Controlled Vapor–Solid Deposition of Millimeter‐Size Single Crystal 2D Bi2O2Se for High‐Performance Phototransistors

Atomically thin 2D materials have received intense interest both scientifically and technologically. Bismuth oxyselenide (Bi₂O₂Se) is a semiconducting 2D material with high electron mobility and good stability, making it promising for high‐performance electronics and optoelectronics. Here, an ambient‐pressure vapor–solid (VS) deposition approach for the growth of millimeter‐size 2D Bi₂O₂Se single crystal domains with thicknesses down to one monolayer is reported. The VS‐grown 2D Bi₂O₂Se has good crystalline quality, chemical uniformity, and stoichiometry. Field‐effect transistors (FETs) are fabricated using this material and they show a small contact resistivity of 55.2 Ω cm measured by a transfer line method. Upon light irradiation, a phototransistor based on the Bi₂O₂Se FETs exhibits a maximum responsivity of 22 100 AW^?1, which is a record among currently reported 2D semiconductors and approximately two orders of magnitude higher than monolayer MoS₂. The Bi₂O₂Se phototransistor shows a gate tunable photodetectivity up to 3.4 × 10¹⁵ Jones and an on/off ratio up to ≈10⁹, both of which are much higher than phototransistors based on other 2D materials reported so far. The results of this study indicate a method to grow large 2D Bi₂O₂Se single crystals that have great potential for use in optoelectronic applications.     

Sensitive and Ultrabroadband Phototransistor Based on Two‐Dimensional Bi2O2Se Nanosheets

Bi₂O₂Se, a high‐mobility and air‐stable 2D material, has attracted substantial attention for application in integrated logic electronics and optoelectronics. However, achieving an overall high performance over a wide spectral range for Bi₂O₂Se‐based devices remains a challenge. A broadband phototransistor with high photoresponsivity (R) is reported that comprises high‐quality large‐area ( ≈ 180 µm) Bi₂O₂Se nanosheets synthesized via a modified chemical vapor deposition method with a face‐down configuration. The device covers the ultraviolet (UV), visible (Vis), and near‐infrared (NIR) wavelength ranges (360–1800 nm) at room temperature, exhibiting a maximum R of 108 696 A W^?1 at 360 nm. Upon illumination at 405 nm, the external quantum efficiency, R, and detectivity (D*) of the device reach up to 1.5 × 10⁷%, 50055 A W^?1, and 8.2 × 10¹² Jones, respectively, which is attributable to a combination of the photogating, photovoltaic, and photothermal effects. The devices reach a ?3 dB bandwidth of 5.4 kHz, accounting for a fast rise time (τ_rise) of 32 µs. The high sensitivity, fast response time, and environmental stability achieved simultaneously in these 2D Bi₂O₂Se phototransistors are promising for high‐quality UV and IR imaging applications.     

Broadband Bi2O2Se Photodetectors from Infrared to Terahertz

2D Bi₂O₂Se has shown great potential in photodetector from visible to infrared (IR) owing to its high mobility, ambient stability, and layer-tunable bandgaps. However, for the terahertz (THz) band with longer wavelength and richer spectral information, there are few reports on the research of THz detection based on 2D materials. Herein, an antenna-assisted Bi₂O₂Se photodetector is constructed to achieve broadband photodetection from IR to THz ranges driven by multi-mechanism of electromagnetic waves to electrical conversion. The good tradeoff between the bandgap and high mobility results in a broad spectral detection. In the IR region, the nonequilibrium carriers result from photo-induced electron-hole pairs in the Bi₂O₂Se body. While in the THz region, the carriers are caused by the injected electrons from the metal electrodes by the electromagnetic-induced well. The Bi₂O₂Se photodetector achieves a broadband responsivity of 58 A W^-1 at 1550 nm, 2.7 × 10⁴ V W^-1 at 0.17 THz, and 1.9 × 10⁸ V W^-1 at 0.029 THz, respectively. Surprisingly, an ultrafast response time of 476 ns and a quite low noise equivalent power of 0.2 pW Hz^−1/2 are acquired at room temperature. Our researches exhibit promising prospects of Bi₂O₂Se in broadband detection, THz imaging, and ultrafast sensing.     

Submillimeter 2D Bi2Se3 Flakes toward High‐Performance Infrared Photodetection at Optical Communication Wavelength

Infrared detection at optical communication wavelength is of great significance because of their diverse commercial and military communication applications. The layered Bi₂Se₃ with a narrow band gap of 0.3 eV is regarded as a promising candidate toward high‐performance terahertz to infrared applications. However, the controllable synthesis of large‐size ultrathin Bi₂Se₃ flakes remains a challenge owing to complex nucleation process and infrared telecommunication photodetectors based on Bi₂Se₃ flakes are rarely reported. Here, large size (submillimeter: 0.2–0.4 mm in lateral dimensions) and ultrathin (thickness: 3 nm to few nanometers) 2D Bi₂Se₃ flakes with high crystal quality are obtained by suppressing the nucleation density. More importantly, back‐gate field‐effect transistor based on Bi₂Se₃ flake exhibits an ultrahigh on/off current ratio of 10⁶ and competitive mobility of 39.4 cm² V^?1 s^?1. Moreover, excellent on/off ratio of 972.5, responsivity of 23.8 A W^?1, and external quantum efficiency of 2035% are obtained from Bi₂Se₃‐based photodetector at 1456 nm in the E‐band of the telecommunication range. With controlled morphology and excellent photoresponse performance, the Bi₂Se₃ photodetector shows great potential in the optoelectronic field including communications, military, and remote sensing.     

All‐Layered 2D Optoelectronics: A High‐Performance UV–vis–NIR Broadband SnSe Photodetector with Bi2Te3 Topological Insulator Electrodes

Nanoelectronics is in urgent demand of exceptional device architecture with ultrathin thickness below 10 nm and dangling‐bond‐free surface to break through current physical bottleneck and achieve new record of integration level. The advance in 2D van der Waals materials endows scientists with new accessibility. This study reports an all‐layered 2D Bi₂Te₃‐SnSe‐Bi₂Te₃ photodetector, and the broadband photoresponse of the device from ultraviolet (370 nm) to near‐infrared (808 nm) is demonstrated. In addition, the optimized responsivity reaches 5.5 A W^?1, with the corresponding eternal quantum efficiency of 1833% and detectivity of 6 × 10¹⁰ cm Hz^1/2 W^?1. These figures‐of‐merits are among the best values of the reported all‐layered 2D photodetectors, which are several orders of magnitude higher than those of the previous SnSe photodetectors. The superior device performance is attributed to the synergy of highly conductive surface state of Bi₂Te₃ topological insulator, perfect band alignment between Bi₂Te₃ and SnSe as well as small interface potential fluctuation. Meanwhile, the all‐layered 2D device is further constructed onto flexible mica substrate and its photoresponse is maintained roughly unchanged upon 60 bending cycles. The findings represent a fundamental scenario for advancement of the next generation high performance and high integration level flexible optoelectronics.     

Self-Driven WSe2/Bi2O2Se Van der Waals Heterostructure Photodetectors with High Light On/Off Ratio and Fast Response

Benefiting from the superior electron mobility and good air-stability, the emerging layered bismuth oxyselenide (Bi₂O₂Se) nanosheet has received considerable attention with the promising prospects for electronics and optoelectronics applications. However, the high charge carrier concentration and bolometric effect of Bi₂O₂Se give rise to the high dark current and relatively slow photoresponse, which severely impede further improvement of the performance of Bi₂O₂Se based photodetectors. Here, a WSe₂/Bi₂O₂Se Van der Waals p-n heterostructure is reported with a pronounced rectification ratio of 10⁵ and a low reverse dark current of 10⁻¹¹ A, as well as an enhanced light on/off ratio up to 618 under 532 nm light illumination. The device also exhibits a fast response speed of 2.6 µs and a broadband detection capability from 365 to 2000 nm due to the efficient charge separation and strong interlayer coupling at the interface of the two flakes. Importantly, the built-in potential in the WSe₂/Bi₂O₂Se heterostructure offers a competitive self-powered photodetector with the light on/off ratio above 10⁵ and a photovoltaic responsivity of 284 mA W⁻¹. The WSe₂/Bi₂O₂Se heterostructure shows promising potentials for high-performance self-driven photodetector applications.     

Seed‐Induced Vertical Growth of 2D Bi2O2Se Nanoplates by Chemical Vapor Transport

As two‐dimensional (2D) layered materials attract more attention owing to their unique optical, electrical, and thermal properties, there are persistent efforts to grow high‐quality 2D layered materials for fundamental research and device applications. While large‐area 2D layered materials with high crystal quality can be obtained through chemical vapor transport, the strong binding between 2D layered materials and substrates poses a significant challenge for attempts to reveal their intrinsic properties and to use these 2D building blocks for constructing advanced heterostructured devices. Therefore, it would be ideal to grow high‐quality 2D materials with minimized contact and binding with substrate. Through both calculation and experiment, it is demonstrated that by introducing a seed layer at the nucleation stage, the crystallographic disregistry and the corresponding adhesion energy between 2D materials and substrate can be altered, resulting in a change of crystal surface in contact with the substrate, and therefore vertical growth of 2D materials on substrates. As an example, it is demonstrated that with Bi₂O₃ serving as a seed layer, vertical growth of 2D plates of Bi₂O₂Se on mica substrates can be realized. These vertically grown 2D nanoplates of Bi₂O₂Se can be conveniently transferred with their thermal properties investigated for the first time.     

Large‐Size Growth of Ultrathin SnS2 Nanosheets and High Performance for Phototransistors

2D SnS₂ nanosheets have been attracting intensive attention as one potential candidate for the modern electronic and/or optoelectronic fields. However, the controllable large‐size growth of ultrathin SnS₂ nanosheets still remains a great challenge and the photodetectors based on SnS₂ nanosheets suffer from low responsivity, thus hindering their further applications so far. Herein, an improved chemical vapor deposition route is provided to synthesize large‐size SnS₂ nanosheets, the side length of which can surpass 150 μm. Then, ultrathin SnS₂ nanosheet‐based phototransistors are fabricated, which achieve high photoresponsivities up to 261 A W^?1 (with a fast rising time of 20 ms and a falling time of 16 ms) in air and 722 A W^?1 in vacuum, respectively. Furthermore, the effects of back‐gate voltage and air adsorbates on the optoelectronic properties of the SnS₂ nanosheet have been systematically investigated. In addition, a high‐performance flexible photodetector based on SnS₂ nanosheet is also fabricated with a high responsivity of 34.6 A W^?1.     

Out-of-Plane Resistance Switching of 2D Bi2O2Se at the Nanoscale

Two-dimensional (2D) bismuth oxyselenide (Bi₂O₂Se) with high electron mobility shows great potential for nanoelectronics. Although the in-plane properties of Bi₂O₂Se have been widely studied, its out-of-plane electrical transport behavior remains elusive, despite its importance in fabricating devices with new functionality and high integration density. Here, the out-of-plane electrical properties of 2D Bi₂O₂Se at nanoscale are revealed by conductive atomic force microscope. This work finds that hillocks with tunable heights and sizes are formed on Bi₂O₂Se after applying a vertical electric field. Intriguingly, such hillocks are conductive in the vertical direction, resulting in a previously unknown out-of-plane resistance switching in thick Bi₂O₂Se flakes while ohmic conductive characteristic in thin ones. Furthermore, the transformation is observed from bipolar to stable unipolar conduction in thick Bi₂O₂Se flake possessing such hillocks, suggesting its potential to function as a selector in vertical devices. This work reveals the unique out-of-plane transport behavior of 2D Bi₂O₂Se, providing the basis for fabricating vertical devices based on this emerging 2D material.     

Bidirectional All‐Optical Synapses Based on a 2D Bi2O2Se/Graphene Hybrid Structure for Multifunctional Optoelectronics

Neuromorphic computing has been extensively studied to mimic the brain functions of perception, learning, and memory because it may overcome the von Neumann bottleneck. Here, with the light‐induced bidirectional photoresponse of the proposed Bi₂O₂Se/graphene hybrid structure, its potential use in next‐generation neuromorphic hardware is examined with three distinct optoelectronic applications. First, a photodetector based on a Bi₂O₂Se/graphene hybrid structure presents positive and negative photoresponsibility of 88 and ?110 A W^?1 achieved by the excitation of visible wavelength and ultraviolet wavelength light at intensities of 1.2 and 0.3 mW cm^?2, respectively. Second, this unique photoresponse contributes to the realization of all optically stimulated long‐term potentiation or long‐term depression to mimic synaptic short‐term plasticity and long‐term plasticity, which are attributed to the combined effect of photoconductivity, bolometric, and photoinduced desorption. Third, the devices are applied to perform digital logic functions, such as “AND” and “OR,” using full light modulation. The proposed Bi₂O₂Se/graphene‐based optoelectronic device represents an innovative and efficient building block for the development of future multifunctional artificial neuromorphic systems.