Lateral Monolayer MoSe2–WSe2 p–n Heterojunctions with Giant Built‐In Potentials

2D transition metal dichalcogenides (TMDs) have exhibited strong application potentials in new emerging electronics because of their atomic thin structure and excellent flexibility, which is out of field of tradition silicon technology. Similar to 3D p–n junctions, 2D p–n heterojunctions by laterally connecting TMDs with different majority charge carriers (electrons and holes), provide ideal platform for current rectifiers, light‐emitting diodes, diode lasers and photovoltaic devices. Here, growth and electrical studies of atomic thin high‐quality p–n heterojunctions between molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) by one‐step chemical vapor deposition method are reported. These p–n heterojunctions exhibit high built‐in potential (≈0.7 eV), resulting in large current rectification ratio without any gate control for diodes, and fast response time (≈6 ms) for self‐powered photodetectors. The simple one‐step growth and electrical studies of monolayer lateral heterojunctions open up the possibility to use TMD heterojunctions for functional devices.     

Tunable Polarity Behavior and Self‐Driven Photoswitching in p‐WSe2/n‐WS2 Heterojunctions

Van der Waals (vdW) p–n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next‐generation of electronics and optoelectronics devices. Here, it is reported that the WSe₂/WS₂ p–n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n‐“anti‐bipolar”–p‐ambipolar is observed in the WSe₂/WS₂ heterojunctions owing to the successive work of conducting channels of junctions, p‐WSe₂ and n‐WS₂ on the electrical transport of the whole systems. The type‐II band alignment obtained from first principle calculations and built‐in potential in this vdW heterojunction can also facilitate the efficient electron–hole separation, thus enabling the significant photovoltaic effect and a much enhanced self‐driven photoswitching response in this system.     

Multifunctional van der Waals Broken‐Gap Heterojunction

The finite energy band‐offset that appears between band structures of employed materials in a broken‐gap heterojunction exhibits several interesting phenomena. Here, by employing a black phosphorus (BP)/rhenium disulfide (ReS₂) heterojunction, the tunability of the BP work function (Φ _BP) with variation in flake thickness is exploited in order to demonstrate that a BP‐based broken‐gap heterojunction can manifest diverse current‐transport characteristics such as gate tunable rectifying p–n junction diodes, Esaki diodes, backward‐rectifying diodes, and nonrectifying devices as a consequence of diverse band‐bending at the heterojunction. Diversity in band‐bending near heterojunction is attributed to change in the Fermi level difference (Δ) between BP and ReS₂ sides as a consequence of Φ _BP modulation. No change in the current transport characteristics in several devices with fixed Δ also provides further evidence that current‐transport is substantially impacted by band‐bending at the heterojunction. Optoelectronic experiments on the Esaki diode and the p–n junction diode provide experimental evidence of band‐bending diversity. Additionally, the p⁺–n–p junction comprising BP (38 nm)/ReS₂/BP(5.8 nm) demonstrates multifunctionality of binary and ternary inverters as well as exhibiting the behavior of a bipolar junction transistor with common‐emitter current gain up to 50.     

Direct Synthesis of a Self‐Assembled WSe2/MoS2 Heterostructure Array and its Optoelectrical Properties

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next‐generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self‐assembled WSe₂/MoS₂ heterostructures through facile solution‐based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe₂ wires are selectively stacked over the MoS₂ wires at a specific angle, which enables the formation of parallel‐ and cross‐aligned heterostructures. The realized WSe₂/MoS₂‐based p–n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W^?1 and response speed of 16 µs. As a feasible application, a WSe₂/MoS₂‐based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution‐based growth of hierarchical structures with various alignments could offer a method for the further development of large‐area electronic and optoelectronic applications.     

Carbon‐Nanotube‐Confined Vertical Heterostructures with Asymmetric Contacts

Van der Waals (vdW) heterostructures have received intense attention for their efficient stacking methodology with 2D nanomaterials in vertical dimension. However, it is still a challenge to scale down the lateral size of vdW heterostructures to the nanometer and make proper contacts to achieve optimized performances. Here, a carbon‐nanotube‐confined vertical heterostructure (CCVH) is employed to address this challenge, in which 2D semiconductors are asymmetrically sandwiched by an individual metallic single‐walled carbon nanotube (SWCNT) and a metal electrode. By using WSe₂ and MoS₂, the CCVH can be made into p‐type and n‐type field effect transistors with high on/off ratios even when the channel length is 3.3 nm. A complementary inverter was further built with them, indicating their potential in logic circuits with a high integration level. Furthermore, the Fermi level of SWCNTs can be efficiently modulated by the gate voltage, making it competent for both electron and hole injection in the CCVHs. This unique property is shown by the transition of WSe₂ CCVH from unipolar to bipolar, and the transition of WSe₂/MoS₂ from p–n junction to n–n junction under proper source–drain biases and gate voltages. Therefore, the CCVH, as a member of 1D/2D mixed heterostructures, shows great potentials in future nanoelectronics and nano‐optoelectronics.     

Synthesis of 2D Layered BiI3 Nanoplates,BiI3/WSe2van der Waals Heterostructures and Their Electronic,Optoelectronic Properties

Two–dimensional layered materials (2DLMs) have attracted considerable recent interest as a new material platform for fundamental materials science and potential new technologies. Here we report the growth of layered metal halide materials and their optoelectronic properties. BiI₃ nanoplates can be readily grown on SiO₂/Si substrates with a hexagonal geometry, with a thickness in the range of 10–120 nm and a lateral dimension of 3–10 µm. Transmission electron microscopy and electron diffraction studies demonstrate that the individual nanoplates are high quality single crystals. Micro‐Raman studies show characteristic A _g band at ≈115 cm^?1 with slight red‐shift with decreasing thickness, and micro‐photoluminescence studies show uniform emission around 690 nm with blue‐shift with decreasing thickness. Electrical transport studies of individual nanoplates show n‐type semiconductor characteristics with clear photoresponse. Further, the BiI₃ can be readily grown on other 2DLMs (e.g., WSe₂) to form van der Waals heterostructures. Electrical transport measurements of BiI₃/WSe₂ vertical heterojunctions demonstrate p–n diode characteristics with gate‐tunable rectification behavior and distinct photovoltaic effect. The synthesis of the BiI₃ nanoplates can expand the library of 2DLMs and enable a wider range of van der Waals heterostructures.     

WSe2 Photovoltaic Device Based on Intramolecular p–n Junction

High quality p–n junctions based on 2D layered materials (2DLMs) are urgent to exploit, because of their unique properties such as flexibility, high absorption, and high tunability which may be utilized in next‐generation photovoltaic devices. Based on transfer technology, large amounts of vertical heterojunctions based on 2DLMs are investigated. However, the complicated fabrication process and the inevitable defects at the interfaces greatly limit their application prospects. Here, an in‐plane intramolecular WSe₂ p–n junction is realized, in which the n‐type region and p‐type region are chemically doped by polyethyleneimine and electrically doped by the back‐gate, respectively. An ideal factor of 1.66 is achieved, proving the high quality of the p–n junction realized by this method. As a photovoltaic detector, the device possesses a responsivity of 80 mA W^?1 (≈20% external quantum efficiency), a specific detectivity of over 10¹¹ Jones and fast response features (200 µs rising time and 16 µs falling time) at zero bias, simultaneously. Moreover, a large open‐circuit voltage of 0.38 V and an external power conversion efficiency of ≈1.4% realized by the device also promises its potential in microcell applications.     

Reconfigurable Diodes Based on Vertical WSe2 Transistors with van der Waals Bonded Contacts

New device concepts can increase the functionality of scaled electronic devices, with reconfigurable diodes allowing the design of more compact logic gates being one of the examples. In recent years, there has been significant interest in creating reconfigurable diodes based on ultrathin transition metal dichalcogenide crystals due to their unique combination of gate‐tunable charge carriers, high mobility, and sizeable band gap. Thanks to their large surface areas, these devices are constructed under planar geometry and the device characteristics are controlled by electrostatic gating through rather complex two independent local gates or ionic‐liquid gating. In this work, similar reconfigurable diode action is demonstrated in a WSe₂ transistor by only utilizing van der Waals bonded graphene and Co/h‐BN contacts. Toward this, first the charge injection efficiencies into WSe₂ by graphene and Co/h‐BN contacts are characterized. While Co/h‐BN contact results in nearly Schottky‐barrier‐free charge injection, graphene/WSe₂ interface has an average barrier height of ≈80 meV. By taking the advantage of the electrostatic transparency of graphene and the different work‐function values of graphene and Co/h‐BN, vertical devices are constructed where different gate‐tunable diode actions are demonstrated. This architecture reveals the opportunities for exploring new device concepts.     

Single Atomically Sharp Lateral Monolayer p‐n Heterojunction Solar Cells with Extraordinarily High Power Conversion Efficiency

The recent development of 2D monolayer lateral semiconductor has created new paradigm to develop p‐n heterojunctions. Albeit, the growth methods of these heterostructures typically result in alloy structures at the interface, limiting the development for high‐efficiency photovoltaic (PV) devices. Here, the PV properties of sequentially grown alloy‐free 2D monolayer WSe₂‐MoS₂ lateral p‐n heterojunction are explores. The PV devices show an extraordinary power conversion efficiency of 2.56% under AM 1.5G illumination. The large surface active area enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic with only 5% reduction of efficiency at incident angles up to 75°. Modeling studies demonstrate the PV devices comply with typical principles, increasing the feasibility for further development. Furthermore, the appropriate electrode‐spacing design can lead to environment‐independent PV properties. These robust PV properties deriving from the atomically sharp lateral p‐n interface can help develop the next‐generation photovoltaics.     

Metal‐Guided Selective Growth of 2D Materials: Demonstration of a Bottom‐Up CMOS Inverter

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large‐area electronics and circuits strongly relies on wafer‐scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal‐guided selective growth (MGSG), is reported. The success of control over the transition‐metal‐precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p‐ and n‐type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom‐up complementary metal‐oxide‐semiconductor inverter based on p‐type WSe₂ and n‐type MoSe₂ is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.     

Tunable Tribotronic Dual‐Gate Logic Devices Based on 2D MoS2 and Black Phosphorus

With the Moore's law hitting the bottleneck of scaling‐down in size (below 10 nm), personalized and multifunctional electronics with an integration of 2D materials and self‐powering technology emerge as a new direction of scientific research. Here, a tunable tribotronic dual‐gate logic device based on a MoS₂ field‐effect transistor (FET), a black phosphorus FET and a sliding mode triboelectric nanogenerator (TENG) is reported. The triboelectric potential produced from the TENG can efficiently drive the transistors and logic devices without applying gate voltages. High performance tribotronic transistors are achieved with on/off ratio exceeding 106 and cutoff current below 1 pA μm^–1. Tunable electrical behaviors of the logic device are also realized, including tunable gains (improved to ≈13.8) and power consumptions (≈1 nW). This work offers an active, low‐power‐consuming, and universal approach to modulate semiconductor devices and logic circuits based on 2D materials with TENG, which can be used in microelectromechanical systems, human–machine interfacing, data processing and transmission.     

Black‐Phosphorus‐Based Orientation‐Induced Diodes

Despite many decades of research of diodes, which are fundamental components of electronic and photoelectronic devices with p–n or Schottky junctions using bulk or 2D materials, stereotyped architectures and complex technological processing (doping and multiple material operations) have limited future development. Here, a novel rectification device, an orientation‐induced diode, assembled using only few‐layered black phosphorus (BP) is investigated. The key to its realization is to utilize the remarkable anisotropy of BP in low dimensions and change the charge‐transport conditions abruptly along the different crystal orientations. Rectification ratios of 6.8, 22, and 115 can be achieved in cruciform BP, cross‐stacked BP junctions, and BP junctions stacked with vertical orientations, respectively. The underlying physical processes and mechanisms can be explained using “orientation barrier” band theory. The theoretical results are experimentally confirmed using localized scanning photocurrent imaging. These orientation‐induced optoelectronic devices open possibilities for 2D anisotropic materials with a new degree of freedom to improve modulation in diodes.     

Mobility Engineering in Vertical Field Effect Transistors Based on Van der Waals Heterostructures

Vertical integration of 2D layered materials to form van der Waals heterostructures (vdWHs) offers new functional electronic and optoelectronic devices. However, the mobility in vertical carrier transport in vdWHs of vertical field‐effect transistor (VFET) is not yet investigated in spite of the importance of mobility for the successful application of VFETs in integrated circuits. Here, the mobility in VFET of vdWHs under different drain biases, gate biases, and metal work functions is first investigated and engineered. The traps in WSe₂ are the main source of scattering, which influences the vertical mobility and three distinct transport mechanisms: Ohmic transport, trap‐limited transport, and space‐charge‐limited transport. The vertical mobility in VFET can be improved by suppressing the trap states by raising the Fermi level of WSe₂. This is achieved by increasing the injected carrier density by applying a high drain voltage, or decreasing the Schottky barrier at the graphene/WSe₂ and metal/WSe₂ junctions by applying a gate bias and reducing the metal work function, respectively. Consequently, the mobility in Mn vdWH at +50 V gate voltage is about 76 times higher than the initial mobility of Au vdWH. This work enables further improvements in the VFET for successful application in integrated circuits.     

Lateral 2D WSe2 p–n Homojunction Formed by Efficient Charge-Carrier-Type Modulation for High-Performance Optoelectronics

As unique building blocks for next-generation optoelectronics, high-quality 2D p–n junctions based on semiconducting transition metal dichalcogenides (TMDs) have attracted wide interest, which are urgent to be exploited. Herein, a novel and facile electron doping of WSe₂ by cetyltrimethyl ammonium bromide (CTAB) is achieved for the first time to form a high-quality intramolecular p–n junction with superior optoelectronic properties. Efficient manipulation of charge carrier type and density in TMDs via electron transfer between Br⁻ in CTAB and TMDs is proposed theoretically by density functional theory (DFT) calculations. Compared with the intrinsic WSe₂ photodetector, the switching light ratio (I_light/I_dark) of the p–n junction device can be enhanced by 10³, and the temporal response is also dramatically improved. The device possesses a responsivity of 30 A W⁻¹, with a specific detectivity of over 10¹¹ Jones. In addition, the mechanism of charge transfer in CTAB-doped 2D WSe₂ and WS₂ are investigated by designing high-performance field effect transistors. Besides the scientific insight into the effective manipulation of 2D materials by chemical doping, this work presents a promising applicable approach toward next-generation photoelectronic devices with high efficiency.     

Suppression of Persistent Photoconductivity of Rubrene Crystals using Gate‐Tunable Rubrene/Bi2Se3 Diodes with Photoinduced Negative Differential Resistance

Organic single‐crystalline semiconductors show great potential in high‐performance photodetectors. However, they suffer from persistent photoconductivity (PPC) due to the charge trapping, which has severely hindered high‐speed imaging applications. Here, a universal strategy of solving the PPC by integrating with topological insulator Bi₂Se₃ is provided. The rubrene/Bi₂Se₃ heterojunctions are selected as an example for general demonstration due to the reproducibly high mobility and broad optoelectronic applications of rubrene crystals. By virtue of high carrier concentration on Bi₂Se₃ surface and the strong built‐in electrical field, the photoresponse of the heterotransistor is significantly reduced for more than two orders (from over 10 s to 54 ms), meanwhile the photoresponsivity can reach 124 A W^?1. To the best of knowledge, this operating speed is among the fastest responses in organic–inorganic heterojunctions. The heterotransistor also shows unique negative differential resistance under positive gate bias, which can be explained by photoinduced de‐trapping of electron trap states in the bulk rubrene crystals. Besides, the rubrene/Bi₂Se₃ heterojunction behaves as a gate‐tunable backward‐like diode due to the inhomogenous carrier distribution in the thick rubrene crystal and inversion of relative Fermi level positions. The findings demonstrate versatile functionalities of the rubrene/Bi₂Se₃ heterojunctions for various emerging optoelectronic applications.     

Gate‐Tunable Graphene–WSe2 Heterojunctions at the Schottky–Mott Limit

Metal–semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. Here, a graphene–WSe₂ p‐type Schottky junction, which exhibits a lack of Fermi level pinning, is studied. The Schottky junction displays near‐ideal diode characteristics with large gate tunability and small leakage currents. Using a gate electrostatically coupled to the WSe₂ channel to tune the Schottky barrier height, the Schottky–Mott limit is probed in a single device. As a special manifestation of the tunable Schottky barrier, a diode with a dynamically controlled ideality factor is demonstrated.     

Nanoparticle‐Enhanced Silver‐Nanowire Plasmonic Electrodes for High‐Performance Organic Optoelectronic Devices

Improved performance in plasmonic organic solar cells (OSCs) and organic light‐emitting diodes (OLEDs) via strong plasmon‐coupling effects generated by aligned silver nanowire (AgNW) transparent electrodes decorated with core–shell silver–silica nanoparticles (Ag@SiO₂NPs) is demonstrated. NP‐enhanced plasmonic AgNW (Ag@SiO₂NP–AgNW) electrodes enable substantially enhanced radiative emission and light absorption efficiency due to strong hybridized plasmon coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) modes, which leads to improved device performance in organic optoelectronic devices (OODs). The discrete dipole approximation (DDA) calculation of the electric field verifies a strongly enhanced plasmon‐coupling effect caused by decorating core–shell Ag@SiO₂NPs onto the AgNWs. Notably, an electroluminescence efficiency of 25.33 cd A^?1 (at 3.2 V) and a power efficiency of 25.14 lm W^?1 (3.0 V) in OLEDs, as well as a power conversion efficiency (PCE) value of 9.19% in OSCs are achieved using hybrid Ag@SiO₂NP–AgNW films. These are the highest values reported to date for optoelectronic devices based on AgNW electrodes. This work provides a new design platform to fabricate high‐performance OODs, which can be further explored in various plasmonic and optoelectronic devices.     

Tunable SnSe2/WSe2 Heterostructure Tunneling Field Effect Transistor

The burgeoning 2D semiconductors can maintain excellent device electrostatics with an ultranarrow channel length and can realize tunneling by electrostatic gating to avoid deprivation of band‐edge sharpness resulting from chemical doping, which make them perfect candidates for tunneling field effect transistors. Here this study presents SnSe₂/WSe₂ van der Waals heterostructures with SnSe₂ as the p‐layer and WSe₂ as the n‐layer. The energy band alignment changes from a staggered gap band offset (type‐II) to a broken gap (type‐III) when changing the negative back‐gate voltage to positive, resulting in the device operating as a rectifier diode (rectification ratio ~10⁴) or an n‐type tunneling field effect transistor, respectively. A steep average subthreshold swing of 80 mV dec^?1 for exceeding two decades of drain current with a minimum of 37 mV dec^?1 at room temperature is observed, and an evident trend toward negative differential resistance is also accomplished for the tunneling field effect transistor due to the high gate efficiency of 0.36 for single gate devices. The I _ON/I _OFF ratio of the transfer characteristics is >10⁶, accompanying a high ON current >10^?5 A. This work presents original phenomena of multilayer 2D van der Waals heterostructures which can be applied to low‐power consumption devices.     

WSe2 Homojunction Devices: Electrostatically Configurable as Diodes,MOSFETs, and Tunnel FETs for Reconfigurable Computing

In this paper, electrostatically configurable 2D tungsten diselenide (WSe₂) electronic devices are demonstrated. Utilizing a novel triple‐gate design, a WSe₂ device is able to operate as a tunneling field‐effect transistor (TFET), a metal–oxide–semiconductor field‐effect transistor (MOSFET) as well as a diode, by electrostatically tuning the channel doping to the desired profile. The implementation of scaled gate dielectric and gate electrode spacing enables higher band‐to‐band tunneling transmission with the best observed subthreshold swing (SS) among all reported homojunction TFETs on 2D materials. Self‐consistent full‐band atomistic quantum transport simulations quantitatively agree with electrical measurements of both the MOSFET and TFET and suggest that scaling gate oxide below 3 nm is necessary to achieve sub‐60 mV dec^?1 SS, while further improvement can be obtained by optimizing the spacers. Diode operation is also demonstrated with the best ideality factor of 1.5, owing to the enhanced electrostatic control compared to previous reports. This research sheds light on the potential of utilizing electrostatic doping scheme for low‐power electronics and opens a path toward novel designs of field programmable mixed analog/digital circuitry for reconfigurable computing.     

Enhanced Stability and Tunable Photoluminescence in Perovskite CsPbX3/ZnS Quantum Dot Heterostructure

All‐inorganic perovskite CsPbX₃ (X = Cl, Br, I) and related materials are promising candidates for potential solar cells, light emitting diodes, and photodetectors. Here, a novel architecture made of CsPbX₃/ZnS quantum dot heterodimers synthesized via a facile solution‐phase process is reported. Microscopic measurements show that CsPbX₃/ZnS heterodimer has high crystalline quality with enhanced chemical stability, as also evidenced by systematic density functional theory based first‐principles calculations. Remarkably, depending on the interface structure, ZnS induces either n‐type or p‐type doping in CsPbX₃ and both type‐I and type‐II heterojunctions can be achieved, leading to rich electronic properties. Photoluminescence measurement results show a strong blue‐shift and decrease of recombination lifetime with increasing sulfurization, which is beneficial for charge diffusion in solar cells and photovoltaic applications. These findings are expected to shed light on further understanding and design of novel perovskite heterostructures for stable, tunable optoelectronic devices.