Giant Enhancement of Fluorescence Emission by Fluorination of Porous Graphene with High Defect Density and Subsequent Application as Fe3+ Ion Sensors.

Defect-mediated nonradiative recombination in traditional semiconductors, such as porous graphene, tremendously lowers the fluorescence emission, thus greatly restricting their applications in more extensive fields. Here, we report that the fluorescence emission of porous graphene with a high defect density has a giant enhancement (about two orders of magnitude) by a direct and simple fluorination strategy, showing a fine defect-tolerance characteristic. Meanwhile, the corresponding fluorocarbon bonds with excellent thermostability (over 500 °C in N2 even air) also bring about good stability. The photophysical origins during the whole photoluminescence evolution are further investigated. In the excitation process, the coexistence of fluorine and aromatic regions in fluorinated porous graphene (FPG) contributes to producing a new electronic band gap structure to match the maximum excitation wavelength, then numerous excitons generate, which is a precondition for strong fluorescence emission. In the emission process, weak electron-phonon interactions, large rigidity, and constrained electron at the defects in FPG greatly reduce nonradiative recombination loss. Moreover, fluorine at the defects also reduces interlayer interactions among FPG nanosheets and resists the influence of absorbed impurities, thereby further restricting nonradiative recombination pathway. Highly fluorescent FPG has been utilized as a fascinating tool to achieve sensitive and naked-eye detection of Fe3+ ions with a high selectivity. The fluorescence quenching efficiency reaches 24% even with an ultralow concentration of Fe3+ (0.06 μM), and that increases to 84% when the concentration of Fe3+ is 396 μM.