Disentangling size effects and spectral inhomogeneity in carbon nanodots by ultrafast dynamical hole-burning

Carbon nanodots (CDs) are a novel family of nanomaterials exhibiting unique optical properties. In particular, their bright and tunable fluorescence redefines the paradigm of carbon as a "black" material and is considered very appealing for many applications. While the field keeps growing, understanding CDs fundamental properties and relating them to their variable structures becomes more and more critical. Two crucial problems concern the effect of size on the electronic structure of CDs, and to what extent their optical properties are influenced by structural disorder. Furthermore, it remains largely unclear whether traditional concepts borrowed from the photo-physics of semiconductor quantum dots can be applied to any type of CDs. We used femtosecond optical hole burning to address the excited-state properties of a family of CDs with the specific structure of β-C3N4. The experiments provide compelling evidence of the dramatic effects of structural heterogeneity on the optical spectra, and reveal the remarkably simple pattern of the electronic transitions of these CDs, normally obscured by disorder. Moreover, the data conclusively clarify the different effects of the nanometric size and of the disordered surface structure on the fluorescence tunability, ruling out for these CDs any quantum confinement effect comparable to semiconductor quantum dots.