The synthesis of two-dimensional alloy systems such as MoS₂(1-x)Se₂x has gained significant attention due to the ability to tailor their electronic and optical properties through controlled composition tuning. These alloys belong to the family of transition metal dichalcogenides (TMDCs), where the chalcogen atoms (S, Se) are partially substituted to form a homogeneous or phase-segregated structure depending on growth conditions. Confined growth techniques, including chemical vapor deposition (CVD) and metal-organic chemical vapor deposition (MOCVD), are widely employed to achieve precise control over alloy composition, layer thickness, and lateral dimensions.

Precursor mixing ratios play a critical role in determining the final composition of the alloy. In the case of MoS₂(1-x)Se₂x, the ratio of sulfur to selenium precursors directly influences the relative incorporation of S and Se into the lattice. For example, using solid precursors such as MoO₃, sulfur powder, and selenium powder, the vapor-phase transport of these species can be regulated by adjusting their relative quantities in the reaction zone. A higher partial pressure of selenium vapor relative to sulfur results in increased Se incorporation, leading to a higher x value in MoS₂(1-x)Se₂x. Studies have shown that maintaining a precise precursor ratio is essential to avoid unintended phase segregation or non-uniform alloying.

Temperature is another critical parameter that affects both the growth kinetics and the composition of the alloy. The growth temperature influences the reactivity of the precursors and the mobility of adatoms on the substrate surface. For MoS₂(1-x)Se₂x, typical CVD growth temperatures range between 650°C and 850°C. At lower temperatures, sulfur tends to incorporate more readily due to its higher vapor pressure compared to selenium, leading to sulfur-rich alloys. As the temperature increases, selenium incorporation becomes more favorable, shifting the composition toward higher x values. However, excessively high temperatures may induce undesirable effects such as chalcogen evaporation or decomposition of the TMDC lattice.

Phase segregation is a major challenge in the synthesis of 2D alloy systems. Due to differences in bond energies and atomic radii between sulfur and selenium, the system may exhibit a tendency toward phase separation rather than forming a homogeneous alloy. The miscibility gap between MoS₂ and MoSe₂ can lead to the formation of domains with varying compositions, which can adversely affect electronic properties. To mitigate this, growth conditions must be carefully optimized to promote uniform alloy formation. Strategies such as rapid quenching, controlled cooling rates, and the use of inert gas flow to stabilize the growth environment have been employed to suppress phase segregation.

Bandgap engineering is one of the most compelling applications of 2D alloy systems. The bandgap of MoS₂(1-x)Se₂x can be continuously tuned from approximately 1.55 eV (pure MoS₂) to 1.41 eV (pure MoSe₂) by varying the selenium content. This tunability arises from the compositional dependence of the electronic structure, where the increasing Se content reduces the bandgap due to the higher energy of Se p-orbitals compared to S p-orbitals. The ability to precisely control the bandgap makes these alloys highly attractive for optoelectronic applications, including photodetectors, light-emitting diodes, and solar cells. Additionally, strain engineering can further modulate the bandgap, offering additional degrees of freedom for device design.

The confined growth of 2D alloy systems also enables the fabrication of lateral and vertical heterostructures with sharp interfaces. By spatially controlling the precursor delivery during growth, compositionally graded structures or abrupt heterojunctions can be achieved. Such structures are useful for designing devices with built-in electric fields or tailored carrier transport properties. For instance, lateral heterostructures of MoS₂ and MoSe₂ have been demonstrated for use in photovoltaic applications, where the band alignment facilitates efficient charge separation.

Despite the progress in synthesis techniques, challenges remain in achieving large-area, uniform 2D alloy films with precise compositional control. Variability in precursor delivery, substrate interactions, and thermal gradients can lead to inhomogeneities across the sample. Advanced growth techniques, such as atomic layer deposition (ALD) or pulsed laser deposition (PLD), are being explored to improve uniformity and scalability. Additionally, in-situ characterization methods, including Raman spectroscopy and photoluminescence mapping, are essential for real-time monitoring of alloy composition and quality.

In summary, the synthesis of 2D alloy systems like MoS₂(1-x)Se₂x via confined growth techniques offers a powerful platform for bandgap engineering and optoelectronic applications. Precursor mixing ratios and growth temperature are key parameters that determine alloy composition, while phase segregation remains a critical challenge requiring precise control. The ability to tune electronic properties through alloying opens new possibilities for next-generation semiconductor devices, provided that growth methodologies continue to advance in reproducibility and scalability.