Introduction

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as tantalizingly promising materials for the next generation of high-performance technology. These materials have a wide range of potential applications in fields such as electronics, photonics, and computing by virtue of their high carrier mobility and tunable bandgaps. To realize the full potential of 2D TMDs, it is essential to produce high-quality, large-scale single-crystal films. Such films would allow for their integration with silicon (Si)-based electronics, enabling the optimization for better performance. Obtaining large-area single-crystal growth of 2D TMDs involves seamlessly stitching together tens of millions of 2D domains that are all aligned in the same direction. This can be accomplished by using a single crystalline substrate and ensuring that the lattices of the 2D materials and underlying substrates are well matched. One feasible approach to achieve this is through epitaxy, which allows for controlling the orientation of 2D TMD domains during their nucleation. In traditional epitaxy growth, the epilayer of three-dimensional (3D) materials interacts covalently with the substrate due to the existence of dangling bonds on the surfaces, and strong chemical bonding forms at the interface, which determines the orientation of the epitaxial layer. Thus, a lattice mismatch of less than 5–10% is required to achieve conventional epitaxy. On the contrary, the van der Waals (vdW) epitaxy of TMD materials results in weak interaction due to the lack of surface dangling bonds. Consequently, TMDs whose lattices have threefold symmetry grow antiparallel domains and thus the twin boundaries when the domains stitch with each other. The antiparallel domain formation is due to the binding energy degeneracy, and only when it is broken can the unidirectionally aligned domains be grown. Step-guided epitaxy was proposed to overcome energy degeneracy, where the edge of the step along the specific direction on the substrate acts as preferential nucleation sites that would guide the unidirectional alignment of 2D materials at a large scale.

Epitaxy of 2D TMD wafers with continuous single crystallinity

Single-crystal 2D TMDs on oxide substrates

The large-scale growth of single-crystal TMD films on insulating oxide substrates (e.g., SiO2, TiO2, and Al2O3) is essential for develo** next-generation ultrathin and flexible electronic and optoelectronic devices. Among the oxide substrates, large-scale growth of 2D TMDs on SiO2 substrates would enable batch fabrication and seamless integration of atomically thin high-performance transistors, memories, and phototransistors on Si-based devices without film transfer. However, the amorphous nature of SiO2 makes it difficult to achieve epitaxial growth, which requires meticulous symmetry and lattice matching between epilayers and growth substrates. The result is the formation of small and randomly oriented TMD domains that are less than ideal for industrial scaling. Despite strenuous efforts, the size of single-crystalline MoS2 domains on SiO2 remains limited.1,2 On the contrary, when 2D TMDs are grown on insulating oxide substrates (typically single crystal in nature), the domain’s shapes and orientations can be engineered by their epitaxy interaction with the underlying substrate.3 Such Such crystalline oxide substrates thus are employed for growing 2D TMDs, in addition to their relatively high thermal stability, chemical inertness, and atomically flat surface. These unique material properties facilitate precursor migration during chemical vapor deposition (CVD), thereby improving the thickness uniformity of the resulting 2D TMD film. For example, single-crystal strontium titanate (SrTiO3) and titanium dioxide (TiO2) substrates have been used for TMD growth as shown in Figure 1.4,17 However, the current results from these growth processes show either two distinct growth orientations (0° and 60°) on GaN or the formation of small nanocrystals with uniform width on Si(001) surfaces pretreated with phosphine. To fully utilize semiconductor substrates for large-scale production and achieve single-crystal TMDs, additional efforts and advancements are necessary.24,26 Defect density can be controlled during the synthesis stage or post-synthesis treatments. The former is mainly achieved by stoichiometry deviations or do**. In contrast, the latter can create or heal defective sites by any method, such as ion bombardment, plasma treatment, and chemical treatment.27,31

Figure 10
figure 10

During-synthesis methods of engineering the defects in 2D materials: (a) Left: Scanning tunneling microscopy images of a chemical vapor deposition (CVD)-WS2. Right: Hydroxide vapor-phase deposition-WS2.30 (b) Left: X-ray photoelectron spectroscopy data of Mo 3d for CVD MoS2 and O-MoS2. Right: Typical photoluminescence (PL) spectra of O-MoS2, SM-MoS2, and SE-MoS2 on SiO2/Si substrate showing PL enhancement of O-MoS2.31

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In parallel, post-synthesis treatments for vacancy healing have been explored, such as annealing of 2D TMD materials under a chalcogenide environment. In Figure 11a, atomic-resolution STM images show the monolayer 2D MoS2−xOx with oxygen substitution of sulfur vacancies after exposure to air and annealing in a hydrogen sulfide (H2S) atmosphere at 200°C. STM results indicate that 2D MoS2−xOx crystals can be reduced to the original state of defect-free MoS2 via efficient surface chemistry engineering.25 Similar approaches, including high-pressure annealing under only a sulfur environment, show remarkable recovery of the crystal quality in MoS2.32 Not only the dry method of gas annealing has been used, but also the wet solution treatment has been explored. As shown in Figure 11b, bilayered MoS2 is assembled by a dry transfer. Then, self-healing of sulfur vacancies spontaneously occurs by virtue of assembling sulfur adatom clusters on the MoS2 basal plane through a nonoxidizing acid poly(4-styrene sulfonate), thus enabling the hydrogenation process. The resulting PL spectra indicate the restoration of the MoS2 basal plane after the chemical treatment.33 Moreover, plasma treatments are utilized to modify chalcogen defect concentrations in monolayer TMDs, enabling the addition of new functionalities. For instance, by controlling ion energy and sputtering time in a helium plasma, the properties of MoS2 can be adjusted from being semiconducting to exhibiting metallic-like behavior through the introduction of S defects. Similarly, p-type MoS2 can be obtained by subjecting it to oxygen plasma treatment.34,35,36,37

Figure 11
figure 11

Post-synthesis methods of engineering the defects in 2D materials: (a) Scanning tunneling microscopy images of 2D MoS2−x before (left) and after (right) 30 min. annealing at 200°C in H2S.25 (b) Left: Optical microscopy image of the as-grown and self-healed samples. Scale bar = 5 mm. Right: Photoluminescence (PL) spectra acquired from different regions are highlighted in the inset.33

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Conclusion and outlook

To date, large-scale and single-crystal TMD monolayers have been successfully obtained on single-crystal metal or insulator substrates by the CVD method via two routes: (1) nucleation and growth of a single nucleus on a substrate and (2) the seamless coalescence of unidirectionally aligned 2D domains on a tailored substrate. Despite the tremendous efforts in achieving large-scale and high-quality CVD-TMDs and their integration into industrial fabrication and applications, the growth mechanisms, especially the nucleation of single-crystal 2D TMDs, have not been thoroughly unraveled, making precise and controllable growth difficult to achieve at large scale. Meanwhile, engineering defects in 2D TMDs is a fascinating area for maximizing their potential. Although defects from grain boundaries can be eliminated with single-crystal growth, intrinsic vacancies still impact material quality. In-depth studies of reaction chemistries and post-synthesis treatments are crucial for defect engineering in 2D TMDs. Additionally, new or modified designs in automated CVD systems are necessary due to challenges in maintaining consistent chalcogenide-to-metal concentrations and achieving uniform deposition of evaporated molecules. This limitation makes it difficult to control defects or utilize current laboratory-scale CVD growth methods for industrial applications. Furthermore, plasma treatments offer an additional avenue for defect manipulation and functionalization in 2D TMDs, providing exciting opportunities for tailoring their properties and adding new functionalities. Collaborative efforts between academia and industry are essential to advance scalable and reliable production methods for TMDs and bridge the gap between laboratory discoveries and fabrication processes. Further advancements in the growth of 2D vertical and lateral heterostructures with high crystallinity and large scale hold significant potential for a wide range of optoelectronic and electronic devices. These advancements offer opportunities to develop high-performance logic gates and digital integrated circuits, including complementary metal oxide semiconductor (CMOS) inverters. TMD-based field-effect transistors (FETs) have already demonstrated promising characteristics, such as higher on/off current ratios and lower subthreshold swings compared to traditional silicon-based FETs. However, integrating TMDs with CMOS technology requires bridging the gap between laboratory innovations and fabrication production. This calls for collaboration between academia and industry to develop scalable and reliable production methods for TMDs.