Raman spectroscopy serves as a powerful, non-destructive analytical tool for characterizing carbon nanomaterials, offering insights into their structural, electronic, and vibrational properties. The technique is particularly valuable for graphene, carbon nanotubes (CNTs), and related materials due to its sensitivity to sp² hybridization, defects, layer thickness, and doping effects. Key spectral features include the G-band, D-band, and 2D-band, each providing distinct information about the material's quality and properties.

The G-band, appearing around 1580 cm⁻¹, arises from the in-plane vibrational mode of sp²-bonded carbon atoms. It is a doubly degenerate (LO and TO) phonon mode at the Brillouin zone center (Γ-point) and is present in all graphitic materials. The G-band's position, intensity, and linewidth reflect the degree of crystallinity, strain, and doping. For monolayer graphene, the G-band is sharp and symmetric, while in bilayer or few-layer graphene, it may broaden or split due to interlayer interactions. In metallic CNTs, the G-band splits into G⁺ and G⁻ peaks due to curvature-induced symmetry breaking, with the G⁻ peak exhibiting a Breit-Wigner-Fano line shape from electron-phonon coupling.

The D-band, near 1350 cm⁻¹, is a disorder-induced feature activated by defects, edges, or structural imperfections that break translational symmetry. It involves an intervalley double-resonant Raman process, where a defect scatters an electron from the K to K' point in the Brillouin zone, coupling with a phonon of momentum q ≈ K. The D-band intensity relative to the G-band (I_D/I_G ratio) quantifies defect density. For graphene, an increase in I_D/I_G initially correlates with defect concentration but eventually decreases at very high defect densities due to amorphization. In CNTs, the D-band serves as a proxy for sidewall defects or functionalization.

The 2D-band (or G'-band), centered around 2700 cm⁻¹, is a second-order process involving two phonons with opposite momentum. Unlike the D-band, it does not require defects and is intrinsic to graphitic materials. In monolayer graphene, the 2D-band is a single sharp Lorentzian peak, while in bilayer graphene, it evolves into a four-component structure due to interlayer coupling. The 2D-band's shape, position, and intensity ratio to the G-band (I_2D/I_G) are critical for determining layer count and stacking order. For example, I_2D/I_G > 2 typically indicates monolayer graphene, while ratios below 1 suggest multilayer systems.

Raman spectroscopy also detects doping effects in carbon nanomaterials. Electron or hole doping shifts the G-band position due to changes in the Fermi level and electron-phonon coupling. In graphene, n-doping upshifts the G-band, while p-doping downshifts it. The 2D-band is less sensitive to doping but may exhibit broadening or shifts at high carrier concentrations. For CNTs, the radial breathing mode (RBM) between 100–300 cm⁻¹ provides additional information about tube diameter and metallicity.

Analysis protocols for quality assessment involve systematic evaluation of spectral parameters. For graphene, the absence of a D-band indicates high crystallinity, while the 2D-band shape confirms layer uniformity. In CNTs, the RBM, G-band, and D-band collectively assess diameter distribution, metallic/semiconducting ratio, and defect density. For doped systems, combined analysis of G-band shifts and the width of the 2D-band helps quantify carrier concentration. Electrical properties correlate with Raman features; for instance, high I_D/I_G ratios in graphene correspond to reduced carrier mobility due to defect scattering, while G-band splitting in CNTs distinguishes metallic from semiconducting behavior.

Contrasting with other carbon forms in the taxonomy, Raman spectra vary significantly. Graphene oxide (G28) exhibits a prominent D-band due to oxygenated defects and a broadened G-band from sp³ hybridization. Reduced graphene oxide shows partial recovery of the 2D-band as sp² domains are restored. Carbon quantum dots (G27) display weak or absent 2D-bands but may show fluorescence overlapping with Raman signals. Diamond-like carbon films (G30) lack the 2D-band entirely, with a broadened G-band and possible T-band (~1050 cm⁻¹) from sp³ content. Carbon nanohorns (G32) exhibit spectra resembling CNTs but with additional disorder-induced features. Graphitic carbon nitride (G33) shows distinct peaks around 700–800 cm⁻¹ from triazine ring vibrations, absent in graphene or CNTs.

In summary, Raman spectroscopy provides a comprehensive fingerprint of carbon nanomaterials, enabling defect quantification, layer analysis, and doping assessment. The technique's versatility makes it indispensable for quality control and property optimization across diverse applications, from electronics to energy storage. By correlating spectral features with material properties, researchers can tailor synthesis and processing conditions to achieve desired performance metrics.