5.2. Anti-counterfeiting applications
Compared with organic luminescent molecules or inorganic semiconductor quantum dots, CDs have high luminous stability, creating opportunities to replace the aforementioned organic materials or quantum dots. However, their tendency to aggregate by π−π stacking between adjacent layers, resulting in PL quenching in the solid-state hinders their widespread application in optoelectronic devices. Such π−π stacking causes an increase in uncontrolled electroluminescence due to intermolecular excimer formation, which prevents the formation of pure emission colors in light-emitting devices. Aggregated luminous CDs provide a bright prospect for addressing these problems. Park et al.[95] used the functionalization agent to reduce π−π stacking between adjacent CDs, thus achieving quenching-resistant PL in the solid-state (Figure 7C). CDs-based white emitting diodes exhibited efficient down-conversion for white light emission with a correlated-color temperature of 5612 K and a high color rendering index of 86.2 at Commission Internationale de l Éclairage (CIE) coordinate of (0.333, 0.359). In another example,[34] the CIE of LED based on the solid-state luminous CDs was (0.285, 0.341). Shao et al. [30] synthesized LEDs with various emission colors utilizing self-quenching-resistant solid-state fluorescent CDs. By combining a blue-CDs powder and a self-quenching-resistant solid-state fluorescent CDs powder, a white light LED with the CIE coordinate of (0.31, 0.31) was realized. In the other two examples,[88, 96] the carbon nano ring systems were used to prevent aggregation-induced PL quenching and to fabricate high-efficiency LEDs (Figure 7D).