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394 Handbook on the Physics and Chemistry of Rare Earths (Carlos et al., 2009). Among many interesting applications, organic–inorganic hybrids can be interesting as host matrices for Ln3+ molecular thermometry (Milla ́n et al., 2016). Thanks to their dual character, hybrids can gather the physical properties of inorganic materials and the processability of organic polymers (Escribano et al., 2008; Sanchez et al., 2005, 2011). Besides, they can also benefit from the syn- thetic facilities of sol–gel process that can be performed at room temperature and, like the polymerization of organic monomers, can be activated in a variety of forms. An example is the thermometric system proposed by us in 2010 (Brites et al., 2010). The matrix was a diureasil hybrid framework formed by a siliceous backbone in which PEG chains of different lengths are grafted on both sides by urea bridges (Carlos et al., 2000; de Zea Bermudez et al., 1998, 1999). The encapsulation of the complexes was performed by mixing them with the alkoxysilane diureasil hybrid precursors in solution. Then, the hydrolysis and condensation reactions of the alkoxy groups produce the intercrossing of the polymer chains forming a solid organic–inorganic hybrid framework with a uniform distribution of the Ln3+ thermometric complexes. The hybrids, pro- cessed as monoliths or thick and thin films, can be applied on the object on which the temperature must be determined by spraying, dip, or spin coating. Diureasil hybrid films were used as self-referenced and efficient luminescent probes to map temperature in microelectronic circuits and optoelectronic devices (Brites et al., 2010, 2013a,c; Ferreira et al., 2013). The temperature mapping using Ln3+-doped hybrid matrices was reported by Brites et al. using a diureasil film doped with [Eu(btfa)3(MeOH)(bpeta)] and [Tb(btfa)3(MeOH)(bpeta)] b-diketonate chelates (Brites et al., 2010). The absolute emission quantum yield is 0.160.02 and Sm1⁄41.9%K1, at 201 K, diminishing to 0.7–0.4%K1 in the 300–350 K range. The high spatial resolution achievable with this system was demonstrated with a simple emis- sion detector system consisting on scanning with an optical probe (Fig. 25), which improved that recorded with a state-of-the-art commercial IR thermal camera. With a small-scanning step the resolution can be improved up to 0.42 mm (see Section 3.3.1 for the discussion on spatial resolution of lumines- cent thermometers), below the Rayleigh limit of diffraction (1.89 mm) asso- ciated to the fibers diameter (200 mm). Afterward, the same research group measured the temperature gradient between the two Mach–Zehnder arms using diureasil films embedded the [Eu(btfa)3(MeOH)(bpeta)] and [Tb(btfa)3(MeOH)(bpeta)] complexes (Ferreira et al., 2013). When compared with the IR camera, the diureasil thermometer presents a better spatial resolu- tion (28 mm instead of 128 mm) due to the significant temperature averaging of the camera that produces a smoothed readout of the temperature profile. This intrinsic limitation of the IR camera is critical to evaluate the tempera- ture gradient in the region where the Mach–Zehnder was fabricated, which critically precludes the output modeling and prediction of the device optical performance (Ferreira et al., 2013).PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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