Down and upconversion luminescence in Pr3+/Yb3+ -codoped α-NaYF4 nanocrystals

Cubic α-NaYF₄ nanometer-sized crystals singly doped with Pr³⁺ and codoped with Pr³⁺ and 20 % Yb³⁺ were synthesized by an original coprecipitation route at 60 °C during 1 h. The obtained nanoparticles exhibited primary particles showing cubic shape with mean sizes around ∼49 nm. Down and up-conversion mechanisms were studied versus the Pr³⁺ concentration in singly Pr³⁺ doped and Pr³⁺, Yb³⁺ codoped materials. Blue/red and NIR excited emission spectra as well as kinetics of both ³P₀ and ¹D₂ levels of Pr³⁺ have been studied qualitatively. DC and UC were evidenced indicating that Pr³⁺→Pr³⁺ and Pr³⁺↔Yb³⁺ energy transfers occur, which are strongly dependent on the Pr³⁺ concentration. The obtained results indicate that both Pr³⁺ and Yb³⁺ play the role of sensitizer or acceptor, leading to the modification of the DC/UC efficiency ratio due to a feedback energy transfer. Such assumption is discussed in the frame of three levels system. It gives insights into factors involved in the concentration quenching in both DC and UC materials based on the Pr³⁺/Yb³⁺ pairs.