Post-Synthetic Graphitization and Photoluminescence Tuning of Carbon Dots from L-Glutamic Acid

Abstract

Carbon dots are promising luminescent nanostructures. Their availability, based on inexpensive precursors and their intrinsic fluorescent nature, place them as potential choices against traditional quantum dots in some applications. However, the carbon dots nature is nowadays not well defined and understood. They are usually hybrid nanomaterials in terms of structure and composition, which hinders the knowledge of their photoluminescence mechanisms. Herein, the solvent-free pyrolysis of L-glutamic acid by microwave synthesis leads to an incomplete graphitization of the precursor resulting in loose polymer-like nanohybrids. Further experiments confirm that these carbon dots (CD) can be crosslinked and graphitized in a second heating step when using temperatures above 160 °C, reaching a plateau of graphitization over 200 °C. An exhaustive analysis by thermogravimetric analysis, Raman and X-ray photoelectron spectroscopy highlights the structural changes undergo from 140 to 200 °C, where an increase of stability is observed due to the graphitization process. The condensation and decarboxylation reactions with loss of hydrophilic functional groups like amine and carboxylic acids explain the formation of sp2 domains and the concomitant loss of solubility. The annealed CD exhibit a broader and slightly red-shifted emission band, with lower emission quantum yields, when compared to the initial nanohybrids.