Van der Waals heterostructures constructed from transition metal dichalcogenides (TMDCs) like MoS2, WS2, and other layered materials such as graphene or hexagonal boron nitride (hBN) represent a frontier in condensed matter physics and materials science. These heterostructures exploit weak interlayer van der Waals forces to stack atomically thin layers with precise control, enabling the exploration of novel electronic, optical, and quantum phenomena. Unlike conventional epitaxial heterostructures, van der Waals assembly circumvents lattice mismatch constraints, allowing the integration of disparate materials with tailored functionalities. Key aspects of these systems include interfacial charge transfer, band alignment engineering, moiré superlattices, and emergent properties like exciton trapping, which collectively define their potential for next-generation optoelectronic and quantum devices.

Interfacial charge transfer is a defining characteristic of TMDC-based heterostructures. When two dissimilar materials like MoS2 and graphene are brought into contact, their Fermi levels equilibrate, leading to electron redistribution across the interface. For instance, graphene’s high carrier mobility and zero bandgap facilitate efficient charge injection into MoS2, enhancing its photoresponse in photodetectors. Studies have shown that the charge transfer magnitude depends on the work function difference between the layers, doping levels, and interfacial cleanliness. In MoS2/graphene systems, photoexcited electrons in MoS2 can transfer to graphene within femtoseconds, leaving behind holes that form long-lived charge-separated states. This effect is exploited in photovoltaic and photocatalytic applications where efficient charge separation is critical. Similarly, in WS2/hBN structures, the insulating nature of hBN suppresses interfacial recombination, prolonging exciton lifetimes in WS2.

Band alignment plays a pivotal role in determining the electronic and optical behavior of these heterostructures. Type-II band alignment, where the conduction band minimum of one material aligns with the valence band maximum of another, is particularly common in TMDC-based systems. For example, MoS2/WS2 heterostructures exhibit a type-II alignment, leading to spatially indirect excitons with reduced recombination rates. The band offsets can be tuned via layer thickness, strain, or external electric fields, offering a versatile platform for designing optoelectronic devices. In MoS2/graphene heterostructures, the absence of a bandgap in graphene creates a Schottky barrier at the interface, influencing carrier injection efficiency. Precise measurement techniques such as scanning tunneling spectroscopy and photoluminescence mapping have revealed that even subtle changes in interlayer distance or twist angle can modulate band alignment significantly.

Moiré patterns arise when two stacked layers exhibit a small lattice mismatch or relative twist angle, resulting in a superlattice potential that modulates electronic states. In TMDC heterostructures like twisted MoS2/WS2, moiré patterns can localize excitons, creating periodic arrays of quantum emitters. These patterns also lead to flat electronic bands near certain magic angles, fostering correlated electron phenomena such as Mott insulation or superconductivity. For instance, a twist angle of approximately 60° in bilayer MoS2 can induce flat bands near the valence band edge, enhancing electron-electron interactions. The moiré potential also influences exciton diffusion, with trapped excitons exhibiting longer lifetimes due to reduced overlap with phonon modes. Such effects are harnessed in quantum light sources and single-photon emitters.

Emergent phenomena in these heterostructures include exciton trapping, interlayer excitons, and hybridized electronic states. In MoS2/hBN systems, the dielectric contrast between the layers localizes excitons within the TMDC, increasing their binding energy and stability. Interlayer excitons, where electrons and holes reside in separate layers, exhibit unique valley physics and extended lifetimes, making them suitable for valleytronics. Hybridized states at the interface of graphene and TMDCs can lead to enhanced spin-orbit coupling, enabling spin-polarized charge transport. These phenomena are often probed using ultrafast spectroscopy or magneto-optical measurements, revealing insights into many-body interactions and quantum coherence.

Fabrication techniques for van der Waals heterostructures fall into two broad categories: dry transfer and direct growth. Dry transfer methods involve mechanically exfoliating individual layers and stacking them using deterministic placement tools under optical microscopy guidance. This approach offers high flexibility in layer selection and twist angle control but may introduce contaminants or strain. Polymer-assisted transfer, using materials like polycarbonate or PDMS, minimizes residues but requires careful optimization of adhesion forces. Direct growth methods, such as chemical vapor deposition (CVD), enable scalable production of heterostructures but face challenges in achieving clean interfaces and precise alignment. For example, sequential CVD growth of WS2 on hBN can produce high-quality monolayers, though controlling nucleation sites remains a hurdle. Hybrid approaches, combining growth and transfer, are increasingly adopted to balance scalability with interface quality.

The unique properties of TMDC-based van der Waals heterostructures have spurred applications in photodetectors, LEDs, and quantum devices. Photodetectors leveraging MoS2/graphene heterostructures achieve high responsivity and broadband spectral response due to efficient charge transfer and graphene’s zero bandgap. LEDs utilizing interlayer excitons in WSe2/MoS2 stacks emit light at energies below the individual layer bandgaps, enabling tunable wavelengths. Quantum devices exploit moiré-trapped excitons or correlated electron states for single-photon emission or topological phases. Future directions include exploring twisted multilayer systems, integrating unconventional materials like magnetic TMDCs, and developing large-area assembly techniques for industrial adoption.

In summary, van der Waals heterostructures involving TMDCs and layered materials offer a rich platform for investigating interfacial phenomena, band engineering, and emergent quantum effects. Advances in fabrication and characterization continue to unlock their potential, bridging fundamental science with technological innovation. The interplay of charge transfer, moiré physics, and exciton dynamics underscores their versatility, paving the way for breakthroughs in optoelectronics, quantum information, and beyond.