Modulation of Junction Modes in SnSe2/MoTe2 Broken-Gap van der Waals Heterostructure for Multifunctional Devices

Highly Responsive and Self-Powered Photodetector Based on PtSe2/MoS2 Heterostructure

In recent years, 2D materials and their heterostructures have started to offer an ideal platform for high-performance photodetection devices. In this work, a highly responsive, self-powered photodetector based on PtSe2/MoS2 van der Waals heterostructure is demonstrated. The device achieves a noteworthy wide band spectral response from visible (405 nm) range to the near infrared region (980 nm). The remarkable photoresponsivity and external quantum efficiency up to 4.52 A/W, and 1880% are achieved, respectively, at 405 nm illumination with fast response time of 20 ms. In addition, the photodetector exhibits a decent photoresponsivity of 33.4 mA/W at zero bias, revealing the photodetector works well in the self-driven mode. Our work suggests that a PtSe2/MoS2 heterostructure could be a potential candidate for the high-performance photodetection applications.

Phase-Centric MOCVD Enabled Synthetic Approaches for Wafer-Scale 2D Tin Selenides

Following an initial nucleation stage at the flake level, atomically thin film growth of a van der Waals material is promoted by ultrafast lateral growth and prohibited vertical growth. To produce these highly anisotropic films, synthetic or post-synthetic modifications are required, or even a combination of both, to ensure large-area, pure-phase, and low-temperature deposition. A set of synthetic strategies is hereby presented to selectively produce wafer-scale tin selenides, SnSex (both x = 1 and 2), in the 2D forms. The 2D-SnSe2 films with tuneable thicknesses are directly grown via metal-organic chemical vapor deposition (MOCVD) at 200 °C, and they exhibit outstanding crystallinities and phase homogeneities and consistent film thickness across the entire wafer. This is enabled by excellent control of the volatile metal-organic precursors and decoupled dual-temperature regimes for high-temperature ligand cracking and low-temperature growth. In contrast, SnSe, which intrinsically inhibited from 2D growth, is indirectly prepared by a thermally driven phase transition of an as-grown 2D-SnSe2 film with all the benefits of the MOCVD technique. It is accompanied by the electronic n-type to p-type crossover at the wafer scale. These tailor-made synthetic routes will accelerate the low-thermal-budget production of multiphase 2D materials in a reliable and scalable fashion.

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.

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.

Two-dimensional HfS2-ZrS2 lateral heterojunction FETs with high rectification and photocurrent

Multifunctional devices are an indispensable choice to fulfil the increasing demand for miniaturized and integrated circuit systems. However, bulk material-based devices encounter the challenge of miniaturized all-in-one systems with multiple functions. In this study, we designed a field effect transistor (FET) based on a monolayer HfS2-ZrS2 lateral heterojunction. It possesses simultaneous and obvious rectifying behavior and photodetection characteristics in the visible light region, such as the rectification ratio of ∼1012, photocurrent density of 13.3 nA m-1, responsivity of 57 mA W-1, and extinction ratio of 108. Notably, the rectification ratio of the single-gate FET is larger than that of the dual-gate FET under the negative gate voltage. These results indicate that monolayer lateral heterojunction-based FETs can provide an effective route to integrate rectifying and photodetection functions in single optoelectronic nanodevices.

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.

Highly Tunable Negative Differential Resistance Device Based on Insulator-to-Metal Phase Transition of Vanadium Dioxide

Negative differential resistance (NDR) based on the band-to-band tunneling (BTBT) mechanism has recently shown great potential in improving the performance of various electronic devices. However, the applicability of conventional BTBT-based NDR devices is restricted by their insufficient performance due to the limitations of the NDR mechanism. In this study, we develop an insulator-to-metal phase transition (IMT)-based NDR device that exploits the abrupt resistive switching of vanadium dioxide (VO₂) to achieve a high peak-to-valley current ratio (PVCR) and peak current density (J_peak) as well as controllable peak and valley voltages (V_peak/valley). When a phase transition is induced in VO₂, the effective voltage bias on the two-dimensional channel is decreased by the reduction in the VO₂ resistance. Accordingly, the effective voltage adjustment induced by the IMT results in an abrupt NDR. This NDR mechanism based on the abrupt IMT results in a maximum PVCR of 71.1 through its gate voltage and VO₂ threshold voltage tunability characteristics. Moreover, V_peak/valley is easily modulated by controlling the length of VO₂. In addition, a maximum J_peak of 1.6 × 10⁶ A/m² is achieved through light-tunable characteristics. The proposed IMT-based NDR device is expected to contribute to the development of various NDR devices for next-generation electronics.

A two-dimensional Te/ReS2 van der Waals heterostructure photodetector with high photoresponsivity and fast photoresponse

Two-dimensional (2D) semiconductors are the building blocks for high-performance optoelectronic devices. However, the performance of photoconductive photodetectors based on 2D semiconductors is hampered by low photoresponsivity and large dark current. Herein, a van der Waals heterostructure (vdWH) composed of rhenium disulfide (ReS₂) and tellurium (Te) is fabricated. The Te/ReS₂ vdWH photodetector exhibits a sensitive and broadband photoresponse and has high photoresponse on/off ratios under ultraviolet and visible light illumination, especially over 10² in visible light. The Te/ReS₂ vdWH photodetector achieves the responsivity of 7.9 A W^-1 at 365 nm, 3.02 A W^-1 at 450 nm, 2.37 A W^-1 at 532 nm, and 2.45 A W^-1 at 660 nm. In addition, the device achieves a high specific detectivity of 10¹¹ Jones and a fast photoresponse speed of 11.9 μs. Such high responsivity could be attributed to the efficient absorption of phonons by the Te/ReS₂ vdWH and the high-quality heterostructure interfaces with a small amount of trap states. The highly crystalline structure of Te/ReS₂ with a low density of defects reduces the grain boundary scattering, leading to the rapid diffusion of charge carriers. Moreover, the Te/ReS₂ vdWH device exhibits a photovoltaic effect and can be employed as a self-powered photodetector (SPPD), which is sensitive to visible light of 450 nm, 532 nm, and 660 nm. Our findings demonstrate that the Te/ReS₂ vdWH photodetector is an ideal building block for the next-generation electronic and optoelectronic devices in practical applications.

Emerging Trends in 2D TMDs Photodetectors and Piezo-Phototronic Devices

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.

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

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

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

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

Chemical Vapor Deposition of Quaternary 2D BiCuSeO p-Type Semiconductor with Intrinsic Degeneracy

2D BiCuSeO is an intrinsic p-type degenerate semiconductor due to its self-doping effect, which possesses great potential to fabricate high-performance 2D-2D tunnel field-effect transistors (TFETs). However, the controllable synthesis of multinary 2D materials by chemical vapor deposition (CVD) is still a challenge due to the restriction of thermodynamics. Here, the CVD synthesis of quaternary 2D BiCuSeO nanosheets is realized. As-grown BiCuSeO nanosheets with thickness down to ≈6.1 nm (≈7 layers) and domain size of ≈277 µm show excellent ambient stability. Intrinsic p-type degeneracy of BiCuSeO, capable of maintaining even in a few layers, is comprehensively unveiled. By varying the thicknesses and temperatures, the carrier concentration of BiCuSeO nanosheets can be adjusted in the range of 10¹⁹ to 10²¹ cm^-3 , and the Hall mobility of BiCuSeO is ≈191 cm² V^-1 s^-1 (at 2 K). Furthermore, taking advantage of the p-type degeneracy of BiCuSeO, a prototypical BiCuSeO/MoS₂ TFET is fabricated. The emergence of the negative differential resistance trend and multifunctional diodes by modulating the gate voltage and temperature reveal the great practical implementation potential of BiCuSeO nanosheets. These results pave way for the CVD synthesis of multinary 2D materials and rational design of high-performance tunnel devices.

Doping Engineering in the MoS2 /SnSe2 Heterostructure toward High-Rejection-Ratio Solar-Blind UV Photodetection

The intentionally designed band alignment of heterostructures and doping engineering are keys to implement device structure design and device performance optimization. According to the theoretical prediction of several typical materials among the transition metal dichalcogenides (TMDs) and group-IV metal chalcogenides, MoS₂ and SnSe₂ present the largest staggered band offset. The large band offset is conducive to the separation of photogenerated carriers, thus MoS₂ /SnSe₂ is a theoretically ideal candidate for fabricating photodetector, which is also verified in the experiment. Furthermore, in order to extend the photoresponse spectrum to solar-blind ultraviolet (SBUV), doping engineering is adopted to form an additional electron state, which provides an extra carrier transition channel. In this work, pure MoS₂ /SnSe₂ and doped MoS₂ /SnSe₂ heterostructures are both fabricated. In terms of the photoelectric performance evaluation, the rejection ratio R₂₅₄ /R₅₃₂ of the photodetector based on doped MoS₂ /SnSe₂ is five orders of magnitude higher than that of pure MoS₂ /SnSe₂ , while the response time is obviously optimized by 3 orders. The results demonstrate that the combination of band alignment and doping engineering provides a new pathway for constructing SBUV photodetectors.

Type-I Heterostructure Based on WS2/PtS2 for High-Performance Photodetectors

van der Waals (vdW) heterodiodes composed of two-dimensional (2D) layered materials led to a new prospect in photoelectron diodes and photovoltaic devices. Existing studies have shown that Type-I heterostructures have great potential to be used as photodetectors; however, the tunneling phenomena in Type-I heterostructures have not been fully revealed. Herein, a highly efficient nn⁺ WS₂/PtS₂ Type-I vdW heterostructure photodiode is constructed. The device shows an ultrahigh reverse rectification ratio of 10⁵ owing to the transmission barrier-induced low reverse current. A unilateral depletion region is formed on WS₂, which inhibits the recombination of carriers at the interface and makes the external quantum efficiency (EQE) of the device reach 67%. Due to the tunneling mechanism of the device, which allows the co-existence of a large photocurrent and a low dark current, this device achieves a light on/off ratio of over 10⁵. In addition, this band design allows the device to maintain a high detectivity of 4.53 × 10¹⁰ Jones. Our work provides some new ideas for exploring new high-efficiency photodiodes.

Enhanced Photodetection Range from Visible to Shortwave Infrared Light by ReSe2/MoTe2 van der Waals Heterostructure

Type II vertical heterojunction is a good solution for long-wavelength light detection. Here, we report a rhenium selenide/molybdenum telluride (n-ReSe2/p-MoTe2) photodetector for high-performance photodetection in the broadband spectral range of 405–2000 nm. Due to the low Schottky barrier contact of the ReSe2/MoTe2 heterojunction, the rectification ratio (RR) of ~102 at ±5 V is realized. Besides, the photodetector can obtain maximum responsivity (R = 1.05 A/W) and specific detectivity (D* = 6.66 × 1011 Jones) under the illumination of 655 nm incident light. When the incident wavelength is 1550–2000 nm, a photocurrent is generated due to the interlayer transition of carriers. This compact system can provide an opportunity to realize broadband infrared photodetection.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

Polarity-Tunable Photocurrent through Band Alignment Engineering in a High-Speed WSe2/SnSe2 Diode with Large Negative Responsivity

Excellent light-matter interaction and a wide range of thickness-tunable bandgaps in layered vdW materials coupled by the facile fabrication of heterostructures have enabled several avenues for optoelectronic applications. Realization of high photoresponsivity at fast switching speeds is a critical challenge for 2D optoelectronics to enable high-performance photodetection for optical communication. Moving away from conventional type-II heterostructure pn junctions towards a WSe₂/SnSe₂ type-III configuration, we leverage the steep change in tunneling current along with a light-induced heterointerface band shift to achieve high negative photoresponsivity, while the fast carrier transport under tunneling results in high speed. In addition, the photocurrent can be controllably switched from positive to negative values, with ∼10⁴× enhancement in responsivity, by engineering the band alignment from type-II to type-III using either the drain or the gate bias. This is further reinforced by electric-field dependent interlayer band structure calculations using density functional theory. The high negative responsivity of 2 × 10⁴ A/W and fast response time of ∼1 μs coupled with a polarity-tunable photocurrent can lead to the development of next-generation multifunctional optoelectronic devices.

Energy-Band Engineering by Remote Doping of Self-Assembled Monolayers Leads to High-Performance IGZO/p-Si Heterostructure Photodetectors

Metal oxide semiconductors are of great interest for enabling advanced photodetectors. However, operational instability and the absence of an appropriate doping technique hinder practical development and commercialization. Here, a strategy is proposed to dramatically increase the conventional photodetection performance, having superior stability in operational and environmental atmospheres. By performing energy-band engineering through an octadecylphosphonic acid (ODPA) self-assembled-monolayer-based doping treatment, the proposed indium-gallium-zinc oxide (IGZO)/p-Si heterointerface devices exhibit greatly enhance the photoresponsive characteristics, including a photoswitching current ratio with a 100-fold increase, and photoresponsivity and detectivity with a 15-fold increase each. The observed ODPA doping effects are investigated through comprehensive analysis with X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM). Furthermore, the proposed photodetectors, fabricated at a 4 in. wafer scale, demonstrate its excellent operation robustness with consistent performance over 237 days and 20 000 testing cycles.

Nano-Thermal Analysis of Defect-Induced Surface Pre-Melting in 2D Tellurium

Thermal properties, such as thermal conductivity, heat capacity, and melting temperature, influence the efficiency and stability of two-dimensional (2D) material applications. However, existing studies on thermal characteristics-except for thermal conductivity-are insufficient for 2D materials. Here, we investigated the melting temperature of 2D Tellurium (2D Te) using the nano-thermal analysis technique and found anomalous behavior that occurs before the melting temperature is reached. The theoretical calculations present surface pre-melting in 2D Te and Raman scattering measurements suggest that defects in 2D Te accelerate surface pre-melting. Understanding the pre-melting surface characteristics of 2D Te will provide valuable information for practical applications.

Ultrafast and Highly Stable Photodetectors Based on p-GeSe/n-ReSe2 Heterostructures

Two-dimensional transition-metal dichalcogenide (2D-TMD) semiconductors and their van der Waals heterostructures (vdWHs) have attracted great attention because of their tailorable band-engineering properties and provide a propitious platform for next-generation extraordinary performance energy-harvesting devices. Herein, we reported unique and unreported germanium selenide/rhenium diselenide (p-GeSe/n-ReSe₂) 2D-TMD vdWH photodetectors for extremely sensitive and high-performance photodetection in the broadband spectral range (visible and near-infrared range). A high and gate-tunable rectification ratio (RR) of 7.34 × 10⁵ is achieved, stemming from the low Schottky barrier contacts and sharp interfaces of the p-GeSe/n-ReSe₂ 2D-TMD vdWHs. In addition, a noticeably high responsivity (R = 2.89 × 10⁵ A/W) and specific detectivity (D* = 4.91 × 10¹³ Jones), with good external quantum efficiency (EQE = 6.1 × 10⁵) are obtained because of intralayer and interlayer transition of excitations, enabling the broadband photoresponse (λ = 532-1550 nm) at room temperature. Furthermore, fast response times of 16-20 μs are estimated under the irradiated laser of λ = 1550 nm because of interlayer exciton transition. Such a TMD-based compact system offers an opportunity for the realization of high-performance broadband infrared photodetectors.

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

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

2D-1D mixed-dimensional heterostructures: progress, device applications and perspectives

Two-dimensional (2D) materials have attracted broad interests and been extensively exploited for a variety of functional applications. Moreover, one-dimensional (1D) atomic crystals can also be integrated into 2D templates to create mixed-dimensional heterostructures, and the versatility of combinations provides 2D-1D heterostructures plenty of intriguing physical properties, making them promising candidate to construct novel electronic and optoelectronic nanodevices. In this review, we first briefly present an introduction of relevant fabrication methods and structural configurations for 2D-1D heterostructures integration. We then discuss the emerged intriguing physics, including high optical absorption, efficient carrier separation, fast charge transfer and plasmon-exciton interconversion. Their potential applications such as electronic/optoelectronic devices, photonic devices, spintronic devices and gas sensors, are also discussed. Finally, we provide a brief perspective for the future opportunities and challenges in this emerging field.

Room-Temperature Negative Differential Resistance in Surface-Supported Metal-Organic Framework Vertical Heterojunctions

The advances of surface-supported metal-organic framework (SURMOF) thin-film synthesis have provided a novel strategy for effectively integrating metal-organic framework (MOF) structures into electronic devices. The considerable potential of SURMOFs for electronics results from their low cost, high versatility, and good mechanical flexibility. Here, the first observation of room-temperature negative differential resistance (NDR) in SURMOF vertical heterojunctions is reported. By employing the rolled-up nanomembrane approach, highly porous sub-15 nm thick HKUST-1 films are integrated into a functional device. The NDR is tailored by precisely controlling the relative humidity (RH) around the device and the applied electric field. The peak-to-valley current ratio (PVCR) of about two is obtained for low voltages (<2 V). A transition from a metastable state to a field emission-like tunneling is responsible for the NDR effect. The results are interpreted through band diagram analysis, density functional theory (DFT) calculations, and ab initio molecular dynamics simulations for quasisaturated water conditions. Furthermore, a low-voltage ternary inverter as a multivalued logic (MVL) application is demonstrated. These findings point out new advances in employing unprecedented physical effects in SURMOF heterojunctions, projecting these hybrid structures toward the future generation of scalable functional devices.

Ferroelectric-tuned van der Waals heterojunction with band alignment evolution

Van der Waals integration with abundant two-dimensional materials provides a broad basis for assembling functional devices. In a specific van der Waals heterojunction, the band alignment engineering is crucial and feasible to realize high performance and multifunctionality. Here, we design a ferroelectric-tuned van der Waals heterojunction device structure by integrating a GeSe/MoS₂ VHJ and poly (vinylidene fluoride-trifluoroethylene)-based ferroelectric polymer. An ultrahigh electric field derived from the ferroelectric polarization can effectively modulate the band alignment of the GeSe/MoS₂ heterojunction. Band alignment transition of the heterojunction from type II to type I is demonstrated. The combination of anisotropic GeSe with MoS₂ realizes a high-performance polarization-sensitive photodetector exhibiting low dark current of approximately 1.5 pA, quick response of 14 μs, and high detectivity of 4.7 × 10¹² Jones. Dichroism ratios are also enhanced by ferroelectric polarization in a broad spectrum from visible to near-infrared. The ferroelectric-tuned GeSe/MoS₂ van der Waals heterojunction has great potential for multifunctional detection applications in sophisticated light information sensing. More profoundly, the ferroelectric-tuned van der Waals heterojunction structure provides a valid band-engineering approach to creating versatile devices.

Silicon/2D-material photodetectors: from near-infrared to mid-infrared

Two-dimensional materials (2DMs) have been used widely in constructing photodetectors (PDs) because of their advantages in flexible integration and ultrabroad operation wavelength range. Specifically, 2DM PDs on silicon have attracted much attention because silicon microelectronics and silicon photonics have been developed successfully for many applications. 2DM PDs meet the imperious demand of silicon photonics on low-cost, high-performance, and broadband photodetection. In this work, a review is given for the recent progresses of Si/2DM PDs working in the wavelength band from near-infrared to mid-infrared, which are attractive for many applications. The operation mechanisms and the device configurations are summarized in the first part. The waveguide-integrated PDs and the surface-illuminated PDs are then reviewed in details, respectively. The discussion and outlook for 2DM PDs on silicon are finally given.

Unprecedentedly Uniform, Reliable, and Centimeter-Scale Molybdenum Disulfide Negative Differential Resistance Photodetectors

Negative differential resistance (NDR) can be applied to various devices such as reflection amplifiers, relaxation oscillators, and neuromorphic devices. However, the development of NDR photodetectors with uniformity, stability, and reproducibility for use in practical applications is still lacking. Herein, we demonstrate highly reliable NDR photodetectors by constructing a MoS₂/p-Si heterostructure. Owing to the formation of a MoS₂ layer with uniform thickness by the plasma-enhanced sulfurization process, a 100% yield with high uniformity (peak-to-valley ratio = 1.195 ± 0.065) was achieved for 120 devices. Furthermore, the proposed NDR photodetectors exhibit unprecedented high cycle-to-cycle endurance, which maintains their NDR characteristics through 100 000 consecutive sweeps without operational failure. This work paves the way for the development of a reliable NDR device and reports unprecedented results of high uniformity, reproducibility, and robustness for practical applications.

Highly Sensitive, Ultrafast, and Broadband Photo-Detecting Field-Effect Transistor with Transition-Metal Dichalcogenide van der Waals Heterostructures of MoTe2 and PdSe2

Recently, van der Waals heterostructures (vdWHs) based on transition-metal dichalcogenides (TMDs) have attracted significant attention owing to their superior capabilities and multiple functionalities. Herein, a novel vdWH field-effect transistor (FET) composed of molybdenum ditelluride (MoTe2 ) and palladium diselenide (PdSe2 ) is studied for highly sensitive photodetection performance in the broad visible and near-infrared (VNIR) region. A high rectification ratio of 6.3 × 105 is obtained, stemming from the sharp interface and low Schottky barriers of the MoTe2 /PdSe2 vdWHs. It is also successfully demonstrated that the vdWH FET exhibits highly sensitive photo-detecting abilities, such as noticeably high photoresponsivity (1.24 × 105 A W-1 ), specific detectivity (2.42 × 1014 Jones), and good external quantum efficiency (3.5 × 106 ), not only due to the intra-TMD band-to-band transition but also due to the inter-TMD charge transfer (CT) transition. Further, rapid rise (16.1 µs) and decay (31.1 µs) times are obtained under incident light with a wavelength of 2000 nm due to the CT transition, representing an outcome one order of magnitude faster than values currently in the literature. Such TMD-based vdWH FETs would improve the photo-gating characteristics and provide a platform for the realization of a highly sensitive photodetector in the broad VNIR region.

Broken-Gap PtS2/WSe2 van der Waals Heterojunction with Ultrahigh Reverse Rectification and Fast Photoresponse

Broken-gap van der Waals (vdW) heterojunctions based on 2D materials are promising structures to fabricate high-speed switching and low-power multifunctional devices thanks to its charge transport versus quantum tunneling mechanism. However, the tunneling current is usually generated under both positive and negative bias voltage, resulting in small rectification and photocurrent on/off ratio. In this paper, we report a broken-gap vdW heterojunction PtS₂/WSe₂ with a bilateral accumulation region design and a big band offset by utilizing thick PtS₂ as an effective carrier-selective contact, which exhibits an ultrahigh reverser rectification ratio approaching 10⁸ and on/off ratio over 10⁸ at room temperature. We also find excellent photodetection properties in such a heterodiode with a large photocurrent on/off ratio over 10⁵ due to its ultralow forward current and a comparable photodetectivity of 3.8 × 10¹⁰ Jones. In addition, the response time of such a photodetector reaches 8 μs owing to the photoinduced tunneling mechanism and reduced interface trapping effect. The proposed heterojunction not only demonstrates the high-performance broken-gap heterodiode but also provides in-depth understanding of the tunneling mechanism in the development of future electronic and optoelectronic applications.

Design and tailoring of two-dimensional Schottky, PN and tunnelling junctions for electronics and optoelectronics

Owing to their superior carrier mobility, strong light-matter interactions, and flexibility at the atomically thin thickness, two-dimensional (2D) materials are attracting wide interest for application in electronic and optoelectronic devices, including rectifying diodes, transistors, memory, photodetectors, and light-emitting diodes. At the heart of these devices, Schottky, PN, and tunneling junctions are playing an essential role in defining device function. Intriguingly, the ultrathin thickness and unique van der Waals (vdW) interlayer coupling in 2D materials has rendered enormous opportunities for the design and tailoring of various 2D junctions, e.g. using Lego-like hetero-stacking, surface decoration, and field-effect modulation methods. Such flexibility has led to marvelous breakthroughs during the exploration of 2D electronics and optoelectronic devices. To advance further, it is imperative to provide an overview of existing strategies for the engineering of various 2D junctions for their integration in the future. Thus, in this review, we provide a comprehensive survey of previous efforts toward 2D Schottky, PN, and tunneling junctions, and the functional devices built from them. Though these junctions exhibit similar configurations, distinct strategies have been developed for their optimal figures of merit based on their working principles and functional purposes.

Astability versus Bistability in van der Waals Tunnel Diode for Voltage Controlled Oscillator and Memory Applications

van der Waals (vdW) tunnel junctions are attractive because of their atomically sharp interface, gate tunability, and robustness against lattice mismatch between the successive layers. However, the negative differential resistance (NDR) demonstrated in this class of tunnel diodes often exhibits noisy behavior with low peak current density and lacks robustness and repeatability, limiting their practical circuit applications. Here, we propose a strategy of using a 1L-WS₂ as an optimum tunnel barrier sandwiched in a broken gap tunnel junction of highly doped black phosphorus (BP) and SnSe₂. We achieve high yield tunnel diodes exhibiting highly repeatable, ultraclean, and gate-tunable NDR characteristics with a signature of intrinsic oscillation, and a large peak-to-valley current ratio (PVCR) of 3.6 at 300 K (4.6 at 7 K), making them suitable for practical applications. We show that the thermodynamic stability of the vdW tunnel diode circuit can be tuned from astability to bistability by altering the constraint through choosing a voltage or a current bias, respectively. In the astable mode under voltage bias, we demonstrate a compact, voltage-controlled oscillator without the need for an external tank circuit. In the bistable mode under current bias, we demonstrate a highly scalable, single-element, one-bit memory cell that is promising for dense random access memory applications in memory intensive computation architectures.