Hexagonal boron nitride (hBN) is a layered material with a structure analogous to graphite, consisting of alternating boron and nitrogen atoms arranged in a honeycomb lattice. Each layer is held together by strong in-plane covalent bonds and weak out-of-plane van der Waals forces, enabling easy exfoliation into thin sheets. The lattice parameters of hBN are approximately a = 0.2504 nm and c = 0.6661 nm, with a interlayer spacing of around 0.333 nm. The material exhibits a wide bandgap of 5.9–6.0 eV, making it an excellent electrical insulator. Its crystallographic symmetry belongs to the P6₃/mmc space group, and unlike graphene, hBN is non-conductive due to the ionic character of the B-N bonds.
Several synthesis methods are employed to produce high-quality hBN. Mechanical exfoliation, similar to the Scotch tape method used for graphene, can isolate few-layer or monolayer hBN from bulk crystals. This technique yields pristine flakes but is limited by low throughput. Chemical vapor deposition (CVD) is a scalable alternative, typically involving ammonia borane or borazine as precursors. Growth occurs on metal substrates like copper or nickel at temperatures between 800–1100°C, with the choice of catalyst influencing layer uniformity. Physical vapor deposition (PVD), including pulsed laser deposition and sputtering, is another approach, though it often requires post-annealing to improve crystallinity. For bulk hBN, high-temperature, high-pressure methods (1800–2000°C, 5–6 GPa) produce crystals with fewer defects, while solution-based exfoliation in solvents like N-methyl-2-pyrrolidone (NMP) offers a low-cost route for industrial applications.
The thermal conductivity of hBN is exceptionally high, reaching up to 400–500 W/m·K in the in-plane direction for bulk crystals, surpassing many metals. This anisotropy arises from the strong sp² bonding within layers versus weak interlayer interactions, with cross-plane conductivity dropping to roughly 2–30 W/m·K. Such properties make hBN ideal for thermal management in electronics, where it dissipates heat efficiently in devices like high-power transistors or LEDs. Its thermal stability up to 1000°C in air further enhances its utility in extreme environments.
Electrically, hBN is an insulator with a resistivity exceeding 10¹⁵ Ω·cm and a dielectric strength of ~0.7 V/nm. These characteristics prevent current leakage in electronic components, making it a preferred gate dielectric or spacer material in 2D heterostructures. The absence of dangling bonds on its surface reduces charge scattering, improving carrier mobility in adjacent materials like graphene or transition metal dichalcogenides (TMDCs). Additionally, hBN’s atomically flat surface minimizes roughness-induced defects, critical for preserving quantum coherence in nanoscale devices.
As a lubricant, hBN outperforms graphite in high-temperature or oxidizing conditions due to its chemical inertness. The layers shear easily under stress, with a friction coefficient as low as 0.1–0.2, and it remains stable up to 900°C without degradation. This lubricity is exploited in aerospace and automotive applications where conventional oils fail.
In optoelectronics, hBN’s wide bandgap and low optical absorption across ultraviolet to infrared wavelengths make it an excellent substrate or encapsulation layer. It enhances photoluminescence yield in TMDCs by suppressing non-radiative recombination, and its hyperbolic phonon polaritons enable sub-diffractional light confinement for nanophotonics. The material also serves as a tunnel barrier in light-emitting devices, facilitating efficient carrier injection.
For thermal management, hBN films are integrated into flexible heat spreaders or composites to cool high-density electronics. Its compatibility with polymers allows for lightweight, thermally conductive coatings in wearable devices or batteries. In power electronics, hBN substrates reduce thermal resistance in GaN-based high-electron-mobility transistors (HEMTs), extending device lifetimes.
Research continues to optimize hBN synthesis for large-area, defect-free growth and explore its quantum applications, such as single-photon emitters from atomic defects. Its combination of thermal, electrical, and mechanical properties ensures a pivotal role in advancing next-generation technologies.