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

Epitaxial Growth of Two-Dimensional MoO2-MoSe2 Metal-Semiconductor Heterostructures for Schottky Diodes

The metal-semiconductor interface fabricated by conventional methods often suffers from contamination, degrading transport performance. Herein, we propose a one-pot chemical vapor deposition (CVD) process to create a two-dimensional (2D) MoO2-MoSe2 heterostructure by growing MoO2 seeds under a hydrogen environment, followed by depositing MoSe2 on the surface and periphery. The ultraclean interface is verified by cross-sectional scanning transmission electron microscopy and photoluminescence. Along with the high work function of semimetallic MoO2 (Ef = -5.6 eV), a high-rectification Schottky diode is fabricated based on this heterostructure. Furthermore, the Schottky diode exhibits an excellent photovoltaic effect with a high open-circuit voltage of 0.26 eV and ultrafast photoresponse, owing to the naturally formed metal-semiconductor contact with suppressed pinning effect. Our method paves the way for the fabrication of an ultraclean 2D metal-semiconductor interface, without defects or contamination, offering promising prospects for future nanoelectronics.

Controlled Epitaxial Growth of (hk1)-Sb2Se3 Film on Cu9S5 Single Crystal via Post-Annealing Treatment for Photodetection Application

Antimony selenide (Sb2Se3) is a promising semiconductor for photodetector applications due to its unique photovoltaic properties. Achieving optimal carrier transport in (001)-Sb2Se3 by the material of contacting substrate requires in-depth study. In this paper, the induced growth of Sb2Se3 films from (hk0) to (hk1) planes is achieved on digenite (Cu9S5) films by post-annealing treatment. The flake-like and flower-like morphologies on the surface of Sb2Se3 films are caused by different thicknesses of the Cu9S5 films, which are related to the (hk0) and (hk1) planes of Sb2Se3 surface. The epitaxial growth of Sb2Se3 films on (105)-Cu9S5 surfaces exhibits thickness dependence. The results inform research into the controlled induced growth of low-dimensional materials. The device of Sb2Se3/Cu9S5/Si has good broadband response (visible to near-infrared), self-powered characteristics, and stability. As the crystalline quality of the Sb2Se3 film increases along the (hk1) plane, the carrier transport is enhanced correspondingly. Under the 980 nm light irradiation, the device has an excellent switching ratio of 2 × 104 at 0 bias, with responsivity, detectivity, and response time up to 17 µA W-1, 1.48 × 107 Jones, and 355/490 µs, respectively. This suggests that Sb2Se3 is suitable for self-powered photodetectors and related optical and optoelectronic devices.

2D Lateral Heterojunction Arrays with Tailored Interface Band Bending

Two-dimensional (2D) lateral heterojunction arrays, characterized by well-defined electronic interfaces, hold significant promise for advancing next-generation electronic devices. Despite this potential, the efficient synthesis of high-density lateral heterojunctions with tunable interfacial band alignment remains a challenging. Here, a novel strategy is reported for the fabrication of lateral heterojunction arrays between monolayer Si2Te2 grown on Sb2Te3 (ML-Si2Te2@Sb2Te3) and one-quintuple-layer Sb2Te3 grown on monolayer Si2Te2 (1QL-Sb2Te3@ML-Si2Te2) on a p-doped Sb2Te3 substrate. The site-specific formation of numerous periodically arranged 2D ML-Si2Te2@Sb2Te3/1QL-Sb2Te3@ML-Si2Te2 lateral heterojunctions is realized solely through three epitaxial growth steps of thick-Sb2Te3, ML-Si2Te2, and 1QL-Sb2Te3 films, sequentially. More importantly, the precisely engineering of the interfacial band alignment is realized, by manipulating the substrate's p-doping effect with lateral spatial dependency, on each ML-Si2Te2@Sb2Te3/1QL-Sb2Te3@ML-Si2Te2 junction. Atomically sharp interfaces of the junctions with continuous lattices are observed by scanning tunneling microscopy. Scanning tunneling spectroscopy measurements directly reveal the tailored type-II band bending at the interface. This reported strategy opens avenues for advancing lateral epitaxy technology, facilitating practical applications of 2D in-plane heterojunctions.

Phase-Controlled Growth of 1T'-MoS2 Nanoribbons on 1H-MoS2 Nanosheets

2D heterostructures are emerging as alternatives to conventional semiconductors, such as silicon, germanium, and gallium nitride, for next-generation electronics and optoelectronics. However, the direct growth of 2D heterostructures, especially for those with metastable phases still remains challenging. To obtain 2D transition metal dichalcogenides (TMDs) with designed phases, it is highly desired to develop phase-controlled synthetic strategies. Here, a facile chemical vapor deposition method is reported to prepare vertical 1H/1T' MoS2 heterophase structures. By simply changing the growth atmosphere, semimetallic 1T'-MoS2 can be in situ grown on the top of semiconducting 1H-MoS2, forming vertical semiconductor/semimetal 1H/1T' heterophase structures with a sharp interface. The integrated device based on the 1H/1T' MoS2 heterophase structure displays a typical rectifying behavior with a current rectifying ratio of ≈103. Moreover, the 1H/1T' MoS2-based photodetector achieves a responsivity of 1.07 A W-1 at 532 nm with an ultralow dark current of less than 10-11 A. The aforementioned results indicate that 1H/1T' MoS2 heterophase structures can be a promising candidate for future rectifiers and photodetectors. Importantly, the approach may pave the way toward tailoring the phases of TMDs, which can help us utilize phase engineering strategies to promote the performance of electronic devices.

Twisted van der Waals Quantum Materials: Fundamentals, Tunability, and Applications

Twisted van der Waals (vdW) quantum materials have emerged as a rapidly developing field of two-dimensional (2D) semiconductors. These materials establish a new central research area and provide a promising platform for studying quantum phenomena and investigating the engineering of novel optoelectronic properties such as single photon emission, nonlinear optical response, magnon physics, and topological superconductivity. These captivating electronic and optical properties result from, and can be tailored by, the interlayer coupling using moiré patterns formed by vertically stacking atomic layers with controlled angle misorientation or lattice mismatch. Their outstanding properties and the high degree of tunability position them as compelling building blocks for both compact quantum-enabled devices and classical optoelectronics. This paper offers a comprehensive review of recent advancements in the understanding and manipulation of twisted van der Waals structures and presents a survey of the state-of-the-art research on moiré superlattices, encompassing interdisciplinary interests. It delves into fundamental theories, synthesis and fabrication, and visualization techniques, and the wide range of novel physical phenomena exhibited by these structures, with a focus on their potential for practical device integration in applications ranging from quantum information to biosensors, and including classical optoelectronics such as modulators, light emitting diodes, lasers, and photodetectors. It highlights the unique ability of moiré superlattices to connect multiple disciplines, covering chemistry, electronics, optics, photonics, magnetism, topological and quantum physics. This comprehensive review provides a valuable resource for researchers interested in moiré superlattices, shedding light on their fundamental characteristics and their potential for transformative applications in various fields.

Epitaxial van der Waals contacts of 2D TaSe2-WSe2 metal-semiconductor heterostructures

The electronic contact between two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors and metal electrodes is a formidable challenge due to the undesired Schottky barrier, which severely limits the electrical performance of TMD devices and impedes the exploration of their unconventional physical properties and potential electronic applications. In this study, we report a two-step chemical vapor deposition (CVD) growth of 2D TaSe2-WSe2 metal-semiconductor heterostructures. Raman mapping confirms the precise spatial modulation of the as-grown 2D TaSe2-WSe2 heterostructures. Transmission electron microscopy (TEM) characterization reveals that this two-step method provides a high-quality and clean interface of the 2D TaSe2-WSe2 heterostructures. Meanwhile, the upper 1T-TaSe2 is formed heteroepitaxially on/around the pre-synthesized 2H-WSe2 monolayers, exhibiting an epitaxial relationship of (20-20)TaSe2//(20-20)WSe2 and [0001]TaSe2//[0001]WSe2. Furthermore, characterization studies using a Kelvin probe force microscope (KPFM) and electrical transport measurements present compelling evidence that the 2D metal-semiconductor heterostructures under investigation can improve the performance of electrical devices. These results bear substantial significance in augmenting the properties of field-effect transistors (FETs), leading to notable improvements in FET mobility and on/off ratio. Our study not only broadens the horizons of direct growth of high-quality 2D metal-semiconductor heterostructures but also sheds light on potential applications in future high-performance integrated circuits.

Two-Dimensional Halide Pb-Perovskite-Double Perovskite Epitaxial Heterostructures

Epitaxial heterostructures of two-dimensional (2D) halide perovskites offer a new platform for studying intriguing structural, optical, and electronic properties. However, difficulties with the stability of Pb- and Sn-based heterostructures have repeatedly slowed the progress. Recently, Pb-free halide double perovskites are gaining a lot of attention due to their superior stability and greater chemical diversity, but they have not been successfully incorporated into epitaxial heterostructures for further investigation. Here, we report epitaxial core-shell heterostructures via growing Pb-free double perovskites (involving combinations of Ag(I)-Bi(III), Ag-Sb, Ag-In, Na-Bi, Na-Sb, and Na-In) around Pb perovskite 2D crystals. Distinct from Pb-Pb and Pb-Sn perovskite heterostructures, growths of the Pb-free shell at 45° on the (100) surface of the lead perovskite core are observed in all Pb-free cases. The in-depth structural analysis carried out with electron diffraction unequivocally demonstrates the growth of the Pb-free shell along the [110] direction of the Pb perovskite, which is likely due to the relatively lower surface energy of the (110) surface. Furthermore, an investigation of anionic interdiffusion across heterostructure interfaces under the influence of heat was carried out. Interestingly, halide anion diffusion in the Pb-free 2D perovskites is found to be significantly suppressed as compared to Pb-based 2D perovskites. The great structural tunability and excellent stability of Pb-free perovskite heterostructures may find uses in electronic and optoelectronic devices in the near future.

Controllable Synthesis of 2H-1T' Mox Re(1- x ) S2 Lateral Heterostructures and Their Tunable Optoelectronic Properties

Constructing heterostructures and doping are valid ways to improve the optoelectronic properties of transition metal dichalcogenides (TMDs) and optimize the performance of TMDs-based photodetectors. Compared with transfer techniques, chemical vapor deposition (CVD) has higher efficiency in preparing heterostructures. As for the one-step CVD growth of heterostructures, cross-contamination between the two materials may occur during the growth process, which may provide the possibility of one-step simultaneous realization of controllable doping and formation of alloy-based heterostructures by finely tuning the growth dynamics. Here, 2H-1T' Mox Re(1- x ) S2 alloy-to-alloy lateral heterostructures are synthesized through this one-step CVD growth method, utilizing the cross-contamination and different growth temperatures of the two alloys. Due to the doping of a small amount of Re atoms in 2H MoS2 , 2H Mox Re(1- x ) S2 has a high response rejection ratio in the solar-blind ultraviolet (SBUV) region and exhibits a positive photoconductive (PPC) effect. While the 1T' Mox Re(1- x ) S2 formed by heavily doping Mo atoms into 1T' ReS2 will produce a negative photoconductivity (NPC) effect under UV laser irradiation. The optoelectronic property of 2H-1T' Mox Re(1- x ) S2 -based heterostructures can be modulated by gate voltage. These findings are expected to expand the functionality of traditional optoelectronic devices and have potential applications in optoelectronic logic devices.

Ultrafast-Programmable 2D Homojunctions Based on van der Waals Heterostructures on a Silicon Substrate

The development of electrically ultrafast-programmable semiconductor homojunctions can lead to transformative multifunctional electronic devices. However, silicon-based homojunctions are not programmable so that alternative materials need to be explored. Here 2D, multi-functional, lateral homojunctions made of van der Waals heterostructures with a semi-floating-gate configuration on a p++ Si substrate feature atomically sharp interfaces and can be electrostatically programmed in nanoseconds, more than seven orders of magnitude faster than other 2D-based homojunctions. By applying voltage pulses with different polarities, lateral p-n, n+ -n and other types of homojunctions can be formed, varied, and reversed. The p-n homojunctions possess a high rectification ratio of up to ≈105 and can be dynamically switched between four distinct conduction states with the current spanning over nine orders of magnitude, enabling them to function as logic rectifiers, memories, and multi-valued logic inverters. Built on a p++ Si substrate, which acts as the control gate, the devices are compatible with Si technology.

Two dimensional NbSe2/Nb2O5 metal-semiconductor heterostructure-based photoelectrochemical photodetector with fast response and high flexibility

Two dimensional (2D) metal-semiconductor heterostructures are promising for high-performance optoelectronic devices due to fast carrier separation and transportation. Considering the superior metallic characteristics accompanied by high electrical conductivity in NbSe₂, surface oxidation provides a facile way to form NbSe₂/Nb₂O₅ metal-semiconductor heterostructures. Herein, size-dependent NbSe₂/Nb₂O₅ nanosheets were achieved by a liquid phase exfoliation method and a gradient centrifugation strategy. These NbSe₂/Nb₂O₅ heterostructure-based photodetectors show high responsivity with 23.21 μA W^-1, fast response time of millisecond magnitude, and wide band detection ability in the UV-Vis region. It is noticeable that the photocurrent density is sensitive to the surface oxygen layer due to the oxygen-sensitized photoconduction mechanism. The flexible testing of the NbSe₂/Nb₂O₅ heterostructure-based PEC-type photodetectors exhibits high photodetection performance even after bending and twisting. Beyond that, the solid-state PEC-type NbSe₂/Nb₂O₅ photodetector also achieves relatively stable photodetection and high stability. This work promotes the application of 2D NbSe₂/Nb₂O₅ metal-semiconductor heterostructures in flexible optoelectronic devices.

Substitutional p-Type Doping in NbS2 -MoS2 Lateral Heterostructures Grown by MOCVD

Monolayer MoS2 has attracted significant attention owing to its excellent performance as an n-type semiconductor from the transition metal dichalcogenide (TMDC) family. It is however strongly desired to develop controllable synthesis methods for 2D p-type MoS2 , which is crucial for complementary logic applications but remains difficult. In this work, high-quality NbS2 -MoS2 lateral heterostructures are synthesized by one-step metal-organic chemical vapor deposition (MOCVD) together with monolayer MoS2 substitutionally doped by Nb, resulting in a p-type doped behavior. The heterojunction shows a p-type transfer characteristic with a high on/off current ratio of ≈104 , exceeding previously reported values. The band structure through the NbS2 -MoS2 heterojunction is investigated by density functional theory (DFT) and quantum transport simulations. This work provides a scalable approach to synthesize substitutionally doped TMDC materials and provides an insight into the interface between 2D metals and semiconductors in lateral heterostructures, which is imperative for the development of next-generation nanoelectronics and highly integrated devices.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Atomic transistors based on seamless lateral metal-semiconductor junctions with a sub-1-nm transfer length

The edge-to-edge connected metal-semiconductor junction (MSJ) for two-dimensional (2D) transistors has the potential to reduce the contact length while improving the performance of the devices. However, typical 2D materials are thermally and chemically unstable, which impedes the reproducible achievement of high-quality edge contacts. Here we present a scalable synthetic strategy to fabricate low-resistance edge contacts to atomic transistors using a thermally stable 2D metal, PtTe₂. The use of PtTe₂ as an epitaxial template enables the lateral growth of monolayer MoS₂ to achieve a PtTe₂-MoS₂ MSJ with the thinnest possible, seamless atomic interface. The synthesized lateral heterojunction enables the reduced dimensions of Schottky barriers and enhanced carrier injection compared to counterparts composed of a vertical 3D metal contact. Furthermore, facile position-selected growth of PtTe₂-MoS₂ MSJ arrays using conventional lithography can facilitate the design of device layouts with high processability, while providing low contact resistivity and ultrashort transfer length on wafer scales.

Epitaxy of NiTe2 on WS2 for the p-Type Schottky Contact and Increased Photoresponse

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have great potential applications in the electronic and optoelectronic devices. Nevertheless, due to the difficulty in the efficient doping of atomic-thickness TMDCs or Fermi level pinning (FLP) effects at the metal/semiconductor interface, most TMDC devices exhibit the n-type conduction polarity, which significantly limits their functional applications based on the p-n junction. Here, 2D semi-metal NiTe₂ nanosheets were epitaxially grown on the WS₂ monolayer by a two-step chemical vapor deposition route. The microstructure and optical characterizations confirm that the vertically stacked NiTe₂/WS₂ heterostructures are formed by van der Waals epitaxy. Interestingly, p-type WS₂ field-effect transistors can be obtained with the hole mobility of ∼4.22 cm²/V·s, when the epitaxial NiTe₂ sheets act as the source/drain electrodes. This is attributed to the decreased FLP effect and hence the low potential barrier for holes at the van der Waals contacts. Furthermore, the photodetectors based on the heterostructures show a 2 orders of magnitude increase in the switch ratio, responsivity, and detectivity and a 1 order of magnitude increase in the rise and decay speeds relative to those based on pristine WS₂. This work paves the way to realize the p-type contact for monolayer WS₂ with significantly enhanced optoelectronic performance.

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their heterostructures have attracted significant interest in both academia and industry because of their unusual physical and chemical properties. They offer numerous applications, such as electronic, optoelectronic, and spintronic devices, in addition to energy storage and conversion. Atomic and structural modifications of van der Waals layered materials are required to achieve unique and versatile properties for advanced applications. This review presents a discussion on the atomic-scale and structural modifications of 2D TMDs and their heterostructures via post-treatment. Atomic-scale modifications such as vacancy generation, substitutional doping, functionalization and repair of 2D TMDs and structural modifications including phase transitions and construction of heterostructures are discussed. Such modifications on the physical and chemical properties of 2D TMDs enable the development of various advanced applications including electronic and optoelectronic devices, sensing, catalysis, nanogenerators, and memory and neuromorphic devices. Finally, the challenges and prospects of various post-treatment techniques and related future advanced applications are addressed.

Switchable Photoresponse Mechanisms Implemented in Single van der Waals Semiconductor/Metal Heterostructure

van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been extensively studied for functional applications, and most of the reported devices work with sole mechanism. The emerging metallic 2D materials provide us new options for building functional vdW heterostructures via rational band engineering design. Here, we investigate the vdW semiconductor/metal heterostructure built with 2D semiconducting InSe and metallic 1T-phase NbTe2, whose electron affinity χInSe and work function ΦNbTe2 almost exactly align. Electrical characterization verifies exceptional diode-like rectification ratio of >103 for the InSe/NbTe2 heterostructure device. Further photocurrent mappings reveal the switchable photoresponse mechanisms of this heterostructure or, in other words, the alternative roles that metallic NbTe2 plays. Specifically, this heterostructure device works in a photovoltaic manner under reverse bias, whereas it turns to phototransistor with InSe channel and NbTe2 electrode under high forward bias. The switchable photoresponse mechanisms originate from the band alignment at the interface, where the band bending could be readily adjusted by the bias voltage. In addition, a conceptual optoelectronic logic gate is proposed based on the exclusive working mechanisms. Finally, the photodetection performance of this heterostructure is represented by an ultrahigh responsivity of ∼84 A/W to 532 nm laser. Our results demonstrate the valuable application of 2D metals in functional devices, as well as the potential of implementing photovoltaic device and phototransistor with single vdW heterostructure.

Wafer-Scale Synthesis of WS2 Films with In Situ Controllable p-Type Doping by Atomic Layer Deposition

Wafer-scale synthesis of p-type TMD films is critical for its commercialization in next-generation electro/optoelectronics. In this work, wafer-scale intrinsic n-type WS₂ films and in situ Nb-doped p-type WS₂ films were synthesized through atomic layer deposition (ALD) on 8-inch α-Al₂O₃/Si wafers, 2-inch sapphire, and 1 cm² GaN substrate pieces. The Nb doping concentration was precisely controlled by altering cycle number of Nb precursor and activated by postannealing. WS₂ n-FETs and Nb-doped p-FETs with different Nb concentrations have been fabricated using CMOS-compatible processes. X-ray photoelectron spectroscopy, Raman spectroscopy, and Hall measurements confirmed the effective substitutional doping with Nb. The on/off ratio and electron mobility of WS₂ n-FET are as high as 10⁵ and 6.85 cm² V⁻¹ s⁻¹, respectively. In WS₂ p-FET with 15-cycle Nb doping, the on/off ratio and hole mobility are 10 and 0.016 cm² V⁻¹ s⁻¹, respectively. The p-n structure based on n- and p- type WS₂ films was proved with a 10⁴ rectifying ratio. The realization of controllable in situ Nb-doped WS₂ films paved a way for fabricating wafer-scale complementary WS₂ FETs.

Atomically thin telluride multiheterostructures: toward spatial modulation of bandgaps

Lateral multiheterostructures with spatially modulated bandgaps have great potential for applications in high-performance electronic, optoelectronic and thermoelectric devices. Multiheterostructures based on transition metal tellurides are especially promising due to their tunable bandgap in a wide range and the rich variety of structural phases. However, the synthesis of telluride-based multiheterostructures remains a challenge due to the low activity of tellurium and the poor thermal stability of tellurium alloys. In this work, we synthesized monolayer WSe_2-2xTe_2x/WSe_2-2yTe_2y (x > y) multiheterostructures in situ using chemical vapor deposition (CVD). Photoluminescence analysis and Raman mapping confirm the spatial modulation of the bandgap in the radial direction. Furthermore, field-effect transistors with the channels parallel (type I) and perpendicular (type II) to the multiheterostructure rings were fabricated. Type I transistors exhibit enhanced ambipolar transport, due to the low energy bridges between the source and drain. Remarkably, the photocurrents in type I transistors are two orders of magnitude higher than those in type II transistors, which can be attributed to the fact that the photovoltaic photocurrents generated at the two heterojunctions are summed together in type I transistors, but they cancel each other in type II transistors. These multiheterostructures will provide a new platform for novel electronic/photonic devices with potential applications in broadband light sensing, highly sensitive imaging and ultrafast optoelectronic integrated circuits.

Superconducting 2D NbS2 Grown Epitaxially by Chemical Vapor Deposition

Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting great attention because of their interesting low-temperature properties such as superconductivity, magnetism, and charge density waves (CDW). However, further studies and practical applications are being slowed down by difficulties in synthesizing high-quality materials with a large grain size and well-determined thickness. In this work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D NbS₂ crystals on a sapphire substrate, with a thickness-dependent structural phase transition. NbS₂ crystals are epitaxially aligned by the underlying c-plane sapphire resulting in high-quality growth. The thickness of NbS₂ is well controlled by growth parameters to be between 1.5 and 10 nm with a large grain size of up to 500 μm. As the thickness increases, we observe in our NbS₂ a transition from a metallic 3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electrical transport measurements. A Berezinskii-Kosterlitz-Thouless (BKT) superconducting transition occurs in the CVD-grown 2H-phase NbS₂ below the transition temperature (T_c) of 3 K. Our work demonstrates thickness and phase-controllable synthesis of high-quality superconducting 2D NbS₂, which is imperative for its practical applications in next-generation TMDC-based electrical devices.

Salt-assisted chemical vapor deposition of two-dimensional transition metal dichalcogenides

Salt-assisted chemical vapor deposition (SA-CVD), which uses halide salts (e.g., NaCl, KBr, etc.) and molten salts (e.g., Na2MoO4, Na2WO4, etc.) as precursors, is one of the most popular methods favored for the fabrication of two-dimensional (2D) materials such as atomically thin metal chalcogenides, graphene, and h-BN. In this review, the distinct functions of halogens (F, Cl, Br, I) and alkali metals (Li, Na, K) in SA-CVD are first clarified. Based on the current development in SA-CVD growth and its related reaction modes, the existing methods are categorized into the Salt 1.0 (halide salts-based) and Salt 2.0 (molten salts-based) techniques. The achievements, advantages, and limitations of each technique are discussed in detail. Finally, new perspectives are proposed for the application of SA-CVD in the synthesis of 2D transition metal dichalcogenides for advanced electronics.

NIR -I and NIR-II irradiation tumor ablation using NbS2 nanosheets as the photothermal agent

Photothermal therapy has been considered a powerful means of cancer therapy due to its minimal invasiveness, effectiveness, and convenience. Although promising, the therapeutic effects are greatly limited as they rely on the photothermal agent (PTA). It is urgent to develop new PTAs with high photothermal conversion performance, especially under irradiation in the long-wavelength biowindows. Herein, a dual-biowindow-responsive PTA made of NbS₂-PVP nanosheets was fabricated to be used both in the first near-infrared (NIR-I) and the second near-infrared (NIR-II) biowindows. With excellent hydrophilicity and biocompatibility, the nanosheets could effectively convert the near-infrared (NIR) light into heat, showing prominent photothermal stability. The calculated photothermal conversion efficiencies reached 59.2% (under NIR-I excitation) and 69.1% (under NIR-II excitation), respectively, which are comparable to those of metallic PTAs. The NbS₂-PVP nanosheets had low cytotoxicity and could trigger strong photothermal treatment and cause cancer cell death upon irradiation by NIR-I or NIR-II light in vitro. Moreover, we have also demonstrated the highly efficient tissue ablation and tumor inhibition capability of NbS₂-PVP nanosheets in vivo. This work explores an effective PTA of two-dimensional nanomaterials in NIR-I and NIR-II biowindows and offers a reference for the design of new kinds of PTAs.

One-Step Synthesis of NbSe2/Nb-Doped-WSe2 Metal/Doped-Semiconductor van der Waals Heterostructures for Doping Controlled Ohmic Contact

van der Waals heterostructures (vdWHs) of metallic (m-) and semiconducting (s-) transition-metal dichalcogenides (TMDs) exhibit an ideal metal/semiconductor (M/S) contact in a field-effect transistor. However, in the current two-step chemical vapor deposition process, the synthesis of m-TMD on pregrown s-TMD contaminates the van der Waals (vdW) interface and hinders the doping of s-TMD. Here, NbSe₂/Nb-doped-WSe₂ metal-doped-semiconductor (M/d-S) vdWHs are created via a one-step synthesis approach using a niobium molar ratio-controlled solution-phase precursor. The one-step growth approach synthesizes Nb-doped WSe₂ with a controllable doping concentration and metal/doped-semiconductor vdWHs. The hole carrier concentration can be precisely controlled by controlling the Nb/(W + Nb) molar ratio in the precursor solution from ∼3 × 10¹¹/cm² at Nb-0% to ∼1.38 × 10¹²/cm² at Nb-60%; correspondingly, the contact resistance R_C value decreases from 10 888.78 at Nb-0% to 70.60 kΩ.μm at Nb-60%. The Schottky barrier height measurement in the Arrhenius plots of ln(I_sat/T²) versus q/K_BT demonstrated an ohmic contact in the NbSe₂/W_xNb_1-xSe₂ vdWHs. Combining p-doping in WSe₂ and M/d-S vdWHs, the mobility (27.24 cm² V^-1 s^-1) and on/off ratio (2.2 × 10⁷) are increased 1238 and 4400 times, respectively, compared to that using the Cr/pure-WSe₂ contact (0.022 cm² V^-1 s^-1 and 5 × 10³, respectively). Together, the R_C value using the NbSe₂ contact shows 2.46 kΩ.μm, which is ∼29 times lower than that of using a metal contact. This method is expected to guide the synthesis of various M/d-S vdWHs and applications in future high-performance integrated circuits.

Vortex-Oriented Ferroelectric Domains in SnTe/PbTe Monolayer Lateral Heterostructures

Heterostructures formed from interfaces between materials with complementary properties often display unconventional physics. Of especial interest are heterostructures formed with ferroelectric materials. These are mostly formed by combining thin layers in vertical stacks. Here the first in situ molecular beam epitaxial growth and scanning tunneling microscopy characterization of atomically sharp lateral heterostructures between a ferroelectric SnTe monolayer and a paraelectric PbTe monolayer are reported. The bias voltage dependence of the apparent heights of SnTe and PbTe monolayers, which are closely related to the type-II band alignment of the heterostructure, is investigated. Remarkably, it is discovered that the ferroelectric domains in the SnTe surrounding a PbTe core form either clockwise or counterclockwise vortex-oriented quadrant configurations. In addition, when there is a finite angle between the polarization and the interface, the perpendicular component of the polarization always points from SnTe to PbTe. Supported by first-principles calculation, the mechanism of vortex formation and preferred polarization direction is identified in the interaction between the polarization, the space charge, and the strain effect at the horizontal heterointerface. The studies bring the application of 2D group-IV monochalcogenides on in-plane ferroelectric heterostructures a step closer.

Hybrid System Combining Two-Dimensional Materials and Ferroelectrics and Its Application in Photodetection

Photodetectors are one of the most important components for a future "Internet-of-Things" information society. Compared to the mainstream semiconductor-based photodetectors, emerging devices based on two-dimensional (2D) materials and ferroelectrics as well as their hybrid systems have been extensively studied in recent decades due to their outstanding performances and related interesting physical, electrical, and optoelectronic phenomena. In this paper, we review the photodetection based on 2D materials and ferroelectric hybrid systems. The fundamentals of 2D and ferroelectric materials as well as the interaction in the hybrid system will be introduced. Ferroelectricity modulated optoelectronic properties in the hybrid system will be discussed in detail. After the basics and figures of merit of photodetectors are summarized, the 2D-ferroelectrics devices with different structures including p-n diodes, Schottky diodes, and field-effect transistors will be reviewed and compared. The polarization of ferroelectrics offers the possibility of the modulation and enhancement of the photodetection in the hybrid detectors, which will be discussed in depth. Finally, the challenges and perspectives of the photodetectors based on 2D ferroelectrics will be proposed. This Review outlines the important aspects of the recent development of the hybrid system of 2D and ferroelectric materials, which could interact with each other and thus lead to photodetectors with higher performances. Such a Review will be helpful for the research of emerging physical phenomena and for the design of multifunctional nanoscale electronic and optoelectronic devices.

Preparation Engineering of Two-Dimensional Heterostructures via Bottom-Up Growth for Device Applications

Two-dimensional heterostructures with tremendous electronic and optoelectronic properties hold great promise for nanodevice integrations and applications owing to the wide tunable characteristics. Toward this end, developing construction strategies in allusion to large-scale production of high-quality heterostructures is critical. The mainstream preparation routes are representatively classified into two categories of top-down and bottom-up approaches. Nonetheless, the relatively low reproductivity and the limitation for lateral heterostructure formations of top-down methods at the present stage inherently impeded their further developments. To surmount these obstacles, assembling heterostructures via miscellaneous bottom-up preparation protocols has emerged as a potential solution, attributed to the controllability and clean interface. Three typical approaches of chemical/physical vapor deposition, solution synthesis, and growth under ultrahigh vacuum conditions have shown promise due to the possibilities for preparing heterostructures with predesigned structures, clean interfaces, and the like. Therefore, bottom-up preparation engineering of heterostructures in two dimensions for further device applications is of vital importance. Moreover, heterostructure integrations by these methods have experienced a period of flourishing development in the past few years. In this review, the classical bottom-up growth routes, characterization methods, and latest progress of diverse heterostructures and further device applications are overviewed. Finally, the challenges and opportunities are discussed.

Two-Dimensional Lateral Heterostructures Made by Selective Reaction on a Patterned Monolayer MoS2 Matrix

Two-dimensional (2D) heterostructures have attracted widespread attention for their promising prospects in the fields of electronics and optoelectronics. However, in order to truly realize 2D-material-based integrated circuits, precisely controllable fabrication of 2D heterostructures is crucial and urgently needed. Here, we demonstrate an ex situ growth method of MoSe₂/MoS₂ lateral heterostructures by selective selenization of a laser-scanned, ultrathin oxidized region (MoO_x) on a monolayer MoS₂ matrix. In our method, monolayer MoS₂ is scanned by a laser with a pre-designed pattern, where the laser-scanned MoS₂ is totally oxidized into MoO_x. The oxidized region is then selenized in a furnace, while the unoxidized MoS₂ region remains unchanged, delivering a MoSe₂/MoS₂ heterostructure. Unlike in situ laser direct growth methods, our method separates the laser-scanned process from the selenization process, which avoids the long time of point-by-point selenization of MoS₂ by laser, making the efficiency of the synthesis greatly improved. The formation process of the heterostructure is studied by Raman spectroscopy and Auger electron spectroscopy. This simple and controllable approach to lateral heterostructures with desired patterns paves the way for fast and mass integration of devices based on 2D heterostructures.

Recent Advances in 2D Group VB Transition Metal Chalcogenides

2D materials have received considerable research interest owing to their abundant material systems and remarkable properties. Among them, 2D group VB transition metal chalcogenides (GVTMCs) stand out as emerging 2D metallic materials and significantly broaden the research scope of 2D materials. 2D GVTMCs have great advantages in electrical transport, 2D magnetism, charge density wave, sensing, catalysis, and charge storage, making them attractive in the fields of functional devices and energy chemistry. In this review, the recent progress of 2D GVTMCs is summarized systematically from fundamental properties, growth methodologies to potential applications. The challenges and prospects are also discussed for future research in this field.

2D Homojunctions for Electronics and Optoelectronics

In the post-Moore era, 2D materials with rich physical properties have attracted widespread attention from the scientific and industrial communities. Among 2D materials, the 2D homojunctions are of great promise in designing novel electronic and optoelectronic devices due to their unique geometries and properties such as homogeneous components, perfect lattice matching, and efficient charge transfer at the interface. In this article, a pioneering review focusing on the structural design and device application of 2D homojunctions such as p-n homojunctions, heterophase homojunctions, and layer-engineered homojunctions is provided. The preparation strategies to construct 2D homojunctions including vapor-phase deposition, lithium intercalation, laser irradiation, chemical doping, electrostatic doping, and photodoping are summarized in detail. Specifically, a careful review on the applications of the 2D homojunctions in electronics (e.g., field-effect transistors, rectifiers, and inverters) and optoelectronics (e.g., light-emitting diodes, photovoltaics, and photodetectors) is provided. Eventually, the current challenges and future perspectives are commented for promoting the rapid development of 2D homojunctions.

Core@shell and lateral heterostructures composed of SnS and NbS2

The spatial arrangement of heterostructures based on two-dimensional layered materials is important in controlling their electronic and optoelectronic properties. In this contribution, by controlling the reaction kinetics and thus the nucleation and growth sequence of p-type SnS and metallic NbS2, controllable preparation of both SnS@NbS2 core@shell and SnS/NbS2 lateral heterostructures was realized. The SnS@NbS2 core@shell heterostructures were further applied in photodetectors, and interestingly, a negative photoresponse was observed due to the Seebeck effect exerted on the NbS2 shell. Compared with the pure metallic NbS2, the SnS@NbS2 core@shell heterostructures showed a 15 times increased signal-to-noise ratio and much improved photocurrent stability, largely due to the charge and heat transfer between the SnS core and NbS2 shell.

Interface chemistry of two-dimensional heterostructures - fundamentals to applications

Two-dimensional heterostructures (2D HSs) have emerged as a new class of materials where dissimilar 2D materials are combined to synergise their advantages and alleviate shortcomings. Such a combination of dissimilar components into 2D HSs offers fascinating properties and intriguing functionalities attributed to the newly formed heterointerface of constituent components. Understanding the nature of the surface and the complex heterointerface of HSs at the atomic level is crucial for realising the desired properties, designing innovative 2D HSs, and ultimately unlocking their full potential for practical applications. Therefore, this review provides the recent progress in the field of 2D HSs with a focus on the discussion of the fundamentals and the chemistry of heterointerfaces based on van der Waals (vdW) and covalent interactions. It also explains the challenges associated with the scalable synthesis and introduces possible methodologies to produce large quantities with good control over the heterointerface. Subsequently, it highlights the specialised characterisation techniques to reveal the heterointerface formation, chemistry and nature. Afterwards, we give an overview of the role of 2D HSs in various emerging applications, particularly in high-power batteries, bifunctional catalysts, electronics, and sensors. In the end, we present conclusions with the possible solutions to the associated challenges with the heterointerfaces and potential opportunities that can be adopted for innovative applications.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.

Two-Dimensional Metallic Vanadium Ditelluride as a High-Performance Electrode Material

Two-dimensional (2D) metallic transition-metal dichalcogenides (MTMDCs) are considered as ideal electrode materials for enhancing the device performances of 2D semiconducting transition-metal dichalcogenides, due to their similar atomic structures and complementary electronic properties. Vanadium ditelluride (VTe₂) behaves as a fascinating material in MTMDCs family, presenting room-temperature ferromagnetism, charge density waves order, and topological property. However, its practical applications in universal electrode/energy-related fields remain unexplored. Herein, we achieved the direct synthesis of ultrathin, large-domain, and thickness-tunable 1T-VTe₂ nanosheets on an easily available mica substrate by chemical vapor deposition (CVD). We further uncover that the CVD-derived 1T-VTe₂ can serve as a high-performance electrode material thanks to its ultrahigh conductivity. Accordingly, a 6 times higher field-effect mobility (∼47.5 cm² V^-1 s^-1) was achieved in 1T-VTe₂-contacted monolayer MoS₂ devices than that using a conventional Ti/Au electrode (∼8.1 cm² V^-1 s^-1). Moreover, the CVD-synthesized 1T-VTe₂ nanosheets are revealed to present excellent electrocatalytic activity for hydrogen evolution reaction. These results should propel the direct application of CVD-grown 2D MTMDCs as high-performance electrode materials in all 2D materials related devices.

Interfaces determine the nucleation and growth of large NbS2 single crystals

The synthesis of large NbS₂ single crystals benefits from Na–Nb–O droplet interfaces, which reduces nucleation density and increases growth rate.

Seamlessly Splicing Metallic Sn x Mo1- x S2 at MoS2 Edge for Enhanced Photoelectrocatalytic Performance in Microreactor

Accurate design of the 2D metal-semiconductor (M-S) heterostructure via the covalent combination of appropriate metallic and semiconducting materials is urgently needed for fabricating high-performance nanodevices and enhancing catalytic performance. Hence, the lateral epitaxial growth of M-S Sn x Mo1- x S2/MoS2 heterostructure is precisely prepared with in situ growth of metallic Sn x Mo1- x S2 by doping Sn atoms at semiconductor MoS2 edge via one-step chemical vapor deposition. The atomically sharp interface of this heterostructure exhibits clearly distinguished performance based on a series of characterizations. The oxygen evolution photoelectrocatalytic performance of the epitaxial M-S heterostructure is 2.5 times higher than that of pure MoS2 in microreactor, attributed to the efficient electron-hole separation and rapid charge transfer. This growth method provides a general strategy for fabricating seamless M-S lateral heterostructures by controllable doping heteroatoms. The M-S heterostructures show increased carrier migration rate and eliminated Fermi level pinning effect, contributing to their potential in devices and catalytic system.

Two-Dimensional Metallic NiTe2 with Ultrahigh Environmental Stability, Conductivity, and Electrocatalytic Activity

Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDCs) supply a versatile platform for investigating newfangled physical issues and developing potential applications in electronics/spintronics/electrocatalysis. Among these, NiTe₂ (a type-II Dirac semimetal) possesses a Dirac point near its Fermi level. However, as-prepared 2D MTMDCs are mostly environmentally unstable, and little attention has been paid to synthesizing such materials. Herein, a general chemical vapor deposition (CVD) approach has been designed to prepare thickness-tunable and large-domain (∼1.5 mm) 1T-NiTe₂ on an atomically flat mica substrate. Significantly, ultrahigh conductivity (∼1.15 × 10⁶ S m^-1) of CVD-synthesized 1T-NiTe₂ and high catalytic activity in pH-universal hydrogen evolution reaction have been uncovered. More interestingly, the 2D 1T-NiTe₂ maintains robust environmental stability for more than one year and even after a variety of harsh treatments. These results hereby fill an existing research gap in synthesizing environmentally stable 2D MTMDCs, making fundamental progress in developing 2D MTMDC-based devices/catalysts.

Enhanced Optoelectronic Performance of CVD-Grown Metal-Semiconductor NiTe2/MoS2 Heterostructures

Van der Waals (vdW) heterostructures are the fundamental blocks for two-dimensional (2D) electronic and optoelectronic devices. In this work, a high-quality 2D metal-semiconductor NiTe₂/MoS₂ heterostructure is prepared by a two-step chemical vapor deposition (CVD) growth. The back-gated field-effect transistors (FETs) and photodetectors based on the heterostructure show enhanced electronic and optoelectronic performance than that of a pristine MoS₂ monolayer, owing to the better heterointerface in the former device. Especially, this photodetector based on the metal-semiconductor heterostructure shows 3 orders faster rise time and decay time than that of the pristine MoS₂ under the same fabrication procedure. The enhancement of electronic behavior and optoelectronic response by the epitaxial growth of metallic vdW layered materials can provide a new method to improve the performance of optoelectronic devices.

Transition-Metal Substitution-Induced Lattice Strain and Electrical Polarity Reversal in Monolayer WS2

The physical and chemical properties of transition metal dichalcogenides can be effectively tuned by doping or alloying, which is essential for their practical applications. However, the microstructure evolutions and their effects on the physical properties induced by alloying from hetero-atoms with different outermost electronic structures are still unclear. Here, we synthesized Nb-substituted WS₂ with various Nb concentrations showing unusual changes of optical behaviors and continuous electrical polarity reversal. The fully softened Raman mode, rapidly quenched photoluminescence, and severe electron scattering can be attributed to the combined effects of charge doping and lattice strain caused by atomic Nb doping. Three types of substitution modes of Nb atoms in the WS₂ lattice were observed directly from atomic-resolution scanning transmission electron microscopy. Density functional theory calculations further confirm the role of lattice strain in the evolutions of optical and electrical characteristics. With increasing Nb concentration, n-type, ambipolar, and p-type field-effect transistors can be achieved, indicating the capacity of this doping method to engineer the properties of two-dimensional materials for future electronic applications.

General synthesis of two-dimensional van der Waals heterostructure arrays

Two-dimensional van der Waals heterostructures (vdWHs) have attracted considerable interest^1-4. However, most vdWHs reported so far  are created by an arduous micromechanical exfoliation and manual restacking process⁵, which-although versatile for proof-of-concept demonstrations^6-16 and fundamental studies^17-30-is clearly not scalable for practical technologies. Here we report a general synthetic strategy for two-dimensional vdWH arrays between metallic transition-metal dichalcogenides (m-TMDs) and semiconducting TMDs (s-TMDs). By selectively patterning nucleation sites on monolayer or bilayer s-TMDs, we precisely control the nucleation and growth of diverse m-TMDs with designable periodic arrangements and tunable lateral dimensions at the predesignated spatial locations, producing a series of vdWH arrays, including VSe₂/WSe₂, NiTe₂/WSe₂, CoTe₂/WSe₂, NbTe₂/WSe₂, VS₂/WSe₂, VSe₂/MoS₂ and VSe₂/WS₂. Systematic scanning transmission electron microscopy studies reveal nearly ideal vdW interfaces with widely tunable moiré superlattices. With the atomically clean vdW interface, we further show that the m-TMDs function as highly reliable synthetic vdW contacts for the underlying WSe₂ with excellent device performance and yield, delivering a high ON-current density of up to 900 microamperes per micrometre in bilayer WSe₂ transistors. This general synthesis of diverse two-dimensional vdWH arrays provides a versatile material platform for exploring exotic physics and promises a scalable pathway to high-performance devices.

Revealing stable geometries and magic clusters of hexagonal boron nitride in the nucleation of chemical vapor deposition growth on Ni(111)/Cu(111) surfaces: a theoretical study

To improve the quality of chemical vapor deposition (CVD)-prepared hexagonal boron nitride (h-BN), it is essential to understand the growth mechanism, particularly to learn the structures as well as their stabilities and kinetic evolutions of the formed clusters in the initial growth stage. Herein, we performed systematic studies on the stabilities of various geometries of different-/identical-sized BN clusters on (111) surfaces of Ni and Cu by density functional theory simulations. The results show that the stable configurations of different-sized clusters are those containing the most normal hexagons composed with alternate B and N atoms. There exist ultra-stable magic clusters on the (111) surfaces of both the metals. On Ni(111), the geometries of the magic clusters are composed of hexagons arranged in the core-shell structure, while they contain tetragons on the Cu(111) surface. The ultra-high stabilities of the magic clusters can be attributed to the comprehensive effect from the core-shell structure, high symmetry, edged atoms, and adsorption sites. The stable geometries of different-sized clusters as well as magic clusters present the vital roles of metal substrates in CVD-synthesis of h-BN and provide instructive information in improving the quality of h-BN by selecting appropriate metal substrates.

P-type Doping in Large-Area Monolayer MoS2 by Chemical Vapor Deposition

Molybdenum disulfide (MoS₂) with excellent properties has been widely reported in recent years. However, it is a great challenge to achieve p-type conductivity in MoS₂ because of its native stubborn n-type conductivity. Substitutional transition metal doping has been proved to be an effective approach to tune their intrinsic properties and enhance device performance. Herein, we report the growth of Nb-doping large-area monolayer MoS₂ by a one-step salt-assisted chemical vapor deposition method. Electrical measurements indicate that Nb doping suppresses n-type conductivity in MoS₂ and shows an ambipolar transport behavior after annealing under the sulfur atmosphere, which highlights the p-type doping effect via Nb, corresponding to the density functional theory calculations with Fermi-level shifting to valence band maximum. This work provides a promising approach of two-dimensional materials in electronic and optoelectronic applications.

Multi-gate memristive synapses realized with the lateral heterostructure of 2D WSe2 and WO3

The development of novel synaptic device architectures with a high order of synaptic plasticity can provide a breakthrough toward neuromorphic computing. Herein, through the thermal oxidation of two-dimensional (2D) WSe₂, unique memristive synapses based on the lateral heterostructure of 2D WSe₂ and WO₃, with multi-gate modulation characteristics, are firstly demonstrated. An intermediate transition layer in the heterostructure is observed through transmission electron microscopy. Raman spectroscopy and detailed electrical measurements provide insights into the mechanism of memristive behavior, revealing that the protons injected into/removed from the intermediate transition layer account for the memristive behavior. This novel memristive synapse can be used to emulate two neuron-based synaptic functions, like post-synaptic current, short-term plasticity and long-term plasticity, with remarkable linearity, symmetry, and an ultralow energy consumption of ∼2.7 pJ per spike. More importantly, the synaptic plasticity between the drain and source electrodes can be effectively modulated by the gate voltage and visible light in a four-terminal configuration. Such multi-gate tuning of the synaptic plasticity cannot be accomplished by any previously reported multi-gate synaptic devices that only mimic two neuron-based synapses. This new synaptic architecture with electrical and optical modulation enables a realistic emulation of biological synapses whose synaptic plasticity can be additionally regulated by the surrounding astrocytes, greatly improving the recognition accuracy and processing capacity of artificial neuristors, and paving a new way for highly efficient neuromorphic computation devices.

Edge-Epitaxial Growth of InSe Nanowires toward High-Performance Photodetectors

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their unique geometries and physical properties. However, it is challenging to synthesize semiconducting nanowires directly on a SiO₂ /Si substrate due to lattice mismatch. Here, a catalysis-free approach is developed to achieve direct synthesis of long and straight InSe nanowires on SiO₂ /Si substrates through edge-homoepitaxial growth. Parallel InSe nanowires are achieved further on SiO₂ /Si substrates through controlling growth conditions. The underlying growth mechanism is attributed to a selenium self-driven vapor-liquid-solid process, which is distinct from the conventional metal-catalytic vapor-liquid-solid method widely used for growing Si and III-V nanowires. Furthermore, it is demonstrated that the as-grown InSe nanowire-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A W^-1 , ultrahigh detectivity of 1.57 × 10¹⁴ Jones, and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as-grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge-homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO₂ /Si substrates.

Scalable Two-Dimensional Lateral Metal/Semiconductor Junction Fabricated with Selective Synthetic Integration of Transition-Metal-Carbide (Mo2C)/-Dichalcogenide (MoS2)

The construction of manufacturable, stable, high-quality metal/semiconductor junction structures is of fundamental importance to implement higher-level devices and circuit systems. Owing to the unique features of two-dimensional (2D) materials, namely, that intralayer atoms are covalently bonded, whereas interlayer atoms are held together by weak attractive interactions, there are several studies on the fabrication and identification of the peculiar properties of various 2D heterostructures. However, large-scale 2D lateral metal/semiconductor junction structures with acceptable levels of manufacturability and quality have not yet been demonstrated, which is among the critical technological hurdles to overcome for the realization of 2D material-based electronic and photonic devices. This paper reports the fabrication of a manufacturable large-scale metal (Mo₂C)/semiconductor (MoS₂) junction via selective synthetic integration and a lithographically patterned SiO₂ masking layer. It is demonstrated that whereas chemical conversion to Mo₂C occurs in the exposed chemical vapor deposition-grown MoS₂ part, the MoS₂ layer under the SiO₂ masking layer is protected from chemical conversion, so that a scalable Mo₂C/MoS₂ heterostructure is integrated down to nanometer-scale dimensions. Excellent contact resistance of 2.1 kΩ·μm is achieved from this lateral junction structure, providing a manufacturable and highly stable metal/semiconductor building block for real implementation of 2D material-based nanoscale device integration.

Approaching ohmic contact to two-dimensional semiconductors

Two-dimensional semiconductors have attracted immense research interests owing to their intriguing properties and promising applications in electronic and optoelectronic devices. However, the performance of these devices is drastically hindered by the large Schottky barrier at the electric contact interface, which is hardly tunable due to the Fermi level pinning effect. In this review, we will analyze the root causes of the contact problems for the two-dimensional semiconductor devices and summarize the strategies on the basis of different contact geometries, aiming to lift out the Fermi level pinning effect and achieve the ohmic contact. Moreover, the remarkable improvement of the device performance thanks to these optimized contacts will be emphasized. At the end, the merits and limitations of these strategies will be discussed as well, which potentially gives a guideline for handling the electric contact issues in two-dimensional semiconductors devices.

Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD-grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.

Two-Dimensional NbS2 Gas Sensors for Selective and Reversible NO2 Detection at Room Temperature

Transition metal dichalcogenides (TMDs) have attracted enormous attention in diverse research fields. Especially, gas sensors are considered in a promising application exploiting TMDs. However, the studies are confined to only major TMDs such as MoS₂ and WS₂. Particularly, the chemoresistive sensing properties of two-dimensional (2D) NbS₂ have never been explored. For the first time, we report room temperature NO₂ sensing characteristics of 2D NbS₂ nanosheets and the sensing mechanisms using first-principles calculations based on density functional theory. The results demonstrate that the NbS₂ edges possessing different configurations depending on synthetic conditions differ in the sensing ability of the TMD nanosheets. This study not only broadens the potential of 2D NbS₂ for gas sensing applications, but also presents the important role of edge configuration of TMDs depending on synthetic conditions for further studies.

High-Concentration Niobium-Substituted WS2 Basal Domains with Reconfigured Electronic Band Structure for Hydrogen Evolution Reaction

Extrinsically controlling the intrinsic activity and stability of two-dimensional (2D) semiconducting materials by substitutional doping is crucial for energy-related applications. However, an in situ transition-metal doping strategy for uniform and large-area chemical vapor deposited 2D semiconductors remains a formidable challenge. Here, we successfully synthesize highly uniform niobium-substituted tungsten disulfide (Nb-WS₂) monolayers, with a doping concentration of nearly 7% and sizes reaching 100 μm, through a metal dopant precursor route, using salt-catalyzed chemical vapor deposition (CVD). Our results reveal unusual effects in the structural, optical, electronic, and electrocatalysis characteristics of the Nb-WS₂ monolayer. The Nb dopants readily induce a band restructuring effect, providing the most active site with a hydrogen adsorption energy of 0.175 eV and hence greatly improving its hydrogen evolution activity_. The combined advantages of the unusual physics and chemistry by in situ CVD doping technique open the possibility in designing 2D-material-based electronics and catalysts of novel functionalities.