Revealing graphitic nitrogen participating in p–π conjugated domain as emissive center of red carbon dots and applied to red room-temperature phosphorescence

Abstract

Synthesizing high-efficiency red carbon dots (R-CDs) and exploring their photoluminescence mechanism remain challenges as well as critical issues hindering the development of red room-temperature phosphorescence (RTP) materials of CDs. Herein, a series of highly efficient R-CDs are prepared from citric acid and formamide by solvothermal method on account of the protective and destructive effects of solvents. The larger sp² conjugated domains of the carbon core and the high content of graphitic N are identified as the key factors for R-CDs to exhibit red fluorescence. Surface groups also contribute to red emission by means of the formation of the N, O-related defect state. By establishing N-doping models and using DFT calculation, it is further demonstrated that the strong p–π conjugation effect caused by the non-bonding electrons of graphitic N would facilitate HOMO–LUMO splitting of the carbon core, thus leading to a significant decrease in the HOMO–LUMO gap. Energy level splitting could lead to a certain degree of energy level crossing, which could contribute to effective intersystem crossing and produce red RTP. From this, the potential applications of R-CDs in the information encryption field are proposed and demonstrated.