Layer-selective growth of 2D hexagonal boron nitride using two-step chemical vapor deposition

Hexagonal boron nitride (h-BN) has recently become a promising and popular material due to its wide applications, such as deep ultraviolet optoelectronic devices, novel nonvolatile memory technology, room temperature quantum emitter, and dielectric in devices based on two-dimensional (2D) materials. However, realizing uniform h-BN thin films thicker than monolayer remains a major challenge, which is important in both fundamental and practical studies. In this work, we report the selective growth of large-area bilayer h-BN using a two-step chemical vapor deposition (CVD) approach. Through constructing a confined space in the Cu foils/sapphire stack, controlled synthesis of bilayer h-BN is achieved. Systematic characterization verified the formation of bilayer h-BN and the mechanism of CVD growth is also studied. We find that the in situ annealing between the two steps could efficiently facilitate the selective growth, and the local oxidation and reduction process could create preferred nucleation sites for uniform bilayer h-BN. Our results could enrich the understanding of the synthesis mechanism and lay the foundation for controllable synthesis of high-quality h-BN.