Despite many efforts, the mechanisms of light absorption and emission of small fluorescent carbon nanoparticles (C-dots) are still unresolved and are a subject of active discussion. In this work we address the question as to whether the fluorescence is a collective property of these nanoparticles or they are composed of assembled individual emitters. Selecting three types of C-dots with "violet", "blue" and "green" emissions and performing a detailed study of fluorescence intensity, lifetime and time-resolved anisotropy as a function of excitation and emission wavelengths together with the effect of viscogen and dynamic fluorescence quencher, we demonstrate that the C-dots represent assemblies of surface-exposed fluorophores. They behave as individual emitters, display electronic anisotropy, do not exchange their excited-state energies via homo-FRET and possibly display sub-nanosecond intra-particle mobility.

No abstract

The unexpected discovery of the photoluminescence of carbon nanoparticles attracted the attention of many researchers and resulted in their use in a variety of applications. However, the origin of their emission is still obscure, and the majority of the discussions on the subject focus on their molecular and/or excitonic emissive states. We performed cryogenic studies down to 10 K and did not observe any signatures of suppressed molecular relaxation - the spectra remained broad, showing large unaltered Stokes shifts and temperature-independent emission intensities and lifetimes below 80 K with a weak dependence above this value. We demonstrated that the most general features of carbon nanoparticles, the very large Stokes shifts and considerable differences between the absorption and excitation spectra, are the result of the formation of a dynamic defect, the self-trapped Frenkel exciton. It looks like the distorted domain of the H-aggregate due to the exciton-lattice interaction and the local overheating caused by the exciton relaxation. In addition, at low temperatures the long-lifetime spectral component was found and was attributed to phosphorescence. The obtained results strongly support the excitonic nature of the fluorescence of nanocarbon materials.