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

摘要

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 S-1(0) -> P-3(1) 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.