Behavior of Raman D band for pyrocarbons with crystallite size in the 2-5 nm range
Résumé
The pyrocarbon materials investigated here are examples of disordered graphene-based carbons whose crystallite size $L_a$ ranges from 2 to 5 nm. This $L_a$ size range is between two different Raman behaviors, one for which the D band broadens (for $L_a$< 2 nm) and the other for which the D band sharpens (for $L_a$ > 5 nm) respectively, with increasing $L_a$ . To fully understand the nature of the G band signal, we checked its wavelength behavior from UV to near IR. We demonstrated that the Raman spectrum is well fitted with simply two Lorentzians with various respective contributions centered at the wavenumber of the D band and a Breit-Wigner-Fano shape for the G band. Each intensity contribution for the D band varies linearly with $L_a$ between 2 and 5 nm while the total D band intensity is nearly constant for excitation wavelengths ranging from 0.532 to 0.638 μm. In contrast, the integrated intensity ratio D/G follows the well-known $L_a^{-1}$ law. The two subbands building the D band are related to the lifetime of the electrons involved in the double resonance process which can be scattered twice. Their respective occurrences therefore depend on the crystallite size $L_a$, when below ≈ 5 nm.