Disclosing the emissive surface traps in green-emitting carbon nanodots

The bright photoluminescence of surface-functionalized carbon nanoparticles, known as carbon nanodots (CDs), has been studied for more than a decade because of its fundamental photo-physical interest and strong technological potential. However, the essential nature of the electronic states involved in their typical light emission remains very elusive. Here, we provide conclusive evidence that surface carboxylic moieties are the key to CD fluorescence. The synergy of nanosecond and femtosecond optical studies, cryogenic fluorescence, computational investigations and chemical engineering of a strategically chosen model CD system, allows to demonstrate that their visible-light transitions are due to the electron transfer from nitrogen atoms of the core to specific charge trap states strongly localized on –COOH surface groups. These results clarify a long-standing open problem in the photo-physics of carbon dots, and help to establish more solid foundations for the understanding of their optical response.