Millimeter-scale exfoliation of hBN with tunable flake thickness

TL;DR

This paper introduces a metal thin-film exfoliation technique for producing large, high-quality hBN flakes with tunable thickness, addressing a key challenge in scalable 2D material fabrication for advanced device applications.

Contribution

A novel metal thin-film exfoliation method enabling millimeter-scale, high-yield, and tunable-thickness hBN flakes, improving over traditional scotch tape techniques.

Findings

Achieved millimeter-scale hBN flakes with 1-7 layers.

Demonstrated high crystallinity and quality of exfoliated hBN.

Identified residual tensile stress as a key factor in controlling flake thickness.

Abstract

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.