Chemical nature of redox-controlled photoluminescence of graphene quantum dots by post-synthesis treatment

Abstract

Graphene quantum dots (GQDs) have attracted considerable attention because of their unique photoluminescence (PL) properties. Nowadays, several approaches have been reported to improve PL quantum yield (PLQY) and regulate PL colors for their different applications. However, most reports show that higher oxygen content leads to lower PLQY. Here, we report a novel approach to enhance PLQY at high oxygen content. Both oxidation and reduction of GQDs have been demonstrated to improve the PLQY of GQDs and control their PL colors after they were first prepared. The oxidation treatment using hydrogen peroxide (H2O2) and ultraviolet (UV) light, enhanced the PLQY of GQDs from 1.51 ± 0.02% to 3.99 ± 0.02%, and the PL color could also be tuned from green to yellow-green under UV irradiation. When the UV light was removed, reduction reaction occurred immediately, which further improved the PLQY to 10.37 ± 0.01% and changed the PL color to blue. Since there was no heteroatom introduced and the GQDs maintained their original size and concentration, these treated GQDs allow us to combine the detailed structural and optical studies to testify the chemical nature of the observed PL: the PL originated from different surface states and the specific hybridization of states from the surface functional groups and the connected graphene core is responsible to specific PL colors.