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

Direct orientational epitaxy of wafer-scale 2D van der Waals heterostructures of metal dichalcogenides

Two-dimensional (2D) van der Waals (vdW) heterostructures have emerged as a groundbreaking candidate for future integrated circuits due to their tunable band structures, atomically sharp interfaces and seamless compatibility with complementary metal-oxide-semiconductor technologies. Despite their promise, existing synthesis methods, such as mechanical transfer and vapor-phase conversion, struggle to achieve the high-quality, scalable production for practical applications. In response to these longstanding challenges, our study unveils for the first time the direct epitaxial growth of wafer-scale 2D vdW heterostructures (MoS[Formula: see text]/SnS[Formula: see text]) with exceptional quality and uniformity. This achievement is made possible through fundamentally enhancing the adsorption interactions between intermediates and the underlying material. The heterostructures display pristine, defect-free interfaces, consistent crystal orientation and wafer-level thickness uniformity. The Raman peak shifts of MoS[Formula: see text] and SnS[Formula: see text] are constrained to below 0.5 cm[Formula: see text] across the entire wafer, with intensity deviations maintained within an impressive 2%, and thickness uniformity surpassing 99.5%. Owing to their exceptional crystallinity and interface quality, the heterostructures demonstrate extraordinary electron and hole transfer capabilities, showcasing a prominent rectification effect and an astounding responsivity of [Formula: see text] A/W, averaged from 30 devices. Our study signifies a pivotal advancement for the integration of 2D materials into semiconductor technologies, paving the way for next-generation integrated circuits.

Next-Generation Image Sensors Based on Low-Dimensional Semiconductor Materials

With the rapid advancement of technology of big data and artificial intelligence (AI), the exponential increase in visual information leads to heightened demands for the quality and analysis of imaging results, rendering traditional silicon-based image sensors inadequate. This review serves as a comprehensive overview of next-generation image sensors based on low-dimensional semiconductor materials encompassing 0D, 1D, 2D materials, and their hybrids. It offers an in-depth introduction to the distinctive properties exhibited by these materials and delves into the device structures tailored specifically for image sensor applications. The classification of novel image sensors based on low-dimensional materials, in particular for transition metal dichalcogenides (TMDs), covering the preparation methods and corresponding imaging characteristics, is explored. Furthermore, this review highlights the diverse applications of low-dimensional materials in next-generation image sensors, encompassing advanced imaging sensors, biomimetic vision sensors, and non-von Neumann imaging systems. Lastly, the challenges and opportunities encountered in the development of next-generation image sensors utilizing low-dimensional semiconductor materials, paving the way for further advancements in this rapidly evolving field, are proposed.

Influence of the Carrier Gas Flow in the CVD Synthesis of 2-Dimensional MoS2 Based on the Spin-Coating of Liquid Molybdenum Precursors

Atomically thin molybdenum disulfide (MoS2) is a two-dimensional semiconductor with versatile applications. The recent adoption of liquid molybdenum precursors in chemical vapor deposition has contributed significantly to the reproducible wafer-scale synthesis of MoS2 monolayer and few-layer films. In this work, we study the effects of the carrier gas flow rate on the properties of two-dimensional molybdenum disulfide grown by liquid-precursor-intermediate chemical vapor deposition on SiO2/Si substrates. We characterized the samples using Optical Microscopy, Scanning Electron Microscopy, Raman spectroscopy, and Photoluminescence spectroscopy. We analyzed samples grown with different nitrogen carrier flows, ranging from 150 to 300 sccm, and discussed the effect of carrier gas flows on their properties. We found a correlation between MoS2 flake lateral size, shape, and number of layers, and we present a qualitative growth model based on changes in sulfur provision caused by different carrier flows. We show how the use of liquid precursors can allow for the synthesis of homogeneous, single-layer flakes up to 100 µm in lateral size by optimizing the gas flow rate. These results are essential for gaining a deeper understanding of the growth process of MoS2.

Polarized Raman spectroscopy study of CVD-grown Cr2S3 flakes: unambiguous identification of phonon modes

We report a systematic Raman spectroscopy investigation of chemical vapor deposited 2D nonlayered Cr2S3, with both linearly and circularly polarized light over a wide temperature range (5-300 K). Temperature-dependent Raman spectra exhibit a good linear relationship between the peak positions of the phonon modes and temperature. Angle-resolved polarized Raman spectra reveal the polarization-dependent optical response of in-plane and out-of-plane phonon modes. Helicity-dependent Raman investigations complete definite assignment of all the phonon modes observed in the Raman spectra of 2D nonlayered Cr2S3 by the optical selection rule based on a Raman tensor. Our work realizes clear phonon mode identification over a wide temperature range for the emerging material 2D Cr2S3, an important representative of nonlayered 2D system with unique properties for optoelectronic and spintronic applications.

Solution-Processable Route for Large-Area Uniform 2D Semiconductor Nanofilms

The semiconductor thin film engineering technique plays a key role in the development of advanced electronics. Printing uniform nanofilms on freeform surfaces with high efficiency and low cost is significant for actual industrialization in electronics. Herein, a high-throughput colloidal printing (HTCP) strategy is reported for fabricating large-area and uniform semiconductor nanofilms on freeform surfaces. High-throughput and uniform printing rely on the balance of atomization and evaporation, as well as the introduced thermal Marangoni flows of colloidal dispersion, that suppresses outward capillary flows. Colloidal printing with in situ heating enables the fast fabrication of large-area semiconductor nanofilms on freeform surfaces, such as SiO2/Si, Al2O3, quartz glass, poly(ethylene terephthalate) (PET), Al foil, plastic tube, and Ni foam, expanding their technological applications where substrates are essential. The printed SnS2 nanofilms are integrated into thin-film semiconductor gas sensors with one of the fastest responses (8 s) while maintaining the highest sensitivity (Rg/Ra = 21) (toward 10 ppm NO2), as well as an ultralow limit of detection (LOD) of 46 ppt. The ability to print uniform semiconductor nanofilms on freeform surfaces with high-throughput promises the development of next-generation electronics with low cost and high efficiency.

Resistive gas sensors for the detection of NH3gas based on 2D WS2, WSe2, MoS2, and MoSe2: a review

Transition metal dichalcogenides (TMDs) with a two-dimensional (2D) structure and semiconducting features are highly favorable for the production of NH3gas sensors. Among the TMD family, WS2, WSe2, MoS2, and MoSe2exhibit high conductivity and a high surface area, along with high availability, reasons for which they are favored in gas-sensing studies. In this review, we have discussed the structure, synthesis, and NH3sensing characteristics of pristine, decorated, doped, and composite-based WS2, WSe2, MoS2, and MoSe2gas sensors. Both experimental and theoretical studies are considered. Furthermore, both room temperature and higher temperature gas sensors are discussed. We also emphasized the gas-sensing mechanism. Thus, this review provides a reference for researchers working in the field of 2D TMD gas sensors.

Ion adsorption promotes Frank-van der Merwe growth of 2D transition metal tellurides

Reliable synthesis methods for high-quality, large-sized, and uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) are crucial for their device applications. However, versatile approaches to growing high-quality, large-sized, and uniform 2D transition-metal tellurides are rare. Here, we demonstrate an ion adsorption strategy that facilitates the Frank-van der Merwe growth of 2D transition-metal tellurides. By employing this method, we grow MoTe2 and WTe2 with enhanced lateral size, reduced thickness, and improved uniformity. Comprehensive characterizations confirm the high quality of as-grown MoTe2. Moreover, various characterizations verify the adsorption of K+ and Cl- ions on the top surface of MoTe2. X-ray photoelectron spectroscopy (XPS) analysis reveals that the MoTe2 is stoichiometric without K+ and Cl- ions and exhibits no discernable oxidation after washing. This top surface control strategy provides a new controlling knob to optimize the growth of 2D transition-metal tellurides and holds the potential for generalized to other 2D materials.

Direct growth of single-chiral-angle tungsten disulfide nanotubes using gold nanoparticle catalysts

Transition metal dichalcogenide (TMD) nanotubes offer a unique platform to explore the properties of TMD materials at the one-dimensional limit. Despite considerable efforts thus far, the direct growth of TMD nanotubes with controllable chirality remains challenging. Here we demonstrate the direct and facile growth of high-quality WS2 and WSe2 nanotubes on Si substrates using catalytic chemical vapour deposition with Au nanoparticles. The Au nanoparticles provide unique accommodation sites for the nucleation of WS2 or WSe2 shells on their surfaces and seed the subsequent growth of nanotubes. We find that the growth mode of nanotubes is sensitive to the temperature. With careful temperature control, we realize ~79% WS2 nanotubes with single chiral angles, with a preference of 30° (~37%) and 0° (~12%). Moreover, we demonstrate how the geometric, electronic and optical properties of the synthesized WS2 nanotubes can be modulated by the chirality. We anticipate that this approach using Au nanoparticles as catalysts will facilitate the growth of TMD nanotubes with controllable chirality and promote the study of their interesting properties and applications.

Chemical Vapor Deposition of a Single-Crystalline MoS2 Monolayer through Anisotropic 2D Crystal Growth on Stepped Sapphire Surface

Recently, a step-flow growth mode has been proposed to break the inherent molybdenum disulfide (MoS2) crystal domain bimodality and yield a single-crystalline MoS2 monolayer on commonly employed sapphire substrates. This work reveals an alternative growth mechanism during the metal-organic chemical vapor deposition (MOCVD) of a single-crystalline MoS2 monolayer through anisotropic 2D crystal growth. During early growth stages, the epitaxial symmetry and commensurability of sapphire terraces rather than the sapphire step inclination ultimately govern the MoS2 crystal orientation. Strikingly, as the MoS2 crystals continue to grow laterally, the sapphire steps transform the MoS2 crystal geometry into diamond-shaped domains presumably by anisotropic diffusion of ad-species and facet development. Even though these MoS2 domains nucleate on sapphire with predominantly bimodal 0 and 60° azimuthal rotation, the individual domains reach lateral dimensions of up to 200 nm before merging seamlessly into a single-crystalline MoS2 monolayer upon coalescence. Plan-view transmission electron microscopy reveals the single-crystalline nature across 50 μm by 50 μm inspection areas. As a result, the median carrier mobility of MoS2 monolayers peaks at 25 cm2 V-1 s-1 with the highest value reaching 28 cm2 V-1 s-1. This work details synthesis-structure correlations and the possibilities to tune the structure and material properties through substrate topography toward various applications in nanoelectronics, catalysis, and nanotechnology. Moreover, shape modulation through anisotropic growth phenomena on stepped surfaces can provide opportunities for nanopatterning for a wide range of materials.

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T' Transition Metal Dichalcogenide Nanosheets

The past few decades have witnessed a notable increase in transition metal dichalcogenide (TMD) related research not only because of the large family of TMD candidates but also because of the various polytypes that arise from the monolayer configuration and layer stacking order. The peculiar physicochemical properties of TMD nanosheets enable an enormous range of applications from fundamental science to industrial technologies based on the preparation of high-quality TMDs. For polymorphic TMDs, the 1T/1T' phase is particularly intriguing because of the enriched density of states, and thus facilitates fruitful chemistry. Herein, we comprehensively discuss the most recent strategies for direct synthesis of phase-pure 1T/1T' TMD nanosheets such as mechanical exfoliation, chemical vapor deposition, wet chemical synthesis, atomic layer deposition, and more. We also review frequently adopted methods for phase engineering in TMD nanosheets ranging from chemical doping and alloying, to charge injection, and irradiation with optical or charged particle beams. Prior to the synthesis methods, we discuss the configuration of TMDs as well as the characterization tools mostly used in experiments. Finally, we discuss the current challenges and opportunities as well as emphasize the promising fields for the future development.

Substrate Engineering for Chemical Vapor Deposition Growth of Large-Scale 2D Transition Metal Dichalcogenides

The large-scale production of 2D transition metal dichalcogenides (TMDs) is essential to realize their industrial applications. Chemical vapor deposition (CVD) has been considered as a promising method for the controlled growth of high-quality and large-scale 2D TMDs. During a CVD process, the substrate plays a crucial role in anchoring the source materials, promoting the nucleation and stimulating the epitaxial growth. It thus significantly affects the thickness, microstructure, and crystal quality of the products, which are particularly important for obtaining 2D TMDs with expected morphology and size. Here, an insightful review is provided by focusing on the recent development associated with the substrate engineering strategies for CVD preparation of large-scale 2D TMDs. First, the interaction between 2D TMDs and substrates, a key factor for the growth of high-quality materials, is systematically discussed by combining the latest theoretical calculations. Based on this, the effect of various substrate engineering approaches on the growth of large-area 2D TMDs is summarized in detail. Finally, the opportunities and challenges of substrate engineering for the future development of 2D TMDs are discussed. This review might provide deep insight into the controllable growth of high-quality 2D TMDs toward their industrial-scale practical applications.

Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T'-phase semimetal contact electrodes. By transforming polycrystalline 1T'-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T' semimetals and 2H semiconductors. Work function modulation of 1T'-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

Modulated wafer-scale WS2 films based on atomic-layer-deposition for various device applications

Tungsten disulfide (WS2) is promising for potential applications in transistors and gas sensors due to its high mobility and high adsorption of gas molecules onto edge sites. This work comprehensively studied the deposition temperature, growth mechanism, annealing conditions, and Nb doping of WS2 to prepare high-quality wafer-scale N- and P-type WS2 films by atomic layer deposition (ALD). It shows that the deposition and annealing temperature greatly influence the electronic properties and crystallinity of WS2, and insufficient annealing will seriously reduce the switch ratio and on-state current of the field effect transistors (FETs). Besides, the morphologies and carrier types of WS2 films can be controlled by adjusting the processes of ALD. The obtained WS2 films and the films with vertical structures were used to fabricate FETs and gas sensors, respectively. Among them, the Ion/Ioff ratio of N- and P-type WS2 FETs is 105 and 102, respectively, and the response of N- and P-type gas sensors is 14% and 42% under 50 ppm NH3 at room temperature, respectively. We have successfully demonstrated a controllable ALD process to modify the morphology and doping behavior of WS2 films with various device functionalities based on acquisitive characteristics.

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.

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.

Flexible Optical Synapses Based on In2Se3/MoS2 Heterojunctions for Artificial Vision Systems in the Near-Infrared Range

Near-infrared (NIR) synaptic devices integrate NIR optical sensitivity and synaptic plasticity, emulating the basic biomimetic function of the human visual system and showing great potential in NIR artificial vision systems. However, the lack of semiconductor materials with appropriate band gaps for NIR photodetection and effective strategies for fabricating devices with synaptic behaviors limit the further development of NIR synaptic devices. Here, a two-terminal NIR synaptic device consisting of the In₂Se₃/MoS₂ heterojunction has been constructed, and it exhibits fundamental synaptic functions. The reduced band gap and potential barrier of In₂Se₃/MoS₂ heterojunctions are essential for NIR synaptic plasticity. In addition, the NIR synaptic properties of In₂Se₃/MoS₂ heterojunctions under strain have been studied systematically. The ΔEPSC of the In₂Se₃/MoS₂ synaptic device can be improved from 38.4% under no strain to 49.0% under a 0.54% strain with a 1060 nm illumination for 1 s at 100 mV. Furthermore, the artificial NIR vision system consisting of a 10 × 10 In₂Se₃/MoS₂ device array has been fabricated, exhibiting image sensing, learning, and storage functions under NIR illumination. This research provides new ideas for the design of flexible NIR synaptic devices based on 2D materials and presents many opportunities in artificial intelligence and NIR vision systems.

Air-stable van der Waals PtTe2 conductors with high current-carrying capacity and strong spin-orbit interaction

High-performance van der Waals (vdW) integrated electronics and spintronics require reliable current-carrying capacity. However, it is challenging to achieve high current density and air-stable performance using vdW metals owing to the fast electrical breakdown triggered by defects or oxidation. Here, we report that spin-orbit interacted synthetic PtTe₂ layers exhibit significant electrical reliability and robustness in ambient air. The 4-nm-thick PtTe₂ synthesized at a low temperature (∼400°C) shows intrinsic metallic transport behavior and a weak antilocalization effect attributed to the strong spin-orbit scattering. Remarkably, PtTe₂ sustains a high current density approaching ≈31.5 MA cm⁻², which is the highest value among electrical interconnect candidates under oxygen exposure. Electrical failure is caused by the Joule heating of PtTe₂ rather than defect-induced electromigration, which was achievable by the native TeO_x passivation. The high-quality growth of PtTe₂ and the investigation of its transport behaviors lay out essential foundations for the development of emerging vdW spin-orbitronics.

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The synthesized PtTe₂ had a self-passivated surface under exposure to air

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Magnetoconductance study proved the realization of a 2D confined quantum system

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PtTe₂ sustained a remarkably high current density (∼31.5 MA cm⁻²) under air atmosphere

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The native TeO_x passivation retarded the defect-induced electromigration of PtTe₂

Poly(vinyl alcohol)-Assisted Exfoliation of van der Waals Materials

We report a highly efficient and easily transferable poly(vinyl alcohol) (PVA)-assisted exfoliation method, which allows one to obtain van der Waals materials on large scales, e.g., centimeter-scale graphite flakes and hundred-micrometer-scale several layers of ZnIn₂S₄ and BN. The present exfoliation scheme is nondestructive, and the materials prepared by PVA-assisted exfoliation can be directly fabricated into devices. This exfoliation approach could be helpful in overcoming the preparation bottleneck for large-scale applications of two-dimensional (2D) materials.

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.

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.

Growth of bilayer MoTe2 single crystals with strong non-linear Hall effect

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies. Bilayer TMDTs are non-centrosymmetric and have unique topological properties compared to monolayer or trilayer, but a controllable way to prepare bilayer MoTe₂ crystal has not been achieved to date. Herein, we achieve the layer-by-layer growth of large-area bilayer and trilayer 1T′ MoTe₂ single crystals and centimetre-scale films by a two-stage chemical vapor deposition process. The as-grown bilayer MoTe₂ shows out-of-plane ferroelectric polarization, whereas the monolayer and trilayer crystals are non-polar. In addition, we observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer T_d phase MoTe₂ under time reversal-symmetric conditions, while these vanish for thicker layers. For a fixed input current, bilayer T_d MoTe₂ produces the largest second harmonic output voltage among the thicker crystals tested. Our work therefore highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers.

2D transition metal ditellurides exhibit nontrivial topological phases, but the controlled bottom-up synthesis of these materials is still challenging. Here, the authors report the layer-by-layer growth of large-area bilayer and trilayer 1T’ MoTe2 films, showing thickness-dependent ferroelectricity and nonlinear Hall effect.

Recent Progress in Fabrication and Physical Properties of 2D TMDC-Based Multilayered Vertical Heterostructures

Two-dimensional (2D) vertical heterojunctions (HSs), which are usually fabricated by vertically stacking two layers of transition metal dichalcogenide (TMDC), have been intensively researched during the past years. However, it is still an enormous challenge to achieve controllable preparation of the TMDC trilayer or multilayered van der Waals (vdWs) HSs, which have important effects on physical properties and device performance. In this review, we will introduce fundamental features and various fabrication methods of diverse TMDC-based multilayered vdWs HSs. This review focuses on four fabrication methods of TMDC-based multilayered vdWs HSs, such as exfoliation, chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD). The latest progress in vdWs HS-related novel physical phenomena are summarized, including interlayer excitons, long photocarrier lifetimes, upconversion photoluminescence, and improved photoelectrochemical catalysis. At last, current challenges and prospects in this research field are provided.

The role of defects presenting in graphitic SiC sheets and their consequences in the exfoliation of layers - a first principles approach

Recently, there has been a growing interest in exploring new 2D nanostructures, due to their unique electronic and optical properties. An atomically thin SiC sheet, which has a honeycomb structure similar to BN, as well as being a direct band gap semiconductor, is one such candidate. Despite several theoretical reports predicting the structural and dynamical stability of 2D SiC nanostructures, few experimental reports have been reported so far. In the present work, we demonstrated by employing first principles density functional theory calculations that the role of self defects on the exfoliation of SiC layers can be understood by studying monolayer, bilayer and trilayer 2D SiC systems. From our work, it can be seen that the dangled C atom on the removal of a Si atom in the SiC layer prefers to interact with an adjacent layer, owing to the compensation of the charges, whereas, a dangled Si atom (in the carbon vacancy case) in the SiC layer compensates its additional charge within the layer by forming a Si-Si bond. We concluded that the exfoliation process of SiC is significantly affected by Si vacancies, rather than the presence of carbon vacancies. This work also provides an intuitive idea to synthesise 2D SiC nanostructures as it has interesting structural and electronic properties.

Colloidal Synthesis of MoSe2/WSe2 Heterostructure Nanoflowers via Two-Step Growth

The ability to control the active edge sites of transition metal dichalcogenides (TMDs) is crucial for modulating their chemical activity for various electrochemical applications, including hydrogen evolution reactions. In this study, we demonstrate a colloidal synthetic method to prepare core-shell-like heterostructures composed of MoSe₂ and WSe₂ via a two-step sequential growth. By overgrowing WSe₂ on the surface of preexisting MoSe₂ nanosheet edges, MoSe₂-core/WSe₂-shell heterostructures were successfully obtained. Systematic comparisons of the secondary growth time and sequential order of growth suggest that the low synthetic temperature conditions allow the stable overgrowth of shells rich in WSe₂ on top of the core of MoSe₂ with low Gibbs formation energy. The electrochemical analysis confirms that the catalytic activity correlates to the core-shell composition variation. Our results propose a new strategy to control the edge site activity of TMD materials prepared by colloidal synthesis, which is applicable to diverse electrochemical applications.

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.

Visualizing Van der Waals Epitaxial Growth of 2D Heterostructures

Understanding the growth mechanisms of 2D van der Waals (vdW) heterostructures is of great importance in exploring their functionalities and device applications. A custom-built system integrating physical vapor deposition and optical microscopy/Raman spectroscopy is employed to study the dynamic growth processes of 2D vdW heterostructures in situ. This allows the identification of a new growth mode with a distinctly different growth rate and morphology from those of the conventional linear growth mode. A model that explains the difference in morphologies and quantifies the growth rates of the two modes by taking the role of surface diffusion into account is proposed. A range of material combinations including CdI₂ /WS₂ , CdI₂ /MoS₂ , CdI₂ /WSe₂ , PbI₂ /WS₂ , PbI₂ /MoS₂ , PbI₂ /WSe₂ , and Bi₂ Se₃ /WS₂ is systematically investigated. These findings may be generalized to the synthesis of many other 2D heterostructures with controlled morphologies and physical properties, benefiting future device applications.

Substitutionally Doped MoSe2 for High-Performance Electronics and Optoelectronics

2D materials, of which the carrier type and concentration are easily tuned, show tremendous superiority in electronic and optoelectronic applications. However, the achievements are still quite far away from practical applications. Much more effort should be made to further improve their performance. Here, p-type MoSe₂ is successfully achieved via substitutional doping of Ta atoms, which is confirmed experimentally and theoretically, and outstanding homojunction photodetectors and inverters are fabricated. MoSe₂ p-n homojunction device with a low reverse current (300 pA) exhibits a high rectification ratio (10⁴ ). The analysis of dark current reveals the domination of the Shockley-Read-Hall (SRH) and band-to-band tunneling (BTB) current. The homojunction photodetector exhibits a large open-circuit voltage (0.68 V) and short-circuit currents (1 µA), which is suitable for micro-solar cells. Furthermore, it possesses outstanding responsivity (0.28 A W^-1 ), large external quantum efficiency (42%), and a high signal-to-noise ratio (≈10⁷ ). Benefiting from the continuous energy band of homojunction, the response speed reaches up to 20 µs. Besides, the Ta-doped MoSe₂ inverter exhibits a high voltage gain (34) and low power consumption (127 nW). This work lays a foundation for the practical application of 2D material devices.

Atomic-Layer-Deposition-Based 2D Transition Metal Chalcogenides: Synthesis, Modulation, and Applications

Transition metal chalcogenides (TMCs) are a large family of 2D materials with different properties, and are promising candidates for a wide range of applications such as nanoelectronics, sensors, energy conversion, and energy storage. In the research of new materials, the development and investigation of industry-compatible synthesis techniques is of key importance. In this respect, it is important to study 2D TMC materials synthesized by the atomic layer deposition (ALD) technique, which is widely applied in industries. In addition to the synthesis of 2D TMCs, ALD is used to modulate the characteristic of 2D TMCs such as their carrier density and morphology. So far, the improvement of thin film uniformity without oxidation and the synthesis of low-dimensional nanomaterials on 2D TMCs have been the research focus. Herein, the synthesis and modulation of 2D TMCs by ALD is described, and the characteristics of ALD-based TMCs used in nanoelectronics, sensors, and energy applications are discussed.

Defect activated optical Raman modes in single layer MoSe2

In the last decade, transition metal dichalcogenides (TMDs) have been intensively synthesized/studied thus linking their morphological aspect to their physical properties, and consequently leading to the understanding of the possible benefits of defects in such materials. Nevertheless, for future applications, quantifying and identifying defects in TMDs is still a milestone to reach in order to better employ these materials in optoelectronic devices. Raman Spectroscopy has been successfully employed in graphene to quantify punctual or line defects. In this paper, we bombarded monolayer MoSe₂with He ions and found out the existence of three defect activated Raman bands around 250-300 cm^-1. Density functional theory calculations were employed to obtain the electronic and phonon dispersion bands, making it possible to infer that these bands arise from inter-valley Raman double resonance processes. Interestingly, the same punctual defect model, that allows one to predict the defect concentration at which graphene starts to become amorphous, also works for TMDs. Hence, this work opens the door to the macroscopic quantification of defects in TMDs, which is essential for technological applications.

Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films

Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and ReaxFF reactive force field-based molecular dynamics simulations, we provide insights into WS₂ crystal growth mechanisms, providing a direct link between synthetic conditions and microstructure. Dark-field TEM imaging of coalesced monolayer WS₂ films illuminates defect arrays that atomic-resolution STEM imaging identifies as translational grain boundaries. Electron diffraction and high-resolution imaging reveal that the films have nearly a single orientation with imperfectly stitched domains that tilt out-of-plane when released from the substrate. Imaging and ReaxFF simulations uncover two types of translational mismatch, and we discuss their origin related to relatively fast growth rates. Statistical analysis of >1300 facets demonstrates that microstructural features are constructed from nanometer-scale building blocks, describing the system across sub-Ångstrom to multimicrometer length scales.

Recent progress on kinetic control of chemical vapor deposition growth of high-quality wafer-scale transition metal dichalcogenides

2D transition metal dichalcogenides (TMDs) have attracted significant attention due to their unique physical properties. Chemical vapor deposition (CVD) is generally a promising method to prepare ideal TMD films with high uniformity, large domain size, good single-crystallinity, etc., at wafer-scale for commercial uses. However, the CVD-grown TMD samples often suffer from poor quality due to the improper control of reaction kinetics and lack of understanding about the phenomenon. In this review, we focus on several key challenges in the controllable CVD fabrication of high-quality wafer-scale TMD films and highlight the importance of the control of precursor concentration, nucleation density, and oriented growth. The remaining difficulties in the field and prospective directions of the related topics are further summarized.

Low-Loss Integrated Nanophotonic Circuits with Layered Semiconductor Materials

Monolayer transition-metal dichalcogenides with direct bandgaps are emerging candidates for optoelectronic devices, such as photodetectors, light-emitting diodes, and electro-optic modulators. Here we report a low-loss integrated platform incorporating molybdenum ditelluride monolayers with silicon nitride photonic microresonators. We achieve microresonator quality factors >3 × 10⁶ in the telecommunication O- to E-bands. This paves the way for low-loss, hybrid photonic integrated circuits with layered semiconductors, not requiring heterogeneous wafer bonding.

Tailoring the structure of MoS2 using ball-milled MoO3 powders: hexagonal, triangular, and fullerene-like shapes

Single and few-layered MoS₂ materials have attracted attention due to their outstanding physicochemical properties with potential applications in optoelectronics, catalysis, and energy storage. In the past, these materials have been produced using the chemical vapor deposition (CVD) method using MoO₃ films and powders as Mo precursors. In this work, we demonstrate that the size and morphology of few-layered MoS₂ nanostructures can be controlled, modifying the Mo precursor mechanically. We synthesized few-layered MoS₂ materials using MoO₃ powders previously exposed to a high-energy ball milling treatment by the salt-assisted CVD method. The MoO₃ powders milled for 30, 120, and 300 min were used to synthesize sample MoS₂-30, MoS₂-120, and MoS₂-300, respectively. We found morphologies mainly of hexagons (MoS₂-30), triangles (MoS₂-120), and fullerenes (MoS₂-300). The MoS₂ nanostructures and MoO₃ powders were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray diffraction, and thermogravimetric analysis. It was found that MoO₃ milled powders exhibit oxygen loss and decrease in crystallite size as milling time increases. Oxygen deficiency in the Mo precursor prevents the growth of large MoS₂ crystals and a large number of milled MoO_3-x  + NaCl promote greater nucleation sites for the formation of MoS₂, achieving a high density of nanoflakes in the 2H and 3R phases, with diameter sizes in the range of ∼30-600 nm with 1-12 layers. Photoluminescence characterization at room temperature revealed a direct bandgap and exciting trends for the different MoS₂ samples. We envisage that our work provides a route for modifying the structure and optical properties for future device design via precursor engineering.

Atomic layer deposition of tungsten sulfide using a new metal-organic precursor and H2S: thin film catalyst for water splitting

Transition metal dichalcogenides (TMDs) are extensively researched in the past few years due to their two-dimensional layered structure similar to graphite. This group of materials offers tunable optoelectronic properties depending on the number of layers and therefore have a wide range of applications. Tungsten disulfide (WS₂) is one of such TMDs that has been studied relatively less compared to MoS₂. Herein, WS _x thin films are grown on several types of substrates by atomic layer deposition (ALD) using a new metal-organic precursor [tris(hexyne) tungsten monocarbonyl, W(CO)(CH₃CH₂C≡CCH₂CH₃)₃] and H₂S molecules at a relatively low temperature of 300 °C. The typical self-limiting film growth by varying both, precursor and reactant, is obtained with a relatively high growth per cycle value of ∼0.13 nm. Perfect growth linearity with negligible incubation period is also evident in this ALD process. While the as-grown films are amorphous with considerable S-deficiency, they can be crystallized as h-WS₂ film by post-annealing in the H₂S atmosphere above 700 °C as observed from x-ray diffractometry analysis. Several other analyses like Raman and x-ray photoelectron spectroscopy, transmission electron microscopy, UV-vis. spectroscopy are performed to find out the physical, optical, and microstructural properties of as-grown and annealed films. The post-annealing in H₂S helps to promote the S content in the film significantly as confirmed by the Rutherford backscattering spectrometry. Extremely thin (∼4.5 nm), as-grown WS _x films with excellent conformality (∼100% step coverage) are achieved on the dual trench substrate (minimum width: 15 nm, aspect ratio: 6.3). Finally, the thin films of WS _x (as-grown and 600/700 °C annealed) on W/Si and carbon cloth substrate are investigated for electrochemical hydrogen evolution reaction (HER). The as-grown WS _x shows poor performance towards HER and is attributed to the S-deficiency, amorphous character, and oxygen contamination of the WS _x film. Annealing the WS _x film at 700 °C results in the formation of a crystalline layered WS₂ phase, which significantly improves the HER performance of the electrode. The study reveals the importance of sulfur content and crystallinity on the HER performance of W-based sulfides.

Synthesis of high quality 2D carbide MXene flakes using a highly purified MAX precursor for ink applications

The practical application of 2D MXenes in electronic and energy fields has been hindered by the severe variation in the quality of MXene products depending on the parent MAX phases, manufacturing techniques, and preparation parameters. In particular, their synthesis has been impeded by the lack of studies reporting the synthesis of high-quality parent MAX phases. In addition, controllable and uniform deposition of 2D MXenes on various large-scale substrates is urgently required to use them practically. Herein, a method of pelletizing raw materials could synthesize a stoichiometric Ti₃AlC₂ MAX phase with high yield and processability, and fewer impurities. The Ti₃AlC₂ could be exfoliated into 1-2-atom-thick 2D Ti₃C₂T _x flakes, and their applicability was confirmed by the deposition and additional alignment of the 2D flakes with tunable thickness and electrical properties. Moreover, a practical MXene ink was fabricated with rheological characterization. MXene ink exhibited much better thickness uniformity while retaining excellent electrical performances (e.g., sheet resistance, electromagnetic interference shielding ability) as those of a film produced by vacuum filtration. The direct functional integration of MXenes on various substrates is expected to initiate new and unexpected MXene-based applications.

Recent progress in the CVD growth of 2D vertical heterostructures based on transition-metal dichalcogenides

This review summarizes recent advances in the controllable CVD growth of 2D TMDC vertical heterostructures under four different strategies.

Effect of temperature and exfoliation time on the properties of chemically exfoliated MoS2 nanosheets

A systematic investigation of the experimental conditions for the chemical exfoliation of MoS₂ using n-butyllithium as intercalating agent has been carried out to unravel the effect of reaction time and temperature for maximizing the percentage of monolayer thick-flakes and achieve a control over the content of metallic 1T vs. semiconductive 2H phases, thereby tuning the electrical properties of ultrathin MoS₂ few-layer thick films.

Low-Power and Ultra-Thin MoS2 Photodetectors on Glass

Integration of low-power consumer electronics on glass can revolutionize the automotive and transport sectors, packaging industry, smart building and interior design, healthcare, life science engineering, display technologies, and many other applications. However, direct growth of high-performance, scalable, and reliable electronic materials on glass is difficult owing to low thermal budget. Similarly, development of energy-efficient electronic and optoelectronic devices on glass requires manufacturing innovations. Here, we accomplish both by relatively low-temperature (<600 °C) metal-organic chemical vapor deposition growth of atomically thin MoS₂ on multicomponent glass and fabrication of low-power phototransistors using atomic layer deposition (ALD)-grown, high-k, and ultra-thin (∼20 nm) Al₂O₃ as the top-gate dielectric, circumventing the challenges associated with the ALD nucleation of oxides on inert basal planes of van der Waals materials. The MoS₂ photodetectors demonstrate the ability to detect low-intensity visible light at high speed and low energy expenditure of ∼100 pico Joules. Furthermore, low device-to-device performance variation across the entire 1 cm² substrate and aggressive channel length scalability confirm the technology readiness level of ultra-thin MoS₂ photodetectors on glass.

Phase Variations and Layer Epitaxy of 2D PdSe2 Grown on 2D Monolayers by Direct Selenization of Molecular Pd Precursors

Two-dimensional (2D) materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe₂ and Pd₂Se₃ are two of the noble transition-metal chalcogenides with excellent physical properties that have recently emerged as promising materials for electronics, optoelectronics, catalyst, and sensors. This research presents a feasible approach to synthesize PdSe₂ and Pd₂Se₃ with inherently asymmetric structure on honeycomb lattice 2D monolayer substrates of graphene and MoS₂. We directly deposit a molecular transition-metal precursor complex on the surface of the 2D substrates, followed by low-temperature selenization by chemical vapor flow. Parameter control leads to tuning of the material from monolayer nanocrystals with Pd₂Se₃ phase, to continuous few-layer PdSe₂ films. Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations, and heteroepitaxy at the atomic level. PdSe₂ with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe₂ and monolayer Pd₂Se₃ are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe₂ and underlying 2D substrate was also revealed by ADF-STEM. These results offer both nanoscale and atomic-level insights into direct growth of van der Waals heterostructures, as well as an innovative method for 2D synthesis by predetermined nucleation.

In Situ Cleaning and Fluorination of Black Phosphorus for Enhanced Performance of Transistors with High Stability

Most two-dimensional (2D) semiconductors suffer from intrinsic instability under ambient conditions, especially 2D black phosphorus (BP). Although much effort has been made to study the passivation of 2D materials against corrosion by oxygen and water molecules, facile and effective passivation with long-term stability is still challenging; in particular, selective passivation, which is critical for integration into nanoelectronics, is still lacking. Here, we develop a novel passivation route for BP using a fluorinated self-assembled thin film of PFSA (perfluorosulfonic acid, PFSA), where the surface modifier with high hydrophobicity on BP presents extremely stable characteristics over five months under ambient conditions. Moreover, we report for the first time in situ cleaning and selective fluorination of only BP flakes on a SiO₂/Si substrate by a spin-coating process followed by ultrasonication, which was attributed to the formation of P-F covalent bonds on the BP surface. Selectively fluorinated BP shows not only enhanced stability in air but also electrical properties of the BP field-effect transistor (FET), with the on-current of the BP FET increasing and presenting enhanced carrier mobility (125 cm² V^-1 s^-1) and on/off ratio (10⁴). This significant finding sheds light on fabricating vertical 2D heterostructures to realize high performance and reliability with versatile 2D materials. This work demonstrates an emerging passivation approach for long-term stability together with superior electrical properties, which paves the way for integrating 2D semiconductors into critical channel materials in FETs that are favorable for next-generation digital logic circuits.

Surface Characterization of MoS2 Atomic Layers Mechanically Exfoliated on a Si Substrate

Mo disulfide overlayers with the thickness exceeding 1.77 nm were obtained on Si substrates through mechanical exfoliation. The resulting Mo disulfide flakes were then analyzed ex situ using combination of Auger electron spectroscopy (AES), elastic-peak electron spectroscopy (EPES) and scanning electron microscopy (SEM) in order to characterize their surface chemical composition, electron transport phenomena and surface morphology. Prior to EPES measurements, the Mo disulfide surface was sputter-cleaned and amorphized by 3 kV argon ions, and the resulting S/Mo atomic ratio varied in the range 1.80-1.88, as found from AES measurements. The SEM images revealed single crystalline small-area (up to 15 μm in lateral size) Mo disulfide flakes having polygonal or near-triangular shapes. Such irregular-edged flakes exhibited high crystal quality and thickness uniformity. The inelastic mean free path (IMFP), characterizing electron transport, was evaluated from the relative EPES using Au reference material for electron energies E = 0.5-2 keV. Experimental IMFPs, λ, determined for the AES-measured surface compositions were approximated by the simple function λ = kEp, where k = 0.0289 and p = 0.946 were fitted parameters. Additionally, these IMFPs were compared with IMFPs resulting from the two methods: (i) present calculations based on the formalism of the Oswald et al. model; (ii) the predictive equation of Tanuma et al. (TPP-2M) for the measured Mo0.293S0.551C0.156 surface composition (S/Mo = 1.88), and also for stoichiometric MoS2 composition. The fitted function was found to be reasonably consistent with the measured, calculated and predicted IMFPs. We concluded that the measured IMFP value at 0.5 keV was only slightly affected by residual carbon contamination at the Mo disulfide surface.

Universal Precise Growth of 2D Transition-Metal Dichalcogenides in Vertical Direction

Two-dimensional transition-metal dichalcogenides (TMDs) have been one of the hottest focus of materials due to the most beneficial electronic and optoelectronic properties. Up to now, one of the big challenges is the synthesis of large-area layer-number-controlled single-crystal films. However, the poor understanding of the growth mechanism seriously hampers the progress of the scalable production of TMDs with precisely tunable thickness at an atomic scale. Here, the growth mechanisms in the vertical direction were systemically studied based on the density functional theory (DFT) calculation and an advanced chemical vapor deposition (CVD) growth. As a result, the U-type relation of the TMD layer number to the ratio of metal/chalcogenide is confirmed by the capability of ultrafine tuning of the experimental conditions in the CVD growth. In addition, high-quality uniform monolayer, bilayer, trilayer, and multilayer TMDs in a large area (8 cm²) were efficiently synthesized by applying this modified CVD. Although bilayer TMDs can be obtained at both high and low ratios of metal/chalcogenide based on the suggested mechanism, they demonstrate significantly different optical and electronic transport properties. The modified CVD strategy and the proposed mechanism should be helpful for synthesizing and large-area thickness-controlled TMDs and understanding their growth mechanism and could be used in integrated electronics and optoelectronics.

Doping of MoTe2 via Surface Charge Transfer in Air

Doping is a key process by which the concentration and type of majority carriers can be tuned to achieve desired conduction properties. The common way of doping is via bulk impurities, as in the case of silicon. For van der Waals bonded semiconductors, control over bulk impurities is not as well developed, because they may either migrate between the layers or bond with the surfaces or interfaces becoming undesired scattering centers for carriers. Herein, we investigate by means of Kelvin probe force microscopy (KPFM) and density functional theory calculations (DFT) the doping of MoTe2 via surface charge transfer occurring in air. Using DFT, we show that oxygen molecules physisorb on the surface and increase its work function (compared to pristine surfaces) toward p-type behavior, which is consistent with our KPFM measurements. The surface charge transfer doping (SCTD) driven by adsorbed oxygen molecules can be easily controlled or reversed through thermal annealing of the entire sample. Furthermore, we also demonstrate local control of the doping by contact electrification. As a reversible and controllable nanoscale physisorption process, SCTD can thus open new avenues for the emerging field of 2D electronics.

Growth Control of WS2: From 2D Layer by Layer to 3D Vertical Standing Nanowalls

Large area 2D WS₂ has been grown successfully by radio frequency magnetron sputtering (RFMS) method. First, in order to investigate the pressure dependence on the grown WS₂ samples, WS₂ were grown at 5 different growth pressures, 5, 10, 15, 20, and 25 mTorr. It has been observed that the surface morphology changes for the samples grown at higher growth pressures, 15, 20, and 25 mTorr. Vertically standing nanowall (NW)-like structures have been formed at these relatively high growth pressures. It has also been observed that the (002) plane is highly dominant, which means layer by layer growth parallel to the substrate, for the sample grown at 20 mTorr. X-ray photoelectron spectroscopy (XPS) measurements revealed an increasing atomic percentage of the S element to W element, S/W, ratio in thin films, as the growth pressure increases. Growth dynamics of WS₂ has been investigated by time-dependent-growth WS₂ samples, 5, 10, 20, 40, and 80 s under 20 mTorr pressure. It has been shown by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy that a highly smooth surface has been achieved in the samples grown for the duration of 5 and 10 s. Raman mapping measurements on the sample grown at 5 s have revealed large area homogeneous growth. As the growth time gets longer, the NWs emerge on the surface at some nucleation points. Only the peak that belongs to the (002) plane has been observed for samples grown at 5 and 10 s by the X-ray diffraction (XRD) measurements. XRD measurements have revealed the appearance of turbostratic peaks of (11l) and (10l) as the thickness increases. Photoluminescence measurements have indicated near-band-edge emission centered at 630 nm for only 5 and 10 s samples.

Controlled Growth of Large-Area Bilayer Tungsten Diselenides with Lateral P-N Junctions

Bilayer two-dimensional (2D) van der Waals (vdW) materials are attracting increasing attention due to their predicted high quality electronic and optical properties. Here, we demonstrate dense, selective growth of WSe₂ bilayer flakes by chemical vapor deposition with the use of a 1:10 molar mixture of sodium cholate and sodium chloride as the growth promoter to control the local diffusion of W-containing species. A large fraction of the bilayer WSe₂ flakes showed a 0 (AB) and 60° (AA') twist between the two layers, whereas Moiré 15 and 30° twist angles were also observed. Well-defined monolayer-bilayer junctions were formed in the as-grown bilayer WSe₂ flakes, and these interfaces exhibited p-n diode rectification and an ambipolar transport characteristic. This work provides an efficient method for the layer-controlled growth of 2D materials, in particular, 2D transition metal dichalcogenides, and promotes their applications in next-generation electronic and optoelectronic devices.

Electrically Robust Single-Crystalline WTe2 Nanobelts for Nanoscale Electrical Interconnects

As the elements of integrated circuits are downsized to the nanoscale, the current Cu-based interconnects are facing limitations due to increased resistivity and decreased current-carrying capacity because of scaling. Here, the bottom-up synthesis of single-crystalline WTe2 nanobelts and low- and high-field electrical characterization of nanoscale interconnect test structures in various ambient conditions are reported. Unlike exfoliated flakes obtained by the top-down approach, the bottom-up growth mode of WTe2 nanobelts allows systemic characterization of the electrical properties of WTe2 single crystals as a function of channel dimensions. Using a 1D heat transport model and a power law, it is determined that the breakdown of WTe2 devices under vacuum and with AlO x capping layer follows an ideal pattern for Joule heating, far from edge scattering. High-field electrical measurements and self-heating modeling demonstrate that the WTe2 nanobelts have a breakdown current density approaching ≈100 MA cm-2, remarkably higher than those of conventional metals and other transition-metal chalcogenides, and sustain the highest electrical power per channel length (≈16.4 W cm-1) among the interconnect candidates. The results suggest superior robustness of WTe2 against high-bias sweep and its possible applicability in future nanoelectronics.