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

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.