Fundamental Optical Processes in Armchair Carbon Nanotubes

TL;DR

This study uses advanced separation and spectroscopic techniques to investigate the optical properties of armchair carbon nanotubes, revealing their strong excitonic absorption and clarifying the origins of specific Raman features.

Contribution

It demonstrates that density gradient ultracentrifugation effectively enriches armchair nanotubes and characterizes their excitonic optical absorption properties.

Findings

Density gradient ultracentrifugation enriches armchair nanotubes.

Armchair nanotubes exhibit strong excitonic interband absorption.

The broad G^{-} Raman feature arises from non-armchair metallic nanotubes.

Abstract

We have used post-synthesis separation methods based on density gradient ultracentrifugation and DNA-based ion-exchange chromatography to produce aqueous suspensions strongly enriched in armchair nanotubes for spectroscopic studies. Through resonant Raman spectroscopy of the radial breathing mode phonons, we provide macroscopic and unambiguous evidence that density gradient ultracentrifugation can enrich armchair nanotubes. Furthermore, using conventional, optical absorption spectroscopy in the near-infrared and visible range, we show that interband absorption in armchair nanotubes is strongly excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine that observation of the broad, lower frequency (G^{-}) feature is a result of resonance with non-armchair "metallic" nanotubes. These findings regarding the fundamental optical absorption and scattering processes in metallic carbon nanotubes lay the foundation for further spectroscopic studies to probe many-body physical phenomena in one dimension.