Molten Salt Synthesized CaTa4O11:Er3+/Yb3+ with Superior Upconversion Luminescence.

Low phonon tantalate-based phosphors with a layer structure are considered to have excellent upconversion luminescence (UCL) intensity, which could be reduced due to the existence of impure phase defects and inappropriate doped rare earth ions. To improve their UCL performance, we have prepared single-phase CaTa4O11:Er3+/Yb3+ samples by a molten salt synthesis (MSS) using KCl as the reaction medium and compared its UCL properties with counterparts made by a conventional solid-state reaction (SSR) in this study. We have demonstrated that the MSS samples have a much higher UCL intensity under 980 nm laser excitation than the SSR ones due to accurate replacement of Ca2+ sites by Er3+/Yb3+ in high-purity single-phase MSS samples. We have further enhanced the green UCL intensity of the MSS samples by 1.57 times via acid picking (AP). Under 980 nm laser excitation at a high powder density of 61.3 W/cm2, the green UCL intensity of the MSS-AP samples can reach 3.72 times that of the SSR-AP samples. For potential luminescence thermometry applications, the maximum absolute sensitivity (SA) of the MSS-AP samples reaches 0.01316 K-1 at 501 K based on the luminescence intensity ratio. This study shows that CaTa4O11:Er3+/Yb3+ phosphors prepared by the MSS method are single-phase samples with excellent pure green UCL as a suitable temperature sensing material.

Cooling Condition Effect on Spectral Properties and Smartphone-Based Anticounterfeiting Application of Zn3Ga2Ge2O10/Cr3+ Phosphors.

The influence of cooling history for the Zn3Ga2Ge2O10/Cr3+ phosphors prepared by solid state reaction on the spectral properties was discovered, and an anticounterfeiting scheme based on the identification with smartphone was proposed and experimentally demonstrated using the studied phosphors. A combination of color-tunable visible fluorescence emission and near-infrared (NIR) afterglow emission in Zn3Ga2Ge2O10/x mol % Cr3+(x = 0, 0.05, 1, 2, 3, and 4) phosphors to achieve multimode anticounterfeiting was reported. It is found that with the increasing Cr3+ concentrations, the visible emission can be tuned from green, light pink, and light red to deep red under 254 nm ultraviolet (UV) excitation. This phenomenon is related to the formation of oxygen vacancies in the host during the process of natural cooling and the characteristic emission of Cr3+. In addition, the persistent time of the Cr3+ emission centered at 700 nm can be also tuned by various Cr3+ concentrations. A possible mechanism was deduced to explain the afterglow phenomenon. Lastly, a flower pattern applied in anticounterfeiting was fabricated using the Zn3Ga2Ge2O10/x mol % Cr3+ (x = 0, 0.05, 1, 2, 3, and 4) phosphors to present tunable color and NIR afterglow signals at different excitation modes, and the camera of smartphone was chosen as a detection tool to take the NIR images. The results obtained above suggest that the prepared phosphors at natural cooling condition have great potential in affording advanced optical anticounterfeiting.

Influence of Er3+ concentration and Ln3+ on the Judd-Ofelt parameters in LnOCl (Ln = Y, La, Gd) phosphors.

The optical transition properties of trivalent rare earth (RE3+) doped luminescent materials have received extensive attention. The Judd-Ofelt theory is an effective tool for exploring the optical transition properties for the 4f-4f transitions of lanthanides. The aim of this work is to discover the effect of Er3+ concentration and different Ln3+ ions on the Judd-Ofelt parameters in LnOCl:Er3+ (Ln = Y, La, Gd) phosphors. Oxychloride LnOCl:Er3+ (Ln = Y, La, Gd) phosphors were produced via a single displacement reaction technique. The Judd-Ofelt calculation procedure for RE3+ doped powders was modified and then adopted to obtain the Judd-Ofelt parameters of Er3+ in the studied phosphors. Meanwhile, a new route for examining the Judd-Ofelt calculation quality was proposed and used. It was found that the Er3+ doping concentration slightly affects the optical transition properties of Er3+ in YOCl and LaOCl, but greatly affects the optical transition properties in GdOCl. Moreover, it was also found that the optical transition properties of Er3+ depend also on Ln3+ (Ln = Y, La, Gd) though the crystal structure of these compounds is similar. The Judd-Ofelt parameters of Er3+ are the smallest in LaOCl:Er3+, medium in YOCl:Er3+, but the biggest in GdOCl:Er3+ when the doping concentration is the same.

Fluorescence decay route of optical transition calculation for trivalent rare earth ions and its application for Er3+-doped NaYF4 phosphor.

Usually, the optical transition properties of trivalent rare earth (RE) ions in transparent hosts can be quantitatively investigated in the framework of Judd-Ofelt theory. A standard and commonly accepted calculation procedure based on the absorption spectrum has already been established. However, it is hard to assess the optical transition properties of trivalent RE ions doped in powdered and film materials owing to the difficulty in the absolute absorption spectrum measurements. In this work, we proposed a new route to calculate the Judd-Ofelt parameters of trivalent RE ion-doped materials in any morphological and shaped forms. In this method, the fluorescence decay values bridging the radiative transition rates and the Judd-Ofelt parameters were used. As an application example of the proposed Judd-Ofelt calculation method, the Judd-Ofelt parameters of Er3+ in NaYF4 were calculated via the proposed route, and it was found that the obtained results are in reasonable accordance with those derived from other routes. It was also proved that this proposed Judd-Ofelt calculation method is a practicable and effective route for evaluating optical transition properties of trivalent RE ions in non-transparent hosts as long as the fluorescence decay values can be measured.

Blue-Green-Yellow Color-Tunable Luminescence of Ce3+-, Tb3+-, and Mn2+-Codoped Sr3YNa(PO4)3F via Efficient Energy Transfer.

Tunable blue-green-yellow emitting Sr3YNa(PO4)3F (SYNPF):Ce3+,Tb3+,Mn2+ phosphors have been prepared via a high-temperature solid-state reaction method. The structural and luminescent properties, energy transfer (ET) mechanism, and thermal quenching of the samples are investigated in detail. The ET from Ce3+ to Tb3+ and Mn2+ in SYNPF is identified as the interaction of the electric dipole-quadrupole with ET efficiencies of 81.6% and 69.3%, respectively. The critical distances for Ce3+/Tb3+ and Ce3+/Mn2+ are calculated through the spectral overlap method to be 9.54 and 10.92 Å. Under UV excitation, high-efficiency tunable blue-green-yellow emissions from Ce3+ to Tb3+ and Mn2+ can be obtained. Moreover, white light can also be achieved by adjusting the stoichiometry of Ce3+ and Mn2+ properly in the SYNPF phosphor. In addition, the temperature-dependent luminescence spectra exhibit good thermal quenching behaviors for the as-prepared phosphors. The above results suggest that SYNPF:Ce3+,Tb3+,Mn2+ phosphors can act as color-tunable emission phosphors for potential applications in WLEDs.

Detection of nitroaromatics in aqueous media based on luminescence resonance energy transfer using upconversion nanoparticles as energy donors.

Upconversion nanoparticle (UCNP)-based luminescence resonance energy transfer (LRET) systems are a powerful tool widely used to detect organic molecules or metal ions because of their simplicity and high sensitivity. The sandwich structure NaYF4:Er3+,Yb3+@NaYF4@NH2 UCNPs, as a highly selective and sensitive aqueous probe for detecting nitroaromatics, has been designed and prepared by a cothermolysis method and modified with polyetherimide to acquire amine groups on the surface of the core/shell UCNPs. The detection principle of nitroaromatics is based on LRET, which forms the Meisnheimer complex between the electron-deficient cyclobenzene of nitroaromatics and the electron-rich amino group on the surface of the sandwich structure UCNPs. As a consequence, nitroaromatics can be brought into close proximity to the sandwich structure UCNPs. With the increase of nitroaromatics (2,4,6-trinitrophenol and 2,4,6-trinitrotoluene) concentrations, the sandwich structure NaYF4:Er3+,Yb3+@NaYF4@NH2 UCNPs display a dramatic luminescent quenching effect at 407 nm and 540 nm under 980 nm excitation. The luminescent quenching intensity of the sandwich structure UCNPs is linearly correlated to the concentration of the nitroaromatics. The detection limit of 2,4,6-trinitrophenol (TNP) and 2,4,6-trinitrotoluene (TNT) are 0.78 and 0.77 ng ml-1, respectively. Therefore, the sandwich structure of NaYF4:Er3+,Yb3+@NaYF4@NH2 UCNPs can act as a valuable probe to detect nitroaromatics in public safety and security conditions.

Reply to the 'Comment on "A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the ß-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation"' by D. Zhang, Q. Xu and Y. Zhang, Phys. Chem. Chem. Phys., 2019, 21, DOI: 10.1039/C8CP07577H.

In this Reply, we truthfully respond to the comments on our recent paper entitled "A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the ß-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation" published in Phys. Chem. Chem. Phys. [Y. Zhang, B. Chen, S. Xu, X. Li, J. Zhang, J. Sun, X. Zhang, H. Xia, R. Hua, A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the beta-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation, Phys. Chem. Chem. Phys., 20, 2018, 15876-15883]. In the Comment, the authors oppugned partial calculation results we reported in our original paper, thus we redid the calculations and compared the presently obtained results with the original ones and the author provided ones. The recalculations and comparisons confirmed that our calculations are reproducible and the results are correct. In the Comment, the authors also made some comments on the Judd-Ofelt calculation approaches for powdered samples reported by other researchers. Following the authors' train of thought we added some supplements to the comments to understand the application strategy of Judd-Ofelt theory. Furthermore, we extended some points of view regarding the fluorescence lifetime measurements the authors presented in the Comment.

Highly stable and tunable white luminescence from Ag-Eu3+ co-doped fluoroborate glass phosphors combined with violet LED.

Ag-Eu3+ co-doped fluoroborate glass phosphors doped with various Eu3+-concentrations were prepared by a melt-quenching technique. The luminescent properties of these glass phosphors were characterized by excitation and emission spectra. Broad excitation and emission bands located, respectively, at 300-450 nm and 390-700 nm originating from silver aggregates were observed. Strong red emissions were detected under 404 nm violet light-emitting diode (LED) excitation for those Ag-Eu3+ co-doped samples. It was found that these red emissions of Eu3+ well compensated the deficiency of the red spectral components in glasses containing Ag aggregates. In addition, it was confirmed that stable white light could be achieved from the combination of a specific Ag-Eu3+ co-doped fluoroborate glass phosphor and LEDs with different output wavelengths. By adjusting the luminescence intensity ratio of the glass phosphor to the 404 nm violet LED, tunable emitting color was realized, and the studied glass phosphors showed excellent emitting color stability toward LED drive currents. Our results demonstrated that this kind of easy fabrication, low-cost, and highly stable Ag-Eu3+ co-doped fluoroborate glass phosphors had potential application in white LED.

NIR Downconversion and Energy Transfer Mechanisms in Tb3+/Yb3+ Codoped Na5Lu9F32 Single Crystals.

Tb3+/Yb3+ codoped Na5Lu9F32 single crystals with near-infrared (NIR) emission are achieved by an improved Bridgman approach. Energy transfer from Tb3+ to Yb3+ ions is affirmed by the photoluminescence (PL) emission spectra and decay curves characterization. On the basis of the analysis of energy transfer rate dependence on the Yb3+ concentration, the interaction between Tb3+ and Yb3+ ions in Na5Lu9F32 single crystals is confirmed through the one-to-one energy transfer process. Results demonstrate that the prepared Na5Lu9F32 single crystals might be promising candidates to convert sunlight to improve the performance of the silicon solar cells.

A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the ß-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation.

It is difficult to calculate the Judd-Ofelt (J-O) parameters for trivalent rare earth (RE)-doped powders due to the unavailable absorption spectrum that is necessarily used in the conventional J-O calculation procedure. In this study, a universal method starting from the diffuse-reflection spectrum for calculating the J-O parameters of RE3+-doped powdered samples was proposed. In this proposed method, by taking the Kubelka-Munk function into account, the absorption cross-section spectrum was derived from the diffuse-reflection spectrum in the RE3+-doped powdered sample using the connection between the absorption cross section and the radiative transition rate of RE3+. Then, the J-O parameters might be calculated from the absorption cross-section spectrum via the traditional J-O calculation technique. The NaYF4:Er3+/Yb3+ and NaYF4:Er3+ phosphors were prepared via an auto-combustion-assisted fluoridation technique, and the J-O calculation was carried out for the obtained samples. The obtained J-O parameters were compared with those reported in the literature and also verified by comparing the calculated radiative transition lifetimes with the experimental values. Finally, it was deduced that the proposed J-O calculation route was practicable.

Mesoporous silica coating NaYF4:Yb,Er@NaYF4 upconversion nanoparticles loaded with ruthenium(II) complex nanoparticles: Fluorometric sensing and cellular imaging of temperature by upconversion and of oxygen by downconversion.

Accurate detection of temperature and oxygen concentration at the cellular level is important in tumor diagnosis. Multifunctional nanocomposites are described that consist of upconversion luminescent nanoparticles capped with mesoporous silica and loaded with an oxygen-sensitive luminescent ruthenium complex. The nanocomposites of type NaYF4:Yb,Er@NaYF4@mSiO2-Ru have two modes of operation: Its red downconversion luminescence (at excitation/emission peaks of 455/606 nm) is quenched by oxygen (O2), and this is used to sense and image O2. The green upconversion luminescence (typically acquired at excitation/emission wavelengths of 980/525 and 544 nm), in turn, is used to measure temperature. The nanocomposites were then applied to dual mode in-vitro imaging of temperature and O2 in hepatocellular carcinoma cells (HepG-2). Graphical abstract A new multifunctional nanocomposite of mesoporous silica coating NaYF4:Yb,Er@NaYF4 upconversion nanopaticless loaded with ruthenium (II) complex has been designed with both green upconversion and red downconversion luminescence for temperature detection, oxygen sensing and dual mode cell imaging.

NaYF4:Sm3+/Yb3+@NaYF4:Er3+/Yb3+ core-shell structured nanocalorifier with optical temperature probe.

A core-shell structure with a NaYF4:Sm3+/Yb3+ core for photothermal conversion nanocalorifier and a NaYF4:Er3+/Yb3+ shell as temperature probe for potential applications in photothermal therapy (PTT) were synthesized by a thermal decomposition technique of rare-earth oleate complexes. The optical temperature reading-out property for the NaYF4:Sm3+/Yb3+@NaYF4:Er3+/Yb3+ core-shell structure was systematically investigated and it was found that in comparison with pure NaYF4:Er3+/Yb3+ particles, the temperature sensing performance of the NaYF4:Er3+/Yb3+ shell did not become worse due to the presence of NaYF4:Sm3+/Yb3+ core. Furthermore, the photothermal conversion behavior for core-shell nanoparticles was successfully examined by dint of temperature sensing of the NaYF4:Er3+/Yb3+ shell, and it was found that an excitation-power-density-dependent temperature increase of up to several tens degrees can be achieved. All the experimental results suggested that the core-shell structure may be an excellent nanocalorifier candidate for advanced temperature-controllable PTT.

Enhanced luminescence at 2.7 µm of Na5Lu9F32 single crystals co-doped Er3+/Pr3+ grown by Bridgman method.

Na5Lu9F32 single crystals co-doped with 1 mol. % Er3+ and various concentrations (0.1 mol. %, 0.3 mol. %, 0.5 mol. %, 0.7 mol. %, and 1 mol. %) of Pr3+ were successfully grown using the Bridgman method. Optical spectroscopic investigations of the obtained single crystal were reported for the absorption, emission, and luminous decay. The obtained single crystal appears with almost no absorption at the 2.7 µm band, attributable to the OH- bond. According to the Judd-Ofelt (J-O) theory, the J-O intense parameters of Er3+ ions were calculated. Under the 980 nm LD pumping, an obviously enhanced emission at 2.7 µm was obtained in the Er3+/Pr3+ co-doped crystal compared with the Er3+ singly doped crystal due to the energy transfer from Er3+ to Pr3+. The most intense emission at 2.7 µm was obtained when the doping concentrations of Er3+ and Pr3+ were 1 mol. % and 0.5 mol. % in the present research. The maximum emission cross-section and gain cross-section at 2.7 µm were also estimated. Moreover, using the Dexter theory, the energy transfer microscopic parameters have been calculated, and the decay curve fitting using the Inokuti-Hirayama expression indicated the dipole-dipole energy transfer from Er3+ to Pr3+ ions.

Anomalous Temperature-Dependent Upconversion Luminescence of α-NaYF4:Yb³âº/Er³âº Nanocrystals Synthesized by a Microwave-Assisted Hydrothermal Method.

Yb³âº/Er³âºco-doped cubic-(α-) phase NaYF4 nanocrystals were prepared through a microwave- assisted hydrothermal method. Temperature-dependent upconversion luminescence (UCL) and sensing properties were systematically studied. It is interesting that anomalous temperature- dependent UCL behavior is observed. With increasing temperature (303-573 K), the UCL intensity of Er³âº does not quench monotonously but reaches a minimum around 483 K and then increases. However, it was found that the UCL spectra change in a different way with decreasing temperature (573-303 K) from the one measured with increasing temperature. The fluorescence intensity ratio of ²H11/2 --> 4I15/2 to 4S3/2 --> 4I15/2 at any measured temperature point remains almost constant in all measurement processes, indicating the consistency of temperature in each spectrum measurement at all temperature points regardless of the heating or the cooling process in our experiments. The results demonstrate that NaYF4:Yb³âº/Er³âº UC nanocrystal has good sensing stability and may have potential application in the nanoscale thermal sensor.

Hydrothermal Synthesis and Luminescence Properties of Eu²âº- and Eu³âº-Doped SrAIF5 Nanorods.

Eu²âº- and Eu³âº-doped SrAIF5 nanorods were synthesized via a hydrothermal process. The crystal structure and morphology of the final products were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The prepared nanorods' diameters range from 40 to 50 nm, and lengths range from 400 nm to 2 µm along with the doped concentration of rare earth. The f-f transitions of Eu²âº can be observed in the SrAlF5:Eu²âº nanorods at room temperature, and the photo-luminescent (PL) properties of SrAlF5:Eu³âº nanorods are also described.

Preparation, Energy Transfer Characteristics and Colorimetric Properties of Na2Dy4(WO4)7:Eu³âº Nanophosphors.

Eu³âº-doped Na2Dy4(WO4)7 nanophosphors were synthesized via a co-precipitation method. The crystal structure and morphology of the nanophosphors were characterized by X-ray diffraction analysis and field emission scanning electron microscopy, respectively. For all Eu³âº doping concentrations, spherical particles with an average diameter of about 40 nm and consisting of a pure Na2Dy4(WO4)7 phase were obtained. The excitation and emission properties of the nanophosphors were examined by fluorescence spectroscopy and the energy transfer between Eu³âº and Dy³âº was studied. Furthermore, the influence of the Eu³âº concentration on the color coordinates and luminescence intensity is discussed. The nanophosphors could easily be excited by ultraviolet (UV) and near-UV light corresponding to the f-f transitions of Dy³âº. The color coordinates of the nanophosphors light emission are in the white light region and the Commission Internationale de l'Éclairage (CIE) coordinates of the nanophosphors light emission varied between (0.331, 0.354) and (0.405, 0.368) depending on the Eu³âº doping concentration.

Excellent optical thermometry based on single-color fluorescence in spherical NaEuF4 phosphor.

Temperature-dependent luminescence of spherical NaEuF4 phosphors with different particle sizes was studied. The thermally coupled 5D0 and 5D1 level of Eu³âº was observed. The linear dependence of emission intensities of 5D0 level of NaEuF4 phosphor on temperature confirmed the excellent temperature sensing performance. Sensitivity up to 0.43% is achieved via decreasing the particle size, which is higher than that of reported thermometry based on upconversion of lanthanide ions. Moreover, the original luminescent intensity of 90% was recovered after 10 temperature-changed cycles, indicating good sensing stability. Therefore, spherical NaEuF4 phosphor might be a promising candidate for optical temperature sensors.

Electrospinning preparation and luminescence properties of terbium complex/polymer composite fibers.

The terbium complex Tb(acac)2AA (acac: acetyl acetone, AA: acrylic acid) was incorporated into polystyrene (PS) matrix and electrospun into various composite fibers. The mass ratios of rare-earth complex to polymer were fixed in all composite fibers. The diameters of the composite fibers decrease with increase in the collection distance during electrospinning. The luminescence properties of the Tb(acac)2AA/PS composite fibers were studied in comparison to those of the pure Tb complex. The excitation bands became narrower and blue-shifted compared with that of the pure complex in all of the Tb(acac)2AA/PS composite fibers. The fluorescence lifetime of the 5D4 state of Tb3+ ion in the composite fibers was shorter than that of the Tb complex. The effect of the fiber diameter on the luminescence properties was also studied. As a result, the fluorescence lifetimes of the 5D4 state in all of the Tb(acac)2AA/PS composite fibers decreased gradually with decrease in diameter.

[Differentiated fluorescence centers in europium acetylacetonate hydrate doped poly methyl methacrylate].

Poly methyl methacrylate (PMMA) was doped with europium acetylacetonate hydrate. Judd-Ofelt parameters Ω2 (19.73 x 10(-20) cm2) and Ω4(2.19 x 10(-20) cm2) indicate a high inversion asymmetry and strong covalent environment arounc Eu3+ in PMMA. The maximum stimulated emission cross-sections for the 5D0 --> 7F(J); (J = l, 2 and 4) transitions in EAH doped PMMA were calculated to be 0.38 x 10(-21), 4.90 x 10(-21) and 0.36 x 10(-21) cm2, respectively. Efficient purplish-red and red fluorescence was obtained from ediropium acetylacetonate hydrate doped PMMA under 365 and 254 nm excitation respectively, indicating that it can be used as UV sensitive components for fiber optic sensors. Because of the suitable refractive index difference between doped sample and pure PMMA, it is expected to fabricate standard 9 µm/125 µm optical fiber which supports multimode transmission, providing basis of further development for medical fiber, flexible communication fiber and fiber optic sensor.

Lead-free double perovskite Cs2NaInCl6 nanocrystals doped with Sb3+ and Tb3+ for copper ions detection in lubricating oil.

Detecting heavy metal copper ions in lubricating oil holds immense significance for assessing mechanical wear and predicting mechanical failure. While perovskite nanocrystals offer high sensitivity in detecting copper ions, traditional lead halide perovskites suffer from lead toxicity defects. Lead-free perovskites, like Cs2NaInCl6, avoid the issue of lead toxicity but display lower luminescence intensity due to the presence of forbidden optical transitions. To address these issues, this study synthesized Cs2NaInCl6 nanocrystals (NCs) co-doped with Sb3+ and Tb3+ ions for copper ions detection in lubricating oil. The introduction of Sb3+ effectively reduced the band gap of the Cs2NaInCl6 host, creating an energy transfer pathway for Tb3+ emission via self-trapped excitations (STEs). Moreover, the doping of Tb3+ ions resulted in the suppression of STEs emission due to electron transfer from STEs to Tb3+. The emission of Tb3+ increased initially and then decreased with the increasing Tb3+ concentration, peaking at 40 %. Finally, Cs2NaInCl6: 2.5 %Sb3+, 40 %Tb3+ NCs were employed as probes for copper ions detection, exhibiting superior sensitivity and selectivity compared to similar probes. The presence of copper ions introduced competition between copper and Tb3+ for electrons from STEs, consequently leading to the quenching of multiple emission intensities associated with STEs and Tb3+. This method shows promising potential in predicting mechanical failure.