Synthesis and luminescence properties of Yb3+, Tm3+ and Ho3+ co-doped SrGd2(WO4)2(MoO4)2 nano-crystal

Novel up-conversion luminescent SrGd₂(WO₄)₂(MoO₄)₂: Yb³⁺/Tm³⁺/Ho³⁺ nano-crystals were synthesized by hydrothermal method. The composition ratio of rare earth had been investigated. It indicated that when C_Yb³⁺ = 10 mol% and C_Yb³⁺/C_Tm³⁺/C_Ho³⁺ = 10:1.5:2, the emission intensities were the highest. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and up-conversion luminescence spectra were used to characterize SrGd₂(WO₄)₂(MoO₄)₂: Yb³⁺/Tm³⁺/Ho³⁺ nano-crystals and they showed that the sample had high degree of crystallinity, the sample was tetragonal system, and the grain size of the sample was about 56 nm. Three emission peaks, including blue emission peak, green emission peak and red emission peak were observed at 477, 543 and 651 nm corresponding to ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions of Tm³⁺, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺ respectively. All the emission peaks were observed by excitation of 980 nm semiconductor laser. The relationship between up-conversion intensity and excitation power revealed that blue emission at 477 nm was a three-photon absorption process, green emission at 543 nm and red emission at 651 nm was a two-photon absorption process. The quantum yields of the sample were near 3.2%.     

Yellow lighting upconversion from Yb3+/Ho3+ co-doped CaMoO4

Yellow upconversion (UC) luminescence is observed in Ho³⁺/Yb³⁺ co-doped CaMoO₄ synthesized by complex citrate-gel method. Under 980 nm excitation, Ho³⁺/Yb³⁺ co-doped CaMoO₄ exhibited yellow emission based on green emission near 543 nm generated by ⁴F₄, ⁵S₂ → ⁵I8 transition and strong red emission around 656 nm generated by ⁵F₅ → ⁵I₈ transition, which are assigned to the intra 4f transitions of Ho³⁺ ions. The optimum doping concentration of Ho³⁺ and Yb³⁺ was investigated for highest upconversion luminescence. Based on pump power dependence, upconversion mechanism of Ho³⁺/Yb³⁺ co-doped CaMoO₄ was studied in detail.     

Processing and electrical properties of (1 − x)[(1 − y)(Pb(Mg1/3Nb2/3)O3)–y(Pb(Yb1/2Nb1/2)O3)]–xPbTiO3 ceramics

Ferroelectric ceramics in the vicinity of morphotropic phase boundary (MPB) with compositions represented as (1 ? x)[(1 ? y)(Pb(Mg_1/3Nb_2/3)O₃)–y(Pb(Yb_1/2Nb_1/2)O₃)]–xPbTiO₃ were prepared by solid state reaction. The addition of PYbN to PMN–PT decreased the sintering temperature from 1200 °C (y = 0.25) to 1000 °C (y = 0.75). The PT content, where the MPB was observed, increased with the PYbN addition. A remanent polarization value of 28.5 µC/cm² and a coercive field value of 11 kV/cm were measured from 0.62[0.25PMN–0.75PYbN]–0.38PT ceramics, which were close to the ones measured from PMN–0.32PT ceramics. In addition, the Curie temperature was found to increase with PYbN additions.     

Frequency up-conversion luminescence in Yb3+–Ho3+ co-doped PbxCd1−xF2 nano-crystals precipitated transparent oxyfluoride glass-ceramics

Oxyfluoride glasses were developed with composition 30SiO₂·15AlO_1.5·28PbF₂·22CdF₂·(4.9−x)GdF₃·0.1HoF₃·xYbF₃ (x=0, 0.1, 0.2, 0.5, 1, 2, 3, and 4) in mol%. Powder X-ray diffraction analysis revealed that the heat-treatments of the oxyfluoride glasses at the first crystallization temperature cause the precipitation of Yb³⁺–Ho³⁺ co-doped fluorite-type nano-crystals of about 17.8 nm in diameter in the glass matrix. These transparent glass-ceramics exhibited very strong green up-conversion luminescence due to the Ho³⁺: (⁵F₄, ⁵S₂)→⁵I₈ transition under 980 nm excitation. The intensity of the green up-conversion luminescence in the glass-ceramics was much stronger than that in the precursor oxyfluoride glass. The reasons for the highly efficient Ho³⁺ up-conversion luminescence in the oxyfluoride glass-ceramics are discussed.     

Controllable phase/morphology tailoring of REF3 and NaREF4 (RE = La-Lu,Y), and insights into the up-conversion luminescence of GdF3:Yb3+/Tm3+ spheres

Four typical rare earth fluorides of hexagonal/orthorhombic REF₃ and hexagonal/cubic NaREF₄ (RE = La-Lu, Y, excluding radioactive Pm), have been hydrothermally synthesized from mixed solutions of the rare earth nitrate solutions and ammonium fluoride (NH₄F), in the presence of EDTA-2Na. The phase and the morphology are closely related to the ionic radius of RE³⁺. Large RE³⁺ ions, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺ and Sm³⁺, crystallize in hexagonal REF₃ (nanocrystals), and middle RE³⁺ ions (RE = Eu-Dy) crystallize in orthorhombic REF₃ (micron-sized spheres/polyhedral micro-crystallites). For small RE³⁺ ions, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ and Y³⁺, they tend to being incorporated into the host lattice of cubic NaREF₄ (submicron spheres). However, the smallest Lu³⁺ tends to crystallizing in orthorhombic LuF₃. Varying the F^-/RE³⁺ molar ratio (R) and the reaction temperature can efficiently induce the phase/morphology evolution of rare earth fluorides. Increasing R up to 10–30 yielded hexagonal NaREF₄ (RE = Dy, Tb, Gd, Eu, Sm, micron prisms), although the original products were orthorhombic REF₃ (RE = Dy, Tb, Gd, Eu) and hexagonal SmF₃ at R = 5. However, increasing R is inefficient to the phase transition for large RE elements of La, Ce, Pr, Nd. Lowering the reaction temperature from 180 °C to 80 °C gave rise to the phase transition from orthorhombic LuF₃ to cubic NaLuF₄. GdF₃:Yb³⁺,Tm³⁺ spheres exhibited intense blue emission at ~476 nm (¹G₄ → ³H₆ transition of Tm³⁺), deep-red emission at ~646 nm (¹G₄→³F₄ transition of Tm³⁺), and near-infrared emission at ~695 nm (³F_2,3 → ³H₆ transitions of Tm³⁺) under the 980 nm infrared excitation. The intense near-infrared emission under the infrared excitation suggested that they might hold great potential in bio imaging application.     

Microstructure and upconversion luminescence of Yb<Superscript>3+</Superscript> and Ho<Superscript>3+</Superscript> co-doped BST thick films

Ba_0.8Sr_0.2TiO₃ (BST) thick films co-doped with Yb³⁺ and Ho³⁺ were fabricated by the screen printing techniques on alumina substrates. The structure and morphology of the BST thick films were studied by XRD and SEM, respectively. After sintered at 1240 °C for 100 min the BST thick films are polycrystalline with a perovskite structure. The upconversion luminescence properties of the RE-doped BST thick films under 800 nm excitation at room temperature were investigated. The upconversion emission bands centered at 470 and 534 nm corresponding to ⁵F₁ → ⁵I₈ and ⁵F₄ → ⁵I₈ transition, respectively were observed, and the upconversion mechanisms were discussed. The dependence of the upconversion emission intensity upon the Ho³⁺ ions concentration was also examined; the emission intensity reaches a maximum value in the sample with 2 mol% Yb³⁺ and 0.250 mol% Ho³⁺ ions. All the results show that the BST thick films co-doped with Yb³⁺ and Ho³⁺ may have potential use for photoelectric devices.     

Synthesis of NaLn(WO4)2 phosphors via a new phase-conversion protocol and investigation of up/down conversion photoluminescence

The two-dimensional (2D) crystallite morphology and low OH⁻/Ln³⁺ molar ratio of Ln₂(OH)₄SO₄·2H₂O (Ln = lanthanide) make it an ideal precursor for materials synthesis via phase conversion, which was manifested in this work by the direct generation of well-defined NaLn(WO₄)₂ phosphor particles via hydrothermal reaction with Na₂WO₄. Kinetics study showed that pure NaLn(WO₄)₂ can be produced by reaction at 180 °C for ~24 h or at 200 °C for ~6 h under WO₄²⁻/Ln³⁺ = 10 M ratio. Morphology analysis revealed that, though NaLn(WO₄)₂ evolved via re-precipitation, the layered crystal structure and 2D crystallite morphology of the precursor could have templated the nucleation/growth of NaLn(WO₄)₂, leading to uniform particles (~4–5 μm) of a unique microdisc-like morphology. Under 394 nm excitation, the Ln = La_0.95Eu_0.05 phosphor showed down-conversion luminescence having an absolute quantum yield of ~35.4%, a fluorescence lifetime of ~1.13 ms for its 616 nm main emission, and color coordinates of around (0.63, 0.37). Under 978 nm laser excitation, the Ln = La_0.97Yb_0.02Ho_0.01 and Ln = La_0.97Yb_0.02Er_0.01 phosphors exhibited the strongest up-conversion (UC) luminescence at ~660 nm (the ⁵F₅ → ⁵I₈ transition of Ho³⁺) and 551 nm (the ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺), average fluorescence lifetimes of ~178.3 and 82.3 µs for the above emissions, and chromaticity coordinates of around (0.62, 0.38) and (0.25, 0.72), respectively. The two UC phosphors were also analyzed to exhibit UC luminescence through a two-photon mechanism.     

Magnetic properties of RE2Mo2O7 pyrochlores

The magnetic properties of RE₂Mo₂O₇ (RE=Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺) and of solid solutions of the type (Nd_1?x A _x )₂Mo₂O₇ (A=Yb or Er; 0.05≤x≤0.4) have been studied using an ac mutual inductance bridge. All the compounds were found to exhibit magnetic ordering in the neighborhood of or below 77 K. The data have been analyzed using (i) the available susceptibility data on Y₂Mo₂O₇, (ii) a ferromagnetic coupling among the Mo⁴⁺ ions to obtain the contribution from the RE³⁺ ion, and (iii) the available susceptibility data on RE₂Ti₂O₇ to obtain the contribution from the Mo⁴⁺ ions. It was found that procedure (iii) gave the most satisfactory explanation of the magnetic ordering. The results indicated that (1) the Mo⁴⁺ ions become ordered magnetically, (2) the behavior of the RE³⁺ ions is almost the same as in the isostructural RE₂Ti₂O₇ and RE₂V₂O₇ compounds, and (3) the susceptibility values differ appreciably as we go from Nd³⁺ to Yb³⁺, possibly due to narrowing of the π* Mo-O conduction band.     

Up-conversion luminescence and near-infrared quantum cutting of Ho3+/Yb3+ co-doped hexagonal NaGdF4 phosphors

In this paper, Ho³⁺/Yb³⁺ co-doped Sodium Gadolinium Fluoride (NaGdF₄) phosphors were synthesized by a facile and efficient hydrothermal method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy spectrum (EDS), and photoluminescence (PL). The results of XRD and SEM indicated that the prepared NaGdF₄ samples have the hexagonal structure and good orientation in the case of changing the concentration of Yb³⁺. The results of pump power dependence of up-conversion intensity indicated that the energy transfer mechanism from Yb³⁺ to Ho³⁺ is a two-photon process. The energy transfer between Ho³⁺ and Yb³⁺ under 448 nm excitation was determined by PL spectra and fluorescence lifetimes. All the results proved that the doping of Yb³⁺ ions can effectively improve the up-conversion luminescence of NaGdF₄ phosphors and achieve quantum cutting efficiency of up to 179.8%. Overall, NaGdF₄:Ho³⁺/Yb³⁺ have potential application in the field of optoelectronic materials.

     

Crystal growth and characterization of Tm doped mixed rare-earth aluminum perovskite

In this work, we present results of structural characterization and optical properties including radio luminescence of (Lu_xGd_yY_0.99?x?yTm_0.01)AP single crystal scintillators for (x, y) = (0.30, 0.19), (0, 0.19) and (0, 0) grown by the micro-pulling-down (μ-PD) method. The grown crystals were single phase materials with perovskite structure (Pbnm) as confirmed by XRD and had a good crystallinity. The distribution of the crystal constituents in growth direction was evaluated, and significant segregation of Lu and Gd was detected in (Lu_0.30Gd_0.19Y_0.50Tm_0.01)AP sample. The crystals demonstrated 70% transmittance in visible wavelength range and some absorption bands due to Tm³⁺, Gd³⁺ and color centers were exhibited in 190–900 nm. The radioluminescence measurement under X-ray irradiation demonstrated several emission peaks ascribed to 4f–4f transitions of Tm³⁺ and Gd³⁺. The ratio of emission intensity in longer wavelength range was increased when Y was replaced by Lu or Gd.     

Structure and piezoelectric properties of new (Bi0.5Na0.5)1?x?yBax(Yb0.5Na0.5)yTiO3 lead-free ceramics

New lead-free ceramics (Bi_0.5Na_0.5)_1−x−yBa_x(Yb_0.5Na_0.5)_yTiO₃ (x = 0.02–0.10 and y = 0–0.04) have been prepared by an ordinary sintering technique and their structure and piezoelectric properties have been studied. X-ray diffraction shows that Ba²⁺ and Yb³⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure and a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases is formed at 0.04 < x < 0.10. The partial substitutions of Ba²⁺ and Yb³⁺ for A-site ions of Bi_0.5Na_0.5TiO₃ decrease effectively the coercive field E _c and improve significantly the remanent polarization P _r. The ceramics with x = 0.06 and y = 0–0.02 situate within the MPB and possess the lower E _c and larger P _r, and thus exhibit optimum piezoelectric properties: d ₃₃ = 155–171 pC/N and k _p = 29.2–36.7%. The temperature dependences of the dielectric and ferroelectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above T _d.     

Structure and electrical properties of Bi0.5Na0.5TiO3-Y0.5Na0.5TiO3-BaTiO3 lead-free piezoelectric ceramics

New (1 – x ? y)Bi_0.5Na_0.5TiO₃-xY_0.5Na_0.5TiO₃-yBaTiO₃ lead-free ceramics have been prepared by a conventional ceramic fabrication technique, and their structure and electrical properties have been studied. A morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. As compared to pure Bi_0.5Na_0.5TiO₃ ceramic, the partial substitutions of Y³⁺ for Bi³⁺ and Ba²⁺ for (Bi_0.5Na_0.5)²⁺ in the A-sites of Bi_0.5Na_0.5TiO₃ lower effectively the coercive field E _c and increase the remanent polarization P _r of the ceramics. Because of low E _c, large P _r and the MPB, the ceramics with x = 0–0.02 and y = 0.06 exhibit the optimum piezoelectric properties: d ₃₃ = 155–159 pC/N and k _p = 28.8–36.7%. The temperature dependences of dielectric properties of the ceramics show relaxor-like behaviors. The ferroelectric properties at different temperature suggest that the ceramics may contain both the polar and non-polar regions near/above T _d.     

Effect of processing parameters on structural,morphological and optical Y2O3:Yb3+/Ho3+ powders characteristics

Rod-like and flake-like up-converting Y₂O₃:Yb³⁺/Ho³⁺ particles which are composed of nanoparticles with size less than 100 nm, are prepared by a simple hydrothermal processing at 473 K (3 h) followed by additional thermal treatment at 1373 K (3 and 12 h). The effect of precursor pH value on the formation of Y₂O₃:Yb³⁺/Ho³⁺ is followed through X-ray powder diffractometry (XRPD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Structural refinement confirms formation of the cubic bixbyte structure (S.G. Ia-3) with the non-uniform accommodation of dopants at C₂ and S₆ cationic sites. Under 978 nm laser excitation, strong green (530–570 nm) up-conversion is observed in all samples. The emission shows a decrease in intensity with an increase in external temperature, indicating FIR (fluorescence intensity ratio) based temperature sensing behavior of 0.52% for the ⁵F₄ → ⁵I₈/⁵S₂ → ⁵I₈ transitions.     

Spectroscopic properties of HoAl3(BO3)4 single crystal

The Judd–Ofelt theory has been applied to analyze absorption spectra of Ho³⁺ ion in HoAl₃(BO₃)₄ measured in spectral range 300–700 nm at room temperature. The Judd–Ofelt spectroscopic parameters have been determined as: Ω₂ = 18.87 × 10⁻²⁰ cm², Ω₄ = 17.04 × 10⁻²⁰ cm², Ω₆ = 9.21 × 10⁻²⁰ cm². These parameters have been used to calculate radiative lifetimes and branching ratios of the luminescence manifolds. Three luminescent bands were found in the spectral range 450–700 nm ascribed to transitions from the ⁵F₅, (⁵F₄, ⁵S₂) and ³K₈ states to the ground state ⁵I₈. Experimental intensities of these luminescence transitions were compared with those calculated by using Judd–Ofelt theory and the system of kinetic equations for populations of starting luminescing states. Probabilities of radiativeless transitions were evaluated from this comparison.     

White upconverted luminescence of Ho/Yb/Tm tri-doped Gd2Mo3O9 phosphors

Yb³⁺/Tm³⁺/Ho³⁺ tri-doped Gd₂Mo₃O₉ phosphors were synthesized by the high-temperature solid-state method. Under 980 nm near-infrared excitation, the white-light emission can be observed, which is consists of the blue, green, and red UC emissions. The green and red emission at 547 nm and 660 nm originated from the transition of Ho³⁺ (⁵S₂, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈) and the blue emission at 475 nm attributed to the transition of Tm³⁺ (⁵G₄ → ⁵H₆). In this experiment, we selected the optimum concentration ratio of the three rare earths for the bright white emission. The Commission internationale de L’Eclairage (CIE) coordinates for the samples were calculated, and chromaticity coordinates were very close to white light regions. We find that the calculated CIE color coordinates of the Yb³⁺/Tm³⁺/Ho³⁺ tri-doped Gd₂Mo₃O₉ phosphors changed with the incident pump power from 400 mW/cm² to 1000 mW/cm². The upconversion luminescence mechanism of the samples was discussed on its spectral. The white light may be proved to be a candidate material for applications in various fields.     

Efficient up-conversion emission and energy transfer in LaAlO3 doped with Er3+, Ho3+, and Yb3+ ions

The spectroscopic properties of LaAlO₃ polycrystals doped with Er³⁺, Ho³⁺ and Yb³⁺ ions have been investigated. Very efficient up-conversion emission occurs upon IR excitation. The strongest luminescence has been observed for a sample doped with Er³⁺, Ho³⁺, and Yb³⁺ ions simultaneously and annealed at 1500 °C. An efficient energy transfer to Yb³⁺ ions is observed when Er³⁺ or Ho³⁺ ions are excited. The energy transfer mechanisms are proposed.     

Enhancement of green emission for Al-doped YBO3:Tb

High-quality Y_1−x−yAl_xTb_yBO₃ (0 ≤ x ≤ 0.1, 0.04 ≤ y ≤ 0.16) phosphor powders with fine size, spherical and regular morphology, and non-agglomeration were successfully prepared by ultrasonic spray pyrolysis. The blue emission from the ⁵D₃→⁷F_J (J = 4, 5, and 6) transition for the Y_0.94−xAl_xTb_0.06BO₃ phosphor was quenched. The optimal concentration of Tb³⁺ was reduced by doping Al³⁺ into the Y_1−yTb_yBO₃ phosphor, i.e., y = 0.1 and 0.12 for Y_0.975−yAl_0.025Tb_yBO₃ and Y_1−yTb_yBO₃, respectively. The Al³⁺ doping was highly effective for improving the photoluminescence characteristics. The photoluminescence emission intensity of the Al³⁺-doped Y_0.915Al_0.025Tb_0.06BO₃ phosphor at 543 nm was about three times stronger than that of the Al³⁺-free Y_0.94Tb_0.06BO₃ phosphor.     

Preparation of Eu3+-Y3+ co-doping red-emitting phosphors for white-light emitting diodes (W-LEDs) application and investigation of their optical characteristics

Eu³⁺-Y³⁺ co-doping Ca_0.54Sr_0.34?1.5x Eu_0.08Y _x (MoO₄) _y (WO₄)_1?y (x = 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20. y = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0), have been prepared and their luminescent properties are investigated to seek a new red-emitting phosphor for ultraviolet-light emitting diodes chips. In absorption efficiency, luminescent intensity and chromaticity coordinates, Ca_0.54Sr_0.22(MoO₄)_0.2(WO₄)_0.8:0.08Eu³⁺, 0.08Y³⁺ phosphor is better than commercial Y₂O₂S:Eu³⁺ phosphor excited by 390?C405 nm light emitting diodes chip.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

Up-conversion luminescent characteristics of Yb3+/Tb3+/Ho3+ tri-doped phosphate glasses for solid-state lighting

Yb³⁺/Tb³⁺/Ho³⁺ tri-doped phosphate glasses with a chemical composition (in mol %) of 20Na₂O–42ZnO–28P₂O₅–10B₂O₃ were combined by the conventional melt quenching technique. The physical properties, glass network structure, transmittance spectrum, up-conversion (UC) emission spectra, energy transfer mechanism, and chromaticity change of the prepared glasses were systematically studied. The UC emission spectra results show that the strongest UC luminescence is observed for 2.0Yb³⁺/1.0Tb³⁺/0.15Ho³⁺ (mol %) composition. When Yb³⁺ ions as sensitizer, the energy transfer of Ho³⁺?→?Tb³⁺ ions happens. The physical properties indicate that the Yb³⁺, Tb³⁺, and Ho³⁺ ions are beneficial for a compact glass structure. It can be observed that the glasses have a strong light transmittance in the visible range, and its light transmittance is about 90%. Furthermore, Yb³⁺/Tb³⁺/Ho³⁺ tri-doped phosphate glass materials have better thermostability and chroma stability, these findings have significant implications for the use of advanced solid-state lighting.