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Lanthanides in Luminescent Thermometry Chapter 281 397 Eu/Tb ratio, Sa1⁄418.8%K1 for 0.1% Eu, corresponding to Sm1⁄41.39%K1 at 283 K (calculated by us). However, the thermometer precision decreases as the correlation coefficient of the linear fit degrades to r21⁄40.9894. The postfunctionalization method was also used to synthesize tunable white- with Sa 1⁄4 47.98% K1 at 140 K (that corresponds to Sm 1⁄4 3.01% K1 at 180 K, calculated by us) (Zhou and light emitters nanocrystalline MOFs Yan, 2015). 5.4 Upconverting Nanoparticles Like in molecular thermometers, UCNPs-based thermometers are based on one or two emission bands showing a different variation with temperature. However, the excitation mechanism (Fig. 20) is quite different as the light is captured by a sensitizer ion in a host crystal (Sedlmeier et al., 2012). A detailed description of the mechanisms of the upconversion process can be found in several recent reviews (Bettinelli et al., 2015; Burbano et al., 2015; Chen et al., 2014; Liu, 2015; Liu et al., 2013; Wang and Liu, 2009; Zhou et al., 2015a,b). Moreover, the sensitizer, usually Yb3+, that has a large absorption cross section, absorbs IR light (980 nm) transferring the energy to the activator in a double- or triple-photon processes. The activator releases this energy by multiple emission bands at shorter wavelengths (visible) that have different response to temperature variations. Common activators in UCNPs (eg, Er3+, Tm3+, Ho3+, Tb3+, and Eu3+) are characterized by a ladder-like energy level structure and they are incorporated in the host lattice as dopants in a ratio lower than 2 mol% to minimize cross-relaxation energy losses. The 2F7/2!2F5/2 transition of Yb3+ is resonant with many f–f transi- tions of the activators, thus facilitating efficient energy transfer. Suitable host compounds are CaF2, NaYF4, NaGdF4, NaY2F5O, LaF3, Y2O3, Gd2O3, ZnO, Al2O3, Y2Ti2O7, Y3A5O12, YNbO4, CaMoO4, BaTiO3, SrWO4, KLu(WO4)2, and LnPO4. Like MOFs, UCNPs have a crystalline structure, and their thermometric response is very reproducible. On the other hand, they need high excitation power and they are normally operated with laser light sources. They also have lower sensitivities than hybrid NPs made with the same lanthanide ions (Dong et al., 2012). Host compounds are sparingly soluble and form crystals of nanometric size by simple coprecipitation in water at moderate temperatures. However, this method yields high size dispersion, low crystallinity, and agglomeration. These drawbacks can be overcome by performing the synthesis at high tem- perature, Fig. 26. A frequent synthesis method is the hydrothermal reaction of lanthanide salts and fluorides that can be assisted by microwaves heating. They are also prepared by thermal decomposition of organic precursors in organic solvents at high temperature in the presence of surfactants. This method yields very low size dispersion, but it is obviously more expensive.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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