Influence on the Optical and Electronic Properties of Graphene Quantum Dots Originating from the S-Doping Site: A Theoretical Investigation

The optical and electronic properties of sulfur-doped graphene quantum dots (GQDs) have been theoretically investigated using DFT/TD-DFT methods in water (B3LYP/6-31G/SMD). Both edge and core sites for doping have been considered. Simulation results reveal that edge-doped sulfydryl (-SH) presents no significant influence on the energy level distribution compring with the pristine GQDs which results in idential absorption spectra, however, the nonzero oscillator strength (f) of S₀-S₁ transition of edge-doped S-GQDs enables fluorescence emission (@∼450 nm) whereas the GQDs yield no fluorescence emission based on Kasha's rule. Moreover, the S doping of GQDs at the core sites dramatically raises the HOMO energy level and regulates the electron transfer process and the corresponding density of states during electron transition. This eventually decreases the HOMO-LUMO gaps, enabling strong fluorescence emission in the near-infrared region, which suggests that the S-GQDs at the skeleton sites have great potential for application in the bioimaging field.