Gate‐Controlled BP–WSe2 Heterojunction Diode for Logic Rectifiers and Logic Optoelectronics

p–n junctions play an important role in modern semiconductor electronics and optoelectronics, and field‐effect transistors are often used for logic circuits. Here, gate‐controlled logic rectifiers and logic optoelectronic devices based on stacked black phosphorus (BP) and tungsten diselenide (WSe₂) heterojunctions are reported. The gate‐tunable ambipolar charge carriers in BP and WSe₂ enable a flexible, dynamic, and wide modulation on the heterojunctions as isotype (p–p and n–n) and anisotype (p–n) diodes, which exhibit disparate rectifying and photovoltaic properties. Based on such characteristics, it is demonstrated that BP–WSe₂ heterojunction diodes can be developed for high‐performance logic rectifiers and logic optoelectronic devices. Logic optoelectronic devices can convert a light signal to an electric one by applied gate voltages. This work should be helpful to expand the applications of 2D crystals.     

Ultrahigh‐Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2/Graphene/SnS2 p–g–n Junctions

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next‐generation optoelectronics since they can be stacked layer‐by‐layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice‐mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h‐BN/p‐MoTe₂/graphene/n‐SnS₂/h‐BN p–g–n junction, fabricated by a layer‐by‐layer dry transfer, demonstrates high‐sensitivity, broadband photodetection at room temperature. The combination of the MoTe₂ and SnS₂ of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built‐in electric field for high‐efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5?7‐layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W^?1 with fast photoresponse and specific detectivity up to ≈10¹³ Jones in the ultraviolet–visible–near‐infrared spectrum. This result suggests that the vdW p–g–n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh‐sensitivity and broadband photonic detectors.     

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.     

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.     

Homogeneous 2D MoTe2 CMOS Inverters and p–n Junctions Formed by Laser‐Irradiation‐Induced p‐Type Doping

Among all typical transition‐metal dichalcogenides (TMDs), the bandgap of α‐MoTe₂ is smallest and is close to that of conventional 3D Si. The properties of α‐MoTe₂ make it a favorable candidate for future electronic devices. Even though there are a few reports regarding fabrication of complementary metal–oxide‐semiconductor (CMOS) inverters or p–n junction by controlling the charge‐carrier polarity of TMDs, the fabrication process is complicated. Here, a straightforward selective doping technique is demonstrated to fabricate a 2D p–n junction diode and CMOS inverter on a single α‐MoTe₂ nanoflake. The n‐doped channel of a single α‐MoTe₂ nanoflake is selectively converted to a p‐doped region via laser‐irradiation‐induced MoO_x doping. The homogeneous 2D MoTe₂ CMOS inverter has a high DC voltage gain of 28, desirable noise margin (NM_H = 0.52 V_DD, NM_L = 0.40 V_DD), and an AC gain of 4 at 10 kHz. The results show that the doping technique by laser scan can be potentially used for future larger‐scale MoTe₂ CMOS circuits.     

Homogeneous 2D MoTe2 p–n Junctions and CMOS Inverters formed by Atomic‐Layer‐Deposition‐Induced Doping

Recently, α‐MoTe₂, a 2D transition‐metal dichalcogenide (TMD), has shown outstanding properties, aiming at future electronic devices. Such TMD structures without surface dangling bonds make the 2D α‐MoTe₂ a more favorable candidate than conventional 3D Si on the scale of a few nanometers. The bandgap of thin α‐MoTe₂ appears close to that of Si and is quite smaller than those of other typical TMD semiconductors. Even though there have been a few attempts to control the charge‐carrier polarity of MoTe₂, functional devices such as p–n junction or complementary metal–oxide–semiconductor (CMOS) inverters have not been reported. Here, we demonstrate a 2D CMOS inverter and p–n junction diode in a single α‐MoTe₂ nanosheet by a straightforward selective doping technique. In a single α‐MoTe₂ flake, an initially p‐doped channel is selectively converted to an n‐doped region with high electron mobility of 18 cm² V^?1 s^?1 by atomic‐layer‐deposition‐induced H‐doping. The ultrathin CMOS inverter exhibits a high DC voltage gain of 29, an AC gain of 18 at 1 kHz, and a low static power consumption of a few nanowatts. The results show a great potential of α‐MoTe₂ for future electronic devices based on 2D semiconducting materials.     

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.     

Edge‐Epitaxial Growth of 2D NbS2‐WS2 Lateral Metal‐Semiconductor Heterostructures

2D metal‐semiconductor heterostructures based on transition metal dichalcogenides (TMDs) are considered as intriguing building blocks for various fields, such as contact engineering and high‐frequency devices. Although, a series of p–n junctions utilizing semiconducting TMDs have been constructed hitherto, the realization of such a scheme using 2D metallic analogs has not been reported. Here, the synthesis of uniform monolayer metallic NbS₂ on sapphire substrate with domain size reaching to a millimeter scale via a facile chemical vapor deposition (CVD) route is demonstrated. More importantly, the epitaxial growth of NbS₂‐WS₂ lateral metal‐semiconductor heterostructures via a “two‐step” CVD method is realized. Both the lateral and vertical NbS₂‐WS₂ heterostructures are achieved here. Transmission electron microscopy studies reveal a clear chemical modulation with distinct interfaces. Raman and photoluminescence maps confirm the precisely controlled spatial modulation of the as‐grown NbS₂‐WS₂ heterostructures. The existence of the NbS₂‐WS₂ heterostructures is further manifested by electrical transport measurements. This work broadens the horizon of the in situ synthesis of TMD‐based heterostructures and enlightens the possibility of applications based on 2D metal‐semiconductor heterostructures.     

Recent Developments in Controlled Vapor‐Phase Growth of 2D Group 6 Transition Metal Dichalcogenides

An overview of recent developments in controlled vapor‐phase growth of 2D transition metal dichalcogenide (2D TMD) films is presented. Investigations of thin‐film formation mechanisms and strategies for realizing 2D TMD films with less‐defective large domains are of central importance because single‐crystal‐like 2D TMDs exhibit the most beneficial electronic and optoelectronic properties. The focus is on the role of the various growth parameters, including strategies for efficiently delivering the precursors, the selection and preparation of the substrate surface as a growth assistant, and the introduction of growth promoters (e.g., organic molecules and alkali metal halides) to facilitate the layered growth of (Mo, W)(S, Se, Te)₂ atomic crystals on inert substrates. Critical factors governing the thermodynamic and kinetic factors related to chemical reaction pathways and the growth mechanism are reviewed. With modification of classical nucleation theory, strategies for designing and growing various vertical/lateral TMD‐based heterostructures are discussed. Then, several pioneering techniques for facile observation of structural defects in TMDs, which substantially degrade the properties of macroscale TMDs, are introduced. Technical challenges to be overcome and future research directions in the vapor‐phase growth of 2D TMDs for heterojunction devices are discussed in light of recent advances in the field.     

Thermodynamically Stable Synthesis of Large‐Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n–n Heterojunction Devices

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large‐area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large‐scale and highly crystalline molybdenum disulfide monolayers using a solution‐processed precursor deposition. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean monolayer films and large crystal sizes up to 500 µm. As a result, the photoluminescence exhibits only a small full‐width‐half‐maximum of 48 meV, comparable to that of exfoliated and suspended monolayer crystals. It is confirmed that this growth procedure can be extended to the synthesis of other TMDC monolayers, and robust MoS₂/WS₂ heterojunction devices are easily prepared using this synthetic procedure due to the large‐sized crystals. The heterojunction device shows a fast response time (≈45 ms) and a significantly high photoresponsivity (≈40 AW^?1) because of the built‐in potential and the majority‐carrier transport at the n–n junction. These findings indicate an efficient pathway for the fabrication of high‐performance 2D optoelectronic devices.     

2D Ruddlesden–Popper Perovskites Microring Laser Array

3D organic–inorganic hybrid perovskites have featured high gain coefficients through the electron–hole plasma stimulated emission mechanism, while their 2D counterparts of Ruddlesden–Popper perovskites (RPPs) exhibit strongly bound electron–hole pairs (excitons) at room temperature. High‐performance solar cells and light‐emitting diodes (LEDs) are reported based on 2D RPPs, whereas light‐amplification devices remain largely unexplored. Here, it is demonstrated that ultrafast energy transfer along cascade quantum well (QW) structures in 2D RPPs concentrates photogenerated carriers on the lowest‐bandgap QW state, at which population inversion can be readily established enabling room‐temperature amplified spontaneous emission and lasing. Gain coefficients measured for 2D RPP thin‐films (≈100 nm in thickness) are found about at least four times larger than those for their 3D counterparts. High‐density large‐area microring arrays of 2D RPPs are fabricated as whispering‐gallery‐mode lasers, which exhibit high quality factor (Q ≈ 2600), identical optical modes, and similarly low lasing thresholds, allowing them to be ignited simultaneously as a laser array. The findings reveal that 2D RPPs are excellent solution‐processed gain materials potentially for achieving electrically driven lasers and ideally for on‐chip integration of nanophotonics.     

Spatial Mapping of Hot‐Spots at Lateral Heterogeneities in Monolayer Transition Metal Dichalcogenides

Lateral heterogeneities in atomically thin 2D materials such as in‐plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano‐ and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot‐spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS₂ and WS₂ do not largely impact the distribution of heat dissipation, while GBs of MoS₂ appreciably localize heating in the device. High‐resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure–property‐processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all‐2D electronic circuitry with enhanced functionalities, lifetime, and performance.     

Hybridizing Plasmonic Materials with 2D‐Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light–matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross‐section of 2D‐TMDs inhibits the light–matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D‐TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D‐TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D‐TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon‐enhanced interaction in 2D‐TMDs is briefly described and state‐of‐the‐art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.     

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p–n Diodes

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p–n HJ of GeSe/MoS₂ is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>10⁴) through the potential barrier in the vertical‐direction tunneling of HJs is observed. The negative differential transconductance with high peak‐to‐valley ratio (>10⁵) due to the series resistance change of GeSe, MoS₂, and HJs at different gate voltages is observed. Moreover, strong and broad‐band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS₂ HJs are expected to be important for understanding the carrier transport and photoresponse of 2D‐material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.     

Large‐Scale Growth and Field‐Effect Transistors Electrical Engineering of Atomic‐Layer SnS2

2D layers of metal dichalcogenides are of considerable interest for high‐performance electronic devices for their unique electronic properties and atomically thin geometry. 2D SnS₂ nanosheets with a bandgap of ≈2.6 eV have been attracting intensive attention as one potential candidate for modern electrocatalysis, electronic, and/or optoelectronic fields. However, the controllable growth of large‐size and high‐quality SnS₂ atomic layers still remains a challenge. Herein, a salt‐assisted chemical vapor deposition method is provided to synthesize atomic‐layer SnS₂ with a large crystal size up to 410 µm and good uniformity. Particularly, the as‐fabricated SnS₂ nanosheet‐based field‐effect transistors (FETs) show high mobility (2.58 cm² V^?1 s^?1) and high on/off ratio (≈10⁸), which is superior to other reported SnS₂‐based FETs. Additionally, the effects of temperature on the electrical properties are systematically investigated. It is shown that the scattering mechanism transforms from charged impurities scattering to electron–phonon scattering with the temperature. Moreover, SnS₂ can serve as an ideal material for energy storage and catalyst support. The high performance together with controllable growth of SnS₂ endow it with great potential for future applications in electrocatalysis, electronics, and optoelectronics.     

High‐Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure

Van der Waals heterostructures based on 2D layered materials have received wide attention for their multiple applications in optoelectronic devices, such as solar cells, light‐emitting devices, and photodiodes. In this work, high‐performance photovoltaic photodetectors based on MoTe₂/MoS₂ vertical heterojunctions are demonstrated by exfoliating‐restacking approach. The fundamental electric properties and band structures of the junction are revealed and analyzed. It is shown that this kind of photodetectors can operate under zero bias with high on/off ratio (>10⁵) and ultralow dark current (≈3 pA). Moreover, a fast response time of 60 µs and high photoresponsivity of 46 mA W^?1 are also attained at room temperature. The junctions based on 2D materials are expected to constitute the ultimate functional elements of nanoscale electronic and optoelectronic applications.     

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.     

Highly Oriented Thin Films of 2D Ruddlesden‐Popper Hybrid Perovskite toward Superfast Response Photodetectors

2D hybrid perovskites have shown great promise in the photodetection field, due to their intriguing attributes stemming from unique structural architectures. However, the great majority of detectors based on this 2D system possess a relatively low response speed (≈ms), making it extremely urgent to develop new candidates for superfast photodetection. Here, a new organic–inorganic hybrid perovskite, (PA)₂(FA)Pb₂I₇ (EFA, where PA is n‐pentylaminium and FA is formamidine), which features the 2D Ruddlesden–Popper type perovskite framework that is composed of the corner‐sharing PbI₆ octahedra is reported. Significantly, photodetectors fabricated on highly oriented thin films, which exhibit a perfect orientation parallel to 2D inorganic perovskite layers, exhibit a superfast response time up to ≈2.54 ns. To the best of the knowledge, this figure‐of‐merit catches up with that of the top‐ranking commercial materials, and sets a new record for 2D hybrid perovskite photodetectors. Moreover, extremely high photodetectivity (≈1.73 × 10¹⁴ Jones, under an incident power intensity of ≈46 µW cm^?2), considerable switching ratios (>10³), and low dark current (≈10 pA) are also achieved in the detector, indicating its great potential for high‐efficiency photodetection. These results shed light on the possibilities to explore new 2D candidates for assembling future high‐performance optoelectronic devices.     

Synthesis of In‐Plane Artificial Lattices of Monolayer Multijunctions

Recently, monolayers of van der Waals materials, including transition metal dichalcogenides (TMDs), are considered ideal building blocks for constructing 2D artificial lattices and heterostructures. Heterostructures with multijunctions of more than two monolayer TMDs are intriguing for exploring new physics and materials properties. Obtaining in‐plane heterojunctions of monolayer TMDs with atomically sharp interfaces is very significant for fundamental research and applications. Currently, multistep synthesis for more than two monolayer TMDs remains a challenge because decomposition or compositional alloying is thermodynamically favored at the high growth temperature. Here, a multistep chemical vapor deposition (CVD) synthesis of the in‐plane multijunctions of monolayer TMDs is presented. A low growth temperature synthesis is developed to avoid compositional fluctuations of as‐grown TMDs, defects formations, and interfacial alloying for high heterointerface quality and thermal stability of monolayer TMDs. With optimized parameters, atomically sharp interfaces are successfully achieved in the synthesis of in‐plane artificial lattices of the WS₂/WSe₂/MoS₂ at reduced growth temperatures. Growth behaviors as well as the heterointerface quality are carefully studied in varying growth parameters. Highly oriented strain patterns are found in the second harmonic generation imaging of the TMD multijunctions, suggesting that the in‐plane heteroepitaxial growth may induce distortion for unique material symmetry.     

Merits and Challenges of Ruddlesden–Popper Soft Halide Perovskites in Electro‐Optics and Optoelectronics

Following the rejuvenation of 3D organic–inorganic hybrid perovskites, like CH₃NH₃PbI₃, (quasi)‐2D Ruddlesden–Popper soft halide perovskites R₂A_n?1Pb_nX_3n+1 have recently become another focus in the optoelectronic and photovoltaic device community. Although quasi‐2D perovskites were first introduced to stabilize optoelectronic/photovoltaic devices against moisture, more interesting properties and device applications, such as solar cells, light‐emitting diodes, white‐light emitters, lasers, and polaritonic emission, have followed. While delicate engineering design has pushed the performance of various devices forward remarkably, understanding of the fundamental properties, especially the charge‐transfer process, electron–phonon interactions, and the growth mechanism in (quasi)‐2D halide perovskites, remains limited and even controversial. Here, after reviewing the current understanding and the nexus between optoelectronic/photovoltaic properties of 2D and 3D halide perovskites, the growth mechanisms, charge‐transfer processes, vibrational properties, and electron–phonon interactions of soft halide perovskites, mainly in quasi‐2D systems, are discussed. It is suggested that single‐crystal‐based studies are needed to deepen the understanding of the aforementioned fundamental properties, and will eventually contribute to device performance.