Van der Waals Bipolar Junction Transistor Using Vertically Stacked Two‐Dimensional Atomic Crystals

The majority of microelectronic devices rely on a p‐n junction. The process of making such a junction is complicated, and it is difficult to make layers that form a junction with an atomic thickness. In this study, bipolar junctions are made by using 2D atomic crystalline layers and even a single layer in which 2D layers adhere together to form a heterostructure via van der Waals forces. A vertical 2D bipolar junction transistor (V2D‐BJT) is studied for the first time. It uses an MoS₂/WSe₂/MoS₂ heterostructure and has an n‐p‐n configuration that exhibits a maximum common‐base current gain of ≈0.97 and a stable common‐emitter current gain (β) of 12 with a nanowatt power consumption. In the first attempt at gas sensing, it shows outstanding performance, exhibiting a very fast response and recovery time (9 and 35 s, respectively) with a power dissipation of only 2 nW. This study demonstrates the potential application of the V2D‐BJT in nanowatt power amplifiers as well as fast‐response and low‐power gas sensors.     

Salt‐Assisted Growth of P‐type Cu9S5 Nanoflakes for P‐N Heterojunction Photodetectors with High Responsivity

P‐n junctions based on two dimensional (2D) van der Waals (vdW) heterostructure are one of the most promising alternatives in next‐generation electronics and optoelectronics. By choosing different 2D transition metal dichalcogenides (TMDCs), the p‐n junctions have tailored energy band alignments and exhibit superior performance as photodetectors. The p‐n diodes working at reverse bias commonly have high detectivity due to suppressed dark current but suffer from low responsivity resulting from small quantum efficiency. Greater build‐in electric field in the depletion layer can improve the quantum efficiency by reducing recombination of charge carriers. Herein, Cu₉S₅, a novel p‐type semiconductor with direct bandgap and high optical absorption coefficient, is synthesized by salt‐assisted chemical vapor deposition (CVD) method. The high density of holes in Cu₉S₅ endows the constructed p‐n junction, Cu₉S₅/MoS₂, with strong build‐in electric field according to Anderson heterojunction model. Consequently, the Cu₉S₅/MoS₂ p‐n heterojunction has low dark current at reverse bias and high photoresponse under illumination due to the efficient charge separation. The Cu₉S₅/MoS₂ photodetector exhibits good photodetectivity of 1.6 × 10¹² Jones and photoresponsivity of 76 A W^?1 under illumination. This study demonstrates Cu₉S₅ as a promising p‐type semiconductor for high‐performance p‐n heterojunction diodes.     

Piezo‐Phototronic Effect for Enhanced Flexible MoS2/WSe2 van der Waals Photodiodes

The recent discoveries of transition‐metal dichalcogenides (TMDs) as novel 2D electronic materials hold great promise to a rich variety of artificial van der Waals (vdWs) heterojunctions and superlattices. Moreover, most of the monolayer TMDs become intrinsically piezoelectric due to the lack of structural centrosymmetry, which offers them a new degree of freedom to interact with external mechanical stimuli. Here, fabrication of flexible vdWs p–n diode by vertically stacking monolayer n‐MoS₂ and a few‐layer p‐WSe₂ is achieved. Electrical measurement of the junction reveals excellent current rectification behavior with an ideality factor of 1.68 and photovoltaic response is realized. Performance modulation of the photodiode via piezo‐phototronic effect is also demonstrated. The optimized photoresponsivity increases by 86% when introducing a −0.62% compressive strain along MoS₂ armchair direction, which originates from realigned energy‐band profile at MoS₂/WSe₂ interface under strain‐induced piezoelectric polarization charges. This new coupling mode among piezoelectricity, semiconducting, and optical properties in 2D materials provides a new route to strain‐tunable vdWs heterojunctions and may enable the development of novel ultrathin optoelectronics.     

Patterned Growth of P‐Type MoS2 Atomic Layers Using Sol–Gel as Precursor

2D layered MoS₂ has drawn intense attention for its applications in flexible electronic, optoelectronic, and spintronic devices. Most of the MoS₂ atomic layers grown by conventional chemical vapor deposition techniques are n‐type due to the abundant sulfur vacancies. Facile production of MoS₂ atomic layers with p‐type behavior, however, remains challenging. Here, a novel one‐step growth has been developed to attain p‐type MoS₂ layers in large scale by using Mo‐containing sol–gel, including 1% tungsten (W). Atomic‐resolution electron microscopy characterization reveals that small tungsten oxide clusters are commonly present on the as‐grown MoS₂ film due to the incomplete reduction of W precursor at the reaction temperature. These omnipresent small tungsten oxide clusters contribute to the p‐type behavior, as verified by density functional theory calculations, while preserving the crystallinity of the MoS₂ atomic layers. The Mo containing sol–gel precursor is compatible with the soft‐lithography techniques, which enables patterned growth of p‐type MoS₂ atomic layers into regular arrays with different shapes, holding great promise for highly integrated device applications. Furthermore, an atomically thin p–n junction is fabricated by the as‐prepared MoS₂, which shows strong rectifying behavior.     

Infrared Proximity Sensors Based on Photo-Induced Tunneling in van der Waals Integration

Infrared (IR) detectors based on photo-induced tunneling in van der Waals heterostructures (vdWHs) of graphene/h-BN/graphene or MoS₂/h-BN/graphene exhibit extremely low dark currents owing to a large electron barrier. However, a lack of tunneling barrier materials except for h-BN for 2D vdWHs limits their further enhancement. In this study, a broadband detection is reported with high sensitivity and fast photoresponse of IR proximity sensor by a vdW integration (2D-3D) of graphene or MoS₂, with NiO/Ni as the IR absorber and hole selective transport layer/counter electrode. The low Schottky barrier height of the reported junctions suppresses dark current with a high detectivity ≈ 10¹⁴ Jones and generates a photocurrent by transporting photo-excited carriers through a low hole barrier at a wide wavelength. Two types of integrated IR proximity sensor applications are developed: a passive sensor (MoS₂/NiO/Ni) for the near-IR (NIR) range and an active sensor (Gr/NiO/Ni) for the mid-IR (MIR) range. The former shows a broadband photoresponse to reflect the NIR, while the latter absorbs human body irradiation (2–16 µm wavelength) with a fast photoresponse of 3.5 s (rise time) and 1.8 s (fall time). The fabricated sensors utilize low power, broadband detection, high sensitivity, fast photoresponse, and large-scale area at room temperature.     

Van der Waals p–n Junction Based on an Organic–Inorganic Heterostructure

Organic–inorganic heterostructures are an emerging topic that is very interesting for optoelectronics. Here, non‐conventional p–n junctions are investigated using organic rubrene single crystal and 2D MoS₂ as the p‐ and n‐type semiconducting materials, respectively. The current‐rectifying behavior is clearly observed in the junction device. The rectification ratio can be electrically tuned by the gate voltage due to the 2D nature of the heterostructure. The devices also show good photoresponse properties with a photoresponsivity of ≈500 mA W^?1 and a fast response time. These findings suggest a new route to facilitate the design of nanoelectronic and optoelectronic devices based on layered inorganics and organics.     

Nonvolatile Floating‐Gate Memories Based on Stacked Black Phosphorus–Boron Nitride–MoS2 Heterostructures

Research on van der Waals heterostructures based on stacked 2D atomic crystals is intense due to their prominent properties and potential applications for flexible transparent electronics and optoelectronics. Here, nonvolatile memory devices based on floating‐gate field‐effect transistors that are stacked with 2D materials are reported, where few‐layer black phosphorus acts as channel layer, hexagonal boron nitride as tunnel barrier layer, and MoS₂ as charge trapping layer. Because of the ambipolar behavior of black phosphorus, electrons and holes can be stored in the MoS₂ charge trapping layer. The heterostructures exhibit remarkable erase/program ratio and endurance performance, and can be developed for high‐performance type‐switching memories and reconfigurable inverter logic circuits, indicating that it is promising for application in memory devices completely based on 2D atomic crystals.     

Size‐Dependent Optical Absorption of Layered MoS2 and DNA Oligonucleotides Induced Dispersion Behavior for Label‐Free Detection of Single‐Nucleotide Polymorphism

Size‐dependent optical absorption of semiconductive (2H) layered molybdenum disulfide (MoS₂), exhibiting great discrimination abilities to single‐ and double‐stranded DNA (ssDNA) and (dsDNA), is studied. In the presence of high concentration of salt, layered MoS₂ trends to aggregate rapidly, leading to the increases of sizes in both vertical and lateral dimensions of the nanosheets, which results from the interplay between van der Waals attraction and electrical double‐layer repulsion. Meanwhile, the aggregation behavior of layered MoS₂ is remarkably inhibited by the synergistic effects of DNA oligonucleotides. ssDNA can adsorb on the surface of layered MoS₂, resulting in a great dispersion, even in the presence of high concentration of salt, while the dispersion behavior is weakened when ssDNA is replaced by dsDNA. Whereas compared to graphene with zero bandgap energy, layered MoS₂, with semiconductive properties, exhibits great characteristic optical absorption in visible wavelength region devoted to exploring the aggregation behavior of layered MoS₂. Therefore, DNA oligonucleotides induced size control of layered MoS₂, contributing to the regular change of its characteristic absorption in visible region, is considered a label‐free bioassay for the detection of single‐nucleotide polymorphism. Due to its easy operation and high specificity, it is expected that the proposed assay holds great promise for further applications.     

Controllable Switching between Highly Rectifying Schottky and p–n Junctions in an Ionic MoS2 Device

Semiconductor junctions are of great significance for the development of electronic and optoelectronic devices. Here, controllable switching is demonstrated from a Schottky junction to a p–n junction in a partially ionic liquid-gated MoS₂ device with two types of metal contacts. Excellent rectification behavior with a current on-off ratio exceeding 10⁶ is achieved in both Schottky and p–n junction modes. The formation of Schottky junction at the Pd electrode/MoS₂ contact and p–n junction at the p-MoS₂/n-MoS₂ interface is revealed by spatially resolved photocurrent mappings. The switching between the two junctions under ionic gate modulation is correlated with the evolution of the energy band, further validated by the finite element simulation. The device exhibits excellent photodetection properties in the p–n junction mode, including an open circuit voltage up to 0.84 V, a responsivity of 0.24 A W⁻¹, a specific detectivity of 1.7 × 10¹¹ Jones, a response time of hundreds of microseconds and a linear dynamic range of up to 91 dB. The electric field control of such high-performance Schottky and p–n junctions opens up fresh perspectives for studying the behavior of junction and the development of 2D electronic devices.     

Mixed‐Dimensional 1D ZnO–2D WSe2 van der Waals Heterojunction Device for Photosensors

2D layered van der Waals (vdW) atomic crystals are an emerging class of new materials that are receiving increasing attention owing to their unique properties. In particular, the dangling‐bond‐free surface of 2D materials enables integration of differently dimensioned materials into mixed‐dimensional vdW heterostructures. Such mixed‐dimensional heterostructures herald new opportunities for conducting fundamental nanoscience studies and developing nanoscale electronic/optoelectronic applications. This study presents a 1D ZnO nanowire (n‐type)–2D WSe₂ nanosheet (p‐type) vdW heterojunction diode for photodetection and imaging process. After amorphous fluoropolymer passivation, the ZnO–WSe₂ diode shows superior performance with a much‐enhanced rectification (ON/OFF) ratio of over 10⁶ and an ideality factor of 3.4–3.6 due to the carbon–fluorine (C? F) dipole effect. This heterojunction device exhibits spectral photoresponses from ultraviolet (400 nm) to near infrared (950 nm). Furthermore, a prototype visible imager is demonstrated using the ZnO–WSe₂ heterojunction diode as an imaging pixel. To the best of our knowledge, this is the first demonstration of an optoelectronic device based on a 1D–2D hybrid vdW heterojunction. This approach using a 1D ZnO–2D WSe₂ heterojunction paves the way for the further development of electronic/optoelectronic applications using mixed‐dimensional vdW heterostructures.     

Atomically Thin MoS2: A Versatile Nongraphene 2D Material

Two‐dimensional inorganic materials are emerging as a premiere class of materials for fabricating modern electronic devices. The interest in 2D layered transition metal dichalcogenides is especially high. Particularly, 2D MoS₂ is being heavily researched due to its novel functionalities and its suitability for a wide range of electronic and optoelectronic applications. In this article, the progress in mono/few layer(s) MoS₂ research is reviewed by focusing primarily on the layer dependent evolution of crystal, phonon, and electronic structure. The review includes extensive detail into the methodologies adapted for single or few layer(s) MoS₂ growth. Further, the review covers the versatility of 2D MoS₂ for a broad range of device applications. Recent advancements in the field of van der Waals heterostructures are also highlighted at the end of the review.     

Progressive NO2 Sensors with Rapid Alarm and Persistent Memory-Type Responses for Wide-Range Sensing Using Antimony Triselenide Nanoflakes

Antimony triselenide (Sb₂Se₃) nanoflake-based nitrogen dioxide (NO₂) sensors exhibit a progressive bifunctional gas-sensing performance, with a rapid alarm for hazardous highly concentrated gases, and an advanced memory-type function for low-concentration (<1 ppm) monitoring repeated under potentially fatal exposure. Rectangular and cuboid shaped Sb₂Se₃ nanoflakes, comprising van der Waals planes with large surface areas and covalent bond planes with small areas, can rapidly detect a wide range of NO₂ gas concentrations from 0.1 to 100 ppm. These Sb₂Se₃ nanoflakes are found to be suitable for physisorption-based gas sensing owing to their anisotropic quasi-2D crystal structure with extremely enlarged van der Waals planes, where they are humidity-insensitive and consequently exhibit an extremely stable baseline current. The Sb₂Se₃ nanoflake sensor exhibits a room-temperature/low-voltage operation, which is noticeable owing to its low energy consumption and rapid response even under a NO₂ gas flow of only 1 ppm. As a result, the Sb₂Se₃ nanoflake sensor is suitable for the development of a rapid alarm system. Furthermore, the persistent gas-sensing conductivity of the sensor with a slow decaying current can enable the development of a progressive memory-type sensor that retains the previous signal under irregular gas injection at low concentrations.     

Al‐Doped Black Phosphorus p–n Homojunction Diode for High Performance Photovoltaic

2D layered materials based p–n junctions are fundamental building block for enabling new functional device applications with high efficiency. However, due to the lack of controllable doping technique, state‐of‐the‐art 2D p–n junctions are predominantly made of van der Waals heterostructures or electrostatic gated junctions. Here, the authors report the demonstration of a spatially controlled aluminum doping technique that enables a p–n homojunction diode to be realized within a single 2D black phosphorus nanosheet for high performance photovoltaic application. The diode achieves a near‐unity ideality factor of 1.001 along with an on/off ratio of ≈5.6 × 10³ at a low bias of 2 V, allowing for low‐power dynamic current rectification without signal decay or overshoot. When operated under a photovoltaic regime, the diode's dark current can be significantly suppressed. The presence of a built‐in electric field additionally gives rise to temporal short‐circuit current and open‐circuit voltage under zero external bias, indicative of its enriched functionalities for self‐powered photovoltaic and high signal‐to‐noise photodetection applications.     

Novel and Enhanced Optoelectronic Performances of Multilayer MoS2–WS2 Heterostructure Transistors

Van der Waals heterostructures designed by assembling isolated two‐dimensional (2D) crystals have emerged as a new class of artificial materials with interesting and unusual physical properties. Here, the multilayer MoS₂–WS₂ heterostructures with different configurations are reported and their optoelectronic properties are studied. It is shown that the new heterostructured material possesses new functionalities and superior electrical and optoelectronic properties that far exceed the one for their constituents, MoS₂ or WS₂. The vertical transistor exhibits a novel rectifying and bipolar behavior, and can also act as photovoltaic cell and self‐driven photodetector with photo‐switching ratio exceeding 10³. The planar device also exhibits high field‐effect ON/OFF ratio (>10⁵), high electron mobility of 65 cm²/Vs, and high photo­responsivity of 1.42 A/W compared to that in isolated multilayer MoS₂ or WS₂ nanoflake transistors. The results suggest that formation of MoS₂–WS₂ heterostructures could significantly enhance the performance of optoelectronic devices, thus open up possibilities for future nanoelectronic, photovoltaic, and optoelectronic applications.     

Anisotropic Growth of Nonlayered CdS on MoS2 Monolayer for Functional Vertical Heterostructures

2D semiconductors have emerged as a crucial material for use in next‐generation optotelectronics. Similar to microelectronic devices, 2D vertical heterostructures will most likely be the elemental components for future nanoscale electronics and optotelectronics. To date, the components of mostly reported 2D van der Waals heterostructures are restricted to layer crystals. In this work, it is demonstrated that nonlayered semiconductors of CdS can be epitaxially grown on to 2D layered MoS₂ substrate to form a new quasi vertical heterostructure with clean interface by chemical vapor deposition. Photodetectors based on this CdS/MoS₂ heterostructure show broader wavelength response and ≈50‐fold improvement in photoresponsivity, compared to the devices fabricated from MoS₂ monolayer only. This research opens up a way to fabricate a variety of functional quasi heterostructures from nonlayered semiconductors.     

AsP/InSe Van der Waals Tunneling Heterojunctions with Ultrahigh Reverse Rectification Ratio and High Photosensitivity

Van der Waals heterojunctions made of 2D materials offer competitive opportunities in designing and achieving multifunctional and high‐performance electronic and optoelectronic devices. However, due to the significant reverse tunneling current in such thin p–n junctions, a low rectification ratio along with a large reverse current is often inevitable for the heterojunctions. Here, a vertically stacked van der Waals heterojunction (vdWH) tunneling device is reported consisting of black arsenic phosphorus (AsP) and indium selenide (InSe), which shows a record high reverse rectification ratio exceeding 10⁷ along with an unusual ultralow forward current below picoampere and a high current on/off ratio over 10⁸ simultaneously at room temperature under the proper band alignment design of both the Schottky junction and the heterojunction. Therefore, the vdWH tunneling device can function as an ultrasensitive photodetector with an ultrahigh light on/off ratio of 1 × 10⁷, a comparable responsivity of around 1 A W^?1, and a high detectivity over 1 × 10¹² Jones in the visible wavelength range. Furthermore, the device exhibits a clear photovoltaic effect and shows a spectral detection capability up to 1550 nm. The work sheds light on developing future electronic and optoelectronic multifunctional devices based on the van der Waals integration of 2D materials with designed band alignment.     

High Surface Area MoS2/Graphene Hybrid Aerogel for Ultrasensitive NO2 Detection

A MoS₂/graphene hybrid aerogel synthesized with two‐dimensional MoS₂ sheets coating a high surface area graphene aerogel scaffold is characterized and used for ultrasensitive NO₂ detection. The combination of graphene and MoS₂ leads to improved sensing properties with the graphene scaffold providing high specific surface area and high electrical and thermal conductivity and the single to few‐layer MoS₂ sheets providing high sensitivity and selectivity to NO₂. The hybrid aerogel is integrated onto a low‐power microheater platform to probe the gas sensing performance. At room temperature, the sensor exhibits an ultralow detection limit of 50 ppb NO₂. By heating the material to 200 °C, the response and recovery times to reach 90% of the final signal decrease to <1 min, while retaining the low detection limit. The MoS₂/graphene hybrid also shows good selectivity for NO₂ against H₂ and CO, especially when compared to bare graphene aerogel. The unique structure of the hybrid aerogel is responsible for the ultrasensitive, selective, and fast NO₂ sensing. The improved sensing performance of this hybrid aerogel also suggests the possibility of other 2D material combinations for further sensing applications.     

High‐Sensitivity Floating‐Gate Phototransistors Based on WS2 and MoS2

In recent years, 2D layered materials have been considered as promising photon absorption channel media for next‐generation phototransistors due to their atomic thickness, easily tailored single‐crystal van der Waals heterostructures, ultrafast optoelectronic characteristics, and broadband photon absorption. However, the photosensitivity obtained from such devices, even under a large bias voltage, is still unsatisfactory until now. In this paper, high‐sensitivity phototransistors based on WS₂ and MoS₂ are proposed, designed, and fabricated with gold nanoparticles (AuNPs) embedded in the gate dielectric. These AuNPs, located between the tunneling and blocking dielectric, are found to enable efficient electron trapping in order to strongly suppress dark current. Ultralow dark current (10^?11 A), high photoresponsivity (1090 A W^?1), and high detectivity (3.5 × 10¹¹ Jones) are obtained for the WS₂ devices under a low source/drain and a zero gate voltage at a wavelength of 520 nm. These results demonstrate that the floating‐gate memory structure is an effective configuration to achieve high‐performance 2D electronic/optoelectronic devices.     

Ultrathin Non‐van der Waals Magnetic Rhombohedral Cr2S3: Space‐Confined Chemical Vapor Deposition Synthesis and Raman Scattering Investigation

Two dimensional (2D) magnetic materials display enormous application potential in spintronic fields. However, most of currently reported magnetic materials are van der Waals layered structure that is easy to be isolated via exfoliation method. By contrast, the studies on non‐van der Waals ultrathin magnetic materials are rare, largely due to the difficulty in fabrication. Rhombohedral Cr₂S₃, an intensively studied antiferromagnetic transition metal chalcogenide with Neel temperature of ≈120 K, has a typical non‐van der Waals structure. Restricted by the strong covalent bonding in all the three dimensions of non‐van der Waals structure, the synthesis of ultrathin Cr₂S₃ single crystals is still a challenge that is not achieved yet. Besides, the study on the Raman modes of rhombohedral Cr₂S₃ is also absent. Herein, by employing space‐confined chemical vapor deposition strategy, ultrathin rhombohedral Cr₂S₃ single crystals with a thickness down to ≈2.5 nm for the first time are successfully grown. Moreover, a systematically investigation is also conducted on the Raman vibrations of ultrathin rhombohedral Cr₂S₃. With the aid of angle‐resolved polarized Raman technique, the Raman modes of rhombohedral Cr₂S₃ for the first time based on crystal symmetry and Raman selection rules are rationally assigned.     

Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors

The high‐bias electrical characteristics of back‐gated field‐effect transistors with chemical vapor deposition synthesized bilayer MoS₂ channel and Ti Schottky contacts are discussed. It is found that oxidized Ti contacts on MoS₂ form rectifying junctions with ≈0.3 to 0.5 eV Schottky barrier height. To explain the rectifying output characteristics of the transistors, a model is proposed based on two slightly asymmetric back‐to‐back Schottky barriers, where the highest current arises from image force barrier lowering at the electrically forced junction, while the reverse current is due to Schottky‐barrier‐limited injection at the grounded junction. The device achieves a photoresponsivity greater than 2.5 A W^?1 under 5 mW cm^?2 white‐LED light. By comparing two‐ and four‐probe measurements, it is demonstrated that the hysteresis and persistent photoconductivity exhibited by the transistor are peculiarities of the MoS₂ channel rather than effects of the Ti/MoS₂ interface.