Hexagonal boron nitride (hBN) has emerged as a critical substrate for two-dimensional material heterostructures due to its unique structural and electronic properties. Its atomically flat surface, absence of dangling bonds, and low density of charge traps make it an ideal platform for hosting graphene, transition metal dichalcogenides (TMDCs), and other 2D materials. These characteristics significantly enhance carrier mobility and reduce scattering, enabling high-performance electronic and optoelectronic structures. The interaction between hBN and layered materials also introduces opportunities for moiré pattern formation and strain engineering, which can modulate electronic and optical properties in novel ways.
The surface of hBN is exceptionally smooth, with roughness typically on the order of a few picometers. This flatness minimizes extrinsic scattering sources that degrade carrier transport in supported 2D materials. Unlike conventional oxide substrates such as silicon dioxide, hBN does not possess dangling bonds on its surface. These dangling bonds, if present, act as charge traps and scattering centers, leading to reduced mobility in supported materials. The absence of such defects in hBN results in a cleaner interface, preserving the intrinsic electronic properties of overlying graphene or TMDCs. Studies have shown that graphene on hBN can exhibit carrier mobilities exceeding 100,000 cm²/Vs at low temperatures, approaching the theoretical limit for suspended graphene.
Another critical advantage of hBN is its low density of charged impurities. Many substrates introduce unintentional doping or charge inhomogeneity due to trapped charges at the interface. In contrast, hBN has a wide bandgap of approximately 6 eV, making it highly insulating and resistant to charge transfer unless intentionally modified. This property ensures that the electronic behavior of the overlying 2D material remains dominated by intrinsic factors rather than substrate-induced disorder. For TMDCs like MoS₂, WS₂, or WSe₂, the use of hBN as a substrate reduces charge localization and improves photoluminescence quantum yield by suppressing non-radiative recombination pathways.
The lattice mismatch between hBN and common 2D materials plays a crucial role in determining the heterostructure's properties. Graphene has a lattice constant of approximately 2.46 Å, while hBN has a slightly larger lattice constant of about 2.50 Å. This small mismatch leads to the formation of moiré superlattices when the two materials are stacked with a relative twist angle. These moiré patterns create periodic potential modulations that can alter the electronic band structure, leading to phenomena such as secondary Dirac points in graphene or modulated excitonic behavior in TMDCs. The periodicity of the moiré pattern depends on the twist angle, with smaller angles producing larger superlattices. At specific angles, such as 0° or 60°, commensurate stacking occurs, while intermediate angles result in incommensurate structures with varying degrees of strain and electronic modulation.
Precise alignment techniques are essential for controlling the twist angle between hBN and the overlying 2D material. Dry transfer methods using polymer stamps or pick-and-place techniques allow for manual or semi-automated stacking with angular precision down to fractions of a degree. Advanced alignment systems incorporate real-time optical microscopy or electron microscopy to monitor the relative orientation during transfer. The ability to control the twist angle with high precision enables the engineering of tailored electronic properties, such as bandgap opening in graphene or tuning of interlayer excitons in TMDC heterobilayers.
Strain engineering is another important aspect of hBN-supported heterostructures. The mechanical coupling between hBN and 2D materials can induce strain due to differences in thermal expansion coefficients or intentional mechanical deformation. hBN's high mechanical strength and flexibility make it an excellent medium for strain transfer without introducing defects. Local strain can modify bandgaps, shift optical transitions, or even create pseudo-magnetic fields in graphene. For example, uniaxial strain in graphene on hBN can break sublattice symmetry, leading to anisotropic electronic transport. In TMDCs, strain can tune the exciton energy and enhance valley polarization effects, which are critical for valleytronic applications.
The dielectric environment provided by hBN also influences the optical and electronic properties of adjacent 2D materials. hBN has a dielectric constant of approximately 3–4, which is lower than many conventional oxides. This reduces dielectric screening compared to high-κ substrates, leading to stronger Coulomb interactions in TMDCs. Enhanced exciton binding energies and reduced charge screening improve the stability of excitonic complexes, making hBN-encapsulated structures ideal for studying many-body effects in 2D semiconductors. Additionally, the refractive index contrast between hBN and air or other dielectrics can be exploited to design optical cavities or waveguides for enhanced light-matter interaction.
The thermal properties of hBN further contribute to its effectiveness as a substrate. With a high thermal conductivity of several hundred W/mK, hBN efficiently dissipates heat generated in active 2D material devices. This is particularly important for high-power applications or optoelectronic devices where localized heating can degrade performance. The thermal interface resistance between hBN and graphene or TMDCs is also relatively low, ensuring efficient thermal transport across the heterostructure.
Interfacial cleanliness is paramount for achieving optimal performance in hBN-based heterostructures. Contaminants such as polymer residues or airborne hydrocarbons can introduce additional scattering and degrade electronic quality. Encapsulation techniques, where the 2D material is sandwiched between two hBN layers, help preserve the interface quality by shielding it from environmental exposure. Fully encapsulated graphene or TMDC devices exhibit more consistent and reproducible behavior compared to exposed structures.
The role of hBN extends beyond passive substrate effects. In some configurations, hBN actively participates in the heterostructure's electronic behavior. For instance, in graphene-hBN-graphene tunnel junctions, hBN acts as an atomically thin barrier enabling coherent quantum tunneling. The thickness uniformity and defect-free nature of hBN are critical for achieving high tunneling efficiency and low leakage currents. Similarly, in TMDC heterostructures, hBN can serve as a tunnel barrier or dielectric spacer in vertical devices, enabling precise control over interlayer coupling.
In summary, hexagonal boron nitride serves as an exceptional substrate for 2D material heterostructures due to its atomically flat surface, low defect density, and excellent interfacial properties. Its influence on carrier mobility, moiré pattern formation, and strain engineering enables the realization of high-performance electronic and optoelectronic systems. The ability to precisely control alignment and stacking further enhances the versatility of hBN-supported structures, making it a cornerstone of modern 2D materials research.