Mechanisms behind excitation- and concentration-dependent multicolor photoluminescence in graphene quantum dots

Despite numerous efforts, the mechanism behind multicolor photoluminescence (PL) in graphene quantum dots (GQDs) is still controversial. A deep insight into the origin of the multicolor emissions in GQDs is quite necessary for modulating their luminescence to facilitate the better use of this fluorescent material. Herein, GQDs with amino, carboxyl, and ammonium carboxylate groups were synthesized. The as-prepared GQDs exhibited intriguing excitation- and concentration-dependent multicolor PL characteristics. By regulating the excitation wavelength or concentration of GQDs, specific luminescence colors including blue, cyan, green, yellow, and even orange can be obtained. Systematic structural and optical studies indicated that the graphene basal plane and different functional groups dominantly exhibited nN 2P-σ*, π-π*, nO 2p-π* (-COOH), nO 2p-π* (-COO-) and nN 2p-π* electronic transitions, which appeared as multi-fluorescent centers that gave rise to the excitation-dependent multicolor PL. The occurrence of different types of electronic transitions and their color emissions were proved by pH-dependent PL measurements. In addition, systematic optical and morphology analyses revealed that GQDs could self-assemble into J-type aggregates with different morphologies and sizes as the concentration increased, and the observed concentration-dependent multicolor PL can be ascribed to aggregation-mediated energy level reconstruction in GQDs. Our findings further suggest that the competition among various fluorescent centers and self-aggregation processes dominated the luminescent properties of GQDs. This work will contribute to understand the origins of excitation- and concentration-dependent multicolor emissions in GQDs, which is also highly instructive for broadening the application fields of GQDs.