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

TL;DR

This study uses low frequency Raman spectroscopy to analyze few-layer hBN crystals, revealing layer-dependent vibrational modes that enable precise thickness determination in ultra-thin sheets.

Contribution

It demonstrates the effectiveness of low frequency Raman spectroscopy for quantifying the number of layers in ultra-thin hBN crystals, with detailed analysis of vibrational mode shifts.

Findings

Layer-dependent Raman mode frequencies identified

Discrete vibrational modes observed in 3-layer hBN

Raman spectroscopy accurately determines layer thickness

Abstract

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.