Resonant Raman Spectroscopy of Armchair Carbon Nanotubes: Absence of "Metallic" Signature

TL;DR

This study uses resonant Raman spectroscopy on enriched armchair carbon nanotubes to show that the broad G- peak, previously linked to metallic nanotubes, is absent in armchair structures, clarifying the structure-peak relationship.

Contribution

It provides direct experimental evidence that the broad G- peak is not present in armchair nanotubes, challenging previous assumptions about its association with metallicity.

Findings

Broad G- peak absent in armchair nanotubes

G- peak appears only when resonant with non-armchair metals

Strong correlation between G-band lineshape and nanotube structure

Abstract

The appearance of a broad peak at $\sim$1550 cm$^{-1}$ (G$^{-}$ peak) in carbon nanotube resonant Raman scattering spectra has been conventionally attributed to the presence of metallic nanotubes. Here, we present resonant Raman measurements on macroscopic nanotube ensembles enriched in armchair species prepared through density gradient ultracentrifugation. Our data clearly demonstrate that the broad G$^{-}$ mode is absent for armchair structures and appears only when the excitation laser is resonant with non-armchair "metals." Due to the large number ($\sim$10$^{10}$) of nanotubes across several armchair species probed, our work firmly establishes a general correlation between G-band lineshape and nanotube structure.