A Raman study to obtain crystallite size of carbon materials: A better alternative to the Tuinstra–Koenig law

By varying excitation energy from ultraviolet to infrared, Raman spectra obtained from both cokes and pyrocarbons revealed the evolution of each band. While the spectra of different samples can be very similar at given wavelength, the wavelength dependence should be considered for correlating spectral features to crystallite size La. The D′ band was found to vanish in the UV range. The D band was fitted with one or two Lorentzians for crystallite sizes larger than ∼5 nm or below respectively, both centered on the same wavenumber. Both the D band intensity and integrated intensity were accurately obtained, and used to question the range of use of the Tuinstra–Koenig law. The G band shape is well fitted and its width increases monotonously with 2 < La < 10 nm. The energy dependence of ID/IG was found to vary with the excitation energy EL   as EL-b with 1 < b < 4 depending on both La and the sample type. We question the validity of the empirical laws from the literature for having been obtained on a limited sampling and wrongly considered to remain valid over the full ranges of both La and wavelengths. Considering the G band width instead is emphasized.