Vertical Barrier Heterostructures for Reliable and High-Performance Self-Powered Infrared Detection

With their fascinating properties, emerging two-dimensional (2D) materials offer innovative ways to prepare high-performance infrared (IR) detectors. However, the current performance of 2D IR photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, sharply raised contact resistance, and deteriorated metal conductivity at nanoscale. Here, we introduce a vertical barrier heterojunction with a structure of PtSe2/GaAs that combines the excellent optoelectronic properties of transition metal sulfides with topological semi-metals, which allows for an adjustable bandgap and high carrier mobility. The heterojunction was fabricated using the wet transfer method. The heterostructures show significant rectification behaviors and photovoltaic effects, which allow it to operate as a self-driven photodetector at zero bias. The photoresponse parameters at 850 nm with zero bias voltage are 67.2 mA W-1, 6.7 × 1012 Jones, 9.8%, 3.8 × 105, 164 μs, and 198 μs for the responsivity, specific detectivity, external quantum efficiency, Ilight/Idark ratio, rise time, and fall time, respectively. Moreover, the heterojunction is highly sensitive to a wide spectral band from ultraviolet to near-infrared (360-1550 nm). At the same time, this heterostructure demonstrates significant potential for applications in IR polarized light detection and room-temperature high-resolution IR imaging. The excellent properties of the heterojunction make it well-suited for high-performance, self-powered IR detection.

Radical n-p Conduction Switching and Significant Photoconductivity Enhancement in NbOI2 via Pressure-Modulated Peierls Distortion

The absence of intrinsic p-type 2D layered semiconductors has hampered the development of 2D devices, particularly in complementary metal-oxide-semiconductor (CMOS) devices and integrated circuits. Developing practical p-type semiconductors and advanced modulation techniques for precise carrier control is paramount to advancing electronic devices and systems. Here, by applying pressure to continuously tune the Peierls distortion in NbOI2, we effectively control the polarity and concentration of carriers and significantly enhance its photoelectric properties. The results demonstrate that by suppressing the off-center displacement of Nb atoms along the in-plane b direction under pressure, NbOI2 undergoes a semiconductor-to-semiconductor phase transition from C2 to C2/m, leading to a significant transition from n-type to p-type carrier behavior. Additionally, the gradual inhibition of internal interactions within Nb-Nb dimers along the in-plane c direction under high pressure facilitates electron delocalization, substantially enhancing the photoelectric properties. The photocurrent is increased by more than 3 orders of magnitude under xenon irradiation, and the spectral response range is continuously red-shifted and extended to 1450 nm. These findings highlight the potential of pressure engineering to adjust photoelectric properties effectively and flexibly, offering valuable insights for designing high-performance p-type two-dimensional semiconductors.

An Au25 nanocluster/MoS2 vdWaals heterojunction phototransistor for chromamorphic visual-afterimage emulation

Color vision relies on three cone photoreceptors that are sensitive to different wavelengths of light. The interaction of three incident light wavelengths over time creates a fascinating color coupling perception, termed chromamorphic computing. However, the realization of this fascinating characteristic in semiconductor devices remains a great challenge. Herein, a mixed-dimensional optoelectronic transistor based on a novel metal nanocluster Au25(SC12H25)18 and two-dimensional MoS2 van der Waals (vdWaals) heterojunction is proposed for chromamorphic visual-afterimage emulation with red-green-blue three-color spatiotemporal coupling perception. This distinguished molecular-like electronic level of Au25 nanoclusters allows the transistor to have visible light-sensitive properties, endowing it with the ability to perceive color information. Moreover, the chromamorphic functions are realized using a color spatiotemporal coupling approach. By utilizing the photogating effect of light stimulus, the device exhibits visual experience-dependent plasticity in accordance with the Bienenstock-Cooper-Munro (BCM) learning rule. Most importantly, for the first time, intriguing visual afterimages could be implemented using a color sensitization approach based on a close relationship between visual persistence and negative afterimages. These results represent an important step towards a new generation of intelligent visual color perception systems for human-computer interaction, bionic robots, etc.

Quantitative photocurrent scanning probe microscopy on PbS quantum dot monolayers

Photoconductive atomic force microscopy can probe monolayers of PbS/perovskite quantum dots (QDs) with a contact area of 1-3 QDs in stable and reproducible acquisition conditions for I/V curves and photocurrent maps. From the measurements, quantitative values for the barrier height, built-in voltage, diffusion constant and ideality factor are deduced with high precision. The data analysis is based on modelling a superposition of the drift current of the photo-excited charges and a diffusion current across the interface barriers, providing physical insight into the underlying processes. Besides looking into PbS/perovskite on an indium tin oxide substrate, it is shown how the photocurrent is modified by changing either the QD ligand (to thiocyanate) or the substrate (to micro- and nanostructured gold). The dependence of the photocurrent on the light irradiance is found to follow a power law with an exponent of 0.64. Generally, quantitative measurements with high spatial resolution (on the single QD level) can provide significant insight into the processes in nanostructured hybrid optoelectronic components.

Harnessing Persistent Photocurrent in a 2D Semiconductor-Polymer Hybrid Structure: Electron Trapping and Fermi Level Modulation for Optoelectronic Memory

Recently, 2D semiconductor-based optoelectronic memory has been explored to overcome the limitations of conventional von Neumann architectures by integrating optical sensing and data storage into one device. Persistent photocurrent (PPC), essential for optoelectronic memory, originates from charge carrier trapping according to the Shockley-Read-Hall (SRH) model in 2D semiconductors. The quasi-Fermi level position influences the activation of charge-trapping sites. However, the correlation between quasi-Fermi level modulations and PPC in 2D semiconductors has not been extensively studied. In this study, we demonstrate optoelectronic memory based on a 2D semiconductor-polymer hybrid structure and confirm that the underlying mechanism is charge trapping, as the SRH model explains. Under light illumination, electrons transfer from polyvinylpyrrolidone to p-type tungsten diselenide, resulting in high-level injection and majority carrier-type transitions. The quasi-Fermi level shifts upward with increasing temperature, improving PPC and enabling optoelectronic memory at 433 K. Our findings offer valuable insights into optimizing 2D semiconductor-based optoelectronic memory.

Development of Self-Doped Monolayered 2D MoS2 for Enhanced Photoresponsivity

Transition metal dichalcogenides (TMDs) exist in two distinct phases: the thermodynamically stable trigonal prismatic (2H) and the metastable octahedral (1T) phase. Phase engineering has emerged as a potent technique for enhancing the performance of TMDs in optoelectronics applications. Nevertheless, understanding the mechanism of phase transition in TMDs and achieving large-area synthesis of phase-controlled TMDs continue to pose significant challenges. This study presents the synthesis of large-area monolayered 2H-MoS2 and mixed-phase 1T/2H-MoS2 by controlling the growth temperature in the chemical vapor deposition (CVD) method without use of a catalyst. The field-effect transistors (FETs) devices fabricated with 1T/2H-MoS2 mixed-phase show an on/off ratio of 107. Photo response devices fabricated with 1T/2H-MoS2 mixed-phase show ≈55 times enhancement in responsivity (from 0.32 to 17.4 A W-1) and 102 times increase in the detectivity (from 4.1 × 1010 to 2.48 × 1012 cm Hz W-1) compare to 2H-MoS2. Introducing the metallic 1T phase within the 2H phase contributes additional carriers to the material, which prevents the electron-hole recombination and thereby increases the carrier density in the 1T/2H-MoS2 mixed-phase in comparison to 2H-MoS2. This work provides insights into the self-doping effects of 1T phase in 2H MoS2, enabling the tuning of 2D TMDs properties for optoelectronic applications.

The Rise of Xene Hybrids

Xenes, mono-elemental atomic sheets, exhibit Dirac/Dirac-like quantum behavior. When interfaced with other 2D materials such as boron nitride, transition metal dichalcogenides, and metal carbides/nitrides/carbonitrides, it enables them with unique physicochemical properties, including structural stability, desirable bandgap, efficient charge carrier injection, flexibility/breaking stress, thermal conductivity, chemical reactivity, catalytic efficiency, molecular adsorption, and wettability. For example, BN acts as an anti-oxidative shield, MoS2 injects electrons upon laser excitation, and MXene provides mechanical flexibility. Beyond precise compositional modulations, stacking sequences, and inter-layer coupling controlled by parameters, achieving scalability and reproducibility in hybridization is crucial for implementing these quantum materials in consumer applications. However, realizing the full potential of these hybrid materials faces challenges such as air gaps, uneven interfaces, and the formation of defects and functional groups. Advanced synthesis techniques, a deep understanding of quantum behaviors, precise control over interfacial interactions, and awareness of cross-correlations among these factors are essential. Xene-based hybrids show immense promise for groundbreaking applications in quantum computing, flexible electronics, energy storage, and catalysis. In this timely perspective, recent discoveries of novel Xenes and their hybrids are highlighted, emphasizing correlations among synthetic parameters, structure, properties, and applications. It is anticipated that these insights will revolutionize diverse industries and technologies.

Growth of Monolayer MoS2 Flakes via Close Proximity Re-Evaporation

We report a two-step growth process of MoS2 nanoflakes using a low-pressure chemical vapor deposition technique. In the first step, a MoS2 layer was synthesized on a c-plane sapphire substrate. This layer was subsequently re-evaporated at a higher temperature to form mono- or few-layer MoS2 flakes. As a result, the close proximity re-evaporation enabled the growth of pristine MoS2 nanoflakes. Atomic force microscopy analysis confirmed the synthesis of nanoclusters/nanoflakes with lateral dimensions of over 10 μm and a flake height of approximately 1.3 nm, demonstrating bi-layer MoS2, whereas transmission electron microscopy analysis revealed triangular MoS2 nanoflakes, with a diffraction pattern proving the presence of single crystalline hexagonal MoS2. Raman data revealed the typical modes of high-quality MoS2 nanoflakes. Finally, we presented the photocurrent dependence of a MoS2-based photoresist under illumination with light-emitting diode of 405 nm wavelength. The measured current-voltage dependence across various luminous flux outlined the sensitivity of MoS2 to polarized light and thus opens further opportunities for applications in high-performance photodetectors with polarization sensitivity.

Bias-dependent photoresponse of Td-WTe2grown by chemical vapor deposition

The type-II Weyl semimetal Td-WTe2is one of the wonder materials for high-performance optoelectronic devices. We report the self-powered Td-WTe2photodetectors and their bias-dependent photoresponse in the visible region (405, 520, 638 nm) driven by the bulk photovoltaic effect. The device shows the responsivity of 15.8 mAW-1and detectivity of 5.2 × 109Jones at 520 nm. Besides, the response time of the WTe2photodetector shows the strong bias-voltage dependent property. This work offers a physical reference for understanding the photoresponse process of Td-WTe2photodetectors.

Electrically tunable giant Nernst effect in two-dimensional van der Waals heterostructures

The Nernst effect, a transverse thermoelectric phenomenon, has attracted significant attention for its potential in energy conversion, thermoelectrics and spintronics. However, achieving high performance and versatility at low temperatures remains elusive. Here we demonstrate a large and electrically tunable Nernst effect by combining the electrical properties of graphene with the semiconducting characteristics of indium selenide in a field-effect geometry. Our results establish a new platform for exploring and manipulating this thermoelectric effect, showcasing the first electrical tunability with an on/off ratio of 103. Moreover, photovoltage measurements reveal a stronger photo-Nernst signal in the graphene/indium selenide heterostructure compared with individual components. Remarkably, we observe a record-high Nernst coefficient of 66.4 μV K-1 T-1 at ultralow temperatures and low magnetic fields, an important step towards applications in quantum information and low-temperature emergent phenomena.

2D Free-Standing GeS1-xSex with Composition-Tunable Bandgap for Tailored Polarimetric Optoelectronics

Germanium-based monochalcogenides (i.e., GeS and GeSe) with desirable properties are promising candidates for the development of next-generation optoelectronic devices. However, they are still stuck with challenges, such as relatively fixed electronic band structure, unconfigurable optoelectronic characteristics, and difficulty in achieving free-standing growth. Herein, it is demonstrated that two-dimensional (2D) free-standing GeS1-xSex (0 ≤ x ≤ 1) nanoplates can be grown by low-pressure rapid physical vapor deposition (LPRPVD), fulfilling a continuously composition-tunable optical bandgap and electronic band structure. By leveraging the synergistic effect of composition-dependent modulation and free-standing growth, GeS1-xSex-based optoelectronic devices exhibit significantly configurable hole mobility from 6.22 × 10-4 to 1.24 cm2V-1s⁻1 and tunable responsivity from 8.6 to 311 A W-1 (635 nm), as x varies from 0 to 1. Furthermore, the polarimetric sensitivity can be tailored from 4.3 (GeS0.29Se0.71) to 1.8 (GeSe) benefiting from alloy engineering. Finally, the tailored imaging capability is also demonstrated to show the application potential of GeS1-xSex alloy nanoplates. This work broadens the functionality of conventional binary materials and motivates the development of tailored polarimetric optoelectronic devices.

Van der Waals Schottky Junction Photodetector with Ultrahigh Rectifying Ratio and Switchable Photocurrent Generation

Metal-semiconductor junctions play an important role in the development of electronic and optoelectronic devices. A Schottky junction photodetector based on two-dimensional (2D) materials is promising for self-powered photodetection with fast response speed and large signal-to-noise ratio. However, it usually suffers from an uncontrolled Schottky barrier due to the Fermi level pinning effect arising from the interface states. In this work, all-2D Schottky junctions with near-ideal Fermi level depinning are realized, attributed to the high-quality interface between 2D semimetals and semiconductors. We further demonstrate asymmetric diodes based on multilayer graphene/MoS2/PtSe2 with a current rectification ratio exceeding 105 and an ideality factor of 1.2. Scanning photocurrent mapping shows that the photocurrent generation mechanism in the heterostructure switches from photovoltaic effect to photogating effect at varying drain biases, indicating both energy conversion and optical sensing are realized in a single device. In the photovoltaic mode, the photodetector is self-powered with a response time smaller than 100 μs under the illumination of a 405 nm laser. In the photogating mode, the photodetector exhibits a high responsivity up to 460 A/W originating from a high photogain. Finally, the photodetector is employed for single-pixel imaging, demonstrating its high-contrast photodetection ability. This work provides insight into the development of high-performance self-powered photodetectors based on 2D Schottky junctions.

Graphene photodetectors integrated with silicon and perovskite quantum dots

Photodetectors (PDs) play a crucial role in imaging, sensing, communication systems, etc. Graphene (Gr), a leading two-dimensional material, has demonstrated significant potential for photodetection in recent years. However, its relatively weak interaction with light poses challenges for practical applications. The integration of silicon (Si) and perovskite quantum dots (PQDs) has opened new avenues for Gr in the realm of next-generation optoelectronics. This review provides a comprehensive investigation of Gr/Si Schottky junction PDs and Gr/PQD hybrid PDs as well as their heterostructures. The operating principles, design, fabrication, optimization strategies, and typical applications of these devices are studied and summarized. Through these discussions, we aim to illuminate the current challenges and offer insights into future directions in this rapidly evolving field.

Two-dimensional topological semimetals: an emerging candidate for terahertz detectors and on-chip integration

The importance of terahertz (THz) detection lies in its ability to provide detailed information in a non-destructive manner, making it a valuable tool across various domains including spectroscopy, communication, and security. The ongoing development of THz detectors aims to enhance their sensitivity, resolution and integration into compact and portable devices such as handheld scanners or integrated communication chips. Generally, two-dimensional (2D) materials are considered potential candidates for device miniaturization but detecting THz radiation using 2D semiconductors is generally difficult due to the ultra-small photon energy. However, this challenge is being addressed by the advent of topological semimetals (TSM) with zero-bandgap characteristics. These semimetals offer low-energy excitations in proximity to the Dirac point, which is particularly important for applications requiring a broad detection range. Their distinctive band structures with linear energy-momentum dispersion near the Fermi level also lead to high electron mobility and low effective mass. The presence of topologically protected dissipationless conducting channels and self-powered response provides a basis for low-energy integration. In order to establish paradigms for semimetal-based THz detectors, this review initially offers an analytical summary of THz detection principles. Then, the review demonstrates the distinct design of devices, the excellent performance derived from the topological surface state and unique band structures in TSM. Finally, we outline the prospective avenues for on-chip integration of TSM-based THz detectors. We believe this review can promote further research on the new generation of THz detectors and facilitate advancements in THz imaging, spectroscopy, and communication systems.

Scanning Probe Microscopies for Characterizations of 2D Materials

2D materials are intriguing due to their remarkably thin and flat structure. This unique configuration allows the majority of their constituent atoms to be accessible on the surface, facilitating easier electron tunneling while generating weak surface forces. To decipher the subtle signals inherent in these materials, the application of techniques that offer atomic resolution (horizontal) and sub-Angstrom (z-height vertical) sensitivity is crucial. Scanning probe microscopy (SPM) emerges as the quintessential tool in this regard, owing to its atomic-level spatial precision, ability to detect unitary charges, responsiveness to pico-newton-scale forces, and capability to discern pico-ampere currents. Furthermore, the versatility of SPM to operate under varying environmental conditions, such as different temperatures and in the presence of various gases or liquids, opens up the possibility of studying the stability and reactivity of 2D materials in situ. The characteristic flatness, surface accessibility, ultra-thinness, and weak signal strengths of 2D materials align perfectly with the capabilities of SPM technologies, enabling researchers to uncover the nuanced behaviors and properties of these advanced materials at the nanoscale and even the atomic scale.

High-Performance Contact-Doped WSe2 Transistors Using TaSe2 Electrodes

Two-dimensional (2D) transitional metal dichalcogenides (TMDs) have garnered significant attention due to their potential for next-generation electronics, which require device scaling. However, the performance of TMD-based field-effect transistors (FETs) is greatly limited by the contact resistance. This study develops an effective strategy to optimize the contact resistance of WSe2 FETs by combining contact doping and 2D metallic electrode materials. The contact regions were doped using a laser, and the metallic TaSe2 flakes were stacked on doped WSe2 as electrodes. Doping the contact areas decreases the depletion width, while introducing the TaSe2 contact results in a lower Schottky barrier. This method significantly improves the electrical performance of the WSe2 FETs. The doped WSe2/TaSe2 contact exhibits an ultralow Schottky barrier height of 65 meV and a contact resistance of 11 kΩ·μm, which is a 50-fold reduction compared to the conventional Cr/Au contact. Our method offers a way on fabricating high-performance 2D FETs.

Design of Aluminum Doped ZnO/Phenyl-C-Butyric Acid Methyl Ester/Tungsten Disulfide (AZO/PCBM/WS2) Heterojunction-Based Device for Applications in Solar Cells and Broadband Self-Powered Photodetectors

The domain related to the use of renewable energy is showing high interest among the researchers in the current days, and light energy can be considered as one of such. Solar cells are the kind of devices that use light energy in an efficient way to generate electricity. Hence, designing and developing a solar cell requires meticulous attention to get an optimum output. This work focuses on designing and optimization of aluminum dope ZnO/phenyl-C-butyric acid methyl ester/tungsten disulfide (AZO/PCBM/WS2) heterojunction-based device for applications in solar cells. Moreover, considering the high demand of optoelectronics like photodetectors, this work also uncovers the capabilities of the device in the applications of broadband self-powered photodetectors. The optimized values of several parameters of the device such as the thickness and doping density of the WS2 layer, the defect density of the WS2 layer, and the interface PCBM/WS2 were 2 μm, 1017 cm-3, 1014 cm-3, and 1010 cm-2, respectively. These optimal conditions facilitated in obtaining fill factor and efficiency values of 84.78 and 23.92%, respectively. Further, the photodetection performance was evaluated with the parameters like responsivity and detectivity. The maximum calculated responsivity of the device at 0 V bias was 0.63 A/W at 880 nm, whereas the maximum detectivity was 1.54 × 1013 Jones. This study also extensively focuses on the in-depth understanding of the physics associated with the movement of the charge carriers under the illuminated conditions by studying the band diagrams and the electric fields of the heterojunctions at various conditions.

Giant Negative Photoresponse in van der Waals Graphene/AgBiP2Se6/Graphene Trilayer Heterostructures

The positive photoconductive (PPC) effect is a well-established primary detection mechanism employed by photodetectors. In contrast, the negative photoconductive (NPC) effect is not extensively investigated thus far, and research on the NPC effect is still in its early stage. Herein, a quaternary van der Waals material, AgBiP2Se6 atomic layers, is discovered to achieve a giant NPC effect. Through experimental observations in a Graphene/AgBiP2Se6/ Graphene-based vertical photodetector, an irreversible conversion is identified from common PPC photoresponse to atypical NPC photoresponse. Notably, this device demonstrates an exceptionally high negative responsivity (R) of 4.9 × 105 A W-1, surpassing the previous records for NPC photodetectors. Additionally, it exhibits remarkable optoelectronic performances, including an external quantum efficiency of 1.3 × 108% and a detectivity (D) of 3.60 × 1012 Jones. The exceptionally high NPC photoresponse observed in this device can be attributed to the swift suppression of photogenerated free carriers at robust recombination centers situated at significant depths, induced by the elevated drain-source voltage bias. The remarkably high NPC photoresponse also positions AgBiP2Se6 as a promising 2D material for multifunctional optoelectronic devices and an excellent platform for systematic exploration of the NPC effect.

Strain-Enhanced Large-Area Monolayer MoS2 Photodetectors

In this study, we show a direct correlation between the applied mechanical strain and an increase in monolayer MoS2 photoresponsivity. This shows that tensile strain can improve the efficiency of monolayer MoS2 photodetectors. The observed high photocurrent and extended response time in our devices are indicative that devices are predominantly governed by photogating mechanisms, which become more prominent with applied tensile strain. Furthermore, we have demonstrated that a nonencapsulated MoS2 monolayer can be used in strain-based devices for many cycles and extensive periods of time, showing endurance under ambient conditions without loss of functionality. Such robustness emphasizes the potential of MoS2 for further functionalization and utilization of different flexible sensors.

Recent advance of high-quality perovskite nanostructure and its application in flexible photodetectors

Flexible photodetectors (PDs) have garnered increasing attention for their potential applications in diverse fields, including weather monitoring, smart robotics, smart textiles, electronic eyes, wearable biomedical monitoring devices, and so on. Notably, perovskite nanostructures have emerged as a promising material for flexible PDs due to their distinctive features, such as a large optical absorption coefficient, tunable band gap, extended photoluminescence decay time, high carrier mobility, low defect density, long exciton diffusion lengths, strong self-trapped effect, good mechanical flexibility, and facile synthesis methods. In this review, we first introduce various synthesis methods for perovskite nanostructures and elucidate their corresponding optical and electrical properties, encompassing quantum dots, nanocrystals, nanowires, nanobelts, nanosheets, single-crystal thin films, polycrystalline thin films, and nanostructured arrays. Furthermore, the working mechanism and key performance parameters of optoelectronic devices are summarized. The review also systematically compiles recent advancements in flexible PDs based on various nanostructured perovskites. Finally, we present the current challenges and prospects for the development of perovskite nanostructures-based flexible PDs.

Recent Advances in Broadband Photodetectors from Infrared to Terahertz

The growing need for the multiband photodetection of a single scene has promoted the development of both multispectral coupling and broadband detection technologies. Photodetectors operating across the infrared (IR) to terahertz (THz) regions have many applications such as in optical communications, sensing imaging, material identification, and biomedical detection. In this review, we present a comprehensive overview of the latest advances in broadband photodetectors operating in the infrared to terahertz range, highlighting their classification, operating principles, and performance characteristics. We discuss the challenges faced in achieving broadband detection and summarize various strategies employed to extend the spectral response of photodetectors. Lastly, we conclude by outlining future research directions in the field of broadband photodetection, including the utilization of novel materials, artificial microstructure, and integration schemes to overcome current limitations. These innovative methodologies have the potential to achieve high-performance, ultra-broadband photodetectors.

High-performance MoS2phototransistors with Hf1-xAlxO back-gate dielectric layer grown by plasma enhanced atomic layer deposition

Molybdenum sulfide (MoS2) as an emerging optoelectronic material, shows great potential for phototransistors owing to its atomic thickness, adjustable band gap, and low cost. However, the phototransistors based on MoS2have been shown to have some issues such as large gate leakage current, and interfacial scattering, resulting in suboptimal optoelectronic performance. Thus, Al-doped hafnium oxide (Hf1-xAlx) is proposed to be a dielectric layer of the MoS2-based phototransistor to solve this problem because of the relatively higher crystallization temperature and dielectric constant. Here, a high-performance MoS2phototransistor with Hf1-xAlxO gate dielectric layer grown by plasma-enhanced atomic layer deposition has been fabricated and studied. The results show that the phototransistor exhibits a high responsivity of 2.2 × 104A W-1, a large detectivity of 1.7 × 1017Jones, a great photo-to-dark current ratio of 2.2 × 106%, and a high external quantum efficiency of 4.4 × 106%. The energy band alignment and operating mechanism were further used to clarify the reason for the enhanced MoS2phototransistor. The suggested MoS2phototransistors could provide promising strategies in further optoelectronic applications.

Enhanced Photogating Gain in Scalable MoS2 Plasmonic Photodetectors via Resonant Plasmonic Metasurfaces

Absorption of photons in atomically thin materials has become a challenge in the realization of ultrathin, high-performance optoelectronics. While numerous schemes have been used to enhance absorption in 2D semiconductors, such enhanced device performance in scalable monolayer photodetectors remains unattained. Here, we demonstrate wafer-scale integration of monolayer single-crystal MoS2 photodetectors with a nitride-based resonant plasmonic metasurface to achieve a high detectivity of 2.58 × 1012 Jones with a record-low dark current of 8 pA and long-term stability over 40 days. Upon comparison with control devices, we observe an overall enhancement factor of >100; this can be attributed to the local strong EM field enhanced photogating effect by the resonant plasmonic metasurface. Considering the compatibility of 2D semiconductors and hafnium nitride with the Si CMOS process and their scalability across wafer sizes, our results facilitate the smooth incorporation of 2D semiconductor-based photodetectors into the fields of imaging, sensing, and optical communication applications.

Liquid-Metal-Assisted Synthesis of Patterned GaN Thin Films for High-Performance UV Photodetectors Array

GaN's outstanding physical characteristics allow for a wide range of applications in numerous industries. Although individual GaN-based ultraviolet (UV) photodetectors are the subject of in-depth research in recent decades, the demand for photodetectors array is rising as a result of advances in optoelectronic integration technology. However, as a prerequisite for constructing GaN-based photodetectors array, large-area, patterned synthesis of GaN thin films remains a certain challenge. This work presents a facile technique for pattern growing high-quality GaN thin films for the assembly of an array of high-performance UV photodetectors. This technique uses UV lithography, which is not only very compatible with common semiconductor manufacturing techniques, but also enables precise patterning modification. A typical detector has impressive photo-response performance under 365 nm irradiation, with an extremely low dark current of 40 pA, a high Ilight /Idark ratio over 105 , a high responsivity of 4.23 AW-1 , and a decent specific detectivity of 1.76 × 1012 Jones. Additional optoelectronic studies demonstrate the strong homogeneity and repeatability of the photodetectors array, enabling it to serve as a reliable UV image sensor with enough spatial resolution. These outcomes highlight the proposed patterning technique's enormous potential.

Low-Dimensional-Materials-Based Photodetectors for Next-Generation Polarized Detection and Imaging

The vector characteristics of light and the vectorial transformations during its transmission lay a foundation for polarized photodetection of objects, which broadens the applications of related detectors in complex environments. With the breakthrough of low-dimensional materials (LDMs) in optics and electronics over the past few years, the combination of these novel LDMs and traditional working modes is expected to bring new development opportunities in this field. Here, the state-of-the-art progress of LDMs, as polarization-sensitive components in polarized photodetection and even the imaging, is the main focus, with emphasis on the relationship between traditional working principle of polarized photodetectors (PPs) and photoresponse mechanisms of LDMs. Particularly, from the view of constitutive equations, the existing works are reorganized, reclassified, and reviewed. Perspectives on the opportunities and challenges are also discussed. It is hoped that this work can provide a more general overview in the use of LDMs in this field, sorting out the way of related devices for "more than Moore" or even the "beyond Moore" research.

Recent advances in 2D transition metal dichalcogenide-based photodetectors: a review

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a highly promising platform for the development of photodetectors (PDs) owing to their remarkable electronic and optoelectronic properties. Highly effective PDs can be obtained by making use of the exceptional properties of 2D materials, such as their high transparency, large charge carrier mobility, and tunable electronic structure. The photodetection mechanism in 2D TMD-based PDs is thoroughly discussed in this article, with special attention paid to the key characteristics that set them apart from PDs based on other integrated materials. This review examines how single TMDs, TMD-TMD heterostructures, TMD-graphene (Gr) hybrids, TMD-MXene composites, TMD-perovskite heterostructures, and TMD-quantum dot (QD) configurations show advanced photodetection. Additionally, a thorough analysis of the recent developments in 2D TMD-based PDs, highlighting their exceptional performance capabilities, including ultrafast photo response, ultrabroad detectivity, and ultrahigh photoresponsivity, attained through cutting-edge methods is provided. The article conclusion highlights the potential for ground-breaking discoveries in this fast developing field of research by outlining the challenges faced in the field of PDs today and providing an outlook on the prospects of 2D TMD-based PDs in the future.

Photogating-assisted tunneling boosts the responsivity and speed of heterogeneous WSe2/Ta2NiSe5 photodetectors

Photogating effect is the dominant mechanism of most high-responsivity two-dimensional (2D) material photodetectors. However, the ultrahigh responsivities in those devices are intrinsically at the cost of very slow response speed. In this work, we report a WSe2/Ta2NiSe5 heterostructure detector whose photodetection gain and response speed can be enhanced simultaneously, overcoming the trade-off between responsivity and speed. We reveal that photogating-assisted tunneling synergistically allows photocarrier multiplication and carrier acceleration through tunneling under an electrical field. The photogating effect in our device features low-power consumption (in the order of nW) and shows a dependence on the polarization states of incident light, which can be further tuned by source-drain voltages, allowing for wavelength discrimination with just a two-electrode planar structure. Our findings offer more opportunities for the long-sought next-generation photodetectors with high responsivity, fast speed, polarization detection, and multi-color sensing, simultaneously.

Next-Generation Photodetectors beyond Van Der Waals Junctions

With the continuous advancement of nanofabrication techniques, development of novel materials, and discovery of useful manipulation mechanisms in high-performance applications, especially photodetectors, the morphology of junction devices and the way junction devices are used are fundamentally revolutionized. Simultaneously, new types of photodetectors that do not rely on any junction, providing a high signal-to-noise ratio and multidimensional modulation, have also emerged. This review outlines a unique category of material systems supporting novel junction devices for high-performance detection, namely, the van der Waals materials, and systematically discusses new trends in the development of various types of devices beyond junctions. This field is far from mature and there are numerous methods to measure and evaluate photodetectors. Therefore, it is also aimed to provide a solution from the perspective of applications in this review. Finally, based on the insight into the unique properties of the material systems and the underlying microscopic mechanisms, emerging trends in junction devices are discussed, a new morphology of photodetectors is proposed, and some potential innovative directions in the subject area are suggested.

Two-Dimensional Oxide Crystals for Device Applications: Challenges and Opportunities

Atomically thin two-dimensional (2D) oxide crystals have garnered considerable attention because of their remarkable physical properties and potential for versatile applications. In recent years, significant advancements have been made in the design, preparation, and application of ultrathin 2D oxides, providing many opportunities for new-generation advanced technologies. This review focuses on the controllable preparation of 2D oxide crystals and their applications in electronic and optoelectronic devices. Based on their bonding nature, the various types of 2D oxide crystals are first summarized, including both layered and nonlayered crystals, as well as their current top-down and bottom-up synthetic approaches. Subsequently, in terms of the unique physical and electrical properties of 2D oxides, recent advances in device applications are emphasized, including photodetectors, field-effect transistors, dielectric layers, magnetic and ferroelectric devices, memories, and gas sensors. Finally, conclusions and future prospects of 2D oxide crystals are presented. It is hoped that this review will provide comprehensive and insightful guidance for the development of 2D oxide crystals and their device applications.

Self-Powered Programmable van der Waals Photodetectors with Nonvolatile Semifloating Gate

Tunable photovoltaic photodetectors are of significant relevance in the fields of programmable and neuromorphic optoelectronics. However, their widespread adoption is hindered by intricate architectural design and energy consumption challenges. This study employs a nonvolatile MoTe2/hexagonal boron nitride/graphene semifloating photodetector to address these issues. Programed with pulsed gate voltage, the MoTe2 channel can be reconfigured from an n+-n to a p-n homojunction and the photocurrent transition changes from negative to positive values. Scanning photocurrent mapping reveals that the negative and positive photocurrents are attributed to Schottky junction and p-n homojunction, respectively. In the p-n configuration, the device demonstrates self-driven, linear, rapid response (∼3 ms), and broadband sensitivity (from 405 to 1500 nm) for photodetection, with typical performances of responsivity at ∼0.5 A/W and detectivity ∼1.6 × 1012 Jones under 635 nm illumination. These outstanding photodetection capabilities emphasize the potential of the semifloating photodetector as a pioneering approach for advancing logical and nonvolatile 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.

Stability of Nanometer-Thick Layered Gallium Chalcogenides and Improvements via Hydrogen Passivation

The gallium monochalcogenides family, comprising gallium sulfide (GaS), gallium selenide (GaSe), and gallium telluride (GaTe), is capturing attention for its applications in energy storage and production, catalysis, photonics, and optoelectronics. This interest originates from their properties, which include an optical bandgap larger than those of most common transition metal dichalcogenides, efficient light absorption, and significant carrier mobility. For any application, stability to air exposure is a fundamental requirement. Here, we perform a comparative study of the stability of layered GaS, GaSe, and GaTe nanometer-thick films down to a few layers with the goal of identifying the most suitable Ga chalcogenide for future integration in photonic and optoelectronic devices. Our study unveils a trend of decreasing air stability from sulfide to selenide and finally to telluride. Furthermore, we demonstrate a hydrogen passivation process to prevent the oxidation of GaSe with a higher feasibility and durability than other state-of-the-art passivation methods proposed in the literature.

A review on advanced band-structure engineering with dynamic control for nonvolatile memory based 2D transistors

With advancements in information technology, an enormous amount of data is being generated that must be quickly accessible. However, conventional Si memory cells are approaching their physical limits and will be unable to meet the requirements of intense applications in the future. Notably, 2D atomically thin materials have demonstrated multiple novel physical and chemical properties that can be used to investigate next-generation electronic devices and breakthrough physical limits to continue Moore's law. Band structure is an important semiconductor parameter that determines their electrical and optical properties. In particular, 2D materials have highly tunable bandgaps and Fermi levels that can be achieved through band structure engineering methods such as heterostructure, substrate engineering, chemical doping, intercalation, and electrostatic doping. In particular, dynamic control of band structure engineering can be used in recent advancements in 2D devices to realize nonvolatile storage performance. This study examines recent advancements in 2D memory devices that utilize band structure engineering. The operational mechanisms and memory characteristics are described for each band structure engineering method. Band structure engineering provides a platform for developing new structures and realizing superior performance with respect to nonvolatile memory.

A high-performance long-wave infrared photodetector based on a WSe2/PdSe2 broken-gap heterodiode

Layered narrow bandgap quasi-two-dimensional (2D) transition metal dichalcogenides (TMDs) demonstrated excellent performance in long-wave infrared (LWIR) detection. However, the low light on/off ratio and specific detectivity (D*) due to the high dark current of the device fabricated using a single narrow bandgap material hindered its wide application. Herein, we report a type-III broken-gap band-alignment WSe2/PdSe2 van der Waals (vdW) heterostructure. The heterodiode device has a prominently low dark current and exhibits a high photoresponsivity (R) of 55.3 A W-1 and a high light on/off ratio >105 in the visible range. Notably, the WSe2/PdSe2 heterodiode shows an excellent uncooled LWIR response, with an R of ∼0.3 A W-1, a low noise equivalence power (NEP) of 4.5 × 10-11 W Hz-1/2, and a high D* of 1.8 × 108 cm Hz1/2 W-1. This work provides a new approach for designing high-performance room-temperature operational LWIR photodetectors.

High photoresponsivity MoS2phototransistor through enhanced hole trapping HfO2gate dielectric

Phototransistor using 2D semiconductor as the channel material has shown promising potential for high sensitivity photo detection. The high photoresponsivity is often attributed to the photogating effect, where photo excited holes are trapped at the gate dielectric interface that provides additional gate electric field to enhance channel charge carrier density. Gate dielectric material and its deposition processing conditions can have great effect on the interface states. Here, we use HfO2gate dielectric with proper thermal annealing to demonstrate a high photoresponsivity MoS2phototransistor. When HfO2is annealed in H2atmosphere, the photoresponsivity is enhanced by an order of magnitude as compared with that of a phototransistor using HfO2without annealing or annealed in Ar atmosphere. The enhancement is attributed to the hole trapping states introduced at HfO2interface through H2annealing process, which greatly enhances photogating effect. The phototransistor exhibits a very large photoresponsivity of 1.1 × 107A W-1and photogain of 3.3 × 107under low light illumination intensity. This study provides a processing technique to fabricate highly sensitive phototransistor for low optical power detection.

Effect of external electric field on the electronic properties of the AlAs/SiC van der Waals heterostructure

Type-II van der Waals (vdW) heterostructures are regarded as the optimum candidates for unipolar electronic device applications due to their capacity for spontaneous electron-hole separation. Here, we studied the electronic properties of the AlAs/SiC vdW heterostructure via density functional theory calculations. Results show that the conduction band minimum (CBM) and valence band maximum (VBM) of this heterostructure are mainly contributed by different materials, illustrating that the AlAs/SiC heterostructure has a type-II band alignment. Interestingly, this heterostructure possesses flat valence bands near the Fermi level. In addition, under the modulation of external electric field ranging between -1 V Å-1∼0.8 V Å-1, the band gap of the heterostructure can be tuned continuously, while the band structure maintains a stable type-II band alignment with flat top valence bands. When the electric field exceeds -1 or 0.8 V Å-1, the heterostructure transitions from semiconductor material to metal, indicating the tunability of electronic properties under external fields. These results indicate that the AlAs/SiC heterostructure shows great potential for application in high-performance optoelectronic devices and a strong correlation may exist in this system.

Positive and Negative Photoconductivity in Ir Nanofilm-Coated MoO3 Bias-Switching Photodetector

In this study, we delved into the influence of Ir nanofilm coating thickness on the optical and optoelectronic behavior of ultrathin MoO3 wafer-scale devices. Notably, the 4 nm Ir coating showed a negative Hall voltage and high carrier concentration of 1.524 × 1019 cm-3 with 0.19 nm roughness. Using the Kubelka-Munk model, we found that the bandgap decreased with increasing Ir thickness, consistent with Urbach tail energy suggesting a lower level of disorder. Regarding transient photocurrent behavior, all samples exhibited high stability under both dark and UV conditions. We also observed a positive photoconductivity at bias voltages of >0.5 V, while at 0 V bias voltage, the samples displayed a negative photoconductivity behavior. This unique aspect allowed us to explore self-powered negative photodetectors, showcasing fast response and recovery times of 0.36/0.42 s at 0 V. The intriguing negative photoresponse that we observed is linked to hole self-trapping/charge exciton and Joule heating effects.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Towards high photoresponse of perovskite nanowire/copper phthalocyanine heterostructured photodetector

One-dimensional nanowire structures composed of perovskite are widely recognized for their exceptional optoelectronic performance and mechanical properties, making them a popular area of investigation in photodetection research. In this work, a perovskite nanowire/copper phthalocyanine heterojunction-based photodetector was fabricated, which exhibits high photoresponse in the visible-near-infrared region. The incorporation of a heterojunction significantly enhanced the photoelectric performance. Specifically, the photoresponsivity and external quantum efficiency of the nanowire-based device were elevated from 58.5 A W-1and 1.35 × 104% to 84.5 A W-1and 1.97 × 104% at 532 nm, respectively. The enhanced photoresponse of the heterojunction device can be attributed to the unique microstructure of nanowire arrays. The wrapping of the nanowires by copper phthalocyanine forms heterojunctions with a larger dissociation area, which facilitated exciton dissociation and enhanced device performance. This work provides a promising example for optimizing the performance of nanowire devices.

Photoconductivity Enhancement in Atomically Thin Molybdenum Disulfide through Local Doping from Confined Water

Two-dimensional transition metal dichalcogenide (TMDC) materials have shown great potential for usage in opto-electronic devices, especially down to the regime of a few layers to a single layer. However, at these limits, the material properties can be strongly influenced by the interfaces. By using photoconductive atomic force microscopy, we show a local enhancement of photoconductivity at the nanoscale in bilayer molybdenum disulfide on mica, where water is confined between the TMDC and the substrate. We have found that the structural phase of the water influences the doping level and thus the tunneling barrier at the nanojunction. This leads to an increase in photocurrent and enhanced photopower generation.

Probing the Electronic and Opto-Electronic Properties of Multilayer MoS2 Field-Effect Transistors at Low Temperatures

Transition metal dichalcogenides (TMDs)-based field-effect transistors (FETs) are being investigated vigorously for their promising applications in optoelectronics. Despite the high optical response reported in the literature, most of them are studied at room temperature. To extend the application of these materials in a photodetector, particularly at a low temperature, detailed understanding of the photo response behavior of these materials at low temperatures is crucial. Here we present a systematic investigation of temperature-dependent electronic and optoelectronic properties of few-layers MoS2 FETs, synthesized using the mechanical exfoliation of bulk MoS2 crystal, on the Si/SiO2 substrate. Our MoS2 FET show a room-temperature field-effect mobility μFE ~40 cm2·V-1·s-1, which increases with decreasing temperature, stabilizing at 80 cm2·V-1·s-1 below 100 K. The temperature-dependent (50 K < T < 300 K) photoconductivity measurements were investigated using a continuous laser source λ = 658 nm (E = 1.88 eV) over a broad range of effective illuminating laser intensity, Peff (0.02 μW < Peff < 0.6 μW). Photoconductivity measurements indicate a fractional power dependence of the steady-state photocurrent. The room-temperature photoresponsivity (R) obtained in these samples was found to be ~2 AW-1, and it increases as a function of decreasing temperature, reaching a maximum at T = 75 K. The optoelectronic properties of MoS2 at a low temperature give an insight into photocurrent generation mechanisms, which will help in altering/improving the performance of TMD-based devices for various applications.

Synthesis of violet phosphorus with large lateral sizes to facilitate nano-device fabrications

Violet phosphorus has been proven to be the most stable phosphorus allotrope and has attracted much attention recently. The growth of violet phosphorus with large lateral sizes is crucial to obtain good quality violet phosphorene for nanodevice fabrication. Herein, a large number of violet phosphorus plates have been produced from molten lead using an optimized method to achieve red bronze luster. The crystal structure of the as-produced violet phosphorus was determined by single-crystal X-ray diffraction to be monoclinic with the space group P2/n (13) (CSD-2160375), identical to the one from the chemical vapor transport method (CSD-1935087). The as-produced violet phosphorus plates were found to have lateral sizes of 1.30 ± 0.41 mm². The violet phosphorus plates were easily exfoliated and directly transferred to silicon substrates to facilitate building of a back-gate field-effect transistor. A hole mobility of 2.308 cm² V^-1 s^-1 was obtained from a violet phosphorus nanosheet with a thickness of 52 nm under ambient conditions. The absolute responsivity of 130 mA W^-1 with a fast response time of 27 ms was also obtained under the irradiation of a 530 nm laser.

Integration of Self-Passivated Topological Electrodes for Advanced 2D Optoelectronic Devices

With the rapid development of two-dimensional semiconductor technology, the inevitable chemical disorder at a typical metal-semiconductor interface has become an increasingly serious problem that degrades the performance of 2D semiconductor optoelectronic devices. Herein, defect-free van der Waals contacts have been achieved by utilizing topological Bi₂ Se₃ as the electrodes. Such clean and atomically sharp contacts avoid the consumption of photogenerated carriers at the interface, enabling a markedly boosted sensitivity as compared to counterpart devices with directly deposited metal electrodes. Typically, the device with 2D WSe₂ channel realizes a high responsivity of 20.5 A W^-1 , an excellent detectivity of 2.18 × 10¹²  Jones, and a fast rise/decay time of 41.66/38.81 ms. Furthermore, high-resolution visible-light imaging capability of the WSe₂ device is demonstrated, indicating its promising application prospect in future optoelectronic systems. More inspiringly, the topological electrodes are universally applicable to other 2D semiconductor channels, including WS₂ and InSe, suggesting its broad applicability. These results open fascinating opportunities for the development of high-performance electronics and optoelectronics.

TiS3 Nanoribbons: A Novel Material for Ultra-Sensitive Photodetection across Extreme Temperature Ranges

Photodetectors that can operate over a wide range of temperatures, from cryogenic to elevated temperatures, are crucial for a variety of modern scientific fields, including aerospace, high-energy science, and astro-particle science. In this study, we investigate the temperature-dependent photodetection properties of titanium trisulfide (TiS₃)- in order to develop high-performance photodetectors that can operate across a wide range of temperatures (77 K-543 K). We fabricate a solid-state photodetector using the dielectrophoresis technique, which demonstrates a quick response (response/recovery time ~0.093 s) and high performance over a wide range of temperatures. Specifically, the photodetector exhibits a very high photocurrent (6.95 × 10^-5 A), photoresponsivity (1.624 × 10⁸ A/W), quantum efficiency (3.3 × 10⁸ A/W·nm), and detectivity (4.328 × 10¹⁵ Jones) for a 617 nm wavelength of light with a very weak intensity (~1.0 × 10^-5 W/cm²). The developed photodetector also shows a very high device ON/OFF ratio (~32). Prior to fabrication, the TiS₃ nanoribbons were synthesized using the chemical vapor technique and characterized according to their morphology, structure, stability, and electronic and optoelectronic properties; this was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and a UV-Visible-NIR spectrophotometer. We anticipate that this novel solid-state photodetector will have broad applications in modern optoelectronic devices.

Multiple-State Nonvolatile Memory Based on Ultrathin Indium Oxide Film via Liquid Metal Printing

In this work, the ultrathin two-dimensional (2D) indium oxide (InO_x) with a large area of more than 100 μm² and a high degree of uniformity was automatically peeled off from indium by the liquid-metal printing technique. Raman and optical measurements revealed that 2D-InO_x has a polycrystalline cubic structure. By altering the printing temperature which affects the crystallinity of 2D-InO_x, the mechanism of the existence and disappearance of memristive characteristics was established. The tunable characteristics of the 2D-InO_x memristor with reproducible one-order switching was manifest from the electrical measurements. Further adjustable multistate characteristics of the 2D-InO_x memristor and its resistance switching mechanism were evaluated. A detailed examination of the memristive process demonstrated the Ca²⁺ mimic dynamic in 2D-InO_x memristors as well as the fundamental principles underlying biological and artificial synapses. These surveys allow us to comprehend a 2D-InO_x memristor using the liquid-metal printing technique and could be applied to future neuromorphic applications and in the field of revolutionary 2D material exploration.

Helical polymers for dissymmetric circularly polarized light imaging

Control of the spin angular momentum (SAM) carried in a photon provides a technologically attractive element for next-generation quantum networks and spintronics1-5. However, the weak optical activity and inhomogeneity of thin films from chiral molecular crystals result in high noise and uncertainty in SAM detection. Brittleness of thin molecular crystals represents a further problem for device integration and practical realization of chiroptical quantum devices6-10. Despite considerable successes with highly dissymmetric optical materials based on chiral nanostructures11-13, the problem of integration of nanochiral materials with optical device platforms remains acute14-16. Here we report a simple yet powerful method to fabricate chiroptical flexible layers via supramolecular helical ordering of conjugated polymer chains. Their multiscale chirality and optical activity can be varied across the broad spectral range by chiral templating with volatile enantiomers. After template removal, chromophores remain stacked in one-dimensional helical nanofibrils producing a homogeneous chiroptical layer with drastically enhanced polarization-dependent absorbance, leading to well-resolved detection and visualization of SAM. This study provides a direct path to scalable realization of on-chip detection of the spin degree of freedom of photons necessary for encoded quantum information processing and high-resolution polarization imaging.

Visible Near-Infrared Photodetection Based on Ta2NiSe5/WSe2 van der Waals Heterostructures

The increasing interest in two-dimensional materials with unique crystal structures and novel band characteristics has provided numerous new strategies and paradigms in the field of photodetection. However, as the demand for wide-spectrum detection increases, the size of integrated systems and the limitations of mission modules pose significant challenges to existing devices. In this paper, we present a van der Waals heterostructure photodetector based on Ta₂NiSe₅/WSe₂, leveraging the inherent characteristics of heterostructures. Our results demonstrate that this detector exhibits excellent broad-spectrum detection ability from the visible to the infrared bands at room temperature, achieving an extremely high on/off ratio, without the need for an external bias voltage. Furthermore, compared to a pure material detector, it exhibits a fast response and low dark currents (~3.6 pA), with rise and fall times of 278 μs and 283 μs for the response rate, respectively. Our findings provide a promising method for wide-spectrum detection and enrich the diversity of room-temperature photoelectric detection.

Giant Superlinear Power Dependence of Photocurrent Based on Layered Ta2 NiS5 Photodetector

Photodetector based on two-dimensional (2D) materials is an ongoing quest in optoelectronics. 2D photodetectors are generally efficient at low illuminating power but suffer severe recombination processes at high power, which results in the sublinear power-dependent photoresponse and lower optoelectronic efficiency. The desirable superlinear photocurrent is mostly achieved by sophisticated 2D heterostructures or device arrays, while 2D materials rarely show intrinsic superlinear photoresponse. This work reports the giant superlinear power dependence of photocurrent based on multilayer Ta₂ NiS₅ . While the fabricated photodetector exhibits good sensitivity (3.1 mS W^-1 per □) and fast photoresponse (31 µs), the bias-, polarization-, and spatial-resolved measurements point to an intrinsic photoconductive mechanism. By increasing the incident power density from 1.5 to 200 µW µm^-2 , the photocurrent power dependence varies from sublinear to superlinear. At higher illuminating conditions, prominent superlinearity is observed with a giant power exponent of γ = 1.5. The unusual photoresponse can be explained by a two-recombination-center model where density of states of the recombination centers (RC) effectively closes all recombination channels. The photodetector is integrated into camera for taking photos with enhanced contrast due to superlinearity. This work provides an effective route to enable higher optoelectronic efficiency at extreme conditions.

How to characterize figures of merit of two-dimensional photodetectors

Photodetectors based on two-dimensional (2D) materials have been the focus of intensive research and development over the past decade. However, a gap has long persisted between fundamental research and mature applications. One of the main reasons behind this gap has been the lack of a practical and unified approach for the characterization of their figures of merit, which should be compatible with the traditional performance evaluation system of photodetectors. This is essential to determine the degree of compatibility of laboratory prototypes with industrial technologies. Here we propose general guidelines for the characterization of the figures of merit of 2D photodetectors and analyze common situations when the specific detectivity, responsivity, dark current, and speed can be misestimated. Our guidelines should help improve the standardization and industrial compatibility of 2D photodetectors.

Advances in the Field of Two-Dimensional Crystal-Based Photodetectors

Two-dimensional (2D) materials have sparked intense interest among the scientific community owing to their extraordinary mechanical, optical, electronic, and thermal properties. In particular, the outstanding electronic and optical properties of 2D materials make them show great application potential in high-performance photodetectors (PDs), which can be applied in many fields such as high-frequency communication, novel biomedical imaging, national security, and so on. Here, the recent research progress of PDs based on 2D materials including graphene, transition metal carbides, transition-metal dichalcogenides, black phosphorus, and hexagonal boron nitride is comprehensively and systematically reviewed. First, the primary detection mechanism of 2D material-based PDs is introduced. Second, the structure and optical properties of 2D materials, as well as their applications in PDs, are heavily discussed. Finally, the opportunities and challenges of 2D material-based PDs are summarized and prospected. This review will provide a reference for the further application of 2D crystal-based PDs.