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

Eu 3+ was ideal red emitter due to its efficient red emitting transitions 5 D 0 → 7 F j and Ce 3+ was most common sensitizer due to its strong parity-allowed absorption bands . However, direct sensitization of Ce 3+ on Eu 3+ (SCE) in hexagonal NaYF 4 was very weak and unpractical. In this work, rate equations were introduced to elaborate the details of energy transfer (ET) among Ce 3+ , Gd 3+ and Eu 3+ in NaYF 4 , 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 Eu 3+ concentration over 0.03, SCE was drastically dimished; on the contrary, it could be 60 folds enhanced by tri-doping Gd 3+ ions. The rate equations of sensitizers were established and solved by calculating the lifetimes. It was found out that, while increasing Gd 3+ concentration to 0.4, ET Ce→Gd could compete against ET Ce→Eu to capture 73.8% of energy absorped by Ce 3+ , 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.