Synthesis and Study of New Luminescent Carbon Particles with High Emission Quantum Yield

Luminescent carbon nanoparticles (CNPs) having a high mass yield of carbon material and emission quantum yield are synthesized from lentil grain flour via thermal carbonization, followed by treatment in nitric acid, and by the hydrothermal method. The luminescent and optical properties, solubility, and stability of solutions of various CNPs during interaction between particles and electromagnetic radiation are studied and analyzed by photoluminescence and optical absorption methods. When carbonization temperatures are 400 and 500°C, the mass yields of carbon materials are 34.6 and 29.7%, respectively. The amount of CNPs whose sizes are less than 50 nm is 7.2–12.5%, whereas the proportion of particles soluble in pure water is no more than 0.8% of the weight of a carbon material. New particles having a mass yield of 81–104% and high solubility of CNPs in water are formed as a result of special treatment of carbon material powders in HNO₃. The solubility of CNP powders is strongly dependent on temperature and pH of a solvent; some samples achieve 30.3 g/L at 96.9°C. The energy of the direct band gap of CNPs is 3.325–3.445 eV, and the carbon nuclei of the particles contain structural defects and various surface groups of high concentration. Some absorption bands, including those at 271 (NH1) and 370 nm (NH2), appear in the optical absorption spectra after hydrothermal treatment of all CNPs in the presence of ammonia or urea. The structural features of the NH1 and NH2 bands, the photoluminescence spectra, and the magnitudes and the stability of quantum yields of different CNPs are thoroughly studied depending on temperature and duration of hydrothermal treatment. The maximum quantum yield of the best CNP samples during excitation with 406 nm laser radiation is 39.3%, which is a promising indicator for practical use.