  As a two-dimensional (2D) dielectric material, hexagonal boron nitride (hBN)
is in high demand for applications in photonics, nonlinear optics, and
nanoelectronics. Unfortunately, the high-throughput preparation of
macroscopic-scale, high-quality hBN flakes with controlled thickness is an
ongoing challenge, limiting device fabrication and technological integration.
Here, we present a metal thin-film exfoliation method to prepare hBN flakes
with millimeter-scale dimension, near-unity yields, and tunable flake thickness
distribution from 1-7 layers, a substantial improvement over scotch tape
exfoliation. The single crystallinity and high quality of the exfoliated hBN
are demonstrated with optical microscopy, atomic force microscopy, Raman
spectroscopy, and second harmonic generation. We further explore a possible
mechanism for the effectiveness and selectivity based on thin-film residual
stress measurements, density functional theory calculations, and transmission
electron microscopy imaging of the deposited metal films. We find that the
magnitude of the residual tensile stress induced by thin film deposition plays
a key role in determining exfoliated flake thickness in a manner which closely
resembles 3D semiconductor spalling. Lastly, we demonstrate that our
exfoliated, large-area hBN flakes can be readily incorporated as encapsulating
layers for other 2D monolayers. Altogether, this method brings us one step
closer to the high throughput, mass production of hBN-based 2D photonic,
optoelectronic, and quantum devices.
