The origin of emissive states of carbon nanoparticles derived from ensemble-averaged and single-molecular studies

At present, there is no consensus understanding on the origin of photoluminescence of carbon nanoparticles, particularly the so-called carbon dots. Providing comparative analysis of spectroscopic studies in solution and on a single-molecular level, we demonstrate that these particles behave collectively as fixed single dipoles and probably are the quantum emitter entities. Their spectral and lifetime heterogeneity in solutions is explained by variation of the local chemical environment within and around luminescence centers. Hence, the carbon dots possess a unique hybrid combination of fluorescence properties peculiar to dye molecules, their conjugates and semiconductor nanocrystals. It is proposed that their optical properties are due to generation of H-aggregate-type excitonic states with their coherence spreading over the whole nanoparticles.