Synthesis of graphene oxide dots for excitation-wavelength independent photoluminescence at high quantum yields

This study reports on the synthesis of graphene oxide dots (GODs, 2.5 +/- 0.5 nm) exhibiting excitation-wavelength independent photoluminescence (PL) at 530 nm. The GODs, which are of high uniformity and crystallinity, are produced by mildly oxidizing thermally-reduced GO sheets under sonication. The GOD aqueous suspension yields a maximal PL quantum yield (QY) of 16% under excitation at 470 nm. This PL can be ascribed to the irradiative excitation of electrons from the non-bonding oxygen (n) states to the graphene anti-bonding pi orbital with subsequent relaxation of the electrons to the n ground states. Nitrogen-doping reduces vacancy defects and donates electrons to compensate for the unbalanced charge in p-type GODs, thereby increasing the PL QY to 22% for the nitrogen-doped GODs (NGODs). Treating the unadorned GODs and NGODs with submerged liquid plasma in tetrahydrofuran suppresses charge leakage from the carbonyl groups on the graphene periphery and increases the QYs to 42% and 50%, respectively. The GODs could be used as phosphors for the generation of white light by combining green emissions (530 nm) with violet light used for excitation. The present study demonstrates facile synthesis of high-quality green-emitting GODs and an effective method for the repair of vacancy defects and the stabilization of oxygen functionalities to enhance PL emission from GODs.