Grain boundaries in chemical-vapor-deposited atomically thin hexagonal boron nitride

Atomically thin hexagonal boron nitride (h-BN) exhibits a wide band gap, as well as excellent thermal and chemical stability, and thus has been used in ultraviolet light emission and as building blocks for two-dimensional (2D) heterostructures. Large-area h-BN films for technical applications can now be produced by chemical vapor deposition (CVD). Unfortunately, grain boundaries (GBs) are ubiquitously introduced as a result of the coalescence of grains with different crystallographic orientations. It is well known that the properties of materials largely depend on GB structures. Here, we carried out a systematic study on the GB structures in CVD-grown polycrystalline h-BN monolayer films with a transmission electron microscope. Interestingly, most of these GBs are revealed to be formed via overlapping between neighboring grains, which are distinct from the covalently bonded GBs as commonly observed in other 2D materials. Further density functional theory calculations show that hydrogen plays an essential role in overlapping GB formation. This work provides an in-depth understanding of the microstructures and formation mechanisms of GBs in CVD-grown h-BN films, which should be informative in guiding the precisely controlled synthesis of large-area single-crystalline h-BN and other 2D materials.