摘要： Graphene quantum dots (GQDs) have attract broad interest during the last several years. One of the mysteries for GQDs is the excitation-dependent emission spectrum. In this work, we tried to reveal the fundamental understanding of this issue by investigating the spectroscopy of individual GQDs. We measured emission spectra on individual GQDs and white-light N-doped GQDs (IGQDs) prepared by electron beam irradiation, with different excitation lights at 488 nm, 532 nm, and 633 nm. We found narrow spectra for both individual GQDs and IGQDs compare to the ensemble spectra. The spectrum by summing up spectra of individual dots is almost consistent with ensemble spectrum in solution. More importantly, different dots were observed in the same sample with different excitation wavelengths indicating the heterogeneity of the GQDs structure. Together with the crystal structure characterization of the IGQDs, we can conclude that the luminescence is not from the band-to-band transition but from defect states. These results directly proved the heterogeneity of the GQDs materials revealing the mechanism of the spectroscopic mystery. The observed narrow emission spectrum of individual dots also shows their potential application for light emitting devices after purification.