Solid-State Emissive Carbon Quantum Dots (CQDs) with a Long Wavelength Emission

Carbon quantum dots (CQDs)—growing stars of the nanocarbon family—have received attention owing to their intriguing photoluminescence (PL), stability, and biocompatibility. However, CQDs may suffer from serious aggregation-caused quenching (ACQ) of PL, specifically in powder or thin film form. Moreover, most engineering applications are solid-state; therefore, it is important to design solid-state emissive CQDs, potentially defeating ACQ. Previous reviews highlighted the strategies for solid-state emissive CQDs; however, less focus has been given to CQDs with emissions at longer wavelengths. This review summarizes recent advances (specifically in the last two years), focusing on long wavelengths, including yellow-, orange-, and red-emissive fluorescence. Furthermore, the synthesis method, quantum yield, and mechanism of fluorescence are explained. In addition, a detailed summary of synthesis parameters and their role in emission tuning is highlighted. Finally, the future directions and potential applications of solid-state CQDs for emerging applications were discussed.