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Powders of Y3NbO7:xDy3+ (x = 0.5, 1, 1.5, 3 and 5 mol%) were produced through a solid-state process. X-ray diffraction investigations confirm the fluorite-type structure (space group ??3̅?) of powders with crystallite size in the range of 19–60 nm. Both the photoluminescence excitation and emission spectra revealed the presence of defects within the material. With different excitation wavelengths, the emission spectra exhibited distinct emission patterns. At each excitation wavelength, the emission was quenched at Dy3+ concentration higher than 1 mol%. The decay time measurements of the highest intensity emission revealed a progressive decrease from 0.472 milliseconds for x = 0.5 mol% to 0.246 milliseconds for x = 5 mol%. The CIE chromaticity coordinates investigation revealed that the emission color may be altered by varying the excitation wavelength, ranging from blue (excitation at 333 nm) and near-white (excitation at 353 nm and 390 nm) to orange (excitation at 457 nm). The chromaticity of emission under 353 nm and 390 nm excitation validated the material's suitability as an almost-white phosphor.

Herein, we demonstrate the photoluminescence properties of Dy3+-activated YNbO4, LuNbO4, and mixed YxLu1−xNbO4:Dy3+ (x=0.25, 0.5, 0.75) phosphors. For this purpose, fve samples with a fxed Dy3+ concentration (2 mol%) were prepared by the solid-state reaction method. X-ray difraction measurements showed that all phosphors crystallize in a monoclinic fergusonite-beta-(Y) structure with a C2/c space group. Scanning electron microscopy clearly shows that samples are composed of dense, well-developed micron-sized, cube-shaped grains with rounded edges. The photoluminescent emission spectra feature Dy3+ peaks at standard positions corresponding to transitions from the 4 F9/2 excited emitting level to the 6 HJ (J=15/2; 13/2; 11/2 and 9/2) lower levels with two dominant emission bands placed in the blue (~479 nm, B) and yellow (~576 nm, Y) spectral region. It is observed that with Lu increase in the host lattice Y/B ratio decreases toward the desired ratio of unity to obtain white light. To evaluate the suitability of these phosphors for use in solid-state lighting, their photoluminescence emission was analyzed in detail by calculating CIE coordinates, correlated color temperature (CCT) and Delta u,v (DUV). It is shown that CIE chromaticity coordinates of all Dy3+-activated YxLu1−xNbO4 samples (x=0, 0.25, 0.5, 0.75, and 1) fall into the white portion of the diagram and that with the increase of Lu in the host lattice color becomes whiter. CCT values for all samples are in the cooler 4000–4500 K range with positive DUVs indicating that color points are placed above the black body curve. The average lifetime of 4 F9/2 level is calculated to be~0.2 ms for all Dy3+-activated YxLu1−xNbO4 samples, indicating that there is no infuence of the Y-to-Lu ratio in the host niobate material on the luminescence kinetics.

Luminescence thermometry has emerged as a very versatile optical technique for remote temperature measurements, exhibiting a wide range of applicability spanning from cryogenic temperatures to 2000 K. This technology has found extensive utilization across many disciplines. In the last thirty years, there has been significant growth in the field of luminous thermometry. This growth has been accompanied by the development of temperature read-out procedures, the creation of luminescent materials for very sensitive temperature probes, and advancements in theoretical understanding. This review article primarily centers on luminescent nanoparticles employed in the field of luminescence thermometry. In this paper, we provide a comprehensive survey of the recent literature pertaining to the utilization of lanthanide and transition metal nanophosphors, semiconductor quantum dots, polymer nanoparticles, carbon dots, and nanodiamonds for luminescence thermometry. In addition, we engage in a discussion regarding the benefits and limitations of nanoparticles in comparison with conventional, microsized probes for their application in luminescent thermometry.

Luminescent Eu3+-doped SrF2 nanoparticles were synthesized using microwave-assisted synthesis. Their surfaces were modified either by polyethylene glycol or mercaptopropionic acid to enhance their dispersibility in aqueous media and colloidal formation. X-ray diffraction analysis confirmed single-phase cubic structure in both types of hydrophilic-modified Sr0.9Eu0.1F2 luminescent powders. The average crystallite size of nanoparticles is found to be ∼13 nm. Scanning electron microscopy of the representative PEG2000-modified sample revealed that particles form agglomerates composed of densely packed nanoparticles. The microstructure at a local level was investigated by transmission electron microscopy showing the presence of sphere-like nanoparticles with an average particle size of 12.5 nm and 14 nm for PEG-modified and MPA-modified Sr0.9Eu0.1F2, respectively. Both PEG2000 and MPA-modified Sr0.9Eu0.1F2 water dispersions show strong red Eu3+ emission under 405 nm excitation that is quenched in the presence of the primary NP fertilizer, ammonium dihydrogen phosphate, NH4H2PO4. The limit of detection was calculated to be ∼19 mM for both hydrophilic-modified Sr0.9Eu0.1F2 luminescent nanoparticles.

Polycrystalline LiInO2 was synthesized by the solid-state method and was characterized using combined experimental and computational methods. The results of experimental measurements were compared with the computational analysis of lattice vibrations based on DFT calculations. A good agreement between experimental and theoretical frequencies of LiInO2 structure vibration mode was obtained. © 2023, Publishing House of the Romanian Academy. All rights reserved.

As yttrium niobates, the Y2O3–Nb2O5 combination exists in two forms: YNbO4 and Y3NbO7, and the latter has been studied as a host for photoluminescent ions to a limited extent [1-3]. In this research, the powders of Y3NbO7:xDy3+ (x = 0.5, 1, 1.5, 3 and 5 mol%) were produced through a solid-state process. X-ray diffraction investigations confirmed the fluorite-type structure (space group ) of powders with crystallite size in the range of 19–60 nm. Both the photoluminescence excitation and emission spectra revealed the presence of defects within the material. With different excitation wavelengths, the emission spectra exhibited distinct emission patterns. At each excitation wavelength, the emission was quenched at Dy3+ concentration higher than 1mol%. The decay time measurements of the highest intensity emission revealed a progressive decrease from 0.472 milliseconds for x = 0.5 mol% to 0.246 milliseconds for x = 5 mol%. The CIE chromaticity coordinates investigation revealed that the emission color may be altered by varying the excitation wavelength, ranging from blue (excitation at 333 nm) and nearwhite (excitation at 353 nm and 390 nm) to orange (excitation at 457 nm). The chromaticity of emission under 353 nm and 390 nm excitation validated the material's suitability as an almostwhite phosphor.

Oxide compounds of the niobate type with the composition ANbO4 (A = Lu, Y, La, Gd) have promising properties as inorganic luminescence host materials [1-5]. The luminescence process of converting lower-energy, long-wavelength NIR light into higher-energy UV or VIS light, known as the luminescence up-conversion (UC) phenomenon, finds wide application in optical and sensor devices and bio-medical imaging and therapy [6]. The UC of rare-earth (RE)-based materials appears mainly through excited-state absorption, energy-transfer upconversion, and photon avalanche mechanisms, the latter of which is rarely found. Niobates ANbO4 (A = Lu, Y, La, Gd) show energy-transfer UC luminescence when doped with Er3+, Ho3+, and Tm3+ ions as activators and often co-doped with Yb3+ ions as sensitizers [7]. The choice of yttrium niobate (YNbO4) as the matrix and rare-earth ions of erbium/ytterbium for UC activation seems preferable because YNbO4 has low phonon energy, is both chemically and thermally stable as a host material, with a fairly simple synthesis, while Er3+ as an activator has a suitable energy level spacing and can be excited by several IR wavelengths, and Yb3+ as a sensitizer has a higher probability of absorption under 980 nm excitation which can increase the energy-transfer UC [8]. In our recent study, erbium (1 at. %) and ytterbium (2 at. %) doped yttrium niobate (YNbO4:Er,Yb) nanocrystallites of about 31 nm were synthesized using a combustion-assisted solid-state method. Scanning electron microscopy showed uniformity of shape and particle size of several microns and no RE-dopants affecting the microstructure. Energy dispersive spectroscopy mapping confirmed the satisfactory quality of doping. Emission spectra at 980 nm excitation showed a total of 4 Er3+ emission bands from the UC process. To determine the variation of emission characteristics with temperature, the phosphor thermometry decay measurement method was used. The quality and potential of YNbO4:Er,Yb as a tempe

The Mn5+-activated Ca6Ba(PO4)4O phosphor is described in this paper as a versatile nearinfrared luminescence thermometry probe in a physiological temperature range. It is focused on several steady-state approaches to luminescence thermometry operating entirely in the first (excitation ex=635 nm) and the second (emission) biological transparency windows. This material was synthesized by a solid-state reaction using CaCO3, BaCO3, (NH4)H2PO4 and MnO as precursor materials. Starting chemicals were thoroughly mixed in an agate mortar for 1 hour with an appropriate amount of etanol. A mixture of the raw materials was heated at 600⁰C for 6 hours, ground in an agate mortar, and further calcinated at 1280⁰C for 10h. The exploited features of Ca6Ba(PO4)4O:Mn5+ emission spectra were (Figure 1): i) luminescence intensity ratio (LIR) between spin-allowed 3T2 → 3A2 and spin forbidden 1E → 3A2 (Sr MAX=2.04%K-1 @ 20 oC, dT<0.36 K); ii) LIR between Stokes peak corresponding to the bending deformation vibronic mode (O-Mn-O) and spin forbidden 1E → 3A2 (Sr MAX=0.57 %K-1 @ 100 oC, dT<2 K above 50 oC); iii) LIR between Stokes and Anti-Stokes peaks corresponding to the bending deformation vibronic mode (O-Mn-O) (Sr MAX=0.52 %K-1 @ 100 oC, dT3 K); iv) red-shift of the position of the 1E → 3A2 peak (Sa=0.21 cm-1K -1, dT<0.33 K) and v) the broadening of the full with at half maximum (FWHM) of the same peak with the temperature increase (Sr MAX=0.34 %K-1 @ 100 oC, dT<0.38 K). The steady-state approach facilitated quick measurements of emission spectra and, consequently, a large number of acquired spectra (50) for each temperature point, which were used to experimentally determine the temperature resolution for each thermometry method.

Yttrium niobate (YNbO4) is considered to be a self-activated phosphor with a strong emission band in the blue region originating from the NbO4 3- group, but also an effective host for various luminescent-based doping-altered applications with different rare-earth ions [1-4]. Here, (Y0.8-z,Mez)NbO4:Er,Yb (z = 0, 0.1, 0.2) phosphors with 0.05 erbium and 0.15 ytterbium as up-conversion (UC) activator and sensitizer, respectively, and different metal ions (Me) replacements of Y3+ ions that launch the host local lattice disorders, were synthesized by the traditional solid-state reaction method. The starting metal oxide precursors in the appropriate molar ratio were mixed in a ball mill at 100 rpm for 8 hours, then preannealed at 800 °C for 4 hours, and finally annealed at 1200 °C for 4 hours. The morphology of the synthesized phosphors and their particle size and shape were examined by scanning electron microscopy. X-ray diffraction measurements were used to confirm the crystal structure of samples. The emission spectra of Y0.8Er0.05Yb0.15NbO4 at room temperature under the excitation of 980 nm show green and red visible UC and near-infrared emission bands of very low intensity, which originate from energy transfer UC processes between Yb3+ and Er3+ ions and which are attributed to (2H11/2, 4S3/2) → 4I15/2, 4F9/2 → 4I15/2, and 4I9/2 → 4I15/2 transitions of Er3+ ions, respectively. The lifetime of the 4S3/2 excited energy level of Er3+ in Y0.8Er0.05Yb0.15NbO4 that depends on the host and plays an essential role in infrared to visible UC is 0.198 ms. UC mechanisms, the intensity of the UC emission, and changes in lifetime caused by different Me-ions entering the host lattice were discussed.

A promising lithium-indium-oxide (LiInO2) wide band-gap semiconductor for scintillating detection, photoluminescence, and photocatalysis [1-3] was prepared by a mechanochemical solid-state synthetic procedure that can be found elsewhere [3]. Its structure and morphology were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. SEM images show agglomerates of relatively uniform size of around 300 nm spherical-shaped particles of LiInO2 powder, while the XRD pattern confirmed the formation of the nanocrystalline tetragonal structure withamdI /41 space group (no. 141) symmetry. Detailed vibration analysis, together with the assignments of the band modes, was performed through the best-fit match of the experimental and density functional theory (DFT) calculated Raman spectrum. Geometry optimizations and vibrational frequencies calculations were conducted using B97-1 functional correlation [4] and LanL2DZ was used as a basis set.

When activated by Dy3+, Na0.25K0.25Bi0.50TiO3 (NKBT), piezoelectric powders show strong luminescence in the blue and yellow spectral range. Emissions of this material can be effectively utilized for both luminescence intensity ratio and lifetime-based readouts of temperature. Photoluminescence measurements over a temperature range of 293–483 K show that the luminescence intensity ratio temperature readout has maximal relative sensitivity of 1.93% K−1 at 380 K, while the relative sensitivity of the lifetime temperature readout reaches 1.1% K−1 at 480 K. For this study, materials were synthesized by a solid-state reaction using TiO2, Bi2O3, Na2CO3, K2CO3 and Dy2O3 as precursors. X-ray diffraction measurements showed that the NKBT sample crystallized in the A-site substituted distorted perovskite rhombohedral structure (R3c symmetry). The photoluminescence spectra showed characteristic emission bands of Dy3+ ions centered at 457 nm (4I15/2 → 6H15/2), 478 nm (4F9/2 → 6H15/2), 574 nm (4F9/2 → 6H13/2) and 663 nm (4F9/2 → 6H11/2). The ratio of emissions from 4F9/2 and 4I15/2 excited states to the 6H15/2 ground state was used as a luminescence intensity ratio indicator of temperature, while the decay of emission from 4F9/2 → 6H13/2 transition was used as a lifetime indicator of temperature. CIE coordinates x = 0.326 and y = 0.361 calculated from room temperature emission spectra show the perspective of this material for use in white light emission devices.

Herein, fine powders of yellow pigments based on BiVO4 were successfully prepared in two crystalline forms, tetragonal zircon-structured and monoclinic scheelite-structured, through a ethylene glycol-assisted method subjected to a thermal treatment at two different temperatures. The obtained materials were brightly colored with different hues of yellow, exhibited high reflection in the near-infrared region and showed visible luminescence under excitation by UV light. The new preparation method had a considerable effect on chromatic properties of the prepared scheelite-structured BiVO4 pigments. The sample with the most vivid and bright shade of yellow was found to have the L*a*b* and L*C* ab h* ab color coordinates of (87.28, 0.37, 91.53) and (87.28, 91.53, 89.79), which are indicative of exceptionally good chromatic properties superior and/or comparable to those of other inorganic yellow pigments, both commercially available and recently described. In addition, the NIR reflectance of this powder was very high (≥ 80%). © 2018 Elsevier Ltd and Techna Group S.r.l.

Dy3+-and Nd3+-doped YNbO4 powders were prepared using a mechanochemical method followed by annealing, and their photoluminescent properties were examined to assess the possibility of their use in phosphor thermometry. Luminescence intensity ratio technique was used as a temperature readout scheme for both YNba(4):Dy3+ and YNbO4:Nd3+ samples. The crystalline structure of the samples is confirmed by XRD measurements, while SEM was used for the microstructural and compositional characterization. Sample doped with Dy3+ ions emitting in the blue region has been investigated in the 293-773 K temperature range and exhibits maximal relative sensitivity of 1.97% K-1 at 300 K. YNbO4:Nd3+ sample showed intense NIR emission which fall into the first biological window. When heated from 303 up to 473 K relative temperature sensitivity of 0.28%K-1 at 300 K was achieved. Values of temperature sensitivities show that YNbO4 has a large potential for the development of thermographic phosphors in the blue and NIR spectral ranges when doped with trivalent Dy and Nd, respectively. (C) 2017 Elsevier B.V. All rights reserved.