Carbon dots (CDs) have become one of most promising fluorescent materials in recent days, because of their promising photoluminescence and photocatalytic properties. However, the practical applicabilities for emissive and catalytic devices are still debatable, because of the lack of fundamental understanding behind the structure–property correlations. Herein, we have developed different types of nitrogen-doped CDs (N-CDs) by varying different nitrogen-containing precursors through a simple bottom-up based carbonization technique. Depending on the nature of nitrogen atom precursor, we are able to critically control the subpopulations of various intrinsic constituents of N-CDs, i.e., aromatic domains, amorphous domains, and small molecular fluorophores inside N-CDs. Detailed structural and elemental features have been correlated with the underpinning photophysical processes by means of steady-state and time-resolved fluorescence spectroscopy. In addition, the effect of temperature on overall photoluminescence properties has been corroborated with the internal structure of N-CDs. Finally, we have investigated the photocatalytic properties and the detailed photocatalysis mechanisms by scavenging the active species originated upon light irradiation. Results suggest that photocatalytic efficiency is maximum at a larger extent of amorphous domains and in the presence of nitrogen atoms specifically located at the edges, while photoluminescence intensity is higher at larger extent of molecular fluorophores and aromatic domains. Therefore, these fundamental investigations will open up new possibilities considering the optimizations of heteroatom functionalized CDs for their on-demand applicabilities in emitting as well as photocatalytic devices.

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Unlike the traditional fluorescent material, carbon dots (CDs) have unique photoluminescent properties, which directly depend on several synthesis parameters during the bottom-up carbonization process. Overall photoluminescence properties of CDs are mainly regulated by the three major emissive domains of CDs, that is, (a) small molecular fluorophores, (b) graphitic aromatic domains, and (c) amorphous domains and/or surface states. However, the extent of carbonization is extremely crucial as it directs the relative populations of the three emissive domains and their interplay, which eventually regulates the overall photo-physics of carbon-based nanomaterials. Therefore, it is highly desirable to explore the molecular-level stepwise transformations of small precursor molecules to zero-dimensional CDs and eventually their critical optimization in emitting, catalytic, and optoelectronic devices. Herein, we have investigated the stepwise growth process of zero-dimensional N-functionalized CDs from small molecular precursors–citric acid and ammonia. In-depth molecular insight into the evolution of chromophore centers has been gained through detailed structure–property correlation. Structural and elemental features have been illustrated by employing proton nuclear magnetic resonance, Fourier-transform infrared, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. Furthermore, the intrinsic molecular-level transformation of CDs is nicely correlated with the evolution of the intriguing photophysical properties by detailed steady-state and time-resolved spectroscopy. In addition, the extent of aromaticity and the internal rigidity during the growth process have also been illustrated by temperature-dependent fluorescence spectroscopy. Overall, the current fundamental study will be extremely crucial for the development of CDs and their molecular-level optimization for on-demand applications.

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