Generalized Mechanistic Model for the Chemical Vapor Deposition of 2D Transition Metal Dichalcogenide Monolayers

Van der Waals Epitaxy of III-Nitride Semiconductors Based on 2D Materials for Flexible Applications

III-nitride semiconductors have attracted considerable attention in recent years owing to their excellent physical properties and wide applications in solid-state lighting, flat-panel displays, and solar energy and power electronics. Generally, GaN-based devices are heteroepitaxially grown on c-plane sapphire, Si (111), or 6H-SiC substrates. However, it is very difficult to release the GaN-based films from such single-crystalline substrates and transfer them onto other foreign substrates. Consequently, it is difficult to meet the ever-increasing demand for wearable and foldable applications. On the other hand, sp² -bonded two-dimensional (2D) materials, which exhibit hexagonal in-plane lattice arrangements and weakly bonded layers, can be transferred onto flexible substrates with ease. Hence, flexible III-nitride devices can be implemented through such 2D release layers. In this progress report, the recent advances in the different strategies for the growth of III-nitrides based on 2D materials are reviewed, with a focus on van der Waals epitaxy and transfer printing. Various attempts are presented and discussed herein, including the different kinds of 2D materials (graphene, hexagonal boron nitride, and transition metal dichalcogenides) used as release layers. Finally, current challenges and future perspectives regarding the development of flexible III-nitride devices are discussed.

Growth of Large-Area Homogeneous Monolayer Transition-Metal Disulfides via a Molten Liquid Intermediate Process

Growth of large-area, uniform, and high-quality monolayer transition-metal dichalcogenides (TMDs) for practical and industrial applications remains a long-standing challenge. The present study demonstrates a modified predeposited chemical vapor deposition (CVD) process by employing an annealing procedure before sulfurization, which helps in achieving large-area, highly uniform, and high-quality TMDs on various substrates. The annealing procedure resulted in a molten liquid state of the precursors in the CVD process, which not only facilitated a uniform redistribution of the precursor on the substrate (avoid the aggregation) because of the uniform redistribution of the liquid precursor on the substrate but more importantly avoided the undesired multilayer growth via the self-limited lateral supply precursors mechanism. A 2 in. uniform and continuous monolayer WS₂ film has been synthesized on the SiO₂/Si substrate. Moreover, uniform monolayer WS₂ single crystals can be prepared on more general and various substrates including sapphire, mica, quartz, and Si₃N₄ using the same growth procedure. Besides, this growth mechanism can be generalized to synthesize other monolayer TMDs such as MoS₂ and MoS₂/WS₂ heterostructures. Hence, the present method provides a generalized attractive strategy to grow large-area, uniform, single-layer two-dimensional (2D) materials. This study has significant implications in the advancement of batch production of various 2D-material-based devices for industrial and commercial applications.

Revealing the Grain Boundary Formation Mechanism and Kinetics during Polycrystalline MoS2 Growth

Controllable synthesis of MoS₂ with desired grain morphology via chemical vapor deposition (CVD) or physical vapor deposition (PVD) remains a challenge. Hence, it is important to understand polycrystalline growth of MoS₂ and further provide guidelines for its CVD/PVD growth. Here, we formulate a kinetic Monte Carlo (kMC) model aiming at predicting the grain boundary (GB) formation in the CVD/PVD growth of polycrystalline MoS₂. In the kMC model, the grain growth is via kink nucleation and propagation, whose energetic parameters and initial nucleus details are either from first-principles calculations or from experiments. Using the kMC model, we perform extensive simulations to predict the GB formation by using two, three, four, and five initial nuclei and compare the simulation results with previous experimental results. The obtained GB morphologies are in an excellent agreement with those experimental results. These agreements suggest that the proposed kMC model can correctly capture the mechanism and kinetics of GB formation. In particular, we reveal that the formation of smooth/rough GB is dictated by the two growth vectors for the kink propagation at the two associated grain edges, which is validated by our high-resolution scanning transmission electron microscopy images for PVD growth of MoS₂ grains. Besides, we have made predictions beyond reproducing experimental observations, including the growth with artificially designed nuclei, the morphology transformation by tuning the Mo and S sources, and the formation of high-quality single-crystalline monolayer MoS₂ by using single-crystalline substrates with vicinal steps. Our kMC model may serve as a powerful predictive tool for the CVD/PVD growth of monolayer MoS₂ with desired GB configurations.

Bilayer Lateral Heterostructures of Transition-Metal Dichalcogenides and Their Optoelectronic Response

Two-dimensional lateral heterojunctions based on monolayer transition-metal dichalcogenides (TMDs) have received increasing attention given that their direct band gap makes them very attractive for optoelectronic applications. Although bilayer TMDs present an indirect band gap, their electrical properties are expected to be less susceptible to ambient conditions, with higher mobilities and density of states when compared to monolayers. Bilayers and few-layers single domain devices have already demonstrated higher performance in radio frequency and photosensing applications. Despite these advantages, lateral heterostructures based on bilayer domains have been less explored. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS₂-WS₂ and MoSe₂-WSe₂ monodomains. The heterojunctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layers. A phenomenological mechanism is proposed to explain the growth mode with self-limited thickness that happens within a certain window of growth conditions. With respect to their as-grown monolayer counterparts, bilayer lateral heterostructures yield nearly 1 order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ∼10³ times larger than those extracted from the as-grown monolayers, in addition to room-temperature electroluminescence. The improved performance of bilayer heterostructures significantly expands the potential of two-dimensional materials for optoelectronics.

Mo Concentration Controls the Morphological Transitions from Dendritic to Semicompact, and to Compact Growth of Monolayer Crystalline MoS2 on Various Substrates

The domain morphology in the growth of transition-metal dichalcogenides (TMDCs) is mostly triangular but rarely dendritic. Here, we report a robust chemical vapor deposition method to fabricate atomic-thin 2H-phase MoS₂ dendrites on several single-crystalline substrates with different lattice structures, such as rutile-TiO₂(001), SrTiO₃(001), and sapphire(0001). It is found that by tuning the concentration of Mo adatoms, the morphology of MoS₂ domains on these substrates evolves from tridentate dendrites at a low Mo concentration to semicompact fractal domains at an intermediate Mo concentration, and to a compact triangular shape at a high Mo concentration. First-principles calculations reveal that the edge diffusion barrier of Mo is comparable to the attachment barrier, inhibiting fast Mo atom diffusion along the edge. Kinetics Monte Carlo simulations with varying Mo concentrations well reproduce the experimental results. Our combined experimental and theoretical analyses evidently show that the growth of MoS₂ dendritic domains at a low Mo concentration is a nonequilibrium process, which is dominated by the kinetics of Mo adatoms. Our study presents an effective route to control the morphology of TMDCs by simply tuning the transition-metal adatom concentration.

The effect of disordered substrate on crystallization in 2D

In this work, the effect of amorphous substrate on crystallization is addressed. By performing Monte-Carlo simulations of solid on solid models, we explore the effect of the disorder on crystal growth. The disorder is introduced via local geometry of the lattice, where local connectivity and transition rates are varied from site to site. A comparison to an ordered lattice is accomplished and for both, ordered and disordered substrates, an optimal growth temperature is observed. Moreover, we find that under specific conditions the disordered substrate may have a beneficial effect on crystal growth, i.e. better crystallization as a direct consequence of the presence of disorder.

Controlled fractal growth of transition metal dichalcogenides

We report controlled fractal growth of atomically thin transition metal dichalcogenides (TMDCs) by chemical vapor deposition, with morphological evolution from dendritic to triangular. Several important growth parameters controlling the fractal dimensions were identified, including the relaxation rate, adhesion coefficient, diffusion anisotropy and growth time. A model based on nucleation, diffusion limited aggregation and relaxation was proposed to explain the morphological evolution. The results of the computational simulation based on this model are in good agreement with the experimental results. Our study sheds light on the growth mechanism of TMDCs and paves the way for growth of TMDCs with improved controllability.

Nucleation dynamics of single crystal WS2 from droplet precursors uncovered by in-situ monitoring

Transition metal dichalcogenides (TMDs) attract intence attention due to its unique optoelectrical features. Recent progress in production stage of TMD enables us to synthesis uniform and large area TMD with mono layer thickness. Elucidation of growth mechanism is a challenge to improve the crystallinity of TMD, which is regargeded as a next crutial subject in the production stage. Here we report novel diffusion and nucleation dynamics during tungsten disulphide (WS₂) growth. The diffusion length (L_d) of the precursors have been measured with unique nucleation control methods. It was revealed that the L_d reaches up to ~750 μm. This ultra-long diffusion can be attributed to precursor droplets observed during in-situ monitoring of WS₂ growth. The integrated synthesis of >35,000 single crystals and monolayer WS₂ was achieved at the wafer scale based on this model. Our findings are highly significant for both the fundamental study of droplet-mediated crystal growth and the industrial application of integrated single-crystal TMDs.

ZnO-Controlled Growth of Monolayer WS2 through Chemical Vapor Deposition

Monolayer tungsten disulfide (2D WS₂) films have attracted tremendous interest due to their unique electronic and optoelectronic properties. However, the controlled growth of monolayer WS₂ is still challenging. In this paper, we report a novel method to grow WS₂ through chemical vapor deposition (CVD) with ZnO crystalline whisker as a growth promoter, where partially evaporated WS₂ reacts with ZnO to form ZnWO₄ by-product. As a result, a depletion region of W atoms and S-rich region is formed which is favorable for subsequent monolayer growth of WS₂, selectively positioned on the silicon oxide substrate after the CVD growth.

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

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

Employing a Bifunctional Molybdate Precursor To Grow the Highly Crystalline MoS2 for High-Performance Field-Effect Transistors

Growth of the large-sized and high-quality MoS₂ single crystals for high-performance low-power electronic applications is an important step to pursue. Despite the significant improvement made in minimizing extrinsic MoS₂ contact resistance based on interfacial engineering of the devices, the electron mobility of field-effect transistors (FETs) made of a synthetic monolayer MoS₂ is yet far below the expected theoretical values, implying that the MoS₂ crystal quality needs to be further improved. Here, we demonstrate the high-performance two-terminal MoS₂ FETs with room-temperature electron mobility up to ∼90 cm² V^-1 s^-1 based on the sulfurization growth of the bifunctional precursor, sodium molybdate dihydrate. This unique transition-metal precursor, serving as both the crystalline Mo source and seed promotor (sodium), could facilitate the lateral growth of the highly crystalline monolayer MoS₂ crystals (edge length up to ∼260 μm). Substrate surface treatment with oxygen plasma prior to the deposition of the Mo precursor is fundamental to increase the wettability between the Mo source and the substrate, promoting the thinning and coalescence of the source clusters during the growth of large-sized MoS₂ single crystals. The control of growth temperature is also an essential step to grow a strictly monolayer MoS₂ crystal. A proof-of-concept for thermoelectric device integration utilizing monolayer MoS₂ sheds light on its potential in low-voltage and self-powered electronics.

Atomic-Scale Structural Modification of 2D Materials

2D materials have attracted much attention since the discovery of graphene in 2004. Due to their unique electrical, optical, and magnetic properties, they have potential for various applications such as electronics and optoelectronics. Owing to thermal motion and lattice growth kinetics, different atomic-scale structures (ASSs) can originate from natural or intentional regulation of 2D material atomic configurations. The transformations of ASSs can result in the variation of the charge density, electronic density of state and lattice symmetry so that the property tuning of 2D materials can be achieved and the functional devices can be constructed. Here, several kinds of ASSs of 2D materials are introduced, including grain boundaries, atomic defects, edge structures, and stacking arrangements. The design strategies of these structures are also summarized, especially for atomic defects and edge structures. Moreover, toward multifunctional integration of applications, the modulation of electrical, optical, and magnetic properties based on atomic-scale structural modification are presented. Finally, challenges and outlooks are featured in the aspects of controllable structure design and accurate property tuning for 2D materials with ASSs. This work may promote research on the atomic-scale structural modification of 2D materials toward functional applications.

Doping Nanoscale Graphene Domains Improves Magnetism in Hexagonal Boron Nitride

Carbon doping can induce unique and interesting physical properties in hexagonal boron nitride (h-BN). Typically, isolated carbon atoms are doped into h-BN. Herein, however, the insertion of nanometer-scale graphene quantum dots (GQDs) is demonstrated as whole units into h-BN sheets to form h-CBN. The h-CBN is prepared by using GQDs as seed nucleations for the epitaxial growth of h-BN along the edges of GQDs without the assistance of metal catalysts. The resulting h-CBN sheets possess a uniform distrubution of GQDs in plane and a high porosity macroscopically. The h-CBN tends to form in small triangular sheets which suggests an enhanced crystallinity compared to the h-BN synthesized under the same conditions without GQDs. An enhanced ferromagnetism in the h-CBN emerges due to the spin polarization and charge asymmetry resulting from the high density of CN and CB bonds at the boundary between the GQDs and the h-BN domains. The saturation magnetic moment of h-CBN reaches 0.033 emu g^-1 at 300 K, which is three times that of as-prepared single carbon-doped h-BN.

Kinetics of Graphene and 2D Materials Growth

During the last 10 years, remarkable achievements on the chemical vapor deposition (CVD) growth of 2D materials have been made, but the understanding of the underlying mechanisms is still relatively limited. Here, the current progress on the understanding of the growth kinetics of 2D materials, especially for their CVD synthesis, is reviewed. In order to present a complete picture of 2D materials' growth kinetics, the following factors are discussed: i) two types of growth modes, namely attachment-limited growth and diffusion-limited growth; ii) the etching of 2D materials, which offers an additional degree of freedom for growth control; iii) a number of experimental factors in graphene CVD synthesis, such as structure of the substrate, pressure of hydrogen or oxygen, temperature, etc., which are found to have profound effects on the growth kinetics; iv) double-layer and few-layer 2D materials' growth, which has distinct features different from the growth of single-layer 2D materials; and v) the growth of polycrystalline 2D materials by the coalescence of a few single crystalline domains. Finally, the current challenges and opportunities in future 2D materials' synthesis are summarized.

Addressing the isomer cataloguing problem for nanopores in two-dimensional materials

The presence of extended defects or nanopores in two-dimensional (2D) materials can change the electronic, magnetic and barrier membrane properties of the materials. However, the large number of possible lattice isomers of nanopores makes their quantitative study a seemingly intractable problem, confounding the interpretation of experimental and simulated data. Here we formulate a solution to this isomer cataloguing problem (ICP), combining electronic-structure calculations, kinetic Monte Carlo simulations, and chemical graph theory, to generate a catalogue of unique, most-probable isomers of 2D lattice nanopores. The results demonstrate remarkable agreement with precise nanopore shapes observed experimentally in graphene and show that the thermodynamic stability of a nanopore is distinct from its kinetic stability. Triangular nanopores prevalent in hexagonal boron nitride are also predicted, extending this approach to other 2D lattices. The proposed method should accelerate the application of nanoporous 2D materials by establishing specific links between experiment and theory/simulations, and by providing a much-needed connection between molecular design and fabrication.

Electrochemical Polishing of Two-Dimensional Materials

Two-dimensional (2D) layered materials demonstrate their exquisite properties such as high temperature superconductivity, superlubricity, charge density wave, piezotronics, flextronics, straintronics, spintronics, valleytronics, and optoelectronics, mostly, at the monolayer limit. Following initial breakthroughs based on micromechanically exfoliated 2D monolayers, significant progress has been made in recent years toward the bottom-up synthesis of large-area monolayer 2D materials such as MoS₂ and WS₂ using physical vapor deposition and chemical vapor deposition techniques in order to facilitate their transition into commercial technologies. However, the nucleation and subsequent growth of the secondary, tertiary, and greater numbers of vertical layers poses a significant challenge not only toward the realization of uniform monolayers but also toward maintaining their consistent electronic and optoelectronic properties which change abruptly when transitioning from the monolayer to multilayer form. Chemical or physical techniques which can remove the unwanted top layers without compromising the material quality will have tremendous consequences toward the development of atomically flat, large-area, uniform monolayers of 2D materials. Here, we report a simple, elegant, and self-limiting electrochemical polishing technique that can thin down any arbitrary thickness of 2D material, irrespective of whether these are obtained using powder vapor transport or mechanical exfoliation, into their corresponding monolayer form at room temperature within a few seconds without compromising their atomistic integrity. The effectiveness of this electrochemical polishing technique is inherent to 2D transition-metal dichalcogenides owing to the stability of their basal planes, enhanced edge reactivity, and stronger than van der Waals interaction with the substrate. Our study also reveals that 2D monolayers are chemically more robust and corrosion resistant compared to their bulk counterparts in similar oxidative environments, which enables electrochemical polishing of such materials down to a monolayer.

Direct observation of the CVD growth of monolayer MoS2 using in situ optical spectroscopy

Real-time monitoring is essential for understanding and precisely controlling of growth of two-dimensional transition metal dichalcogenide (2D TMDC) materials. However, it is very challenging to carry out such studies during chemical vapor deposition (CVD). Here, we report the first, real time, in situ study of the CVD growth of 2D TMDCs. More specifically, the CVD growth of a molybdenum disulfide (MoS₂) monolayer on sapphire substrates has been monitored in situ using differential transmittance spectroscopy (DTS). The growth of the MoS₂ monolayer can be precisely followed by observation of the evolution of the characteristic optical features. Consequently, a strong correlation between the growth rate of the MoS₂ monolayer and the temperature distribution in the CVD reactor has been revealed. Our results demonstrate the great potential of real time, in situ optical spectroscopy to assist the precisely controlled growth of 2D semiconductor materials.

Mechanical properties of molybdenum diselenide revealed by molecular dynamics simulation and support vector machine

Despite the spurring interests in two-dimensional transition metal dichalcogenide (TMDC) materials, knowledge on the mechanical properties of one of their important member, i.e., molybdenum diselenide (MoSe2) is scarce and remains an open topic. In this work, the mechanical properties of h-MoSe2 and t-MoSe2 were systematically investigated using classical molecular dynamics (MD) simulations combined with machine learning (ML) techniques. The effects of chirality, temperature and strain rate on fracture strain, fracture strength and Young's modulus were characterized in both armchair and zigzag directions. For h-MoSe2, the fracture strengths were 13.6 and 13.0 GPa for armchair and zigzag chiralities, respectively, at 1 K and strain rate of 5 × 10-4 ps-1; the corresponding fracture strains were 0.23 and 0.27. The Young's moduli in armchair and zigzag directions exhibited similar values of 100.9 and 99.5 GPa, respectively. For t-MoSe2, much lower fracture strengths of 6.1 and 6.3 GPa, fracture strains of 0.13 and 0.15, and Young's moduli of 83.7 and 83.0 GPa were predicted under the same conditions. A total of 700 MD simulation cases were calculated under different impact factors and initial conditions, which were subsequently fed into the support vector machine (SVM) algorithm for ML modeling. After training, the ML model could predict the mechanical properties of both MoSe2 types given only the input features such as chirality, temperature and strain rate.

Doping engineering and functionalization of two-dimensional metal chalcogenides

Two-dimensional (2D) layered metal chalcogenides (MXs) have significant potential for use in flexible transistors, optoelectronics, sensing and memory devices beyond the state-of-the-art technology. To pursue ultimate performance, precisely controlled doping engineering of 2D MXs is desired for tailoring their physical and chemical properties in functional devices. In this review, we highlight the recent progress in the doping engineering of 2D MXs, covering that enabled by substitution, exterior charge transfer, intercalation and the electrostatic doping mechanism. A variety of novel doping engineering examples leading to Janus structures, defect curing effects, zero-valent intercalation and deliberately devised floating gate modulation will be discussed together with their intriguing application prospects. The choice of doping strategies and sources for functionalizing MXs will be provided to facilitate ongoing research in this field toward multifunctional applications.

Uniform Vapor-Pressure-Based Chemical Vapor Deposition Growth of MoS2 Using MoO3 Thin Film as a Precursor for Coevaporation

Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS₂) by using thermally evaporated thin films of molybdenum trioxide (MoO₃) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO₃ thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS₂ throughout the substrate. These high-quality crystals are as large as 95 μm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 10⁶ under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS₂ for future semiconductor device applications.

Growth Mechanisms and Electronic Properties of Vertically Aligned MoS2

Thin films of layered semiconductors emerge as highly promising materials for energy harvesting and storage, optoelectronics and catalysis. Their natural propensity to grow as oriented crystals and films is one of their distinct properties under recent focal interest. Specifically, the reaction of transition metal films with chalcogen vapor can result in films of vertically aligned (VA) layers, while metal-oxides react with chalcogens in vapor phase to produce horizontally aligned crystals and films. The growth mechanisms of vertically oriented films are not yet fully understood, as well as their dependence on the initial metal film thickness and growth conditions. Moreover, the resulting electronic properties and the role of defects and disorder had not yet been studied, despite their critical influence on catalytic and device performance. In this work, we study the details of oriented growth of MoS₂ with complementary theoretical and experimental approaches. We present a general theoretical model of diffusion-reaction growth that can be applied to a large variety of layered materials synthesized by solid-vapor reaction. Moreover, we inspect the relation of electronic properties to the structure of vertically aligned MoS₂ and shed light on the density and character of defects in this material. Our measurements on Si-MoS₂ p-n hetero-junction devices point to the existence of polarizable defects that impact applications of vertical transition-metal dichalcogenide materials.

Synthesis of 2D transition metal dichalcogenides by chemical vapor deposition with controlled layer number and morphology

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have stimulated the modern technology due to their unique and tunable electronic, optical, and chemical properties. Therefore, it is very important to study the control parameters for material preparation to achieve high quality thin films for modern electronics, as the performance of TMDs-based device largely depends on their layer number, grain size, orientation, and morphology. Among the synthesis methods, chemical vapor deposition (CVD) is an excellent technique, vastly used to grow controlled layer of 2D materials in recent years. In this review, we discuss the different growth routes and mechanisms to synthesize high quality large size TMDs using CVD method. We highlight the recent advances in the controlled growth of mono- and few-layer TMDs materials by varying different growth parameters. Finally, different strategies to control the grain size, boundaries, orientation, morphology and their application for various field of are also thoroughly discussed.

Microstructure and Elastic Constants of Transition Metal Dichalcogenide Monolayers from Friction and Shear Force Microscopy

Optical and electrical properties of 2D transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. This study demonstrates how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS₂ ) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable the fourth-order elastic constants of monolayer WS₂ to be acquired experimentally. The results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs and insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such 2D semiconductors.

Dislocation driven spiral and non-spiral growth in layered chalcogenides

Two-dimensional materials have shown great promise for implementation in next-generation devices. However, controlling the film thickness during epitaxial growth remains elusive and must be fully understood before wide scale industrial application. Currently, uncontrolled multilayer growth is frequently observed, and not only does this growth mode contradict theoretical expectations, but it also breaks the inversion symmetry of the bulk crystal. In this work, a multiscale theoretical investigation aided by experimental evidence is carried out to identify the mechanism of such an unconventional, yet widely observed multilayer growth in the epitaxy of layered materials. This work reveals the subtle mechanistic similarities between multilayer concentric growth and spiral growth. Using the combination of experimental demonstration and simulations, this work presents an extended analysis of the driving forces behind this non-ideal growth mode, and the conditions that promote the formation of these defects. Our study shows that multilayer growth can be a result of both chalcogen deficiency and chalcogen excess: the former causes metal clustering as nucleation defects, and the latter generates in-domain step edges facilitating multilayer growth. Based on this fundamental understanding, our findings provide guidelines for the narrow window of growth conditions which enables large-area, layer-by-layer growth.

Revealing the microscopic CVD growth mechanism of MoSe2 and the role of hydrogen gas during the growth procedure

Understanding the microscopic mechanisms for the nucleation and growth of two-dimensional molybdenum diselenide (2D MoSe₂) via chemical vapor deposition (CVD) is crucial towards the precisely controlled growth of the 2D material. In this work, we employed a joint use of transmission electron microscopy and CVD, in which the 2D MoSe₂ were directly grown on a graphene membrane based on grids, that enables the microstructural characterization of as-grown MoSe₂ flakes. We further explore the role of hydrogen gas and find: in an argon ambient, the primary products are few-layer MoSe₂ flakes, along with MoO _x nanoparticles; while with the introduction of H₂, single-layer MoSe₂ became the dominant product during the CVD growth. Quantitative analysis of the effects of H₂ flow rate on the flake sizes, and areal coverage was also given. Nevertheless, we further illuminated the evolution of shape morphology and edge structures of single-layer MoSe₂, and proposed the associated growth routes during a typical CVD process.

Control of the Nucleation Density of Molybdenum Disulfide in Large-Scale Synthesis Using Chemical Vapor Deposition

Atmospheric pressure chemical vapor deposition (CVD) is presently a promising approach for preparing two-dimensional (2D) MoS₂ crystals at high temperatures on SiO₂/Si substrates. In this work, we propose an improved CVD method without hydrogen, which can increase formula flexibility by controlling the heating temperature of MoO₃ powder and sulfur powder. The results show that the size and coverage of MoS₂ domains vary largely, from discrete triangles to continuous film, on substrate. We find that the formation of MoS₂ domains is dependent on the nucleation density of MoS₂. Laminar flow theory is employed to elucidate the cause of the different shapes of MoS₂ domains. The distribution of carrier gas speeds at the substrate surface leads to a change of nucleation density and a variation of domain morphology. Thus, nucleation density and domain morphology can be actively controlled by adjusting the carrier gas flow rate in the experimental system. These results are of significance for understanding the growth regulation of 2D MoS₂ crystals.

One-pot growth of two-dimensional lateral heterostructures via sequential edge-epitaxy

Two-dimensional heterojunctions of transition-metal dichalcogenides have great potential for application in low-power, high-performance and flexible electro-optical devices, such as tunnelling transistors, light-emitting diodes, photodetectors and photovoltaic cells. Although complex heterostructures have been fabricated via the van der Waals stacking of different two-dimensional materials, the in situ fabrication of high-quality lateral heterostructures with multiple junctions remains a challenge. Transition-metal-dichalcogenide lateral heterostructures have been synthesized via single-step, two-step or multi-step growth processes. However, these methods lack the flexibility to control, in situ, the growth of individual domains. In situ synthesis of multi-junction lateral heterostructures does not require multiple exchanges of sources or reactors, a limitation in previous approaches as it exposes the edges to ambient contamination, compromises the homogeneity of domain size in periodic structures, and results in long processing times. Here we report a one-pot synthetic approach, using a single heterogeneous solid source, for the continuous fabrication of lateral multi-junction heterostructures consisting of monolayers of transition-metal dichalcogenides. The sequential formation of heterojunctions is achieved solely by changing the composition of the reactive gas environment in the presence of water vapour. This enables selective control of the water-induced oxidation and volatilization of each transition-metal precursor, as well as its nucleation on the substrate, leading to sequential edge-epitaxy of distinct transition-metal dichalcogenides. Photoluminescence maps confirm the sequential spatial modulation of the bandgap, and atomic-resolution images reveal defect-free lateral connectivity between the different transition-metal-dichalcogenide domains within a single crystal structure. Electrical transport measurements revealed diode-like responses across the junctions. Our new approach offers greater flexibility and control than previous methods for continuous growth of transition-metal-dichalcogenide-based multi-junction lateral heterostructures. These findings could be extended to other families of two-dimensional materials, and establish a foundation for the development of complex and atomically thin in-plane superlattices, devices and integrated circuits.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures

Two-dimensional (2D) materials have attracted increasing research interest because of the abundant choice of materials with diverse and tunable electronic, optical, and chemical properties. Moreover, 2D material based heterostructures combining several individual 2D materials provide unique platforms to create an almost infinite number of materials and show exotic physical phenomena as well as new properties and applications. To achieve these high expectations, methods for the scalable preparation of 2D materials and 2D heterostructures of high quality and low cost must be developed. Chemical vapor deposition (CVD) is a powerful method which may meet the above requirements, and has been extensively used to grow 2D materials and their heterostructures in recent years, despite several challenges remaining. In this review of the challenges in the CVD growth of 2D materials, we highlight recent advances in the controlled growth of single crystal 2D materials, with an emphasis on semiconducting transition metal dichalcogenides. We provide insight into the growth mechanisms of single crystal 2D domains and the key technologies used to realize wafer-scale growth of continuous and homogeneous 2D films which are important for practical applications. Meanwhile, strategies to design and grow various kinds of 2D material based heterostructures are thoroughly discussed. The applications of CVD-grown 2D materials and their heterostructures in electronics, optoelectronics, sensors, flexible devices, and electrocatalysis are also discussed. Finally, we suggest solutions to these challenges and ideas concerning future developments in this emerging field.

Toward a Mechanistic Understanding of Vertical Growth of van der Waals Stacked 2D Materials: A Multiscale Model and Experiments

Vertical stacking of monolayers via van der Waals (vdW) interaction opens promising routes toward engineering physical properties of two-dimensional (2D) materials and designing atomically thin devices. However, due to the lack of mechanistic understanding, challenges remain in the controlled fabrication of these structures via scalable methods such as chemical vapor deposition (CVD) onto substrates. In this paper, we develop a general multiscale model to describe the size evolution of 2D layers and predict the necessary growth conditions for vertical (initial + subsequent layers) versus in-plane lateral (monolayer) growth. An analytic thermodynamic criterion is established for subsequent layer growth that depends on the sizes of both layers, the vdW interaction energies, and the edge energy of 2D layers. Considering the time-dependent growth process, we find that temperature and adatom flux from vapor are the primary criteria affecting the self-assembled growth. The proposed model clearly demonstrates the distinct roles of thermodynamic and kinetic mechanisms governing the final structure. Our model agrees with experimental observations of various monolayer and bilayer transition metal dichalcogenides grown by CVD and provides a predictive framework to guide the fabrication of vertically stacked 2D materials.

A Novel and Facile Route to Synthesize Atomic-Layered MoS2 Film for Large-Area Electronics

High-quality and large-area molybdenum disulfide (MoS₂ ) thin film is highly desirable for applications in large-area electronics. However, there remains a challenge in attaining MoS₂ film of reasonable crystallinity due to the absence of appropriate choice and control of precursors, as well as choice of suitable growth substrates. Herein, a novel and facile route is reported for synthesizing few-layered MoS₂ film with new precursors via chemical vapor deposition. Prior to growth, an aqueous solution of sodium molybdate as the molybdenum precursor is spun onto the growth substrate and dimethyl disulfide as the liquid sulfur precursor is supplied with a bubbling system during growth. To supplement the limiting effect of Mo (sodium molybdate), a supplementary Mo is supplied by dissolving molybdenum hexacarbonyl (Mo(CO)₆ ) in the liquid sulfur precursor delivered by the bubbler. By precisely controlling the amounts of precursors and hydrogen flow, full coverage of MoS₂ film is readily achievable in 20 min. Large-area MoS₂ field effect transistors (FETs) fabricated with a conventional photolithography have a carrier mobility as high as 18.9 cm² V^-1 s^-1 , which is the highest reported for bottom-gated MoS₂ -FETs fabricated via photolithography with an on/off ratio of ≈10⁵ at room temperature.

Substrate Lattice-Guided Seed Formation Controls the Orientation of 2D Transition-Metal Dichalcogenides

Two-dimensional (2D) transition-metal dichalcogenide (TMDC) semiconductors are important for next-generation electronics and optoelectronics. Given the difficulty in growing large single crystals of 2D TMDC materials, understanding the factors affecting the seed formation and orientation becomes an important issue for controlling the growth. Here, we systematically study the growth of molybdenum disulfide (MoS₂) monolayer on c-plane sapphire with chemical vapor deposition to discover the factors controlling their orientation. We show that the concentration of precursors, that is, the ratio between sulfur and molybdenum oxide (MoO₃), plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS₂ monolayers. High S/MoO₃ ratio is needed in the early stage of growth to form small seeds that can align easily to the substrate lattice structures, while the ratio should be decreased to enlarge the size of the monolayer at the next stage of the lateral growth. Moreover, we show that the seeds are actually crystalline MoS₂ layers as revealed by high-resolution transmission electron microscopy. There exist two preferred orientations (0° or 60°) registered on sapphire, confirmed by our density functional theory simulation. This report offers a facile technique to grow highly aligned 2D TMDCs and contributes to knowledge advancement in growth mechanism.

A kinetic Monte Carlo simulation method of van der Waals epitaxy for atomistic nucleation-growth processes of transition metal dichalcogenides

Controlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.

Shining a light on transition metal chalcogenides for sustainable photovoltaics

Transition metal chalcogenides are an important family of materials that have received significant interest in recent years as they have the potential for diverse applications ranging from use in electronics to industrial lubricants. One of their most exciting properties is the ability to generate electricity from incident light. In this perspective we will summarise and highlight the key results and challenges in this area and explain how transition metal chalcogenides are a good choice for future sustainable photovoltaics.

Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition

We show that hexagonal domains of monolayer tungsten disulfide (WS₂) grown by chemical vapor deposition (CVD) with powder precursors can have discrete segmentation in their photoluminescence (PL) emission intensity, forming symmetric patterns with alternating bright and dark regions. Two-dimensional maps of the PL reveal significant reduction within the segments associated with the longest sides of the hexagonal domains. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. Monolayers of WS₂ hexagonal islands transferred to new substrates still exhibit this PL segmentation, ruling out local strain in the regions as the dominant cause. High-power laser irradiation causes preferential degradation of the bright segments by sulfur removal, indicating the presence of a more defective region that is higher in oxidative reactivity. Atomic force microscopy (AFM) images of topography and amplitude modes show uniform thickness of the WS₂ domains and no signs of segmentation. However, AFM phase maps do show the same segmentation of the domain as the PL maps and indicate that it is caused by some kind of structural difference that we could not clearly identify. These results provide important insights into the spatially varying properties of these CVD-grown transition metal dichalcogenide materials, which may be important for their effective implementation in fast photo sensors and optical switches.

Aligned MoO2/MoS2 and MoO2/MoTe2 Freestanding Core/Shell Nanoplates Driven by Surface Interactions

Controlling the growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is an important step toward utilizing these materials for either electronics or catalysis. Here, we report a new surface-templated growth method that enables the fabrication of MoO₂/MoS₂ and MoO₂/MoTe₂ core/shell nanoplates epitaxially aligned on (0001)-oriented 4H-silicon carbide and sapphire substrates. These heterostructures are characterized by a variety of techniques to identify the chemical and structural nature of the interface. Scanning electron microscopy shows that the nanoplates feature 3-fold symmetry indicative of epitaxial growth. Raman spectroscopy indicates that the MoO₂/MoS₂ nanoplates are composed of co-localized MoO₂ and MoS₂, and transmission electron microscopy confirms that the nanoplates feature MoO₂ cores with 2D MoS₂ coatings. Locked-coupled X-ray diffraction shows that the interfacial planes of the MoO₂ nanoplate cores belong to the {010} and {001} families. This method may be further generalized to create novel nanostructured interfaces with single-crystal substrates.

Temperature-Related Morphological Evolution of MoS2 Domains on Graphene and Electron Transfer within Heterostructures

Other than the well-known sulfurization of molybdate compound to synthesize molybdenum disulfide (MoS₂ ) layers, the dynamic process in the whole crystalline growth from nuclei to triangular domains has been rarely experimentally explored. Here, a competing sulfur-capture principle jointly with strict epitaxial mechanism is first proposed for the initial topography evolution and the final intrinsic highly oriented growth of triangular MoS₂ domains with Mo or S terminations on the graphene (Gr) template. Additionally, potential distributions on MoS₂ domains and bare Gr are presented to be different due to the charge transfer within heterostructures. The findings offer the mechanism of templated growth of 2D transition metal dichalcogenides, and provide general principles in syntheses of vertical 2D heterostructures that can be applied to electronics.

Stacking sequence dependent photo-electrocatalytic performance of CVD grown MoS2/graphene van der Waals solids

New layered solids by the combinatorial stacking of different atomic layers are emanating as novel candidates for energy efficient devices. Here, sequentially stacked single layer graphene-molybdenum disulfide (MoS₂) van der Waals (vdW) solids are demonstrated for their efficacy in the catalysis of hydrogen evolution reaction (HER), and importance of their stacking order in tuning the photo-electrocatalytic (PEC) efficiency is unraveled. Single layer graphene and a few layered MoS₂ stacked vdW solids based transparent flexible electrodes were prepared, and a particular stacking sequence where top-graphene: bottom-MoS₂/polydimethylsiloxane (PDMS) geometry (MSGR) exhibited the lowest onset and over potentials and a very high exchange current density (j ₀ ∼ 245 ± 1 μA cm^-2) in acidic HER in comparison to the individual layers and other stacked configuration (MoS₂ on top of graphene on PDMS, GRMS). The HER studies under dark and white light illuminations were conducted to explore the PEC responses of the devices. The augmented HER performance of MSGR is further confirmed from the charge transfer resistance measurements using electrochemical impedance spectroscopy. Role of graphene plasmonics and MoS₂ to graphene electron transfer were studied, and this study unravels the importance of a new factor, stacking order of vdW layers, while designing novel devices from the layered solids.

Booming Development of Group IV-VI Semiconductors: Fresh Blood of 2D Family

As an important component of 2D layered materials (2DLMs), the 2D group IV metal chalcogenides (GIVMCs) have drawn much attention recently due to their earth-abundant, low-cost, and environmentally friendly characteristics, thus catering well to the sustainable electronics and optoelectronics applications. In this instructive review, the booming research advancements of 2D GIVMCs in the last few years have been presented. First, the unique crystal and electronic structures are introduced, suggesting novel physical properties. Then the various methods adopted for synthesis of 2D GIVMCs are summarized such as mechanical exfoliation, solvothermal method, and vapor deposition. Furthermore, the review focuses on the applications in field effect transistors and photodetectors based on 2D GIVMCs, and extends to flexible devices. Additionally, the 2D GIVMCs based ternary alloys and heterostructures have also been presented, as well as the applications in electronics and optoelectronics. Finally, the conclusion and outlook have also been presented in the end of the review.

Dominance of Dispersion Interactions and Entropy over Electrostatics in Determining the Wettability and Friction of Two-Dimensional MoS2 Surfaces

The existence of partially ionic bonds in molybdenum disulfide (MoS₂), as opposed to covalent bonds in graphene, suggests that polar (electrostatic) interactions should influence the interfacial behavior of two-dimensional MoS₂ surfaces. In this work, using molecular dynamics simulations, we show that electrostatic interactions play a negligible role in determining not only the equilibrium contact angle on the MoS₂ basal plane, which depends solely on the total interaction energy between the surface and the liquid, but also the friction coefficient and the slip length, which depend on the spatial variations in the interaction energy. While the former is found to result from the exponential decay of the electric potential above the MoS₂ surface, the latter results from the trilayered sandwich structure of the MoS₂ monolayer, which causes the spatial variations in dispersion interactions in the lateral direction to dominate over those in electrostatic interactions in the lateral direction. Further, we show that the nonpolarity of MoS₂ is specific to the two-dimensional basal plane of MoS₂ and that other planes (e.g., the zigzag plane) in MoS₂ are polar with respect to interactions with water, thereby illustrating the role of edge effects, which could be important in systems involving vacancies or nanopores in MoS₂. Finally, we simulate the temperature dependence of the water contact angle on MoS₂ to show that the inclusion of entropy, which has been neglected in recent mean-field theories, is essential in determining the wettability of MoS₂. Our findings reveal that the basal planes in graphene and MoS₂ are unexpectedly similar in terms of their interfacial behavior.

Lateral Versus Vertical Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Thermodynamic Insight into MoS2

Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) that possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodynamic and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated density functional theory (DFT) and calculation of phase diagram (CALPHAD) modeling approach is employed to provide thermodynamic insight into lateral versus vertical growth of the prototypical 2D material MoS2. Various DFT energies are predicted from the layer-dependent MoS2, 2D flake-size related mono- and bilayer MoS2, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral versus vertical growth of 2D islands. For example, the monolayer MoS2 flake in a small 2D lateral size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS2 flake becomes stable with increasing 2D lateral size. The critical 2D flake-size of phase stability between mono- and bilayer MoS2 is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure-temperature-composition (P-T-x) growth windows are predicted for MoS2, indicating that the formation of MoS2 flake with reduced size appears in the middle but close to the lower T and higher P "Gas + MoS2" phase region. It further suggests that Mo diffusion is a controlling factor for MoS2 growth owing to its extremely low diffusivity compared to that of sulfur. Calculated MoS2 energies, Mo and S diffusivities, and size-dependent P-T-x growth windows are in good accord with available experiments, and the present data provide quantitative insight into the controlled growth of 2D layered MoS2.