Efficient Room-Temperature Phosphorescence from Nitrogen-Doped Carbon Dots in Composite Matrices

Carbon dots (CDs) have various attractive properties and potential applications, but there is much less attention paid to their phosphorescent phenomenon and mechanism. Herein, we prepared a kind of highly efficient CD-based phosphorescent material by a subtle design method that incorporated N-doped CDs (NCDs) into composite matrices (the melting recrystallization urea and biuret from the heating urea) by a one-pot heating treatment for the mixture of urea and NCDs. Through systematic investigation, C=N bonds on the surface of the NCDs can create new energy level structures, and for the first time, evidence that shows they are the origin of phosphorescence is presented. Composite matrices play a dual role to suppress the vibrational dissipation of long-lived triplets by combining the rigidity of the melting recrystallization urea and hydrogen bonding between biuret and NCDs, which have obvious advantages over a single-component matrix. The results show the obtained materials have an ultralong phosphorescent lifetime of 1.06 s under 280 nm excitation and a high phosphorescent quantum yield of 7% under 360 nm excitation in air, which are the highest values recorded for the CD-based materials. These CD-based room-temperature phosphorescent materials have also shown potential in white light-emitting diodes and data security.