Free molecular-flow modulated synthesis of hexagonal boron nitride monolayers

Atomically smooth hexagonal boron nitride (h-BN) films are needed for emerging applications of two-dimensional (2D) devices based on van der Waals heterostructures. Currently, it is difficult to prepare high-quality, largearea single crystalline h-BN monolayers by standard chemical vapor deposition. Here, we develop a free-molecular-flow growth model for h-BN synthesis by constructing a narrow gap within Cu/supporting substrates stacking, improving upon earlier studies by increasing domain sizes up to ∼80 μm. Further, the edges of h-BN varying from negative-curved to straight, positive-curved, and even round were reliably modified by varying the precursor heating temperature. Moreover, the merging processes of h-BN films are experimentally investigated, demonstrating the Bravais law is applicable for the coalescence of h-BN domains. This work not only offers a promising strategy for highquality h-BN growth and insight into its growth dynamics which sheds light on reliable edge controllability and possible properties modification of other 2D crystals, but also enriches the understanding of the classical crystal growth theory and extends its applicability into the growth and evolution of 2D crystals.