Unusual High Degree of Unperturbed Environment in the Interior of Single-Wall Carbon Nanotubes

Double wall carbon nanotubes were prepared by vacuum annealing of single wall carbon nanotubes filled with ${\mathrm{C}}_{60}$. Strong evidence is provided for a highly defect free and unperturbed environment in the interior of the tubes. This is concluded from unusual narrow Raman lines for the radial breathing mode of the inner tubes. Lorentzian linewidths scale down to $0.35{\mathrm{c}\mathrm{m}}^{-1}$ which is almost $10$ times smaller than linewidths reported so far for this mode. A splitting is observed for the majority of the Raman lines. It is considered to originate from tube-tube interaction between one inner tube and several different outer tubes. The highest RBM frequency detected is $484\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$ corresponding to a tube diameter of only $0.50\mathrm{n}\mathrm{m}$. Labeling of the Raman lines with the folding vector is provided for all inner tubes. This labeling is supported by density functional calculations.