Stacking sequence determines Raman intensities of observed interlayer shear modes in 2D layered materials - A general bond polarizability model

TL;DR

This paper demonstrates how stacking sequence influences the Raman intensities and frequency shifts of interlayer shear modes in 2D layered materials, using a bond polarizability model validated by experiments and calculations.

Contribution

It introduces a general bond polarizability model that predicts stacking-dependent Raman intensity trends in 2D materials, validated by experiments and first-principles calculations.

Findings

Raman frequencies blue shift for AB stacking and red shift for ABC stacking with increasing layers.

Stacking sequence determines which interlayer shear modes have the largest Raman intensity.

The model accurately predicts stacking effects across various 2D materials.

Abstract

2D layered materials have recently attracted tremendous interest due to their fascinating properties and potential applications. The interlayer interactions are much weaker than the intralayer bonds, allowing the as-synthesized materials to exhibit different stacking sequences (e.g. ABAB, ABCABC), leading to different physical properties. Here, we show that regardless of the space group of the 2D material, the Raman frequencies of the interlayer shear modes observed under the typical configuration blue shift for AB stacked materials, and red shift for ABC stacked materials, as the number of layers increases. Our predictions are made using an intuitive bond polarizability model which shows that stacking sequence plays a key role in determining which interlayer shear modes lead to the largest change in polarizability (Raman intensity); the modes with the largest Raman intensity determining the frequency trends. We present direct evidence for these conclusions by studying the Raman modes in few layer graphene, MoS2, MoSe2, WSe2 and Bi2Se3, using both first principles calculations and Raman spectroscopy. This study sheds light on the influence of stacking sequence on the Raman intensities of intrinsic interlayer modes in 2D layered materials in general, and leads to a practical way of identifying the stacking sequence in these materials.