RESUMO
Radioluminescence and visible photoluminescence tunability features from a single Tm3+-doped yttrium tantalate phosphor prepared by a soft sol-gel method designed to afford cubic Y3TaO7 and monoclinic M'-YTaO4 crystalline phases are reported. The annealing temperature influenced the crystallization kinetics and stabilized a preferential phase. To investigate how the crystalline phase affected the Tm3+ optical properties, excitation and emission spectra in the visible range were recorded for the samples annealed at 900 or 1100 °C. Inhomogeneous broadening in the emission spectra was due to the structural disorder of the Y3TaO7 phase. Energy transfer between the yttrium tantalate host and Tm3+ ions was observed upon CT band excitation. Under UV light, an intense and tunable cyan to blue emission ascribed to both the Tm3+ transitions 1D2 â 3F4 and 1G4 â 3H6 also emerged and could be observed by the naked eye. The lifetime decay curves demonstrated the occupation of distinct sites and that the symmetry sites occupied by Tm3+ ions in the Y3TaO7 host have higher lifetime values than in the M'-YTaO4 phase. A radioluminescence study was carried out to evaluate the yttrium tantalate scintillation performance, which was considerably enhanced in the presence of the M'-YTaO4 phase. Intense white light emission displaying a large color correlated temperature range could be obtained by controlling the delay time for the time-resolved measurements and upon an orange-emitting phosphor addition. All the above-mentioned structural and photoluminescence properties make these Tm3+-doped yttrium tantalates potential candidates for photonic applications, particularly integrated w-LED systems.
RESUMO
Glass and glass-ceramics containing nanocrystals of Bi2Te4O11 cubic phase co-doped with Er3+ and Yb3+ were prepared by heat treatment of the precursor tellurite glass and investigated for optical applications. Lanthanide doped tellurite glass and glass-ceramics have been extensively investigated because of their optical and photoluminescence performance for technological photonic applications. Er3+ and Er3+/Yb3+ doped TeO2-GeO2-K2O-Bi2O3 tellurite glass compositions were prepared by the conventional melt-quenching method. Photoluminescence results showed the important role played by Yb3+ ions when co-doping with Er3+ ions in comparison with the Er3+ single-doped glass. Due to their larger absorption cross-section, Yb3+ species significantly absorbs 980 nm photons and effectively transfers them to Er3+ ions via a set of mechanisms including ground-state absorption (GSA), excited-state absorption (ESA), and energy transfer upconversion (ETU). Er3+/Yb3+ co-doped sample was chosen for the synthesis of transparent glass-ceramics by controlled heat treatment above Tg for 5 to 120 min. X-ray diffraction patterns, high-resolution transmission electron microscopy (TEM) images, and selected area electron diffraction (SAED) from Er3+/Yb3+ co-doped glass-ceramic samples were used to verify the nanocrystal precipitation, crystalline phase, and chemical nature. The structural change resulting from the crystallization of Bi2Te4O11 nanocrystals was evaluated by the Raman shift of the bands between 300-500 cm-1, which are assigned to the formation of Bi-O-Te linkages and the reduction of [TeO3] depolymerized units. The effects of HT time on the glass-ceramic's optical and upconversion photoluminescence properties were studied in the visible range under excitation at 980 nm in terms of the energy transfer mechanisms from Yb3+ to Er3+. Results indicate that Er3+/Yb3+ co-doped tellurite glass and glass-ceramics are potential candidates for photonic applications in lighting, energy conversion, and luminescent solar cell concentrators.