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.
This paper provides the detailed study of (nano)particle's size effect on structural and luminescent properties of LaPO 4 :Eu 3+ synthesized by four different methods: high temperature solid-state, co-precipitation, reverse micelle and colloidal. These methods delivered monoclinic monazite-phase submicron particles (> 100 nm), 4 × 20 nm nanorods and 5 nm spheres (depending on the annealing temperature), 2 × 15 nm nanorods, and ultrasmall spheres (2 nm), respectively. The analysis of emission intensity dependence on Eu 3+ concentration showed that quenching concentration increases with a decrease of the particle size. The critical distance for energy transfer between Eu 3+ ions is found to be 18.2 Å, and the dipole-dipole interaction is the dominant mechanism responsible for the concentration quenching of emission. With the increase in Eu 3+ concentration, the unit-cell parameter slightly increases to accommodate larger Eu 3+ ions at sites of smaller La 3+ ions. Photoluminescent emission spectra presented four characteristic bands in the red spectral region: at 592 nm (5 D 0 → 7 F 1), at 612 nm (5 D 0 → 7 F 2), at 652 nm (5 D 0 → 7 F 3) and at 684 nm (5 D 0 → 7 F 4), while in small colloidal nanoparticles additional emission bands from host defects appear at shorter wavelengths. Intensities of f-f electronic transitions change with particles size due to small changes in symmetry around europium sites, while emission bandwidths increase with the reduction of particle size due to increased structural disorder. Judd-Ofelt analysis showed that internal quantum yield of Eu 3+ emission is strongly influenced by particle's morphology.
The emission of Er3+ provides three combinations of emission bands suitable for ratiometric luminescence thermometry. Two combinations utilize ratios of visible emissions (2H11/2→4I15/2 at 523 nm/ 4S3/2→4I15/2 at 542 nm and 4F7/2→4I15/2 at 485 nm/ 4S3/2→4I15/2 at 545 nm), while emissions from the third combination are located in near-infrared, e.g., in the first biological window (2H11/2→4I13/2 at 793 nm/ 4S3/2→4I13/2 at 840 nm). Herein, we aimed to compare thermometric performances of these three different ratiometric readouts on account of their relative sensitivities, resolutions, and repeatability of measurements. For this aim, we prepared Yb3+,Er3+:YF3 nanopowders by oxide fluorination. The structure of the materials was confirmed by X-ray diffraction analysis and particle morphology was evaluated from FE-SEM measurements. Upconversion emission spectra were measured over the 293–473 K range upon excitation by 980 nm radiation. The obtained relative sensitivities on temperature for 523/542, 485/542, and 793/840 emission intensity ratios were 1.06 ± 0.02, 2.03 ± 0.23, and 0.98 ± 0.10%K−1 with temperature resolutions of 0.3, 0.7, and 1.8 K, respectively. The study showed that the higher relative temperature sensitivity does not necessarily lead to the more precise temperature measurement and better resolution, since it may be compromised by a larger uncertainty in measurement of low-intensity emission bands.
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.
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