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.

Figure 1

Raman response of the radial breathing mode of double-wall carbon nanotubes, excited by different lasers as indicated (hr stands for high resolution). The top spectrum was recorded for 90 K, all other spectra for 20 K. The response is intensity calibrated.

Figure 2

List plot of observed Raman lines for the radial breathing mode (RBM) of the inner tubes. Lasers used for excitation are noted on the right-hand side in eV. Heavy and light circles indicate the strongest and the second strongest peaks in the spectrum. Full and open squares represent the geometrically allowed semiconducting and metallic tubes, respectively. The numbers are the components of the folding vector.

Figure 3

Assignment of the observed Raman lines to the folding vectors $(n,m)$. M indicates metallic tubes. The horizontal bars describe the width of the splitting due to intertube interaction. Seven lasers were selected for the presentation as indicated. Intensities for the different lasers are not drawn to scale in this figure. The resolution of the spectrum recorded with 1064 nm (Fourier-transform–Raman) was only $2{\mathrm{c}\mathrm{m}}^{-1}$ and recording was at 110 K.

Figure 4

Density of states for a narrow metallic tube as calculated by ab initio and by tight binding. In the former case the structure between the two first Van Hove singularities can provide visibility to the Raman lines, even for laser energies below the $E_{11}^{\mathrm{m}}$ gap.