The growth of 2D heterostructures via van der Waals epitaxy has emerged as a pivotal technique for engineering materials with tailored electronic, optical, and mechanical properties. Unlike conventional epitaxial growth, which requires stringent lattice matching between materials, van der Waals epitaxy leverages weak interlayer forces to stack atomically thin layers without the constraints of covalent bonding. This enables the integration of disparate 2D materials, such as graphene/hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDCs) like MoS₂/WS₂, into functional heterostructures with minimal defects and high interfacial quality.

A critical factor in van der Waals epitaxy is the accommodation of lattice mismatch between adjacent layers. In traditional epitaxy, even small lattice mismatches can introduce strain and dislocations, degrading device performance. However, van der Waals heterostructures tolerate mismatches as high as 20-30% due to the absence of strong interfacial bonds. For example, graphene and hBN exhibit a lattice mismatch of approximately 1.7%, yet they form atomically sharp interfaces with minimal strain transfer. Similarly, MoS₂ and WS₂, with a mismatch of around 4%, can be stacked without significant structural defects. The weak interlayer interactions allow each material to retain its intrinsic properties while enabling novel phenomena at the interface.

Interfacial cleanliness is another essential consideration for high-quality heterostructure growth. Contaminants such as adsorbates, oxides, or organic residues can disrupt van der Waals interactions and introduce scattering centers, impairing electronic transport. To mitigate this, growth techniques often employ in situ cleaning processes, such as high-temperature annealing in ultrahigh vacuum or inert gas environments. For instance, chemical vapor deposition (CVD) of graphene on hBN typically involves pre-treatment of the hBN substrate at temperatures exceeding 800°C to remove surface impurities. Advanced transfer methods, such as polymer-free direct stacking, further reduce contamination by eliminating intermediate handling steps.

Sequential growth techniques are widely used to fabricate van der Waals heterostructures with precise layer control. One common approach involves the CVD growth of individual 2D layers followed by deterministic transfer onto a target substrate. For example, graphene can be grown on copper foil, transferred onto hBN, and subsequently overlaid with TMDCs like MoS₂ to form a multi-layered stack. Alternatively, some studies have demonstrated direct sequential growth, where multiple materials are synthesized in a single CVD process by modulating precursor gases and temperatures. This method reduces interfacial contamination but requires careful optimization of growth parameters to prevent unintended alloying or degradation of underlying layers.

Twistronics, the study of angle-dependent phenomena in twisted 2D heterostructures, has gained significant attention due to the unique electronic properties arising from moiré patterns. For instance, rotating graphene relative to hBN by a specific angle (e.g., 0° or 30°) can induce superlattice potentials, leading to Hofstadter’s butterfly spectra or correlated insulating states. Similarly, twisting two TMDC layers, such as MoS₂/WS₂, can create moiré excitons with enhanced binding energies and spatially modulated optical responses. The precision of twist angle control is critical, with deviations as small as 0.1° significantly altering electronic behavior. Van der Waals epitaxy enables such fine-tuning by allowing post-growth rotation and alignment of layers.

Optoelectronic applications of 2D heterostructures benefit from their tunable band alignment and strong light-matter interactions. Type-II band alignment in MoS₂/WS₂ heterostructures, for example, facilitates efficient charge separation, making them ideal for photodetectors and solar cells. The interlayer excitons formed at these interfaces exhibit long lifetimes and valley-polarized emission, enabling novel light-emitting devices. Graphene/hBN heterostructures, on the other hand, are prized for their high carrier mobility and low optical absorption, serving as transparent electrodes or ultrafast photodetectors. The ability to engineer these properties through layer selection and stacking order underscores the versatility of van der Waals epitaxy.

Challenges remain in scaling up production and achieving uniform heterostructures over large areas. Variability in growth conditions, such as temperature gradients or precursor flow rates, can lead to inhomogeneous layer thicknesses or incomplete coverage. Recent advances in roll-to-roll CVD and spatial atomic layer deposition (ALD) aim to address these issues by enabling continuous, high-throughput synthesis. Additionally, the development of in situ characterization tools, such as Raman spectroscopy or electron microscopy during growth, provides real-time feedback for process optimization.

The future of van der Waals epitaxy lies in expanding the library of compatible materials and exploring new heterostructure configurations. Emerging 2D materials, such as Janus TMDCs (where the two chalcogen layers are different) or magnetic CrI₃, offer additional degrees of freedom for designing multifunctional devices. Integrating these materials into complex heterostructures could unlock unprecedented functionalities, from room-temperature superconductivity to ultra-low-power electronics.

In summary, van der Waals epitaxy has revolutionized the fabrication of 2D heterostructures by overcoming the limitations of lattice mismatch and enabling precise control over interfacial properties. Its applications in twistronics and optoelectronics highlight the potential for next-generation devices with tailored quantum behaviors and enhanced performance. As growth techniques continue to advance, the scalability and reproducibility of these heterostructures will further solidify their role in modern semiconductor technology.