Theory of the tangential G-band feature in the Raman spectra of metallic carbon nanotubes

TL;DR

This paper presents a theoretical analysis of the G-band features in Raman spectra of metallic carbon nanotubes, explaining the observed Lorentzian and BWF lineshapes through phonon-plasmon coupling, and matching experimental trends.

Contribution

It introduces a model based on optical plasmon coupling to explain the G-band spectral features in metallic nanotubes, challenging previous semi-acoustic plasmon explanations.

Findings

The optical plasmon explains both Lorentzian and BWF lineshapes.

The BWF peak shifts with nanotube diameter, matching experiments.

The theory refutes the role of gapless semi-acoustic plasmon.

Abstract

The tangential G-band in the Raman spectra of a metallic single-wall carbon nanotube shows two peaks: a higher frequency component having the Lorentzian shape and a lower-frequency component of lower intensity with a Breit-Wigner-Fano (BWF)-type lineshape. This interesting feature has been analyzed on the basis of phonon-plasmon coupling in a nanotube. It is shown that while the gapless semi-acoustic plasmon cannot account for the observed spectra as claimed by other investigators, the low-lying optical plasmon corresponding to the tangential motion of the electrons on the nanotube surface can explain the observed features. In particular, this theory can explain occurrence of both the Lorentzian and BWF lineshapes in the G-band Raman spectra of metallic single-wall carbon nanotubes. Furthermore, the theory shows that the BWF peak moves to higher frequency, has a lower intensity and a lower half width at higher diameters of the nanotube. All these features are in agreement with experimental observations.