Structure and photoluminescence evolution of nanodots during pyrolysis of citric acid: from molecular nanoclusters to carbogenic nanoparticles

Citric-acid-derived carbon nanodots are increasingly being explored as novel fluorescent nanomaterials due to their strong photoluminescence (PL). However, an accurate picture of the formation of carbon nanodots and an exhaustive structure–property correlation are still lacking. Herein we present a systematic investigation of the formation mechanism of carbon nanodots by following the pyrolysis of a citric acid–diethylenetriamine precursor at different temperatures. The collective nanodots are investigated by dynamic rheological measurements, exhibiting a strong pyrolytic temperature dependence of the viscoelastic properties. Atomic force microscopy, transmission electron microscopy, and Raman spectroscopy reveal that the synthesized “dots” at different pyrolytic temperatures are different in essence, and the transition of their chemical structure from molecular clusters to carbogenic nanoparticles during pyrolysis is highly verified. We find that a molecular fluorophore with intense PL predominates at low temperature (<250 °C), but a newly created quasi-molecular fluorophore with blue-shifted and decreased PL quantum yield predominates at high temperatures (300 °C). Time-resolved photoluminescence spectroscopy suggests that the strong PL suppression at high temperature is due mostly to a dramatic increase in the nonradiative decay rate of the quasi-molecular electronic state.