Observation of out-of-plane vibrations in few-layer graphene

TL;DR

This study reports the detection of out-of-plane layer breathing mode vibrations in few-layer graphene with different stacking orders, using Raman spectroscopy to analyze their unique spectral features and phonon interactions.

Contribution

It is the first to identify and analyze layer breathing mode vibrations in FLG with various stacking orders through Raman spectroscopy and double resonance theory.

Findings

LBM vibrations observed in 2-6 layer graphene

Distinct Raman features for different stacking orders

Multiple peaks in LOZO' Raman band linked to phonon interactions

Abstract

We report the observation of layer breathing mode (LBM) vibrations in few-layer graphene (FLG) samples of thickness from 2 to 6 layers, exhibiting both Bernal (AB) and rhombohedral (ABC) stacking order. The LBM vibrations are identified using a Raman combination band lying around 1720 cm-1. From double resonance theory, we identify the feature as the LOZO' combination mode of the out-of-plane LBM (ZO') and the in-plane longitudinal optical mode (LO). The LOZO' Raman band is found to exhibit multiple peaks, with a unique line shape for each layer thickness and stacking order. These complex line shapes of the LOZO'-mode arise both from the material-dependent selection of different phonons in the double-resonance Raman process and from the detailed structure of the different branches of LBM in FLG.