Transition metal dichalcogenides (TMDCs) such as MoS2, WS2, and WSe2 have gained significant attention due to their unique electronic, optical, and mechanical properties when synthesized as monolayers. The synthesis of high-quality monolayer TMDCs is critical for fundamental studies and potential applications. Several techniques have been developed to produce these materials, each with distinct advantages and limitations. The most prominent methods include mechanical exfoliation, chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). The choice of synthesis method depends on factors such as scalability, crystal quality, and intended use.

Mechanical exfoliation, inspired by the Scotch tape method used for graphene, is one of the simplest techniques to obtain monolayer TMDCs. This method involves peeling bulk TMDC crystals using adhesive tape to transfer thin flakes onto a substrate, such as silicon dioxide or sapphire. The process can yield high-quality monolayers with minimal defects due to the absence of chemical reactions or high-temperature processing. However, mechanical exfoliation suffers from low yield and poor scalability, making it unsuitable for large-area applications. The randomness in flake size and thickness also poses challenges for uniformity. Despite these limitations, exfoliated monolayers are widely used in fundamental research due to their excellent electronic properties.

Chemical vapor deposition (CVD) is a widely adopted method for producing large-area monolayer TMDCs with better control over uniformity and scalability. In a typical CVD process, solid precursors such as molybdenum or tungsten oxides are vaporized and reacted with chalcogen sources (e.g., sulfur or selenium) at elevated temperatures (650–1000°C) in a tube furnace. The reaction occurs on a substrate, often sapphire or SiO2/Si, where the precursors nucleate and grow into continuous monolayers. The choice of substrate is crucial, as lattice matching and surface energy influence nucleation density and crystal orientation. CVD-grown TMDCs exhibit good electronic performance, with mobilities comparable to exfoliated samples. However, challenges remain in controlling defect density, grain boundaries, and layer uniformity over large areas. Variations in precursor flow rates, temperature gradients, and gas pressure can lead to non-uniform growth or multilayer formation.

Metal-organic chemical vapor deposition (MOCVD) offers improved precursor control compared to conventional CVD, enabling more precise monolayer growth. In MOCVD, metal-organic precursors such as molybdenum hexacarbonyl or tungsten hexacarbonyl are used alongside chalcogen sources, allowing for lower growth temperatures (400–700°C) and better stoichiometric control. The use of volatile organometallic precursors enhances gas-phase reactions, leading to more uniform nucleation and reduced defect densities. MOCVD is particularly advantageous for producing wafer-scale TMDC films with high reproducibility, making it suitable for industrial applications. However, the high cost of metal-organic precursors and the complexity of the process limit its widespread adoption. Additionally, residual carbon contamination from the precursors can degrade electronic properties if not properly managed.

Molecular beam epitaxy (MBE) is an ultra-high vacuum technique that provides atomic-level control over TMDC growth, making it ideal for producing high-purity monolayers with minimal defects. In MBE, elemental sources of transition metals and chalcogens are evaporated in a controlled manner, allowing for precise layer-by-layer deposition. The slow growth rates and low processing temperatures (200–500°C) minimize thermal stress and defect formation. MBE-grown TMDCs exhibit excellent crystallinity and sharp interfaces, which are critical for quantum confinement studies and heterostructure engineering. However, MBE is limited by its low throughput and high equipment costs, restricting its use to specialized research rather than large-scale production. The requirement for ultra-high vacuum conditions also complicates the integration of MBE-grown films with standard semiconductor processing techniques.

The choice of substrate plays a critical role in determining the quality of monolayer TMDCs across all synthesis methods. Sapphire (Al2O3) is commonly used due to its high thermal stability and lattice matching with TMDCs, promoting epitaxial growth. Silicon dioxide (SiO2/Si) substrates are also popular for their compatibility with existing semiconductor technologies but may introduce strain due to thermal expansion mismatches. Recent studies have explored the use of hexagonal boron nitride (hBN) as a substrate to reduce charge scattering and improve electronic performance. The interaction between the substrate and the growing TMDC layer influences nucleation density, grain size, and defect formation, making substrate engineering an active area of research.

Precursor selection and growth conditions further dictate the quality of synthesized monolayers. In CVD and MOCVD, the ratio of transition metal to chalcogen precursors must be carefully balanced to avoid non-stoichiometric compounds or unwanted phases. Temperature gradients and gas flow dynamics within the reaction chamber affect nucleation density and film uniformity. Post-growth treatments such as annealing in chalcogen-rich environments can passivate defects and improve crystal quality.

Scalability remains a key challenge in the synthesis of monolayer TMDCs. While CVD and MOCVD offer better prospects for large-area growth compared to exfoliation or MBE, achieving uniform films over wafer-scale dimensions with minimal defects is still an ongoing effort. Advances in precursor delivery systems, substrate engineering, and process automation are expected to address these challenges.

In summary, mechanical exfoliation provides high-quality monolayers for research but lacks scalability. CVD and MOCVD offer better control over large-area growth, though defect management remains a concern. MBE delivers exceptional purity and precision but is limited by cost and throughput. The continued refinement of these techniques will be essential for unlocking the full potential of monolayer TMDCs in future technologies.