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 NaLu1-xYxGeO4. 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 NaLu1-xYxGeO4:0.01Bi3+ and NaLu1-xYxGeO4:0.01Bi3+,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.@en

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 Bi2+ 2P1/2 ground state in NaYGeO4 and NaLuGeO4 by thermoluminescence study. The electron trap depth produced by Bi3+ codopant in NaLu1-xYxGeO4:0.003Bi3+,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 Bi2+ 2P1/2 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.@en

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. @en

In this thesis, we have studied two types of charge carrier capturing and detrapping processes: (a) electron capturing and electron liberation; (b) hole capturing and hole liberation. Both the (a) and (b) processes can be utilized for the rational design of afterglow and storage phosphors in different compounds.@en

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 LiLu1−xYxSiO4:0.01Ce3+,0.01Ln3+ and LiLu1−xYxSiO4:0.01Bi3+,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 LiLu0.5Y0.5SiO4:0.01Ce3+, 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 LiLu1−xYxSiO4 and LiLu0.25Y0.75Si1−yGeyO4:0.01Bi3+ solid solutions. The TL intensity of the optimized LiLu0.25Y0.75SiO4:0.01Bi3+ 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 LiLu0.5Y0.5SiO4:0.01Ce3+,0.01Sm3+ and LiLu0.25Y0.75SiO4:0.01Bi3+ phosphors dispersed in silicone gel imaging plates.@en

Various methods for deliberate design electron and hole trapping materials are explored with a study on double lanthanide doped rare earth ortho phosphates. Cerium acts as recombination center while lanthanide codopants as electron trapping centers in LaPO4:0.005Ce3+,0.005Ln3+. The electron trap depth generated by lanthanide codopants can be tailored by the choice of lanthanide, and for fixed set of lanthanide dopants like in Gd1-xLaxPO4:0.005Ce3+,0.005Ho3+ solid solutions by changing x leading to conduction band (CB) engineering. Here, the electrons liberated from Ho2+ recombine through the conduction band at Ce4+ to yield Ce3+ 5d-4f emission. In contrast, samarium, europium and ytterbium are recombination centers, while Tb3+ and Pr3+ act as hole trapping centers in double lanthanide doped YPO4. For Tb3+ and Pr3+ codopants recombination is realized via hole release rather than the more common reported electron release. The holes recombine via the valence band with the electrons trapped at Yb2+, Sm2+, or Eu2+ to generate 4f-4f luminescence from Yb3+, Sm3+, or Eu3+. Lu3+ was introduced in YPO4 to tailor the valence band (VB) energy and to tune the hole trap depths of Tb3+ and Pr3+ in Y1-xLuxPO4:0.005Ln3+ solid solutions. Our results promote the deliberate design electron and hole trapping materials from deep understanding of trap level locations and on the transport and trapping processes of charge carriers.@en

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 Y1-xLuxPO4 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.@en