Fluorescence response from the surface states of nitrogen-doped carbon nanodots: evidence of a heterogeneous population of molecular-sized fluorophores.

Fluorescent Nitrogen-doped Carbon Nanodots (NCDs) of ∼4 nm diameter were prepared by acid-driven microwave irradiation of DMF solvent. Spectroscopic studies of the NCDs demonstrated that excitation of the carbon core did not contribute any fluorescence emission. Instead, the emission originated exclusively from the surface states. The fluorescence featured a prominent red edge excitation shift (REES), while changing the excitation wavelength over ∼0.5 eV indicated the emergence of different emitter species in the temporal evolution of fluorescence. These results combined to indicate a large degree of heterogeneity in the population of these surface-localized emitters, so that working with different excitation energies produced different sets of excited surface fluorophores that evolved independently of each other. Fluorescence anisotropy dynamics in the NCDs was attributable to the reorientational motion of the surface fluorophores which was decoupled from the rotational diffusion of the carbon core of the NCD. The anisotropy decay rates revealed that the fluorophores had size comparable to typical organic fluorophores, irrespective of excitation energy.