Protonation and sonication effects on aggregation sensitive Raman features of single wall carbon nanotubes

The G-band and the G′-band (∼2600–2700 cm−1) Raman spectra of single walled carbon nanotubes (SWCNTs) in a sodium dodecyl sulphate aqueous solution were investigated as a function of sonication energy density (ρ). The relative intensity of the G-band feature around ∼1520–1560 cm−1 (G−) and the linewidth (Γ) of the G′-band decrease monotonically with ρ where the former dependence is probably due to charge transfer (promoted by sonication induced pH decrease/solubilisation) and consequently not an unambiguous indication of solubilisation. In contrast Γ is shown to be independent of pH, which suggests that the linewidth of the G′-band may be an important indicator of the aggregation state of SWCNTs.