Suppressing Nucleation in Metal-Organic Chemical Vapor Deposition of MoS2 Monolayers by Alkali Metal Halides

Substrate-induced strain in molybdenum disulfide grown by aerosol-assisted chemical vapor deposition

Transition metal dichalcogenides have been extensively studied in recent years because of their fascinating optical, electrical, and catalytic properties. However, low-cost, scalable production remains a challenge. Aerosol-assisted chemical vapor deposition (AACVD) provides a new method for scalable thin film growth. In this study, we demonstrate the growth of molybdenum disulfide (MoS2) thin films using AACVD method. This method proves its suitability for low-temperature growth of MoS2thin films on various substrates, such as glass, silicon dioxide, quartz, silicon, hexagonal boron nitride, and highly ordered pyrolytic graphite. The as-grown MoS2shows evidence of substrate-induced strain. The type of strain and the morphology of the as-grown MoS2highly depend on the growth substrate's surface roughness, crystallinity, and chemical reactivity. Moreover, the as-grown MoS2shows the presence of both direct and indirect band gaps, suitable for exploitation in future electronics and optoelectronics.

Lung cancer cells detection by a photoelectrochemical MoS2 biosensing chip

This research aims to explore the potential application of this approach in the production of biosensor chips. The biosensor chip is utilized for the identification and examination of early-stage lung cancer cells. The findings of the optical microscope were corroborated by the field emission scanning electron microscopy, which provided further evidence that the growth of MoS2 is uniform and that there is minimal disruption in the electrode, hence minimizing the likelihood of an open circuit creation. Furthermore, the bilayer structure of the produced MoS2 has been validated through the utilization of Raman spectroscopy. A research investigation was undertaken to measure the photoelectric current generated by three various types of clinical samples containing lung cancer cells, specifically the CL1, NCI-H460, and NCI-H520 cell lines. The findings from the empirical analysis indicate that the coefficient of determination (R-Square) for the linear regression model was approximately 98%. Furthermore, the integration of a double-layer MoS2 film resulted in a significant improvement of 38% in the photocurrent, as observed in the device's performance.

Unleashing the potential of tungsten disulfide: Current trends in biosensing and nanomedicine applications

The discovery of graphene ignites a great deal of interest in the research and advancement of two-dimensional (2D) layered materials. Within it, semiconducting transition metal dichalcogenides (TMDCs) are highly regarded due to their exceptional electrical and optoelectronic properties. Tungsten disulfide (WS2) is a TMDC with intriguing properties, such as biocompatibility, tunable bandgap, and outstanding photoelectric characteristics. These features make it a potential candidate for chemical sensing, biosensing, and tumor therapy. Despite the numerous reviews on the synthesis and application of TMDCs in the biomedical field, no comprehensive study still summarizes and unifies the research trends of WS2 from synthesis to biomedical applications. Therefore, this review aims to present a complete and thorough analysis of the current research trends in WS2 across several biomedical domains, including biosensing and nanomedicine, covering antibacterial applications, tissue engineering, drug delivery, and anticancer treatments. Finally, this review also discusses the potential opportunities and obstacles associated with WS2 to deliver a new outlook for advancing its progress in biomedical research.

Large-area single-crystal TMD growth modulated by sapphire substrates

Transition metal dichalcogenides (TMDs) have recently attracted extensive attention due to their unique physical and chemical properties; however, the preparation of large-area TMD single crystals is still a great challenge. Chemical vapor deposition (CVD) is an effective method to synthesize large-area and high-quality TMD films, in which sapphires as suitable substrates play a crucial role in anchoring the source material, promoting nucleation and modulating epitaxial growth. In this review, we provide an insightful overview of different epitaxial mechanisms and growth behaviors associated with the atomic structure of sapphire surfaces and the growth parameters. First, we summarize three epitaxial growth mechanisms of TMDs on sapphire substrates, namely, van der Waals epitaxy, step-guided epitaxy, and dual-coupling-guided epitaxy. Second, we introduce the effects of polishing, cutting, and annealing processing of the sapphire surface on the TMD growth. Finally, we discuss the influence of other growth parameters, such as temperature, pressure, carrier gas, and substrate position, on the growth kinetics of TMDs. This review might provide deep insights into the controllable growth of large-area single-crystal TMDs on sapphires, which will propel their practical applications in high-performance nanoelectronics and optoelectronics.

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

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.

Controlled Synthesis and Accurate Doping of Wafer-Scale 2D Semiconducting Transition Metal Dichalcogenides

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.

Large area MoS2 thin film growth by direct sulfurization

In this study, we present the growth of monolayer MoS₂ (molybdenum disulfide) film. Mo (molybdenum) film was formed on a sapphire substrate through e-beam evaporation, and triangular MoS₂ film was grown by direct sulfurization. First, the growth of MoS₂ was observed under an optical microscope. The number of MoS₂ layers was analyzed by Raman spectrum, atomic force microscope (AFM), and photoluminescence spectroscopy (PL) measurement. Different sapphire substrate regions have different growth conditions of MoS₂. The growth of MoS₂ is optimized by controlling the amount and location of precursors, adjusting the appropriate growing temperature and time, and establishing proper ventilation. Experimental results show the successful growth of a large-area single-layer MoS₂ on a sapphire substrate through direct sulfurization under a suitable environment. The thickness of the MoS₂ film determined by AFM measurement is about 0.73 nm. The peak difference between the Raman measurement shift of 386 and 405 cm^-1 is 19.1 cm^-1, and the peak of PL measurement is about 677 nm, which is converted into energy of 1.83 eV, which is the size of the direct energy gap of the MoS₂ thin film. The results verify the distribution of the number of grown layers. Based on the observation of the optical microscope (OM) images, MoS₂ continuously grows from a single layer of discretely distributed triangular single-crystal grains into a single-layer large-area MoS₂ film. This work provides a reference for growing MoS₂ in a large area. We expect to apply this structure to various heterojunctions, sensors, solar cells, and thin-film transistors.

Nucleation and Growth of Monolayer MoS2 at Multisteps of MoO2 Crystals by Sulfurization

Two-dimensional (2D) materials and their heterostructures are promising for next-generation optoelectronics, spintronics, valleytronics, and electronics. Despite recent progress in various growth studies of 2D materials, mechanical exfoliation of flakes is still the most common method to obtain high-quality 2D materials because precisely controlling material growth and synthesizing a single domain during the growth process of 2D materials, for the desired shape and quality, is challenging. Here, we report the nucleation and growth behaviors of monolayer MoS₂ by sulfurizing a faceted monoclinic MoO₂ crystal. The MoS₂ layers nucleated at the thickness steps of the MoO₂ crystal and grew epitaxially with crystalline correlation to the MoO₂ surface. The epitaxially grown MoS₂ layer expands outwardly on the SiO₂ substrate, resulting in a monolayer single-crystal film, despite multiple nucleations of MoS₂ layers on the MoO₂ surface owing to several thickness steps. Although the photoluminescence of MoS₂ is quenched owing to efficient charge transfer between MoS₂ and metallic MoO₂, the MoS₂ stretched out to the SiO₂ substrate shows a high carrier mobility of (15 cm² V^-1 s^-1), indicating that a high-quality monolayer MoS₂ film can be grown using the MoO₂ crystal as a seed and precursor. Our work shows a method to grow high-quality MoS₂ using a faceted MoO₂ crystal and provides a deeper understanding of the nucleation and growth of 2D materials on a step-like surface.

Vapor-Liquid-Solid Growth of Site-Controlled Monolayer MoS2 Films Via Pressure-Induc ed Supercritical Phase Nucleation

In this study, a novel pressure-induced supercritical phase nucleation method is proposed to synthesize monolayer MoS₂ films, which is promoter free and can avoid contamination of films derived from these heterogeneous promoters in most of the existing techniques. The low-crystallinity and size-controlled MoO₂(acac)₂ particles are recrystallized on the substrate via the pressure-sensitive solvent capacity of supercritical CO₂ and these particles are used as growth sites. The size of single-crystal MoS₂ on the substrate is found to be dependent on the wetting area of the pyrolyzed precursor droplets (MoO₂) on the surface, and the formation of continuous films with high coverage is mainly controlled by the coalescence of MoO₂ droplets. It is enhanced by the increase of the nucleation site density, which can be adjusted by the supersaturation of the supercritical fluid solution. Our findings pave a new way for the controllable growth of MoS₂ and other two-dimensional materials and provide sufficient and valuable evidence for vapor-liquid-solid growth.

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

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

Sb-Mediated Tuning of Growth- and Exciton Dynamics in Entirely Catalyst-Free GaAsSb Nanowires

Vapor-liquid-solid (VLS) growth is the mainstream method in realizing advanced semiconductor nanowires (NWs), as widely applied to many III-V compounds. It is exclusively explored also for antimony (Sb) compounds, such as the relevant GaAsSb-based NW materials, although morphological inhomogeneities, phase segregation, and limitations in the supersaturation due to Sb strongly inhibit their growth dynamics. Fundamental advances are now reported here via entirely catalyst-free GaAsSb NWs, where particularly the Sb-mediated effects on the NW growth dynamics and physical properties are investigated in this novel growth regime. Remarkably, depending on GaAsSb composition and nature of the growth surface, both surfactant and anti-surfactant action is found, as seen by transitions between growth acceleration and deceleration characteristics. For threshold Sb-contents up to 3-4%, adatom diffusion lengths are increased ≈sevenfold compared to Sb-free GaAs NWs, evidencing the significant surfactant effect. Furthermore, microstructural analysis reveals unique Sb-mediated transitions in compositional structure, as well as substantial reduction in twin defect density, ≈tenfold over only small compositional range (1.5-6% Sb), exhibiting much larger dynamics as found in VLS-type GaAsSb NWs. The effect of such extended twin-free domains is corroborated by ≈threefold increases in exciton lifetime (≈4.5 ns) due to enlarged electron-hole pair separation in these phase-pure NWs.

Untangling the intertwined: metallic to semiconducting phase transition of colloidal MoS2 nanoplatelets and nanosheets

2D semiconducting transition metal dichalcogenides (TMDCs) are highly promising materials for future spin- and valleytronic applications and exhibit an ultrafast response to external (optical) stimuli which is essential for optoelectronics. Colloidal nanochemistry on the other hand is an emerging alternative for the synthesis of 2D TMDC nanosheet (NS) ensembles, allowing for the control of the reaction via tunable precursor and ligand chemistry. Up to now, wet-chemical colloidal syntheses yielded intertwined/agglomerated NSs with a large lateral size. Here, we show a synthesis method for 2D mono- and bilayer MoS₂ nanoplatelets with a particularly small lateral size (NPLs, 7.4 nm ± 2.2 nm) and MoS₂ NSs (22 nm ± 9 nm) as a reference by adjusting the molybdenum precursor concentration in the reaction. We find that in colloidal 2D MoS₂ syntheses initially a mixture of the stable semiconducting and the metastable metallic crystal phase is formed. 2D MoS₂ NPLs and NSs then both undergo a full transformation to the semiconducting crystal phase by the end of the reaction, which we quantify by X-ray photoelectron spectroscopy. Phase pure semiconducting MoS₂ NPLs with a lateral size approaching the MoS₂ exciton Bohr radius exhibit strong additional lateral confinement, leading to a drastically shortened decay of the A and B exciton which is characterized by ultrafast transient absorption spectroscopy. Our findings represent an important step for utilizing colloidal TMDCs, for example small MoS₂ NPLs represent an excellent starting point for the growth of heterostructures for future colloidal photonics.

A critical review of fabrication challenges and reliability issues in top/bottom gated MoS2field-effect transistors

The voyage of semiconductor industry to decrease the size of transistors to achieve superior device performance seems to near its physical dimensional limitations. The quest is on to explore emerging material systems that offer dimensional scaling to match the silicon- based technologies. The discovery of atomic flat two-dimensional materials has opened up a completely new avenue to fabricate transistors at sub-10 nanometer level which has the potential to compete with modern silicon-based semiconductor devices. Molybdenum disulfide (MoS₂) is a two-dimensional layered material with novel semiconducting properties at atomic level seems like a promising candidate that can possibly meet the expectation of Moore's law. This review discusses the various 'fabrication challenges' in making MoS₂based electronic devices from start to finish. The review outlines the intricate challenges of substrate selection and various synthesis methods of mono layer and few-layer MoS₂. The review focuses on the various techniques and methods to minimize interface defect density at substrate/MoS₂interface for optimum MoS₂-based device performance. The tunable band-gap of MoS₂with varying thickness presents a unique opportunity for contact engineering to mitigate the contact resistance issue using different elemental metals. In this work, we present a comprehensive overview of different types of contact materials with myriad geometries that show a profound impact on device performance. The choice of different insulating/dielectric gate oxides on MoS₂in co-planar and vertical geometry is critically reviewed and the physical feasibility of the same is discussed. The experimental constraints of different encapsulation techniques on MoS₂and its effect on structural and electronic properties are extensively discussed.

Self-assembled GA-Repeated Peptides as a Biomolecular Scaffold for Biosensing with MoS2 Electrochemical Transistors

Biosensors with two-dimensional materials have gained wide interest due to their high sensitivity. Among them, single-layer MoS2 has become a new class of biosensing platform owing to its semiconducting property. Immobilization of bioprobes directly onto the MoS2 surface with chemical bonding or random physisorption has been widely studied. However, these approaches potentially cause a reduction of conductivity and sensitivity of the biosensor. In this work, we designed peptides that spontaneously align into monomolecular-thick nanostructures on electrochemical MoS2 transistors in a non-covalent fashion and act as a biomolecular scaffold for efficient biosensing. These peptides consist of repeated domains of glycine and alanine in the sequence and form self-assembled structures with sixfold symmetry templated by the lattice of MoS2. We investigated electronic interactions of self-assembled peptides with MoS2 by designing their amino acid sequence with charged amino acids at both ends. Charged amino acids in the sequence showed a correlation with the electrical properties of single-layer MoS2, where negatively charged peptides caused a shift of threshold voltage in MoS2 transistors and neutral and positively charged peptides had no significant effect on the threshold voltage. The transconductance of transistors had no decrease due to the self-assembled peptides, indicating that aligned peptides can act as a biomolecular scaffold without degrading the intrinsic electronic properties for biosensing. We also investigated the impact of peptides on the photoluminescence (PL) of single-layer MoS2 and found that the PL intensity changed sensitively depending on the amino acid sequence of peptides. Finally, we demonstrated a femtomolar-level sensitivity of biosensing using biotinylated peptides to detect streptavidin.

High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing

Nanopores in two-dimensional (2D) membranes hold immense potential in single-molecule sensing, osmotic power generation, and information storage. Recent advances in 2D nanopores, especially on single-layer MoS2, focus on the scalable growth and manufacturing of nanopore devices. However, there still remains a bottleneck in controlling the nanopore stability in atomically thin membranes. Here, we evaluate the major factors responsible for the instability of the monolayer MoS2 nanopores. We identify chemical oxidation and delamination of monolayers from their underlying substrates as the major reasons for the instability of MoS2 nanopores. Surface modification of the substrate and reducing the oxygen from the measurement solution improves nanopore stability and dramatically increases their shelf-life. Understanding nanopore growth and stability can provide insights into controlling the pore size, shape and can enable long-term measurements with a high signal-to-noise ratio and engineering durable nanopore devices.

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

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

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.

Electrical spectroscopy of defect states and their hybridization in monolayer MoS2

Defects in solids are unavoidable and can create complex electronic states that can significantly influence the electrical and optical properties of semiconductors. With the rapid progress in the integration of 2D semiconductors in practical devices, it is imperative to understand and characterize the influence of defects in this class of materials. Here, we examine the electrical response of defect filling and emission using deep level transient spectroscopy (DLTS) and reveal defect states and their hybridization in a monolayer MOCVD-grown material deposited on CMOS-compatible substrates. Supported by aberration-corrected STEM imaging and theoretical calculations, we find that neighboring sulfur vacancy pairs introduce additional shallow trap states via hybridization of individual vacancy levels. Even though such vacancy pairs only represent ~10% of the total defect concentration, they can have a substantial influence on the off currents and switching slopes of field-effect transistors based on 2D semiconductors. Our technique, which can quantify the energy states of different defects and their interactions, allows rapid and nondestructive electrical characterization of defect states important for the defect engineering of 2D semiconductors.

2D Materials-Based Static Random-Access Memory

2D transition-metal dichalcogenide semiconductors, such as MoS₂ and WSe₂ , with adequate bandgaps are promising channel materials for ultrascaled logic transistors. This scalability study of 2D material (2DM)-based field-effect transistor (FET) and static random-access memory (SRAM) cells analyzing the impact of layer thickness reveals that the monolayer 2DM FET with superior electrostatics is beneficial for its ability to mitigate the read-write conflict in an SRAM cell at scaled technology nodes (1-2.1 nm). Moreover, the monolayer 2DM SRAM exhibits lower cell read access time and write time than the bilayer and trilayer 2DM SRAM cells at fixed leakage power. This simulation predicts that the optimization of 2DM SRAM designed with state-of-the-art contact resistance, mobility, and equivalent oxide thickness leads to excellent stability and operation speed at the 1-nm node. Applying the nanosheet (NS) gate-all-around (GAA) structure to 2DM further reduces cell read access time and write time and improves the area density of the SRAM cells, demonstrating a feasible scaling path beyond Si technology using 2DM NSFETs. In addition to the device design, the process challenges for 2DM NSFETs, including the cost-effective stacking of 2DM layers, formation of electrical contacts, suspended 2DM channels, and GAA structures, are also discussed.

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.

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.

Field-Effect Transistor Based on 2D Microcrystalline MoS2 Film Grown by Sulfurization of Atomically Layer Deposited MoO3

Atomically thin molybdenum disulfide (MoS₂) is a promising channel material for next-generation thin-body field-effect transistors (FETs), which makes the development of methods allowing for its controllable synthesis over a large area an essential task. Currently, one of the cost-effective ways of its synthesis is the sulfurization of preliminary grown oxide- or metallic film. However, despite apparent progress in this field, the electronic quality of the obtained MoS₂ is inferior to that of exfoliated samples, making the detailed investigation of the sulfurized films' properties of great interest. In this work, we synthesized continuous MoS₂ films with a thickness of ≈2.2 nm via the sulfurization of an atomic-layer-deposited MoO₃ layer. X-ray photoelectron spectroscopy, transmission electron microscopy, and Raman spectroscopy indicated the appropriate chemical composition and microcrystalline structure of the obtained MoS₂ films. The semiconductor quality of the synthesized films was confirmed by the fabrication of a field-effect transistor (FET) with an I_on/I_off ratio of ≈40, which was limited primarily by the high contact resistance. The Schottky barrier height at the Au/MoS₂ interface was found to be ≈1.2 eV indicating the necessity of careful contact engineering. Due to its simplicity and cost-effectiveness, such a technique of MoS₂ synthesis still appears to be highly attractive for its applications in next-generation microelectronics. Therefore, further research of the electronic properties of films obtained via this technique is required.

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.

Recent Progress in Phase Regulation, Functionalization, and Biosensing Applications of Polyphase MoS2

The disulfide compounds of molybdenum (MoS₂ ) are layered van der Waals materials that exhibit a rich array of polymorphic structures. MoS₂ can be roughly divided into semiconductive phase and metallic phase according to the difference in electron filling state of the 4d orbital of Mo atom. The two phases show completely different properties, leading to their diverse applications in biosensors. But to some extent, they compensate for each other. This review first introduces the relationship between phase state and the chemical/physical structures and properties of MoS₂ . Furthermore, the synthetic methods are summarized and the preparation strategies for metastable phases are highlighted. In addition, examples of electronic and chemical property designs of MoS₂ by means of doping and surface modification are outlined. Finally, studies on biosensors based on MoS₂ in recent years are presented and classified, and the roles of MoS₂ with different phases are highlighted. This review offers references for the selection of materials to construct different types of biosensors based on MoS₂ , and provides inspiration for sensing performance enhancement.

Gas-Phase Alkali Metal-Assisted MOCVD Growth of 2D Transition Metal Dichalcogenides for Large-Scale Precise Nucleation Control

Advances in large-area and high-quality 2D transition metal dichalcogenides (TMDCs) growth are essential for semiconductor applications. Here, the gas-phase alkali metal-assisted metal-organic chemical vapor deposition (GAA-MOCVD) of 2D TMDCs is reported. It is determined that sodium propionate (SP) is an ideal gas-phase alkali-metal additive for nucleation control in the MOCVD of 2D TMDCs. The grain size of MoS₂ in the GAA-MOCVD process is larger than that in the conventional MOCVD process. This method can be applied to the growth of various TMDCs (MoS₂ , MoSe₂ , WSe₂ , and WSe₂ ) and the generation of large-scale continuous films. Furthermore, the growth behaviors of MoS₂ under different SP and oxygen injection time conditions are systematically investigated to determine the effects of SP and oxygen on nucleation control in the GAA-MOCVD process. It is found that the combination of SP and oxygen increases the grain size and nucleation suppression of MoS₂ . Thus, the GAA-MOCVD with a precise and controllable supply of a gas-phase alkali metal and oxygen allows achievement of optimum growth conditions that maximizes the grain size of MoS₂ . It is expected that GAA-MOCVD can provide a way for batch fabrication of large-scale atomically thin electronic devices based on 2D semiconductors.

Optical and Material Characteristics of MoS2/Cu2O Sensor for Detection of Lung Cancer Cell Types in Hydroplegia

In this study, n-type MoS₂ monolayer flakes are grown through chemical vapor deposition (CVD), and a p-type Cu₂O thin film is grown via electrochemical deposition. The crystal structure of the grown MoS₂ flakes is analyzed through transmission electron microscopy. The monolayer structure of the MoS₂ flakes is verified with Raman spectroscopy, multiphoton excitation microscopy, atomic force microscopy, and photoluminescence (PL) measurements. After the preliminary processing of the grown MoS₂ flakes, the sample is then transferred onto a Cu₂O thin film to complete a p-n heterogeneous structure. Data are confirmed via scanning electron microscopy, SHG, and Raman mapping measurements. The luminous energy gap between the two materials is examined through PL measurements. Results reveal that the thickness of the single-layer MoS₂ film is 0.7 nm. PL mapping shows a micro signal generated at the 627 nm wavelength, which belongs to the B2 excitons of MoS₂ and tends to increase gradually when it approaches 670 nm. Finally, the biosensor is used to detect lung cancer cell types in hydroplegia significantly reducing the current busy procedures and longer waiting time for detection. The results suggest that the fabricated sensor is highly sensitive to the change in the photocurrent with the number of each cell, the linear regression of the three cell types is as high as 99%. By measuring the slope of the photocurrent, we can identify the type of cells and the number of cells.

Additive-Assisted Growth of Scaled and Quality 2D Materials

2D materials are increasingly becoming key components in modern electronics because of their prominent electronic and optoelectronic properties. The central and premise to the entire discipline of 2D materials lie in the high-quality and scaled preparations. The chemical vapor deposition (CVD) method offers compelling benefits in terms of scalability and controllability in shaping large-area and high-quality 2D materials. The past few years have witnessed development of numerous CVD growth strategies, with the use of additives attracting substantial attention in the production of scaled 2D crystals. This review provides an overview of different additives used in CVD growth of 2D materials, as well as a methodical demonstration of their vital roles. In addition, the intrinsic mechanisms of the production of scaled 2D crystals with additives are also discussed. Lastly, reliable guidance on the future design of optimal CVD synthesis routes is provided by analyzing the accessibility, pricing, by-products, controllability, universality, and commercialization of various additives.

Low-Power Artificial Neural Network Perceptron Based on Monolayer MoS2

Machine learning and signal processing on the edge are poised to influence our everyday lives with devices that will learn and infer from data generated by smart sensors and other devices for the Internet of Things. The next leap toward ubiquitous electronics requires increased energy efficiency of processors for specialized data-driven applications. Here, we show how an in-memory processor fabricated using a two-dimensional materials platform can potentially outperform its silicon counterparts in both standard and nontraditional Von Neumann architectures for artificial neural networks. We have fabricated a flash memory array with a two-dimensional channel using wafer-scale MoS₂. Simulations and experiments show that the device can be scaled down to sub-micrometer channel length without any significant impact on its memory performance and that in simulation a reasonable memory window still exists at sub-50 nm channel lengths. Each device conductance in our circuit can be tuned with a 4-bit precision by closed-loop programming. Using our physical circuit, we demonstrate seven-segment digit display classification with a 91.5% accuracy with training performed ex situ and transferred from a host. Further simulations project that at a system level, the large memory arrays can perform AlexNet classification with an upper limit of 50 000 TOpS/W, potentially outperforming neural network integrated circuits based on double-poly CMOS technology.

Tuning the morphology of 2D transition metal chalcogenides via oxidizing conditions

Two-dimensional transition metal chalcogenides (TMCs) are emerging as an intriguing platform to realize nascent properties in condensed matter physics, materials science and device engineering. Controllable growing of TMCs becomes increasingly important, especially for the layer number, doping, and morphology. Here, we successfully tune the morphology of MoS₂, MoSe₂, WS₂and WSe₂, from homogenous films to individual single crystalline grains only via changing the oxidizing growth conditions. The oxidization degrees are determined by the oxygen that adsorbed on substrates and the oxygen concentrations in reaction gas together. We find the homogenous films are easily formed under the reductive conditions, triangular grains prefer the weak oxidizing conditions, and medium oxidizing conditions bring in dendritic grains with higher oxygen doping and inhomogenous photoluminescence intensities from edge to interior regions shown in the dendritic grains. These growth rules under different oxidizing conditions are easily generalized to other TMCs, which also show potential for growing specific TMCs with designed oxygen doping levels.

Epitaxy of 2D Materials toward Single Crystals

Two-dimensional (2D) materials exhibit unique electronic, optical, magnetic, mechanical, and thermal properties due to their special crystal structure and thus have promising potential in many fields, such as in electronics and optoelectronics. To realize their real applications, especially in integrated devices, the growth of large-size single crystal is a prerequisite. Up to now, the most feasible way to achieve 2D single crystal growth is the epitaxy: growth of 2D materials of one or more specific orientations with single-crystal substrate. Only when the 2D domains have the same orientation, they can stitch together seamlessly and single-crystal 2D films can be obtained. In this view, four different epitaxy modes of 2D materials on various substrates are presented, including van der Waals epitaxy, edge epitaxy, step-guided epitaxy, and in-plane epitaxy focusing on the growth of graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenide (TMDC). The lattice symmetry relation and the interaction between 2D materials and the substrate are the key factors determining the epitaxy behaviors and thus are systematically discussed. Finally, the opportunities and challenges about the epitaxy of 2D single crystals in the future are summarized.

A Review on Chemical Vapour Deposition of Two-Dimensional MoS2 Flakes

Two-dimensional (2D) materials such as graphene, transition metal dichalcogenides, and boron nitride have recently emerged as promising candidates for novel applications in sensing and for new electronic and photonic devices. Their exceptional mechanical, electronic, optical, and transport properties show peculiar differences from those of their bulk counterparts and may allow for future radical innovation breakthroughs in different applications. Control and reproducibility of synthesis are two essential, key factors required to drive the development of 2D materials, because their industrial application is directly linked to the development of a high-throughput and reliable technique to obtain 2D layers of different materials on large area substrates. Among various methods, chemical vapour deposition is considered an excellent candidate for this goal thanks to its simplicity, widespread use, and compatibility with other processes used to deposit other semiconductors. In this review, we explore the chemical vapour deposition of MoS2, considered one of the most promising and successful transition metal dichalcogenides. We summarize the basics of the synthesis procedure, discussing in depth: (i) the different substrates used for its deposition, (ii) precursors (solid, liquid, gaseous) available, and (iii) different types of promoters that favour the growth of two-dimensional layers. We also present a comprehensive analysis of the status of the research on the growth mechanisms of the flakes.

Recent Advances in Processing and Applications of Heterobimetallic Oxide Thin Films by Aerosol-assisted Chemical Vapor Deposition

The fabrication of smart, efficient, and innovative devices critically needs highly refined thin-film nanomaterials; therefore, facile, scalable, and economical methods of thin films production are highly sought-after for the sustainable growth of the hi-tech industry. The chemical vapor deposition (CVD) technique is widely implemented at the industrial level due to its versatile features. However, common issues with a precursor, such as reduced volatility and thermal stability, restrict the use of CVD to produce novel and unique materials. A modified CVD approach, named aerosol-assisted CVD (AACVD), has been the center of attention due to its remarkable tendency to fabricate uniform, homogenous, and distinct nano-architecture thin films in an uncomplicated and straightforward manner. Above all, AACVD can utilize any custom-made or commercially available precursors, which can be transformed into a transparent solution in a common organic solvent; thus, a vast array of compounds can be used for the formation of nanomaterial thin films. This review article highlights the importance of AACVD in fabricating heterobimetallic oxide thin films and their potential in making energy production (e. g., photoelectrochemical water splitting), energy storage (e. g., supercapacitors), and environmental protection (e. g., electrochemical sensors) devices. A heterobimetallic oxide system involves two metallic species either in a composite, solid solution, or metal-doped metal oxides. Moreover, the AACVD tunable parameters, such as temperature, deposition time, and precursor, which drastically affect thin films microstructure and their performance in device applications, are also discussed. Lastly, the key challenges and issues of scaling up AACVD to the industrial level and processing for emerging functional materials are also highlighted.

Superconducting 2D NbS2 Grown Epitaxially by Chemical Vapor Deposition

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

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

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

Revealing the Brønsted-Evans-Polanyi relation in halide-activated fast MoS2 growth toward millimeter-sized 2D crystals

Achieving large-size two-dimensional (2D) crystals is key to fully exploiting their remarkable functionalities and application potentials. Chemical vapor deposition growth of 2D semiconductors such as monolayer MoS2 has been reported to be activated by halide salts, for which various investigations have been conducted to understand the underlying mechanism from different aspects. Here, we provide experimental evidence showing that the MoS2 growth dynamics are halogen dependent through the Brønsted-Evans-Polanyi relation, based on which we build a growth model by considering MoS2 edge passivation by halogens, and theoretically reproduce the trend of our experimental observations. These mechanistic understandings enable us to further optimize the fast growth of MoS2 and reach record-large domain sizes that should facilitate practical applications.

Wafer-Scale Uniform Growth of an Atomically Thin MoS2 Film with Controlled Layer Numbers by Metal-Organic Chemical Vapor Deposition

The growth control of a molybdenum disulfide (MoS₂) thin film, including the number of layers, growth rate, and electrical property modulation, remains a challenge. In this study, we synthesized MoS₂ thin films using the metal-organic chemical vapor deposition (MOCVD) method with a 2 inch wafer scale and achieved high thickness uniformity according to the positions on the substrate. In addition, we successfully controlled the number of MoS₂ layers to range from one to five, with a growth rate of 10 min per layer. The layer-dependent optical and electrical properties were characterized by photoluminescence, Raman spectroscopy, differential reflectance spectroscopy, and field effect transistors. To guide the growth of MoS₂, we summarized the relation between the growth aspects and the precursor control in the form of a growth map. Reference to this growth map enabled control of the growth rate, domain density, and domain size according to the application purposes. Finally, we confirmed the electrical performance of MOCVD-grown MoS₂ with five layers under a high-κ dielectric environment, which exhibited an on/off current ratio of 10^∼6 and a maximum field effect mobility of 8.6 cm² V^-1 s^-1.

Oxidation and Degradation of WS2 Monolayers Grown by NaCl-Assisted Chemical Vapor Deposition: Mechanism and Prevention

The preservation of two-dimensional WS₂ in the environment is a concern for researchers. In addition to water vapor and oxygen, the latest research points out that degradation is directly related to light absorption. Based on the selection rules of nonlinear optics, two-photon absorption is dipole forbidden in the exciton 1s states, but second-harmonic generation (SHG) is allowed with virtual transitions. According to this mechanism, we proved that SHG is an optical detection method with non-photooxidative damage and energy characteristics. With this detection method, we can explore the oxidation and degradation mechanisms of WS₂ grown by NaCl-assisted chemical vapor deposition in its original state. The WS₂ monolayers that use NaCl to assist in growth have undergone different degradation processes, starting to oxidize from random positions in the triangular flake. We use a photocatalytic reaction to explain the photo-induced degradation mechanism with sulfur vacancies. It was further found that WS₂ grown with NaCl assistance is hydrolyzed in a dark and high-humidity environment, which does not occur in pure WS₂. Finally, we demonstrated that changing the direction of the sapphire substrate relative to the gas flow direction to grow NaCl-assisted WS₂ can greatly improve its stability in the ambient atmosphere, even when exposed to light. The optimal geometric structures and ground state energies are investigated by the density functional theory-based calculations. According to the orientation and symmetry of NaCl-assisted WS₂, we can expect that it will have a better growth quality when the gas flow direction is perpendicular to the [112̄0] direction of the sapphire substrate. This contributes to the nucleation and subsequent growth of NaCl-assisted WS₂. This research provides a more stable optical inspection method than other established methods and greatly improves the operational stability of NaCl-assisted WS₂ under environmental conditions.

Two-dimensional materials as solid-state nanopores for chemical sensing

Solid-state nanopores as a versatile alternative to biological nanopores have grown tremendously over the last two decades. They exhibit unique characteristics including mechanical robustness, thermal and chemical stability, easy modifications and so on. Moreover, the pore size of a solid-state nanopore could be accurately controlled from sub-nanometers to hundreds of nanometers based on the experimental requirements, presenting better adaptability than biological nanopores. Two-dimensional (2D) materials with single layer thicknesses and highly ordered structures have great potential as solid-state nanopores. In this perspective, we introduced three kinds of substrate-supported 2D material solid-state nanopores, including graphene, MoS₂ and MOF nanosheets, which exhibited big advantages compared to traditional solid-state nanopores and other biological counterparts. Besides, we suggested the fabrication and modulation of 2D material solid-state nanopores. We also discussed the applications of 2D materials as solid-state nanopores for ion transportation, DNA sequencing and biomolecule detection.

Growth mechanism and atomic structure of group-IIA compound-promoted CVD-synthesized monolayer transition metal dichalcogenides

Developing promoters that can boost the growth quality, efficiency, and robustness of two-dimensional (2D) transition metal dichalcogenides is significant for their industrial applications. Herein a new group (group IIA) of promoters in the periodic table has been disclosed, whose chlorides (especially CaCl₂ and SrCl₂) exhibit a versatile promoting effect on the CVD growth of various TMD monolayers, including hexagonal MoS₂, MoSe₂, Re doped MoS₂, and triclinic ReS₂. The promoting effect of group IIA promoters relies on the appropriate dose and is strongly substrate-dependent. The performances of five typical group IA-IIA metal chlorides are ranked by quantitative investigations, displaying periodic variations closely related to the electronegativities of the metal elements. A brand-new acid-base match model is proposed, attributing the promoting mechanism to an increase of the substrate basicity due to the usage of promoters, thus leading to the sufficient adsorption of the acidic precursor. Aberration-corrected annular dark field scanning transmission electron microscopy (ADF-STEM) was applied, unveiling anomalous grain boundaries (GBs) with a low density of coincident sites in the as-grown ReS₂ and detailed atomic configurations of Re doped MoS₂. This work expands the promoter library and gives an insight into GB engineering for the CVD growth of 2D TMDs.

A Roadmap for Disruptive Applications and Heterogeneous Integration Using Two-Dimensional Materials: State-of-the-Art and Technological Challenges

This Mini Review attempts to establish a roadmap for two-dimensional (2D) material-based microelectronic technologies for future/disruptive applications with a vision for the semiconductor industry to enable a universal technology platform for heterogeneous integration. The heterogeneous integration would involve integrating orthogonal capabilities, such as different forms of computing (classical, neuromorphic, and quantum), all forms of sensing, digital and analog memories, energy harvesting, and so forth, all in a single chip using a universal technology platform. We have reviewed the state-of-the-art 2D materials such as graphene, transition metal dichalcogenides, phosphorene and hexagonal boron nitride, and so forth, and how they offer unique possibilities for a range of futuristic/disruptive applications. Besides, we have discussed the technological and fundamental challenges in enabling such a universal technology platform, where the world stands today, and what gaps are required to be filled.

Gas-Phase "Prehistory" and Molecular Precursors in Monolayer Metal Dichalcogenides Synthesis: The Case of MoS2

Two-dimensional MoS₂ is one of the most promising materials for nanoelectronics due to its semiconducting nature and plethora of extraordinary properties. The main method for mass production of large-scale, high-quality MoS₂ monolayers is chemical vapor deposition (CVD). Yet, the details of the chemistry occurring during the synthesis remain largely unknown, hindering process optimization. Combining ab initio molecular dynamics (AIMD) simulations and first-principles calculations allows us to explore the complete processes of MoS₂ monolayer growth at the atomic level. We find that solid MoO₃ precursor sublimates forming ringlike molecules, such as Mo₃O₉, which can later be regarded as gas-phase Mo-carrier reactants, undergoing sulfurization in three main stages: ring opening, chain breaking as the rate-limiting step, and further sulfurization. The fully sulfurized MoS₆ molecule emerges as an immediate gas precursor to the crystal growth, as it reacts to join the MoS₂-layer edge, with the release of a S₄ molecule. Our comprehensive study provides detailed insights into the microscopic reaction mechanisms of MoS₂ CVD growth and guidance for optimizing the synthesis parameters for transition metal dichalcogenides.

Engineering Wafer-Scale Epitaxial Two-Dimensional Materials through Sapphire Template Screening for Advanced High-Performance Nanoelectronics

In view of its epitaxial seeding capability, c-plane single crystalline sapphire represents one of the most enticing, industry-compatible templates to realize manufacturable deposition of single crystalline two-dimensional transition metal dichalcogenides (MX₂) for functional, ultrascaled, nanoelectronic devices beyond silicon. Despite sapphire being atomically flat, the surface topography, structure, and chemical termination vary between sapphire terraces during the fabrication process. To date, it remains poorly understood how these sapphire surface anomalies affect the local epitaxial registry and the intrinsic electrical properties of the deposited MX₂ monolayer. Therefore, molybdenum disulfide (MoS₂) is deposited by metal-organic chemical vapor deposition (MOCVD) in an industry-standard epitaxial reactor on two types of c-plane sapphire with distinctly different terrace and step dimensions. Complementary scanning probe microscopy techniques reveal an inhomogeneous conductivity profile in the first epitaxial MoS₂ monolayer on both sapphire templates. MoS₂ regions with poor conductivity correspond to sapphire terraces with uncontrolled topography and surface structure. By intentionally applying a substantial off-axis cut angle (1° in this work), the sapphire terrace width and step height-and thus also surface structure-become more uniform across the substrate and MoS₂ conducts the current more homogeneously. Moreover, these effects propagate into the extrinsic MoS₂ device performance: the field-effect transistor variability reduces both within and across wafers at higher median electron mobility. Carefully controlling the sapphire surface topography and structure proves an essential prerequisite to systematically study and control the MX₂ growth behavior and capture the influence on its structural and electrical properties.

Synthesis of High-Performance Monolayer Molybdenum Disulfide at Low Temperature

The large-area synthesis of high-quality MoS₂ plays an important role in realizing industrial applications of optoelectronics, nanoelectronics, and flexible devices. However, current techniques for chemical vapor deposition (CVD)-grown MoS₂ require a high synthetic temperature and a transfer process, which limits its utilization in device fabrications. Here, the direct synthesis of high-quality monolayer MoS₂ with the domain size up to 120 µm by metal-organic CVD (MOCVD) at a temperature of 320 °C is reported. Owing to the low-substrate temperature, the MOCVD-grown MoS₂ exhibits low impurity doping and nearly unstrained properties on the growth substrate, demonstrating enhanced electronic performance with high electron mobility of 68.3 cm² V^-1 s^-1 at room temperature. In addition, by tuning the precursor ratio, a better understanding of the MoS₂ growth process via a geometric model of the MoS₂ flake shape, is developed, which can provide further guidance for the synthesis of 2D materials.

Strategies, Status, and Challenges in Wafer Scale Single Crystalline Two-Dimensional Materials Synthesis

The successful exfoliation of graphene has given a tremendous boost to research on various two-dimensional (2D) materials in the last 15 years. Different from traditional thin films, a 2D material is composed of one to a few atomic layers. While atoms within a layer are chemically bonded, interactions between layers are generally weak van der Waals (vdW) interactions. Due to their particular dimensionality, 2D materials exhibit special electronic, magnetic, mechanical, and thermal properties, not found in their 3D counterparts, and therefore they have great potential in various applications, such as 2D materials-based devices. To fully realize their large-scale practical applications, especially in devices, wafer scale single crystalline (WSSC) 2D materials are indispensable. In this review, we present a detailed overview on strategies toward the synthesis of WSSC 2D materials while highlighting the recent progress on WSSC graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenide (TMDC) synthesis. The challenges that need to be addressed in future studies have also been described. In general, there have been two distinct routes to synthesize WSSC 2D materials: (i) allowing only one nucleus on a wafer scale substrate to be formed and developed into a large single crystal and (ii) seamlessly stitching a large number of unidirectionally aligned 2D islands on a wafer scale substrate, which is generally single crystalline. Currently, the synthesis of WSSC graphene has been realized by both routes, and WSSC hBN and MoS₂ have been synthesized by route (ii). On the other hand, the growth of other WSSC 2D materials and WSSC multilayer 2D materials still remains a big challenge. In the last section, we wrap up this review by summarizing the future challenges and opportunities in the synthesis of various WSSC 2D materials.

Super-resolved Optical Mapping of Reactive Sulfur-Vacancies in Two-Dimensional Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) represent a class of semiconducting two-dimensional (2D) materials with exciting properties. In particular, defects in 2D-TMDs and their molecular interactions with the environment can crucially affect their physical and chemical properties. However, mapping the spatial distribution and chemical reactivity of defects in liquid remains a challenge. Here, we demonstrate large area mapping of reactive sulfur-deficient defects in 2D-TMDs in aqueous solutions by coupling single-molecule localization microscopy with fluorescence labeling using thiol chemistry. Our method, reminiscent of PAINT strategies, relies on the specific binding of fluorescent probes hosting a thiol group to sulfur vacancies, allowing localization of the defects with an uncertainty down to 15 nm. Tuning the distance between the fluorophore and the docking thiol site allows us to control Föster resonance energy transfer (FRET) process and reveal grain boundaries and line defects due to the local irregular lattice structure. We further characterize the binding kinetics over a large range of pH conditions, evidencing the reversible adsorption of the thiol probes to the defects with a subsequent transitioning to irreversible binding in basic conditions. Our methodology provides a simple and fast alternative for large-scale mapping of nonradiative defects in 2D materials and can be used for in situ and spatially resolved monitoring of the interaction between chemical agents and defects in 2D materials that has general implications for defect engineering in aqueous condition.

Two-Step Growth of Uniform Monolayer MoS2 Nanosheets by Metal-Organic Chemical Vapor Deposition

To achieve large area growth of transition metal dichalcogenides of uniform monolayer thickness, we demonstrate metal-organic chemical vapor deposition (MOCVD) growth under low pressure followed by a high-temperature sulfurization process under atmospheric pressure (AP). Following sulfurization, the MOCVD-grown continuous MoS₂ film transforms into compact triangular crystals of uniform monolayer thickness as confirmed from the sharp distinct photoluminescence peak at 1.8 eV. Raman and X-ray photoelectron spectroscopies confirm that the structural disorders and chalcogen vacancies inherent to the as-grown MOCVD film are substantially healed and carbon/oxygen contaminations are heavily suppressed. The as-grown MOCVD film has a Mo/S ratio of 1:1.6 and an average defect length of ∼1.56 nm, which improve to 1:1.97 and ∼21 nm, respectively, upon sulfurization. The effect of temperature and duration of the sulfurization process on the morphology and stoichiometry of the grown film is investigated in detail. Compared to the APCVD growth, this two-step growth process shows more homogenous distribution of the triangular monolayer MoS₂ domains across the entire substrate, while demonstrating comparable electrical performance.

Analyzing growth kinematics and fractal dimensions of molybdenum disulfide films

Though the positive role of alkali halides in realizing large area growth of transition metal dichalcogenide layers has been validated, the film-growth kinematics has not yet been fully established. This work presents a systematic analysis of the MoS₂morphology for films grown under various pre-treatment conditions of the substrate with sodium chloride (NaCl). At an optimum NaCl concentration, the domain size of the monolayer increased by almost two orders of magnitude compared to alkali-free growth of MoS₂. The results show an inverse relationship between fractal dimension and areal coverage of the substrate with monolayers and multi-layers, respectively. Using the Fact-Sage software, the role of NaCl in determining the partial pressures of Mo- and S-based compounds in gaseous phase at the growth temperature is elucidated. The presence of alkali salts is shown to affect the domain size and film morphology by affecting the Mo and S partial pressures. Compared to alkali-free synthesis under the same growth conditions, MoS₂film growth assisted by NaCl results in ≈81% of the substrate covered by monolayers. Under ideal growth conditions, at an optimum NaCl concentration, nucleation was suppressed, and domains enlarged, resulting in large area growth of MoS₂monolayers. No evidence of alkali or halogen atoms were found in the composition analysis of the films. On the basis of Raman spectroscopy and photoluminescence measurements, the MoS₂films were found to be of good crystalline quality.

Growth-Etch Metal-Organic Chemical Vapor Deposition Approach of WS2 Atomic Layers

Metal-organic chemical vapor deposition (MOCVD) is one of the main methodologies used for thin-film fabrication in the semiconductor industry today and is considered one of the most promising routes to achieve large-scale and high-quality 2D transition metal dichalcogenides (TMDCs). However, if special measures are not taken, MOCVD suffers from some serious drawbacks, such as small domain size and carbon contamination, resulting in poor optical and crystal quality, which may inhibit its implementation for the large-scale fabrication of atomic-thin semiconductors. Here we present a growth-etch MOCVD (GE-MOCVD) methodology, in which a small amount of water vapor is introduced during the growth, while the precursors are delivered in pulses. The evolution of the growth as a function of the amount of water vapor, the number and type of cycles, and the gas composition is described. We show a significant domain size increase is achieved relative to our conventional process. The improved crystal quality of WS₂ (and WSe₂) domains wasis demonstrated by means of Raman spectroscopy, photoluminescence (PL) spectroscopy, and HRTEM studies. Moreover, time-resolved PL studies show very long exciton lifetimes, comparable to those observed in mechanically exfoliated flakes. Thus, the GE-MOCVD approach presented here may facilitate their integration into a wide range of applications.