Aggregation-induced emission enhancement of carbon quantum dots and applications in light emitting devices

Intensive research efforts have been made on fluorescent carbon quantum dots (CQDs); however, it is still a challenge to realize highly emissive solid-state luminescence due to the aggregation-induced self-quenching. In order to fundamentally overcome this shortcoming, it is critical to develop CQDs with aggregation-induced emission enhancement (AIEE) to meet the requirements of both high photoluminescence (PL) efficiency and optical output. In this study, a solid film of CQDs exhibiting a high PL quantum yield (QY) of 61% was obtained by restricting the nonradiative transitions in the aggregation state. On the contrary, the PLQY of CQD solution was very low (ca. 5%), in which most of the excited CQDs nonradiatively relaxed to the ground state via the motions of surface groups. The AIEE effect was convincingly supported by the time-resolved PL. The PL decay contained only a single radiative lifetime of 10.98 ns in the solid state, while in solution, besides the radiative lifetime of 11.26 ns, the PL decay also contained a fast parameter of 3.5 ns attributed to the nonradiative motions of surface groups. The AIEE enabled high PL efficiency and illuminance simultaneously, significantly promoting the optoelectronic applications of solid state CQDs. A bright blue LED using the highly emissive CQDs as a color convertor (conversion efficiency = 42%) is demonstrated.