Graphene quantum dots (GQDs) containing N atoms were successfully synthesized using a facile, inexpensive, and environmentally friendly hydrothermal reaction of urea and citric acid, and the effect of the GQDs’ C–N configurations on their photoluminescence (PL) properties were investigated. High-resolution transmission electron microscopy (HR-TEM) images confirmed that the dots were spherical, with an average diameter of 2.17 nm. X-ray photoelectron spectroscopy (XPS) analysis indicated that the C–N configurations of the GQDs substantially affected their PL intensity. Increased PL intensity was obtained in areas with greater percentages of pyridinic-N and lower percentages of pyrrolic-N. This enhanced PL was attributed to delocalized π electrons from pyridinic-N contributing to the C system of the GQDs. On the basis of energy electron loss spectroscopy (EELS) and UV-Vis spectroscopy analyses, we propose a PL mechanism for hydrothermally synthesized GQDs.

Pyrrolic-N-rich carbon dots (CDs) that exhibit an absorption peak in the first near-infrared (NIR) window region were developed using a one-step microwave-assisted hydrothermal synthesis. A high concentration of urea enabled the introduction of a large amount of pyrrolic-nitrogen on the CD surfaces. Upon optimization of the experimental conditions, the absorption peak of the CDs red-shifted from 550 to 650 nm. The resulting pyrrolic-N-rich CDs exhibited photothermal effects with high NIR photothermal efficiency (54.3%) and photoluminescence. The prepared CDs, which show a first NIR window absorption peak, photoluminescence, and negative surface charge, have the potential to be used as multifunctional nanocarriers for cell imaging and drug delivery and as photothermal agents in cancer therapy.

Synthesized transparent CQD–PVA composite films performed stable UV-A blocking, even after exposure to UV light for several days and elevated temperature.

How fast is the reaction of N-doped C-dot formation?