Carbon Dot/Al2O3 Nanocomposites with an Enhanced Lifetime, Phosphorescence Quantum Efficiency, and Stability for Anticounterfeiting Applications

A short lifetime, low phosphorescence quantum efficiency (PQE), and unstable room-temperature phosphorescence (RTP) in oxidants and solvents are the core problems in the development of carbon dot (CD)-based RTP materials for anticounterfeiting applications. Herein, a series of CDs/Al2O3 nanocomposites with excitation-dependent RTP properties were fabricated by a sol–gel process followed by high-temperature treatment. The crystalline structure and rigid Al–O network of the Al2O3 matrix could readily benefit the stabilization of the triplet excited state of CDs. The Al–O–C interactions between the CDs and Al2O3 favor the facilitation of intersystem crossing (ISC) to effectively populate triplet excitons, which contributes to the RTP with a low energy gap. As a result, an ultralong lifetime of up to 1.4 s (observed by the naked eye for 22 s) and a high PQE of 12.78% were achieved by CDs/Al2O3-500 and CDs/Al2O3-300, respectively. The results demonstrated that the oxidants, polar solvents, and strong acids and alkalis almost had no effect on the RTP emission of the CDs/Al2O3 nanocomposites. Taking advantage of the ultralong phosphorescence afterglow, excitation-dependent multicolor RTP emission, and excellent stability, graphics of the Eight Trigrams were present for advanced anticounterfeiting via time-resolved luminescence imaging based on different CDs/Al2O3 nanocomposites.