Novel and easy access to highly luminescent Eu and Tb doped ultra-small CaF, SrF and BaF nanoparticles - structure and luminescence.

A universal fast and easy access at room temperature to transparent sols of nanoscopic Eu and Tb doped CaF, SrF and BaF 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 CaF:Eu10 via SrF:Eu10 to BaF: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 Ce and Tb results in a huge increase (>50 times) of the green luminescence intensity due to energy transfer Ce → Tb. In this case, the luminescence intensity is higher for CaF than for SrF, due to a lower spatial distance of the rare earth ions.