Photoluminescent Carbon Quantum Dots: Synthetic Approaches and Photophysical Properties

A number of synthetic methodologies and applications of carbon quantum dots (CQDs) have been reported since they were first discovered nearly two decades ago. Unlike metal‐based or semiconductor‐based (e. g., metal chalcogenides) quantum dots (MSQDs), CQDs have the unique feature of being prepared through a variety of synthetic protocols, which are typically understood from considerations of reaction models and photoluminescence mechanisms. Consequently, this brief review article describes quantum dots, in general, and CQDs, in particular, from various viewpoints: (i) their definition, (ii) their photophysical properties, and (iii) the superiority of CQDs over MSQDs. Where possible, comparisons are made between CQDs and MSQDs. First, however, the review begins with a general brief description of quantum dots (QDs) as nanomaterials (sizes≤10 nm), followed by a short description of MSQDs and CQDs. Described subsequently are the various top‐down and bottom‐up approaches to synthesize CQDs followed by their distinctive photophysical properties (emission spectra; quantum yields, Φs). Carbon quantum dots (CQDs) synthesized in four different solvents or mixed solvents with 1,3,6‐trinitropyrene as the carbon precursor: blue‐emitting (DMF/H2O), green‐emitting (EtOH/CH3CO2H), yellow‐emitting (EtOH), and red‐emitting (DMF). Photograph shows the photoluminescent CQDs dispersed in toluene under UV light, and normalized emission spectra with bands at 460, 517, 581, and 620 nm, respectively.