Synthesis and modification of pristine and nitrogen-doped carbon dots by combining template pyrolysis and oxidation

The template pyrolysis synthesis, as a bottom-up approach, and oxidation, as a top-down approach, were combined to produce pristine and nitrogen doped carbon dots. Varying the precursor type, synthesis temperature and oxidation time the color of carbon dots was tuned from green to orange because of the bandgap changes and formation of interband states. TEM, Raman, and XPS data revealed the graphene core of carbon dots with the shell of carboxyl and hydroxyl functionalities. The bulk and surface N-doping of the carbon dots affected their photoluminescence in a different way because of the opposite effect on the π-electron system: the core modification led to a blue shift, while the surface amination resulted in a red shift. A blue shift was also observed for the undoped dots with increasing the synthesis temperature, while more prolonged oxidation led to a red shift. Excitation with different wavelengths revealed the inhomogeneity of photoluminescence sites in nitrogen-doped carbon dots.