RESUMO
The influence of erbium ion concentration on the optical properties of BaF2:ErF3 crystals was investigated. Four ErF3 concentration (0.05, 0.08, 0.15 and 0.5 mol% ErF3)-doped BaF2 crystals were obtained using the Bridgman technique. Room temperature optical absorption in the 250-850 nm spectral range was measured, and the photoluminescence (PL) and decay times were also investigated. The Judd-Ofelt (JO) approximation was used, taking into account four absorption peaks (at 377, 519, 653 and 802 nm). The JO intensity parameters, Ωt (t = 2, 4, 6), were calculated. The influence of the ErF3 concentration on the JO parameters, branching ratio, radiative transition probability and radiative lifetime were studied. The obtained results were compared with measured values and with those reported in the literature. Under excitation at 380 nm, the well-known green (539 nm) and red (668 nm) emissions were obtained. The calculated and experimental radiative lifetimes were in millisecond range for green and red emissions. The intensity of the PL spectra varied with the Er3+ ion concentration. The emission intensity increased linearly or exponentially, depending on the ErF3 concentration. Under excitation at 290 nm, separate to the green and red emissions, a new UV emission band (at 321 nm) was obtained. Other research has not reported the UV emission or the influence of ErF3 concentration on emission behavior.
RESUMO
In this work, several nanostructures (nanopowders and nanostars) of undoped and 1%, 3% and 5% europium (Eu3+)-doped ZnO have been synthesized via coprecipitation method using oxalic acid and sodium hydroxide as precipitation agents. Starting from zinc acetate and europium acetate, nanopowders were obtained by coprecipitation with oxalic acid. ZnO based nanostars were synthesized by coprecipitation of Zn2+ and Eu3+ with hydroxide ions (HO-), when zinc chloride and europium acetate were used as reagents. The structure and morphology of the as-prepared ZnO nanopowders and nanostars were investigated by X-ray diffraction and electron microscopy. Only würtzite structure of ZnO was identified in all the samples based on ZnO. Transmission electron microscopy (TEM) investigations have shown an average particle÷crystallite size range from 23 to 29 nm and polyhedral and spherical morphology with tendency to form aggregates for nanopowders. Cytotoxicity tests on MG-63 cell lines was also performed. Photocatalytic activity of ZnO nanopowders have reached higher values compared to ZnO nanostars. The photocatalytic test indicates that the ZnO nanopowders have better activity than the nanostars, most probably because of the higher specific surface. Doping the ZnO with Eu2O3 does not seem to alter it in a decisive manner. The toxicity results indicated that ZnO nanoparticles (NPs) high toxicity on tumoral cells is also induced by particle size and, consequently, the dissolution of Zn2+ ions is dependent on the size of the particles, increasing with the particles size.