The Raman response of double wall carbon nanotubes

TL;DR

This paper reviews the properties and production methods of double wall carbon nanotubes (DWCNTs), highlighting their high purity, detailed vibrational analysis via Raman spectroscopy, and potential for industrial scaling.

Contribution

It introduces a high-purity, catalyst-free method for producing DWCNTs from peapods and provides detailed vibrational mode analysis for precise structural characterization.

Findings

Inner tubes are grown with high perfection and narrow RBM lines.

Splitting of RBMs indicates interaction between shells.

Reliable assignment of (n,m) indexes to RBMs enables precise diameter determination.

Abstract

Raman spectroscopy on carbon nanotubes (CNT) yields a rich variety of information owing to the close interplay between electronic and vibrational properties. In this paper, we review the properties of double wall carbon nanotubes (DWCNTs). In particular, it is shown that SWCNT encapsulating C$_{60}$, so-called peapods, are transformed into DWCNTs when subject to a high temperature treatment. The inner tubes are grown in a catalyst free environment and do not suffer from impurities or defects that are usually encountered for as-grown SWCNTs or DWCNTs. As a consequence, the inner tubes are grown with a high degree of perfection as deduced from the unusually narrow radial breathing mode (RBM) lines. This apostrophizes the interior of the SWCNTs as a nano-clean room. The mechanism of the inner nanotube production from C$_{60}$ is discussed. We also report recent studies aimed at the simplification and industrial scaling up of the DWCNT production process utilizing a low temperature peapod synthesis method. A splitting of the RBMs of inner tubes is observed. This is related to the interaction between the two shells of the DWCNTs as the same inner tube type can be encapsulated in different outer ones. The sharp appearance of the inner tube RBMs allows a reliable assignment of the tube modes to (n,m) indexes and thus provides a precise determination of the relation between the tube diameter and the RBM frequencies.