A library of atomically thin metal chalcogenides

Photocatalytic Applications of ReS2-Based Heterostructures

ReS2-based heterostructures, which involve the coupling of a narrow band-gap semiconductor ReS2 with other wide band-gap semiconductors, have shown promising performance in energy conversion and environmental pollution protection in recent years. This review focuses on the preparation methods, encompassing hydrothermal, chemical vapor deposition, and exfoliation techniques, as well as achievements in correlated applications of ReS2-based heterostructures, including type-I, type-II heterostructures, and Z-scheme heterostructures for hydrogen evolution, reduction of CO2, and degradation of pollutants. We believe that this review provides an overview of the most recent advances to guide further research and development of ReS2-based heterostructures for photocatalysis.

Preparation, properties and applications of two-dimensional superlattices

As a combination concept of a 2D material and a superlattice, two-dimensional superlattices (2DSs) have attracted increasing attention recently. The natural advantages of 2D materials in their properties, dimension, diversity and compatibility, and their gradually improved technologies for preparation and device fabrication serve as solid foundations for the development of 2DSs. Compared with the existing 2D materials and even their heterostructures, 2DSs relate to more materials and elaborate architectures, leading to novel systems with more degrees of freedom to modulate material properties at the nanoscale. Here, three typical types of 2DSs, including the component, strain-induced and moiré superlattices, are reviewed. The preparation methods, properties and state-of-the-art applications of each type are summarized. An outlook of the challenges and future developments is also presented. We hope that this work can provide a reference for the development of 2DS-related research.

A universal growth method for high-quality phase-engineered germanium chalcogenide nanosheets

Low-dimensional group IV-VI metal chalcogenide-based semiconductors hold great promise for opto-electronic device applications owing to their diverse crystalline phases and intriguing properties related to thermoelectric and ferroelectric effects. Herein, we demonstrate a universal chemical vapor deposition (CVD) growth method to synthesize stable germanium chalcogenide-based (GeS, GeS₂, GeSe, GeSe₂) nanosheets, which increases the library of the p-type semiconductor. The phase transition between different crystalline polytypes can be deterministically controlled by hydrogen concentration in the reaction chamber. Structural characterization and synthesis experiments identify the behavior, where the higher hydrogen concentration promotes the transiton from germanium dichalcogenides to germanium monochalcogenides. The angle-polarized and temperature-dependent Raman spectra demonstrate the strong interlayer coupling and lattice orientation. Based on the optimized growth scheme and systematic comparison of electrical properties, GeSe nanosheet photodetectors were demonstrated, which exhibit superior device performance on SiO₂/Si and HfO₂/Si substrate with a high photoresponsivity up to 10⁴ A W^-1, fast response time less than 15 ms, and high mobility of 3.2 cm² V^-1 s^-1, which is comparable to the mechanically exfoliated crystals. Our results manifest the hydrogen-mediated deposition strategy as a facile control knob to engineer crystalline phases of germanium chalcogenides for high performance optoelectronic devices.

Controllable Synthesis of 2D Materials by Electrochemical Exfoliation for Energy Storage and Conversion Application

2D materials have captured much recent research interest in a broad range of areas, including electronics, biology, sensors, energy storage, and others. In particular, preparing 2D nanosheets with high quality and high yield is crucial for the important applications in energy storage and conversion. Compared with other prevailing synthetic strategies, the electrochemical exfoliation of layered starting materials is regarded as one of the most promising and convenient methods for the large-scale production of uniform 2D nanosheets. Here, recent developments in electrochemical delamination are reviewed, including protocols, categories, principles, and operating conditions. State-of-the-art methods for obtaining 2D materials with small numbers of layers-including graphene, black phosphorene, transition metal dichalcogenides and MXene-are also summarized and discussed in detail. The applications of electrochemically exfoliated 2D materials in energy storage and conversion are systematically reviewed. Drawing upon current progress, perspectives on emerging trends, existing challenges, and future research directions of electrochemical delamination are also offered.

Fully Optical in Operando Investigation of Ambient Condition Electrical Switching in MoS2 Nanodevices

MoS₂ nanoswitches have shown superb ultralow switching energies without excessive leakage currents. However, the debate about the origin and volatility of electrical switching is unresolved due to the lack of adequate nanoimaging of devices in operando. Here, three optical techniques are combined to perform the first noninvasive in situ characterization of nanosized MoS₂ devices. This study reveals volatile threshold resistive switching due to the intercalation of metallic atoms from electrodes directly between Mo and S atoms, without the assistance of sulfur vacancies. A "semi-memristive" effect driven by an organic adlayer adjacent to MoS₂ is observed, which suggests that nonvolatility can be achieved by careful interface engineering. These findings provide a crucial understanding of nanoprocess in vertically biased MoS₂ nanosheets, which opens new routes to conscious engineering and optimization of 2D electronics.

Salt-Assisted Low-Temperature Growth of 2D Bi2 O2 Se with Controlled Thickness for Electronics

Bi₂ O₂ Se is the most promising 2D material due to its semiconducting feature and high mobility, making it propitious channel material for high-performance electronics that demands highly crystalline Bi₂ O₂ Se at low-growth temperature. Here, a low-temperature salt-assisted chemical vapor deposition approach for growing single-domain Bi₂ O₂ Se on a millimeter scale with thicknesses of multilayer to monolayer is presented. Because of the advantage of thickness-dependent growth, systematical scrutiny of layer-dependent Raman spectroscopy of Bi₂ O₂ Se from monolayer to bulk is investigated, revealing a redshift of the A_1g mode at 162.4 cm^-1 . Moreover, the long-term environmental stability of ≈2.4 nm thick Bi₂ O₂ Se is confirmed after exposing the sample for 1.5 years to air. The backgated field effect transistor (FET) based on a few-layered Bi₂ O₂ Se flake represents decent carrier mobility (≈287 cm²  V^-1 s^-1 ) and an ON/OFF ratio of up to 10⁷ . This report indicates a technique to grow large-domain thickness controlled Bi₂ O₂ Se single crystals for electronics.

Excessive Iodine Enabled Ultrathin Inorganic Perovskite Growth at the Liquid-Air Interface

The liquid-air interface offers a platform for the in-plane growth of free-standing materials. However, it is rarely used for inorganic perovskites and ultrathin non-layered perovskites. Herein the liquid-air interfacial synthesis of inorganic perovskite nanosheets (Cs₃ Bi₂ I₉ , Cs₃ Sb₂ I₉ ) is achieved simply by drop-casting the precursor solution with only the addition of iodine. The products are inaccessible without iodine addition. The thickness and lateral size of these nanosheets can be adjusted through the iodine concentration. The high volatility of the iodine spontaneously drives precursors that normally stay in the liquid to the liquid-air interface. The iodine also repairs in situ iodine vacancies during perovskite growth, giving enhanced optical and optoelectronic properties. The liquid-air interfacial growth of ultrathin perovskites provides multi-degree-of-freedom for constructing perovskite-based heterostructures and devices at atomic scale.

A general thermodynamics-triggered competitive growth model to guide the synthesis of two-dimensional nonlayered materials

Two-dimensional (2D) nonlayered materials have recently provoked a surge of interest due to their abundant species and attractive properties with promising applications in catalysis, nanoelectronics, and spintronics. However, their 2D anisotropic growth still faces considerable challenges and lacks systematic theoretical guidance. Here, we propose a general thermodynamics-triggered competitive growth (TTCG) model providing a multivariate quantitative criterion to predict and guide 2D nonlayered materials growth. Based on this model, we design a universal hydrate-assisted chemical vapor deposition strategy for the controllable synthesis of various 2D nonlayered transition metal oxides. Four unique phases of iron oxides with distinct topological structures have also been selectively grown. More importantly, ultra-thin oxides display high-temperature magnetic ordering and large coercivity. Mn_xFe_yCo_3-x-yO₄ alloy is also demonstrated to be a promising room-temperature magnetic semiconductor. Our work sheds light on the synthesis of 2D nonlayered materials and promotes their application for room-temperature spintronic devices.

Wearable chemical sensors based on 2D materials for healthcare applications

Chemical sensors worn on the body could make possible the continuous, noninvasive, and accurate monitoring of vital human signals, which is necessary for remote health monitoring and telemedicine. Attractive for creating high-performance, wearable chemical sensors are atomically thin materials with intriguing physical features, abundant chemistry, and high surface-to-volume ratios. These advantages allow for appropriate material-analyte interactions, resulting in a high level of sensitivity even at trace analyte concentrations. Previous review articles covered the material and device elements of 2D material-based wearable devices extensively. In contrast, little research has addressed the existing state, future outlook, and promise of 2D materials for wearable chemical sensors. We provide an overview of recent advances in 2D-material-based wearable chemical sensors to overcome this deficiency. The structure design, manufacturing techniques, and mechanisms of 2D material-based wearable chemical sensors will be evaluated, as well as their applicability in human health monitoring. Importantly, we present a thorough review of the current state of the art and the technological gaps that would enable the future design and nanomanufacturing of 2D materials and wearable chemical sensors. Finally, we explore the challenges and opportunities associated with designing and implementing 2D wearable chemical sensors.

Fast Fabrication of WS2/Bi2Se3 Heterostructures for High-Performance Photodetection

Two-dimensional (2D) material heterostructures have attracted considerable attention owing to their interesting and novel physical properties, which expand the possibilities for future optoelectronic, photovoltaic, and nanoelectronic applications. A portable, fast, and deterministic transfer technique is highly needed for the fabrication of heterostructures. Herein, we report a fast half-wet poly(dimethylsiloxane) (PDMS) transfer process utilizing the change of adhesion energy with the help of micron-sized water droplets. Using this method, a vertical stacking of the WS2/Bi2Se3 heterostructure with a straddling band configuration is successfully assembled on a fluorophlogopite substrate. Thanks to the complementary band gaps and high efficiency of interfacial charge transfer, the photodetector based on the heterostructure exhibits a superior responsivity of 109.9 A W-1 for a visible incident light at 473 nm and 26.7 A W-1 for a 1064 nm near-infrared illumination. Such high photoresponsivity of the heterostructure demonstrates that our transfer method not only owns time efficiency but also ensures high quality of the heterointerface. Our study may open new pathways to the fast and massive fabrication of various vertical 2D heterostructures for applications in twistronics/valleytronics and other band engineering devices.

Retrieving Grain Boundaries in 2D Materials

The coalescence of randomly distributed grains with different crystallographic orientations can result in pervasive grain boundaries (GBs) in 2D materials during their chemical synthesis. GBs not only are the inherent structural imperfection that causes influential impacts on structures and properties of 2D materials, but also have emerged as a platform for exploring unusual physics and functionalities stemming from dramatic changes in local atomic organization and even chemical makeup. Here, recent advances in studying the formation mechanism, atomic structures, and functional properties of GBs in a range of 2D materials are reviewed. By analyzing the growth mechanism and the competition between far-field strain and local chemical energies of dislocation cores, a complete understanding of the rich GB morphologies as well as their dependence on lattice misorientations and chemical compositions is presented. Mechanical, electronic, and chemical properties tied to GBs in different materials are then discussed, towards raising the concept of using GBs as a robust atomic-scale scaffold for realizing tailored functionalities, such as magnetism, luminescence, and catalysis. Finally, the future opportunities in retrieving GBs for making functional devices and the major challenges in the controlled formation of GB structures for designed applications are commented.

Tunability of the Superconductivity of NbSe2 Films Grown by Two-Step Vapor Deposition

Layered metallic transition-metal dichalcogenides (TMDCs) are ideal platforms for exploring their fascinating electronic properties at two-dimensional limits, such as their charge density wave (CDW) and superconductivity. Therefore, developing ways to improve the crystallization quality of TMDCs is urgently needed. Here we report superconductively tunable NbSe₂ grown by a two-step vapor deposition method. By optimizing the sputtering conditions, superconducting NbSe₂ films were prepared from highly crystalline Nb films. The bilayer NbSe₂ films showed a superconducting transition temperature that was up to 3.1 K. Similar to the salt-assisted chemical vapor deposition (CVD) method, superconducting monolayer NbSe₂ crystals were also grown from a selenide precursor, and the growth strategy is suitable for many other TMDCs. Our growth method not only provides a way to improve the crystalline quality of TMDC films, but also gives new insight into the growth of monolayer TMDCs. It holds promise for exploring two-dimensional TMDCs in fundamental research and device applications.

General low-temperature growth of two-dimensional nanosheets from layered and nonlayered materials

Most of the current methods for the synthesis of two-dimensional materials (2DMs) require temperatures not compatible with traditional back-end-of-line (BEOL) processes in semiconductor industry (450 °C). Here, we report a general BiOCl-assisted chemical vapor deposition (CVD) approach for the low-temperature synthesis of 27 ultrathin 2DMs. In particular, by mixing BiOCl with selected metal powders to produce volatile intermediates, we show that ultrathin 2DMs can be produced at 280-500 °C, which are ~200-300 °C lower than the temperatures required for salt-assisted CVD processes. In-depth characterizations and theoretical calculations reveal the low-temperature processes promoting 2D growth and the oxygen-inhibited synthetic mechanism ensuring the formation of ultrathin nonlayered 2DMs. We demonstrate that the resulting 2DMs exhibit electrical, magnetic and optoelectronic properties comparable to those of 2DMs grown at much higher temperatures. The general low-temperature preparation of ultrathin 2DMs defines a rich material platform for exploring exotic physics and facile BEOL integration in semiconductor industry.

Emerging MoS2 Wafer-Scale Technique for Integrated Circuits

As an outstanding representative of layered materials, molybdenum disulfide (MoS2) has excellent physical properties, such as high carrier mobility, stability, and abundance on earth. Moreover, its reasonable band gap and microelectronic compatible fabrication characteristics makes it the most promising candidate in future advanced integrated circuits such as logical electronics, flexible electronics, and focal-plane photodetector. However, to realize the all-aspects application of MoS2, the research on obtaining high-quality and large-area films need to be continuously explored to promote its industrialization. Although the MoS2 grain size has already improved from several micrometers to sub-millimeters, the high-quality growth of wafer-scale MoS2 is still of great challenge. Herein, this review mainly focuses on the evolution of MoS2 by including chemical vapor deposition, metal-organic chemical vapor deposition, physical vapor deposition, and thermal conversion technology methods. The state-of-the-art research on the growth and optimization mechanism, including nucleation, orientation, grain, and defect engineering, is systematically summarized. Then, this review summarizes the wafer-scale application of MoS2 in a transistor, inverter, electronics, and photodetectors. Finally, the current challenges and future perspectives are outlined for the wafer-scale growth and application of MoS2.

Machine Learning-Assisted Synthesis of Two-Dimensional Materials

Two-dimensional (2D) materials have intriguing physical and chemical properties, which exhibit promising applications in the fields of electronics, optoelectronics, as well as energy storage. However, the controllable synthesis of 2D materials is highly desirable but remains challenging. Machine learning (ML) facilitates the development of insights and discoveries from a large amount of data in a short time for the materials synthesis, which can significantly reduce the computational costs and shorten the development cycles. Based on this, taking the 2D material MoS₂ as an example, the parameters of successfully synthesized materials by chemical vapor deposition (CVD) were explored through four ML algorithms: XGBoost, Support Vector Machine (SVM), Naïve Bayes (NB), and Multilayer Perceptron (MLP). Recall, specificity, accuracy, and other metrics were used to assess the performance of these four models. By comparison, XGBoost was the best performing model among all the models, with an average prediction accuracy of over 88% and a high area under the receiver operating characteristic (AUROC) reaching 0.91. And these findings showed that the reaction temperature (T) had a crucial influence on the growth of MoS₂. Furthermore, the importance of the features in the growth mechanism of MoS₂ was optimized, such as the reaction temperature (T), Ar gas flow rate (R_f), reaction time (t), and so on. The results demonstrated that ML assisted materials preparation can significantly minimize the time spent on exploration and trial-and-error, which provided perspectives in the preparation of 2D materials.

Edge Reconstruction-Dependent Growth Kinetics of MoS2

Understanding the growth mechanisms of multielement two-dimensional (2D) crystals is challenging because of the unbalanced stoichiometry and possible reconstruction of their edges. Here, we present a systematic theoretical study on the chemical vapor deposition (CVD) growth mechanism of MoS₂. We found that the growth kinetics of MoS₂ highly depends on its edge reconstruction determined by concentrations of Mo and S in the growth environment. Based on the calculated energies of nucleation and propagation of various MoS₂ edges, we predicted the transition of a MoS₂ island growth from a regime of a triangle enclosed by Mo-terminated zigzag edges that are passivated by 50% S (Mo-II edges), to a regime of continuous evolution within a triangle, hexagon, and inverted triangle with 75%-S-terminated edges (S-III edges) and Mo-II edges, and finally to a regime of triangles with Mo-terminated zigzag edges that are passivated by 100% S (Mo-III edges) by tuning the growth condition from Mo-rich to S-rich, which provides a reasonable explanation to many experimental observations. This study provides a general guideline on theoretical studies of 2D crystals' growth mechanisms, deepens our understanding on the growth mechanism of multielement 2D crystals, and is beneficial for the controllable synthesis of various 2D crystals.

Epitaxial Growth of High-Quality Monolayer MoS2 Single Crystals on Low-Symmetry Vicinal Au(101) Facets with Different Miller Indices

Epitaxial growth of wafer-scale monolayer semiconducting transition metal dichalcogenide single crystals is essential for advancing their applications in next-generation transistors and highly integrated circuits. Several efforts have been made for the growth of monolayer MoS₂ single crystals on high-symmetry Au(111) and sapphire substrates, while more prototype growth systems still need to be discovered for clarifying the internal mechanisms. Herein, we report the epitaxial growth of unidirectionally aligned monolayer MoS₂ domains and single-crystal films on low-symmetry Au(101) vicinal facets via a facile chemical vapor deposition method. On-site scanning tunneling microscopy observations reveal the formation of a specific rectangular Moiré pattern along the [101̅] step edge of Au(101) and along its perpendicular direction. The perfect lattice constant matching of MoS₂/Au(101) along the substrate high-symmetry directions (i.e., Au[101̅], Au [010]) as well as the step-edge-guiding effect are proposed to facilitate the robust epitaxy. Multiscale characterizations further confirm the domain-boundary-free feature of the monolayer MoS₂ films merged by unidirectionally aligned monolayer domains. This work hereby puts forward a symmetry mismatched epitaxial system for the direct synthesis of monolayer MoS₂ single crystals, which should deepen our understanding about the epitaxy of 2D layered materials and propel their applications in various fields.

New Approaches to Produce Large-Area Single Crystal Thin Films

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

Modifying the Power and Performance of 2-Dimensional MoS2 Field Effect Transistors

Over the past 60 years, the semiconductor industry has been the core driver for the development of information technology, contributing to the birth of integrated circuits, Internet, artificial intelligence, and Internet of Things. Semiconductor technology has been evolving in structure and material with co-optimization of performance-power-area-cost until the state-of-the-art sub-5-nm node. Two-dimensional (2D) semiconductors are recognized by the industry and academia as a hopeful solution to break through the quantum confinement for the future technology nodes. In the recent 10 years, the key issues on 2D semiconductors regarding material, processing, and integration have been overcome in sequence, making 2D semiconductors already on the verge of application. In this paper, the evolution of transistors is reviewed by outlining the potential of 2D semiconductors as a technological option beyond the scaled metal oxide semiconductor field-effect transistors. We mainly focus on the optimization strategies of mobility (μ), equivalent oxide thickness (EOT), and contact resistance (R_C ), which enables high ON current (I_on ) with reduced driving voltage (V_dd ). Finally, we prospect the semiconductor technology roadmap by summarizing the technological development of 2D semiconductors over the past decade.

Modified Spatially Confined Strategy Enabled Mild Growth Kinetics for Facile Growth Management of Atomically-Thin Tungsten Disulfides

Chemical vapor deposition (CVD) has been widely used to produce high quality 2D transitional metal dichalcogenides (2D TMDCs). However, violent evaporation and large diffusivity discrepancy of metal and chalcogen precursors at elevated temperatures often result in poor regulation on X:M molar ratio (M = Mo, W etc.; X = S, Se, and Te), and thus it is rather challenging to achieve the desired products of 2D TMDCs. Here, a modified spatially confined strategy (MSCS) is utilized to suppress the rising S vapor concentration between two aspectant substrates, upon which the lateral/vertical growth of 2D WS2 can be selectively regulated via proper S:W zones correspond to greatly broadened time/growth windows. An S:W-time (SW-T) growth diagram was thus proposed as a mapping guide for the general understanding of CVD growth of 2D WS2 and the design of growth routes for the desired 2D WS2 . Consequently, a comprehensive growth management of atomically thin WS2 is achieved, including the versatile controls of domain size, layer number, and lateral/vertical heterostructures (MoS2 -WS2 ). The lateral heterostructures show an enhanced hydrogen evolution reaction performance. This study advances the substantial understanding to the growth kinetics and provides an effective MSCS protocol for growth design and management of 2D TMDCs.

Salt-Assisted Selective Growth of H-phase Monolayer VSe2 with Apparent Hole Transport Behavior

Vanadium diselenide (VSe₂) exhibits versatile electronic and magnetic properties in the trigonal prismatic (H-) and octahedral (T-) phases. Compared to the metallic T-phase, the H-phase with a tunable semiconductor property is predicted to be a ferrovalley material with spontaneous valley polarization. Herein we report an epitaxial growth of the monolayer 2D VSe₂ on a mica substrate via the chemical vapor deposition (CVD) method by introducing salt in the precursor. Our first-principles calculations suggest that the monolayer H-phase VSe₂ with a large lateral size is thermodynamically favorable. The honeycomb-like structure and the broken symmetry are directly observed by spherical aberration-corrected scanning transmission electron microscopy (STEM) and confirmed by giant second harmonic generation (SHG) intensity. The p-type transport behavior is further evidenced by the temperature-dependent resistance and field-effect device study. The present work introduces a new phase-stable 2D transition metal dichalcogenide, opening the prospect of novel electronic and spintronics device design.

Design Guidelines for Two-Dimensional Transition Metal Dichalcogenide Alloys

Two-dimensional (2D) materials and transition metal dichalcogenides (TMD) in particular are at the forefront of nanotechnology. To tailor their properties for engineering applications, alloying strategies-used successfully for bulk metals in the last century-need to be extended to this novel class of materials. Here we present a systematic analysis of the phase behavior of substitutional 2D alloys in the TMD family on both the metal and the chalcogenide site. The phase behavior is quantified in terms of a metastability metric and benchmarked against systematic computational screening of configurational energy landscapes from First-Principles. The resulting Pettifor maps can be used to identify broad trends across chemical spaces and as starting point for setting up rational search strategies in phase space, thus allowing for targeted computational analysis of properties on likely thermodynamically stable compounds. The results presented here also constitute a useful guideline for synthesis of binary metal 2D TMDs alloys via a range of synthesis techniques.

Zero-Dimensional Cs3BiX6 (X = Br, Cl) Single Crystal Films with Second Harmonic Generation

The development of atomically thin single crystal films is necessary to potential applications in the 2D semiconductor field, and it is significant to explore new physical properties in low-dimensional semiconductors. Since, zero-dimensional (0D) materials without natural layering are connected by strong chemical bonds, it is challengeable to break symmetry and grow 0D Cs₃BiX₆ (X = Br, Cl) single crystal thin films. Here, we report the successful growth of 0D Cs₃BiX₆ (X = Br, Cl) single crystal films using a solvent evaporation crystallization strategy. Their phases and structures are both well evaluated to confirm 0D Cs₃BiX₆ (X = Br, Cl) single crystal films. Remarkably, the chemical potential dependent morphology evolution phenomenon is observed. It gives rise to morphology changes of Cs₃BiBr₆ films from rhombus to hexagon as BiBr₃ concentration increased. Additionally, the robust second harmonic generation signal is detected in the Cs₃BiBr₆ single crystal film, demonstrating the broken symmetry originated from decreased dimension or shape change.

Bipolar Electrochemistry - A Powerful Tool for Micro/Nano-Electrochemistry

The understanding of areas for "classical" electrochemistry (including catalysis, electrolysis and sensing) and bio-electrochemistry at the micro/nanoscale are focus on the continued performance facilitations or the exploration of new features. In the recent 20 years, a different mode for driving electrochemistry has been proposed, which is called as bipolar electrochemistry (BPE). BPE has garnered attention owing to the interesting properties: (i) its wireless nature facilitates electrochemical sensing and high throughput analysis; (ii) the gradient potential distribution on the electrodes surface is a useful tool for preparing gradient surfaces and materials. These permit BPE to be used for modification and analytical applications on a micro/nanoscale surface. This review aims to introduce the principle and classification of BPE and BPE at micro/nanoscale; sort out its applications in electrocatalysis, electrosynthesis, electrophoresis, power supply and so on; explain the confined BPE and summarize its analytical application for single entities (single cells, single particles and single molecules), and discuss finally the important direction of micro/nanoscale BPE.

Looking Beyond 0 and 1: Principles and Technology of Multi-Valued Logic Devices

Ever since the invention of solid-state transistors, binary devices have dominated the electronics industry. Although the binary technology links the natural property of devices to be in the ON or OFF state with two logic levels, it provides the least possible information content per interconnect. Multi-valued logic (MVL) has long been considered as a means of improving the computation efficiency and reducing the power consumption of modern chips. In view of the power density limits of the conventional complementary metal-oxide-semiconductor technology, MVL technologies have recently gained even more attention, and various MVL unit devices based on conventional and emerging materials have been proposed. Herein, the recent achievements toward the development of compact MVL unit devices are reviewed. First, basic principles of MVL technologies are introduced by describing methods of obtaining multiple logic states and discussing radix-related aspects of MVL computation. Next, MVL unit devices are classified and overviewed with emphasis on principles of operation, technologies, and applications. Finally, a comparative discussion of strengths and weaknesses is provided for each class of MVL devices, and the review concludes with the outlook for the MVL field.

The Road for 2D Semiconductors in the Silicon Age

Continued reduction in transistor size can improve the performance of silicon integrated circuits (ICs). However, as Moore's law approaches physical limits, high-performance growth in silicon ICs becomes unsustainable, due to challenges of scaling, energy efficiency, and memory limitations. The ultrathin layers, diverse band structures, unique electronic properties, and silicon-compatible processes of 2D materials create the potential to consistently drive advanced performance in ICs. Here, the potential of fusing 2D materials with silicon ICs to minimize the challenges in silicon ICs, and to create technologies beyond the von Neumann architecture, is presented, and the killer applications for 2D materials in logic and memory devices to ease scaling, energy efficiency bottlenecks, and memory dilemmas encountered in silicon ICs are discussed. The fusion of 2D materials allows the creation of all-in-one perception, memory, and computation technologies beyond the von Neumann architecture to enhance system efficiency and remove computing power bottlenecks. Progress on the 2D ICs demonstration is summarized, as well as the technical hurdles it faces in terms of wafer-scale heterostructure growth, transfer, and compatible integration with silicon ICs. Finally, the promising pathways and obstacles to the technological advances in ICs due to the integration of 2D materials with silicon are presented.

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

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

Integrated Memory Devices Based on 2D Materials

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D-material-based memory-device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain-inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state-of-the-art memory devices made from 2D materials, as well as their implications for brain-inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Uniform-embeddable-distributed Ni3S2 cocatalyst inside and outside a sheet-like ZnIn2S4 photocatalyst for boosting photocatalytic hydrogen evolution

The rational cocatalyst design is considered a significant route to boost the solar-energy conversion efficiency for photocatalytic H₂ generation. However, the traditional cocatalyst-loading on the surface of a photocatalyst easily leads to scarce exposed active sites induced by the agglomeration of cocatalysts and a hindrance of the light absorption of the photocatalyst, thus significantly limiting the enhancement of the photocatalytic H₂-generation performance. Herein, a new concept of uniform-embeddable-distributed cocatalysts is put forward. Consequently, uniform-embeddable-distributed cocatalysts of Ni₃S₂ were designed and constructed inside and outside of the nanosheet-like ZnIn₂S₄ photocatalyst to form a Ni₃S₂/ZnIn₂S₄ binary system (UEDNiS/ZIS). The unique uniform-embeddable-distributed Ni₃S₂ cocatalyst (UEDNiS) could provide abundant exposed active sites, facilitate the spatial separation and ordered transfer of charges inside and outside of ZnIn₂S₄ nanosheets, and reduce the hydrogen-adsorption free energy for photocatalytic H₂-generation reactions. As a result, UEDNiS/ZIS exhibited a high photocatalytic H₂-generation rate of 60 μmol h^-1 under visible-light irradiation, almost 7.8 and 2.8 times higher than pristine ZnIn₂S₄ and the traditional surface-loaded Ni₃S₂/ZnIn₂S₄ (TSLNiS/ZIS), respectively. This work represents a new cocatalyst-design approach to realize high-efficiency hydrogen evolution in binary heterostructured photocatalytic systems.

Emerging laser-assisted vacuum processes for ultra-precision, high-yield manufacturing

Laser technology is a cutting-edge process with a unique photothermal response, precise site selectivity, and remote controllability. Laser technology has recently emerged as a novel tool in the semiconductor, display, and thin film industries by providing additional capabilities to existing high-vacuum equipment. The in situ and in operando laser assistance enables using multiple process environments with a level of complexity unachievable with conventional vacuum equipment. This broadens the usable range of process parameters and directly improves material properties, product precision, and device performance. This review paper examines the recent research trends in laser-assisted vacuum processes (LAVPs) as a vital tool for innovation in next-generation manufacturing processing equipment and addresses the unique characteristics and mechanisms of lasers exclusively used in each study. All the findings suggest that the LAVP can lead to methodological breakthroughs in dry etching, 2D material synthesis, and chemical vapor deposition for optoelectronic devices.

Van der Waals-Interface-Dominated All-2D Electronics

The interface is the device. As the feature size rapidly shrinks, silicon-based electronic devices are facing multiple challenges of material performance decrease and interface quality degradation. Ultrathin 2D materials are considered as potential candidates in future electronics by their atomically flat surfaces and excellent immunity to short-channel effects. Moreover, due to naturally terminated surfaces and weak van der Waals (vdW) interactions between layers, 2D materials can be freely stacked without the lattice matching limit to form high-quality heterostructure interfaces with arbitrary components and twist angles. Controlled interlayer band alignment and optimized interfacial carrier behavior allow all-2D electronics based on 2D vdW interfaces to exhibit more comprehensive functionality and better performance. Especially, achieving the same computing capacity of multiple conventional devices with small footprint all-2D devices is considered to be the key development direction of future electronics. Herein, the unique properties of all-2D vdW interfaces and their construction methods are systematically reviewed and the main performance contributions of different vdW interfaces in 2D electronics are summarized, respectively. Finally, the recent progress and challenges for all-2D vdW electronics are discussed, and how to improve the compatibility of 2D material devices with silicon-based industrial technology is pointed out as a critical challenge.

Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors

Tuning the charge transport properties of two-dimensional transition metal dichalcogenides (TMDs) is pivotal to their future device integration in post-silicon technologies. To date, co-doping of TMDs during growth still proves to be challenging, and the synthesis of doped WSe2, an otherwise ambipolar material, has been mainly limited to p-doping. Here, we demonstrate the synthesis of high-quality n-type monolayered WSe2 flakes using a solid-state precursor for Se, zinc selenide. n-Type transport has been reported with prime electron mobilities of up to 10 cm2 V-1 s-1. We also demonstrate the tuneability of doping to p-type transport with hole mobilities of 50 cm2 V-1 s-1 after annealing in air. n-Doping has been attributed to the presence of Zn adatoms on the WSe2 flakes as revealed by X-ray photoelectron spectroscopy (XPS), spatially resolved time of flight secondary ion mass spectroscopy (SIMS) and angular dark-field scanning transmission electron microscopy (AD-STEM) characterization of WSe2 flakes. Monolayer WSe2 flakes exhibit a sharp photoluminescence (PL) peak at room temperature and highly uniform emission across the entire flake area, indicating a high degree of crystallinity of the material. This work provides new insight into the synthesis of TMDs with charge carrier control, to pave the way towards post-silicon electronics.

Defining shapes of two-dimensional crystals with undefinable edge energies

The equilibrium shape of crystals is a fundamental property of both aesthetic appeal and practical importance: the shape and its facets control the catalytic, light-emitting, sensing, magnetic and plasmonic behaviors. It is also a visible macro-manifestation of the underlying atomic-scale forces and chemical makeup, most conspicuous in two-dimensional (2D) materials of keen current interest. If the crystal surface/edge energy is known for different directions, its shape can be obtained by the geometric Wulff construction, a tenet of crystal physics; however, if symmetry is lacking, the crystal edge energy cannot be defined or calculated and thus its shape becomes elusive, presenting an insurmountable problem for theory. Here we show how one can proceed with auxiliary edge energies towards a constructive prediction, through well-planned computations, of a unique crystal shape. We demonstrate it for challenging materials such as SnSe, which is of C2v symmetry, and even AgNO2 of C1, which has no symmetry at all.

Interface Influence on the Photoelectric Performance of Transition Metal Dichalcogenide Lateral Heterojunctions

The ultrathin feature of two-dimensional (2D) transition metal dichalcogenides (TMDs) has brought special performance in electronic and optoelectronic fields. When vertical and lateral heterojunctions are made using different TMD combinations, the original properties of premier TMDs can be optimized. Especially for lateral heterojunctions, their sharp interface signifies a narrow space charge region, leading to a strong in-plane built-in electric field, which may contribute to high separation efficiency of photogenerated carriers, good rectification behavior, self-powered photoelectric device construction, etc. However, due to the poor controllability over the synthesis process, obtaining a clean and sharp interface of the lateral heterojunction is still a challenge. Herein, we propose a simple chemical vapor deposition (CVD) method, which can effectively separate the growth process of different TMDs, thus resulting in good regulation of the composition change at the junction region. By this method, MoS₂-WS₂ lateral heterojunctions with sharp interfaces have been obtained with good rectification characteristics, ∼10⁵ on/off ratio, 1874% external quantum efficiency, and ∼120 ms photoresponse speed, exhibiting a better photoelectric performance than that of the lateral ones with graded junctions.

Recent advances in metallic transition metal dichalcogenides as electrocatalysts for hydrogen evolution reaction

Layered metallic transition metal dichalcogenides (MTMDs) exhibit distinctive electrical and catalytic properties to drive basal plane activity, and, therefore, they have emerged as promising alternative electrocatalysts for sustainable hydrogen evolution reactions (HERs). A key challenge for realizing MTMDs-based electrocatalysts is the controllable and scalable synthesis of high-quality MTMDs and the development of engineering strategies that allow tuning their electronic structures. However, the lack of a method for the direct synthesis of MTMDs retaining the structural stability limits optimizing the structural design for the next generation of robust electrocatalysts. In this review, we highlight recent advances in the synthesis of MTMDs comprising groups VB and VIB and various routes for structural engineering to enhance the HER catalytic performance. Furthermore, we provide insight into the potential future directions and the development of MTMDs with high durability as electrocatalysts to generate green hydrogen through water-splitting technology.

Periodic Spiral Ripples on VS2 Flakes: A Tip-Enhanced Raman Investigation

Using atmospheric-pressure chemical vapor deposition, we have synthesized vanadium disulfide (VS₂) flakes with a metallic 1T phase that display nanoscale spiral surface ripples. To understand the origin of these chiral patterns in these transition metal dichalcogenides, tip-enhanced Raman spectroscopy and Kelvin probe force microscopies were jointly used to investigate their crystal structure, possible oxidation, and electronic properties, respectively. We found that the surface corrugation consists of small crystalline domains with distinct orientations. The change in local orientation is observed concomitantly with a spectral shift of the lattice modes of VS₂ and results in the formation of grain boundaries between the domains with distinct orientation. Additionally, the periodic surface structure is modulating the work function of VS₂ by 14 meV.

Two-Dimensional Wedge-Shaped Magnetic EuS: Insight into the Substrate Step-Guided Epitaxial Synthesis on Sapphire

Rare earth chalcogenides (RECs) with novel luminescence and magnetic properties offer fascinating opportunities for fundamental research and applications. However, controllable synthesis of RECs down to the two-dimensional (2D) limit still has a great challenge. Herein, 2D wedge-shaped ferromagnetic EuS single crystals are successfully synthesized via a facile molten-salt-assisted chemical vapor deposition method on sapphire. Based on the theoretical simulations and experimental measurements, the mechanisms of aligned growth and wedge-shaped growth are systematically proposed. The wedge-shaped growth is driven by a dual-interaction mechanism, where the coupling between EuS and the substrate steps impedes the lateral growth, and the strong bonding of nonlayered EuS itself facilitates the vertical growth. Through temperature-dependent Raman and photoluminescence characterization, the nanoflakes show a large Raman temperature coefficient of -0.030 cm^-1 K^-1 and uncommon increasing band gap with temperature. More importantly, by low-temperature magnetic force microscopy characterization, thickness variation of the magnetic signal is revealed within one sample, indicating the great potential of the wedge-shaped nanoflake to serve as a platform for highly efficient investigation of thickness-dependent magnetic properties. This work sheds new light on 2D RECs and will offer a deep understanding of 2D wedge-shaped materials.

Wafer-scale MoS2 with water-vapor assisted showerhead MOCVD

Among numerous thin film synthesis methods, metalorganic chemical vapor deposition performed in a showerhead reactor is the most promising one for broad use in scalable and commercially adaptable two-dimensional material synthesis processes. Adapting the most efficient monolayer growth methodologies from tube-furnace systems to vertical-showerhead geometries allows us to overcome the intrinsic process limitations and improve the overall monolayer yield quality. Here, we demonstrate large-area, monolayer molybdenum disulphide growth by combining gas-phase precursor supply with unique tube-furnace approaches of utilizing sodium molybdate pre-seeding solution spincoated on a substrate along with water vapor added during the growth step. The engineered process yields a high-quality, 4-inch scale monolayer film on sapphire wafers. The monolayer growth coverage, average crystal size and defect density were evaluated using Raman and photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and scanning transmission electron microscopy imaging. Our findings provide a direct step forward toward developing a reproducible and large-scale MoS₂ synthesis with commercial showerhead reactors.

SERS enhancement induced by the Se vacancy defects in ultra-thin hybrid phase SnSex nanosheets

Improving the photo-induced charge transfer (PICT) efficiency by adjusting the energy levels difference between adsorbed probe molecules and substrate materials is a key factor for boosting the surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM). Herein, a new route to improve the SERS activity of two-dimensional (2D) selenium and tin compounds (SnSe_x, 1 ≤ x ≤ 2) by the hybrid phase materials is researched. The physical properties and the energy band structure of SnSe_x were analyzed. The enhanced SERS activity of 2D SnSe_x can be attribute to the coupling of the PICT resonance caused by the defect energy levels induced by Se vacancy and the molecular resonance Raman scattering (RRS). This established a relationship between the physical properties and SERS activity of 2D layered materials. The resonance probe molecule, rhodamine (R6G), which is used to detect the SERS performance of SnSe_x nanosheets. The enhancement factor (EF) of R6G on the optimized SnSe_1.35 nanosheets can be as high as 2.6 × 10⁶, with a detection limit of 10^-10 M. The SERS result of the environmental pollution, thiram, shows that the SnSe_x nanosheets have a practical application in trace SERS detection, without the participation of metal particles. These results demonstrate that, through hybrid phase materials, the SERS sensitivity of 2D layered nanomaterials can be improved. It provides a kind of foreground non-metal SERS substrate in monitoring or detecting and provide a deep insight into the chemical SERS mechanism based on 2D layered materials.

Giant Enhancement and Directional Second Harmonic Emission from Monolayer WS2 on Silicon Substrate via Fabry-Pérot Micro-Cavity

Two-dimensional transition metal dichalcogenides (TMDs) possess large second-order optical nonlinearity, making them ideal candidates for miniaturized on-chip frequency conversion devices, all-optical interconnection, and optoelectronic integration components. However, limited by subnanometer thickness, the monolayer TMD exhibits low second harmonic generation (SHG) conversion efficiency (<0.1%) and poor directionality, which hinders their practical applications. Herein, we proposed a Fabry-Pérot (F-P) cavity formed by coupling an atomically thin WS₂ film with a silicon hole matrix to enhance the SH emission. A maximum enhancement (∼1580 times) is achieved by tuning the excitation wavelength to be resonant with the microcavity modes. The giant enhancement is attributed to the strong electric field enhancement in the F-P cavity and the oscillator strength enhancement of excitons from suspended WS₂. Moreover, directional SH emission (divergence angle ∼5°) is obtained benefiting from the resonance of the F-P microcavity. Our research results can provide a practical sketch to develop both high-efficiency and directional nonlinear optical devices for silicon-based on-chip integration optics.

Inkjet printing of two-dimensional van der Waals materials: a new route towards emerging electronic device applications

Two-dimensional (2D) van der Waals (vdW) materials are considered one of the most promising candidates to realize emerging electrical applications. Although until recently, much effort has been dedicated to demonstrating high-performance single 2D vdW devices, associated with rapid progress in 2D vdW materials, demands for their large-scale practical applications have noticeably increased from a manufacturing perspective. Drop-on-demand inkjet printing can be the most feasible solution by exploiting the advantages of layered 2D contacts and advanced 2D vdW ink formulations. This review presents recent achievements in inkjet-printed 2D vdW material-based device applications. A brief introduction to 2D vdW materials and inkjet printing principles, followed by various ink formulation methods, is first presented. Then, the state-of-the-art inkjet-printed 2D vdW device applications and their remaining technical issues are highlighted. Finally, prospects and challenges to be overcome to demonstrate fully inkjet-printed, high-performance 2D vdW devices are also discussed.

Synergy Effect of High-Stability of VS4 Nanorods for Sodium Ion Battery

Sodium-ion batteries (SIBs) have attracted increasing interest as promising candidates for large-scale energy storage due to their low cost, natural abundance and similar chemical intercalation mechanism with lithium-ion batteries. However, achieving superior rate capability and long-life for SIBs remains a major challenge owing to the limitation of favorable anode materials selection. Herein, an elegant one-step solvothermal method was used to synthesize VS₄ nanorods and VS₄ nanorods/reduced graphene oxide (RGO) nanocomposites. The effects of ethylene carbonate/diethyl carbonate(EC/DEC), ethylene carbonate/dimethyl carbonate(EC/DMC), and tetraethylene glycol dimethyl ether (TEGDME) electrolytes on the electrochemical properties of VS₄ nanorods were investigated. The VS₄ nanorods electrodes exhibit high specific capacity in EC/DMC electrolytes. A theoretical calculation confirms the advance of EC/DMC electrolytes for VS₄ nanorods. Significantly, the discharge capacity of VS₄/RGO nanocomposites remains 100 mAh/g after 2000 cycles at a large current density of 2 A/g, indicating their excellent cycling stability. The nanocomposites can improve the electronic conductivity and reduce the Na⁺ diffusion energy barrier, thereby effectively improving the sodium storage performance of the hybrid material. This work offers great potential for exploring promising anode materials for electrochemical applications.

Facilitating Uniform Large-Scale MoS2, WS2 Monolayers, and Their Heterostructures through van der Waals Epitaxy

The fabrication process for the uniform large-scale MoS₂, WS₂ transition-metal dichalcogenides (TMDCs) monolayers, and their heterostructures has been developed by van der Waals epitaxy (VdWE) through the reaction of MoCl₅ or WCl₆ precursors and the reactive gas H₂S to form MoS₂ or WS₂ monolayers, respectively. The heterostructures of MoS₂/WS₂ or WS₂/MoS₂ can be easily achieved by changing the precursor from WCl₆ to MoCl₅ once the WS₂ monolayer has been fabricated or switching the precursor from MoCl₅ to WCl₆ after the MoS₂ monolayer has been deposited on the substrate. These VdWE-grown MoS₂, WS₂ monolayers, and their heterostructures have been successfully deposited on Si wafers with 300 nm SiO₂ coating (300 nm SiO₂/Si), quartz glass, fused silica, and sapphire substrates using the protocol that we have developed. We have characterized these TMDCs materials with a range of tools/techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), micro-Raman analysis, photoluminescence (PL), atomic force microscopy (AFM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and selected-area electron diffraction (SAED). The band alignment and large-scale uniformity of MoS₂/WS₂ heterostructures have also been evaluated with PL spectroscopy. This process and resulting large-scale MoS₂, WS₂ monolayers, and their heterostructures have demonstrated promising solutions for the applications in next-generation nanoelectronics, nanophotonics, and quantum technology.

Ultrathin ferrite nanosheets for room-temperature two-dimensional magnetic semiconductors

The discovery of magnetism in ultrathin crystals opens up opportunities to explore new physics and to develop next-generation spintronic devices. Nevertheless, two-dimensional magnetic semiconductors with Curie temperatures higher than room temperature have rarely been reported. Ferrites with strongly correlated d-orbital electrons may be alternative candidates offering two-dimensional high-temperature magnetic ordering. This prospect is, however, hindered by their inherent three-dimensional bonded nature. Here, we develop a confined-van der Waals epitaxial approach to synthesizing air-stable semiconducting cobalt ferrite nanosheets with thickness down to one unit cell using a facile chemical vapor deposition process. The hard magnetic behavior and magnetic domain evolution are demonstrated by means of vibrating sample magnetometry, magnetic force microscopy and magneto-optical Kerr effect measurements, which shows high Curie temperature above 390 K and strong dimensionality effect. The addition of room-temperature magnetic semiconductors to two-dimensional material family provides possibilities for numerous novel applications in computing, sensing and information storage.

Heterodimensional superlattice with in-plane anomalous Hall effect

Superlattices-a periodic stacking of two-dimensional layers of two or more materials-provide a versatile scheme for engineering materials with tailored properties1,2. Here we report an intrinsic heterodimensional superlattice consisting of alternating layers of two-dimensional vanadium disulfide (VS2) and a one-dimensional vanadium sulfide (VS) chain array, deposited directly by chemical vapour deposition. This unique superlattice features an unconventional 1T stacking with a monoclinic unit cell of VS2/VS layers identified by scanning transmission electron microscopy. An unexpected Hall effect, persisting up to 380 kelvin, is observed when the magnetic field is in-plane, a condition under which the Hall effect usually vanishes. The observation of this effect is supported by theoretical calculations, and can be attributed to an unconventional anomalous Hall effect owing to an out-of-plane Berry curvature induced by an in-plane magnetic field, which is related to the one-dimensional VS chain. Our work expands the conventional understanding of superlattices and will stimulate the synthesis of more extraordinary superstructures.

NaCl Micro-Crystal as a Molecular Mold for Enhanced Synthesis of Planar Phenazines and Their Applications on Chemosensing and a Full-Color Fluorescent Material

NaCl has been successfully used as a template for the synthesis of 2D nanomaterials, but it is seldom used for the construction of flat small organic molecules. Herein, a simple, low-cost, and highly efficient synthesis of phenazines with planar main frames, such as 5-phenyl-5,14-dihydro-5,7,12,14-tetraazapentacene, in the presence of NaCl micro-crystal as a kind of molecular mold is described. The reactants were mixed with NaCl powder and heated to 320 °C for 5 min. Yields >90% were readily achieved after a simple precipitation in water. The effectiveness of NaCl crystal as a mold with HCl was confirmed by comparison with common inorganic salts, SiO₂, and γ-Al₂O₃ with HCl together with combinations including NaNO₃ + HNO₃, Na₂SO₄ + H₂SO₄, NaH₂PO₄ + H₃PO₄, and NaH₂PO₄ + polyphosphoric acid. The mechanism was deduced with the aid of computer simulation, which confirms the stabilization of 5,14-dihydro-5,7,12,14-tetraazapentacene by the NaCl surface. DMSO solution of a product, 1,3-dihydro-imidazo[4,5-b]phenazin-2-one, showed enhanced fluorescence in H₂O, and it was used as a fluorescent probe for pH and Hg²⁺. A full-color material was prepared by mixing precursors of epoxy resin and phenazines, and its fluorescent color could be adjusted by the ratio of phenazines.

Defect-suppressed submillimeter-scale WS2 single crystals with high photoluminescence quantum yields by alternate-growth-etching CVD

Defects, such as uncontrollable vacancies, will intensively degrade the material properties and device performance of CVD-grown transition metal dichalcogenides (TMDs). Although vacancies can be repaired by some post-processing measures, these treatments are usually time-consuming, complicated and may introduce uncontrollable chemical contaminants into TMDs. How to efficiently suppress the uncontrollable defects during CVD growth and acquire intrinsic high-quality CVD-grown TMDs without any after-treatment remains a critical challenge, and has not yet been well resolved. Here, an alternate-growth-etching (AGE) CVD method was demonstrated to fabricate defect-suppressed submillimeter-scale monolayer WS₂ single crystals. Compared with normal CVD, the grain size of the as-grown WS₂ can be enlarged by 4-5 times (∼520 μm) and the growth rate of ∼14.4 μm min^-1 is also at a high level compared to reported results. Moreover, AGE-CVD can efficiently suppress atomic vacancies in WS₂. In every growth-etching cycle, the etching of WS₂ occurs preferentially at the defective sites, which will be healed at the following growth stage. As a result, WS₂ monolayers obtained by AGE-CVD possess higher crystal quality, carrier mobility (8.3 cm² V^-1 s^-1) and PL quantum yield (QY, 52.6%) than those by normal CVD. In particular, such a PL QY is the highest value ever reported for in situ CVD-grown TMDs without any after-treatment, and is even comparable to the values of mechanically exfoliated samples. This AGE-CVD method is also appropriate for the synthesis of other high-quality TMD single crystals on a large-scale.

NaCl-Assisted Temperature-Dependent Controllable Growth of Large-Area MoS2 Crystals Using Confined-Space CVD

Due to its semiconducting nature, controlled growth of large-area chemical vapor deposition (CVD)-grown two-dimensional (2D) molybdenum disulfide (MoS2) has a lot of potential applications in photodetectors, sensors, and optoelectronics. Yet the controllable, large-area, and cost-effective growth of highly crystalline MoS2 remains a challenge. Confined-space CVD is a very promising method for the growth of highly crystalline MoS2 in a controlled manner. Herein, we report the large-scale growth of MoS2 with different morphologies using NaCl as a seeding promoter for confined-space CVD. Changes in the morphologies of MoS2 are reported by variation in the amount of seeding promoter, precursor ratio, and the growth temperature. Furthermore, the properties of the grown MoS2 are analyzed using optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). The electrical properties of the CVD-grown MoS2 show promising performance from fabricated field-effect transistors. This work provides new insight into the growth of large-area MoS2 and opens the way for its various optoelectronic and electronic applications.