Linearly controlled arrangement of $^{13}$C isotopes in single-wall carbon nanotubes

TL;DR

This paper demonstrates a catalyst-free method to grow inner carbon nanotubes with controlled $^{13}$C isotope arrangements, revealing isotope clustering and enabling potential atomic-level engineering of nanotube composition.

Contribution

It introduces a novel growth process for inner SWCNTs with controlled isotope distribution, supported by Raman spectroscopy and semi-empirical calculations.

Findings

Inner nanotubes grown from $^{13}$C labeled toluene show isotope clustering.

Raman spectra reveal a tail indicating non-homogeneous isotope distribution.

No carbon diffusion occurs during inner tube growth.

Abstract

The growth of single wall carbon nanotubes (SWCNT) inside host SWCNTs remains a compelling alternative to the conventional catalyst induced growth processes. It not only provides a catalyst free process but the ability to control the constituents of the inner tube if appropriate starting molecules are used. We report herein the growth of inner SWCNTs from $^{13}$C labeled toluene and natural carbon C$_{60}$. The latter molecule is essentially a stopper which acts to retain the smaller toluene. The Raman spectrum of the inner nanotubes is anomalous as it contains a highly isotope shifted "tail", which cannot be explained by assuming a homogeneous distribution of the isotopes. {\color{black}Semi-empirical} calculations of the Raman modes indicate that this unsual effect is explicable if small clusters of $^{13}$C are assumed. This indicates the absence of carbon diffusion during the inner tube growth. When combined with appropriate molecular recognition, this may enable a molecular engineering of the atomic and isotope composition of the inner tubes.