Raman spectroscopy of double-walled carbon nanotubes treated with H2SO4

In this work, we performed a detailed study of the Raman spectra of double-wall carbon nanotube (DWNT) bucky paper samples. The effects of H2SO4 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 H2SO4 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 H2SO4 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 eV range, for which the acid treatment causes a omega(-)(G) upshift for SWNTs and a downshift for DWNTs. The G(') 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.