Electronic and optical properties of sulfur and nitrogen doped graphene quantum dots: A theoretical study

Abstract

Here we present a theoretical study based on density functional theory (DFT) of the electronic and optical properties of graphene quantum dots (GQDs) doped with sulfur and nitrogen atoms. Unlike the actual literature, which focusses mostly in the edge functionalization of GQDs, our report focusses on the effect of sulfur and nitrogen atomic dopants positioned at the center and near the edge of the GQD carbon backbone structure. According to formation energies, we evidence that doping is more favorable for nitrogen than sulfur, and that the near-edge doping is more favorable than center doping. Regarding the electronic properties, there is a clear shift to lower wavelength in the UV–Vis spectra when using the CAM-B3LYP exchange correlation functional, with respect to B3LYP functional for all the structures, showing a better concordance with experimental data. As a general trend, the major transition observed in the nitrogen doped system is situated at higher wavelength, while in the case of the sulfur doped system, the transition is located at lower wavelength when comparing with the undoped GQDs. This observation is also in good agreement with the experimental trend observed for nitrogen and sulfur-doped GQDs for which a redshift and blueshift in the UV–Vis absorption is typically observed respect to the undoped case, respectively. All of this justify the selected methodology for describing the electronic and optical properties of doped GQDs and provide additional insights to correlate with the optical properties observed experimentally.