Luminescence properties of Yb:Nd:Tm:KY3F10 nanophosphor and thermal treatment effects

Abstract

In this work, we present the spectroscopic properties of KY3F10 (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 1G4 (Tm3+) 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 Nd3+ to Tm3+ 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 802nm. An investigation of the 1G4 level luminescence kinetic of Tm3+ in Yb/Nd/Tm system revealed that the luminescence efficiency (1G4) starts with a very low value (0.38%) for the synthesized nanocrystal (as grown) and strongly increases to 97% after thermal treatment at 550°C for 6h under argon flow. As a consequence of the thermal treatment at T=550°C, the contributions of the (Nd×Tm) (Up1) and (Nd×Yb×Tm) (Up2) upconversion processes to the 1G4 luminescence are 33% (Up1) and 67% for Up2. Up2 process represented by Nd3+ (4F3/2)→Yb3+ (2F7/2) followed by Yb3+ (2F5/2)→Tm (3H4)→Tm3+ (1G4) was previously reported as the main mechanism to produce the blue luminescence in Yb:Nd:Tm:YLiF4 and KY3F10 bulk crystals. Results of X-ray diffraction analysis of nanopowder using the Rietveld method reveled that crystallite sizes remain unchanged (12–14nm) after thermal treatments with T≤400°C, while the 1G4 luminescence efficiency strongly increases from 0.38% (T=25°C) to 12% (T=400°C). Results shown that the Nd3+ ions distribution has a concentration gradient increasing towards the nanoparticle surface allowing the direct (Nd×Tm) (Up1) (78%) in competition with the (Nd×Yb×Tm) (Up2) (22%) upconversions for the synthesized nanocrystals (11nm).