The integration of graphene with other two-dimensional materials such as hexagonal boron nitride (hBN), molybdenum disulfide (MoS₂), and related van der Waals (vdW) heterostructures has opened new frontiers in condensed matter physics and materials science. By leveraging weak interlayer interactions, these stacked systems exhibit unique electronic, optical, and mechanical properties that are not accessible in individual monolayers. The precise control over interfacial charge transfer, moiré superlattices, and twist-angle engineering has enabled breakthroughs in twistronics and optoelectronic applications, though challenges in fabrication and alignment remain significant hurdles.

Van der Waals assembly allows the stacking of atomically thin layers without the constraints of lattice matching, enabling the creation of heterostructures with tailored functionalities. When graphene is combined with hBN, the resulting heterostructure often exhibits improved electronic properties due to the atomically flat and chemically inert surface of hBN, which reduces charge scattering and enhances carrier mobility. The lattice mismatch between graphene and hBN, approximately 1.7%, leads to the formation of moiré patterns, which modulate the electronic potential landscape. These patterns can induce secondary Dirac points in graphene’s band structure, altering its transport properties. Similarly, stacking graphene with transition metal dichalcogenides like MoS₂ introduces strong interfacial charge transfer due to differences in work functions. For instance, graphene typically donates electrons to MoS₂, creating a p-doped graphene layer and enhancing the photoresponse in optoelectronic devices.

Moiré patterns play a critical role in defining the electronic behavior of vdW heterostructures. The periodicity and intensity of these patterns depend on the twist angle between adjacent layers. At specific angles, such as the magic angle near 1.1° for graphene-graphene systems, flat electronic bands emerge, leading to correlated insulating states and superconductivity. While such effects are most prominently studied in bilayer graphene, similar phenomena occur in graphene-hBN and graphene-MoS₂ systems, though the energy scales and interactions differ. Twist-angle engineering thus serves as a powerful tool to tailor electronic properties, but achieving precise angular control remains a fabrication challenge. Current techniques rely on transfer and alignment methods using polymer stamps or micromanipulators, often requiring real-time optical or scanning probe microscopy feedback to ensure accuracy.

Fabrication challenges in assembling these heterostructures are multifaceted. The manual transfer of 2D materials, though widely used, suffers from low yield and poor reproducibility. Contamination at the interfaces, such as polymer residues from transfer processes, can degrade device performance. Automated transfer systems and cleanroom-compatible techniques have been developed to mitigate these issues, but scalability remains a concern. Additionally, achieving uniform interlayer coupling across large areas is difficult due to intrinsic material inhomogeneities and strain-induced deformations. Recent advances in deterministic placement using van der Waals pick-up techniques have improved alignment precision, yet sub-degree angular control is still non-trivial for industrial-scale production.

Interfacial charge transfer in graphene-based heterostructures is another critical aspect governing device performance. In graphene-MoS₂ systems, photoexcitation leads to rapid electron transfer from graphene to MoS₂, making these stacks promising for photodetection and photovoltaic applications. The charge transfer dynamics are influenced by the twist angle, layer thickness, and dielectric environment. For example, increasing the twist angle can reduce the overlap of electronic wavefunctions, thereby modulating the charge transfer rate. Encapsulating graphene with hBN not only enhances mobility but also screens Coulomb interactions, affecting exciton dynamics in adjacent TMD layers. These interactions are exploited in optoelectronic devices, where heterostructures exhibit enhanced photoluminescence, gate-tunable photoresponse, and improved quantum efficiency.

Twistronics, the study of how twist angles influence electronic properties, has become a major research direction in 2D materials. In graphene-hBN heterostructures, the moiré potential can lead to Hofstadter’s butterfly spectra under high magnetic fields, revealing fractal quantum Hall effects. Similarly, small twist angles in graphene-MoS₂ systems can create periodic potential modulations that alter exciton diffusion and recombination pathways. These effects are harnessed in devices such as tunable photodetectors, superlattice-based transistors, and quantum emitters. The ability to dynamically tune twist angles via mechanical deformation adds another layer of functionality, enabling real-time property modulation.

Optoelectronic applications of graphene-based heterostructures benefit from the synergistic effects of combining materials with complementary properties. Graphene’s broadband absorption and high carrier mobility, coupled with the strong light-matter interaction in TMDs, make these stacks ideal for ultrafast photodetectors and flexible optoelectronics. Heterostructures incorporating graphene, hBN, and MoS₂ have demonstrated responsivities exceeding 10⁴ A/W and response times in the picosecond range. The transparent nature of these materials further allows for integration into wearable electronics and transparent conductive films. However, optimizing interfacial defects and ensuring efficient charge extraction remain key challenges for commercial deployment.

The future of van der Waals heterostructures lies in advancing fabrication techniques to achieve higher precision and scalability. Methods such as epitaxial growth of 2D materials on templated substrates and roll-to-roll transfer processes are being explored to overcome current limitations. Additionally, machine learning-assisted alignment and in-situ characterization tools are being developed to improve reproducibility. As control over twist angles and interfacial cleanliness improves, the potential for discovering new quantum phenomena and enabling next-generation devices will expand.

In summary, stacking graphene with hBN, MoS₂, and other 2D materials via van der Waals assembly offers a versatile platform for engineering novel electronic and optoelectronic properties. The interplay of moiré patterns, interfacial charge transfer, and twist-angle manipulation provides a rich landscape for fundamental studies and technological innovations. While fabrication challenges persist, ongoing advancements in alignment techniques and material integration are paving the way for practical applications in twistronics and beyond.