Influence of chemical states of doped nitrogen on photoluminescence intensity of hydrothermally synthesized carbon dots

Nitrogen-containing carbon dots were synthesized from glucose and glucosamine by hydrothermal treatment. The relation between the PL properties of the carbon dots and chemical states of nitrogen in them are discussed. The actual N/C ratio of synthesized samples increased from 0.00 to 0.08 as the nominal molar percentage of glucosamine/(glucose+glucosamine), x_N, was increased from 0 to 100 mol%. The samples were nanoparticles with microscopic local graphite structure. Some nitrogen atoms were doped in the graphite skeleton, and others were present as surface NH₂ groups. The nanoparticles exhibited blue photoluminescence (PL) under near-ultraviolet excitation. The PL peak shifted to longer wavelength as the excitation wavelength increased. PL intensity was proportional to the absorbance of the carbon dots, and both increased with the amount of nitrogen doped in the graphite skeleton. Other carbon dots were synthesized using ammonium hydroxide as a nitrogen source instead of glucosamine. These samples possessed abundant surface NH₂ groups, and exhibited no change in PL intensity with increasing NH₂ content. Nitrogen doped in the graphite skeleton formed donor levels in the energy band of the carbon dots, increasing the amount of absorption centers and enhancing PL.