Polarized Raman analysis of aligned double-walled carbon nanotubes

Polarized Raman spectra were obtained from a rope of aligned double-walled carbon nanotubes (DWNTs). The DWNTs exhibit a similar polarization dependence to individual semiconducting single-walled carbon nanotubes and a stronger depolarization effect than multiwalled carbon nanotubes attributed to their fewer shells. Based on group theory analysis and related theoretical predictions, the mode symmetry was identified and consequently we found that the $G$-band profile consisted of two groups of well-resolved modes related to two constituent tubes of DWNTs. The two constituent tubes generally retain their original mode symmetries and vibrations, suggestive of a weak interlayer interaction.

Figure 1

(a) Photo image of the DWNT rope used in the polarized Raman study, where the symmetry axes are indicated. The inset gives the high-magnification image and the position of laser spot (cross). (b), (c), and (d) HRTEM images of a DWNT bundle, an isolated DWNT, and two parallel DWNTs in the sample, respectively. (e) and (f) RBM and normalized $G$-band spectra of three different samples with similar outer diameters: S1 (black line), S2 (grey line), and S3 (dashed line).

Figure 2

$G$-band Raman spectra of an aligned DWNT bundle in the $VV$ (a) and $VH$ (b) polarization geometries, respectively, at various angles $\theta$ (from the top to the bottom, 0°, 15°, 30°, 45°, 60°, 75°, 90°). The insets in (a) and (b) give the Raman intensity dependence of the $135{\mathrm{cm}}^{-1}$ (filled circles) and $218{\mathrm{cm}}^{-1}$ (open circles) peaks on $\theta$ in the $VV$ and $VH$ polarization geometries.

Figure 3

Line shape analysis for the four different polarized spectra of the DWNT bundles. The frequencies (widths) of the modes in the Figure are displayed in ${\mathrm{cm}}^{-1}$.

Figure 4

Raman intensity dependence of the $1556{\mathrm{cm}}^{-1}$ (filled circles) and $1603{\mathrm{cm}}^{-1}$ (open circles) modes of the DWNT bundles for $VV$ polarization geometry on the measured angle $\theta$.