Exploring Intrinsic Electron-Trapping Centers for Persistent Luminescence in Bi³⁺-Doped LiREGeO₄ (RE = Y, Sc, Lu): Mechanistic Origin from First-Principles Calculations

Abstract

Revealing the nature of intrinsic defects that act as charge-carrier trapping centers for persistent luminescence (PersL) in inorganic phosphors remains a crucial challenge from an experimental perspective. It was recently reported that Bi 3+ -doped LiREGeO 4 (RE = Sc, Y, Lu) compounds displayed strong ultraviolet-A PersL at ∼360 nm with a duration of tens of hours at room temperature. However, the mechanistic origin of the PersL remains to be unveiled. Herein, we carried out a systematic study on optical transitions, formation energies, and charge-transition levels of dopants and intrinsic point defects in these compounds using hybrid density functional theory calculations. The results show that the efficient charging by 254 nm is due to the D-band transition of Bi 3+ and hence the charge carriers pertinent to PersL are electrons originating from the dopants which are involved in the trapping and detrapping processes. The main electron-trapping centers are antisite defects Ge Li 0 , interstitial defects Li i 0 , and dopants Bi 2+ , with the former one responsible for the strong PersL and the latter two for its long-time duration. These findings are further confirmed by comparison with calculated results for isostructural NaLuGeO 4 and LiLuSiO 4 , based on which the roles of Li and Ge elements in forming intrinsic defects with appropriate trap depths for PersL are clarified. Our results not only assist in the understanding of experimental observations but also provide a theoretical basis for the rational design of novel PersL phosphors containing lithium and germanium in the host compound.

Enabling narrowband cyan photoluminescence and long-lasting ultraviolet-A persistent luminescence in Bi3+ single-doped Sr3Sc2Ge3O12 phosphors by selective site occupation

“…Some of them are captured by the energy traps, which are generally lattice defects and impurities and locate close to the bottom of the conduction band (process 2). 54 As the irradiation time increases, the energy traps are fully filled, and the deep traps are filled by the non-radiative relaxation of shallow traps. After stopping the excitation, the stored electrons escape back to the 3 P 1 emitting state owing to room temperature thermal stimulation (process 3).…”

“…Subsequent radiative recombination with the trapped holes at Bi 3+ results in UVA persistent luminescence (process 4). 54,55 Under the excitation of natural sunlight, the high-energy UV radiation in sunlight can promote the ground state electrons of Bi 3+ to the 3 P 1 emitting state (process 5), followed by the filling of the nearby traps through a tunnelling process (process 6). The transfer of electrons to the energy traps through the conduction band with the help of ambient thermal energy is also possible because the 3 P 1 energy level is close to the conduction band.…”

Emerging Ultraviolet Persistent Luminescent Materials

“…Bi 3+ can not only realize UVC PersL but also UVA PersL and UVB PersL depending on the host lattices as shown in Table 1 and Figure 3a. [ 9,14,28,31–38,40–42 ] Its flexibility is attributed to the lattice‐sensitive feature of the 6 s 6 p energy states similar to the 4 f 5 d state of Pr 3+ .…”

“…The olivine‐type UV PersL materials are reported by doping with the emitters such as Bi, [ 35–38,40–42 ] Pr, [ 12 ] or Gd. [ 28 ] The olivine host can be formulated as A Ln BO 4 (generally, A = Li, Na; Ln = Sc, Y, Lu, Gd; B = Si, Ge).…”

“…Figure a displays a schematic energy level diagram of Li Ln GeO 4 ( Ln = Sc, Y, Lu):Bi 3+ . [ 40 ] Upon 254 nm UV irradiation (arrow 1), electrons are excited directly from the ground state of Bi 3+ to the host conduction band (i.e., Bi 3+ D‐band or MMCT excitation) to form Bi 4+ . The liberated electrons are mobile in the CB and captured by intrinsic point defects.…”