Graphitic carbon nitride quantum dots embedded in magnesium fluoride: Achieving blue-violet room-temperature phosphorescence for information encryption and fingerprint detection

Abstract

Graphitic carbon nitride quantum dots (g-CNQDs) with exceptional fluorescence properties have potential applications in various fields, but their phosphorescence characteristics and associated mechanisms have received considerably less attention. This study presents the room-temperature phosphorescence (RTP) from g-CNQDs by embedding them into a magnesium fluoride (MgF₂) matrix. The g-CNQDs were synthesized by hydrothermal treatment of graphitic carbon nitride in an aqueous H₂O₂ solution. Calcining the mixture of g-CNQDs, magnesium nitrate, and ammonium fluoride at 300–700°C resulted in the production of g-CNQDs@MgF₂ composites. Systematic investigations reveal that the phosphorescence originates from the triplet excited states of the π-conjugated structures of tri-s-triazine rings in the core of g-CNQDs. The MgF₂ matrix provides multiple constraints on g-CNQDs through a rigid network and robust covalent and hydrogen bonds, which effectively stabilize the triplet excitons and suppress the non-radiative transitions, enabling long-lasting blue-violet RTP with an optimal lifetime of 214 ms. By adjusting the calcination temperature, the phosphorescence properties of g-CNQDs@MgF₂ can be finely tuned. The composites possess outstanding optical stability against different solvents, strong acids, and bases, and show promising applications in information encryption and fingerprint detection, offering a cost-effective and environmentally friendly alternative to conventional RTP materials.