Thermal Annealing and Doping Induced Tailoring of Phase and Upconversion Luminescence of NaYF4:Yb Er Microcrystals

The influence of Mn2+ ion concentration (x = 0-20 mol%) as well as the role of thermal-annealing temperature (400-600 degrees C) on the structural as well as luminescence properties of NaYF4:Yb, Er (Y: 78-x%, Yb: 20%, Er: 2%) microcrystals prepared via a coprecipitation method is investigated. The cubic phase of the as-prepared NaYF4:Yb, Er (Y: 78%, Yb: 20%, Er: 2%) was found to remain intact upon the addition of the Mn2+ ions, but the thermalannealing elucidates that the phase of the sample depends upon the annealing temperature as well as the Mn2+ ion concentration. Among the Mn2+ ion co-doped samples, 3 mol% doped samples dominant to have a maximum positive influence on the upconversion luminescence of the sample, and a further increase in concentration leads to the concentration-induced quenching of the upconversion luminescence. Moreover, the enhancement factor of green (H-2(11/2) -> I-4(15)/2 and S-4(3/2) -> I-4(15/2)), as well as red (F-4(9/2) -> I-4(15/2)) emission, depend upon the annealing temperature, with a maximum enhancement factor of 5 and 3.12 times for the sample annealed at 400 degrees C, 8.6 and 7.25 times for the sample annealed at 500 degrees C, and 6 and 4 times for the sample annealed at 600 degrees C, as compared to Mn2+ ion undoped samples. The maximum emission strength for the green as well as red is observed for the sample annealed at 600 degrees C and co-doped with 3 mol Mn2+ ions. The laser power-dependent study on all the samples shows that the upconversion process is a multi-photon process, predominantly a two-photon process. [GRAPHICS] .