Vacuum Referred Binding Energy (VRBE)-Guided Design of Orange Persistent Ca₃Si₂O₇:Eu²⁺ Phosphors

Orange persistent phosphors of Ca₃Si₂O₇ (CSO) doped with Eu²⁺ were strategically developed by codoping Sm³⁺ or Tm³⁺. First, a vacuum referred binding energy, VRBE, diagram of Ca₃Si₂O₇ (CSO) was constructed from the measured spectroscopic data. By the zigzag curve of the divalent lanthanide ions in the VRBE diagram, Sm³⁺ and Tm³⁺ ions were predicted to be a suitable electron trap for the persistent luminescence. The initial persistent luminance of CSO:Eu²⁺-Sm³⁺ and CSO:Eu²⁺-Tm³⁺ was found to be 290 times and 9300 times stronger, respectively, compared with CSO:Eu²⁺. By optimizing Eu²⁺ and Tm³⁺ concentrations, the persistent luminescence duration on 0.32 mcd/m² reached approximately 50 min in CSO:Eu²⁺-Tm³⁺. From the VRBE diagram and the persistent luminescence properties, we discuss the persistent mechanism including the charging process, detrapping process, and electron trapping centers.