Direct Imaging of the Nitrogen Rich Edge in Monolayer Hexagonal Boron Nitride and Its Band Structure Tuning
Identification of edge atoms and tracking the edge structure evolution of two-dimensional (2D) crystals at the scale of individual atom is critical to understand the edge-dominated properties and behaviors response to the external field stimulus. Here, the direct imaging of edge configuration atom-by-atom for the monolayer hexagonal boron nitride (h-BN) is demonstrated by aberration-corrected transmission electron microscopy. The tracking of edge atoms’ arrangement reveals that nitrogen atom terminated zigzag type dominates along the edge, naturally leading the nitrogen rich (N-rich) feature in the margin area while keeping the stoichiometric interior of a h-BN monolayer. Two approaches including both top-down fabrication and bottom-up growth are proposed to obtain such novel h-BN flakes with an N-rich ratio larger than 1% when the size shrinks down to the threshold of 25 nm. Further density functional theory calculation shows a new bandgap of ~3 eV is created by the N-rich fact, and the h-BN transfers to a n-type semiconductor by self-doping. The results call for the attention of ultra-small h-BN islands used in intriguing 2D electronic devices with a function of photoresponse to visible light.