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- Alkaline earth metal fluorides (1)
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- Fluorolytic sol–gel synthesis (1)
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In this article, the high potential of the fluorolytic sol–gel process to synthesize nanoscopic rare earth-doped calcium fluoride sols is shown. Through a fluorolytic sol–gel process we manage to achieve spherical monodisperse ~5 nm sized nanoparticles using a simple and reproducible one-pot-wet chemical route at room temperature. The as-synthesized clear sols exhibit an intense red and green luminescence under UV excitation at room temperature. A spectroscopic study of the sols revealed the characteristic transitions 5D0 → 7FJ of Eu3+ and 5D4 → 7FJ of Tb3+, with 5D0 → 7F2 (611 nm) of Eu3+ and 5D4 → 7F4 (581 nm) of Tb3+ as the most prominent transitions. This facile synthetic strategy is also valuable for developing other luminescent nanoparticles.
A universal fast and easy access at room temperature to transparent sols of nanoscopic Eu3+ and Tb3+ doped CaF2, SrF2 and BaF2 particles via the fluorolytic sol–gel synthesis route is presented. Monodisperse quasi-spherical nanoparticles with sizes of 3–20 nm are obtained with up to 40% rare earth doping showing red or green luminescence. In the beginning luminescence quenching effects are only observed for the highest content, which demonstrates the unique and outstanding properties of these materials. From CaF2:Eu10 via SrF2:Eu10 to BaF2:Eu10 a steady increase of the luminescence intensity and lifetime occurs by a factor of ≈2; the photoluminescence quantum yield increases by 29 to 35% due to the lower phonon energy of the matrix. The fast formation process of the particles within fractions of seconds is clearly visualized by exploiting appropriate luminescence processes during the synthesis. Multiply doped particles are also available by this method. Fine tuning of the luminescence properties is achieved by variation of the Ca-to-Sr ratio. Co-doping with Ce3+ and Tb3+ results in a huge increase (>50 times) of the green luminescence intensity due to energy transfer Ce3+ → Tb3+. In this case, the luminescence intensity is higher for CaF2 than for SrF2, due to a lower spatial distance of the rare earth ions.