How to design ultraviolet emitting persistent materials for potential multifunctional applications: a living example of a NaLuGeO4:Bi3+,Eu3+ phosphor
We report on the design of ultraviolet (UV) emitting persistent (PersL) materials. Understanding the luminescence mechanism of Bi3+ ions, the occupation rules of sites, the density functional theory (DFT) calculations and an empirical energy level scheme guided us to select the most appropriate emitters, host and traps. Finally, the NaLuGeO4:Bi3+,Eu3+ phosphor was successfully designed. The experimental results indicated that the NaLuGeO4:Bi3+,Eu3+ material is indeed able to emit excellent UV PersL, which can be recorded for more than 63 h. This exciting result is sufficiently encouraging for the initiation of a more thorough investigation. Accordingly, the excitation temperature-dependent and fading thermoluminescence experiments were conducted, and the trap properties were deeply studied by the initial rising method. The results reveal the PersL mechanism and the significant role of Eu3+ codopants as foreign traps. On the basis of this work, the UV PersL of the as-designed NaLuGeO4:Bi3+,Eu3+ material is certainly promising for some potential multifunctional applications, and the design concepts of this work are indeed effective and feasible for the design of PersL materials.