Luminescence properties of Yb:Nd:Tm:KY sub(3)F sub(10) nanophosphor and thermal treatment effects

In this work, we present the spectroscopic properties of KY sub(3)F sub(10) (KY3F) nanocrystals activated with thulium and codoped with ytterbium and neodymium ions. The most important processes that lead to the thulium upconversion emissions in the blue region were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays and to determine the most important mechanisms involved in the upconversion process that populates super(1)G sub(4) (Tm super(3+)) excited states. Analysis of the energy-transfer processes dynamics using selective pulsed-laser excitations in Yb:Nd:Tm, Nd:KY3F nanocrystals shows that the direct energy transfer from Nd super(3+) to Tm super(3+) ions is the mechanism responsible for the 78% of the blue upconversion luminescence in the Yb:Nd:Tm:KY3F when compared with the Yb:Nd:Tm:KY3F bulk crystal for an laser excitation at 802 nm. An investigation of the super(1)G sub(4) level luminescence kinetic of Tm super(3+) in Yb/Nd/Tm system revealed that the luminescence efficiency ( super(1)G sub(4)) starts with a very low value (0.38%) for the synthesized nanocrystal (as grown) and strongly increases to 97% after thermal treatment at 550 degree C for 6 h under argon flow. As a consequence of the thermal treatment at T=550 degree C, the contributions of the (NdTm) (Up sub(1)) and (NdYbTm) (Up sub(2)) upconversion processes to the super(1)G sub(4) luminescence are 33% (Up sub(1)) and 67% for Up sub(2). Up sub(2) process represented by Nd super(3+) ( super(4)F sub(3/2)) arrow right Yb super(3+) ( super(2)F sub(7/2)) followed by Yb super(3+) ( super(2)F sub(5/2)) arrow right Tm ( super(3)H sub(4)) arrow right Tm super(3+) ( super(1)G sub(4)) was previously reported as the main mechanism to produce the blue luminescence in Yb:Nd:Tm:YLiF sub(4) and KY sub(3)F sub(10) bulk crystals. Results of X-ray diffraction analysis of nanopowder using the Rietveld method reveled that crystallite sizes remain unchanged (12-14 nm) after thermal treatments with T less than or equal to 400 degree C, while the super(1)G sub(4) luminescence efficiency strongly increases from 0.38% (T=25 degree C) to 12% (T=400 degree C). Results shown that the Nd super(3+) ions distribution has a concentration gradient increasing towards the nanoparticle surface allowing the direct (NdTm) (Up sub(1)) (78%) in competition with the (NdYbTm) (Up sub(2)) (22%) upconversions for the synthesized nanocrystals (11 nm).