TMDs) offer advanced strategies for manipulating light-emitting and light-harvesting behaviors. A crucial factor determining the light-material interaction is in the band alignment at the heterojunction interface, particularly the distinctions between type-I and type-II alignments. However, altering the band alignment from one type to another without changing the constituent materials is exceptionally difficult. Here, utilizing Bi2O2Se with a thickness-dependent band gap as a bottom layer, we present an innovative strategy for engineering interfacial band configurations in WS2/Bi2O2Se heterojunctions. In particular, we achieve tuning of the band alignment from type-I (Bi2O2Se straddling WS2) to type-II and finally to type-I (WS2 straddling Bi2O2Se) by increasing the thickness of the Bi2O2Se bottom layer from monolayer to multilayer. We verified this band architecture conversion using steady-state and transient spectroscopy as well as density functional theory calculations. Using this material combination, we further design a sophisticated band architecture incorporating both type-I (WS2 straddles Bi2O2Se, fluorescence-quenched) and type-I (Bi2SeO5 straddles WS2, fluorescence-recovered) alignments in one sample through focused laser beam (FLB). By programming the FLB trajectory, we achieve a predesigned localized fluorescence micropattern on WS2 without changing its intrinsic atomic structure. This effective band architecture design strategy represents a significant leap forward in harnessing the potential of TMD heterojunctions for multifunctional photonic applications.

Prediction of single-layer antimony oxyselenide (Sb2O2Se2): metal-to-semiconductor transition via hydrogenation

In this study, the structural, electronic, vibrational, and mechanical properties of single-layer Antimony Oxyselenide (Sb2O2Se2) and its hydrogenated structure (Sb2O2Se2H2) are investigated by performing density functional theory-based first principles calculations. Geometry optimizations reveal that single-layer Sb2O2Se2crystallizes in tetragonal structure which is shown to possess dynamical stability by means of phonon band dispersions. In addition, the mechanical stability of the predicted single layer is satisfied via the linear-elastic parameters. Electronically, it is revealed that single-layer Sb2O2Se2exhibits metallic behavior whose highest occupied states are found to arise from the surface Se atoms, may be an indication for tuning the electronic features via surface functionalization. For the surface modification of Sb2O2Se2, top of each Se atom is saturated with a H atom and fully hydrogenated single-layer Sb2O2Se2H2is shown to be an in-plane anisotropic structure. Phonon band dispersion calculations indicate the dynamical stability of Sb2O2Se2H2. Mechanically stable Sb2O2Se2H2is found to possess anisotropic linear-elastic behavior, which is much softer than its pristine structure. Moreover, electronically a metallic-to-semiconducting transition is shown to occur as the unoccupied Se-orbitals are saturated via H atoms. Our work offers insights into prediction of a novel single-layer material, namely Sb2O2Se2, and reports the chemically-driven semiconducting behavior via hydrogenation, which may lead to the use of hydrogenated structure in solar cell, photoelectrode, or photocatalyst applications owing to its suitable band gap.

Recent advances in bismuth oxychalcogenide nanosheets for sensing applications

This review offers insights into the fundamental properties of bismuth oxychalcogenides Bi2O2X (X = S, Se, Te) (BOXs), concentrating on recent advancements primarily from studies published over the past five years. It examines the physical characteristics of these materials, synthesis methods, and their potential as critical components for gas sensing, biosensing, and optical sensing applications. Moreover, it underscores the implications of these advancements for the development of military, environmental, and health monitoring devices.

Two-Dimensional Semiconductors and Transistors for Future Integrated Circuits

Silicon transistors are approaching their physical limit, calling for the emergence of a technological revolution. As the acknowledged ultimate version of transistor channels, 2D semiconductors are of interest for the development of post-Moore electronics due to their useful properties and all-in-one potentials. Here, the promise and current status of 2D semiconductors and transistors are reviewed, from materials and devices to integrated applications. First, we outline the evolution and challenges of silicon-based integrated circuits, followed by a detailed discussion on the properties and preparation strategies of 2D semiconductors and van der Waals heterostructures. Subsequently, the significant progress of 2D transistors, including device optimization, large-scale integration, and unconventional devices, are presented. We also examine 2D semiconductors for advanced heterogeneous and multifunctional integration beyond CMOS. Finally, the key technical challenges and potential strategies for 2D transistors and integrated circuits are also discussed. We envision that the field of 2D semiconductors and transistors could yield substantial progress in the upcoming years and hope this review will trigger the interest of scientists planning their next experiment.

Hidden Direct Bandgap of Bi2O2Se by Se Vacancy and Enhanced Direct Bandgap of Bismuth Oxide Overlayer

The Bi2O2Se surfaces are well-known to possess 50% Se vacancies, yet they have shown no in-gap states within the indirect bandgap (∼0.8 eV). We have found that the hidden in-gap states arising from the Se vacancies in a 2 × 1 pattern induce a reduced direct bandgap (∼0.5 eV). Such a reduced direct bandgap is responsible for the high electron mobility of Bi2O2Se. Moreover, the Bi oxide overlayers of the Bi thin films, formed through air exposure and annealing, unexpectedly exhibit a large direct bandgap (∼2.1 eV). The simplified fabrication of Bi oxide overlayers provides promise for improving Bi2O2Se electronic devices and enhancing photocatalytic activity.

Layer-Controlled Growth of Single-Crystalline 2D Bi2O2Se Film Driven by Interfacial Reconstruction

As semiconductor scaling continues to reach sub-nanometer levels, two-dimensional (2D) semiconductors are emerging as a promising candidate for the post-silicon material. Among these alternatives, Bi2O2Se has risen as an exceptionally promising 2D semiconductor thanks to its excellent electrical properties, attributed to its appropriate bandgap and small effective mass. However, unlike other 2D materials, growth of large-scale Bi2O2Se films with precise layer control is still challenging due to its large surface energy caused by relatively strong interlayer electrostatic interactions. Here, we present the successful growth of a wafer-scale (∼3 cm) Bi2O2Se film with precise thickness control down to the monolayer level on TiO2-terminated SrTiO3 using metal-organic chemical vapor deposition (MOCVD). Scanning transmission electron microscopy (STEM) analysis confirmed the formation of a [BiTiO4]1- interfacial structure, and density functional theory (DFT) calculations revealed that the formation of [BiTiO4]1- significantly reduced the interfacial energy between Bi2O2Se and SrTiO3, thereby promoting 2D growth. Additionally, spectral responsivity measurements of two-terminal devices confirmed a bandgap increase of up to 1.9 eV in monolayer Bi2O2Se, which is consistent with our DFT calculations. Finally, we demonstrated high-performance Bi2O2Se field-effect transistor (FET) arrays, exhibiting an excellent average electron mobility of 56.29 cm2/(V·s). This process is anticipated to enable wafer-scale applications of 2D Bi2O2Se and facilitate exploration of intriguing physical phenomena in confined 2D systems.

Electric Control of Helicity-Dependent Photocurrent and Surface Polarity Detection on Two-Dimensional Bi2O2Se Nanosheets

Bismuth oxyselenide (Bi2O2Se) is a two-dimensional (2D) layered semiconductor material with high electron Hall mobility and excellent environmental stability as well as strong spin-orbit interaction (SOI), which has attracted intense attention for application in spintronic and spin optoelectronic devices. However, a comprehensive study of spin photocurrent and its microscopic origin in Bi2O2Se is still missing. Here, the helicity-dependent photocurrent (HDPC) was investigated in Bi2O2Se nanosheets. By analyzing the dependence of HDPC on the angle of incidence, we find that the HDPC originates from surface states with Cs symmetry in Bi2O2Se, which can be attributed to the circular photogalvanic effect (CPGE) and circular photon drag effect (CPDE). It is revealed that the HDPC current almost changes linearly with the source-drain voltage. Furthermore, we demonstrate effective tuning of HDPC in Bi2O2Se by ionic liquid gating, indicating that the spin splitting of the surface electronic structure is effectively tuned. By analyzing the gate voltage dependence of HDPC, we can unambiguously identify the surface polarity and the surface electronic structure of Bi2O2Se. The large HDPC in Bi2O2Se nanosheets and its efficient electrical tuning demonstrate that 2D Bi2O2Se nanosheets may provide a good platform for opto-spintronics devices.

Two dimensional (2D) materials and biomaterials for water desalination; structure, properties, and recent advances

BACKGROUND

An efficient solution to the global freshwater dilemma is desalination. MXene, Molybdenum Disulfide (MoS₂), Graphene Oxide, Hexagonal Boron Nitride, and Phosphorene are just a few examples of two-dimensional (2D) materials that have shown considerable promise in the development of 2D materials for water desalination. However, other promising materials for desalinating water are biomaterials. The benefits of bio-materials are their wide distribution, lack of toxicity, and superior capacity for water desalination.

METHODS

For the rational use of water and the advancement of sustainable development, it is of the utmost importance to research 2D-dimensional materials and biomaterials that are effective for water desalination. The scientific community has concentrated on wastewater remediation using bio-derived materials, such as nanocellulose, chitosan, bio-char, bark, and activated charcoal generated from plant sources, among the various endeavors to enhance access to clean water. Moreover, the 2D-materials and biomaterials may have ushered in a new age in the production of desalination materials and created a promising future.

RESULTS

The present review article focuses on and reviews the progress of 2D materials and biomaterials for water desalination. Their properties, surface, and structure, combined with water desalination applications, are highlighted. Further, the practicability and potential future directions of 2D materials and biomaterials are proposed. Thus, the current work provides information and discernments for developing novel 2D materials and biomaterials for wastewater desalination. Moreover, it aims to promote the contribution and advancement of materials for water desalination, fabrication, and industrial production.

Tunable lattice dynamics and dielectric functions of two-dimensional Bi2O2Se: striking layer and temperature dependent effects

Two-dimensional (2D) Bi₂O₂Se semiconductors with a narrow band gap and ultrahigh mobility have been regarded as an emerging candidate for optoelectronic devices, whereas the ambiguous phonon characteristics and optical properties still limit their future applications. Herein, high-quality centimeter-scale 2D Bi₂O₂Se films are successfully synthesized to disclose the lattice dynamics and dielectric functions under the control of thickness and temperature. It has been demonstrated that the stronger electrostatic Bi-Se interactions result in a stiffened phonon vibration of thicker Bi₂O₂Se layers. Three excitons (E_a, E_b, and E_c) exhibit significant red shifts with layer stacking. Interestingly, the dielectric properties in the visible-near infrared region (E_a and E_b) are dominated by the combined effect of the joint density of states and mass density, whereas the dielectric properties in the ultraviolet region (E_c) are dominated by the exciton effect. Furthermore, the temperature-sensitivity of the phonon frequency and exciton transition energies is revealed to be layer-dependent. In particular, the optical response of E_b excitons exhibits a prominent dependence on temperature, which indicates a promising optical modulation by temperature in the visible spectrum. This study enriches the knowledge about phonon dynamics and dielectric properties for 2D Bi₂O₂Se, which provides an essential reference for high-performance related optoelectronic devices.

Challenges for Nanoscale CMOS Logic Based on Two-Dimensional Materials

For ultra-scaled technology nodes at channel lengths below 12 nm, two-dimensional (2D) materials are a potential replacement for silicon since even atomically thin 2D semiconductors can maintain sizable mobilities and provide enhanced gate control in a stacked channel nanosheet transistor geometry. While theoretical projections and available experimental prototypes indicate great potential for 2D field effect transistors (FETs), several major challenges must be solved to realize CMOS logic circuits based on 2D materials at the wafer scale. This review discusses the most critical issues and benchmarks against the targets outlined for the 0.7 nm node in the International Roadmap for Devices and Systems scheduled for 2034. These issues are grouped into four areas; device scaling, the formation of low-resistive contacts to 2D semiconductors, gate stack design, and wafer-scale process integration. Here, we summarize recent developments in these areas and identify the most important future research questions which will have to be solved to allow for industrial adaptation of the 2D technology.

Green Triplet Self-Trapped Exciton Emission in Layered Rb3Cd2Cl7:Sb3+ Perovskite: Comparison with RbCdCl3:Sb3

Metal halide materials have recently sparked intense research because of their excellent photophysical properties and chemical stability. For example, RbCdCl₃:Sb³⁺ exhibits broad emission at about 600 nm with a high photoluminescence quantum yield (PLQY) over 91% and double emission bands with bright white color. Herein, we obtained a novel Rb and Cd layered perovskite Rb₃Cd₂Cl₇ doped with Sb³⁺, which gives luminescence at 525 nm with a large Stokes shift of 200 nm, originating from a self-trapped exciton (STE). Its PLQY is 57.47%, but its low-temperature PLQY becomes much higher at the same wavelength. When Rb₃Cd₂Cl₇:Sb³⁺ and RbCdCl₃:Sb³⁺ were compared, the two classes of quantum confinement effects by Rb and Cd ions in the lattice were identified to describe their electronic states and different optical properties. These results suggest that properties of Sb-doped cadmium halides could be modified by the structure type and local atomic confinement to find applications as promising luminescent materials for optoelectronic devices.

Defect Engineering in Thickness-Controlled Bi2O2Se-Based Transistors by Argon Plasma Treatment

We present a simple, effective, and controllable method to uniformly thin down the thickness of as-exfoliated two-dimensional Bi₂O₂Se nanoflakes using Ar⁺ plasma treatment. Atomic force microscopy (AFM) images and Raman spectra indicate that the surface morphology and crystalline quality of etched Bi₂O₂Se nanoflakes remain almost unaffected. X-ray photoelectron spectra (XPS) indicate that the O and Se vacancies created during Ar⁺ plasma etching on the top surface of Bi₂O₂Se nanoflakes are passivated by forming an ultrathin oxide layer with UV O₃ treatment. Moreover, a bottom-gate Bi₂O₂Se-based field-effect transistor (FET) was constructed to research the effect of thicknesses and defects on electronic properties. The on-current/off-current (I_on/I_off) ratio of the Bi₂O₂Se FET increases with decreasing Bi₂O₂Se thickness and is further improved by UV O₃ treatment. Eventually, the thickness-controlled Bi₂O₂Se FET achieves a high I_on/I_off ratio of 6.0 × 10⁴ and a high field-effect mobility of 5.7 cm² V^-1 s^-1. Specifically, the variation trend of the I_on/I_off ratio and the electronic transport properties for the bottom-gate Bi₂O₂Se-based FET are well described by a parallel resistor model (including bulk, channel, and defect resistance). Furthermore, the I_ds-V_gs hysteresis and its inversion with UV irradiation were observed. The pulsed gate and drain voltage measurements were used to extract trap time constants and analyze the formation mechanism of different hysteresis. Before UV irradiation, the origin of clockwise hysteresis is attributed to the charge trapping/detrapping of defects at the Bi₂O₂Se/SiO₂ interface and in the Bi₂O₂Se bulk. After UV irradiation, the large anticlockwise hysteresis is mainly due to the tunneling between deep-level oxygen defects in SiO₂ and p⁺⁺-Si gate, which implies the potential in nonvolatile memory.

Giant Modulation of the Electron Mobility in Semiconductor Bi2O2Se via Incipient Ferroelectric Phase Transition

High-mobility layered semiconductors have the potential to enable the next-generation electronics and computing. This paper demonstrates that the ultrahigh electron mobility observed in the layered semiconductor Bi₂O₂Se originates from an incipient ferroelectric transition that endows the material with a robust protection against mobility degradation by Coulomb scattering. Based on first-principles calculations of electron-phonon interaction and ionized impurity scattering, it is shown that the electron mobility of Bi₂O₂Se can reach 10⁴ to 10⁶ cm² V^-1 s^-1 over a wide range of realistic doping concentrations. Furthermore, a small elastic strain of 1.7% can drive the material toward a unique interlayer ferroelectric transition, resulting in a large increase in the dielectric permittivity and a giant enhancement of the low-temperature electron mobility by more than an order of magnitude. These results establish a new route to realize high-mobility layered semiconductors via phase and dielectric engineering.

Van der Waals Epitaxy of Bi2 Te2 Se/Bi2 O2 Se Vertical Heterojunction for High Performance Photodetector

Bismuth oxyselenide (Bi₂ O₂ Se) has emerged as a promising candidate for electronic and optoelectronic applications due to its outstanding electron mobility and ambient stability. However, high dark current and relatively slow photoresponse that originate from high charge carrier concentration as well as bolometric effect in Bi₂ O₂ Se inhibit further improvement of Bi₂ O₂ Se based photodetectors. Here, a one-step van der Waals (vdW) epitaxy synthesis of Bi₂ Te₂ Se/Bi₂ O₂ Se vertical heterojunction with type-II band alignment and high-quality interface is demonstrated. The crystal quality and uniformity of the heterojunction are supported by Raman, transmission electron microscopy and energy dispersive spectroscopy results. A photodetector based on Bi₂ Te₂ Se/Bi₂ O₂ Se heterojunction demonstrates steady photoresponse over a large wavelength range (532-1456 nm), with a high specific responsivity of 2.21 × 10³ A W^-1 at 532 nm and fast response speed of 50 ms. Moreover, field effect regulation allows for further improvement of the photoresponse performance of the heterojunction field effect transistor device, where the responsivity can be increased to 3.34 × 10³ A W^-1 with a 60 V gate voltage. Overall, the one-step vdW epitaxy process is a promising and convenient route towards constructing high quality Bi₂ O₂ Se based heterojunction for improving its photodetection performance.

Recent advances in the fabrication of 2D metal oxides

Atomically thin two-dimensional (2D) metal oxides exhibit unique optical, electrical, magnetic, and chemical properties, rendering them a bright application prospect in high-performance smart devices. Given the large variety of both layered and non-layered 2D metal oxides, the controllable synthesis is the critical prerequisite for enabling the exploration of their great potentials. In this review, recent progress in the synthesis of 2D metal oxides is summarized and categorized. Particularly, a brief overview of categories and crystal structures of 2D metal oxides is firstly introduced, followed by a critical discussion of various synthesis methods regarding the growth mechanisms, advantages, and limitations. Finally, the existing challenges are presented to provide possible future research directions regarding the synthesis of 2D metal oxides. This work can provide useful guidance on developing innovative approaches for producing both 2D layered and non-layered nanostructures and assist with the acceleration of the research of 2D metal oxides.

Understanding the interfacial charge transfer in the CVD grown Bi2O2Se/CsPbBr3 nanocrystal heterostructure and its exploitation in superior photodetection: experiment vs. theory

Efficient charge transfer in a 2D semiconductor heterostructure plays a crucial role in high-performance photodetectors and energy harvesting devices. Non-van der Waals 2D Bi₂O₂Se has enormous potential for high-performance optoelectronics, though very little is known about the interfacial charge transport at the corresponding 2D heterojunction. Herein, we report a combined experimental and theoretical investigation of interfacial charge transfer in the Bi₂O₂Se/CsPbBr₃ heterostructure through various microscopic and spectroscopic tools corroborated with density functional theory calculations. The CVD-grown few-layer Bi₂O₂Se nanosheet possesses high crystallinity and a high absorption coefficient in the visible-near IR region. We integrated the few-layer Bi₂O₂Se nanosheet possessing superior electron mobility and CsPbBr₃ nanocrystals with high light-harvesting capability for efficient broadband photodetection. The band alignment reveals a type-I heterojunction, and the device under reverse bias reveals a fast response time of 12 μs/24 μs (rise time/fall time) and an improved responsivity in the 390 to 840 nm range due to the effective interfacial charge transfer and efficient interlayer coupling at the Bi₂O₂Se/CsPbBr₃ interface. Notably, a photodetector with a better light on/off ratio and a peak responsivity of ∼10³ A W^-1 was achieved in the Bi₂O₂Se/CsPbBr₃ heterostructure due to the synergistic effects in the heterostructure under ambient conditions. The DFT analysis of the density of states and charge density plots in the heterostructure revealed a net transfer of electrons/holes from perovskite nanocrystals to Bi₂O₂Se layers and additional density of states in Bi₂O₂Se. These results are significant for the development of non-van der Waals heterostructure based high-performance low-powered photodetectors.

Surface Diffusion and Epitaxial Self-Planarization for Wafer-Scale Single-Grain Metal Chalcogenide Thin Films

Although wafer-scale single-grain thin films of 2D metal chalcogenides (MCs) have been extensively sought after during the last decade, the grain size of the MC thin films is still limited in the sub-millimeter scale. A general strategy of synthesizing wafer-scale single-grain MC thin films by using commercial wafers (Si, Ge, GaAs) both as metal source and epitaxial collimator is presented. A new mechanism of single-grain thin-film formation, surface diffusion, and epitaxial self-planarization is proposed, where chalcogen elements migrate preferentially along substrate surface and the epitaxial crystal domains flow to form an atomically smooth thin film. Through synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy, the formation of single-grain Si₂ Te₃ , GeTe, GeSe, and GaTe thin films on (111) Si, Ge, and (100) GaAs is verified. The Si₂ Te₃ thin film is used to achieve transfer-free fabrication of a high-performance bipolar memristive electrical-switching device.

Epitaxial growth of large-grain-size ferromagnetic monolayer CrI3 for valley Zeeman splitting enhancement

Two-dimensional (2D) magnetic CrI₃ has received considerable research attention because of its intrinsic features, including insulation, Ising ferromagnetism, and stacking-order-dependent magnetism, as well as potential in spintronic applications. However, the current strategy for the production of ambient-unstable CrI₃ thin layer is limited to mechanical exfoliation, which normally suffers from uncontrollable layer thickness, small size, and low yet unpredictable yield. Here, via a confined vapor epitaxy (CVE) method, we demonstrate the mass production of flower-like CrI₃ monolayers on mica. Interestingly, we discovered the crucial role of K ions on the mica surface in determining the morphology of monolayer CrI₃, reacting with precursors to form a KI_x buffer layer. Meanwhile, the transport agent affects the thickness and size of the as-grown CrI₃. Moreover, the Curie temperature of CrI₃ is greatly affected by the interaction between CrI₃ and the substrate. The monolayer CrI₃ on mica could act as a magnetic substrate for valley Zeeman splitting enhancement of WSe₂. We reckon our work represents a major advancement in the mass production of monolayer 2D CrI₃ and anticipate that our growth strategy may be extended to other transition metal halides.

Growth and Interlayer Engineering of 2D Layered Semiconductors for Future Electronics

Layered materials that do not form a covalent bond in a vertical direction can be prepared in a few atoms to one atom thickness without dangling bonds. This distinctive characteristic of limiting thickness around the sub-nanometer level allowed scientists to explore various physical phenomena in the quantum realm. In addition to the contribution to fundamental science, various applications were proposed. Representatively, they were suggested as a promising material for future electronics. This is because (i) the dangling-bond-free nature inhibits surface scattering, thus carrier mobility can be maintained at sub-nanometer range; (ii) the ultrathin nature allows the short-channel effect to be overcome. In order to establish fundamental discoveries and utilize them in practical applications, appropriate preparation methods are required. On the other hand, adjusting properties to fit the desired application properly is another critical issue. Hence, in this review, we first describe the preparation method of layered materials. Proper growth techniques for target applications and the growth of emerging materials at the beginning stage will be extensively discussed. In addition, we suggest interlayer engineering via intercalation as a method for the development of artificial crystal. Since infinite combinations of the host-intercalant combination are possible, it is expected to expand the material system from the current compound system. Finally, inevitable factors that layered materials must face to be used as electronic applications will be introduced with possible solutions. Emerging electronic devices realized by layered materials are also discussed.

Mobility-Fluctuation-Controlled Linear Positive Magnetoresistance in 2D Semiconductor Bi2O2Se Nanoplates

Linear magnetoresistance is generally observed in polycrystalline zero-gap semimetals and polycrystalline Dirac semimetals with ultrahigh carrier mobility. We report the observation of positive and linear magnetoresistance in a single-crystalline semiconductor Bi₂O₂Se grown by chemical vapor deposition. Both Se-poor and Se-rich Bi₂O₂Se single-crystalline nanoplates display a linear magnetoresistance at high fields. The Se-poor Bi₂O₂Se exhibits a typical 2D conduction feature with a small effective mass of 0.032m₀. The average transport Hall mobility, which is lower than 5500 cm² V^-1 s^-1, is significantly reduced, compared with the ultrahigh quantum mobility as high as 16260 cm² V^-1 s^-1. More interestingly, the pronounced Shubnikov-de Hass oscillations can be clearly observed from the very large and nearly linear magnetoresistance (>500% at 14 T and 2 K) in Se-poor Bi₂O₂Se. A close analysis of the results reveals that the large and linear magnetoresistance observed can be ascribed to the spatial mobility fluctuation, which is strongly supported by Fermi energy inhomogeneity in the nanoplate samples detected using an electrostatic force microscopy images and multiple frequencies in a Shubnikov-de Hass oscillation. On the contrary, the Se-rich Bi₂O₂Se exhibits a transport mobility (<300 cm² V^-1 s^-1) much smaller than that observed in Se-poor samples and shows a much smaller linear magnetoresistance ratio (less than 150% at 14 T and 2 K). More strikingly, no Shubnikov-de Hass oscillations can be observed. Therefore, the linear magnetoresistance in Se-rich Bi₂O₂Se is governed by the average mobility rather than the mobility fluctuation.

Ultrathin Bi2O2S nanosheet near-infrared photodetectors

Recently, a zipper two-dimensional (2D) material Bi₂O₂Se belonging to the layered bismuth oxychalcogenide (Bi₂O₂X: X = S, Se, Te) family, has emerged as an alternate candidate to van der Waals 2D materials for high-performance electronic and optoelectronic applications. This hints towards exploring the other members of the Bi₂O₂X family for their true potential and bismuth oxysulfide (Bi₂O₂S) could be the next member for such applications. Here, we demonstrate for the first time, the scalable room-temperature chemical synthesis and near-infrared (NIR) photodetection of ultrathin Bi₂O₂S nanosheets. The thickness of the freestanding nanosheets was around 2-3 nm with a lateral dimension of ∼80-100 nm. A solution-processed NIR photodetector was fabricated from ultrathin Bi₂O₂S nanosheets. The photodetector showed high performance, under 785 nm laser illumination, with a photoresponsivity of 4 A W^-1, an external quantum efficiency of 630%, and a normalized photocurrent-to-dark-current ratio of 1.3 × 10¹⁰ per watt with a fast response time of 100 ms. Taken together, the findings suggest that Bi₂O₂S nanosheets could be a promising alternative 2D material for next-generation large-area flexible electronic and optoelectronic devices.