A new thermographic nanocrystalline Sr 4 Al 14 O 25 :Mn 4+ ,Tb 3+ phosphor was developed, and the concentrations of both dopants and the synthesis conditions were optimized. The combination of the thermally quenched luminescence from the Mn 4+ ions to the almost temperature-independent emission from Tb 3+ provides a sensitive luminescent thermometer ( S R = 2.8%/°C at 150 °C) with strong emission color variability. In addition, a figure of merit for this luminescence thermochromism was proposed, as the relative sensitivities of the x and y CIE coordinates, which for this phosphor reaches at 150 °C S R ( x ) = 0.6%/°C and S R ( y ) = 0.4%/°C, respectively. Noncontact thermal imaging was demonstrated with this phosphor using a single consumer digital camera and exploiting the ratio of red (R) and green (G) channels of the RGB images, thereby confirming the high application potential of Sr 4 Al 14 O 25 :Mn 4+ ,Tb 3+ nanocrystals for thermal sensing and mapping.

The near-infrared luminescence of Ca6Ba(PO4)4O:Mn5+ is demonstrated and explained. When excited into the broad and strong absorption band that spans the 500–1000 nm spectral range, this phosphor provides an ultranarrow (FWHM = 5 nm) emission centered at 1140 nm that originates from a spin-forbidden 1E → 3A2 transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 μs. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe–Sugano diagram for this phosphor. We found that 1E emission quenches due to the thermally-assisted cross-over with the 3T2 state and that the relatively high Debye temperature of 783 K of Ca6Ba(PO4)4O facilitates efficient emission. Since Ca6Ba(PO4)4O also provides efficient yellow emission of the Eu2+ dopant, we calculated and explained its electronic band structure, the partial and total density of states, effective Mulliken charges of all ions, elastic constants, Debye temperature, and vibrational spectra. Finally, we demonstrated the application of phosphor in a luminescence intensity ratio thermometry and obtained a relative sensitivity of 1.92%K−1 and a temperature resolution of 0.2 K in the range of physiological temperatures.

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The influence of the crystal field strength and host material composition on the spectral position of the emission band and thermal stability of Fe3+ luminescence was investigated in order to develop a highly sensitive luminescence thermometer.

This paper describes Mn 5+ -activated Sr 3 (PO 4 ) 2 and Ba 3 (PO 4 ) 2 phosphors as near-infrared lifetime-based luminescence thermometry probes. Materials were prepared by a solid-state method, and their rhombohedral structures were confirmed by Xray diffraction analysis. Diffuse reflectance measurements showed broad and strong absorption between 650 and 950 nm covering the first biological transparency window and having an absorption maximum at ∼660 nm. By switching Sr with Ba, the following changes in the photoluminescent properties were observed: (i) a red shift of the emission maximum (1173 nm → 1191 nm) and (ii) a decrease in the excited-state lifetime. Thermometric properties of the phosphors were assessed by measuring and analyzing the temperature dependence of the Mn 5+ excited-state lifetime. Lifetime-based luminescence thermometry revealed a relative sensitivity of 0.5% K −1 at 310 K (physiologically relevant range) and a maximal value of ∼1% K −1 at temperatures between 400 and 500 K.

Luminescence (nano)thermometry is an emerging and promising field for remote temperature sensing and thermal imaging of both the surface and interior of objects. While the field is dominated by trivalent...

reliability of temperature readout, which is achievable by 1) increasing the luminescence brightness and/or 2) increasing the relative sensitivity of the luminescent thermometer. [5,6,9] For the materials that are currently most intensively used and studied in luminescence thermometry, i.e., inorganic materials doped with luminescent ions, two strategies are currently employed, 1) the utilization of singly doped materials; and 2) exploitation of Co-doped materials. [10][11][12][13][14][15][16][17][18][19][20][21][22] While the latter approach yields very promising results, there is a significant risk that scaling-up the synthesis of materials to amounts required by industry, the inhomogeneous distribution of dopant ions and their clustering may occur. This may modify the thermometric performance of the phosphor and lead to an unreliable temperature readout. [5,9] On the other hand, for ratio metric temperature measurements based on phosphors doped with one type of luminescent dopant, luminescence intensity ratio from thermally coupled levels is used as a thermometric parameter. [9,[23][24][25] In this approach, the relative sensitivity is proportional to the energy difference ΔE between these levels. [26] It is well known that above ΔE = 2000 cm -1 , the thermal energy required to populate the higher-laying state becomes too high to obtain intense emission. [26] Hence, a further increase in sensitivity is difficult to achieve. In response to the above-mentioned limitations and the requirement to develop luminescent thermometers based on single ion luminescence with high relative sensitivity, the application of materials exhibiting temperature-induced firstorder phase transitions has recently been proposed. [27] In this work, a new luminescent thermometer, whose operating principle exploits a temperature-induced first-order phase transition, is proposed. Although luminescent thermometers using structural phase transition have been described in the literature before, the main limitation of these thermometers is their relatively low sensitivity. [28,29] Therefore, here, LiYO 2 :Nd 3+ nanocrystals have been synthesized for which the temperature increase above ≈290 K induces a structural change from monoclinic (P2 1 /c, Z = 4) to tetragonal (I4 1 /amd, Z = 4) and thus a change in the point symmetry of the crystallographic site of Y 3+ (occupied by the luminescent Nd 3+ ions) from C 2 to D 2d . [30][31][32][33] This symmetry change clearly affects the energy of the Stark levels of the 4 I J mutiplets (Figure S3, Supporting Information) Almost all existing luminescent thermometers rely on the temperaturedependent processes such as multi-phonon relaxation and phonon-assisted energy transfers, thermal population, or coupling between energy levels of ground and excited states of luminescent species (lanthanides, transition metals, quantum dots, fluorescent molecules, etc.). Although such phenomena are in principle suitable for straightforward calibration, aiming to offer high temperature sensitivity, high temperature resol...

Despite the numerous advantages of lanthanide based luminescent thermometers, a major limitation is the limited brightness associated with the low absorption cross section of the 4f-4f transitions. In this paper,...