Structure evolution of hBN grown on molten Cu by regulating precursor flux during chemical vapor deposition

We demonstrate the structure evolution of hexagonal boron nitride (hBN) flakes grown on molten Cu in atmospheric pressure chemical vapor deposition by regulating the flux of precursor. We found that under lower precursor flux, tuned by temperature that controls the sublimation rates, the hBN grains change from triangle to truncated triangle shape with additional B-terminated edges, which could be understood through kinetic Wulff construction, while under higher flux, they form circular shape following deposition-controlled growth and predicted by a phase field modeling. In addition to the monolayer morphology from a single nucleation, adlayer patterns with centered aggregation and diffusive features at high precursor flux are observed and simulated by a two-dimensional (2D) diffusion-reaction model, where the random diffusion and deposition are revealed to be the dominating kinetics. The nucleation density and growth velocity could also be modulated by the ammonia borane heating temperature, where 80 degrees C is found to be optimal for the largest hBN grain size. Our transmission electron microscopy study shows that a misalignment of coalescing grains occurs on such molten Cu substrate, deviated from those observed on molten Au. Our results provide a new tool for the shape and grain size control of 2D materials and the understanding of their growth kinetics for large scale production.