Low frequency Raman spectroscopy of few-atomic-layer thick hBN crystals

Hexagonal boron nitride (hBN) has recently gained a strong interest as a component in engineering van der Waals heterostructures built with dimensional crystals such as graphene. This work reports micro-Raman on hBN flakes made of a few atomic layers, prepared by mechanical. The temperature dependence of the Raman scattering in hBN is first such as to define appropriate measurements conditions for thin layers avoiding undesirable heating induced effects. We focus on the low frequency Raman mode corresponding to the rigid oscillation between adjacent layers, found to be equal to 52. 5 cm-1 in hBN. For hBN sheets with thicknesses below typically 4 nm, the frequency this mode presents discrete values, which are found to decrease down to46. 0 (5) cm-1 for a three-layer hBN, in good agreement with the linear-chain. This makes Raman spectroscopy a relevant tool to quantitatively the number of layers in ultra thin hBN sheets, below 8L.