Authors: A.F. García-Flores, G.G. Lesseux, W. Iwamoto, R.R. Urbano, C. Rettori
Affilation: Universidade Federal do ABC, Brazil
Pages: 59 - 62
Keywords: upconversion, rare-earths, magnetic-properties
In this work we have investigated Yb/Er and Yb/Tm co-doped NaYF4 nanoparticles (NPs) as well as Yb and Er single doped ones by means of luminescence, high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD), magnetization and electron spin resonance (ESR) measurements. These NPs were synthetized following the chemical method. The upconversion emissions of the Yb/Er and Yb/Tm co-doped nanoparticles were observed with naked eye when excited with a laser light of 980 nm. HRTEM and XRD showed a mixture of both cubic (15-25 nm) and hexagonal phase (about 230 nm). From Curie-Weiss fits of the temperature dependence of the magnetization we obtained the oxidation states of Yb3+ (4f13, J = 7/2) and Er3+ (4f11, J = 15/2) also confirmed by the observed ESR signals. As expected for NPs with mixture of cubic and hexagonal phases, the ESR resonances are broader toward high magnetic fields due to the anisotropic contribution of the crystal field to the Kramer ground state g-values. Therefore, our magnetization and ESR measurements confirm that the rare earths are really incorporated in our NaYF4 NPs and that they participate and are responsible for the upconversion process observed in these systems.