Self-Powered Broad-band Photodetectors Based on Vertically Stacked WSe2/Bi2Te3 p-n Heterojunctions

Interfacial diffusion enabled broadband response in photodetector based on In2Se3/GaAs heterojunction

Infrared (IR) photodetectors play a crucial role in modern technologies due to their ability to operate in various environmental conditions. This study developed high-performance In2Se3/GaAs interdiffusion heterostructure photodetectors with broadband response using liquid-phase method. It is believed that an InGaAs layer and In2Se3 have been formed at the interface through the mutual diffusion of elements, resulting in a detection spectral range spanning from 0.45 to 2.7 µm. Consequently, the In2Se3/GaAs photodetector exhibits notably low noise equivalent power of 6.21 × 10-15 WHz-1/2 at 1000 Hz, high photoresponsivity (R) and detectivity (D*) of 16.22 mA/W and 4.01 × 1011 Jones under 0 V with 630 nm wavelength, respectively. At 1550 nm, it achieves a R of 0.43 µAW-1 and D*of 1.07 × 108 Jones under 0 V. This strongly suggests that the interdiffused In2Se3/GaAs heterostructure is a high performance and low-cost material for broadband responsive photodetectors.

Unipolar Barrier Photodetectors Based on Van Der Waals Heterostructure with Ultra-High Light On/Off Ratio and Fast Speed

Unipolar barrier architecture is designed to enhance the photodetector's sensitivity by inducing highly asymmetrical barriers, a higher barrier for blocking majority carriers to depressing dark current, and a low minority carrier barrier without impeding the photocurrent flow through the channel. Depressed dark current without block photocurrent is highly desired for uncooled Long-wave infrared (LWIR) photodetection, which can enhance the sensitivity of the photodetector. Here, an excellent unipolar barrier photodetector based on multi-layer (ML) graphene (G) is developed, WSe2, and PtSe2 (G-WSe2-PtSe2) van der Waals (vdW) heterostructure, in which extremely low dark current of 1.61×10-13 A, a record high light on/off ≈109 are demonstrated at 0 V. Notably, the device exhibits ultrafast response speed with rise time τr = 699 ns and decay time τd = 452 ns and high-power conversion efficiency (η) of 4.87%. The heterostructure demonstrates a broadband photoresponse from 365 nm to LWIR 10.6 µm at room temperature. Notably, the G-WSe2-PtSe2 nBn device demonstrates high photoresponsivity (R) of 1.8 AW-1 with 10.6 µm laser at 1 V bias in ambient air. This unipolar barrier device architecture offers an alternative way for highly sensitive free space communication.

Advances and Challenges in Interfacial Binding Forces for Electrocatalysts

As a practical chemical energy conversion technology, electrocatalysis could be used in fields of energy conversion and environmental protection. In recent years, significant research efforts have been devoted to the design and development of high-performance electrocatalysts because the rational design of catalysts is crucial for enhancing electrocatalytic performance. Creating electrocatalysts by forming interactions between different components at the interface is an important means of controlling and improving performance. Therefore, several common interfacial binding forces used for synthesizing electrocatalysts was systematically summarized in this review for the first time. The discussion revolves around the crucial roles these binding forces play in various electrocatalytic reaction processes. Various characterization techniques capable of proving the existence of these interfacial binding forces was also involved in the review. Finally, some prospects and challenges for designing and researching materials through the utilization of interfacial binding forces were presented.

The Construction and Mechanism Study of High-Speed Carrier Transport Channel in Gallium Arsenide Homojunction Toward High-Performance Photoelectrochemical Photodetector

The energy band structure and surface/interface properties are prerequisite for not only preserving the intrinsic material quality but also manipulating carrier transport behavior for photoelectrochemical (PEC) photodetection. How to precisely design/regulate the band structure and surface/interface properties of semiconductor materials is the key to improving the performance of PEC photodetection. Herein, the quintuple heterotypic homojunction (QH) GaAs film is fabricated with a gradient energy band via plasma-assisted molecular beam epitaxy for constructing a high-speed carrier transport channel in PEC photodetection, which can efficiently drive the separation and transport of photogenerated electron-hole pairs. The designed QH-GaAs-based PEC photodetector exhibits excellent performances, compared with bare i-GaAs, delivering an ultrashort rise/decay times of only 1.1/1.1 ms and a high responsivity of 20.4 mA W-1 at 0 V under 850 nm illumination. Strikingly, an ultrahigh detectivity with 1.46 × 1012 Jones is achieved. More importantly, the QH-GaAs device can stably operate underwater seawater environment. This study provides a novel strategy for designing and fabricating multiple heterotypic homojunction with gradient energy band to boost charge transport dynamics for PEC fields.

High-Performance Gate-Voltage-Tunable Photodiodes Based on Nb2Pd3Se8/WSe2 Mixed-Dimensional Heterojunctions

The mixed-dimensional (MD) van der Waals (vdWs) heterojunction for photodetectors has garnered significant attention owing to its exceptional compatibility and superior quality. Low-dimensional material heterojunctions exhibit unique photoelectric properties attributed to their nanoscale thickness and vdWs contact surfaces. In this work, a novel MD vdWs heterojunction composed of one-dimensional (1D) Nb2Pd3Se8 nanowires and two-dimensional (2D) WSe2 nanosheets is proposed. The heterojunction's energy band engineering is accomplished by manipulating the Fermi level of the bipolar 2D material via gate voltage, resulting in a rectification characteristic that can be adjusted with gate voltage. Under 685 nm laser irradiation, it demonstrates exceptional self-powered photodetection performance, attaining a photoresponsivity of 1.45 A W-1, an ultrahigh detectivity of 6.8 × 1012 Jones, and an ultrafast response time of 37/64 μs at zero bias. In addition, a broadband photodetector from 255 to 1064 nm is realized. These results demonstrate the great potential of Nb2Pd3Se8/WSe2 MD heterostructures for advanced electronic and optoelectronic devices.

PdSe2/2H-MoTe2 Heterojunction Self-Powered Photodetector: Broadband Photodetection and Linear/Circular Polarization Capability

In this research, we introduce a PdSe2/2H-MoTe2 heterojunction photodetector that exhibits both broadband self-powered photodetection and linear/circular polarization detection capabilities. It has a broad spectral response range (covering 375-2200 nm) and reaches a peak sensitivity at 532 nm, exhibiting a notable responsivity of 7.3 × 103 A/W and a substantial specific detectivity of 8.5 × 1012 Jones. Even in the near-infrared region of 1310 nm, it still has a high responsivity of 20 A/W. The self-powered photodetection capabilities of the PdSe2/2H-MoTe2 heterojunction are equally impressive, covering a broad range from 375 to 1550 nm, with a responsivity of 243 mA/W, a specific detectivity of 6.46 × 1010 Jones, a fill factor of 0.8, and an external quantum efficiency of 56.73%. Finally, simultaneous implementation of linear/circular polarization detection on the PdSe2/2H-MoTe2 heterojunction provides a powerful solution for near-infrared full-Stokes polarization detectors with high integration, miniaturization, and portability.

Synthesis, Modulation, and Application of Two-Dimensional TMD Heterostructures

Two-dimensional (2D) transition metal dichalcogenide (TMD) heterostructures have attracted a lot of attention due to their rich material diversity and stack geometry, precise controllability of structure and properties, and potential practical applications. These heterostructures not only overcome the inherent limitations of individual materials but also enable the realization of new properties through appropriate combinations, establishing a platform to explore new physical and chemical properties at micro-nano-pico scales. In this review, we systematically summarize the latest research progress in the synthesis, modulation, and application of 2D TMD heterostructures. We first introduce the latest techniques for fabricating 2D TMD heterostructures, examining the rationale, mechanisms, advantages, and disadvantages of each strategy. Furthermore, we emphasize the importance of characteristic modulation in 2D TMD heterostructures and discuss some approaches to achieve novel functionalities. Then, we summarize the representative applications of 2D TMD heterostructures. Finally, we highlight the challenges and future perspectives in the synthesis and device fabrication of 2D TMD heterostructures and provide some feasible solutions.

Composition Modulation-Mediated Band Alignment Engineering from Type I to Type III in 2D vdW Heterostructures

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe2/Bi2Te3- xSex vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe2/Bi2Te3) to Type III (WSe2/Bi2Se3). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 A W-1 and detectivity of 2.91×1012 Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.

Infrared photodetectors based on wide bandgap two-dimensional transition metal dichalcogenides via efficient two-photon absorption

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention due to their outstanding optoelectronic properties and ease of integration, making them ideal candidates for high-performance photodetectors. However, the excessive width of the bandgap in some 2D TMDs presents a challenge for achieving infrared photodetection. One approach to broaden the photoresponse wavelength range of TMDs is through the utilization of two-photon absorption (TPA) process. Unfortunately, the inefficiency of TPA hinders its application in infrared photodetection. In this study, we propose the design of two photodetectors utilizing high TPA coefficient materials, specifically ReSe2and MoS2, to exploit their TPA capability and extend the photoresponse to the near-infrared region at 1550 nm. The ReSe2photodetector demonstrates an unprecedented responsivity of 43μA W-1, surpassing that of current single-material TPA photodetectors. Similarly, the MoS2photodetector achieves a responsivity of 18μA W-1, comparable to state-of-the-art TPA photodetectors. This research establishes the potential of high TPA coefficient 2D TMDs for infrared photodetection.

Two-dimensional layered material photodetectors: what could be the upcoming downstream applications beyond prototype devices?

With distinctive advantages spanning excellent flexibility, rich physical properties, strong electrostatic tunability, dangling-bond-free surface, and ease of integration, 2D layered materials (2DLMs) have demonstrated tremendous potential for photodetection. However, to date, most of the research enthusiasm has been merely focused on developing novel prototype devices. In the past few years, researchers have also been devoted to developing various downstream applications based on 2DLM photodetectors to contribute to promoting them from fundamental research to practical commercialization, and extensive accomplishments have been realized. In spite of the remarkable advancements, these fascinating research findings are relatively scattered. To date, there is still a lack of a systematic and profound summarization regarding this fast-evolving domain. This is not beneficial to researchers, especially researchers just entering this research field, who want to have a quick, timely, and comprehensive inspection of this fascinating domain. To address this issue, in this review, the emerging downstream applications of 2DLM photodetectors in extensive fields, including imaging, health monitoring, target tracking, optoelectronic logic operation, ultraviolet monitoring, optical communications, automatic driving, and acoustic signal detection, have been systematically summarized, with the focus on the underlying working mechanisms. At the end, the ongoing challenges of this rapidly progressing domain are identified, and the potential schemes to address them are envisioned, which aim at navigating the future exploration as well as fully exerting the pivotal roles of 2DLMs towards the practical optoelectronic industry.

2D materials-based photodetectors combined with ferroelectrics

Photodetectors are essential optoelectronic devices that play a critical role in modern technology by converting optical signals into electrical signals, which are one of the most important sensors of the informational devices in current 'Internet of Things' era. Two-dimensional (2D) material-based photodetectors have excellent performance, simple design and effortless fabrication processes, as well as enormous potential for fabricating highly integrated and efficient optoelectronic devices, which has attracted extensive research attention in recent years. The introduction of spontaneous polarization ferroelectric materials further enhances the performance of 2D photodetectors, moreover, companying with the reduction of power consumption. This article reviews the recent advances of materials, devices in ferroelectric-modulated photodetectors. This review starts with the introduce of the basic terms and concepts of the photodetector and various ferroelectric materials applied in 2D photodetectors, then presents a variety of typical device structures, fundamental mechanisms and potential applications under ferroelectric polarization modulation. Finally, we summarize the leading challenges currently confronting ferroelectric-modulated photodetectors and outline their future perspectives.

Contemporary innovations in two-dimensional transition metal dichalcogenide-based P-N junctions for optoelectronics

Two-dimensional transition metal dichalcogenides (2D-TMDCs) with various physical characteristics have attracted significant interest from the scientific and industrial worlds in the years following Moore's law. The p-n junction is one of the earliest electrical components to be utilized in electronics and optoelectronics, and modern research on 2D materials has renewed interest in it. In this regard, device preparation and application have evolved substantially in this decade. 2D TMDCs provide unprecedented flexibility in the construction of innovative p-n junction device designs, which is not achievable with traditional bulk semiconductors. It has been investigated using 2D TMDCs for various junctions, including homojunctions, heterojunctions, P-I-N junctions, and broken gap junctions. To achieve high-performance p-n junctions, several issues still need to be resolved, such as developing 2D TMDCs of superior quality, raising the rectification ratio and quantum efficiency, and successfully separating the photogenerated electron-hole pairs, among other things. This review comprehensively details the various 2D-based p-n junction geometries investigated with an emphasis on 2D junctions. We investigated the 2D p-n junctions utilized in current rectifiers and photodetectors. To make a comparison of various devices easier, important optoelectronic and electronic features are presented. We thoroughly assessed the review's prospects and challenges for this emerging field of study. This study will serve as a roadmap for more real-world photodetection technology applications.

Improved performance of UV-blue dual-band Bi2O3/TiO2 photodetectors and application of visible light communication with UV light encryption

In this paper, self-powered photodetectors (PDs) with a dual-band photoresponse and excellent photodetection capabilities in complex environments can meet the needs of diverse detection targets, complex environments and diverse tasks. Herein, Bi2O3 nanosheets were deposited on the surface of TiO2 nanorod arrays (NRs) by chemical bath deposition (CBD) to construct self-powered heterojunction PDs with a UV-blue dual-band photoresponse. The nucleation and growth of Bi2O3 nanosheets on TiO2 NRs substrates were controlled by varying the concentration of the complexing agent triethanolamine (TEA) in the precursor solution, which regulated the morphology, crystalline quality and energy band structure as well as the photoelectronic properties of Bi2O3 films. The devices fabricated at a TEA concentration of 0.3 M exhibited excellent self-powered UV-blue dual-band photoresponse characteristics, achieving a photocurrent (Iph) of 144 nA, a responsivity of 1.79 mA W-1 and a detectivity of 5.94 × 1010 Jones under 405 nm illumination at 0 V, which can be attributed to the large built-in electric field (Eb) of Bi2O3/TiO2 heterojunctions, the low interfacial transfer resistance and suitable carrier transport path. In addition, Bi2O3/TiO2 heterojunction PDs with the UV-blue dual-band photoresponse characteristics can be applied in UV-encrypted visible light communication (VLC) with a light-controlled logic gate to improve the security of information transmission.

A waveguide-integrated self-powered van der Waals heterostructure photodetector with high performance at the telecom wavelength

Two-dimensional (2D) materials are attractive candidates for high-performance photodetectors due to their wide operating wavelength and potential to integrate with silicon photonics. However, due to their limited atomic thickness and short carrier lifetime, they suffer from high driving source-drain voltages, weak light-matter interactions and low carrier collection efficiency. Here, we present a high-performance van der Waals (vdWs) heterostructure-based photodetector integrated on a silicon nitride photonic platform combining p-type black phosphorus (BP) and n-type molybdenum disulfide (MoS2). Owing to the efficient carrier separation process and dark current suppression at the junction interface of the vdWs heterostructure, high photodetectivity and a fast response speed can be achieved. A fast response time (∼2.08/3.54 μs), high responsivity (11.26 mA W-1), and a high light on/off ratio (104) operating in the near-infrared telecom band are obtained at zero bias. Our research highlights the great potential of the high-efficiency waveguide-integrated vdWs heterojunction photodetector for integrated optoelectronic systems, such as high-data-rate interconnects operated at standardized telecom wavelengths.

High-Responsivity and Broadband MoS2 Photodetector Using Interfacial Engineering

Combining MoS2 with mature silicon technology is an effective method for preparing high-performance photodetectors. However, the previously studied MoS2/silicon-based heterojunction photodetectors cannot simultaneously demonstrate high responsivity, a fast response time, and broad spectral detection. We constructed a broad spectral n-type MoS2/p-type silicon-based heterojunction photodetector. The SiO2 dielectric layer on the silicon substrate was pretreated with soft plasma to change its thickness and surface state. The pretreated SiO2 dielectric layer and the silicon substrate constitute a multilayer heterostructure with a high carrier concentration and responsiveness. Taking silicon-based and n-type MoS2 heterojunction photodetectors as examples, its responsivity can reach 4.05 × 104 A W1- at 637 nm wavelength with a power density of 2 μW mm-2, and the detectable spectral range is measured from 447 to 1600 nm. This pretreated substrate was proven applicable to other n-type TMDCs, such as MoTe2, ReS2, etc., with certain versatility.

Photoelectric performance of InSe vdW semi-floating gate p-n junction transistor

Semi-floating gate transistors based on vdW materials are often used in memory and programmable logic applications. In this paper, we propose a semi-floating gate photoelectric p-n junction transistor structure which is stacked by InSe/h-BN/Gr. By modulating gate voltage, InSe can be presented as N-type and P-type respectively on different substrates, and then combined into p-n junction. Moreover, InSe/h-BN/Gr device can be switched freely between N-type resistance and p-n junction. The resistance value of InSe resistor and the photoelectric properties of the p-n junction are also sensitively modulated by laser. Under dark conditions, the rectification ratio of p-n junction can be as high as 107. After laser modulation, the device has a response up to 1.154 × 104A W-1, a detection rate up to 5.238 × 1012Jones, an external quantum efficiency of 5.435 × 106%, and a noise equivalent power as low as 1.262 × 10-16W/Hz1/2. It lays a foundation for the development of high sensitivity and fast response rate tunable photoelectric p-n junction transistor.

Self-powered and broadband opto-sensor with bionic visual adaptation function based on multilayer γ-InSe flakes

Visual adaptation that can autonomously adjust the response to light stimuli is a basic function of artificial visual systems for intelligent bionic robots. To improve efficiency and reduce complexity, artificial visual systems with integrated visual adaptation functions based on a single device should be developed to replace traditional approaches that require complex circuitry and algorithms. Here, we have developed a single two-terminal opto-sensor based on multilayer γ-InSe flakes, which successfully emulated the visual adaptation behaviors with a new working mechanism combining the photo-pyroelectric and photo-thermoelectric effect. The device can operate in self-powered mode and exhibit good human-eye-like adaptation behaviors, which include broadband light-sensing image adaptation (from ultraviolet to near-infrared), near-complete photosensitivity recovery (99.6%), and synergetic visual adaptation, encouraging the advancement of intelligent opto-sensors and machine vision systems.

Doping-Free High-Performance Photovoltaic Effect in a WSe2 Lateral p-n Homojunction Formed by Contact Engineering

Two-dimensional transition metal dichalcogenides (TMDs) are promising materials for semiconductor nanodevices owing to their flexibility, transparency, and appropriate band gaps. A variety of optoelectronic and electronic devices based on TMDs p-n diodes have been extensively investigated due to their unique advantages. However, improving their performance is challenging for commercial applications. In this study, we propose a facile and doping-free approach based on the contact engineering of a few-layer tungsten di-selenide to form a lateral p-n homojunction photovoltaic. By combining surface and edge contacts for p-n diode fabrication, the photovoltaic effect is achieved with a high fill factor of ≈0.64, a power conversion efficiency of up to ≈4.5%, and the highest external quantum efficiency with a value of ≈67.6%. The photoresponsivity reaches 283 mA/W, indicating excellent photodiode performance. These results demonstrate that our technique has great potential for application in next-generation optoelectronic devices.

Self-Powered Photodetector with High Efficiency and Polarization Sensitivity Enabled by WSe2/Ta2NiSe5/WSe2 van der Waals Dual Heterojunction

Self-powered photodetectors have triggered widespread attention because of the requirement of Internet of Things (IoT) application and low power consumption. However, it is challenging to simultaneously implement miniaturization, high quantum efficiency, and multifunctionalization. Here, we report a high-efficiency and polarization-sensitive photodetector enabled by two-dimensional (2D) WSe₂/Ta₂NiSe₅/WSe₂ van der Waals (vdW) dual heterojunctions (DHJ) along with a sandwich-like electrode pair. On account of enhanced light collection efficiency and two opposite built-in electric fields at the hetero-interfaces, the DHJ device achieves not only a broadband spectral response of 400-1550 nm but outstanding performance under 635 nm light illumination including an ultrahigh external quantum efficiency (EQE) of 85.5%, a pronounced power conversion efficiency (PCE) of 1.9%, and a fast response speed of 420/640 μs, which is much better than that of the WSe₂/Ta₂NiSe₅ single heterojunction (SHJ). Significantly, based on the strong in-plane anisotropy of 2D Ta₂NiSe₅ nanosheets, the DHJ device shows competitive polarization sensitivities of 13.9 and 14.8 under 635 and 808 nm light, respectively. Furthermore, an excellent self-powered visible imaging capability based on the DHJ device is demonstrated. These results pave a promising platform for realizing self-powered photodetectors with high performance and multifunctionality.

Reconfigurable Artificial Synapse Based on Ambipolar Floating Gate Memory

Artificial synapse networks capable of massively parallel computing and mimicking biological neural networks can potentially improve the processing efficiency of existing information technologies. Semiconductor devices functioning as excitatory and inhibitory synapses are crucial for developing intelligence systems, such as traffic control systems. However, achieving reconfigurability between two working modes (inhibitory and excitatory) and bilingual synaptic behavior in a single transistor remains challenging. This study successfully mimics a bilingual synaptic response using an artificial synapse based on an ambipolar floating gate memory comprising tungsten selenide (WSe₂)/hexagonal boron nitride (h-BN)/ molybdenum telluride (MoTe₂). In this WSe₂/h-BN/MoTe₂ structure, ambipolar semiconductors WSe₂ and MoTe₂ are inserted as channel and floating gates, respectively, and h-BN serves as the tunneling barrier layer. Using either positive or negative pulse amplitude modulations at the control gate, this device with bipolar channel conduction produced eight distinct resistance states. Based on this, we experimentally projected that we could achieve 490 memory states (210 hole-resistance states + 280 electron-resistance states). Using the bipolar charge transport and multistorage states of WSe₂/h-BN/MoTe₂ floating gate memory, we mimicked reconfigurable excitatory and inhibitory synaptic plasticity in a single device. Furthermore, the convolution neural network formed by these synaptic devices can recognize handwritten digits with an accuracy of >92%. This study identifies the unique properties of heterostructure devices based on two-dimensional materials as well as predicts their applicability in advanced recognition of neuromorphic computing.

Photosensitive Dielectric 2D Perovskite Based Photodetector for Dual Wavelength Demultiplexing

Stacked 2D perovskites provide more possibilities for next generation photodetector with more new features. Compared with its excellent optoelectronic properties, the good dielectric performance of metal halide perovskite rarely comes into notice. Here, a bifunctional perovskite based photovoltaic detector capable of two wavelength demultiplexing is demonstrated. In the Black Phosphorus/Perovskite/MoS₂ structured photodetector, the comprehensive utilization of the photosensitive and dielectric properties of 2D perovskite allows the device to work in different modes. The device shows normal continuous photoresponse under 405 nm, while it shows a transient spike response to visible light with longer wavelengths. The linear dynamic range, rise/decay time, and self-powered responsivity under 405 nm can reach 100, 38 µs/50 µs, and 17.7 mA W^-1 , respectively. It is demonstrated that the transient spike photocurrent with long wavelength exposure is related to the illumination intensity and can coexist with normal photoresponse. Two waveband-dependent signals can be identified and used to reflect more information simultaneously. This work provides a new strategy for multispectral detection and demultiplexing, which can be used to improve data transfer rates and encrypted communications. This work mode can inspire more multispectral photodetectors with different stacked 2D materials, especially to the optoelectronic application of the wide bandgap, high dielectric photosensitive materials.

High-Performance and Polarization-Sensitive Imaging Photodetector Based on WS2 /Te Tunneling Heterostructure

Next-generation imaging systems require photodetectors with high sensitivity, polarization sensitivity, miniaturization, and integration. By virtue of their intriguing attributes, emerging 2D materials offer innovative avenues to meet these requirements. However, the current performance of 2D photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, the lack of photoconductive gain, and insufficient photocarrier collection. Here, a tunneling dominant imaging photodetector based on WS₂ /Te heterostructure is reported. This device demonstrates competitive performance, including a remarkable responsivity of 402 A W^-1 , an outstanding detectivity of 9.28 × 10¹³ Jones, a fast rise/decay time of 1.7/3.2 ms, and a high photocurrent anisotropic ratio of 2.5. These outstanding performances can be attributed to the type-I band alignment with carrier transmission barriers and photoinduced tunneling mechanism, allowing reduced interfacial trapping effect, effective photoconductive gains, and anisotropic collection of photocarriers. Significantly, the constructed photodetector is successfully integrated into a polarized light imaging system and an ultra-weak light imaging system to illustrate the imaging capability. These results suggest the promising application prospect of the device in future imaging systems.

Homogeneous in-plane WSe2 P-N junctions for advanced optoelectronic devices

Conventional doping schemes of silicon (Si) microelectronics are incompatible with atomically thick two-dimensional (2D) transition metal dichalcogenides (TMDCs), which makes it challenging to construct high-quality 2D homogeneous p-n junctions. Herein, we adopt a simple yet effective plasma-treated doping method to seamlessly construct a lateral 2D WSe₂ p-n homojunction. WSe₂ with ambipolar transport properties was exposed to O₂ plasma to form WO_x on the surface in a self-limiting process that induces hole doping in the underlying WSe₂via electron transfer. Different electrical behaviors were observed between the as-exfoliated (ambipolar) region and the O₂ plasma-treated (p-doped) region under electrostatic modulation of the back-gate bias (V_BG), which produces a p-n in-plane homojunction. More importantly, a small contact resistance of 710 Ω μm with a p-doped region transistor mobility of ∼157 cm² V^-1 s^-1 was achieved due to the transformation of Schottky contact into Ohmic contact after plasma treatment. This effectively avoids Fermi-level pinning and significantly improves the performance of photodetectors. The resultant WSe₂ p-n junction device thus exhibits a high photoresponsivity of ∼7.1 × 10⁴ mA W^-1 and a superior external quantum efficiency of ∼228%. Also, the physical mechanism of charge transfer in the WSe₂ p-n homojunction was analyzed. Our proposed strategy offers a powerful route to realize low contact resistance and high photoresponsivity in 2D TMDC-based optoelectronic devices, paving the way for next-generation atomic-thickness optoelectronics.

Visible and infrared photodiode based on γ-InSe/Ge van der Waals heterojunction for polarized detection and imaging

Broadband photodetectors are a category of optoelectronic devices that have important applications in modern communication information. γ-InSe is a newly developed two-dimensional (2D) layered semiconductor with an air-stable and low-symmetry crystal structure that is suitable for polarization-sensitive photodetection. Herein, we report a P-N photodiode based on 3D Ge/2D γ-InSe van der Waals heterojunction (vdWH). A built-in electric field is introduced at the p-Ge/n-InSe interface to suppress the dark current and accelerate the separation of photogenerated carriers. Moreover, the heterojunction belongs to the accumulation mode with a well-designed type-II band arrangement, which is suitable for the fast separation of photogenerated carriers. Driven by these advantages, the device exhibits excellent photovoltaic performance within the detection range of 400 to 1600 nm and shows a double photocurrent peak at around 405 and 1550 nm. In particular, the responsivity (R) is up to 9.78 A W^-1 and the specific detectivity (D*) reaches 5.38 × 10¹¹ Jones with a fast response speed of 46/32 μs under a 1550 nm laser. Under blackbody radiation, the room temperature R and D* in the mid-wavelength infrared region are 0.203 A W^-1 and 5.6 × 10⁸ Jones, respectively. Moreover, polarization-sensitive light detection from 405-1550 nm was achieved, with the dichroism ratios of 1.44, 3.01, 1.71, 1.41 and 1.34 at 405, 635, 808, 1310 and 1550 nm, respectively. In addition, high-resolution single-pixel imaging capability is demonstrated at visible and near-infrared wavelengths. This work reveals the great potential of the γ-InSe/Ge photodiode for high-performance, broadband, air-stable and polarization-sensitive photodetection.

Integration of photovoltaic and photogating effects in a WSe2/WS2/p-Si dual junction photodetector featuring high-sensitivity and fast-response

Two-dimensional (2D) material-based van der Waals (vdW) heterostructures with exotic semiconducting properties have shown tremendous potential in next-generation photovoltaic photodetectors. Nevertheless, these vdW heterostructure devices inevitably suffer from a compromise between high sensitivity and fast response. Herein, an ingenious photovoltaic photodetector based on a WSe₂/WS₂/p-Si dual-vdW heterojunction is demonstrated. First-principles calculations and energy band profiles consolidate that the photogating effect originating from the bottom vdW heterojunction not only strengthens the photovoltaic effect of the top vdW heterojunction, but also suppresses the recombination of photogenerated carriers. As a consequence, the separation of photogenerated carriers is facilitated and their lifetimes are extended, resulting in higher photoconductive gain. Coupled with these synergistic effects, this WSe₂/WS₂/p-Si device exhibits both high sensitivity (responsivity of 340 mA W^-1, a light on/off ratio greater than 2500, and a detectivity of 3.34 × 10¹¹ Jones) and fast response time (rise/decay time of 657/671 μs) under 405 nm light illumination in self-powered mode. Finally, high-resolution visible-light and near-infrared imaging capabilities are demonstrated by adopting this dual-heterojunction device as a single pixel, indicating its great application prospects in future optoelectronic systems.

Synthesis of Ultrathin Topological Insulator β-Ag2 Te and Ag2 Te/WSe2 -Based High-Performance Photodetector

β-Ag₂ Te has attracted considerable attention in the application of electronics and optoelectronics due to its narrow bandgap, high mobility, and topological insulator properties. However, it remains a significant challenge to synthesize 2D Ag₂ Te because of the non-layered structure of Ag₂ Te. Herein, the synthesis of large-size, ultrathin single crystal topological insulator 2D Ag₂ Te via the van der Waals epitaxial method for the first time is reported. The 2D Ag₂ Te crystal exhibits p-type conduction behavior with high carrier mobility of 3336 cm² V^-1 s^-1 at room temperature. Taking advantage of the high mobility and perfect electron structure of Ag₂ Te, the Ag₂ Te/WSe₂ heterojunctions are fabricated via mechanical stacking and show an ultrahigh rectification ratio of 2 × 10⁵ . Ag₂ Te/WSe₂ photodetector also exhibits self-driven properties with a fast response speed (40 µs/60 µs) in the near-infrared region. High responsivity (219 mA W^-1 ) and light ON/OFF ratio of 6 × 10⁵ are obtained under the photovoltaic mode. The overall performance of the Ag₂ Te/WSe₂ photodetector is significantly competitive among all reported 2D photodetectors. These results indicate that 2D Ag₂ Te is a promising candidate for future electronic and optoelectronic applications.

Wafer-Scale Growth of Sb2Te3 Films via Low-Temperature Atomic Layer Deposition for Self-Powered Photodetectors

In this work, we demonstrate the performance of a silicon-compatible, high-performance, and self-powered photodetector. A wide detection range from visible (405 nm) to near-infrared (1550 nm) light was enabled by the vertical p-n heterojunction between the p-type antimony telluride (Sb₂Te₃) thin film and the n-type silicon (Si) substrates. A Sb₂Te₃ film with a good crystal quality, low density of extended defects, proper stoichiometry, p-type nature, and excellent uniformity across a 4 in. wafer was achieved by atomic layer deposition at 80 °C using (Et₃Si)₂Te and SbCl₃ as precursors. The processed photodetectors have a low dark current (∼20 pA), a high responsivity of (∼4.3 A/W at 405 nm and ∼150 mA/W at 1550 nm), a peak detectivity of ∼1.65 × 10¹⁴ Jones, and a quick rise time of ∼98 μs under zero bias voltage. Density functional theory calculations reveal a narrow, near-direct, type-II band gap at the heterointerface that supports a strong built-in electric field leading to efficient separation of the photogenerated carriers. The devices have long-term air stability and efficient switching behavior even at elevated temperatures. These high-performance and self-powered p-Sb₂Te₃/n-Si heterojunction photodetectors have immense potential to become reliable technological building blocks for a plethora of innovative applications in next-generation optoelectronics, silicon-photonics, chip-level sensing, and detection.

Optical synaptic devices with ultra-low power consumption for neuromorphic computing

Brain-inspired neuromorphic computing, featured by parallel computing, is considered as one of the most energy-efficient and time-saving architectures for massive data computing. However, photonic synapse, one of the key components, is still suffering high power consumption, potentially limiting its applications in artificial neural system. In this study, we present a BP/CdS heterostructure-based artificial photonic synapse with ultra-low power consumption. The device shows remarkable negative light response with maximum responsivity up to 4.1 × 108 A W-1 at VD = 0.5 V and light power intensity of 0.16 μW cm-2 (1.78 × 108 A W-1 on average), which further enables artificial synaptic applications with average power consumption as low as 4.78 fJ for each training process, representing the lowest among the reported results. Finally, a fully-connected optoelectronic neural network (FONN) is simulated with maximum image recognition accuracy up to 94.1%. This study provides new concept towards the designing of energy-efficient artificial photonic synapse and shows great potential in high-performance neuromorphic vision systems.

Direct Optical Lithography Enabled Multispectral Colloidal Quantum-Dot Imagers from Ultraviolet to Short-Wave Infrared

Complementary metal oxide semiconductor (CMOS) silicon sensors play a central role in optoelectronics with widespread applications from small cell phone cameras to large-format imagers for remote sensing. Despite numerous advantages, their sensing ranges are limited within the visible (0.4-0.7 μm) and near-infrared (0.8-1.1 μm) range , defined by their energy gaps (1.1 eV). However, below or above that spectral range, ultraviolet (UV) and short-wave infrared (SWIR) have been demonstrated in numerous applications such as fingerprint identification, night vision, and composition analysis. In this work, we demonstrate the implementation of multispectral broad-band CMOS-compatible imagers with UV-enhanced visible pixels and SWIR pixels by layer-by-layer direct optical lithography of colloidal quantum dots (CQDs). High-resolution single-color images and merged multispectral images were obtained by using one imager. The photoresponse nonuniformity (PRNU) is below 5% with a 0% dead pixel rate and room-temperature responsivities of 0.25 A/W at 300 nm, 0.4 A/W at 750 nm, and 0.25 A/W at 2.0 μm.

Enhanced self-powered ion-modulated photodetector based on an asymmetric composite structure of superionic conductor RbAg4I5 and graphene

Traditional strategies for self-powered devices face limitations in performance improvement due to the trade-off relationship between different parameters. Here, a new kind of ion-modulation self-powered photodetector is first proposed and fabricated by depositing superionic conductor RbAg₄I₅ on one side of monolayer graphene. The graphene homojunction is successfully formed at the boundary of the asymmetric structure due to the formation of bound states of ions and electrons at the contact interface. This kind of homojunction avoids the trade off between response parameters of traditional self-powered devices because the dissociation of bound states under light irradiation dominates the generation of a photocurrent. The experimental results indicate that the prepared photodetector can achieve great photo response with responsivity of 20 mA/W and a response speed of 700 µs for ultraviolet and visible light when no bias is applied, which is better than most existing graphene-based self-powered devices in single or overall parameters. Further, a semi-quantitative model is systematically established according to the internal mechanism and realizes a good consistency with experimental results. The work provides a new idea and offers the foundation to develop excellent self-powered devices based on superionic materials with good properties in controllability and modulation.

High-Responsivity Multiband and Polarization-Sensitive Photodetector Based on the TiS3/MoS2 Heterojunction

Two-dimensional (2D) material photodetectors have received considerable attention in optoelectronics as a result of their extraordinary properties, such as passivated surfaces, strong light-matter interactions, and broad spectral responses. However, single 2D material photodetectors still suffer from low responsivity, large dark current, and long response time as a result of their atomic-level thickness, large binding energy, and susceptibility to defects. Here, a transition metal trichalcogenide TiS₃ with excellent photoelectric characteristics, including a direct bandgap (1.1 eV), high mobility, high air stability, and anisotropy, is selected to construct a type-II heterojunction with few-layer MoS₂, aiming to improve the performance of 2D photodetectors. An ultrahigh photoresponsivity of the TiS₃/MoS₂ heterojunction of 48 666 A/W at 365 nm, 20 000 A/W at 625 nm, and 251 A/W at 850 nm is achieved under light-emitting diode illumination. The response time and dark current are 2 and 3 orders of magnitude lower than those of the current TiS₃ photodetector with the highest photoresponsivity (2500 A/W), respectively. Furthermore, polarized four-wave mixing spectroscopy and polarized photocurrent measurements verify its polarization-sensitive characteristics. This work confirms the excellent potential of TiS₃/MoS₂ heterojunctions for air-stable, high-performance, polarization-sensitive, and multiband photodetectors, and the excellent type-II TiS₃/MoS₂ heterojunction system may accelerate the design and fabrication of other 2D functional devices.

Van der Waals epitaxial growth and optoelectronics of a vertical MoS2/WSe2 p-n junction

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive attention due to their unique electronic and optical properties. In particular, TMDs can be flexibly combined to form diverse vertical van der Waals (vdWs) heterostructures without the limitation of lattice matching, which creates vast opportunities for fundamental investigation of novel optoelectronic applications. Here, we report an atomically thin vertical p-n junction WSe₂/MoS₂ produced by a chemical vapor deposition method. Transmission electron microscopy and steady-state photoluminescence experiments reveal its high quality and excellent optical properties. Back gate field effect transistor (FET) constructed using this p-n junction exhibits bipolar behaviors and a mobility of 9 cm²/(V·s). In addition, the photodetector based on MoS₂/WSe₂ heterostructures displays outstanding optoelectronic properties (R = 8 A/W, D* = 2.93 × 10¹¹ Jones, on/off ratio of 10⁴), which benefited from the built-in electric field across the interface. The direct growth of TMDs p-n vertical heterostructures may offer a novel platform for future optoelectronic applications.

Te/SnS2 tunneling heterojunctions as high-performance photodetectors with superior self-powered properties

The tunneling heterojunctions made of two-dimensional (2D) materials have been explored to have many intriguing properties, such as ultrahigh rectification and on/off ratio, superior photoresponsivity, and improved photoresponse speed, showing great potential in achieving multifunctional and high-performance electronic and optoelectronic devices. Here, we report a systematic study of the tunneling heterojunctions consisting of 2D tellurium (Te) and Tin disulfide (SnS₂). The Te/SnS₂ heterojunctions possess type-II band alignment and can transfer to type-III one under reverse bias, showing a reverse rectification ratio of about 5000 and a current on/off ratio over 10⁴. The tunneling heterojunctions as photodetectors exhibit an ultrahigh photoresponsivity of 50.5 A W^-1 in the visible range, along with a dramatically enhanced photoresponse speed. Furthermore, due to the reasonable type-II band alignment and negligible band bending at the interface, Te/SnS₂ heterojunctions at zero bias exhibit excellent self-powered performance with a high responsivity of 2.21 A W^-1 and external quantum efficiency of 678%. The proposed heterostructure in this work provides a useful guideline for the rational design of a high-performance self-powered photodetector.

Rational Design of WSe2 /WS2 /WSe2 Dual Junction Phototransistor Incorporating High Responsivity and Detectivity

The excellent semiconducting properties and ultrathin morphological characteristics allow van der Waals (vdW) heterostructures based on 2D materials to be promising channel materials for the next-generation optoelectronic devices, especially in photodetectors. Although various 2D heterostructure-based photodetectors have been developed, the unavoidable trade-off between responsivity and detectivity remains a critical issue for these devices. Here, an ingenious phototransistor based on WSe₂ /WS₂ /WSe₂ dual-vdW heterostructures is constructed, performing both high responsivity and detectivity. In the charge neutrality point (gate voltage of -15 V and bias voltage of 1 V), this device demonstrates a pronounced photosensitivity, accompanying with high detectivity of 1.9 × 10¹⁴  Jones, high responsivity of 35.4 A W^-1 , and fast rise/fall time of 3.2/2.5 ms at 405 nm with power density of 60 µW cm^-2 . Density functional theory calculations, energy band profiles, and optoelectronic characteristics jointly verify that the high performance is ascribed to the distinctive device design, which not only facilitates the separation of photogenerated carriers but also produces a strong photogating effect. As a feasible application, an automotive radar system is demonstrated, proving that the device has considerable potential for application in vehicle intelligent assisted driving.

Flexible 1.3 μm photodetector fabricated with InN nanowires and graphene on overhead projector transparency sheet

We report the first demonstration of flexible photodetectors, operating at the wavelength window of 1.3 μm, fabricated with InN nanowires (NWs) and graphene on an overhead projector transparency (OHP) sheet. The InN NWs, used as an absorption medium for the device, were formed on a Si substrate and exhibited strong emission with a peak wavelength of 1.3 μm at room temperature. They were randomly and horizontally embedded in the graphene sandwich structure functioned as a carrier channel. The photocurrent and photoresponsivity of the flexible photodetector were found to be 1.17 mA and 0.48 A W^-1, respectively, at a voltage of 1 V and a light intensity of 60 mW cm^-2 of a xenon lamp. The photocurrent measured when the photodetector was bent under a strain of 3% was 1.15 mA, which corresponds to 98.3% compared to that before bending. Moreover, the photocurrent and photoresponsivity of the flexible photodetector measured after the 200 cyclic-bending tests are comparable to those measured before bending.

Polarization-Resolved p-Se/n-WS2 Heterojunctions toward Application in Microcomputer System as Multivalued Signal Trigger

Polarization-sensitive photodetectors based on van der Waals heterojunctions (vdWH) have excellent polarization-resolved optoelectronic properties that can enable the applications in polarized light identification and imaging. With the development of optical microcomputer control systems (OMCS), it is crucial and energy efficient to adopt the self-powered and polarization-resolved signal-generators to optimize the circuit design of OMCS. In this work, the selenium (Se) flakes with in-plane anisotropy and p-type character are grown and incorporated with n-type tungsten disulfide (WS₂ ) to construct the type-II vdWH for polarization-sensitive and self-powered photodetectors. Under 405 nm monochrome laser with 1.33 mW cm^-2 power density, the photovoltaic device exhibits superior photodetection performance with the photoelectric conversion efficiency (PCE) of 3.6%, the responsivity (R) of 196 mA W^-1 and the external quantum efficiency (EQE) of about 60%. The strong in-plane anisotropy of Se crystal structure gives rise to the capability of polarized light detection with anisotropic photocurrent ratio of ≈2.2 under the 405 nm laser (13.71 mW cm^-2 ). Benefiting from the well polarization-sensitive and photovoltaic properties, the p-Se/n-WS₂ vdWH is successfully applied in the OMCS as multivalued signal trigger. This work develops the new anisotropic vdWH and demonstrates its feasibility for applications in logic circuits and control systems.

Recent Progress on Graphene Flexible Photodetectors

In recent years, optoelectronics and related industries have developed rapidly. As typical optoelectronics devices, photodetectors (PDs) are widely applied in various fields. The functional materials in traditional PDs exhibit high hardness, and the performance of these rigid detectors is thus greatly reduced upon their stretching or bending. Therefore, the development of new flexible PDs with bendable and foldable functions is of great significance and has much interest in wearable, implantable optoelectronic devices. Graphene with excellent electrical and optical performance constructed on various flexible and rigid substrates has great potential in PDs. In this review, recent research progress on graphene-based flexible PDs is outlined. The research states of graphene conductive films are summarized, focusing on PDs based on single-component graphene and mixed-structure graphene, with a systematic analysis of their optical and mechanical performance, and the techniques for optimizing the PDs are also discussed. Finally, a summary of the current applications of graphene flexible PDs and perspectives is provided, and the remaining challenges are discussed.

High-Throughput Prediction of the Band Gaps of van der Waals Heterostructures via Machine Learning

Van der Waals heterostructures offer an additional degree of freedom to tailor the electronic structure of two-dimensional materials, especially for the band-gap tuning that leads to various applications such as thermoelectric and optoelectronic conversions. In general, the electronic gap of a given system can be accurately predicted by using first-principles calculations, which is, however, restricted to a small unit cell. Here, we adopt a machine-learning algorithm to propose a physically intuitive descriptor by which the band gap of any heterostructures can be readily obtained, using group III, IV, and V elements as examples of the constituent atoms. The strong predictive power of our approach is demonstrated by high Pearson correlation coefficient for both the training (292 entries) and testing data (33 entries). By utilizing such a descriptor, which contains only four fundamental properties of the constituent atoms, we have rapidly predicted the gaps of 7140 possible heterostructures that agree well with first-principles results for randomly selected candidates.

Impact of the Channel Thickness on the Photoresponse of Black Arsenic Mid-Infrared Photodetectors

Recently explored black arsenic is a layered two-dimensional low-symmetry semiconducting material that, owing to its inherent narrow bandgap (∼0.31 eV) in its bulk form, is attractive for mid-infrared optoelectronics. Several studies have been conducted on its structural, charge-transport, and thermal properties for implementation in nanoelectronics. Herein, the thickness-dependent optoelectronic performance of black arsenic devices for mid-infrared wavelengths (2.0-4.0 μm) is investigated. The device was fabricated over an hBN/SiO2/Si substrate using mechanical exfoliation of black arsenic. It is observed that the optoelectronic properties of the devices vary significantly with the thickness of the black arsenic channel of the devices. A peak photoresponsivity of 244 A/W was achieved at 3.00 μm for a 60 nm-thick black arsenic channel. However, the maximum detectivity of 6.14 × 109 Jones was found for a lower thickness (∼25 nm) of black arsenic, along with an excellent (i.e., the least) noise-equivalent power of ∼89 fW/Hz1/2. Our findings reveal that the optoelectronic properties of black arsenic are excellent and can be tuned through thickness control. The promising results suggest the considerable potential of black arsenic in future opto- and nanoelectronic devices.

Controllable Preparation of 2D Vertical van der Waals Heterostructures and Superlattices for Functional Applications

2D van der Waals heterostructures (vdWHs) and superlattices (SLs) with exotic physical properties and applications for new devices have attracted immense interest. Compared to conventionally bonded heterostructures, the dangling-bond-free surface of 2D layered materials allows for the feasible integration of various materials to produce vdWHs without the requirements of lattice matching and processing compatibility. The quality of interfaces in artificially stacked vdWHs/vdWSLs and scalability of production remain among the major challenges in the field of 2D materials. Fortunately, bottom-up methods exhibit relatively high controllability and flexibility. The growth parameters, such as the temperature, precursors, substrate, and carrier gas, can be carefully and comprehensively controlled to produce high-quality interfaces and wafer-scale products of vdWHs/vdWSLs. This review focuses on three types of bottom-up methods for the assembly of vdWHs and vdWSLs with atomically clean and electronically sharp interfaces: chemical/physical vapor deposition, metal-organic chemical vapor deposition, and ultrahigh vacuum growth. These methods can intuitively illustrate the great flexibility and controllability of bottom-up methods for the preparation of vdWHs/vdWSLs. The latest progress in vdWHs and vdWSLs, related physical phenomena, and (opto)electronic devices are summarized. Finally, the authors discuss current challenges and future perspectives in the synthesis and application of vdWHs and vdWSLs.

Self-powered topological insulator Bi2Te3/Ge heterojunction photodetector driven by long-lived excitons transfer

Due to the wide spectral absorption and ultrafast electron dynamical response under optical excitation, topological insulator (TI) was proposed to have appealing application in next-generation photonic and optoelectronic devices. Whereas, the bandgap-free speciality of Dirac surface states usually leads to a quick relaxation of photoexcited carriers, making the transient excitons difficult to manipulate in isolated TIs. Growth of TI Bi₂Te₃/Ge heterostructures can promote the specific lifetime and quantity of long-lived excitons, offering the possibility of designing original near-infrared optoelectronic devices, however, the construction of TI Bi₂Te₃/Ge heterostructures has yet to be investigated. Herein, the high-quality Bi₂Te₃/Ge heterojunction with clear interface was prepared by physical vapor deposition strategy. A significant photoluminescence quenching behaviour was observed by experiments, which was attributed to the spontaneous excitation transfer of electrons at heterointerface via theoretical analysis. Then, a self-powered heterostructure photodetector was fabricated, which demonstrated a maximal detectivity of 1.3 × 10¹¹Jones, an optical responsivity of 0.97 A W^-1, and ultrafast photoresponse speed (12.1μs) under 1064 nm light illumination. This study offers a fundamental understanding of the spontaneous interfacial exciton transfer of TI-based heterostructures, and the as-fabricated photodetectors with excellent performance provided an important step to meet the increasing demand for novel optoelectronic applications in the future.

Epitaxial Topological Insulator Bi2Te3 for Fast Visible to Mid-Infrared Heterojunction Photodetector by Graphene As Charge Collection Medium

Three dimensional topological insulators have a thriving application prospect in broadband photodetectors due to the possessed topological quantum states. Herein, a large area and uniform topological insulator bismuth telluride (Bi₂Te₃) layer with high crystalline quality is directly epitaxial grown on GaAs(111)B wafer using a molecular beam epitaxy process, ensuring efficient out-of-plane carriers transportation due to reduced interface defects influence. By tiling monolayer graphene (Gr) on the as-prepared Bi₂Te₃ layer, a Gr/Bi₂Te₃/GaAs heterojunction array prototype was further fabricated, and our photodetector array exhibited the capability of sensing ultrabroad photodetection wavebands from visible (405 nm) to mid-infrared (4.5 μm) with a high specific detectivity (D*) up to 10¹² Jones and a fast response speed at about microseconds at room temperature. The enhanced device performance can be attributed to enhanced light-matter interaction at the high-quality heterointerface of Bi₂Te₃/GaAs and improved carrier collection efficiency through graphene as a charge collection medium, indicating an application prospect of topological insulator Bi₂Te₃ for fast-speed broadband photodetection up to a mid-infrared waveband. This work demonstrated the potential of integrated topological quantum materials with a conventional functional substrate to fabricate the next generation of broadband photodetection devices for uncooled focal plane array or infrared communication systems in future.

Advances in Self-Powered Ultraviolet Photodetectors Based on P-N Heterojunction Low-Dimensional Nanostructures

Self-powered ultraviolet (UV) photodetectors have attracted considerable attention in recent years because of their vast applications in the military and civil fields. Among them, self-powered UV photodetectors based on p-n heterojunction low-dimensional nanostructures are a very attractive research field due to combining the advantages of low-dimensional semiconductor nanostructures (such as large specific surface area, excellent carrier transmission channel, and larger photoconductive gain) with the feature of working independently without an external power source. In this review, a selection of recent developments focused on improving the performance of self-powered UV photodetectors based on p-n heterojunction low-dimensional nanostructures from different aspects are summarized. It is expected that more novel, dexterous, and intelligent photodetectors will be developed as soon as possible on the basis of these works.

Vertical 1D/2D Heterojunction Architectures for Self-Powered Photodetection Application: GaN Nanorods Grown on Transition Metal Dichalcogenides

Van der Waals (vdW) heterojunctions based on two-dimensional (2D) transition metal dichalcogenide (TMD) materials have attracted the attention of researchers to conduct fundamental investigations on emerging physical phenomena and expanding diverse nano-optoelectronic devices. Herein, the quasi-van der Waals epitaxial (QvdWE) growth of vertically aligned one-dimensional (1D) GaN nanorod arrays (NRAs) on TMDs/Si substrates is reported, and their vdW heterojunctions in the applications of high-performance self-powered photodetection are demonstrated accordingly. Such 1D/2D hybrid systems fully combine the advantages of the strong light absorption of 1D GaN nanoarrays and the excellent electrical properties of 2D TMD materials, boosting the photogenerated current density, which demonstrates a light on/off ratio above 10⁵. The device exhibits a competitive photovoltaic photoresponsivity over 10 A W^-1 under a weak detectable light signal without any external bias, which is attributed to the efficient photogenerated charge separation under the strong built-in potential from the type-II band alignment of GaN NRAs/TMDs. This work presents a QvdWE route to prepare 1D/2D heterostructures for the fabrication of self-powered photodetectors, which shows promising potentials for practical applications of space communications, sensing networks, and environmental monitoring.

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.

Engineering sensitivity and spectral range of photodetection in van der Waals materials and hybrids

Exploration of van der Waals heterostructures in the field of optoelectronics has produced photodetectors with very high bandwidth as well as ultra-high sensitivity. Appropriate engineering of these heterostructures allows us to exploit multiple light-to-electricity conversion mechanisms, ranging from photovoltaic, photoconductive to photogating processes. These mechanisms manifest in different sensitivity and speed of photoresponse. In addition, integrating graphene-based hybrid structures with photonic platforms provides a high gain-bandwidth product, with bandwidths ≫1 GHz. In this review, we discuss the progression in the field of photodetection in 2D hybrids. We emphasize the physical mechanisms at play in diverse architectures and discuss the origin of enhanced photoresponse in hybrids. Recent developments in 2D photodetectors based on room temperature detection, photon-counting ability, integration with Si and other pressing issues, that need to be addressed for these materials to be integrated with industrial standards have been discussed.

Substitutionally Doped MoSe2 for High-Performance Electronics and Optoelectronics

2D materials, of which the carrier type and concentration are easily tuned, show tremendous superiority in electronic and optoelectronic applications. However, the achievements are still quite far away from practical applications. Much more effort should be made to further improve their performance. Here, p-type MoSe₂ is successfully achieved via substitutional doping of Ta atoms, which is confirmed experimentally and theoretically, and outstanding homojunction photodetectors and inverters are fabricated. MoSe₂ p-n homojunction device with a low reverse current (300 pA) exhibits a high rectification ratio (10⁴ ). The analysis of dark current reveals the domination of the Shockley-Read-Hall (SRH) and band-to-band tunneling (BTB) current. The homojunction photodetector exhibits a large open-circuit voltage (0.68 V) and short-circuit currents (1 µA), which is suitable for micro-solar cells. Furthermore, it possesses outstanding responsivity (0.28 A W^-1 ), large external quantum efficiency (42%), and a high signal-to-noise ratio (≈10⁷ ). Benefiting from the continuous energy band of homojunction, the response speed reaches up to 20 µs. Besides, the Ta-doped MoSe₂ inverter exhibits a high voltage gain (34) and low power consumption (127 nW). This work lays a foundation for the practical application of 2D material devices.

Vertically stacked Bi2Se3/MoTe2 heterostructure with large band offsets for nanoelectronics

In recent years, two-dimensional material-based tunneling heterojunctions are emerging as a multi-functional architecture for logic circuits and photodetection owing to the flexible stacking, optical sensitivity, tunable detection band, and highly controllable conductivity behaviors. However, the existing structures are mainly focused on transition or post-transition metal chalcogenides and have been rarely investigated as topological insulator (such as Bi₂Se₃ or Bi₂Te₃)-based tunneling heterostructures. Meanwhile, it is challenging to mechanically exfoliate the topological insulator thin nanoflakes because of the strong layer-by-layer interaction with shorter interlayer spacing. Herein, we report Au-assisted exfoliation and non-destructive transfer method to fabricate large-scale Bi₂Se₃ thin nanosheets. Furthermore, a novel broken-gap tunneling heterostructure is designed by combing 2H-MoTe₂ and Bi₂Se₃via the dry-transfer method. Thanks to the realized band alignment, this ambipolar-n device shows a clear rectifying behavior at V_ds of 1 V. A built-in potential exceeding ∼0.7 eV is verified owing to the large band offsets by comparing the numerical solution of Poisson's equation and the experimental data. Carrier transport is governed by the majority carrier including thermionic emission and the tunneling process through the barrier height. At last, the device shows an ultralow dark current of ∼0.2 pA and a superior optoelectrical performance of I_light/I_dark ratio ≈10⁶, a fast response time of 21 ms, and a specific detectivity of 7.2 × 10¹¹ Jones for a visible light of 405 nm under zero-bias. Our work demonstrates a new universal method to fabricate a topological insulator and paves a new strategy for the construction of novel van der Waals tunneling structures.

Self-driven SnS1-xSex alloy/GaAs heterostructure based unique polarization sensitive photodetectors

With the fast development of semiconductor technology, self-driven devices have become an indispensable part of modern electronic and optoelectronic components. In this field, in addition to traditional Schottky and p-n junction devices, hybrid 2D/3D semiconductor heterostructures provide an alternative platform for optoelectronic applications. Herein we report the growth of 2D SnS_1-xSe_x (x = 0, 0.5, 1) nanosheets and the construction of a hybrid SnS_0.5Se_0.5/GaAs heterostructure based self-driven photodetector. The strong anisotropy of 2D SnS_1-xSe_x is demonstrated theoretically and experimentally. The self-driven photodetector shows high sensitivity to incident light from the visible to near-infrared regime. At the wavelength of 405 nm, the device enables maximum responsivity of 10.2 A W^-1, high detectivity of 4.8 × 10¹² Jones and fast response speed of 0.5/3.47 ms. Impressively, such a heterostructure device exhibits anisotropic photodetection characteristics with the dichroic ratio of ∼1.25 at 405 nm and ∼1.45 at 635 nm. These remarkable features can be attributed to the high-quality built-in potential at the SnS_0.5Se_0.5/GaAs interface and the alloy engineering, which effectively separates the photogenerated carriers and suppresses the deep-level defects, respectively. These results imply the great potential of our SnS_0.5Se_0.5/GaAs heterostructure for high-performance photodetection devices.

Circularly Polarized Photodetectors Based on Chiral Materials: A Review

Circularly polarized light (CPL) plays an important role in many photonic techniques, including tomographic scanning based on circular polarization ellipsometry, optical communication and information of spin, and quantum-based optical calculation and information processing. To fully exploit the functions of CPL in these fields, integrated photoelectric sensors capable of detecting CPL are essential. Photodetectors based on chiral materials can directly detect CPL due to their intrinsic optical activity, without the need to be coupled with polarizers and quarter-wave plates as in conventional photodetectors. This review summarizes the recent research progress in CPL photodetectors based on chiral materials. We first briefly introduce the CPL photodetectors based on different types of chiral materials and their working principles. Finally, current challenges and future opportunities in the development of CPL photodetectors are prospected.