Experimental and theoretical overview on the origin of photoluminescence in carbon nanodots.

Carbon nanodots (CNDs) are extensively explored for bioimaging, sensing, and optoelectronic applications because of their tunable photoluminescence (PL), elevated quantum yield (QY), facile synthesis, and biocompatibility. Nevertheless, despite their discovery in 2004, the origin of their optical characteristics remains ambiguous. Their structural variability results in several emission pathways, such as quantum confinement, surface states, and molecular fluorophores (MFs), which complicates the establishment of a unified structure-property relationship. Inspired by these emerging insights, this review provides a recent comprehensive experimental and theoretical overview of the origin of PL in CNDs over the past two decades. Special emphasis is placed on core-state emission, surface-state emission, MF contributions, and defect-mediated pathways and understanding how variations in environmental conditions modulate PL properties in CNDs. Furthermore, the growing integration of theoretical approaches, especially density functional theory (DFT), with experimental validation is highlighted to elucidate the structure-emission relationship. This review demonstrates that a synergistic theoretical and experimental framework is necessary for uncovering the PL mechanisms and advancing the controlled design of CNDs.