Unraveling electron liberation from Bi2+ for designing Bi3+-based afterglow phosphor for anti-counterfeiting and flexible X-ray imaging

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 NaYGeO 4 . In NaYGeO 4 :0.004Bi 3+ and NaYGeO 4 :0.004Bi 3+ ,0.005Ln 3+ (Ln = Tb or Pr), Bi 3+ appears to act as a shallow electron trap, while Bi 3+ and Ln 3+ 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 NaYGeO 4 and NaLuGeO 4 by thermoluminescence study. The electron trap depth produced by Bi 3+ codopant in NaLu 1-x Y x GeO 4 :0.003Bi 3+ ,0.008 Tb 3+ can be adjusted, by increasing x, resulting in conduction band engineering. By combining Bi 3+ as an electron trap and Bi 3+ and Tb 3+ as the hole traps, excellent X-ray charged afterglow phosphors were developed. The integrated TL intensity of the optimized NaYGeO 4 :0.004Bi 3+ and NaYGeO 4 :0.003Bi 3+ ,0.008Tb 3+ after exposure to X-rays is about 4.5 and 1.1 times higher than that of the state-of-the-art BaFBr(I):Eu 2+ storage phosphor. Intense initial Tb 3+ 4f → 4f afterglow appears in NaYGeO 4 :0.003Bi 3+ ,0.008Tb 3+ and more than 40 h afterglow is measurable in NaYGeO 4 :0.004Bi 3+ and NaYGeO 4 :0.003Bi 3+ , 0.008 Tb 3+ after X-ray charging. We will show proof-of-concept anti-counterfeiting and X-ray imaging applications by using the developed afterglow phosphors and CsPbI 3 quantum dots. This work not only provides experimental evidence on the VRBE in the Bi 2+ 2 P 1/2 ground state in NaYGeO 4 , 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.