Towards information storage by designing both electron and hole detrapping processes in bismuth and lanthanide-doped LiRE(Si,Ge)O₄ (RE = Y, Lu) with high charge carrier storage capacity

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.