Understanding double-resonant Raman scattering in chiral carbon nanotubes: Diameter and energy dependence of the $D$ mode

TL;DR

This paper develops a theoretical model to analyze the double-resonant Raman scattering process in chiral carbon nanotubes, revealing how the $D$ mode depends on diameter and energy, and clarifying previous controversies.

Contribution

The study introduces a symmetry-based, geometry-dependent model that explains the $D$ mode behavior in all chiral nanotubes, regardless of their electronic structure details.

Findings

The $D$ mode frequency depends on nanotube diameter and resonant energy.

Two different diameter dependencies can be observed under different experimental conditions.

All chiral nanotubes can exhibit a $D$ mode, contrary to previous symmetry-based assumptions.

Abstract

We present a theoretical model to describe the double-resonant scattering process in arbitrary carbon nanotubes. We use this approach to investigate the defect-induced $D$ mode in CNTs and unravel the dependence of the $D$-mode frequency on the CNT diameter and on the energy of the resonant optical transition. Our approach is based on the symmetry of the hexagonal lattice and geometric considerations, hence the method is independent of the exact model that is chosen to describe the electronic band structure or the phonon dispersion. We finally clarify the diameter dependence of this Raman mode that was controversely discussed in the past and demonstrate that, depending on the experimental conditions, in general two different dependencies can be measured. We also prove that carbon nanotubes with arbitrary chiral index can exhibit a $D$ mode in their Raman spectrum, in contrast to previous symmetry-based arguments. Furthermore, we give a direct quantification of the curvature-induced phonon frequency corrections of the $D$-mode in carbon nanotubes with respect to graphite.