Role of functionalization in the fluorescence quantum yield of graphene quantum dots

As graphene is sculptured into quantum dots, quantum confinement and edge effects induce a finite energy gap and trigger exotic photoluminescent behavior. However, synthesis of graphene quantum dots (GQDs) with high fluorescence quantum yield and designated emission color remains challenging, due to a lack of knowledge in the exact influences of various structural and chemical factors. Herein, we explore the optical absorption and emission in GQDs with surface functionalization, heteroatom doping, or edge modification. Their fluorescence spectra are systematically compared by time-dependent density functional theory calculations. It shows that the sp(3)-type surface functionalization by O, OH, or F groups can effectively increase the fluorescence intensity by five orders of magnitude with regard to pristine GQDs, ascribed to the localization of excited carriers that enlarges the transition dipole moment for radiative decay. The functional groups also play a key role in fluorescent sensing of toxic metal species with high selectivity and sensitivity.