Luminescence properties of Yb:Nd:Tm:KY 3 F 10 nanophosphor and thermal treatment effects Laércio Gomes a,n , Horácio Marconi da Silva M.D. Linhares a , Rodrigo Uchida Ichikawa b , Luis Gallego Martinez b , Izilda Marcia Ranieri a a Centro de Lasers e Aplicações, Instituto de Pesquisas Energéticas e Nucleares, IPEN-CNEN/SP, Butantã, P.O. Box 11049, São Paulo, SP o5422-970, Brazil b Departamento de Ciências dos Materiais, Instituto de Pesquisas Energéticas e Nucleares, Brazil article info Article history: Received 30 April 2014 Received in revised form 22 August 2014 Accepted 25 August 2014 Available online 6 September 2014 Keywords: Time-resolved luminescence spectroscopy Rare-earth luminescence Energy transfer rate parameters Upconversion luminescence Luminescence efficiency Nanophosphors abstract In this work, we present the spectroscopic properties of KY 3 F 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 1 G 4 (Tm 3 þ ) 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 3 þ to Tm 3 þ 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 1 G 4 level luminescence kinetic of Tm 3 þ in Yb/Nd/Tm system revealed that the luminescence efficiency ( 1 G 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 1C for 6 h under argon flow. As a consequence of the thermal treatment at T ¼550 1C, the contributions of the (Nd  Tm) (Up 1 ) and (Nd  Yb  Tm) (Up 2 ) upconversion processes to the 1 G 4 luminescence are 33% (Up 1 ) and 67% for Up 2 . Up 2 process represented by Nd 3 þ ( 4 F 3/2 )-Yb 3 þ ( 2 F 7/2 ) followed by Yb 3 þ ( 2 F 5/2 )-Tm ( 3 H 4 )-Tm 3 þ ( 1 G 4 ) was previously reported as the main mechanism to produce the blue luminescence in Yb:Nd:Tm:YLiF 4 and KY 3 F 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 r400 1C, while the 1 G 4 luminescence efficiency strongly increases from 0.38% (T ¼25 1C) to 12% (T ¼400 1C). Results shown that the Nd 3 þ ions distribution has a concentration gradient increasing towards the nanoparticle surface allowing the direct (Nd  Tm) (Up 1 ) (78%) in competition with the (Nd  Yb  Tm) (Up 2 ) (22%) upconversions for the synthesized nanocrystals (11 nm). & 2014 Elsevier B.V. All rights reserved. 1. Introduction With the oncoming of the nanoscience and nanotechnology in the past few decades, this new field has exerted great impact on upconversion materials, then studies on the synthesis and proper- ties of upconversion nanophosphors [1] have received intense research interest due to their application as luminescent labels in bioimaging, and as donors in energy transfer systems [2,3]. Within this aim, fluoride crystals doped with trivalent rare earth ions has extensively studied due to generation of efficient multicolor emission lines in the visible when excited by infrared diode laser near 800 and 970 nm (the optical window for biological samples from 700 to 1200 nm). In particular, KY 3 F 10 (KY3F) crystal has been studied as luminescent materials when activated by several RE 3 þ ions, which can easily substitute Y 3 þ ions in a non-center- symmetrical site [C 4v symmetry] [4,5–8]. KY3F is the only com- pound in the KF–YF 3 system that melts congruently without any phase transition. It crystallizes in the cubic fluorite-type structure (Fm3m) with a lattice parameter a ¼ 11.553 Å [8], which constitu- tes an isotropic crystal. In particular, Yb:Nd:Tm:KY 3 F 10 can be synthesized as nanocrystals able to efficiently emit at 482 nm when excited at 802 nm due to its relatively low phonon energy (cut off  500 cm 1 ) that is an important point in avoiding energy loss by non-radiative relaxation involving the 1 G 4 and 3 H 4 excited levels of Tm 3 þ [9,10]. In this paper, nanocrystals of Yb:Nd:Tm:KY3F sizing 11 nm were synthesized by co-precipitation method in aqueous solution. The multiple processes of energy transfer that occur when this material Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2014.08.048 0022-2313/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ55 11 31339380. E-mail address: lgomes@ipen.br (L. Gomes). Journal of Luminescence 157 (2015) 285–292