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

Plasmonic Hot-Electron Effect Enhanced WSe2 Based Transistor Based on Asymmetric Schottky Contacts for Self-Powered Photodetection and Visual Synapse

Schottky interfaces in metal-semiconductor contacts are crucial in optoelectronics, with a focus on enhancing the detection performance. The plasmonic hot-electron effect offers efficient photon-to-electricity conversion, boosting the sensitivity in self-powered photodetectors as well as expanding detection wavelength ranges and improving the functionality of metal-semiconductor-metal (M1-S-M2) structured photodetectors. Utilizing two-dimensional WSe2 nanoflakes, we fabricate a transistor with a M1-S-M2 structure featuring Au and Ag electrodes. Under 405 nm light, we achieve a maximum specific detectivity (D*) of 9.23 × 1011 Jones with a photocurrent density of 4.6 mA cm-2 and a peak on/off ratio of 6.88 × 105. Compared with the Au/WSe2/Au transistor also on the poly(ethylene terephthalate) (PET) substrate, the maximum current density measured for the Au/WSe2/Ag transistor under the light of 405 nm is 1.3 times that of the former, and the D* measured under the light of 1064 nm increases to 20 times the original value; it is now capable of detecting the 1550 nm light, which was undetectable previously. These data clearly demonstrate that the transistor exhibits excellent photodetection performance in the visible and near-infrared spectra. In addition, under biased voltage conditions, the transistor can effectively simulate the visual synaptic behavior under the stimulation of visible light and near-infrared light. Due to its simple structure, wide detection range, excellent light detection performance, and remarkable synaptic plasticity characteristics, this transistor has great potential in various applications of light detection technology and artificial vision systems.

High current density heterojunction bipolar transistors with 3D-GaN/2D-WSe2 as emitter junctions

With the continuous advancement of electronic technology, there is an increasing demand for high-speed, high-frequency, and high-power devices. Due to the inherently small thickness and absence of dangling bonds of two-dimensional (2D) materials, heterojunction bipolar transistors (HBTs) based on 2D layered materials (2DLMs) have attracted significant attention. However, the low current density and limited structural design flexibility of 2DLM-based HBT devices currently hinder their applications. In this work, we present a novel vertical GaN/WSe2/MoS2 HBT with three-dimensional (3D)-GaN/2D-WSe2 as the emitter junction. Harnessing the high carrier concentration and wide bandgap of 3D-GaN, an HBT with a current density of about 260 A cm-2 is obtained. In addition, by selecting an adequate position for the collector electrode, we achieve efficient carrier collection through a collector junction smaller than the emitter junction area, obtaining a common-base current gain of 0.996 and a remarkable common-emitter current gain (β) of 12.4.

Development and challenges of polarization-sensitive photodetectors based on 2D materials

Polarization-sensitive photodetectors based on two-dimensional (2D) materials have garnered significant research attention owing to their distinctive architectures and exceptional photophysical properties. Specifically, anisotropic 2D materials, including semiconductors such as black phosphorus (BP), tellurium (Te), transition metal dichalcogenides (TMDs), and tantalum nickel pentaselenide (Ta2NiSe5), as well as semimetals like 1T'-MoTe2 and PdSe2, and their diverse van der Waals (vdW) heterojunctions, exhibit broad detection spectral ranges and possess inherent functional advantages. To date, numerous polarization-sensitive photodetectors based on 2D materials have been documented. This review initially provides a concise overview of the detection mechanisms and performance metrics of 2D polarization-sensitive photodetectors, which are pivotal for assessing their photodetection capabilities. It then examines the latest advancements in polarization-sensitive photodetectors based on individual 2D materials, 2D vdW heterojunctions, nanoantenna/electrode engineering, and structural strain integrated with 2D structures, encompassing a range of devices from the ultraviolet to infrared bands. However, several challenges persist in developing more comprehensive and functional 2D polarization-sensitive photodetectors. Further research in this area is essential. Ultimately, this review offers insights into the current limitations and challenges in the field and presents general recommendations to propel advancements and guide the progress of 2D polarization-sensitive photodetectors.

2D Programmable Photodetectors Based on WSe2/h-BN/Graphene Heterojunctions

Programmable photovoltaic photodetectors based on 2D materials can modulate optical and electronic signals in parallel, making them particularly well-suited for optoelectronic hybrid dual-channel communication. This work presents a programmable non-volatile bipolar semi-floating gate photovoltaic photodetector (SFG-PD) constructed using tungsten diselenide (WSe2), hexagonal boron nitride (h-BN), and graphene (Gra). By controlling the voltage pulses applied to the control gate, the device generates opposing built-in electric field junctions (p+-p and n-p junctions), enabling reversible switching between positive and negative light responses with a rapid response time of up to 2.02 µs. Moreover, the application of this device is demonstrated in dual-channel optoelectronic hybrid communication, offering a practical solution for achieving high-speed, large-capacity, low-loss, and secure multi-channel communication.

Modulation anisotropy of nanomaterials toward monolithic integrated polarization-sensitive photodetectors

By virtue of the unique ability of providing additional information beyond light intensity and spectra, polarization-sensitive photodetectors could precisely identify targets in several concealed, camouflaged, and non-cooperative backgrounds, making them highly suitable for potential applications in remote sensing, astronomical detection, medical diagnosis, etc. Therefore, to provide a comprehensive design guideline for a wide range of interdisciplinary researchers, this review provides a general overview of state-of-the-art linear, circular, and full-Stokes polarization-sensitive photodetectors. In particular, from the perspectives of technological progress and the development of nanoscience, the detailed discussion focuses on strategies to simplify high-performance polarization-sensitive photodetectors, reducing their size and achieving a smaller volume. In addition, to lay a solid foundation for modulating the properties of future nanostructure-based polarization-sensitive photodetectors, insights into light-matter interactions in low-symmetry materials and asymmetric structures are provided here. Meanwhile, the corresponding opportunities and challenges in this research field are identified.

High Hole Mobility van der Waals Junction Field-Effect Transistors Based on Te/GaAs for Multimode Photodetection and Logic Applications

Recently, interface scattering and low mobility have significantly impeded the performance of two-dimensional (2D) P-type transistors. 2D semiconductor tellurium (Te) has garnered significant interest owing to its unique atomic chain crystal structure, which confers ultrahigh hole mobility. van der Waals heterojunction enhances transistor performance by reducing scattering at the gate-channel interface, attributed to its high-quality interface. In this study, we present Te/gallium arsenide (GaAs) hybrid dimensional JFETs exhibiting sizable on-state currents, elevated transconductance, and mobility as high as 328.4 cm2V-1s-1. Achieving a low-power device, we lowered the threshold voltage from 1.9 to 1 V by modifying the carrier concentration of the gate. Furthermore, enhancing negative photoconductivity on the Te surface is achieved by tuning the depth of the channel depletion region, thereby achieving an enhanced negative photoconductivity mechanism with universal applicability. Based on this, a photodetector featuring both positive and negative photoconductivity and a photovoltaic effect was developed. The negative photoresponsivity and detectivity at 635 nm of the device are -64 AW-1 and 1.41 × 1010 Jones, respectively. Utilizing these properties, we develop Te/GaAs JFET-based logic gate circuits and single-point negative photoconductive imaging applications. This provides a potential research avenue for future logic circuits and optoelectronic devices.

Physisorption-assistant optoelectronic synaptic transistors based on Ta2NiSe5/SnS2 heterojunction from ultraviolet to near-infrared

Neuromorphic computing vision is the most promising technological solution to overcome the arithmetic bottleneck in machine vision applications. All-in-one neuromorphic sensors have been attracting increased attention because they can integrate visual perception, processing, and memory functionalities into one single device. However, the limited responsivity and data retention time of all-in-one neuromorphic sensors usually hinder their potential in multispectral machine vision, especially in the near-infrared (NIR) band which contains critical information for pattern recognition. Here, we demonstrate physisorption-assistant optoelectronic synaptic transistors based on Ta2NiSe5/SnS2 heterojunction, which present tunable synaptic functionality in broadband (375-1310 nm). We propose a strategy about the physisorption-assistant persistent photoconductivity (PAPPC) effect to effectively solve the problem in detecting and storing the NIR light information. Under this strategy, the responsivity and data retention time of our devices were significantly enhanced and prolonged in broadband from 375 to 1310 nm. Further, the devices realize multilevel non-volatile optoelectronic memory through the modulation of several optical and back-gate signals to simulate emotion-controlled learning and memory processes, optical writing-electric erasing, and associative learning. Moreover, we developed a simplified human visual system to simulate color-cognitive perception and memory functions. Our approach offers a route for creating advanced all-in-one neuromorphic sensors and developing neuromorphic computing vision.

Reconfigurable Phototransistors Driven by Gate-Dependent Carrier Modulation in WSe2/Ta2NiSe5 van der Waals Heterojunctions

Reconfigurable field-effect transistors (RFETs) offer notable benefits on electronic and optoelectronic logic circuits, surpassing the integration, flexibility, and cost-efficiency of conventional complementary metal-oxide semiconductor transistors. The low on/off current ratio of these transistors remains a considerable impediment in the practical application of RFETs. To overcome these limitations, a van der Waals heterojunction (vdWH) transistor composed of WSe2/Ta2NiSe5 has been proposed. By modulating a single back-gate voltage and source-drain voltage inputs, the transistor achieves a switchable polarity configuration and bidirectional rectification, making it capable of functioning as a gate-controlled bidirectional half-wave rectifier. The proposed RFET exhibits tunable positive/negative photovoltaic responses, advanced optoelectronic performance, and a gate-voltage-dependent reversal of the photodetector position. Detailed energy band diagram studies have shown that the reconfigurability of the device arises from carrier blockage resulting from the type-I band structure and carrier injection modulated by gate-dependent Schottky barriers. Consequently, the reconfigurable WSe2/Ta2NiSe5 vdWH holds significant promise for advanced multifunctional optoelectronic device applications.

Simulating and Implementing Broadband van der Waals Artificial Visual Synapses Based on Photoconductivity and Pyroconductivity Mechanisms

With advancements in artificial neural networks and information processing technology, a variety of neuromorphic synaptic devices have been proposed to emulate human sensory systems, with vision being a crucial information source. Moreover, as practical applications become increasingly complex, the need for multifunctional visual synapses to expand the application range becomes urgent. This study introduces a MoS2/WSe2 van der Waals (vdW) heterojunction and utilizes it to replicate artificial visual synapses by harnessing the cooperative effect of photoconductivity and pyroconductivity mechanisms. By adjusting the optical power, pulse width, and pulse number of the optical stimulus, the heterojunction effectively simulates synaptic properties. Under the combined action of an external electric field and the built-in electric field (Ebi), the heterojunction exhibits broadband synaptic properties in the visible to near-infrared spectrum (405-1550 nm) while consuming low power of 0.3-1.1 pJ per spike. The heterojunction can detect ultraweak optical signals at 660 nm with an optical power intensity of 14 μW/cm2, displaying a high specific detectivity (D*) of 3.98 × 1011 Jones. Furthermore, at 405, 808, 1064, and 1550 nm, the D* of the heterojunction is 4.16 × 1011, 3.61 × 109, 4.96 × 107, and 1.64 × 107 Jones, respectively. Visual synaptic devices based on the MoS2/WSe2 vdW heterojunction hold significant promise for the future development of integrated sensing and memory processing devices.

Room-Temperature Band-Aligned Infrared Heterostructures for Integrable Sensing and Communication

The demand for miniaturized and integrated multifunctional devices drives the progression of high-performance infrared photodetectors for diverse applications, including remote sensing, air defense, and communications, among others. Nonetheless, infrared photodetectors that rely solely on single low-dimensional materials often face challenges due to the limited absorption cross-section and suboptimal carrier mobility, which can impair sensitivity and prolong response times. Here, through experimental validation is demonstrated, precise control over energy band alignment in a type-II van der Waals heterojunction, comprising vertically stacked 2D Ta2NiSe5 and the topological insulator Bi2Se3, where the configuration enables polarization-sensitive, wide-spectral-range photodetection. Experimental evaluations at room temperature reveal that the device exhibits a self-powered responsivity of 0.48 A·W-1, a specific directivity of 3.8 × 1011 cm·Hz1/2·W-1, a response time of 151 µs, and a polarization ratio of 2.83. The stable and rapid photoresponse of the device underpins the utility in infrared-coded communication and dual-channel imaging, showing the substantial potential of the detector. These findings articulate a systematic approach to developing miniaturized, multifunctional room-temperature infrared detectors with superior performance metrics and enhanced capabilities for multi-information acquisition.

High-Performance Photoinduced Tunneling Self-Driven Photodetector for Polarized Imaging and Polarization-Coded Optical Communication based on Broken-Gap ReSe2/SnSe2 van der Waals Heterojunction

Novel 2D materials with low-symmetry structures exhibit great potential applications in developing monolithic polarization-sensitive photodetectors with small volume. However, owing to the fact that at least half of them presented a small anisotropic factor of ≈2, comprehensive performance of present polarization-sensitive photodetectors based on 2D materials is still lower than the practical application requirements. Herein, a self-driven photodetector with high polarization sensitivity using a broken-gap ReSe2/SnSe2 van der Waals heterojunction (vdWH) is demonstrated. Anisotropic ratio of the photocurrent (Imax/Imin) could reach 12.26 (635 nm, 179 mW cm-2). Furthermore, after a facile combination of the ReSe2/SnSe2 device with multilayer graphene (MLG), Imax/Imin of the MLG/ReSe2/SnSe2 can be further increased up to13.27, which is 4 times more than that of pristine ReSe2 photodetector (3.1) and other 2D material photodetectors even at a bias voltage. Additionally, benefitting from the synergistic effect of unilateral depletion and photoinduced tunneling mechanism, the MLG/ReSe2/SnSe2 device exhibits a fast response speed (752/928 µs) and an ultrahigh light on/off ratio (105). More importantly, MLG/ReSe2/SnSe2 device exhibits excellent potential applications in polarized imaging and polarization-coded optical communication with quaternary logic state without any power supply. This work provides a novel feasible avenue for constructing next-generation smart polarization-sensitive photodetector with low energy consumption.

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.

Two-Dimensional GeS/SnSe2 Tunneling Photodiode with Bidirectional Photoresponse and High Polarization Sensitivity

A two-dimensional (2D) broken-gap (type-III) p-n heterojunction has a unique charge transport mechanism because of nonoverlapping energy bands. In light of this, type-III band alignment can be used in tunneling field-effect transistors (TFETs) and Esaki diodes with tunable operation and low consumption by highlighting the advantages of tunneling mechanisms. In recent years, 2D tunneling photodiodes have gradually attracted attention for novel optoelectronic performance with a combination of strong light-matter interaction and tunable band alignment. However, an in-depth understanding of the tunneling mechanisms should be further investigated, especially for developing electronic and optoelectronic applications. Here, we report a type-III tunneling photodiode based on a 2D multilayered p-GeS/n+-SnSe2 heterostructure, which is first fabricated by the mechanical exfoliation and dry transfer method. Through the Simmons approximation, its various tunneling transport mechanisms dependent on bias and light are demonstrated as the origin of excellent bidirectional photoresponse performance. Moreover, compared to the traditional p-n photodiode, the device enables bidirectional photoresponse capability, including maximum responsivity values of 43 and 8.7 A/W at Vds = 1 and -1 V, respectively, with distinctive photoactive regions from the scanning photocurrent mapping. Noticeably, benefiting from the in-plane anisotropic structure of GeS, the device exhibits an enhanced photocurrent anisotropic ratio of 9, driven by the broader depletion region at Vds = -3 V under 635 nm irradiation. Above all, the results suggest that our designed architecture can be potentially applied to CMOS imaging sensors and polarization-sensitive photodetectors.

Anisotropic Fracture of Two-Dimensional Ta2NiSe5

Anisotropic two-dimensional materials present a diverse range of physical characteristics, making them well-suited for applications in photonics and optoelectronics. While mechanical properties play a crucial role in determining the reliability and efficacy of 2D material-based devices, the fracture behavior of anisotropic 2D crystals remains relatively unexplored. Toward this end, we herein present the first measurement of the anisotropic fracture toughness of 2D Ta2NiSe5 by microelectromechanical system-based tensile tests. Our findings reveal a significant in-plane anisotropic ratio (∼3.0), accounting for crystal orientation-dependent crack paths. As the thickness increases, we observe an intriguing intraplanar-to-interplanar transition of fracture along the a-axis, manifesting as stepwise crack features attributed to interlayer slippage. In contrast, ruptures along the c-axis surprisingly exhibit persistent straightness and smoothness regardless of thickness, owing to the robust interlayer shear resistance. Our work affords a promising avenue for the construction of future electronics based on nanoribbons with atomically sharp edges.

Ultrasensitive Near-Infrared Polarization Photodetectors with Violet Phosphorus/InSe van der Waals Heterostructures

Near-infrared (NIR) polarization photodetectors with two-dimensional (2D) semiconductors and their van der Waals (vdW) heterostructures have presented great impact for the development of a wide range of technologies, such as in the optoelectronics and communication fields. Nevertheless, the lack of a photogenerated charge carrier at the device's interface leads to a poor charge carrier collection efficiency and a low linear dichroism ratio, hindering the achievement of high-performance optoelectronic devices with multifunctionalities. Herein, we present a type-II violet phosphorus (VP)/InSe vdW heterostructure that is predicted via density functional theory calculation and confirmed by Kelvin probe force microscopy. Benefiting from the type-II band alignment, the VP/InSe vdW heterostructure-based photodetector achieves excellent photodetection performance such as a responsivity (R) of 182.8 A/W, a detectivity (D*) of 7.86 × 1012 Jones, and an external quantum efficiency (EQE) of 11,939% under a 1064 nm photon excitation. Furthermore, the photodetection performance can be enhanced by manipulating the device geometry by inserting a few layers of graphene between the VP and InSe (VP/Gr/InSe). Remarkably, the VP/Gr/InSe vdW heterostructure shows a competitive polarization sensitivity of 2.59 at 1064 nm and can be integrated as an image sensor. This work demonstrates that VP/InSe and VP/Gr/InSe vdW heterostructures will be effective for promising integrated NIR optoelectronics.

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.

Pyro-Phototronic Effect-Enhanced Photocurrent of a Self-Powered Photodetector Based on ZnO Nanofiber Arrays/BaTiO3 Films

Self-powered photodetectors (PD) based on ferroelectric materials have gained huge attention because of the spontaneous polarization and unique photovoltaic effect. However, the low photocurrent values and switch ratio of the ferroelectric materials limit their further practical applications in a wide temperature range. In this study, the self-powered ZnO nanofiber array/BaTiO3 (ZnO-NFA/BTO) PD was fabricated by high-ordered ZnO-NFA via electrospinning method deposited on a 300 nm BTO film synthesized using sol-gel method. The electrospinning can prepare ZnO-NFAs with a controllable diameter (100 nm) and orientation and is directly deposited on the quartz at a large scale, which simplifies the fabrication process. This device possesses a greater on/off ratio of 2357 at zero bias than that of BTO PD (3.33) and the ZnO-NFA PD (125) at 0.2 V. The highest responsivity and specific detectivity are 1.41 mA W-1 and 1.48 × 109 Jones at 368 nm under 0 V bias, respectively, which is enhanced about three magnitudes than the pristine BTO PD (1.21 μA W-1 and 1.02 × 109 Jones). The photocurrent of the ZnO-NFA/BTO PD strongly depends on the temperature. After the cooling system and prepolarization processing are both introduced, the largest light current (475 nA) and photovoltaic plateaus (585 nA) are enhanced by about 4417 and 4278% under 368 nm at a power intensity of 4.46 mW cm-2 at 0 °C, respectively. The enhancement of photocurrent is associated with a ferro-pyro-phototronic effect, evidenced by enhanced ferroelectric polarization. The ZnO-NFA/BTO PD can detect weak signals at low power intensity with a wide temperature range of 0-100 °C under 0 V bias. The self-powered ZnO-NFA/BTO PD provides a new and promising way to fabricate high-performance and low-cost photodetectors from inorganic perovskite materials.

Advances in the two-dimensional layer materials for cancer diagnosis and treatment: unique advantages beyond the microsphere

In recent years, two-dimensional (2D) layer materials have shown great potential in the field of cancer diagnosis and treatment due to their unique structural, electronic, and chemical properties. These non-spherical materials have attracted increasing attention around the world because of its widely used biological characteristics. The application of 2D layer materials like lamellar graphene, transition metal dichalcogenides (TMDs), and black phosphorus (BPs) and so on have been developed for CT/MRI imaging, serum biosensing, drug targeting delivery, photothermal therapy, and photodynamic therapy. These unique applications for tumor are due to the multi-variable synthesis of 2D materials and the structural characteristics of good ductility different from microsphere. Based on the above considerations, the application of 2D materials in cancer is mainly carried out in the following three aspects: 1) In terms of accurate and rapid screening of tumor patients, we will focus on the enrichment of serum markers and sensitive signal transformation of 2D materials; 2) The progress of 2D nanomaterials in tumor MRI and CT imaging was described by comparing the performance of traditional contrast agents; 3) In the most important aspect, we will focus on the progress of 2D materials in the field of precision drug delivery and collaborative therapy, such as photothermal ablation, sonodynamic therapy, chemokinetic therapy, etc. In summary, this review provides a comprehensive overview of the advances in the application of 2D layer materials for tumor diagnosis and treatment, and emphasizes the performance difference between 2D materials and other types of nanoparticles (mainly spherical). With further research and development, these multifunctional layer materials hold great promise in the prospects, and challenges of 2D materials development are discussed.