Advanced Synthesis and Unique Properties of 2D Transition Metal Dichalcogenides for Realizing Next-Generation Applications

Abstract

Transition metal dichalcogenides (TMDs) have emerged as a prominent class of two-dimensional (2D) materials, offering a tunable bandgap that addresses the limitations of graphene’s gapless nature. Their exceptional optical and electronic properties have positioned them at the forefront of both fundamental research and advanced device applications. This Review aims to provide a fundamental understanding of the vast family of 2D TMDs while highlighting recent progress in their synthesis techniques, unique properties, and multifunctional applications, particularly in the monolayer and near-monolayer regimes. A significant focus is placed on the challenges associated with emerging synthesis methods and the intriguing properties exhibited by monolayer TMDs, which enable their integration into next-generation photodetectors, optoelectronic devices, and sensors. These advancements enhance the flexibility and performance of monolayered materials, helping to overcome existing limitations.

1. Introduction

The discovery of graphene in 2004 marked the beginning of an exciting era in materials science, revealing the immense potential of 2D materials. Graphene’s extraordinary properties, including high carrier mobility, exceptional electrical conductivity, superior mechanical strength, and ultrahigh thermal conductivity, inspired researchers to explore other 2D materials. Over the years, materials such as black phosphorus, hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDs), silicene, and others have gained prominence due to their unique properties arising from quantum confinement, reduced dimensionality, and surface effects. Among these, TMDs have emerged as a particularly fascinating family, exhibiting a broad range of electronic properties such as layer-dependent bandgaps (transitioning from indirect in bulk to direct in monolayers), high carrier mobility in certain metallic phases, and strong spin–orbit coupling. − Their optical properties include high exciton binding energies, tunable photoluminescence, strong light–matter interactions, valley polarization, and second-harmonic generation. Mechanically, TMDs show excellent flexibility, a high Young’s modulus (up to 270 GPa in monolayers), and the ability to withstand large strains (up to 11%) without degradation. These combined properties make TMDs highly attractive for applications in transistors, photodetectors, flexible electronics, and sensors. The increasing number of yearly publications in this field, as shown in Figure , further underscores the rapidly growing interest and continuous advancements in TMD research. −

1.

Year-wise publication plots for TMD materials (searched by ScienceDirect https://www.sciencedirect.com/).

TMDs are layered materials composed of transition metal atoms (M) sandwiched between two chalcogen atoms (X) in an X-M-X structure. These materials exhibit strong covalent intralayer bonding and weak van der Waals interlayer interactions, enabling facile exfoliation into monolayers. − Their high surface-to-volume ratio, tunable electronic properties, and strong light–matter interactions have placed TMDs at the forefront of next-generation materials research. These characteristics make them indispensable for advanced sensors (e.g., gas, chemical, and biosensors) and optoelectronic devices like solar cells, photodetectors, and light-emitting diodes. −

The TMD family comprises a wide range of materials, including MoS₂, WS₂, TaS₂, NbSe₂, and others, each exhibiting distinct physical and chemical properties. MoS₂, for instance, has become a prototypical TMD due to its direct bandgap of approximately 1.8 eV in the monolayer form, excellent carrier mobility, and remarkable photoluminescence. − These properties make it highly suitable for applications in transistors, photodetectors, and light-emitting diodes. Meanwhile, metallic TMDs like NbSe₂ and TaS₂ exhibit intriguing phenomena, such as superconductivity coexisting with charge density wave (CDW) phases, highlighting their potential for quantum applications. − Furthermore, the interplay between CDW, superconductivity, and magnetic phases in parent and doped compounds provides a robust platform for exploring novel phenomena. For example, the CDW phases in materials such as TaS₂, TaSe₂, and NbSe₂ are strongly influenced by their chemical composition and crystalline structure, with transitions between nearly commensurate and incommensurate phases depending on the material and temperature. Moreover, TMDs such as MoTe₂, NiTe₂, and others serve as platforms for exploring correlated and topological phases of matter. −

A remarkable feature of TMDs lies in their ability to exhibit tunable structural and electronic properties, allowing precise manipulation of their band structure, excitonic behavior, and charge transport characteristics through variations in composition, layer thickness, strain, and external fields. For instance, monolayer TMDs undergo a transition from indirect to direct bandgaps due to quantum confinement, resulting in enhanced optical properties. Similarly, layer-dependent magnetic phenomena, such as ferromagnetic and antiferromagnetic transitions, have been observed in materials like M _1/3TaS₂ and M _1/3NbS₂. These properties are complemented by unique phenomena such as Ising superconductivity, which defies conventional superconducting limits, making TMDs a versatile platform for fundamental physics research and practical applications. −

This Review provides a comprehensive exploration of the rapidly advancing field of 2D TMDs, with a focus on their fundamental structural, mechanical, optical, and magnetic properties. We also examine recent progress in synthesis techniques, including mechanical and chemical exfoliation, which have been instrumental in enhancing the scalability and quality of TMD monolayers. Despite these advancements, achieving large-scale, defect-free, and controlled synthesis remains a major challenge, hindering the widespread industrial integration of TMDs. Unlike previous reviews that concentrate on specific TMD types or isolated applications, this Review offers a broader perspective, capturing the latest breakthroughs and emerging trends across the field. In addition to highlighting these advancements, we critically assess the existing challenges that must be overcome for TMDs to reach their full potential in next-generation technologies. By presenting a holistic and forward-looking analysis, we aim to inspire further research and innovation, driving the continued evolution of this dynamic and promising area of materials science.

2. Structure of TMDs

TMDs exhibit a wide array of structural phases, with the most common being trigonal prismatic (2H), octahedral (1T), monoclinic (1T′), and orthorhombic (1T _d ) configurations, with their comparative unit cell parameters, angles, and space groups shown in Figure (a–d), respectively. − The nature of these phases is influenced by the coordination geometry of the transition metal atoms and the stacking arrangement of the atomic layers. Specifically, the 2H phase is associated with an ABA stacking order, where chalcogen atoms in adjacent atomic planes occupy the same position (A) and are aligned along the direction perpendicular to the layer, providing a layered, periodic structure in Figure (a). This structural configuration leads to the thermodynamic stability of the 2H phase, making it the predominant phase for many TMDs, including NbSe₂, NbS₂, MoS₂, MoSe₂, and WS₂. − The stability of the 2H phase in these materials is a result of the interplay between the coordination of the metal atoms and the specific stacking arrangement of the layers. The commonly observed semiconducting properties of the 2H phase originate from its well-ordered structure. This structure influences electron transport, the bandgap, and optical characteristics, making the 2H phase particularly suitable for applications in electronic devices, light-emitting diodes, transistors, and other optoelectronic technologies.

2.

(a) 2 H phase: side view shows a hexagonal structure with trigonal prismatic coordination of transition metal atoms. (b) 1 T phase: side view reveals an octahedral coordination of transition metal atoms in a planar arrangement. (c) 1 T′ phase: side view depicts a distorted octahedral structure with a zigzag pattern of the atomic layers. (d) 1 T _d phase: side view highlights a monoclinic structure with a tilted and distorted atomic arrangement.

In contrast to the stable 2H phase, the 1T phase adopts a different stacking order, known as ABC, and is characterized by octahedral coordination, as illustrated in Figure (b). The 1T phase is often metastable under ambient conditions but can be stabilized under specific circumstances or through external influences such as temperature, pressure, or doping. The distinct octahedral coordination in the 1T phase contributes to its unique electronic properties, including its potential to host Dirac and Weyl semimetal states, which can be fine-tuned for various applications. A slight distortion in the 1T phase, referred to as the Peierls distortion, can lead to the formation of the monoclinic 1T′ phase as depicted in Figure (c). The 1T′ phase retains the ABC stacking order and exhibits a centrosymmetric arrangement. Notably, the 1T′ phase is characterized by its pronounced electronic anisotropy and potential for charge density wave formation, which significantly influence its transport properties and highlight its potential for innovative uses in electronic and photonic technologies.

Further structural transformations are observed in materials such as MoTe₂, where the 1T′ phase can transition to the 1T _d phase upon cooling below a specific temperature (approximately 280 K). , This transition involves a shift from the centrosymmetric 1T′ structure to the non-centrosymmetric orthorhombic 1T _d structure, which breaks inversion symmetry and introduces new electronic properties. The primary structural difference between the 1T′ and 1T _d phases lies in the unit cell angle γ, which is not 90° in the 1T′ phase. However, the 1T _d phase is particularly noteworthy due to its topologically nontrivial characteristics, making it an intriguing candidate for quantum material research. This phase transition and the structural changes that come with it are important for influencing the material’s electronic, mechanical, topological, and optical properties, as explained later in the section on TMD properties.

3. Synthesis Process

Researchers have developed various techniques to synthesize, isolate, and study 2D TMDs, with each method tailored to specific applications. Among the materials in the TMD family, compounds such as MoS₂, WS₂, ZrS₂, MoSe₂, NbSe₂, TiS₂, NiTe₂, NbTe₂, TaS₂, and WSe₂ have been successfully synthesized in monolayer or few-layer forms. These ultrathin layers enable detailed investigations into their layer-dependent properties, which often exhibit behaviors distinct from their bulk counterparts. This section highlights four primary methods commonly employed to obtain and manipulate monolayer and few-layer TMD structures: mechanical exfoliation (using crystals grown by the chemical vapor transport method), chemical vapor deposition, liquid-phase exfoliation, and wet chemical synthesis. Each technique offers unique advantages in terms of crystal quality, scalability, and structural control, making it indispensable for both fundamental research and practical applications.

3.1. Mechanical Exfoliation

Mechanical exfoliation is a straightforward and efficient technique for isolating monolayer or few-layer TMDs from bulk crystals. , The process typically employs the adhesive Scotch tape method to peel thin layers from the bulk material. Over time, more advanced variants of this technique have been developed, including laser exfoliation and gel-film-based methods utilizing Gel-Pak. These innovations have significantly improved the efficiency and versatility of mechanical exfoliation, broadening its applicability to various research and industrial purposes. The layered crystals used in this process are generally synthesized using the chemical vapor transport (CVT) method, which serves as a fundamental precursor for mechanical exfoliation. Before we dive into the mechanical exfoliation process, the principles of the CVT method are briefly outlined below.

TMD single crystals can be synthesized through two different growth approaches using the CVT method: one without a transport agent, which is feasible for certain lightweight elements and typically yields nanostructures, and the other with a transport agent, which is the standard approach for growing bulk single crystals. The transport-agent-free approach is mainly applicable to compounds with high vapor pressures such as TiS₂. For instance, TiS₂ nanosheets were recently synthesized via CVT without the use of a transport agent. This process critically depends on precise temperature optimization. Titanium and sulfur powders are mixed in a 1:3 atomic ratio using a pestle and mortar, then sealed in a quartz tube under a high vacuum of (10^–5 mbar) using a diffusion pump backed by a rotary pump to eliminate oxygen and moisture, which could lead to undesirable byproducts at high temperatures. − The sealed tube is then placed in a furnace and subjected to temperatures ranging from 400 to 650 °C for approximately 24 h. The synthesis process exhibits a temperature-dependent phase evolution: up to 550 °C, TiS₃ nanostructures form, while above this temperature TiS₂ nanosheets develop. Notably, a gradual morphological transformation is observed with increasing temperature, transitioning from nanosheets to mixed-phase nanosheets/nanoribbons, then to nanoribbons of TiS₃, and finally to TiS₂ nanodiscs.

In contrast, TMD bulk single crystals are synthesized using a transport agent in the CVT process, − as shown in Figure (a) and (b). This method relies on achieving a balanced equilibrium, allowing precursor materials to partially vaporize into the gas phase and subsequently recondense as solid crystals under carefully controlled experimental conditions. − For example, in the case of TiS₂, titanium and sulfur powders are mixed in a stoichiometric ratio with iodine as a transport agent and placed in a horizontal tubular furnace, as illustrated in Figure (b). The growth process is carried out under a controlled temperature gradient of 800–900 °C for 7–14 days. After this period, single crystals formed in the colder zone of the tube. The transport reaction for TiS₂ can be expressed as

$$\begin{array}{r}\mathrm{Ti}(s)+2{\mathrm{I}}_2(g)\rightleftharpoons {\mathrm{TiI}}_4(g)\\ {\mathrm{TiI}}_4(g)+2\mathrm{S}(g)\rightleftharpoons {\mathrm{TiS}}_2(s)+{\mathrm{I}}_2(g)\end{array}$$ 1

3.

(a) Schematic diagram of a tubular furnace showing a ceramic tube which contains a quartz tube placed under a controlled temperature gradient using a temperature controller. (b) A schematic representation of the process where single crystals are grown from powdered material.

At the high-temperature zone (T ₁), titanium (Ti) reacts with iodine (I₂) to form volatile titanium tetraiodide (TiI₄), which then diffuses toward the colder region (T ₂). In this region, the reverse reaction takes place, where TiI₄ reacts with sulfur (S) to reform and deposit TiS₂ as a bulk single crystals. This controlled transport process ensures efficient crystal growth by maintaining a dynamic equilibrium between the forward and backward reactions. The precise tuning of temperature gradients and transport agents plays a crucial role in optimizing the yield and quality of the grown crystals. Similarly to TiS₂, the preparation methods of some other TMD materials using the CVT technique, including their transport agents, growth conditions, durations, and special properties, are summarized in Table . These bulk crystals serve as the foundation for obtaining thin flakes or monolayers through various exfoliation techniques, facilitating fundamental studies and device applications.

1. Chemical Vapor Transport Growth Parameters and Special Properties of Selected TMD Materials Using Transport Agents.

Material	Transport Agent	Temperature Gradient (°C)	Growth Time (day)	Special Properties
1T-TiTe2	I2	690–750	12–13	Pressure-induced topological phase transitions
1T′-MoTe2	Br2	700–900	7–15	Topological superconductor
1T d -WTe2	Br2	680–780	7–8	Topological type-II Weyl semimetal
1T-TiSe2	I2	740–780	12–13	excitonic insulator
2H-NbSe2	I2, SeCl4	700–730	12–15	Ising Superconductor
2H-MoSe2	I2	850–910	11–12	Strong excitonic effects
1T-TaSe2	I2	850–950	10–15	Mott insulator
2H-TiS2	I2	800–900	7–14	High photoresponse
2H-MoS2	I2, Br2	850–1100	7–14	Semiconducting bandgap
2H-TaS2	I2	900–1000	8–10	Ising superconductivity
2H-WS2	I2	950–1030	10–12	High photoluminescence

Scotch tape-based mechanical exfoliation, often considered the gold standard for achieving pristine, highly crystalline nanosheets, remains one of the most effective methods for producing atomically thin layers with minimal contamination. In a typical mechanical exfoliation process, thin TMD crystals are carefully peeled from their bulk counterparts using adhesive materials, such as Scotch tape. The process of obtaining few-layer or monolayer crystals, as illustrated in Figure , begins with cleaving the bulk crystal to form thin layers. Next, Scotch tape is placed on the substrate in an attempt to create atomically thin layers. The peeled layers on the tape are then transferred onto a substrate, where tools such as plastic tweezers are used to gently separate the layers further. After removal of the tape, monolayer (1L) and multilayer TMD nanosheets remain on the substrate, ready for study. This technique has been successfully used to isolate single and few-layer TaS₂, TaSe₂, and WSe₂ on Si/SiO₂ substrates. , Researchers emphasize that nanosheets obtained through mechanical exfoliation retain clean, high-quality structures, making them ideal for investigating inherent thickness-dependent properties such as electronic bandgap, carrier mobility, and vibrational modes. ,

4.

Mechanical exfoliation method for 2D single crystals: (a) Adhesive scotch tape is used to remove layers from the top of the crystal. (b) The tape with attached layers is carefully peeled off. (c) Upon peeling, (d) the bottommost layer is transferred onto the substrate, leaving a thin 2D crystal layer for further analysis.

Beyond traditional Scotch tape-based methods, laser exfoliation has emerged as a promising alternative for isolating few-layer structures, as shown in the procedure in Figure . In 2011, Qian et al. demonstrated the feasibility of using laser exfoliation to produce few-layer graphene from highly ordered pyrolytic graphite (HOPG) under vacuum. , This technique presents a fast and straightforward alternative to mechanical exfoliation, eliminating the risk of glue contamination from the tape. By focusing a laser through a convex lens onto the material surface, intense thermal energy is generated, causing the irradiated area to deform and separate due to the heat. Notably, in this method, femtosecond lasers known for their ultrashort pulse duration are preferred over nanosecond lasers, as FS lasers can minimize thermal defects, thereby preserving the quality of the exfoliated layers. This laser-based approach has successfully produced MoS₂, MoSe₂, WS₂, and WSe₂ nanosheets with minimal defect density, further enriching the toolkit available for fabricating high-quality 2D materials. Another innovative approach to prepare samples utilizes gel films, such as the PF-X4 film supplied by Gel-Pak. In this method, a gel film is first placed on a glass slide to provide a stable platform. The TMD sample initially affixed to Scotch tape is then pressed onto the gel film, facilitating a smooth transfer to the desired substrate. This technique has been effectively applied to TMD materials like WSe₂, MoS₂ and MoSe₂, enabling an additional level of control over sample placement and alignment.

5.

Experimental setup for laser-based exfoliation of bulk TMD materials.

For a concise overview, Table summarizes the three exfoliation processes discussed above, providing a comparative analysis of their effectiveness in isolating and synthesizing TMD monolayers and few-layer structures. These methods encompass a range of approaches for isolating and synthesizing TMD monolayers and few-layer structures, each offering distinct advantages in terms of material quality and preparation precision. However, mechanical exfoliation is limited by its low scalability and inability to produce large quantities of material, making it unsuitable for industrial applications or high-throughput research. These drawbacks highlight the need for alternative techniques, such as chemical vapor deposition (CVD) and wet chemical synthesis, which offer greater scalability and potential for uniform large-area production.

2. Overview of Different Mechanical Exfoliation Methods for Obtaining Few-Layer Structures from Bulk TMD Single Crystals.

Material	Exfoliation Type	Exfoliation Method
NbSe2, MoS2, WS2, TaS2, TaSe2	Adhesive scotch tape	Mechanical exfoliation is a widely used method for obtaining thin flakes of TMDs from bulk single crystals. Typically, adhesive tape is used to peel layers from the bulk material, followed by transfer onto SiO2/Si substrates to ensure optimal optical contrast and device compatibility. To improve stability and prevent degradation, materials such as NbSe2 and TaS2 are often capped with hBN, while MoS2 and WS2 samples undergo cleaning processes to remove residues. The exfoliated flakes are identified using optical microscopy, ensuring high-quality monolayers or few-layer nanosheets suitable for electronic and optoelectronic applications.
MoS2, MoSe2, WS2, WSe2	Laser-based	Laser-based exfoliation offers a fast, contamination-free method for isolating 2D materials. Femtosecond laser pulses generate localized heating, causing material separation with minimal defects. This technique has been successfully applied to TMDs, preserving high-quality nanosheets. ,
WSe2, MoS2, MoSe2	Gel film assistant	The gel film method enhances control over exfoliated flakes. A gel film, like PF-X4, provides a stable surface for smooth transfer, ensuring precise, contamination-free deposition of different TMD materials onto substrates.

3.2. Chemical Vapor Deposition

Chemical vapor deposition (CVD) is a highly versatile and widely employed technique for fabricating thin films or coatings on various substrates. This method relies on the chemical reaction of vapor-phase precursors in a controlled environment to create solid material layers on the substrate’s surface. − The process begins with the introduction of gaseous precursors into a reaction chamber, where they are transported to the substrate, which is heated to a specific temperature to facilitate the reaction, as shown in Figure . This temperature activates the precursors, allowing their chemical transformation to form a thin film. Reaction byproducts, often in gaseous form, are safely removed from the chamber through an exhaust or vacuum system. The CVD process is highly adaptable, offering fine-tuned control over key factors like substrate heating, precursor flow rates, gas composition, and chamber pressure, which enables the synthesis of films with tailored properties such as their thickness, crystallinity, composition, and morphology. Specifically, for TMD materials, tailored CVD processes have been developed to grow MoS₂, WS₂, MoSe₂, and WTe₂ on diverse substrates such as SiO₂/Si, hexagonal boron nitride (h-BN), and sapphire, enabling advancements in flexible electronics, optoelectronic devices, and catalytic applications.

6.

Schematic representation of the chemical vapor deposition (CVD) process.

One of the most remarkable applications of CVD is the growth of 2D TMD materials like MoS₂, with researchers demonstrating its deposition on SiO₂/Si substrates using MoO₃ and sulfur powders as precursors. − However, in this process the substrate preparation played a crucial role in determining film quality, enabling a cost-effective and scalable method for flexible electronics. Further advancements include MoS₂ growth on hexagonal boron nitride (h-BN) substrates, where few-layer h-BN (2–4 layers) provided a stable platform, preserving substrate integrity while supporting high-quality heterostructures. ,

In another demonstration, the CVD growth of WS₂ films on SiO₂ substrates involved detailed procedural optimization to ensure a high film quality. The process began with thorough substrate cleaning using a piranha solution, followed by seed promotion with sodium cholate. , A water/amine solution was used to dissolve WO₃, which served as the tungsten precursor. Sulfur powder was placed upstream of the furnace as the chalcogen source. The furnace was heated to 900 °C, during which sulfur vaporization commenced at approximately 600 °C. Under an argon gas flow, the reaction conditions facilitated the uniform growth of WS₂ layers, which are promising for optoelectronics and catalytic applications due to their exceptional electronic properties.

The fabrication of MoSe₂ layers using CVD has also been refined, as demonstrated in the work of Toby Severs Millard and colleagues. , Their method employed MoO₃ and selenium powders as precursors, with c-plane sapphire substrates acting as the deposition base. By positioning the selenium source upstream and partially covering the MoO₃ crucible to regulate evaporation, precise control over growth conditions was achieved. The synthesis occurred at a furnace temperature of 700 °C under a steady flow of argon and hydrogen, ensuring optimal precursor interactions for the growth of MoSe₂. A crucial innovation in this study was the use of polystyrene instead of poly­(methyl methacrylate) (PMMA) for transferring the grown layers onto another substrate, such as SiO₂/Si precoated with WS₂. The choice of polystyrene ensured the preservation of sample integrity, overcoming the challenges associated with PMMA, which can sometimes leave a residue or compromise the sample quality.

Moreover, the CVD growth of few-layer WTe₂ films was carried out by using a three-zone furnace system. Tellurium (Te) and tungsten hexachloride (WCl₆) powders served as precursors, placed 25 cm apart in separate temperature zones. SiO₂/Si substrates were positioned face-down in a ceramic boat as the growth surface. The chamber was initially evacuated to 0.1 Torr, then purged with high-purity Ar. During deposition, a controlled N₂/H₂ gas mixture facilitated the reaction, with zone temperatures reaching 500 °C in a staggered manner. The growth process was maintained at this temperature for 20 min before naturally cooling to room temperature. The thickness of the WTe₂ films could be tailored by adjusting the amount of WCl₆ precursor and its proximity to the substrate, providing a versatile method for high-quality WTe₂ synthesis.

For a comparative summary, Table presents an overview of the four CVD processes discussed previously, emphasizing their significance in the synthesis of high-quality 2D materials. While CVD enables the controlled growth of uniform monolayers, it faces challenges such as stringent growth conditions, the requirement for specialized substrates, and high costs associated with large-scale production. To address these challenges, liquid-phase exfoliation (LPE) has emerged as a promising alternative. LPE is a cost-effective and scalable method that enables the exfoliation of 2D materials from bulk crystals in suitable solvents without the need for complex setups or high temperatures. This method offers a versatile approach for producing 2D materials, enhancing the precision of CVD in driving advancements in material development.

3. Comprehensive Summary of the Synthesis of a Few-Layer TMDs Using Chemical Vapor Deposition Methods.

Materials	Synthesis Method	Growth Environment
MoS2	Sulfurization MoO3 powder	High-purity MoO3 and sulfur powder were heated at 650 °C for 15 min at a heating rate of 15 °C/min in the presence of N2 flow, resulting in the growth of MoS2 on a SiO2 substrate.
WS2	Sulfurization WO3 powder	WO3 and sulfur powder were heated to 1000 °C and maintained at that temperature for 5 min in an N2 environment, resulting in the growth of WS2 on the SiO2/Si substrate. ,
MoSe2	Selenization MoO3 powder	MoSe2 was grown by the selenization of MoO3 powder at 600 °C for 18 min in the presence of Ag and an H2 gas mixture flow. ,
WTe2	Tellurization WCl6 powder	WTe2 was grown by the tellurization of WCl6 powder at 500 °C for 20 min on a SiO2/Si substrate, with an N2/H2 flow rate of approximately 10 sccm.

3.3. Liquid-Phase Exfoliation

Liquid-phase exfoliation (LPE) is a widely used technique for producing 2D nanosheets from bulk-layered materials. The process involves dispersing bulk crystals into a liquid medium, such as solvents, surfactants, or polymer solutions, to weaken the interlayer van der Waals forces. Mechanical energy, typically provided through ultrasonication or shear mixing, generates forces that separate the layers into thin nanosheets. For bulk graphene, the process is shown in Figure , where the application of mechanical force produces a few-layer or atomic layer of graphite in a suitable liquid medium. This technique is scalable and cost-efficient, making it suitable for the large-scale production of 2D materials such as TMDs, graphene, and h-BN. ,

7.

Liquid-phase exfoliation process: (a) Bulk graphene as the starting material. (b) Bulk graphene is broken into smaller pieces. (c) Mechanical forces in a liquid medium, applied through sonication, separate the particles into thin flakes. (d) Resulting graphene flakes consist of fewer layers of carbon atoms.

In the case of TMDs, materials like MoS₂, MoSe₂, WS₂, MoTe₂, NbSe₂, and TaSe₂ have been successfully exfoliated into few-layer nanosheets using solvents, surfactants, or polymer solutions,. , This method can achieve high concentrations, reaching up to 40 mg/mL. The high-yield production is made possible by direct ultrasonication of bulk powder, where intense sound waves generate shear forces that peel apart the layers, releasing nanosheets into a suspension. This straightforward approach, coupled with solvent stabilization, has enabled the scalable production of TMD nanosheets. However, it also presents challenges such as maintaining uniformity and preventing aggregation during processing.

One of the primary challenges with LPE is the tendency to produce a high degree of polydispersity in terms of nanosheet thickness and lateral size. This variation arises from the unpredictable nature of ultrasonication, which can lead to mixed layer numbers in the final product as well as inconsistencies in the dimensions of the sheets. Such polydispersity complicates the production of uniform, single-layer nanosheets, making it difficult to replicate the highly controlled properties seen in materials derived from mechanical exfoliation.

Efforts to address these limitations have included the intercalation of light atoms (such as lithium) and controlled annealing to enhance the quality and optical properties of LPE-produced nanosheets. In some cases, intercalated TMD nanosheets have demonstrated properties comparable to those of mechanically exfoliated samples after postprocessing, including annealing, making them suitable for flexible optoelectronic applications. , However, these additional steps increase both the preparation time and the production cost, offsetting some of the advantages of LPE’s scalability. Another approach involved purifying LPE-derived suspensions through iterative centrifugation and redispersion cycles. While this technique can enrich the suspension with monolayer nanosheets, it also significantly lengthens the production process and raises costs. ,

Despite these challenges, LPE remains an invaluable technique for the scalable production of 2D nanosheets, as summarized in Table . The method’s adaptability to a wide range of materials and its capacity for relatively high yields make it highly suitable for large-scale applications, where precise monolayer control is less critical. However, its limitations in achieving uniformity and monolayer purity underscore the need for complementary approaches such as wet chemical synthesis, which can provide greater control over layer thickness, morphology, and composition. With ongoing research to refine exfoliation parameters, improve nanosheet purity, and minimize the need for postprocessing, LPE holds significant promise for advancing the integration of TMD nanosheets and other 2D materials into practical devices and functional coatings. As innovations in solvent selection, surfactant application, and sonication techniques emerge, LPE may increasingly offer a balance between scalability and quality, supporting the growing demand for 2D materials across various industries.

4. Liquid-Phase Exfoliation Techniques for Few-Layer TMD Nanosheets.

Materials	Liquid Exfoliation	Exfoliation Method
NbSe2, MoS2, MoSe2, MoTe2, TaSe2, WS2	Sonication in surfactant solution	TMDs are exfoliated using a liquid-phase method by dispersing bulk powder in a solvent such as NMP–water or ethanol–water. The solution is degassed using argon flow (if needed) and then sonicated for 2–8 h under controlled temperature conditions. After exfoliation, the dispersion is centrifuged to remove unexfoliated material, and the supernatant containing nanosheets is collected for further use. ,

3.4. Wet Chemical Synthesis

The wet chemical process has emerged as a promising alternative to mechanical exfoliation for the synthesis of high-purity nanostructured materials, particularly within the realm of 2D materials. − This approach typically involves chemical reactions in solution, where precursors are dissolved and reacted under controlled conditions to form nanosheets. Wet chemical synthesis often employs techniques such as hydrothermal or solvothermal processes, where the temperature and pressure can be adjusted to influence crystal growth. This technique allows for precise control over the structural orientation of the nanosheets, enabling the direct growth of thin layers in a specified direction. Unlike mechanical exfoliation, which typically involves peeling bulk materials to create thin layers, the wet chemical process can be tailored to favor lateral growth while inhibiting vertical expansion, yielding nanosheets with controlled thickness and extensive lateral dimensions. This characteristic is particularly advantageous for applications where well-defined, ultrathin layers are essential, as it facilitates the synthesis of uniform, monolayer, or few-layered structures with minimal defects. Additionally, the wet chemical synthesis method offers remarkable flexibility through diverse approaches, such as solvothermal reactions, hydrothermal growth, and sulfurization or selenization techniques, enabling controlled fabrication of nanostructures such as MoS₂, WS₂, ZrS₂, and other TMDs, each tailored for specific applications in electronics, energy storage, and catalysis.

A notable example of this process was demonstrated by Cheon and colleagues, who successfully produced uniform, disc-shaped ZrS₂ nanosheets with thicknesses of less than 2 nm and tunable lateral dimensions ranging from 20 to 60 nm. Their synthesis involved a reaction between zirconium tetrachloride (ZrCl₄) and carbon disulfide (CS₂) in an oleylamine solution at 300 °C. This method not only yielded ZrS₂ nanosheets but was later expanded to include a broader range of group IV and V transition metal sulfides and selenides, underscoring the versatility of the wet chemical approach. The control over temperature, reaction environment, and choice of precursors enabled precise manipulation of nanosheet dimensions, paving the way for applications in catalysis, energy storage, and electronic devices, where high surface area and thin layer quality are critical.

Hydrothermal synthesis, a subtype of wet chemical methods, has also proven effective in the low-temperature growth of various TMDs, such as MoS₂ and MoSe₂. By utilizing sulfurization and selenization reactions between metal salts and sources like sodium thiosulfate (Na₂S₂O₃) or sodium selenosulfate (Na₂SeSO₃) at temperatures around 140 °C, researchers have been able to achieve controlled growth of these materials. For instance, bilayer MoS₂ nanosheets have been synthesized through a straightforward hydrothermal approach under a carefully maintained molar ratio of 2:1 for sulfur to molybdenum at 180 °C over 50 h. This method employed ammonium molybdate ((NH₂)₂Mo₂O₄.4H₂O) and sulfur powder as sources, resulting in bilayer nanosheets with precise thickness control. The facile hydrothermal route not only enables controlled layer synthesis but also preserves the intrinsic properties of MoS₂, such as its unique electronic structure. Recently, a significant property of MoS₂ was highlighted: its bandgap transitions from an indirect to a direct bandgap as the material is reduced to monolayer thickness, which has far-reaching implications for optoelectronics and photonic applications.

In related studies, Peng and collaborators successfully synthesized WS₂ nanosheets by heating tungsten hexachloride (WCl₆) at 265 °C for 24 h. After heating, the resulting nanosheets were thoroughly washed and centrifuged with deionized water and ethanol to remove any residual impurities before being dried under vacuum. Similar processes have been applied to produce VS₂ and MoSe₂ nanoparticles, with temperatures optimized around 200 °C for uniform particle formation. , This rigorous purification process, followed by vacuum drying, ensures the production of high-purity nanosheets free from contaminants, making them suitable for applications in fields such as catalysis, energy conversion, and electronics where high material quality is essential.

Collectively, these developments in wet chemical synthesis offer substantial advantages over traditional exfoliation techniques, as summarized in Table . They not only enable the scalable production of high-quality nanosheets with tailored thickness and lateral dimensions but also open avenues for synthesizing a diverse array of materials with controlled properties. As a result, wet chemical methods continue to attract significant interest for the development of advanced nanomaterials suited to applications in fields ranging from nanoelectronics to energy storage and catalysis. However, despite its flexibility, wet chemical synthesis often requires complex purification steps to remove residual byproducts or unreacted precursors, which can complicate the production process compared to simpler exfoliation techniques. The continued refinement of these methods, along with an improved understanding of growth mechanisms, will further enhance the versatility and application potential of 2D materials across various high-tech industries.

5. Wet Chemical Synthesis Methods for Selected TMD Materials.

Material	Synthesis Method	Growth Environment
ZrS2	Oleylamine-based synthesis	Zirconium tetrachloride (ZrCl4) reacted with carbon disulfide (CS2) at 300 °C in an oleylamine solution.
MoS2	Hydrothermal synthesis	Ammonium molybdate ((NH2)2Mo2O4.4H2O) and sulfur powder in water, 180 °C for 50 h with a 2:1 sulfur-to-molybdenum molar ratio.
WS2	Solution-phase reaction	Tungsten hexachloride (WCl6) heated at 265 °C for 24 h, followed by centrifugation and washing with deionized water and ethanol.

In summary, various methods for synthesizing and exfoliating TMDs include mechanical exfoliation, chemical vapor deposition (CVD), liquid-phase exfoliation (LPE), and wet chemical synthesis. Mechanical exfoliation, often using adhesive tape, provides high-quality monolayers but lacks scalability. CVD allows for controlled growth of TMDs on various substrates, producing uniform layers with promising applications in flexible electronics; however, it requires precise control over the temperature, pressure, and precursor composition, making large-scale production complex and costly. LPE, utilizing ultrasonication, enables the scalable production of TMD nanosheets, although it often results in polydispersity. Wet chemical synthesis, including hydrothermal methods, offers precise control over the layer thickness and lateral size, enabling the production of high-purity, uniform nanosheets suitable for advanced applications in catalysis, energy storage, and nanoelectronics. However, the necessity of stringent reaction control and postsynthesis processing in wet chemical methods can sometimes pose challenges compared to the relative simplicity of CVD or mechanical exfoliation. Each method presents unique advantages and limitations, contributing to the growing versatility of TMDs in various fields.

4. Properties of 2D TMD Materials

2D TMDs are a class of materials that exhibit unique and remarkable properties due to their two-dimensional nature. These materials showcase excellent mechanical properties, including high flexibility and strength, making them ideal for flexible electronics. They also display intriguing magnetic behaviors, exhibiting either ferromagnetic or antiferromagnetic properties. Additionally, TMDs display superconducting characteristics and a direct bandgap, which contribute to their superior optoelectronic properties, making them promising candidates for applications in electronics, photodetectors, and quantum devices.

4.1. Mechanical Properties of 2D TMDs

2D materials beyond graphene, particularly TMDs, exhibit remarkable mechanical properties, including wide band gaps and exceptional flexibility. − However, despite their potential for advanced applications, these materials remain relatively underexplored. Monolayer MoS₂, a prominent TMD, has been extensively studied for its unique responses under mechanical deformation. Zhao et al. reported through molecular dynamics simulations that the 2H structure of monolayer MoS₂ undergoes a transformation into a quadrilateral phase under large uniaxial tensile deformation along the zigzag direction. This newly formed phase demonstrates stability and exhibits a Young’s modulus approximately 2.5× higher than the untransformed structure. Subsequent experimental investigations revealed a Young’s modulus of approximately 280 GPa and a stiffness of ≈183 N/m for monolayer MoS₂ based on force-deflection curve fittings. These studies also highlighted the elastic nature of MoS₂, with the material returning to its original configuration when the applied deflection remained below a critical threshold. Furthermore, sulfur deficiencies of around 1% have been shown to significantly alter the mechanical properties of MoS₂, transitioning its failure mode from brittle fracture to plastic deformation. Experimental findings indicated a 5 nm plastic region near the crack tip for sulfur-deficient MoS₂, which expanded to 5–10 nm in corrosive environments. Beyond binary TMDs, ternary compounds such as MoS_2x Te_2(1–x) (0 < x < 1) have garnered attention for their tunable mechanical properties. Ying et al. successfully synthesized monolayers of MoS_2x Te_2(1–x) and performed tensile tests using molecular dynamics simulations. Their findings revealed minimal variation in the Young’s modulus for stoichiometric coefficients ranging from 0 to 0.4. However, a notable increase in the modulus was observed for coefficients between 0.6 and 1, with a maximum value of approximately 290 GPa recorded at a stoichiometric coefficient of 1 and a temperature of 100 K. This behavior was attributed to differing stress concentrations caused by the presence of Te and S atoms.

Recent studies have explored the mechanical properties of the near-monolayer 2H phase of TaS₂ using atomic force microscopy (AFM)-based nanoindentation techniques, revealing a Young’s modulus of approximately 86 GPa, which remained consistent across thicknesses ranging from 3.5 to 12.6 nm. Stress–strain curves obtained by applying strain levels up to 25% indicated a breaking strength of 5.07 GPa, about 6% of the measured Young’s modulus. Additionally, DFT calculations simulated stress–strain curves for monolayer crystals under biaxial stretching with results for various TMDs, including 1H-TaS₂, 1H-NbS₂, 1T-NbTe₂, and 1T-TaTe₂, presented in Figure (a). A layer-dependent study of the Young’s modulus for WS₂, WSe₂, and WTe₂ showed a decreasing trend as the number of layers decreased, particularly for materials with 1–3 layers, as illustrated in Figure (b). Furthermore, variations in the Young’s modulus and fracture strength for tungsten-based chalcogenides are shown in Figure (c) and (d), respectively. , Notably, MoS₂ and MoSe₂ exhibit higher fracture toughness compared to graphene and TaS₂, suggesting their suitability for applications requiring enhanced resistance to crack propagation. These findings are crucial for advancing the development of flexible electronics and mechanically reinforced devices based on metallic TMDs and other layered materials. −

8.

(a) Stress–strain curves under biaxial stretching for various 2D materials computed using DFT. (b) Layer-dependent study of the Young’s modulus for WS₂, WSe₂, and WTe₂. (c) Experimentally measured Young’s modulus variation with charge transfer for various metallic TMDs. (d) Fracture strength as a function of layer number for WS₂, WSe₂, and WTe₂. Reprinted with permission from ref . Copyright 2021 The Royal Society of Chemistry. Reprinted with permission from ref . Copyright 2021 American Chemical Society.

Strain variation has also been shown to influence the magnetic and optoelectronic properties of TMDs. Kansara et al. examined the effects of 0–9% strain on NbX₂ (X = S, Se, Te) and observed strain-induced changes in magnetic behavior, including the emergence of ferromagnetism. Wang et al. analyzed strain effects on monolayer WS₂ synthesized via CVD and reported strain-sensitive light emission, making WS₂ a promising candidate for strain sensors and optoelectronic applications. Zhang et al. conducted indentation experiments on suspended multilayers of WSe₂ using atomic force microscopy and measured Young’s modulus of approximately 167 GPa, two-thirds that of WS₂. This enhanced modulus indicates the viability of WSe₂ for flexible optoelectronic devices and nanoelectromechanical systems.

The mechanical properties of other TMDs, such as those of WS₂ and WSe₂, have also been extensively studied. Liu et al. synthesized monolayers of MoS₂ and WS₂ and reported similar elastic moduli of approximately 170 N/m² for both materials. Recent investigations into the bending behavior of TMD monolayers corresponding to transition metal groups IV (Ti, Zr, Hf), VI (Mo, W), and X (Pd, Pt) have revealed significant variations in bending stiffness. For instance, the bending rigidity of MoS₂ was found to be 12.29 eV, with stiffness increasing as the transition metal group progressed from IV to VI. Strain energy, another critical parameter in 2D materials, gives rise to phenomena such as wrinkles, buckles, bubbles, and tents, which significantly impact the mechanical and electronic properties of these materials.

In conclusion, the mechanical properties of TMDs and their ternary derivatives are highly tunable through factors such as strain, compositional variations, and temperature. These materials exhibit a broad spectrum of behaviors, including elastic recovery, ductile deformation, and bandgap modulation, making them promising for advanced applications in electronics, photonics, and flexible devices. However, further exploration and experimental validation are needed to harness their potential.

4.2. Magnetic Properties

The investigation of magnetic properties in TMDs has gained considerable attention because of their possible applications in spintronics, magnetic semiconductors, and low-dimensional magnetic materials. While pristine TMDs are often diamagnetic, recent advancements in doping, alloying, and structural modifications have demonstrated a range of ferromagnetic properties that pave the way for innovative applications.

Intercalation has long been recognized as an effective technique to explore intertwined magnetic and electronic properties in materials. Previous studies on bulk crystals of TaS₂ and NbS₂ have demonstrated that intercalating magnetic atoms such as Fe, Co, Ni, Cr, Mn, and V in 1/3 or 1/4 stoichiometric compositions can induce ferromagnetic or antiferromagnetic ordering. − Recently, Q. He and collaborators were the first to report layer-dependent magnetic properties in TMDs intercalated with magnetic atoms. Using the flux-assisted growth technique, they successfully synthesized few-layer materials such as V_1/3NbS₂, Cr_1/3NbS₂, Mn_1/3NbS₂, Fe_1/3NbS₂, Co_1/3NbS₂, Co_1/3NbSe₂, Fe_1/3TaS₂, and Fe_1/4TaS₂.

Their detailed study focused on the magnetic properties of Fe_1/3TaS₂, with a comparative analysis of Fe_1/4TaS₂. A standard Hall bar device, fabricated using Fe_1/3TaS₂ (as shown in Figure (a)), exhibited ferromagnetic ordering below 19 K and superconductivity at temperatures below 3 K. The magnetic ordering in 2D Fe_1/3TaS₂ was investigated through simultaneous measurements of the longitudinal resistivity (ρ _xx ) and transverse Hall resistivity (ρ _xy ) under varying external magnetic fields and temperatures, with the magnetic field applied along the out-of-plane direction. These measurements revealed hysteresis loops and anomalous Hall effect behavior, as shown in Figure (b–e). Additionally, thickness-dependent measurements of ρ _xx and ρ _xy for layers of various thicknesses (3L, 4L, 6L, 18L, and 33L) demonstrated consistent behavior across all thicknesses, indicating layer-independent magnetic ordering (Figure (f)). Field-dependent variations in magnetoresistance (MR) and Hall resistivity along two different crystallographic directions, along with angle-dependent anomalous Hall effect measurements, provided quantitative insights into magnetic anisotropy energy, as shown in Figure (g) and (h). Furthermore, all of the above measurements, including the temperature-dependent resistivity measurements for various thicknesses (Figure (i)), suggest the presence of long-range ferromagnetism, complex magnetic domain structures, or disorder effects introduced by intercalation, all of which contribute to the layer-independent magnetic transition temperature. Theoretical model calculations further indicated that hybridization between electronic orbitals mediates long-range magnetic interactions among Fe atoms, which is likely responsible for the stable transition temperature observed in these materials. ,,

9.

(a) An optical microscopic image of a standard Hall bar device and a side-view schematic diagram of the heterostructure. (b–e) Hall resistivity and magnetoresistance data for Fe_1/3TaS₂ at various temperatures and for different layer thicknesses (3L and 18L). (f) Layer-dependent transition temperature phase diagram showing that the transition temperature remains nearly constant regardless of the number of layers. (g) Direction-dependent Hall resistivity and magnetoresistance measured at 2 K for both out-of-plane and in-plane directions. (h) The relationship between θ_M and θ_H of magnetic anisotropy energy, analyzed using the Stoner–Wohlfarth model. The inset shows the angular variation of longitudinal and Hall resistivities. (i) The temperature-dependent resistivity for different layers (3L, 4L, 6L, and 18L), demonstrating metallic behavior at zero applied magnetic field. Reproduced with permission from ref . Copyright 2024 Springer Nature.

Moreover, recent developments have also expanded the scope of magnetic TMDs, including newly synthesized materials such as few-layer vanadium disulfide (VS₂). , This material has sparked significant interest due to its intrinsic ferromagnetism, which offers exciting prospects for fabricating ferromagnetic 2D TMDs without the need for introducing additional magnetic atoms or applying tensile strain. The magnetic properties of VS₂ nanosheets, including their magnetic moments and coupling strength, can be tuned by using isotropic strain, enabling precise control over their magnetic behavior. Beyond VS₂, related compounds like VSe₂ and VTe₂ have also been explored, with phonon dispersion calculations confirming the stability of their monolayers. Monte Carlo simulations predict impressively high Curie temperatures of 292, 472, and 553 K for VS₂, VSe₂, and VTe₂ monolayers, respectively, suggesting that these materials exhibit robust ferromagnetism at or well above room temperature. − This discovery opens new avenues for utilizing VX₂-based TMDs in next-generation spintronics and magnetic applications.

Magnetic TMDs, ranging from intercalated compounds like Fe_1/3TaS₂ to intrinsically magnetic monolayers such as VS₂, exhibit remarkable properties driven by their layered structures, tunable magnetic states, and strong electronic-magnetic coupling. These materials enhance our understanding of low-dimensional magnetism and hold promise for spintronics, magnetic storage, and quantum information processing. The versatility of TMDs, achieved through doping, alloying, strain, and defects, underscores their potential for advanced applications, while ongoing research into their stability, scalability, and device integration is essential for practical implementation.

4.3. Superconducting Properties

The superconducting critical temperature (T _c) of 7.2 K in layered NbSe₂ single crystals is observed to decrease as the thickness of the material is reduced. , This thickness-dependent suppression of T _c is a common feature in superconducting thin films and is primarily attributed to the influence of disorder. In the specific case of NbSe₂, experimental studies have also confirmed a modification in the band structure as the material transitions to a single-layer system. This modification is an additional contributing factor to the thickness-dependent behavior of T _c decreased to 3 K at the monolayer limit. Consequently, NbSe₂ serves as an excellent model system to investigate the intricate relationship between collective electronic phenomena and the electronic structure’s dependence on thickness.

In a similar context, for the NbS₂ material, studies have revealed that the superconducting transition temperature decreases with a reduction in the layer number. , This decrease is linked to a reduction in the mobile hole density within thinner flakes, which significantly influences superconductivity. The observed correlation between T _c and thickness in different material systems, fabricated using various methods, highlights a fundamental characteristic of 2D materials. This behavior is attributed to factors such as reduced Coulomb screening, increased localization of Cooper pairs due to enhanced disorder, weaker interlayer Cooper pairing, and a decline in superfluid stiffness. These phenomena collectively explain the generality of thickness-dependent superconductivity in 2D materials, a relationship often observed by the theoretical dependence $T_c\propto \frac{1}{d}$ , where d represents the thickness. This theoretical framework is supported by considerations that the Cooper pair wave functions must satisfy vacuum boundary conditions, yielding predictions consistent with experimental observations.

However, this theoretical explanation does not universally apply to all cases of superconducting materials with thickness-dependent superconducting transition temperatures. A notable exception is observed in few-layer TaS₂ flakes of varying thicknesses. Mechanically exfoliated onto Si/SiO₂ substrates, these flakes demonstrate the persistence of superconductivity down to the thinnest layers investigated, approximately 3.5 nm or five covalent planes. Interestingly, the T _c exhibits a marked increase from 0.5 K in bulk crystals to approximately 2.2 K as the thickness decreases. − Density functional theory (DFT) calculations, along with a simple tight-binding model, suggest that this enhancement in T _c is primarily driven by modifications in the electronic band structure. These changes lead to an increased density of states at the Fermi level, which strengthens the effective electron–phonon coupling, ultimately resulting in a higher superconducting transition temperature. ,

Beyond the thickness-dependent superconducting transition temperature near the monolayer limit, these materials exhibit Ising superconductivity in both noncentrosymmetric and centrosymmetric structures. The primary mechanism driving Ising superconductivity is SOC, though its origin differs between non-centrosymmetric and centrosymmetric structures. Figure illustrates the year-wise reported Ising superconductors along with their upper critical fields.

10.

2D superconductors with enhanced in-plane upper critical fields are categorized based on their properties. Light green represents systems with spin–orbit coupling (SOC), such as δ-doped SrTiO₃ and monolayer Pb films. , Light blue represents type-I Ising superconductors, including monolayer 2H-MoS₂, 2H-NbSe₂, 2H-TaS₂, WS₂, 6ML-TaSe₂, and 6ML-Pb. ,,− Deep blue highlights type-II Ising superconductors, such as 4L-PdTe₂, MoTe₂, and nanosheets of the 1T′ phase of MoS₂. − Beyond these, other superconductors, including stanene and SnSe₂, have also been reported. In the figure, the ratio of the in-plane upper critical field to the Pauli limit is shown year-wise for different materials, with experimentally measured values and theoretical fits extrapolated to 0 K also included.

In non-centrosymmetric structures, 2D TMDs such as MoS₂, NbSe₂, ,− and TaS₂ have garnered significant attention for their unique properties and potential applications in nanoelectronics and nanophotonics. Typically, TMDs have the general formula MX ₂ and exist in multiple structural phases. The 2H phase is particularly significant, with a unit cell composed of two layers where a metal (M) atom is sandwiched between two chalcogen (X) atoms in a trigonal prismatic structure stacked along the c-axis with D _3h symmetry. The weak van der Waals interactions between layers facilitate the isolation of the monolayer MX ₂ using mechanical exfoliation techniques. In the 2D limit, the restriction of electron motion to a plane, coupled with the broken in-plane inversion symmetry of the 2H-MX ₂ lattice, results in strong spin–orbit coupling (SOC). This coupling locks the electron spins to an out-of-plane orientation through an effective Ising Zeeman field with opposite directions for electrons in the K and K′ valleys. This unique field protects the spins of electrons in Cooper pairs from realignment in response to in-plane magnetic fields, enabling a significant enhancement in the in-plane upper critical field (H _c2), often exceeding the Pauli limit by several times.

The superconducting behavior of TMDs such as 2H-TaS₂ and NbSe₂ demonstrates remarkable deviations from the conventional Bardeen–Cooper–Schrieffer (BCS) theory, especially in the monolayer and few-layer limits, as illustrated in Figure (a) and (b). In monolayer NbSe₂, which possesses D _3h symmetry, the interplay of SOC and the absence of inversion symmetry leads to a dramatic enhancement of the upper critical field (H _c2) compared to its bulk counterpart. The effects of the monolayer and multilayer structure are depicted in Figure (c) and (d), respectively. , For example, the T _c in monolayer NbSe₂ decreases to 3 K, but the upper critical field reaches an extraordinary 35 T over six times the Pauli paramagnetic limit. Similarly, in the trilayer limit, T _c reduces to 5.8 K, with H_c2 exceeding three times the Pauli limit, indicating the robustness of SOC-driven Ising superconducting phenomena in the suppression of spin-singlet Cooper pair breaking.

11.

(a) Temperature dependence of the upper critical field H _c2 for TaS₂ near the monolayer limit. (b) The temperature-dependent upper critical field for NbSe₂ exceeds the Pauli limit by nearly six times, as inferred from the monolayer fitting. (c) The spin configuration of electrons in the monolayer is influenced by Ising spin–orbit coupling (SOC). This arises from the interaction with the electric field, which originates from the local breaking of the inversion symmetry. The interaction generates a magnetic field H _SO(k) that aligns the spins in the out-of-plane direction due to the coupling between the electric field (ϵ) and momentum (k); H _SO(k) = k × ϵ. (d) In the multilayer case, interlayer coupling partially restores the inversion symmetry, thereby diminishing or destroying the Ising SOC effect. Reprinted with permission from ref . Copyright 2018 Springer Nature.

Comparable behavior is observed in other TMDs. In monolayer 2H-TaS₂, T _c is reported to be approximately 3 K, while the upper critical field soars to 65 T, as shown in Figure (a), underscoring the influence of SOC and reduced dimensionality on superconducting properties. This extraordinary enhancement in H _c2 far above the conventional BCS limit is attributed to Ising spin–orbit coupling, wherein spins are pinned perpendicular to the crystal plane, providing significant spin protection to Cooper pairs against external magnetic fields. This Ising SOC effect weakens interlayer coupling in the few-layer limit, resulting in the manifestation of unique superconducting phases distinct from those of bulk TMDs.

The role of SOC-induced protection is further highlighted in MoS₂ under an electric double-layer transistor configuration, as explored by Lu and co-workers. Using pulsed high magnetic fields up to 55 T, they observed an enhanced H _c2 that far exceeded the BCS Pauli limit. In this configuration, H _c2 increases with decreasing temperature and saturates at around 52 T at 1.5 K, more than four times the expected BCS limit of 12 T. , This suggests that Ising SOC, combined with strong 2D confinement, is a critical factor in the observed superconducting properties. Furthermore, theoretical and experimental evidence indicates that not only Ising SOC but also the contribution of spin-triplet pairing components plays role in stabilizing high H _c2 in materials such as NbSe₂.

In a centrosymmetric structure, spin-valley locking and the Rashba spin–orbit interaction are absent; instead, other mechanisms, such as electron–phonon coupling and multiband effects, play a vital role in explaining the enhancement of the upper critical magnetic field, as illustrated in Figure (a). For example, nanosheets of 1T′-MoS₂ exhibit a centrosymmetric structure and a superconducting transition temperature of 4.7 K. Interestingly, they violate the conventional superconducting Pauli limit by nearly 3.5 times, indicating unconventional superconducting behavior. Similarly, PdTe₂ is a material that becomes superconducting below T _c 1.7 K and has garnered significant attention due to its unique coexistence of superconductivity and type-II Dirac fermions. PdTe₂ adopts a layered CdI₂-type structure (space group P3̅m1), which preserves the in-plane inversion symmetry. Notably, the superconductivity in PdTe₂ films with such in-plane inversion symmetry persists under a large parallel magnetic field, exceeding the Pauli limit by several times, as shown in Figure (b). These exceptional properties make both 1T′-MoS₂ nanosheets and PdTe₂ promising candidates for studying exotic superconducting mechanisms and potential applications in quantum technologies.

12.

(a) A schematic of type-II Ising superconductivity showing SOC locking spins to orbitals, creating opposite Zeeman-like fields. This protects opposite-spin pairs from in-plane magnetic fields, resulting in exceptionally high upper critical fields. (b) In-plane critical fields for 4-ML (represented by black spheres) and 6-ML (represented by blue spheres) PbTe₂ films. This critical field corresponds to the magnetic field at which the sheet resistance reaches 50% of its normal-state value. Reproduced with permission from ref . Copyright 2020 American Chemical Society.

Interestingly, as the number of layers increases, the strong interlayer coupling in these materials starts to restore the bulk superconducting properties, diminishing the influence of 2D effects and SOC. − This transition highlights the delicate balance among SOC, reduced dimensionality, and interlayer coupling in determining the superconducting behavior of TMDs. These findings underscore the potential of Ising superconductors as platforms for exploring unconventional superconductivity and advancing applications in high-field superconducting technologies.

4.4. Optical Properties of TMDs

Among the various research directions concerning TMDs, their optical properties stand out as a primary area of interest. ,− These optical properties not only provide crucial insights into the underlying quantum mechanical interactions governing TMDs but also play a vital role in determining their suitability for advanced optoelectronic applications such as photodetectors, light-emitting diodes (LEDs), and ultrafast modulators. TMD monolayers (1L), in particular, exhibit direct bandgap behavior, a striking contrast to the indirect bandgap nature of their bulk counterparts. − This transition to a direct bandgap in monolayer TMDs, with conduction band minima and valence band maxima located at the K ⁺ and K ^– points of the Brillouin zone (BZ), respctively, enables them to efficiently emit light. The band structure of 1L TMDs can be understood as a graphene-like hexagonal lattice with broken sublattice symmetry, where alternating metal and chalcogen atoms contribute to the formation of an electronic bandgap and valley-selective optical transition rules for circularly polarized light.

A key feature of the electronic band structure of TMDs is the strong spin–orbit coupling (SOC) from heavy transition metal atoms, which drives band splitting, spin-valley physics, and excitonic stability, shaping their unique quantum properties. The SOC effects lead to a pronounced splitting of the valence band (VB) maxima, known as Δ_so,vb, which varies across different TMDs. For instance, Δ_so,vb ranges from approximately 150 meV in MoS₂ to around 450 meV in WSe₂. This splitting underpins the energy separation between two fundamental interband optical transitions, labeled A and B excitonic resonances, which are intrinsic features of the TMD monolayers. While the conduction band (CB) minima also exhibit SOC-induced splitting (Δ_so,cb), it is relatively smaller due to the dominance of d ⁰ metal orbitals and secondary effects such as p-state admixture from chalcogen atoms. Notably, the interplay of these effects can result in either optically bright or dark ground state transitions depending on the alignment of spin states in the upper VB and lowest CB subbands.

Beyond electrical control, the intrinsic excitonic landscape of monolayer WS₂ and other TMDs is strongly influenced by the interplay between bright and dark excitonic states governed by spin–orbit coupling and valley-selective optical transitions. As illustrated in Figure (a) and (b), photoluminescence spectra measured under varying back-gate voltages (−40 to +40 V) reveal a clear transition from neutral exciton emission at negative voltages to trion emission at positive voltages. The corresponding PL intensity mapping demonstrates the tunability of these excitonic states with electrical doping, highlighting the dynamic exciton-trion interactions in monolayer WS₂. Furthermore, the observation of superlinear (quadratic) emission behavior indicates the presence of biexcitons with substantial binding energies (45 meV), underscoring the complex excitonic phenomena that play a critical role in determining the material’s temperature-dependent optical responses.

13.

Modulation of excitonic emission in monolayer WS₂ through electrical doping at room temperature. (a) Photoluminescence spectra collected from exfoliated monolayer WS₂ under varying back-gate voltages ranging from −40 to +40 V. (b) Two-dimensional PL intensity map illustrating the dependence of emission characteristics on the photon energy and gate voltage. Reproduced with permission from ref . Copyright 2015 American Chemical Society.

Moreover, bright monolayers, such as MoSe₂ and MoTe₂, feature aligned spins that facilitate optically active transitions, whereas dark monolayers like WSe₂ and WS₂ are characterized by antiparallel spin alignment, leading to optically inactive ground states. For materials such as MoS₂, the classification remains less clear due to the small magnitude of Δ_so,cb, which lies in the meV range. − This divergence in optical character has a direct effect on the photoluminescence (PL) behavior of these materials under varying temperature conditions.

In monolayer WSe₂, an unconventional increase in PL intensity with increasing temperature has been observed, which is attributed to the thermal activation of higher-energy excitonic states. These states facilitate radiative recombination by depopulating dark excitonic states and enhancing carrier redistribution, thereby compensating for the optically inactive ground state. − In contrast, monolayer MoSe₂ follows the conventional trend of decreasing PL intensity with temperature, driven by enhanced nonradiative recombination processes that suppress luminescence efficiency. These distinct temperature-dependent PL behaviors highlight the critical role of spin–orbit splitting in dictating excitonic transitions in TMDs.

Recent research has further advanced our understanding of dark and bright excitons in TMD monolayers, revealing their intricate interplay under external perturbations. For instance, the application of in-plane magnetic fields can induce mixing between dark and bright excitonic states, enabling the emission from dark excitons. Experimental measurements have quantified the bright-dark exciton splitting, finding values of approximately 50 meV in WS₂ and WSe₂ and a surprisingly larger splitting of 100 meV in monolayer MoS₂. , These findings align well with theoretical predictions, providing a deeper understanding of the electronic and optical properties of TMDs.

In summary, the optical properties of TMDs are deeply intertwined with their electronic structure, spin–orbit interactions, and external influences, such as temperature and magnetic fields. This intricate relationship highlights their potential for innovative applications in optoelectronics, where their tunable optical characteristics can be harnessed for devices ranging from light emitters to photodetectors, and beyond.

5. Applications of Transition Metal Dichalcogenides

TMDs with their atomically layered structure, exceptional electronic properties, and wide-ranging chemical composition have become a focal point of research for next-generation materials. Their 2D nature and unique properties make them attractive for diverse applications in electronics, optoelectronics, sensing, and energy devices. This Review summarizes these applications and highlights recent advancements in this area.

5.1. Flexible Electronics and Optoelectronics

TMDs, particularly MoS₂, WS₂, and WSe₂, are key contenders for flexible and wearable electronics due to their thin atomic profiles, tunable electronic bandgaps, and mechanical flexibility. , High-performance thin-film transistors (TFTs) based on MoS₂ have achieved an electron mobility of 50 cm²/V·s and current densities exceeding 250 mA/mm. Their mechanical robustness allows for sustained electronic performance over thousands of bending cycles, making them ideal for flexible displays, sensors, and Internet of Things devices.

Recent innovations include flexible wireless systems, where MoS₂ has been used in high-frequency applications, such as radio receivers. The robust electrical performance of BP (black phosphorus), with mobility approaching 1000 cm²/V·s, further extends the scope of TMDs in high-speed flexible electronics, including 5G and microwave applications. BP-based flexible TFTs have demonstrated amplifiers and inverters, paving the way for advanced flexible nano-optoelectronics.

5.2. Photodetectors

TMDs exhibit strong light–matter interactions, making them ideal for photodetectors, solar cells, and quantum applications. Photodetectors are essential in various fields from optical communication to aerospace applications, biomedical imaging, and environmental monitoring. In this connection, though many potential high-performance photodetector materials have already been reported, such as ZnO, TiO₂, SnO₂, 2D graphene, MoS₂, WS₂, and various heterostructures, the demerits of such systems have also been found, including low-band detection light, efficiency, and slow response.

Monolayer TMDs exhibit direct bandgaps, enabling efficient light emission and absorption. The recent advancements in TiS₂-based broadband photodetectors highlight their potential for next-generation optoelectronic devices due to their high responsivity, wide spectral range (UV–visible–NIR), and stable operation over a broad temperature range (−180 to 150 °C). , Hybrid structures of TMDs with other materials such as perovskites are being explored for enhanced solar energy harvesting. TMDs have also been studied for valleytronics, where the valley degree of freedom in their band structure is utilized for information processing. These advancements open possibilities for TMDs in quantum computing and photonic circuits.

5.3. Energy Storage Devices

TMDs have garnered significant attention for energy storage applications due to their layered structures, which facilitate ion intercalation and provide abundant electrochemically active sites. Among them, TiS₂ stands out as a promising electrode material owing to its high electrical conductivity. Few-layer TiS₂ nanocrystals have exhibited excellent supercapacitive properties, with a large proportion of capacitance arising from surface-controlled processes. These nanosheets offer enhanced electrochemical performance compared to that of their bulk counterparts, making them suitable for high-performance energy storage applications.

Another notable material, VS₂, has demonstrated superior electrochemical behavior in supercapacitors due to its intrinsic metallic conductivity. Ultrathin VS₂ nanosheets, assembled into electrode films, have achieved a high areal capacitance of 4760 μF/cm² while maintaining stable performance over extended charge–discharge cycles. This stability, coupled with its high energy storage capability, positions VS₂ as an attractive candidate for next-generation supercapacitors.

Additionally, WS₂-based hybrid structures have shown promising results in flexible energy storage devices. The unique architecture of WO₃/WS₂ nanowire-based electrodes enhances ion absorption through subnanometer gaps between WS₂ layers, resulting in high capacitance, excellent rate capability, and long-term cycling stability. These attributes make WS₂-based systems highly suitable for advanced supercapacitor applications, especially in flexible and wearable energy storage technologies.

5.4. Sensors

Nanoscale TMDs have emerged as highly promising materials for gas, chemical, and biosensing applications due to their exceptional sensitivity, selectivity, and tunable electronic properties. Their high surface-to-volume ratios and defect-rich structures enable enhanced interaction with target analytes, making them ideal transducing components in electrochemical, fluorescent, and colorimetric sensors.

Among various TMDs, WS₂ and VS₂ have shown significant potential in electrochemical biosensing. WS₂-based sensors exhibit high sensitivity in detecting neurotransmitters such as dopamine, with a response of 5.36 μA/μM·cm² and a low detection limit of 0.01 μM, even in the presence of interfering species, such as uric acid. This capability makes them particularly suitable for in situ monitoring of neurotransmitter levels in biological environments. − Additionally, WS₂ nanosheets have been employed for the highly sensitive detection of glycated hemoglobin, a crucial biomarker for diabetes, providing a viable alternative to conventional chromatographic methods.

In addition to biosensing, TMDs have also been widely explored for gas sensing applications. Their high surface area, chemical reactivity, and excellent charge transfer characteristics make them suitable for detecting various toxic and flammable gases, offering great promise for environmental monitoring and industrial safety applications.

5.5. Catalysis and Hydrogen Evolution Reactions (HER)

The edge sites of TMDs, especially MoS₂, act as active centers for electrocatalytic hydrogen evolution reactions (HER). − This property is critical for the development of efficient and sustainable hydrogen production methods. Efforts to enhance catalytic activity involve doping, creating defects, and engineering hybrid materials like MoS₂–graphene composites.

5.6. Emerging Applications

• Thermoelectric Devices: TMDs exhibit high thermoelectric efficiency because of their low thermal conductivity and tunable electrical properties, making them suitable for waste heat recovery and cooling applications.

• Spintronics: TMDs with strong SOC are being explored for spintronic devices, leveraging their Ising superconductivity for high-performance memory and logic circuits.

• Water Splitting and CO ₂ Reduction: The unique electronic and chemical properties of TMDs enable their application in photocatalytic water splitting and CO₂ reduction, addressing global energy and environmental challenges.

6. Conclusion and Future Outlook

The rapid advancements in technology have significantly expanded our understanding of the fundamental properties and applications of two-dimensional materials. These materials exhibit unique physicochemical, optical, electronic, and mechanical characteristics, making them highly attractive for various fields including electronics, energy storage, sensing, and optoelectronics. While graphene initially led the research wave in this domain, other 2D materials, such as TMDs, h-BN, and emerging systems such as black phosphorus, silicene, and germanene, are now receiving substantial attention for their distinct properties. Key advancements, such as the development of TMDs with direct bandgaps and tunable properties through doping, strain, and substrate variations, have enhanced their performance in applications such as semiconductors, flexible electronics, and energy storage devices. In particular, TMDs exhibit potential for ultra-low-power transistors, valleytronics, and high-sensitivity sensors. However, achieving uniform, defect-free growth with precise layer control remains a challenge, especially in scaling up industrial applications.

The integration of artificial intelligence (AI) and machine learning (ML) in predicting properties, optimizing synthesis conditions, and designing tailored 2D materials has opened new frontiers in material science. AI-based systems are being employed to predict band gaps and magnetic ordering and even automate microscopic image analysis. These techniques promise faster and more efficient exploration of novel materials and synthesis methods.

Despite these advancements, several challenges remain. Achieving the production of high-quality 2D materials on a large scale, with properties comparable to those of mechanically exfoliated flakes, is still not fully realized. Key areas that need further focus include:

• Single-layer controllability: Developing reliable methods to grow monolayer films with consistent thickness.

• Defect engineering: Addressing defects to improve electronic and mechanical properties.

• Doping and alloying: Enhancing tunability for applications in optoelectronics and energy devices.

• Stability issues: Resolving the long-term stability concerns for materials like black phosphorus and germanene.

Future research on 2D materials should prioritize the advancement of synthesis techniques to achieve scalable and defect-free growth, enabling the production of high-quality materials. Investigating hybrid structures and heterojunctions will unlock multifunctional device applications, while resolving stability issues for materials such as black phosphorus and germanene will extend their practical use. The integration of AI and ML can further accelerate the exploration and optimization of material properties, refining synthesis conditions, and designing novel devices. Additionally, developing flexible and energy-efficient systems, leveraging the exceptional properties of 2D materials, will cater to emerging applications in electronics, sensors, and energy storage. These efforts, combined with ongoing advancements, are expected to revolutionize electronics, optoelectronics, and energy technologies, paving the way for a new era in materials science and technology.

CRediT: Subrata Mondal resources, supervision, writing - review & editing; Yerumbu Nandakishora supervision.

The authors declare no competing financial interest.