Spectroscopy and Spectroelectrochemistry of Carbon Nanotubes

Raman spectroscopy can detect changes in the C-C bond length, since the RBM varies with diameter and the G band varies with the axial C-C bond length. Moreover, the band intensity also varies as charge transfer occurs, either to or from the nanotubes. Gupta et al [84] monitored the dependence of the C-C bond length in an SWCNT material on charge transfer, for several alkali, halide, and sulfate 

SWCNT bundles containing nanotubes with different diameters are easy to obtain experimentally, but such samples may lead to overlapping RBM modes and complex Raman spectra that are difficult to interpret. In contrast, SWCNTs synthesized via the HiPCO method are favored for spectroelectrochemical studies due to their small diameters, which lead to well-separated RBM peaks, thereby simplifying nanotube chirality assignments significantly [58, 72]. 

The doping-induced changes in the intensity of the Raman signal of carbon nanotubes also have important consequences, that is, they can complicate the quantification of the amount ofthe particular SWCNT or the quantification ofthe defect density in SWCNTs, since the D mode intensity is also strongly affected by doping [86, 87]. 

The doping of carbon nanotubes also causes variations of the frequencies of the Raman features. These doping-induced changes are only subtle in SWCNTs in the case of RBM bands [74], but they are significant for the G and G modes [69, 73, 

The frequency variation of the G mode is rationalized by changes of the C—C bond strength and also changes in the phonon renormalization energy [71]. In the case of nanotube bundles, the changes in the G mode frequency of individual nanotubes contribute to the change of the G band lineshape of nanotube bundles [69, 

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