Size-dependence of Raman scattering from graphene quantum dots: Interplay between shape and thickness

Raman-scattering behaviors have been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The peak frequencies of D and 2D bands are almost irrespective of d, and the intensity of the D band is larger than that of the G band over almost full range of d. These results suggest that GQDs are defective, possibly resulting from the dominant contributions from the edge states at the periphery of GQDs. The G band shows a maximum peak frequency at d = ∼17 nm, whilst the full-width half maximum of the G band and the peak-intensity ratio of the D to G bands are minimized at d = ∼17 nm. Since the average thickness of GQDs (t) is proportional to d, t can act as a factor affecting the d-dependent Raman-scattering behaviors, but they cannot be explained solely by the t variation. We propose that the abrupt changes in the Raman-scattering behaviors of GQDs at d = ∼17 nm originate from size-dependent edge-state variation of GQDs at d = ∼17 nm as d increases.