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

Cubic alpha-NaYF4 nanometer-sized crystals singly doped with Pr3+ and codoped with Pr3+ and 20 % Yb3+ were synthesized by an original coprecipitation route at 60 degrees C during 1 h. The obtained nanoparticles exhibited primary particles showing cubic shape with mean sizes around similar to 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 P-3(0) and D-1(2) 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.