Bi³⁺ acting both as an electron and as a hole trap in La-, Y-, and LuPO₄

Abstract

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.