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

Abstract

Hexagonal boron nitride (hBN) has recently gained a strong interest as a\nstrategic component in engineering van der Waals heterostructures built with\ntwo dimensional crystals such as graphene. This work reports micro-Raman\nmeasurements on hBN flakes made of a few atomic layers, prepared by mechanical\nexfoliation. The temperature dependence of the Raman scattering in hBN is\ninvestigated first such as to define appropriate measurements conditions\nsuitable for thin layers avoiding undesirable heating induced effects. We\nfurther focus on the low frequency Raman mode corresponding to the rigid\nshearing oscillation between adjacent layers, found to be equal to 52.5 cm-1 in\nbulk hBN. For hBN sheets with thicknesses below typically 4 nm, the frequency\nof this mode presents discrete values, which are found to decrease down to\n46.0(5) cm-1 for a three-layer hBN, in good agreement with the linear-chain\nmodel. This makes Raman spectroscopy a relevant tool to quantitatively\ndetermine the number of layers in ultra thin hBN sheets, below 8L.\n