Step-Edge-Guided Nucleation and Growth of Aligned WSe2 on Sapphire via a Layer-over-Layer Growth Mode

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Revisiting the Epitaxial Growth Mechanism of 2D TMDC Single Crystals

Epitaxial growth of 2D transition metal dichalcogenides (TMDCs) on sapphire substrates has been recognized as a pivotal method for producing wafer-scale single-crystal films. Both step-edges and symmetry of substrate surfaces have been proposed as controlling factors. However, the underlying fundamental still remains elusive. In this work, through the molybdenum disulfide (MoS2) growth on C/M sapphire, it is demonstrated that controlling the sulfur evaporation rate is crucial for dictating the switch between atomic-edge guided epitaxy and van der Waals epitaxy. Low-concentration sulfur condition preserves O/Al-terminated step edges, fostering atomic-edge epitaxy, while high-concentration sulfur leads to S-terminated edges, preferring van der Waals epitaxy. These experiments reveal that on a 2 in. wafer, the van der Waals epitaxy mechanism achieves better control in MoS2 alignment (≈99%) compared to the step edge mechanism (<85%). These findings shed light on the nuanced role of atomic-level thermodynamics in controlling nucleation modes of TMDCs, thereby providing a pathway for the precise fabrication of single-crystal 2D materials on a wafer scale.

Eight In. Wafer-Scale Epitaxial Monolayer MoS2

Large-scale, high-quality, and uniform monolayer molybdenum disulfide (MoS2) films are crucial for their applications in next-generation electronics and optoelectronics. Epitaxy is a mainstream technique for achieving high-quality MoS2 films and is demonstrated at a wafer scale up to 4-in. In this study, the epitaxial growth of 8-in. wafer-scale highly oriented monolayer MoS2 on sapphire is reported as with excellent spatial homogeneity, using a specially designed vertical chemical vapor deposition (VCVD) system. Field effect transistors (FETs) based on the as-grown 8-in. wafer-scale monolayer MoS2 film are fabricated and exhibit high performances, with an average mobility and an on/off ratio of 53.5 cm2 V-1 s-1 and 107, respectively. In addition, batch fabrication of logic devices and 11-stage ring oscillators are also demonstrated, showcasing excellent electrical functions. This work may pave the way of MoS2 in practical industry-scale applications.

Molecular beam epitaxy and other large-scale methods for producing monolayer transition metal dichalcogenides

Transition metal dichalcogenide (TMD/TMDC) monolayers have gained considerable attention in recent years for their unique properties. Some of these properties include direct bandgap emission and strong mechanical and electronic properties. For these reasons, monolayer TMDs have been considered a promising material for next-generation quantum technologies and optoelectronic devices. However, for the field to make more gainful advancements and be implemented in devices, high-quality TMD monolayers need to be produced at a larger scale with high quality. In this article, some of the current means to produce larger-scale semiconducting monolayer TMDs will be reviewed. An emphasis will be given to the technique of molecular beam epitaxy (MBE) for two main reasons: (1) there is a growing body of research using this technique to grow TMD monolayers and (2) there is yet to be a body of work that has summarized the current research for MBE monolayer growth of TMDs.

Remote epitaxy of single-crystal rhombohedral WS2 bilayers

Compared to transition metal dichalcogenide (TMD) monolayers, rhombohedral-stacked (R-stacked) TMD bilayers exhibit remarkable electrical performance, enhanced nonlinear optical response, giant piezo-photovoltaic effect and intrinsic interfacial ferroelectricity. However, from a thermodynamics perspective, the formation energies of R-stacked and hexagonal-stacked (H-stacked) TMD bilayers are nearly identical, leading to mixed stacking of both H- and R-stacked bilayers in epitaxial films. Here, we report the remote epitaxy of centimetre-scale single-crystal R-stacked WS2 bilayer films on sapphire substrates. The bilayer growth is realized by a high flux feeding of the tungsten source at high temperature on substrates. The R-stacked configuration is achieved by the symmetry breaking in a-plane sapphire, where the influence of atomic steps passes through the lower TMD layer and controls the R-stacking of the upper layer. The as-grown R-stacked bilayers show up-to-30-fold enhancements in carrier mobility (34 cm2V-1s-1), nearly doubled circular helicity (61%) and interfacial ferroelectricity, in contrast to monolayer films. Our work reveals a growth mechanism to obtain stacking-controlled bilayer TMD single crystals, and promotes large-scale applications of R-stacked TMD.

2D Lateral Heterojunction Arrays with Tailored Interface Band Bending

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

Van der Waals epitaxy of tunable moirés enabled by alloying

The unique physics in moiré superlattices of twisted or lattice-mismatched atomic layers holds great promise for future quantum technologies. However, twisted configurations are thermodynamically unfavourable, making accurate twist angle control during growth implausible. While rotationally aligned, lattice-mismatched moirés such as WSe2/WS2 can be synthesized, they lack the critical moiré period tunability, and their formation mechanisms are not well understood. Here, we report the thermodynamically driven van der Waals epitaxy of moirés with a tunable period from 10 to 45 nanometres, using lattice mismatch engineering in two WSSe layers with adjustable chalcogen ratios. Contrary to conventional epitaxy, where lattice-mismatch-induced stress hinders high-quality growth, we reveal the key role of bulk stress in moiré formation and its unique interplay with edge stress in shaping the moiré growth modes. Moreover, the superlattices display tunable interlayer excitons and moiré intralayer excitons. Our studies unveil the epitaxial science of moiré synthesis and lay the foundations for moiré-based technologies.

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

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

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

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.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

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.

Step engineering for nucleation and domain orientation control in WSe2 epitaxy on c-plane sapphire

Epitaxial growth of two-dimensional transition metal dichalcogenides on sapphire has emerged as a promising route to wafer-scale single-crystal films. Steps on the sapphire act as sites for transition metal dichalcogenide nucleation and can impart a preferred domain orientation, resulting in a substantial reduction in mirror twins. Here we demonstrate control of both the nucleation site and unidirectional growth direction of WSe2 on c-plane sapphire by metal-organic chemical vapour deposition. The unidirectional orientation is found to be intimately tied to growth conditions via changes in the sapphire surface chemistry that control the step edge location of WSe2 nucleation, imparting either a 0° or 60° orientation relative to the underlying sapphire lattice. The results provide insight into the role of surface chemistry on transition metal dichalcogenide nucleation and domain alignment and demonstrate the ability to engineer domain orientation over wafer-scale substrates.

Oriented lateral growth of two-dimensional materials on c-plane sapphire

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) represent the ultimate thickness for scaling down channel materials. They provide a tantalizing solution to push the limit of semiconductor technology nodes in the sub-1 nm range. One key challenge with 2D semiconducting TMD channel materials is to achieve large-scale batch growth on insulating substrates of single crystals with spatial homogeneity and compelling electrical properties. Recent studies have claimed the epitaxy growth of wafer-scale, single-crystal 2D TMDs on a c-plane sapphire substrate with deliberately engineered off-cut angles. It has been postulated that exposed step edges break the energy degeneracy of nucleation and thus drive the seamless stitching of mono-oriented flakes. Here we show that a more dominant factor should be considered: in particular, the interaction of 2D TMD grains with the exposed oxygen-aluminium atomic plane establishes an energy-minimized 2D TMD-sapphire configuration. Reconstructing the surfaces of c-plane sapphire substrates to only a single type of atomic plane (plane symmetry) already guarantees the single-crystal epitaxy of monolayer TMDs without the aid of step edges. Electrical results evidence the structural uniformity of the monolayers. Our findings elucidate a long-standing question that curbs the wafer-scale batch epitaxy of 2D TMD single crystals-an important step towards using 2D materials for future electronics. Experiments extended to perovskite materials also support the argument that the interaction with sapphire atomic surfaces is more dominant than step-edge docking.

p-Type Two-Dimensional Semiconductors: From Materials Preparation to Electronic Applications

Two-dimensional (2D) materials are regarded as promising candidates in many applications, including electronics and optoelectronics, because of their superior properties, including atomic-level thickness, tunable bandgaps, large specific surface area, and high carrier mobility. In order to bring 2D materials from the laboratory to industrialized applications, materials preparation is the first prerequisite. Compared to the n-type analogs, the family of p-type 2D semiconductors is relatively small, which limits the broad integration of 2D semiconductors in practical applications such as complementary logic circuits. So far, many efforts have been made in the preparation of p-type 2D semiconductors. In this review, we overview recent progresses achieved in the preparation of p-type 2D semiconductors and highlight some promising methods to realize their controllable preparation by following both the top-down and bottom-up strategies. Then, we summarize some significant application of p-type 2D semiconductors in electronic and optoelectronic devices and their superiorities. In end, we conclude the challenges existed in this field and propose the potential opportunities in aspects from the discovery of novel p-type 2D semiconductors, their controlled mass preparation, compatible engineering with silicon production line, high-κ dielectric materials, to integration and applications of p-type 2D semiconductors and their heterostructures in electronic and optoelectronic devices. Overall, we believe that this review will guide the design of preparation systems to fulfill the controllable growth of p-type 2D semiconductors with high quality and thus lay the foundations for their potential application in electronics and optoelectronics.

Tungsten Oxide Mediated Quasi-van der Waals Epitaxy of WS2 on Sapphire

Conventional epitaxy plays a crucial role in current state-of-the art semiconductor technology, as it provides a path for accurate control at the atomic scale of thin films and nanostructures, to be used as the building blocks in nanoelectronics, optoelectronics, sensors, etc. Four decades ago, the terms "van der Waals" (vdW) and "quasi-vdW (Q-vdW) epitaxy" were coined to explain the oriented growth of vdW layers on 2D and 3D substrates, respectively. The major difference with conventional epitaxy is the weaker interaction between the epi-layer and the epi-substrates. Indeed, research on Q-vdW epitaxial growth of transition metal dichalcogenides (TMDCs) has been intense, with oriented growth of atomically thin semiconductors on sapphire being one of the most studied systems. Nonetheless, there are some striking and not yet understood differences in the literature regarding the orientation registry between the epi-layers and epi-substrate and the interface chemistry. Here we study the growth of WS2 via a sequential exposure of the metal and the chalcogen precursors in a metal-organic chemical vapor deposition (MOCVD) system, introducing a metal-seeding step prior to the growth. The ability to control the delivery of the precursor made it possible to study the formation of a continuous and apparently ordered WO3 mono- or few-layer at the surface of a c-plane sapphire. Such an interfacial layer is shown to strongly influence the subsequent quasi-vdW epitaxial growth of the atomically thin semiconductor layers on sapphire. Hence, here we elucidate an epitaxial growth mechanism and demonstrate the robustness of the metal-seeding approach for the oriented formation of other TMDC layers. This work may enable the rational design of vdW and quasi-vdW epitaxial growth on different material systems.

Universal epitaxy of non-centrosymmetric two-dimensional single-crystal metal dichalcogenides

The great challenge for the growth of non-centrosymmetric 2D single crystals is to break the equivalence of antiparallel grains. Even though this pursuit has been partially achieved in boron nitride and transition metal dichalcogenides (TMDs) growth, the key factors that determine the epitaxy of non-centrosymmetric 2D single crystals are still unclear. Here we report a universal methodology for the epitaxy of non-centrosymmetric 2D metal dichalcogenides enabled by accurate time sequence control of the simultaneous formation of grain nuclei and substrate steps. With this methodology, we have demonstrated the epitaxy of unidirectionally aligned MoS₂ grains on a, c, m, n, r and v plane Al₂O₃ as well as MgO and TiO₂ substrates. This approach is also applicable to many TMDs, such as WS₂, NbS₂, MoSe₂, WSe₂ and NbSe₂. This study reveals a robust mechanism for the growth of various 2D single crystals and thus paves the way for their potential applications.

Critical Role of Surface Termination of Sapphire Substrates in Crystallographic Epitaxial Growth of MoS2 Using Inorganic Molecular Precursors

A highly reproducible route for the epitaxial growth of single-crystalline monolayer MoS2 on a C-plane sapphire substrate was developed using vapor-pressure-controllable inorganic molecular precursors MoOCl4 and H2S. Microscopic, crystallographic, and spectroscopic analyses indicated that the epitaxial MoS2 film possessed outstanding electrical and optical properties, excellent homogeneity, and orientation selectivity. The systematic investigation of the effect of growth temperature on the crystallographic orientations of MoS2 revealed that the surface termination of the sapphire substrate with respect to the growth temperature determines the crystallographic orientation selectivity of MoS2. Our results suggest that controlling the surface to form a half-Al-terminated surface is a prerequisite for the epitaxial growth of MoS2 on a C-plane sapphire substrate. The insights on the growth mechanism, especially the significance of substrate surface termination, obtained through this study will aid in designing efficient epitaxial growth routes for developing single-crystalline monolayer transition metal dichalcogenides.

New Approaches to Produce Large-Area Single Crystal Thin Films

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

Atomistic Insight into the Epitaxial Growth Mechanism of Single-Crystal Two-Dimensional Transition-Metal Dichalcogenides on Au(111) Substrate

A mechanistic understanding of interactions between atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) and their growth substrates is important for achieving the unidirectional alignment of nuclei and seamless stitching of 2D TMD domains and thus 2D wafers. In this work, we conduct a cross-sectional scanning transmission electron microscopy (STEM) study to investigate the atomic-scale nucleation and early stage growth behaviors of chemical vapor deposited monolayer (ML-) MoS₂ and molecular beam epitaxy ML-MoSe₂ on a Au(111) substrate. Statistical analysis reveals the majority of as-grown domains, i.e., ∼88% for MoS₂ and 90% for MoSe₂, nucleate on surface terraces, with the rest (i.e., ∼12% for MoS₂ and 10% for MoSe₂) on surface steps. Moreover, within the latter case, step-associated nucleation, ∼64% of them are terminated with a Mo-zigzag edge in connection with the Au surface steps, with the rest (∼36%) being S-zigzag edges. In conjunction with ab initio density functional theory calculations, the results confirm that van der Waals epitaxy, rather than the surface step guided epitaxy, plays deterministic roles for the realization of unidirectional ML-MoS₂ (MoSe₂) domains on a Au(111) substrate. In contrast, surface steps, particularly their step height, are mainly responsible for the integrity and thickness of MoS₂/MoSe₂ films. In detail, it is found that the lateral growth of monolayer thick MoS₂/MoSe₂ domains only proceeds across mono-Au-atom high surface steps (∼2.4 Å), but fail for higher ones (bi-Au atom step and higher) during the growth. Our cross-sectional STEM study also confirms the existence of considerable compressive residual strain that reaches ∼3.0% for ML-MoS₂/MoSe₂ domains on Au(111). The present study aims to understand the growth mechanism of 2D TMD wafers.

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

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

Epitaxial growth of inch-scale single-crystal transition metal dichalcogenides through the patching of unidirectionally orientated ribbons

Two-dimensional (2D) semiconductors, especially transition metal dichalcogenides (TMDs), have been envisioned as promising candidates in extending Moore’s law. To achieve this, the controllable growth of wafer-scale TMDs single crystals or periodic single-crystal patterns are fundamental issues. Herein, we present a universal route for synthesizing arrays of unidirectionally orientated monolayer TMDs ribbons (e.g., MoS₂, WS₂, MoSe₂, WSe₂, MoS_xSe_2-x), by using the step edges of high-miller-index Au facets as templates. Density functional theory calculations regarding the growth kinetics of specific edges have been performed to reveal the morphological transition from triangular domains to patterned ribbons. More intriguingly, we find that, the uniformly aligned TMDs ribbons can merge into single-crystal films through a one-dimensional edge epitaxial growth mode. This work hereby puts forward an alternative pathway for the direct synthesis of inch-scale uniform monolayer TMDs single-crystals or patterned ribbons, which should promote their applications as channel materials in high-performance electronics or other fields.

Here, the authors report the direct growth of periodic arrays of 2D semiconductor ribbons by exploiting the step edges of high-miller-index Au facets, showing potential for 2D electronic devices. The synthesized ribbons could also be merged to obtain wafer-scale single-crystal monolayers.

Uniform nucleation and epitaxy of bilayer molybdenum disulfide on sapphire

Two-dimensional transition-metal dichalcogenides (TMDs) are of interest for beyond-silicon electronics^1,2. It has been suggested that bilayer TMDs, which combine good electrostatic control, smaller bandgap and higher mobility than monolayers, could potentially provide improvements in the energy-delay product of transistors^3-5. However, despite advances in the growth of monolayer TMDs^6-14, the controlled epitaxial growth of multilayers remains a challenge¹⁵. Here we report the uniform nucleation (>99%) of bilayer molybdenum disulfide (MoS₂) on c-plane sapphire. In particular, we engineer the atomic terrace height on c-plane sapphire to enable an edge-nucleation mechanism and the coalescence of MoS₂ domains into continuous, centimetre-scale films. Fabricated field-effect transistor (FET) devices based on bilayer MoS₂ channels show substantial improvements in mobility (up to 122.6 cm² V^-1 s^-1) and variation compared with FETs based on monolayer films. Furthermore, short-channel FETs exhibit an on-state current of 1.27 mA μm^-1, which exceeds the 2028 roadmap target for high-performance FETs¹⁶.

The Intrinsic Thermodynamic Difficulty and a Step-Guided Mechanism for the Epitaxial Growth of Uniform Multilayer MoS2 with Controllable Thickness

Multilayer MoS₂ shows superior performance over the monolayer MoS₂ for electronic devices while the growth of multilayer MoS₂ with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS₂ domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-by-layer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics model also suggests that multilayer MoS₂ domains with aligned edges can significantly reduce their free energy and represent a local minimum with very prominent energy advantage on a potential energy surface. However, the nucleation probability of multilayer MoS₂ domains with aligned edges is, if not impossible, extremely rare on flat substrates. Herein, a step-guided mechanism for the growth of uniform multilayer MoS₂ on an epitaxial substrate is theoretically proposed. The steps with proper height on sapphire surface are able to guide the simultaneous nucleation of multilayer MoS₂ with aligned edges and uniform thickness, and promote the continuous growth of multilayer MoS₂ films. The proposed mechanism can be reasonably extended to grow multilayer 2D materials with uniform thickness on epitaxial substrates.

Single-crystal two-dimensional material epitaxy on tailored non-single-crystal substrates

The use of single-crystal substrates as templates for the epitaxial growth of single-crystal overlayers has been a primary principle of materials epitaxy for more than 70 years. Here we report our finding that, though counterintuitive, single-crystal 2D materials can be epitaxially grown on twinned crystals. By establishing a geometric principle to describe 2D materials alignment on high-index surfaces, we show that 2D material islands grown on the two sides of a twin boundary can be well aligned. To validate this prediction, wafer-scale Cu foils with abundant twin boundaries were synthesized, and on the surfaces of these polycrystalline Cu foils, we have successfully grown wafer-scale single-crystal graphene and hexagonal boron nitride films. In addition, to greatly increasing the availability of large area high-quality 2D single crystals, our discovery also extends the fundamental understanding of materials epitaxy.

Scalable Moiré Lattice with Oriented TMD Monolayers

Moiré lattice in artificially stacked monolayers of two-dimensional (2D) materials effectively modulates the electronic structures of materials, which is widely highlighted. Formation of the electronic Moiré superlattice promises the prospect of uniformity among different moiré cells across the lattice, enabling a new platform for novel properties, such as unconventional superconductivity, and scalable quantum emitters. Recently, epitaxial growth of the monolayer transition metal dichalcogenide (TMD) is achieved on the sapphire substrate by chemical vapor deposition (CVD) to realize scalable growth of highly-oriented monolayers. However, fabrication of the scalable Moiré lattice remains challenging due to the lack of essential manipulation of the well-aligned monolayers for clean interface quality and precise twisting angle control. Here, scalable and highly-oriented monolayers of TMD are realized on the sapphire substrates by using the customized CVD process. Controlled growth of the epitaxial monolayers is achieved by promoting the rotation of the nuclei-like domains in the initial growth stage, enabling aligned domains for further grain growth in the steady-state stage. A full coverage and distribution of the highly-oriented domains are verified by second-harmonic generation (SHG) microscopy. By developing the method for clean monolayer manipulation, hetero-stacked bilayer (epi-WS₂/epi-MoS₂) is fabricated with the specific angular alignment of the two major oriented monolayers at the edge direction of 0°/ ± 60°. On account of the optimization for scalable Moiré lattice with a high-quality interface, the observation of interlayer exciton at low temperature illustrates the feasibility of scalable Moiré superlattice based on the oriented monolayers.

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.

Wafer-scale single-orientation 2D layers by atomic edge-guided epitaxial growth

Two-dimensional (2D) layered materials hold tremendous promise for post-Si nanoelectronics due to their unique optical and electrical properties. Significant advances have been achieved in device fabrication and synthesis routes for 2D nanoelectronics over the past decade; however, one major bottleneck preventing their immediate applications has been the lack of a reproducible approach for growing wafer-scale single-crystal films despite tremendous progress in recent experimental demonstrations. In this tutorial review, we provide a systematic summary of the critical factors-including crystal/substrate symmetry and energy consideration-necessary for synthesizing single-orientation 2D layers. In particular, we focus on the discussions of the atomic edge-guided epitaxial growth, which assists in unidirectional nucleation for the wafer-scale growth of single-crystal 2D layers.

Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS2 monolayer on vicinal a-plane sapphire

The growth of wafer-scale single-crystal two-dimensional transition metal dichalcogenides (TMDs) on insulating substrates is critically important for a variety of high-end applications^1-4. Although the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported^5-8, these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts^9-20, the direct growth of wafer-scale single-crystal TMDs on insulating substrates is yet to be realized. Here we report the successful epitaxial growth of two-inch single-crystal WS₂ monolayer films on vicinal a-plane sapphire surfaces. In-depth characterizations and theoretical calculations reveal that the epitaxy is driven by a dual-coupling-guided mechanism, where the sapphire plane-WS₂ interaction leads to two preferred antiparallel orientations of the WS₂ crystal, and sapphire step edge-WS₂ interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS₂ islands. The unidirectional alignment and seamless stitching of WS₂ islands are illustrated via multiscale characterization techniques; the high quality of WS₂ monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS₂ flakes. Our findings offer the opportunity to boost the production of wafer-scale single crystals of a broad range of two-dimensional materials on insulators, paving the way to applications in integrated devices.

Continuous orientated growth of scaled single-crystal 2D monolayer films

Single-crystal 2D materials have attracted a boom of scientific and technological activities. Recently, chemical vapor deposition (CVD) shows great promise for the synthesis of high-quality 2D materials owing to high controllability, high scalability and ultra-low cost. Two types of strategies have been developed: one is single-seed method, which focuses on the ultimate control of the density of nucleation into only one nucleus and the other is a multi-seed approach, which concentrates on the precise engineering of orientation of nuclei into a uniform alignment. Currently, the latter is recognized as a more effective method to meet the demand of industrial production, whereas the oriented domains can seamlessly merge into a continuous single-crystal film in a short time. In this review, we present the detailed cases of growing the representative monocrystalline 2D materials via the single-seed CVD method as well as show its advantages and disadvantages in shaping 2D materials. Then, other typical 2D materials (including graphene, h-BN, and TMDs) are given in terms of the unique feature under the guideline of the multi-seed growth approach. Furthermore, the growth mechanism for the 2D single crystals is presented and the following application in electronics, optics and antioxidation coatings are also discussed. Finally, we outline the current challenges, and a bright development in the future of the continuous orientated growth of scaled 2D crystals should be envisioned.

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.

Recent progress in the synthesis of novel two-dimensional van der Waals materials

The last decade has witnessed the significant progress of physical fundamental research and great success of practical application in two-dimensional (2D) van der Waals (vdW) materials since the discovery of graphene in 2004. To date, vdW materials is still a vibrant and fast-expanding field, where tremendous reports have been published covering topics from cutting-edge quantum technology to urgent green energy, and so on. Here, we briefly review the emerging hot physical topics and intriguing materials, such as 2D topological materials, piezoelectric materials, ferroelectric materials, magnetic materials and twistronic heterostructures. Then, various vdW material synthetic strategies are discussed in detail, concerning the growth mechanisms, preparation conditions and typical examples. Finally, prospects and further opportunities in the booming field of 2D materials are addressed.

Theoretical Study of Chemical Vapor Deposition Synthesis of Graphene and Beyond: Challenges and Perspectives

Two-dimensional (2D) materials have attracted great attention in recent years because of their unique dimensionality and related properties. Chemical vapor deposition (CVD), a crucial technique for thin-film epitaxial growth, has become the most promising method of synthesizing 2D materials. Different from traditional thin-film growth, where strong chemical bonds are involved in both thin films and substrates, the interaction in 2D materials and substrates involves the van der Waals force and is highly anisotropic, and therefore, traditional thin-film growth theories cannot be applied to 2D material CVD synthesis. During the last 15 years, extensive theoretical studies were devoted to the CVD synthesis of 2D materials. This Perspective attempts to present a theoretical framework for 2D material CVD synthesis as well as the challenges and opportunities in exploring CVD mechanisms. We hope that this Perspective can provide an in-depth understanding of 2D material CVD synthesis and can further stimulate 2D material synthesis.

Single-Crystal MoS2 Monolayer Wafer Grown on Au (111) Film Substrates

Monolayer transition metal dichalcogenides (TMDCs) with high crystalline quality are important channel materials for next-generation electronics. Researches on TMDCs have been accelerated by the development of chemical vapor deposition (CVD). However, antiparallel domains and twin grain boundaries (GBs) usually form in CVD synthesis due to the special threefold symmetry of TMDCs lattices. The existence of GBs severely reduces the electrical and photoelectrical properties of TMDCs, thus restricting their practical applications. Herein, the epitaxial growth of single crystal MoS₂ (SC-MoS₂ ) monolayer is reported on Au (111) film across a two-inch c-plane sapphire wafer by CVD. The MoS₂ domains obtained on Au (111) film exhibit unidirectional alignment with zigzag edges parallel to the <110> direction of Au (111). Experimental results indicated that the unidirectional growth of MoS₂ domains on Au (111) is a temperature-guided epitaxial growth mode. The high growth temperature provides enough energy for the rotation of the MoS₂ seeds to find the most favorable orientation on Au (111) to achieve a unidirectional ratio of over 99%. Moreover, the unidirectional MoS₂ domains seamlessly stitched into single crystal monolayer without GBs formation. The progress achieved in this work will promote the practical applications of TMDCs in microelectronics.

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.

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

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

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.

Low-Temperature and High-Quality Growth of Bi2O2Se Layered Semiconductors via Cracking Metal-Organic Chemical Vapor Deposition

Ternary metal-oxy-chalcogenides are emerging as next-generation layered semiconductors beyond binary metal-chalcogenides (i.e., MoS₂). Among ternary metal-oxy-chalcogenides, especially Bi₂O₂Se has been demonstrated in field-effect transistors and photodetectors, exhibiting ultrahigh performance with robust air stability. The growth method for Bi₂O₂Se that has been reported so far is a powder sublimation based chemical vapor deposition. The first step for pursuing the practical application of Bi₂O₂Se as a semiconductor material is developing a gas-phase growth process. Here, we report a cracking metal-organic chemical vapor deposition (c-MOCVD) for the gas-phase growth of Bi₂O₂Se. The resulting Bi₂O₂Se films at very low growth temperature (∼300 °C) show single-crystalline quality. By taking advantage of the gas-phase growth, the precise phase control was demonstrated by modulating the partial pressure of each precursor. In addition, c-MOCVD-grown Bi₂O₂Se exhibits outstanding electrical and optoelectronic performance at room temperature without passivation, including maximum electron mobility of 127 cm²/(V·s) and photoresponsivity of 45134 A/W.

Selective saturation of step-edges as a tool to control the growth of molecular fibres

The concept of bottom-up self-organisation has become a promising alternative for structuring molecular materials, which are hardly accessible by conventional top-down approaches such as lithography due to their limited chemical robustness. While these materials often tend to form three-dimensional, crystalline islands or fibres upon film growth, the size and orientation of such fibres are mainly governed by appropriate preparation conditions as well as microscopic interactions at the interface with the supporting surface. Substrate surface defects such as vacancies or step-edges, which cannot be completely ruled out on real surfaces on the mesoscopic scale, can act as preferred nucleation sites for molecules that leads to parasitic film growth competing with their intrinsic alignment prevailing on an ideal surface. In the present study, we demonstrate for the case of para-quaterphenyl (p-4P) that the presence of azimuthally disordered, fibres on Ag(111) surfaces can be understood as a superposition of step-mediated nucleation and the intrinsic epitaxial fibre growth on ideal surfaces. We validate the concept by purposely exposing the silver substrates briefly to oxygen or even ambient air to passivate the more reactive step-sites, which hampers subsequently grown molecular films to nucleate at these step-edges. This yields a truly epitaxial alignment as well as an enlargement of the fibres present on the whole sample.

Effect of substrate symmetry on the orientations of MoS2 monolayers

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are promising platforms for developing next-generation electronic and optoelectronic devices due to their unique properties. To achieve this, the growth of large single-crystal TMDs is a critical issue. Unraveling the factors affecting the nucleation and domain orientation should hold fundamental significance. Herein, we design the chemical vapor deposition growth of monolayer MoS₂ triangles on Au(111) and Au(100) facets, for exploring the substrate facet effects on the domain orientations. According to multi-scale characterizations, we find that, the obtained triangular MoS₂ domains present two preferential orientations on the six-fold symmetric Au(111) facet, whereas four predominant orientations on the four-fold symmetric Au(100) facet. Using on-site scanning tunneling microscopy, we further reveal the preferred alignments of monolayer MoS₂ triangles along the close-packed directions of both Au(111) and Au(100) facets. Moreover, bunched substrate steps are also found to form along the close-packed directions of the crystal facets, which guides the preferential nucleation of monolayer MoS₂ along the step edges. This work should hereby deepen the understanding of the substrate facet/step effect on the nucleation and orientation of monolayer MoS₂ domains, thus providing fundamental insights into the controllable syntheses of large single-crystal TMD monolayers.

Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS₂ requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS₂ monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS₂ monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS₂ monolayers are largely free of IDBs but instead have translational boundaries that arise when WS₂ domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS₂ monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS₂ field effect transistors exhibited an I_ON/_OFF of ∼10⁷ and mobility of 16 cm²/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

Nitrogen dioxide (NO2), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO2 sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials. Molybdenum disulfide (MoS2) has emerged as a potential candidate for developing next-generation NO2 gas sensors. MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

Defect Engineering in Metastable Phases of Transition-Metal Dichalcogenides for Electrochemical Applications

Metastable metallic phases of transition-metal dichalcogenide (TMD) nanomaterials have displayed excellent performance and emerged as promising candidates for sustainable energy sources low-cost storage and conversion because of their two-dimensional (2D) layered structures and extraordinary physicochemical properties. In order to broaden the range of potential applications, defect engineering is applied to the metastable phases of TMDs for further improvement of their catalytic and electronic properties. According to some recent studies, effective introduction of defects without perturbing the interior conductivity contributes to the development of metastable TMDs. This review provides deep insights into recent progress in electrochemistry using defect engineering in the metastable phases of TMDs. After introducing the structures of metastable phases and methods for defect construction, significant developments in catalysis and energy storage applications are discussed to elucidate structure-function relationships. Key challenges and future directions for defect engineering in the metastable phases of TMDs are also highlighted in the conclusions.

Epitaxial Synthesis of Highly Oriented 2D Janus Rashba Semiconductor BiTeCl and BiTeBr Layers

The family of layered BiTeX (X = Cl, Br, I) compounds are intrinsic Janus semiconductors with giant Rashba-splitting and many exotic surface and bulk physical properties. To date, studies on these materials required mechanical exfoliation from bulk crystals which yielded thick sheets in nonscalable sizes. Here, we report epitaxial synthesis of Janus BiTeCl and BiTeBr sheets through a nanoconversion technique that can produce few triple layers of Rashba semiconductors (<10 nm) on sapphire substrates. The process starts with van der Waals epitaxy of Bi₂Te₃ sheets on sapphire and converts these sheets to BiTeCl or BiTeBr layers at high temperatures in the presence of chemically reactive BiCl₃/BiBr₃ inorganic vapor. Systematic Raman, XRD, SEM, EDX, and other studies show that highly crystalline BiTeCl and BiTeBr sheets can be produced on demand. Atomic level growth mechanism is also proposed and discussed to offer further insights into growth process steps. Overall, this work marks the direct deposition of 2D Janus Rashba materials and offers pathways to synthesize other Janus compounds belonging to MXY family members.

The epitaxy of 2D materials growth

Two dimensional (2D) materials consist of one to a few atomic layers, where the intra-layer atoms are chemically bonded and the atomic layers are weakly bonded. The high bonding anisotropicity in 2D materials make their growth on a substrate substantially different from the conventional thin film growth. Here, we proposed a general theoretical framework for the epitaxial growth of a 2D material on an arbitrary substrate. Our extensive density functional theory (DFT) calculations show that the propagating edge of a 2D material tends to align along a high symmetry direction of the substrate and, as a conclusion, the interplay between the symmetries of the 2D material and the substrate plays a critical role in the epitaxial growth of the 2D material. Based on our results, we have outlined that orientational uniformity of 2D material islands on a substrate can be realized only if the symmetry group of the substrate is a subgroup of that of the 2D material. Our predictions are in perfect agreement with most experimental observations on 2D materials' growth on various substrates known up to now. We believe that this general guideline will lead to the large-scale synthesis of wafer-scale single crystals of various 2D materials in the near future.

Carbon nanotube spiderweb promoted growth of hierarchical transition metal dichalcogenide nanostructures for seamless devices

Hierarchical transition metal dichalcogenide (h-TMDC) nanostructures with abundant active edge sites and good electrical conductivity hold great promise for numerous applications. Here, we report a general method for the chemical synthesis of a series of large-area, free-standing h-TMDC films and their devices by using carbon nanotube (CNT) spiderwebs as both growth promoters and electrical/mechanical reinforcement networks. Our approach allows the seamless integration of h-TMDC nanostructures with abundant active edge sites and CNT networks with good electrical conductivity and mechanical flexibility. As a proof of concept, h-MoSe₂/CNT hybrid films with CNT contacts have been chemically synthesized and applied as flexible electrocatalytic devices for hydrogen evolution reaction (HER). Owing to the seamless connection between the CNT contacts and the electroactive h-TMDC/CNT nanostructures, the flexible electrocatalytic devices exhibited excellent mechanical stability and maintained stable electrocatalytic performance under cyclic bendings. Our method can be readily extended to the large-scale production of various h-TMDC/CNT hybrid films and their seamless devices.

Growth and Grain Boundaries in 2D Materials

Grain boundaries (GBs) are a kind of lattice imperfection widely existing in two-dimensional materials, playing a critical role in materials' properties and device performance. Related key issues in this area have drawn much attention and are still under intense investigation. These issues include the characterization of GBs at different length scales, the dynamic formation of GBs during the synthesis, the manipulation of the configuration and density of GBs for specific material functionality, and the understanding of structure-property relationships and device applications. This review will provide a general introduction of progress in this field. Several techniques for characterizing GBs, such as direct imaging by high-resolution transmission electron microscopy, visualization techniques of GBs by optical microscopy, plasmon propagation, or second harmonic generation, are presented. To understand the dynamic formation process of GBs during the growth, a general geometric approach and theoretical consideration are reviewed. Moreover, strategies controlling the density of GBs for GB-free materials or materials with tunable GB patterns are summarized, and the effects of GBs on materials' properties are discussed. Finally, challenges and outlook are provided.

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

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

Light-matter interactions in two-dimensional layered WSe2 for gauging evolution of phonon dynamics

Phonon dynamics is explored in mechanically exfoliated two-dimensional WSe₂ using temperature-dependent and laser-power-dependent Raman and photoluminescence (PL) spectroscopy. From this analysis, phonon lifetime in the Raman active modes and phonon concentration, as correlated to the energy parameter E ₀, were calculated as a function of the laser power, P, and substrate temperature, T. For monolayer WSe₂, from the power dependence it was determined that the phonon lifetime for the in-plane vibrational mode was twice that of the out-of-plane vibrational mode for P in the range from 0.308 mW up to 3.35 mW. On the other hand, the corresponding relationship for the temperature analysis showed that the phonon lifetime for the in-plane vibrational mode lies within 1.42× to 1.90× that of the out-of-plane vibrational mode over T = 79 K up to 523 K. To provide energy from external stimuli, as T and P were increased, peak broadening in the PL spectra of the A-exciton was observed. From this, a phonon concentration was tabulated using the Urbach formulism, which increased with increasing T and P; consequently, the phonon lifetime was found to decrease. Although phonon lifetime decreased with increasing temperature for all thicknesses, the decay rate in the phonon lifetime in the monolayer (1L) material was found to be 2× lower compared to the bulk. We invoke a harmonic oscillator model to explain the damping mechanism in WSe₂. From this it was determined that the damping coefficient increases with the number of layers. The work reported here sheds fundamental insights into the evolution of phonon dynamics in WSe₂ and should help pave the way for designing high-performance electronic, optoelectronic and thermoelectric devices in the future.