Size‐controlled graphene‐based materials prepared by annealing of pitch‐based cokes: G band phonon line broadening effects due to high pressure, crystallite size, and merging with D′ band
Résumé
The influence of high pressure in the range of 0–15 GPa on the linewidth and the position of the Raman G band is investigated for graphene‐based materials with controlled crystallite sizes, La, obtained from pitch‐based cokes by increasing annealing temperature. It is observed that the linewidth of the G band increases significantly above the critical pressure, Pc ≈ 10 GPa, for all samples, whereas at atmospheric pressure, the linewidth neither depends on variation in the measurement temperature (<400 K) nor on excitation wavelengths. The crystallite size (varied using different annealing temperatures) is found not to influence the critical pressure associated with the modification of carbon hybridization. At the same time, the G band linewidth increases significantly for smaller crystallite sizes depending on the annealing temperature at atmospheric pressure due to the merging of the D′ and G phonon bands. This merging of the bands can be explained by a single phonon involved in two physical Raman processes, which cannot be discriminated.