Enhancing the sensitization of Ce3+ on Eu3+ by tri-doping Gd3+ in hexagonal NaYF4

Eu3+ was ideal red emitter due to its efficient red emitting transitions 5D0→7Fj and Ce3+ was most common sensitizer due to its strong parity-allowed absorption bands. However, direct sensitization of Ce3+ on Eu3+ (SCE) in hexagonal NaYF4 was very weak and unpractical. In this work, rate equations were introduced to elaborate the details of energy transfer (ET) among Ce3+, Gd3+ and Eu3+ in NaYF4, and how the SCE was much enhanced. The pure hexagonal phase of all samples were guaranteed by powder XRD patterns and SEM images, despite the morphology was influenced by rare earth components. While increasing Eu3+ concentration over 0.03, SCE was drastically dimished; on the contrary, it could be 60 folds enhanced by tri-doping Gd3+ ions. The rate equations of sensitizers were established and solved by calculating the lifetimes. It was found out that, while increasing Gd3+ concentration to 0.4, ET Ce→Gd could compete against ET Ce→Eu to capture 73.8% of energy absorped by Ce3+, which strongly favored SCE. The solution also revealed that SCE originated from ET chain Ce→Gd→Eu was much more efficient than direct SCE from ET Ce→Eu, which originated the significant enhancement.