A super energy transfer process based S-shaped cluster in ZnMoO4 phosphors: theoretical and experimental investigation

Abstract

Efficient energy transfer from a sensitizer to an activator in phosphors is very important for white LEDs. Bi³⁺ and Eu³⁺ co-doped red phosphors are potential alternatives for white LEDs. However, energy transfer from Bi³⁺ to Eu³⁺ ions is still not efficient enough in most cases. Here, we have found that every six Zn sites form an S-shaped cluster in the ZnMoO₄ crystal. Two Zn(2) sites will be occupied preferentially in ZnMoO₄ according to the comparison between the calculated and experimental A band positions of Bi³⁺ in the ZnMoO₄ host. Considering the S-shaped clusters and site occupation preference, a super energy transfer process from Bi³⁺ to Eu³⁺ ions is proposed. The distance between the Bi³⁺ and Eu³⁺ ions can be controlled by their total doping concentrations. When their total molar concentration is beyond 1/6, Bi³⁺ and Eu³⁺ begin to sit in two adjacent Zn(2) sites. Thus, a new super energy transfer from Bi³⁺ to Eu³⁺ emerges due to the adjacent Bi³⁺ and Eu³⁺ ions. When excited at 331 or 350 nm, which is assigned to the ¹S₀ → ³P₁ transition of Bi³⁺, the phosphor emits intense red light. The relative intensity is about 6 times higher than that of an ordinary transfer process. It is a good example of how to utilize site occupation preference and provides a new way to design efficient phosphors.