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374 Handbook on the Physics and Chemistry of Rare Earths FIG. 13 (A) Overlap between the 1G4 ! 3F4 and 3F2,3 ! 3H6 Tm3+ transitions in NaYF4:Yb3 +/Tm3+@Pr3+ core–shell NPs (980 nm excitation, 510 K) (Zhou et al., 2014a). The thermometric parameter is defined by integrating the emission spectrum between 625 and 665 nm (I1) and between 655 and 740 nm (I2). (B) Fit of the emission spectrum displayed in (A) using four Gaussians (three for 3F2,3 ! 3H6 and one for 1G4 ! 3F4) to compute a new thermometric parameter using the fitted peak areas. (C) The calibration curves reported by Zhou et al. (2014a) (solid circles) and that computed using the ratio of the areas of the corresponding Gaussian peaks (open circles) are identical, apart from a vertical shift (different B values). The solid and interrupted lines correspond, respectively, to the best fit to data using Eq. (27) (as reported) and Eq. (25) (r2 > 0.998). (D) The relative sensitivity computed using the different calibration curves converge for T > 450 K. For lower temperatures, however, the reported procedure (Zhou et al., 2014a) sig- nificantly underestimates the relative sensitivity. The next step is to calculate the integrated area of each transition by the area of the sum of the Gaussians, for each spectrum recorded at different tempera- tures. As outcome, we are able to compute D, according to Eq. (1), and build a new calibration curve, which is vertically shifted from that reported by the authors (Fig. 13C). Fitting the calculated D to Eq. (25) gives B1⁄49806 and DE 1⁄4 2431 cm1 (r2 > 0.998). In comparing the sensitivity values resulting from the fitting to Eqs. (25) and (27), we realize that the latter underestimates the sensitivity, especially for lower temperatures (Fig. 13D). For this reason, we advocate the use of Eq. (25) instead of Eq. (27) that produces higher sen- sitivity values making clear the comparison between the different emitting centers, independently of the transitions overlap. This is exactly what wasPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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