Fluorescence of carbon dots embedded in a SiO2 host matrix

Carbon dots (CD) are an emerging class of recently discovered carbonaceous nanomaterials, which have attracted a large interest because of their bright and characteristically “tunable” fluorescence, and their potential for suggestive applications. Despite quite an intense research, the fundamental properties of these systems are poorly understood and still lively debated. Here we report on a series of experiments on N-doped CDs prepared by thermal decomposition of citric acid and urea. We studied these CDs by steady-state and nanosecond time-resolved photoluminescence, optical absorption, infrared absorption and atomic force microscopy. CDs (3 nanometers-sized) are found to emit two co-existing and spectrally separated fluorescence signals, likely associated to two completely independent chromophores. We devise a facile sol-gel procedure to embed them in a SiO2 host matrix up to high concentrations (we estimate CD-CD distances <100 nm). Thus we successfully obtain highly fluorescent CD-based monoliths (see photograph) with very good optical quality. We find the characteristic optical properties (e.g. tunability, large quantum yield) of CDs in solution phase to be essentially preserved in SiO2, aside minor perturbations. These similarities imply that the emissions are mostly insensitive to the environment, suggesting that they arise from core electronic transitions rather than to surface states. These SiO2/CD composites also allow us to study the temperature dependence of the emission of CDs. Notably, we found that the usually broad and unstructured fluorescence of CDs undergoes remarkable changes of shape as a function of temperature, and hints of substructures unexpectedly show up in the emission bands (see plot) below 50 K.