First-Principles Modeling of Bottom-Up Synthesis of Carbon Quantum Dots

In this work, we report the results of various scenarios related to the initial stages in the assembly of carbon quantum dots (CQDs) from citric acid (CA) or <i>o</i>-phenylenediamine (OPD). The results of the step-by-step simulations of the synthesis demonstrate that all possible scenarios of CQD assembly are different from those previously proposed. For example, in synthesizing CQDs from citric acid, each addition of a new carbon ring to the growing nanographene leads to the appearance of the carbonyl (C=O) groups on the edges and carboxyl (–COOH) groups in the interior parts of the nanographenes. Even the initial steps of CQD assembly from CA are accompanied by the formation of bushy structures from carboxyl and –CH₂–COOH groups on the edges. On the other hand, in manufacturing CQDs from OPD, the formation of flat nanographenes is extremely energetically favorable. This result is in qualitative agreement with a very high yield of synthesized CQDs from OPD. However, the discussed process of nanographene formation proceeds simultaneously with the oxidation of newly formed nanographenes in a medium of superheated water accompanied by the appearance of C–OH bonds in the internal parts of newly formed <i>sp</i>²- carbon species or even in their etching. For both cases, the scenario of eliminating excessive carboxyl or hydroxyl groups by forming interlayer C–C bonds between two adjacent nanographenes is estimated as possible.