Layered chalcogenide semiconductors, particularly transition metal dichalcogenides (TMDCs), have emerged as a versatile class of materials with applications spanning electronics, optoelectronics, and catalysis. These materials exhibit unique electronic, optical, and chemical properties that are highly dependent on their thickness, stacking order, and heterostructure configurations. Unlike conventional bulk semiconductors, TMDCs such as MoS2, WS2, MoSe2, and WSe2 transition from indirect bandgap behavior in bulk form to direct bandgap characteristics in monolayer configurations, enabling efficient light-matter interactions and tunable electronic transport.

Exfoliation methods play a critical role in isolating high-quality TMDC layers. Mechanical exfoliation, often using adhesive tapes, remains a widely adopted technique for producing pristine monolayers with minimal defects. While this method yields high-quality flakes suitable for fundamental studies, it lacks scalability. Liquid-phase exfoliation offers a more scalable alternative, where bulk crystals are dispersed in solvents and subjected to ultrasonication or shear forces. The choice of solvent, such as N-methyl-2-pyrrolidone (NMP) or isopropanol, significantly impacts the yield and stability of the exfoliated layers. Electrochemical exfoliation has also gained traction, leveraging intercalation of ions such as lithium to weaken interlayer van der Waals forces, followed by mechanical agitation to separate the layers. Each exfoliation method presents trade-offs between flake size, defect density, and scalability, necessitating careful selection based on the intended application.

Heterostructure engineering of TMDCs enables the design of materials with tailored properties. Vertical heterostructures, formed by stacking different TMDC monolayers, exhibit type-II band alignment, facilitating efficient charge separation. For instance, a MoS2/WSe2 heterostructure demonstrates a built-in electric field that drives electrons and holes into separate layers, enhancing photovoltaic and photocatalytic performance. Lateral heterostructures, where different TMDCs are seamlessly connected within the same plane, enable the creation of in-plane p-n junctions with atomically sharp interfaces. The growth of such structures often employs chemical vapor deposition (CVD) with precise control over precursor flow rates and substrate temperatures. The lattice mismatch between different TMDCs, typically below 4%, allows for coherent epitaxial growth without significant strain-induced defects.

Thickness-dependent properties are a hallmark of TMDCs. Monolayer MoS2 exhibits a direct bandgap of approximately 1.8 eV, while bulk MoS2 possesses an indirect bandgap of 1.2 eV. This transition significantly impacts photoluminescence quantum yield, which can exceed 10% in monolayers but is nearly negligible in bulk crystals. Carrier mobility in TMDCs also varies with thickness due to changes in dielectric screening and phonon scattering mechanisms. For example, monolayer MoS2 typically shows mobilities in the range of 1–10 cm²/Vs, while few-layer samples can reach up to 200 cm²/Vs due to reduced Coulomb scattering. The thickness-dependent dielectric constant further influences capacitive coupling in field-effect transistors, with thinner layers offering stronger gate control but higher susceptibility to charge traps.

In electronic applications, TMDCs are explored for their potential in next-generation transistors. The absence of dangling bonds on their surfaces reduces interface scattering, making them attractive for ultrascaled devices. High-performance MoS2 field-effect transistors have demonstrated on/off ratios exceeding 10⁸ and subthreshold swings approaching the thermionic limit of 60 mV/decade at room temperature. The integration of high-k dielectrics such as HfO2 or Al2O3 via atomic layer deposition (ALD) further enhances device performance by mitigating charge trapping and improving gate efficiency. Flexible electronics benefit from TMDCs due to their mechanical robustness, with fracture strains exceeding 10% in monolayer forms, enabling bendable and wearable applications.

Catalytic applications of TMDCs leverage their active edge sites and tunable electronic structures. Monolayer MoS2 exhibits superior hydrogen evolution reaction (HER) activity compared to bulk counterparts due to the exposure of sulfur edges with Gibbs free energy close to thermoneutral. Doping or strain engineering can further optimize these sites for enhanced catalytic performance. For instance, cobalt-doped MoS2 shows a significant reduction in overpotential, achieving current densities of 10 mA/cm² at overpotentials as low as 140 mV. The basal planes of TMDCs, traditionally considered inert, can be activated through defect engineering or phase transitions. The metastable 1T phase of MoS2, stabilized by intercalation or chemical functionalization, demonstrates metallic conductivity and improved catalytic activity over the semiconducting 2H phase.

Environmental stability remains a challenge for TMDCs, particularly in monolayer form. Oxidation and humidity-induced degradation can alter their electronic and catalytic properties. Encapsulation strategies using inert materials such as Al2O3 or hexagonal boron nitride (hBN) have proven effective in prolonging device lifetimes. In catalytic environments, protective coatings must balance stability with maintaining access to active sites, requiring precise control over layer thickness and porosity.

The future of TMDCs lies in advancing synthesis techniques to achieve wafer-scale uniformity and defect control. Progress in area-selective growth and post-growth treatments will enable the integration of TMDCs into mainstream semiconductor fabrication processes. For catalysis, the development of scalable methods to expose and stabilize active edge sites will be crucial for commercial applications. The interplay between thickness, strain, and doping in tuning TMDC properties offers a rich design space for both electronic and catalytic applications, positioning these materials as key enablers of next-generation technologies.