Tunable dual photoluminescence from synthesized urea-based carbon quantum dots: A DFT based simulation on structural insights

The current work focuses on synthesis and characterization of fluorescent Urea-based Carbon Quantum Dots (UCQDs) through a green and simplistic single-step hydrothermal treatment from citric acid and urea combination. The synthesized UCQDs have been characterized by FESEM, X-ray Photoelectron Spectroscopy (XPS), UV-Vis absorption, Photoluminescence, Fourier transform infrared (FTIR) spectroscopy and Life time analysis. Concentration dependent variable particle sizes (4 nm-40 nm) were obtained for prepared UCQDs and their existence were further verified by UV-Vis absorption data and FESEM image analysis. The particle size was found to be directly proportional to the concentration of UCQDs, whereas the decrement of band gap was found to be inversely proportional to the size of UCQDs. Dual emission from UCQDs were appeared from graphite core (π → π*) at 460 nm and from passivated state (n → π*) at 530 nm. Existence of two excited state species were further verified by average lifetimes of 1.89 ns and 1.96 ns. XPS and EDX analysis confirmed the presence of three elements (C, N, O) and there bonding interactions which served as crucial inputs for the designing of potential molecular structure of synthesized UCQDs by theoretical modeling. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) have been used as ab initio methods for the structural insights and spectroscopic analysis of probe UCQDs.