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

Abstract

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 sp2 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 sp2 bonding. Important insights for understanding the optical properties of graphene-based QDs and how they are affected by oxygen-functional groups are shown. Graphene oxide quantum dots (GOQDs) are first synthesized without a reduction process. These GOQDs show clear multicolor emissions depending on the number of oxygen functional groups under single-wavelength excitation. Also, it is demonstrated that the optical properties of reduced GOQDs do not match those of GOQDs, corresponding to irreversible optical origins in oxidation and reduction. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim