Chemical vapor deposition (CVD) has emerged as a leading technique for synthesizing two-dimensional (2D) materials beyond graphene, including transition metal dichalcogenides (TMDCs) and hexagonal boron nitride (hBN). These materials exhibit unique electronic, optical, and mechanical properties, making them suitable for applications in flexible electronics, quantum devices, and optoelectronics. The controlled growth of monolayer and heterostructure films via CVD requires precise optimization of precursors, substrates, and growth conditions to achieve high-quality, large-area films.

Precursor selection plays a critical role in the CVD synthesis of 2D materials. For TMDCs such as MoS2, WS2, MoSe2, and WSe2, common precursors include metal oxides (MoO3, WO3) and chalcogen powders (S, Se). These precursors are typically placed in separate zones of a furnace, where temperature gradients facilitate their vaporization and subsequent reaction on the substrate. For hBN growth, boron and nitrogen sources such as ammonia borane (NH3-BH3) or boric acid (H3BO3) combined with ammonia (NH3) are commonly used. The choice of precursors influences the stoichiometry, crystallinity, and uniformity of the resulting films. For instance, excessive chalcogen flow can lead to multilayered growth, while insufficient supply may result in incomplete reaction and defective films.

Substrate engineering is another crucial factor in CVD growth. The substrate not only provides a surface for nucleation but also influences the crystallographic orientation and strain in the deposited material. Common substrates include SiO2/Si, sapphire, and mica, each offering distinct advantages. Sapphire, with its hexagonal symmetry, promotes epitaxial growth of TMDCs and hBN, leading to aligned domains with reduced grain boundaries. Copper and other metal foils are also employed for their catalytic properties, which lower the energy barrier for precursor decomposition and enhance lateral growth. Recent advances involve the use of engineered substrates with patterned nucleation sites or surface functionalization to control film thickness and domain size. For example, pretreatment with perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) can promote the growth of monolayer MoS2 by reducing the interfacial energy between the substrate and the growing film.

The growth mechanisms of 2D materials via CVD involve a series of steps, including precursor decomposition, adsorption, surface diffusion, and nucleation. For TMDCs, the process typically begins with the reduction of metal oxide precursors to volatile sub-oxides, which then react with chalcogen vapors to form MX2 (where M is a transition metal and X is a chalcogen). The growth kinetics are highly sensitive to temperature, pressure, and gas flow rates. Lower pressures tend to favor monolayer growth by reducing the mean free path of precursor molecules and minimizing gas-phase reactions. In contrast, higher pressures may lead to thicker films due to increased precursor availability. For hBN, the growth mechanism involves the decomposition of boron and nitrogen precursors, followed by surface migration and nucleation. The self-limiting nature of hBN growth on certain substrates, such as copper, enables the formation of uniform monolayers.

Heterostructure fabrication via CVD is an area of significant interest, enabling the integration of multiple 2D materials with tailored electronic and optical properties. Sequential growth, where one material is deposited followed by another, allows for the formation of vertical heterostructures such as MoS2/hBN or WS2/MoS2. Alternatively, simultaneous growth using multiple precursors can yield lateral heterostructures with sharp interfaces. The key challenge lies in controlling the nucleation sites and minimizing intermixing during growth. Substrate temperature and precursor ratios must be carefully tuned to ensure clean interfaces and prevent alloy formation. Recent studies have demonstrated the successful synthesis of MoS2-WS2 lateral heterostructures with atomically sharp boundaries, enabling novel optoelectronic functionalities.

Applications of CVD-grown 2D materials extend across multiple fields. In flexible electronics, TMDCs such as MoS2 serve as active channels in thin-film transistors (TFTs) due to their high carrier mobility and mechanical robustness. The ability to grow these materials directly on flexible substrates like polyimide opens avenues for wearable devices and foldable displays. hBN, with its wide bandgap and exceptional dielectric properties, is used as an insulating layer in 2D electronics, reducing charge scattering and improving device performance. In quantum devices, the defect-free interfaces of CVD-grown TMDCs are exploited for valleytronics and single-photon emitters. Monolayer TMDCs exhibit strong excitonic effects and spin-valley coupling, making them promising candidates for quantum information processing. Additionally, hBN-encapsulated TMDC heterostructures have shown enhanced electronic properties, including high carrier mobility and reduced environmental degradation.

Despite the progress, challenges remain in scaling CVD synthesis for industrial applications. Reproducibility, defect control, and uniformity over large areas are ongoing areas of research. Advances in in-situ monitoring techniques, such as optical spectroscopy and electron microscopy, are providing deeper insights into growth dynamics and enabling real-time process optimization. Furthermore, the development of low-temperature CVD processes compatible with temperature-sensitive substrates will broaden the applicability of 2D materials in next-generation technologies.

In summary, CVD is a versatile and scalable method for synthesizing 2D materials beyond graphene, with precise control over film quality and heterostructure formation. The interplay between precursor chemistry, substrate engineering, and growth kinetics dictates the structural and electronic properties of the resulting materials. As research continues to address existing challenges, CVD-grown TMDCs and hBN are poised to play a transformative role in flexible electronics, quantum devices, and beyond.