Blue room-temperature phosphorescent carbon dot composites with high quantum yield for information encryption and LED light emission

Room temperature phosphorescent (RTP) materials are crucial for various applications such as information encryption, optoelectronic devices, and biological imaging. However, developing metal-free RTP materials with high quantum yield and long-lived blue luminescence remains a significant challenge. In this study, short- wavelength RTP carbon dots (CDs) composites with high quantum yield in blue and cyan colors were prepared by mixing salicylic acid and boric acid precursors in varying mass ratios and pyrolyzing them at the same temperature. Depending on the amount of salicylic acid, the resulting CDs composites were named CDs-x (x = 10, 25, 70, 150), with optimal phosphorescence emission peaks of 400, 400, 455, and 470 nm, respectively. The strategy allowed the luminescent centers of the CDs-x to be confined in a highly rigid boron oxide polycrystalline network and immobilized through the interactions of covalent bonding (C-B) and hydrogen bonding that effectively stabilizes the triplet excited state of CDs-x. As a result, high quantum yield RTP CDs composites were successfully obtained, with the absolute photoluminescence quantum yields up to 95.7 % and phosphorescence quantum yields up to 31.0 %. Moreover, CDs-x (x = 10, 25, 70, 150) had high phosphorescence lifetimes of 862.0, 865.3, 826.9, and 731.9 ms, respectively. Meanwhile, with the increase of salicylic acid, the luminescence centers of CDs-x changed from one to two, resulting in a shift in the color of phosphorescence from blue to cyan. These unique luminescent properties make CDs-x composites ideal for excitation-dependent and time-dependent information encryption strategies under different excitations at 254 and 365 nm. Additionally, these composites have great potential in the preparation of light-emitting diodes, showcasing their importance in the field of optoelectronic devices.