Double-resonant LA phonon scattering in defective graphene and carbon nanotubes

TL;DR

This study investigates the double-resonant LA phonon scattering in defective graphene and carbon nanotubes through Raman spectroscopy, revealing the significance of inner processes and providing a rapid optical method to analyze acoustic phonons.

Contribution

It introduces a detailed analysis of the $D''$ Raman mode, highlighting the role of inner processes and their dependence on excitation energy, layer number, and nanotube diameter.

Findings

The $D''$ mode originates from double-resonant LA phonon scattering.

Inner processes dominate the $D''$ Raman mode.

The method offers a quick way to study acoustic phonons.

Abstract

We present measurements of the $D''$ Raman mode in graphene and carbon nanotubes at different laser excitation energies. The Raman mode around 1050 - 1150\,cm$^{-1}$ originates from a double-resonant scattering process of longitudinal acoustic (LA) phonons with defects. We investigate its dependence on laser excitation energy, on the number of graphene layers and on the carbon nanotube diameter. We assign this Raman mode to so-called 'inner' processes with resonant phonons mainly from the $\Gamma-K$ high-symmetry direction. The asymmetry of the $D''$ mode is explained by additional contributions from phonons next to the $\Gamma-K$ line. Our results demonstrate the importance of inner contributions in the double-resonance scattering process and add a fast method to investigate acoustic phonons in graphene and carbon nanotubes by optical spectroscopy.