  Large-area two-dimensional (2D) materials 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 by
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 (DFT) calculations show that the 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.
