The fundamental mechanisms responsible of the fluorescence capability of CQDs are very debated. Some authors have provided evidence of size-dependent fluorescence properties, suggesting that the emission arises from electronic transitions with the core of the dots, influenced by quantum confinement effects, whereas other works have rather attributed the fluorescence to recombination of surface-trapped charges or proposed a form of coupling between core and surface electronic states. The excitation-dependent fluorescence of CQDs, leading to their characteristic emission tunability, has been mostly linked to the inhomogeneous distribution of their emission characteristics, due to polydispersity, although some works have explained it as a violation of Kasha's rule arising from an unusually slow solvent relaxation.
