Fabrication and optical properties of sulfur- and nitrogen-doped graphene quantum dots by the microwave–hydrothermal approach

Presented in this manuscript are our new research results regarding the fabrication of nitrogen-doped (N-doped) and sulfur-doped (S-doped) graphene quantum dots (GQDs) from citric acid (CA), using the microwave–hydrothermal method. This is a method that uses microwave energy for pyrolysis and carbonization of the precursors of carbon and doped element, followed by a hydrothermal process in NaOH, to yield doped GQDs. Studies using high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy have allowed the confirmation of the fabricated products to be GQDs. Detailed studies of absorption, photoluminescence (PL), PL excitation (PLE) spectra, emission–excitation mapping (EEM) images, and PL spectra at different excitation wavelengths have allowed us to find the best excitation wavelength to obtain the highest PL quantum yield (QY) value from the sample, an example being the QY of the S-GQD samples reaching 40.1%. The absorption mechanism, vibration relaxation process, and non-radiative and radiative electron transition are also explained. Graphical abstract