Charge carrier trapping processes in lanthanide doped LaPO4, GdPO4, YPO4, and LuPO4†
Various methods for deliberate design of electron and hole trapping materials were explored with a study on double lanthanide doped rare earth ortho phosphates. Cerium acts as a recombination center, while lanthanide codopants act 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 a fixed set of lanthanide dopants like in Gd1−xLaxPO4:0.005Ce3+,0.005Ho3+ solid solutions, by changing x, leading to conduction band (CB) engineering. Herein, the electrons liberated from Ho2+ recombined through the conduction band at Ce4+ to yield Ce3+ 5d–4f emission. In contrast, samarium, europium and ytterbium act as recombination centers, while Tb3+ and Pr3+ act as hole trapping centers in double lanthanide doped YPO4, as Tb3+ and Pr3+ codopant recombination is realized via hole release rather than the more commonly reported electron release. The holes recombine via the valence band with the electrons trapped in 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 shed light on the deliberate design of electron and hole trapping materials based on a deep understanding of trap level locations and on the transport and trapping processes of charge carriers.
- This article is part of the themed collection: International Year of the Periodic Table : Lanthanides for Precision Therapy and Beyond