Jorio and Saito, in 2021, are giving a review (open access) about Raman spectroscopy for carbon nanotube applications. "For the specific case of single-wall carbon nanotubes (SWNTs) and also other graphenerelated materials, RS [Raman Spectroscopy] has an extra aspect that makes it even more powerful, which is the highly efficient and selective resonance effects. The resonance effect makes it possible to measure one isolated carbon nanotube by RS and makes it possible to obtain information from the vibrational properties that are usually Raman inactive. Accessing phonons that are Raman inactive (e.g., phonons in the interior of the Brillouin zone) is important to characterize several static and dynamic properties" (Jorio and Saito, and references therein). Differently from the RBM, which is a feature of SWNT only, the G and G' (or 2D) bands are observed in all carbon structures. "What makes these features special for applications is twofold": 1) the high frequencies of the bands due to the small mass of the carbon atoms and 2) metallic structures exhibiting "a strong electron-phonon coupling for a phonon, enabling detailed sensing of the Fermi level position via Raman spectroscopy". The two effects are described by Jorio and Saito. In Fig. (a), Jorio and Saito "show the G-band spectra for graphite (HOPG, top), semiconducting (middle), and metallic (bottom) SWNTs. Although the C-C bond stretching in graphene or graphite exhibits a single Lorentzian peak, SWNTs have two split peaks. This happens because in the flat hexagonal lattice of graphite, the three C-C bond stretching vibration for a carbon atom is isotropic. However, when you impose a curvature in the sp 2 network to form a nanotube, symmetry breaking occurs by a curvatureinduced strain in the direction perpendicular to the tube axis, which lowers the phonon frequency of the C-C vibrations which are preferentially in the circumferential direction. On the other hand, the C-C vibrations in the direction parallel to the nanotube axis do not show the frequency shift" . Consequently, a frequency splitting is induced by the curvature of the layer. "In the case of semiconductor SWNTs, ω + G and ω - G correspond, respectively, to C-C bond stretching in the direction parallel (LO) and perpendicular (iTO) to the nanotube axis, when we consider the phonon propagating direction in the nanotube axis. However, in the case of metallic SWNTs, the assignment of ω + G and ω - G becomes opposite [(iTO) and (LO)] to that for semiconductor nanotubes. This inversion happens because the LO mode becomes soft by the Kohn anomaly effect" (Jorio and Saito, 2021), due to electron-phonon coupling. "This electron-phonon coupling can be tuned by doping a gate electrode on a device, and the phonon-softening phenomenon of the G band is an evidence for metallic SWNT, … Therefore, the analysis of the G-band line shape of metallic SWNTs, … can be used to monitor the Fermi level". See please also the Figure in Jorio and Saito, which is showing how the spectrum changes when the investigated location in the sample is changed.