It is challenging to rational design persistent luminescence and storage phosphors with high storage capacity of electrons and holes after X-ray charging. Such phosphors have potential applications in anti-counterfeiting and X-ray imaging. Here we have combined vacuum referred binding energy diagram (VRBE) construction, photoluminescence spectroscopy, and thermoluminescence to study the trapping processes of charge carriers in NaYGeO₄. In NaYGeO₄:0.004Bi³⁺ and NaYGeO₄:0.004Bi³⁺,0.005Ln³⁺ (Ln = Tb or Pr), Bi³⁺ appears to act as a shallow electron trap, while Bi³⁺ and Ln³⁺ act as deep hole trapping and recombination centres. We will show how to experimentally determine the VRBE in the Bi^{2+ 2}P_1/2 ground state in NaYGeO₄ and NaLuGeO₄ by thermoluminescence study. The electron trap depth produced by Bi³⁺ codopant in NaLu_1-xY_xGeO₄:0.003Bi³⁺,0.008 Tb³⁺ can be adjusted, by increasing x, resulting in conduction band engineering. By combining Bi³⁺ as an electron trap and Bi³⁺ and Tb³⁺ as the hole traps, excellent X-ray charged afterglow phosphors were developed. The integrated TL intensity of the optimized NaYGeO₄:0.004Bi³⁺ and NaYGeO₄:0.003Bi³⁺,0.008Tb³⁺ after exposure to X-rays is about 4.5 and 1.1 times higher than that of the state-of-the-art BaFBr(I):Eu²⁺ storage phosphor. Intense initial Tb³⁺ 4f → 4f afterglow appears in NaYGeO₄:0.003Bi³⁺,0.008Tb³⁺ and more than 40 h afterglow is measurable in NaYGeO₄:0.004Bi³⁺ and NaYGeO₄:0.003Bi³⁺, 0.008 Tb³⁺ after X-ray charging. We will show proof-of-concept anti-counterfeiting and X-ray imaging applications by using the developed afterglow phosphors and CsPbI₃ quantum dots. This work not only provides experimental evidence on the VRBE in the Bi^{2+ 2}P_1/2 ground state in NaYGeO₄, but also shows how to design and develop good afterglow phosphors for anti-counterfeiting and X-ray imaging by deeply studying and controlling the trapping processes of charge carriers in bismuth and/or lanthanides doped inorganic compounds.

Developing a feasible design principle for solid-state materials for persistent luminescence and storage phosphors with high charge carrier storage capacity remains a crucial challenge. Here we report a methodology for such rational design via vacuum referred binding energy (VRBE) diagram aided band structure engineering and crystal synthesis optimization. The ARE(Si,Ge)O₄ (A = Li, Na; RE = Y, Lu) crystal system was selected as a model example. Low-temperature (10 K) photoluminescence excitation and emission spectra of bismuth- and lanthanide-doped ARE(Si,Ge)O₄ system were first systematically studied, and the corresponding VRBE schemes were then established. Guided by these VRBE schemes, Bi³⁺ afterglow and storage phosphor properties were explored in NaLu_1-xY_xGeO₄. By combining Bi³⁺ with Bi³⁺ itself or Eu³⁺, Bi³⁺ appears to act as a deep hole-trapping center, while Bi³⁺ and Eu³⁺ act as less-deep electron traps. Trap depth tunable afterglow and storage were realized in NaLu_1-xY_xGeO₄:0.01Bi³⁺ and NaLu_1-xY_xGeO₄:0.01Bi³⁺,0.001Eu³⁺ by adjusting x, leading to conduction band engineering. More than 28 h of persistent luminescence of Bi³⁺ was measurable in NaYGeO₄:0.01Bi³⁺ due to electron release from Bi²⁺ and recombination with a hole at Bi⁴⁺. The charge carrier storage capacity in NaYGeO₄:0.01Bi³⁺ was discovered to increase ∼7 times via optimizing synthesis condition at 1200 °C during 24 h. The thermoluminescence (TL) intensity of the optimized NaYGeO₄:0.001Bi³⁺ and NaYGeO₄:0.01Bi³⁺,0.001Eu³⁺ is ∼3, and ∼7 times higher than the TL of the state-of-the-art X-ray storage phosphor BaFBr(I):Eu. Proof-of-concept color tuning for anti-counterfeiting application was demonstrated by combining the discovered and optimized NaYGeO₄:0.01Bi³⁺ afterglow phosphor with perovskite CsPbBr₃ and CdSe quantum dots. Information storage application was demonstrated by UV-light- or X-ray-charged NaYGeO₄:0.01Bi³⁺,0.001Eu³⁺ phosphor dispersed in a silicone gel imaging film. This work not only reports excellent storage phosphors but more importantly provides a design principle that can initiate more exploration of afterglow and storage phosphors in a designed way through combining VRBE-scheme-guided band structure engineering and crystal synthesis optimization.

Guided by vacuum referred binding energy (VRBE) diagrams, both the trapping and detrapping processes of electrons and holes are explored in the bismuth and lanthanide-doped LiRE(Si,Ge)O₄ (RE = Y, Lu) family of compounds. The Tm³⁺ electron trap has been combined with the deep hole traps of Ln³⁺ (Ln = Ce, Tb, or Pr) or Bi³⁺ in LiLuSiO₄. During the thermoluminescence readout, the electrons released from Tm²⁺ recombine with holes at Ln⁴⁺ and Bi⁴⁺ to produce typical Ln³⁺ 4f-4f or 5d-4f emission and Bi³⁺ A-band emission. The electron trap depth of lanthanide ions can be tuned by the choice of Ln³⁺ (Ln = Tm or Sm), and for fixed pair of Ln³⁺ and/or Bi³⁺ dopants like in LiLu_1−xY_xSiO₄:0.01Ce³⁺,0.01Ln³⁺ and LiLu_1−xY_xSiO₄:0.01Bi³⁺,0.01Sm³⁺ solid solutions, by adjusting x, resulting in the engineering of the VRBE at the conduction band bottom. The thermoluminescence (TL) intensity of the optimized LiLu_0.5Y_0.5SiO₄:0.01Ce³⁺, 0.005Sm³⁺ is about 8.5 times higher than that of the commercial X-ray BaFBr(I):Eu²⁺ storage phosphor. By combining deep Eu³⁺ or Bi³⁺ electron traps with Ln³⁺ (Ln = Tb or Pr) or Bi³⁺, Ln³⁺ and Bi³⁺ appear to act as less deep hole capturing centres in LiLuSiO₄. Here the recombination is achieved through hole liberation rather than the more commonly reported electron liberation. The holes are released from Ln⁴⁺ and Bi⁴⁺ to recombine with electrons at Eu²⁺ or Bi²⁺ to give characteristic Eu³⁺ 4f-4f and Bi³⁺ A-band emissions. The tailoring of Ln³⁺ and Bi³⁺ hole trap depths by crystal composition modulation is discussed in LiLu_1−xY_xSiO₄ and LiLu_0.25Y_0.75Si_1−yGe_yO₄:0.01Bi³⁺ solid solutions. The TL intensity of the optimized LiLu_0.25Y_0.75SiO₄:0.01Bi³⁺ is ~4.4 times higher than that of the commercial BaFBr(I):Eu²⁺. Proof-of-concept information storage will be demonstrated with X-ray or UV-light charged LiLu_0.5Y_0.5SiO₄:0.01Ce³⁺,0.01Sm³⁺ and LiLu_0.25Y_0.75SiO₄:0.01Bi³⁺ phosphors dispersed in silicone gel imaging plates.

We report a general methodology to the rational design of thermally stimulated short-wave infrared (SWIR) luminescence between ∼900 and 1700 nm by a new combination of using efficient energy transfer from Bi ³⁺ to Nd ³⁺ and an adjustable hole trap depth via valence band engineering. Predictions from a vacuum referred binding energy (VRBE) diagram are combined with the data from optical spectroscopy and thermoluminescence to show the design concept by using bismuth and lanthanide doped rare earth ortho-phosphates as model examples. Nd ³⁺ with its characteristic ⁴ F _3/2 → ⁴ I _j (j = 9/2, 11/2, 13/2) emission in the SWIR range is first selected as the emitting centre. The energy transfer (ET) processes from Bi ³⁺ or Tb ³⁺ recombination centres to Nd ³⁺ are then discussed. Photoluminescence results show that the energy transfer efficiency of Bi ³⁺ → Nd ³⁺ appears to be much higher than of Tb ³⁺ → Nd ³⁺ . To exploit this ET, thermally stimulated Bi ³⁺ A-band emission can then be designed by using Bi ³⁺ as a ∼2.7 eV deep electron trap in YPO ₄ . By combining Bi ³⁺ with Tb ³⁺ , Pr ³⁺ , or Bi ³⁺ itself, the holes trapped at Tb ⁴⁺ , Pr ⁴⁺ , or Bi ⁴⁺ will release earlier than the electrons captured at Bi ²⁺ . On recombination with Bi ²⁺ , Bi ³⁺ in its excited state is formed generating Bi ³⁺ A-band emission. Due to the ET of Bi ³⁺ → Nd ³⁺ 1.06 μm Nd ³⁺ emission appears in YPO ₄ . Herein, the thermally stimulated Nd ³⁺ SWIR emission is achieved by hole release rather than the more commonly reported electron release. The temperature when thermally stimulated Nd ³⁺ SWIR emission appears can further be engineered by changing the Tb ³⁺ or Pr ³⁺ hole trap depth in Y _1−x Lu _x PO ₄ by adjusting x. Such valence band engineering approach can also be applied to other compounds like La _1−x Gd _x PO ₄ and Gd _1−x La _x AlO ₃ solid solutions. Our work opens the avenue to motivate scientists to explore novel SWIR afterglow phosphors in a design way instead of by trial and error approach.

The vacuum referred binding energy (VRBE)-guided design of Bi³⁺-based storage and afterglow materials together with charge carrier trapping processes is explored with a study on bismuth- and lanthanide-doped rare earth ortho-phosphates. By combining Bi³⁺ with the shallow hole trap of Tb³⁺ or Pr³⁺, Bi³⁺ appears to act as a deep electron trap and as a hole recombination center in YPO₄. Combining Bi³⁺ with the deep electron trap of Tm³⁺, Sm³⁺, Yb³⁺, or Eu³⁺, Bi³⁺ appears to act as a shallow hole trap in YPO₄. Here recombination is also realized by means of hole release instead of more commonly reported electron release. Holes are released from Bi⁴⁺ and then recombine through the valence band with the electrons trapped at Ln²⁺ to produce Ln³⁺ 4f-4f emission. Lu³⁺ was introduced into YPO₄ to engineer the valence band (VB) energy and to tailor the hole trap depth of Bi³⁺ in Y_1-xLu_xPO₄ solid solutions. The results show that with increasing x the VRBE at the valence band top moves downward and the hole trap depth of Bi³⁺ increases. With a deep understanding of the Bi²⁺ and Bi³⁺ trap level locations and on the charge carrier trapping process, this work broadens the avenue to explore new persistent luminescence and storage materials using Bi³⁺ both as electron and hole traps.