Ultrafast spectroscopic investigation on fluorescent carbon nanodots: the role of passivation.

Disentangling the respective roles of the surface and core structures in the photocycle of carbon nanodots is a critical open problem in carbon nanoscience. While the need of passivating carbon dot surfaces to obtain efficiently emitting nanoparticles is very well-known in the literature, it is unclear if passivation introduces entirely new surface emitting states, or if it stabilizes existing states making them fluorescent. In this multi-technique femtosecond spectroscopy study, the relaxation dynamics of non-luminescent (non-passivated) carbon dots are directly compared with their luminescent (passivated) counterparts. Non-passivated dots are found to host emissive states, albeit very short-lived and practically incapable of steady-state fluorescence. In contrast, the passivation procedure gives birth to a distinctive new manifold of emitting states, localized on the surface of the dots, and capable of intense, tunable, long-lived fluorescence. It turns out that these surface states are instantaneously populated by photo-excitation, and their subsequent dynamics are entirely independent of core electronic transitions. The experiments reveal the lack of any crosstalk between core- and surface states, at least for certain common types of carbon dots, and open a new perspective on the mechanisms by which surface passivation governs the fluorescence properties of these nanoparticles.