Is the Chain of Oxidation and Reduction Process Reversible in Luminescent Graphene Quantum Dots?

Graphene‐based quantum dots (QDs) have received a tremendous amount of attention as a new type of light‐emitting materials. However, their luminescence origins remain controversial due to extrinsic states of the impurities and disorder structures. Especially, the function of oxygen‐contents should be understood and controlled as a crucial element for tuning the optical properties of graphene‐based QDs. Herein, a series of graphene oxide QDs (GOQDs) with different amounts of oxygen‐contents are first synthesized via a direct oxidation route of graphite nanoparticle and thoroughly compared with a series of reduced GOQDs (rGOQDs) prepared by the conventional chemical reduction. Irreversible emission and different carrier dynamics are observed between the GOQDs and rGOQDs, although both routes show a similar tendency with regard to the variation of oxygen‐functional components. Their luminescence mechanisms are closely associated with different atomic structures. The mechanism for the rGOQDs can be associated with a formation of small sp 2 nanodomains as luminescent centers, whereas those of GOQDs may be composed of oxygen‐islands with difference sizes depending on oxidation conditions surrounded by a large area of sp 2 bonding. Important insights for understanding the optical properties of graphene‐based QDs and how they are affected by oxygen‐functional groups are shown.