  Hexagonal boron nitride (hBN) has recently gained a strong interest as a
strategic component in engineering van der Waals heterostructures built with
two dimensional crystals such as graphene. This work reports micro-Raman
measurements on hBN flakes made of a few atomic layers, prepared by mechanical
exfoliation. The temperature dependence of the Raman scattering in hBN is
investigated first such as to define appropriate measurements conditions
suitable for thin layers avoiding undesirable heating induced effects. We
further focus on the low frequency Raman mode corresponding to the rigid
shearing oscillation between adjacent layers, found to be equal to 52.5 cm-1 in
bulk hBN. For hBN sheets with thicknesses below typically 4 nm, the frequency
of this mode presents discrete values, which are found to decrease down to
46.0(5) cm-1 for a three-layer hBN, in good agreement with the linear-chain
model. This makes Raman spectroscopy a relevant tool to quantitatively
determine the number of layers in ultra thin hBN sheets, below 8L.
