Raman spectroscopy of double-walled carbon nanotubes treated with ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$

In this work, we performed a detailed study of the Raman spectra of double-wall carbon nanotube (DWNT) bucky paper samples. The effects of ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ doping on the electronic and vibrational properties of the DWNTs are analyzed and compared to the corresponding effects on single-wall carbon nanotubes (SWNTs). Analysis of the radial breathing mode (RBM) Raman spectra indicates that the resonance condition for the outer wall nanotubes and the SWNTs are almost the same, indicating that the effect of the inner-outer wall interaction on the transition energies of the outer walls is weak compared to the width of the resonance window for the RBM peaks. The effect of ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ on the RBM frequencies of the outer wall of the DWNTs is stronger for larger diameter nanotubes. In the case of the inner walls, only the metallic nanotubes were affected by the acid treatment, while the RBM peaks for the inner semiconducting nanotubes remained almost unchanged in both frequency and intensity. The $G^{+}$ band was seen to upshift in frequency with ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ doping for both DWNTs and SWNTs. However, the effect of the acid treatment on the $G^{-}$ band frequency for DWNTs was opposite to that of SWNTs in the $2.05–2.15\mathrm{eV}$ range, for which the acid treatment causes a ${\omega }_{G^{-}}$ upshift for SWNTs and a downshift for DWNTs. The $G^{\prime }$ band line shape of the DWNTs is explained in terms of four contributions from different components which are in resonance with the laser excitation. Two of these peaks are more related to the inner wall nanotube while the other two are more related to the outer wall.

Figure 1

(Color online) Kataura plot based on the ETB model (Ref. 11) in the region of transition energies and RBM frequencies for the SWNTs and DWNTs in our samples. The horizontal lines show the laser excitation energies $\left(E_{\text{laser}}\right)$ used in the experiments. The numbers indicate the $2n+m$ families. The black and gray solid lines, respectively, refer to undoped and hole doped nanotubes (0.04 holes/carbon atom) (Ref. 7), as do the closed and open symbols, respectively.

Figure 2

(Color online) RBM spectra for DWNTS for 1.916, 2.052, 2.105, 2.134, 2.186, 2.330, and $2.540\mathrm{eV}$ excitation energies. Solid black (dashed red) lines are for undoped (${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ doped) DWNT samples.

Figure 3

(Color online) (a) $G$ band spectral line shapes for pristine (solid lines) and ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ treated DWNTs (dashed lines). The four best fit Lorentzians for the pristine sample are shown as thin dark lines for $E_{\text{laser}}=2.540\mathrm{eV}$. (b) Best fit frequencies for the $G$ band peaks as a function of the excitation energy for the pristine DWNT samples. Squares and circles are for the $G^{+}$ and $G^{-}$ bands in DWNTs, respectively. For comparison, the stars show the frequencies of these bands obtained for SWNTs. Solid and open symbols are for outer and inner tubes, respectively.

Figure 4

(Color online) Frequency shift with ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ doping of the $G$ band components for (top) SWNTs and (bottom) DWNTs for different laser lines. The dashed lines in each figure are intended to highlight the behavior of nanotubes in family 35. Circles are for the $G_S^{-}$ peak and squares are for the $G_S^{+}$ peak. For DWNTs, solid symbols are for the outer tubes while the open symbols are for the inner wall nanotubes.

Figure 5

(Color online) (a) $G^{\prime }$ band line shape for different laser excitation energies. (b) Frequencies for the different peaks composing the $G^{\prime }$ band and for the $\sim 2450{\mathrm{cm}}^{-1}$ peak corresponding to the $\mathrm{LA}+\mathrm{TO}$ combination mode. The lines show a linear fit to the dependence with excitation energy. The stars (blue online) show the frequencies obtained for SWNTs. The inset shows the $G^{\prime }$ band line shape for $E_{\text{laser}}=2.186\mathrm{eV}$ with the five Lorentzians used to fit it.

Figure 6

(Color online) (a) Ratio between the lower ($G_1^{\prime }$ and $G_2^{\prime }$) and higher ($G_3^{\prime }$ and $G_4^{\prime }$) frequency contributions to the $G^{\prime }$ band line shape in DWNTs as a function of excitation laser energy. Squares are for pristine samples and rhombuses are for acid treated samples.