Transition metal dichalcogenide (TMD) monolayers, such as MoS2 and WS2, have gained significant attention due to their unique structural and electronic properties. Chemical vapor deposition (CVD) has emerged as a leading method for synthesizing large-area, high-quality TMD monolayers with precise control over their growth kinetics and phase composition. The CVD process involves the reaction of transition metal oxide precursors with chalcogen sources under controlled conditions, leading to the formation of atomically thin layers. This article focuses on the critical aspects of CVD synthesis, including precursor selection, substrate interactions, nucleation control, and the challenges associated with stoichiometry and domain size.

Precursor selection plays a pivotal role in determining the quality and uniformity of TMD monolayers. For MoS2 growth, molybdenum trioxide (MoO3) is commonly used as the transition metal precursor due to its high vapor pressure at relatively low temperatures. Sulfur powder serves as the chalcogen source, evaporating at temperatures above 200°C. The stoichiometric ratio between the metal oxide and sulfur is crucial, as an excess or deficiency of sulfur can lead to non-stoichiometric compounds or incomplete conversion. For WS2 growth, tungsten oxide (WO3) is employed alongside sulfur under similar conditions. The choice of precursors also influences the growth rate, with higher precursor evaporation temperatures typically resulting in slower, more controlled deposition.

The substrate material significantly impacts the nucleation and growth of TMD monolayers. Common substrates include silicon dioxide (SiO2), sapphire (Al2O3), and hexagonal boron nitride (h-BN). SiO2 is widely used due to its compatibility with standard semiconductor processes, but its amorphous nature can lead to random nucleation sites. In contrast, sapphire offers a crystalline surface that promotes epitaxial growth, resulting in larger domain sizes. The surface energy and lattice mismatch between the substrate and TMD layer also affect adhesion and strain, which can influence the final film quality. Pretreatment of substrates, such as oxygen plasma cleaning or the use of seeding promoters like perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), can enhance nucleation density and uniformity.

Nucleation control is essential for achieving large, single-crystalline domains of TMD monolayers. The initial stages of growth involve the formation of small nuclei, which subsequently expand and merge into continuous films. The nucleation density is influenced by factors such as temperature, precursor concentration, and substrate surface conditions. Lower precursor concentrations and higher temperatures generally reduce nucleation density, favoring the growth of larger domains. However, excessive temperatures can lead to excessive precursor decomposition and undesirable multilayer formation. The introduction of a temperature gradient or two-step growth process, where nucleation and lateral growth are separated, has been shown to improve domain size and uniformity.

Stoichiometry control remains a challenge in CVD synthesis of TMD monolayers. The reaction between metal oxides and chalcogens must be carefully balanced to avoid the formation of intermediate phases or defects. For example, incomplete sulfurization of MoO3 can result in sub-stoichiometric MoS2-x compounds, which exhibit metallic rather than semiconducting behavior. The use of excess sulfur is common to ensure complete conversion, but this must be optimized to prevent excessive etching of the growing film. In-situ monitoring techniques, such as optical spectroscopy or mass spectrometry, can provide real-time feedback on precursor consumption and reaction progress, enabling better stoichiometric control.

Phase engineering is another critical aspect of TMD monolayer synthesis. TMDs can exist in multiple polymorphs, with the 2H phase being the most stable for semiconducting applications. However, metastable phases such as 1T or 1T' can also form under specific growth conditions. The phase composition is influenced by factors such as temperature, precursor ratio, and substrate interactions. For instance, higher sulfur partial pressures and lower growth temperatures favor the 2H phase, while reducing conditions or the presence of dopants can stabilize metallic phases. Post-growth treatments, such as annealing in chalcogen-rich atmospheres or electron beam irradiation, can further modulate the phase composition.

Domain size and film continuity are key metrics for assessing the quality of CVD-grown TMD monolayers. Large domains with minimal grain boundaries are desirable for electronic applications, as grain boundaries can act as scattering centers and degrade carrier mobility. The lateral size of domains is typically limited by the competition between nucleation and growth kinetics. Strategies to enhance domain size include optimizing precursor flux, reducing nucleation density, and promoting lateral growth through substrate engineering. For example, the use of h-BN substrates has been shown to enable the growth of centimeter-scale single-crystalline MoS2 monolayers due to their low surface energy and lattice matching.

Challenges in reproducibility and scalability remain significant hurdles for the widespread adoption of CVD-grown TMD monolayers. Batch-to-batch variations in precursor purity, substrate preparation, and reactor conditions can lead to inconsistent results. Automated control systems and standardized protocols are being developed to address these issues. Additionally, the transfer of TMD monolayers from growth substrates to target applications introduces further complexities, such as contamination or mechanical damage. Direct growth on functional substrates or the development of transfer-free techniques may provide solutions to these challenges.

In summary, the CVD synthesis of TMD monolayers requires careful consideration of precursor selection, substrate interactions, nucleation control, and phase engineering. Achieving stoichiometric, large-domain films with controlled phase composition is essential for realizing the full potential of these materials. While significant progress has been made, ongoing research is needed to address challenges in reproducibility, scalability, and defect control. Advances in in-situ monitoring and process optimization will further enhance the quality and applicability of CVD-grown TMD monolayers.