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342 Handbook on the Physics and Chemistry of Rare Earths parts of a system are small enough to permit the definition of local tem- perature. However, the results predicted by such theoretical models are hardly comparable to the experimental temperature determination in nanoparticles (NPs). The limitations of contact thermometers for small systems where the spa- tial resolution decreases to the submicron scale have spurred the development of new noncontact (semi-invasive and noninvasive) accurate thermometers with micrometric and nanometric spatial resolution, a challenging research topic under constant development in the last five years (Brites et al., 2012; Jaque and Vetrone, 2012; Wang et al., 2013b; Yue and Wang, 2012). High- resolution thermometry noncontact techniques in micrometer/nanometer range have been cataloged in many different manners, as, for instance, depending on whether they make use of electrical or optical signals or are based on near- or far-field applications (Brites et al., 2012). Independently of the classification, what really matters is a precise identification of the advantages and drawbacks of each method, as well as a proper selection according to the required spatial, temporal, and temperature resolutions. Table 1 presents a summary of the current methods for noncontact high- resolution thermal measurements in micrometer/nanometer range, highlight- ing the advantages and disadvantages of each technique. Among noninvasive spectroscopic methods for determining temperature, the thermal dependence of phosphor luminescence—band shape, peak energy and intensity, and excited states, lifetimes, and risetimes—is one of the most promising accurate techniques (often referred to as thermographic phosphor thermometry). It works remotely with high-detection relative thermal sensitiv- ity (>1%K1) and spatial resolution (<10mm) in short acquisition times (<1ms), even in biological fluids, strong electromagnetic fields, and fast-moving objects (Brites et al., 2010; Jaque and Vetrone, 2012; Wang et al., 2013b). Temperature measurements based on intensity changes require ratiometric readout. The intensity ratio between two transitions is not compromised by the well-known disadvantages of experiments based on the intensity of only one transition, such as the critical dependence on variations of the sensor con- centration, small material inhomogeneities, and optoelectronic drifts of the excitation source and detectors, and, thus, are much more reliable (Brites et al., 2010; Jaque and Vetrone, 2012; Wang et al., 2013b). Luminescent thermal probes are derived from organic dyes, ruthenium complexes, spin crossover NPs, polymers, layered double hydroxides (LDHs), semiconductor quantum dots (QDs), and Ln3+-based materials. Apart from these single-component systems, there are more complex examples in which the temperature dependence of the emission intensity of the probe (and/or the lifetime of a given emitting state) is induced by a second compo- nent. In most of the cases, this temperature-responsive material is a polymer,PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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