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376 Handbook on the Physics and Chemistry of Rare Earths energy levels pairs with larger energy differences, care needs to be taken to ensure that the levels are not too far apart, so that thermalization is no longer observed, as stressed earlier. Additionally, as the energy difference extends, the population (and hence the fluorescence intensity) of the upper thermalized level decreases with concomitant practical problems in measuring signals with low SNR values. As mentioned earlier when the limits of Eq. (1) were dis- cussed, a reasonable operating approach must consider as reliable intensity values only those that are 5% higher than the noise level. Moreover, both Eqs. (29) and (30) are independent of the host matrix that supports the phos- phor, meaning that the same DE gap induces similar Sr and dT values. This is emphasized in a recent publication on NaYF4:Yb3+/Er3+ UC nanowires, nanorods, and nanoplates (Li et al., 2013). Until some time ago, the determination of dT was not very common. How- ever, as the field is growing out of its infancy and develops toward applications, this parameter becomes a key factor to compare different thermometers. In what follows, we will compute dT using Eq. (7) for selected examples of UCNPs. The NPs are, most often, composed of a host crystal (usually fluorides, oxides, phosphates, or sulfides of metal ions) doped with up to three Ln3+ ions (Bettinelli et al., 2015; Chen et al., 2015b; Wang et al., 2010; Zhou et al., 2015a). UCNPs are known to display luminescence that varies strongly in the physiological temperature range, thus suggesting their use in biomedical appli- cations, such as single-particle tracking, bioimaging and therapeutics, and ther- mal sensing (Bettinelli et al., 2015; Fischer et al., 2011; Gu et al., 2013; Il Park et al., 2015; Kumar et al., 2009; Wang and Liu, 2014; Wu et al., 2015; Zhou et al., 2015a). Among Ln3+ ions, Yb3+ and Nd3+ are by far the most used sen- sitizers, due to their large absorption cross sections at 980 and 808 nm, respec- tively, while Er3+, Tm3+, and Ho3+ are the most efficient activators (Bettinelli et al., 2015). The Yb3+/Er3+ pair uses the Yb3+ large absorption cross section at 980 nm and subsequent energy transfer to Er3+ enabling upconversion emis- sion in the green (2H11/2 ! 4I15/2 and 5S3/2 ! 4I15/2 transitions, ca 510–565 nm) and red (4F9/2 ! 4I15/2 transition, ca 620–670 nm) spectral regions. The ratio of the integrated intensities of the 2H11/2 ! 4I15/2 and 5S3/2 ! 4I15/2 transitions is used to sense temperature since the pivotal work of Shinn et al. (1983). Near room temperature, this ratio of integrated intensities permits to get a relative sensitivity of the order of 1%K1 and a minimum temperature uncertainty of 0.5 K, assuming a practical limit of dD/D 1⁄4 0.5% for commercial detectors. In Fig. 15, we compare the thermal uncertainty computed from the published data for a set of illustrative examples of single-center Yb3+/Er3+- and Yb3+/Tm3 +/Gd3+-based thermometers. We suggest comparing the thermal uncertainty of the single-center thermometers by plotting dT as a function of a normalized temperature, Tnor, defined as: kBT2 Tnor 1⁄4 DE (31)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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