The study of engineered van der Waals interfaces has opened new frontiers in condensed matter physics and materials science. By stacking atomically thin layers such as graphene/MoS₂ or hBN/WSe₂ with precise control over their relative orientations, researchers have uncovered a wealth of phenomena arising from interlayer interactions. These heterostructures exhibit unique electronic, optical, and mechanical properties dictated by the van der Waals forces between layers, enabling unprecedented control over quantum states and optoelectronic behavior.
One of the most critical parameters in these systems is the stacking angle between layers. Even minor angular misalignments can drastically alter the electronic structure. For instance, in graphene/MoS₂ heterostructures, a twist angle of 0° (aligned stacking) results in strong interlayer hybridization, while a 30° rotation decouples the layers, leading to nearly independent electronic behavior. At intermediate angles, the mismatch in lattice constants between layers generates moiré superlattices—periodic potential modulations that influence charge carriers. In twisted bilayer graphene, a "magic angle" of approximately 1.1° induces flat electronic bands, resulting in correlated insulator states and superconductivity. Similar effects have been observed in transition metal dichalcogenide (TMDC) heterostructures like MoS₂/WSe₂, where twist angles between 0° and 60° produce tunable moiré potentials.
Moiré patterns in these systems act as artificial superlattices, modifying the band structure and creating new quantum states. For example, in hBN/WSe₂ heterostructures, the moiré wavelength can be adjusted by varying the twist angle, leading to the formation of localized exciton states. The periodicity of the moiré lattice depends on the lattice mismatch and twist angle, following the relation:
λ = a / (2 sin(θ/2)),
where λ is the moiré wavelength, a is the lattice constant, and θ is the twist angle. These patterns can trap excitons, enhance many-body interactions, and even lead to the emergence of moiré exciton-polaritons in strongly coupled light-matter systems.
Interlayer excitons, formed when electrons and holes reside in separate layers, are another hallmark of van der Waals heterostructures. In type-II band-aligned systems like MoS₂/WSe₂, electrons localize in one layer while holes remain in the other, creating spatially indirect excitons with long lifetimes and large dipole moments. These excitons are highly sensitive to external electric and magnetic fields, making them ideal for optoelectronic applications. The binding energy of interlayer excitons is typically lower than that of intralayer excitons, often ranging between 100 and 300 meV, depending on the dielectric environment and interlayer distance. The ability to control their properties through stacking engineering has enabled advances in excitonic devices, such as tunable light emitters and valleytronic systems.
The field of twistronics exploits these angle-dependent phenomena to design materials with on-demand electronic properties. By precisely rotating one layer relative to another, researchers can engineer band structures, induce topological phases, and manipulate correlated electron states. For example, small-angle twists in TMD homobilayers like WSe₂/WSe₂ can create moiré-trapped exciton arrays, while larger angles may lead to hybridization and intervalley coupling. The interplay between twist angle, strain, and dielectric screening further enriches the phase space, allowing for the exploration of novel quantum phases.
Optoelectronic applications of engineered van der Waals interfaces are vast. Heterostructures like graphene/MoS₂ exhibit gate-tunable photoresponse, where the photocurrent can be modulated by adjusting the Fermi level via an external gate voltage. The strong light-matter interaction in TMD-based systems, combined with the ability to tailor interlayer charge transfer, makes them promising for photodetectors with high gain and spectral selectivity. Additionally, the valley degree of freedom in materials like WSe₂ enables valley-polarized light emission and detection, which could be harnessed for valleytronic devices. The integration of hBN as a dielectric spacer or encapsulation layer further enhances device performance by reducing disorder and screening Coulomb interactions.
Beyond conventional optoelectronics, these interfaces have potential in quantum technologies. The long-lived interlayer excitons in MoS₂/WSe₂ can serve as entangled photon sources, while the moiré-trapped states may provide a platform for simulating Hubbard models. The precise control over stacking configurations also facilitates the study of many-body physics, such as Bose-Einstein condensation of excitons or the formation of charge-density waves.
In summary, engineered van der Waals interfaces represent a versatile platform for exploring quantum phenomena and developing next-generation optoelectronic devices. The interplay between twist angle, moiré patterns, and interlayer excitons offers a rich landscape for fundamental research and technological innovation. As fabrication techniques continue to improve, the ability to design heterostructures with atomic precision will further expand the possibilities in twistronics and beyond.