Millimeter-Scale Exfoliation of hBN with Tunable Flake Thickness for Scalable Encapsulation

As a 2D dielectric material, hexagonal boron nitride (hBN) is in high demand for applications in photonic, nonlinear optic, and nanoelectronic devices as an atomically flat and thin dielectric or encapsulation layer. Unfortunately, the high-throughput preparation of macroscopic-scale hBN flakes with selective thickness is an ongoing challenge, limiting device fabrication and technological integration. Here, we use various metal thin films to prepare hBN flakes with millimeter-scale dimension, near-unity yields, and tunable flake thickness distributions from 1 to 7 layers. The single crystallinity and quality of the exfoliated hBN flakes are demonstrated with atomic force microscopy, Raman spectroscopy, and second-harmonic generation. We further explore a possible mechanism for flake-thickness selectivity based on thin-film residual stress measurements and density functional theory calculations. We demonstrate that our exfoliated, large-area hBN flakes can be incorporated as encapsulating layers for MoSe 2 monolayers to effectively protect against photodegradation. This method brings us one step closer to the high-throughput, mass production of hBN-based 2D photonic, optoelectronic, and quantum devices in the future.