From Small Molecules to Zero-Dimensional Carbon Nanodots: Chasing the Stepwise Transformations During Carbonization

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.

Automated summary

This study investigates the stepwise growth process of zero-dimensional N-functionalized carbon dots (CDs) and gains in-depth molecular insight into their evolution of chromophore centers. Structural and elemental features of the CDs are illustrated using various analytical techniques, such as proton nuclear magnetic resonance, Fourier-transform infrared, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The photophysical properties and chemical transformations of the CDs are studied using steady-state and time-resolved spectroscopy. The extent of aromaticity and internal rigidity during the growth process are also illustrated through temperature-dependent fluorescence spectroscopy.