Photoluminescence and electrical properties of Eu3+-doped Na0.5Bi4.5Ti4O15-based ferroelectrics under blue light excitation

Na_0.5Bi_4.5?x Eu _x Ti₄O₁₅ (NBT- _x Eu³⁺) ceramics with x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30 and 0.40 were prepared by conventional ceramics processing. NBT-0.25Eu³⁺ ceramics show the strongest red and orange emissions corresponding to the ⁵D₀ → ⁷F₂ (617 nm) and ⁵D₀ → ⁷F₁ (596 nm) transitions, respectively. The strongest excitation band around 465 nm matches well with the emission wavelength of commercial InGaN-based blue LED chip, indicating that Eu³⁺-doped NBT ceramics may be used as potential environmental friendly red-orange phosphor for W-LEDs application. As an inherent ferroelectric and piezoelectric material, the electrical properties of this potentially multifunctional electro-optical material have been also studied. The introduction of Eu³⁺ distinctly increased the Curie temperature (T _C ) of NBT- _x Eu³⁺ ceramics from 640°C to 711°C as x ranges from 0 to 0.40. For higher temperature applications, the electrical conductivity was also investigated. The conduction of charge carriers in high-temperature range originates from the conducting electrons from the ionization of oxygen vacancies. High T _C and low tanδ makes Eu³⁺-doped NBTceramic also suitable for high temperature piezoelectric sensor applications and electro-optical integration.     

Optical properties of europium-activated hafnium fluoride-based glasses

We have studied the optical absorption, luminescence, and electron paramagnetic resonance of EuF₃- and EuF₂-activated fluorohafnate glasses. The glasses prepared with EuF₂ contain both di- and trivalent europium. The fraction of divalent europium clusters in the glass host decreases with decreasing EuF₂ concentration. Eu²⁺ luminescence in the fluorohafnate glasses is quenched, which is due the overlap of Eu²⁺ excited state levels with the conduction band of the glass, resulting in nonradiative relaxation through Hf³⁺ levels in the conduction band. The Eu³⁺ luminescence spectra contain lines corresponding to transitions from several levels of the ⁵D multiplet to levels of the ⁷F multiplet. The relationship between transitions from different ⁵D levels depends on europium concentration and temperature.     

Temperature effect on the photoluminescence intensity and Eu<Superscript>2+</Superscript> excited state lifetime in EuGa<Subscript>2</Subscript>S<Subscript>4</Subscript> and EuGa<Subscript>2</Subscript>S<Subscript>4</Subscript>:Er<Superscript>3+</Superscript>

The photoluminescence (PL) spectra and Eu²⁺ excited state lifetime of EuGa₂S₄ and EuGa₂S₄:Er³⁺ have been studied in the range 78–500 K. The spectra show a band at 545 nm, due to the 4f ⁶5d → 4f ⁷(⁸ S _7/2) transition. With increasing temperature, the full width at half maximum Γ(T) of the PL band of EuGa₂S₄ and EuGa₂S₄:Er³⁺ crystals increases from 0.15 to 0.22 and from 0.13 to 0.19 eV, respectively. Over the entire temperature range studied, Γ(T) is a linear function of T ^1/2. The 545-nm emission intensity and Eu²⁺ excited state lifetime in EuGa₂S₄ and EuGa₂S₄:Er³⁺ vary exponentially with temperature. The luminescence quenching energies evaluated from the Arrhenius plots of I(10³/T) and τ(10³/T) coincide (0.10 eV) within the error of determination.     

Luminescent properties of europium-activated yttrium gadolinium phosphates

Nanocrystalline Y_{1 ? x ? y} Gd _x Eu _y PO₄ phosphors have been prepared via precipitation from aqueous solutions. From their luminescence and excitation spectra, the intensity ratio I ₆₁₅/I ₅₉₄ of the Eu³⁺ luminescence bands corresponding to electric dipole and magnetic dipole transitions has been determined as a function of Gd³⁺ content. The critical concentration and effective energy transfer radius in Y_{1 ? x ? y} Gd _x Eu _y PO₄ have been evaluated. Excitation of Gd³⁺⁸ S _J ?⁶ D _J and ⁸ S _J ?⁶ J _J transitions to Eu³⁺ luminescence excitation levels in Y_{0.99 ? x} Gd _x Eu_0.01PO₄ involves efficient energy transfer. Under 250-nm excitation, the Eu³⁺ luminescence yield in Y_{0.99 ? x} Gd _x Eu_0.01PO₄ is a factor of 2.5–3 higher than that in Y_0.99Eu_0.01PO₄.     

Luminescence Spectra of Poly(methylmethacrylate)/ZnS:Eu(III),Tb(III) Composites

Colloidal zinc sulfide solutions have been prepared by reacting zinc trifluoroacetate and thioacetamide in methyl methacrylate as a reaction medium, and europium and terbium salts have been added to the solution. Using methyl methacrylate block polymerization, we have synthesized PMMA/ZnS, PMMA/ZnS:Eu(III), PMMA/ZnS:Tb(III), and PMMA/ZnS:Eu(III),Tb(III) composites. The luminescence of the composites is due to charge recombination at energy levels of structural defects and impurities in ZnS and also to ⁵D₀ → ⁷F _j and ⁵D₄ → ⁷F _j electronic transitions of the Eu³⁺ and Tb³⁺ ions. It depends on the composition and structure of the composites, excitation wavelength, and other factors. The mutual effects of the ZnS and the Eu³⁺ and Tb³⁺ ions show up as changes in the position and relative intensity of luminescence bands in the spectra of the composites.     

New,thermally stable Gd<Subscript>11</Subscript>(GeO<Subscript>4</Subscript>)(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>O<Subscript>10</Subscript>-based upconversion phosphors

A series of Gd_11–x–y Yb _x Er _y GeP₃O₂₆ germanate phosphates differing in the ratio of the Yb³⁺ and Er³⁺ active ions have been synthesized, and their luminescence spectra have been measured. According to X-ray diffraction characterization results, all of the synthesized germanate phosphates are single-phase and have a triclinic structure (sp. gr. P1). We have measured upconversion luminescence spectra due to the Er^{3+ 2}H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 radiative transitions in the synthesized gadolinium ytterbium erbium germanate phosphates and determined the luminescence upconversion energy yield (B _en) in Gd_11–x–y Yb _x Er _y GeP₃O₂₆. The effects of the concentrations and ratio of the dopants in the Gd₁₁(GeO₄)(PO₄)₃O₁₀ germanate phosphate host on B _en and the ratio of the luminescence intensities in the red and green spectral regions (R/G) have been assessed.     

Luminescence properties of Bi codoped and P codoped Ca<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript>

New red emitting phosphors, Ca₃(VO₄)₂:Eu³⁺,Bi³⁺, Ca₃((P,V)O₄)₂:Eu³⁺ were synthesized by low temperature solid-state reaction and characterized by X-ray diffraction, scanning electron microscopy, photoluminescence spectra and Fourier transform infrared spectroscopy. The results show that the red emission located at about 613 nm was ascribed to ⁵ D ₀-⁷ F ₂ transition of Eu³⁺. The effect of by Bi doping and by P doping was also investigated systematically.     

Synthesis and luminescence spectra of (Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–YOF):Ln(III) composites

Sols were prepared by reacting yttrium, europium, and terbium trifluoroacetates with thioacetamide in ethyl acetate. The Eu³⁺ and Tb³⁺ concentrations in the sols were 0.10 to 10 wt % relative to yttrium, which corresponded to 0.061 (0.059) to 6.1 (5.9) at % Eu (Tb). The sols were converted into a gel-like state by slowly evaporating the solvent. After ripening, the gels were heat-treated at a temperature of 800°C. X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, and IR spectroscopy results showed that the resultant composites consisted predominantly of a mixture of Y₂O₃ and YOF. The Eu³⁺ and Tb³⁺ ions were shown to substitute for Y³⁺ ions in the crystal lattices of the yttrium oxide and yttrium oxyfluoride. The formation of the (Eu_0.6Y_0.4)₂O₃, Eu₂O₃, and EuOF phases was demonstrated. We determined the types and parameters of the crystal lattices of the synthesized materials in relation to activator concentrations. The luminescence of the composites is due to the ⁵ D ₀ → ⁷ F _j and ⁵ D ₄ → ⁷ F _j electronic transitions of the Eu³⁺ and Tb³⁺ ions and depends on the host and activator compositions, the excitation wavelength, and other factors.     

Large enhancement of upconversion luminescence in Er<Superscript>3+</Superscript>/In<Superscript>3+</Superscript>:Ba<Subscript>0.85</Subscript>Ca<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

A series of In³⁺-doped Ba_0.85Ca_0.15TiO₃:0.75%Er³⁺/xIn³⁺ (BCT:Er/xIn) lead-free piezoelectric ceramics with excellent upconversion luminescence were synthesized by the solid state reaction method. The effects of In³⁺ content on the crystal structure, ferroelectric, dielectric, piezoelectric, and upconversion luminescence properties were systematically studied. Under 980 nm excitation, a giant enhancement of the green emission (550 nm) by 10 times is achieved upon 2.5% mol In³⁺ doping, which is rarely observed in rare-earth ions-doped perovskite ferroelectric materials. The ultraviolet-visible-near infrared absorption measurements show that the In³⁺ doping may improve the dissolution of Er³⁺ ions and modify the isolate-/clustered-Er³⁺ ratio for x?≤?2.5%, resulting in the enhancement of the absorption cross-section, thereby contributing to the enhancement of green luminescence. Unfortunately, the In³⁺ doping suppresses the ferroelectric and piezoelectric properties of the BCT:Er/xIn ceramics. This problem can be resolved by adding a small amount (1 mol%) of Yb³⁺ to the BCT:Er/xIn ceramics to restore their good ferroelectric and piezoelectric properties. Such In³⁺ and rare-earth ions co-doped ceramics with greatly enhanced upconversion luminescence and good ferroelectricity and piezoelectricity may have potential applications in electro-optical devices.     

Upconversion Luminescence of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocrystals Doped with Er<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> Ions

The upconversion luminescence (UCL) of nanocrystalline gadolinium oxide (Gd₂O₃) doped with Er³⁺ and Yb³⁺ ions has been studied in the temperature range of 90–400 K. The nanocrystals were synthesized by chemical vapor deposition and possessed a cubic crystalline structure with an average particle size within 48–57 nm. It is established that the USL intensity in the red (⁴F_9/2 → ⁴I_15/2 transition in Er3+ ion) and green (⁴S_3/2 → ⁴I_15/2 transition) spectral regions depends on the sample temperature and concentration of dopant ions, as well as on the additional structural defects (anion vacancies) created in the crystal lattice by the introduction of Zn²⁺ ions or irradiation with high-energy (10 MeV) electrons. The luminescence efficiency and spectrum of the upconversion phosphor are determined by energy transfer processes.     

Rapid synthesis and enhancement in down conversion emission properties of BaAl<Subscript>2</Subscript>O<Subscript>4</Subscript>:Eu<Superscript>2+</Superscript>,RE<Superscript>3+</Superscript> (RE<Superscript>3+</Superscript>=Y,Pr) nanophosphors

BaAl₂O₄:Eu²⁺,RE³⁺ (RE³⁺=Y, Pr) down conversion nanophosphors were prepared at 600 °C by a rapid gel combustion technique in presence of air using boron as flux and urea as a fuel. A comparative study of the prepared materials was carried out with and without the addition of boric acid. The boric acid was playing the important role of flux and reducer simultaneously. The peaks available in the XPS spectra of BaAl₂O₄:Eu²⁺ at 1126.5 and 1154.8 eV was ascribed to Eu²⁺(3d _5/2) and Eu²⁺(3d _3/2) respectively which confirmed the presence of Eu²⁺ ion in the prepared lattice. Morphology of phosphors was characterized by tunneling electron microscopy. XRD patterns revealed a dominant phase characteristics of hexagonal BaAl₂O₄ compound and the presence of dopants having unrecognizable effects on basic crystal structure of BaAl₂O₄. The addition of boric acid showed a remarkable change in luminescence properties and crystal size of nanophosphors. The emission spectra of phosphors had a broad band with maximum at 490–495 nm due to electron transition from 4f ⁶5d ¹ → 4f ⁷ of Eu²⁺ ion. The codoping of the rare earth (RE³⁺=Y, Pr) ions help in the enhancement of their luminescent properties. The prepared phosphors had brilliant optoelectronic properties that can be properly used for solid state display device applications.     

Synthesis and luminescence spectra of poly(methyl methacrylate)/CdS:Ln(III) composites

Cadmium sulfide was prepared by colloidal synthesis in methyl methacrylate (MMA). Europium and terbium salts were added to the colloidal solutions. Using MMA radical polymerization, we synthesized PMMA/CdS:Eu(III), PMMA/CdS:Tb(III), and PMMA/CdS:Eu(III):Tb(III) luminescent composites. Their luminescence is due to defects in the CdS crystals and the ⁵ D _о → 7Fj and ⁵ D ₄ → ⁷ F _j electronic transitions of the Eu³⁺ and Tb³⁺ ions, respectively. It depends on the composition of the materials, complexation on the surface of the colloidal particles, heat treatment time during synthesis, excitation wavelength, and other factors.     

Synthesis and properties of liquid luminophores based on the POCl<Subscript>3</Subscript>-BiCl<Subscript>3</Subscript>-MCl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> system

Stable BiCl₃-containing solutions of phosphorus oxychloride, activated with UO ₂ ²⁺ and Nd³⁺ ions, can be prepared only in the presence of another Lewis acid MCl _x . The electronic absorption spectra of the liquids prepared and the decay times of the Nd³⁺ luminescence are characteristic of individual solutions based on POCl₃-MCl _x . The radiation-chemical yield of Nd³⁺ in the excited state ⁴ F _3/2 in POCl₃-BiCl₃-MCl _x -²³⁵UO ₂ ²⁺ -Nd³⁺ solutions upon homogeneous excitation with uranium α-particles is lower than in POCl₃-MCl _x -²³⁵UO ₂ ²⁺ -Nd³⁺ solutions at comparable component concentrations. Apparently, Bi³⁺ in solutions based on the POCl₃-BiCl₃-MCl _x system is not incorporated in neodymium- and/or uranyl-containing complexes and remains in the matrix.     

Effect of precursor morphology on the structural properties,optical absorption,and luminescence of BaI<Subscript>2</Subscript>:Eu<Superscript>2+</Superscript>,Eu<Superscript>3+</Superscript>

BaI₂:Eu²⁺,Eu³⁺ powders have been prepared by heat-treating BaCO₃:Eu³⁺ precursor powders of various morphologies in an iodinating agent atmosphere and their structural properties, morphology, optical absorption, and luminescence have been studied. The results demonstrate that the powders thus prepared consist of a mixture of crystalline hydrates of various compositions, dominated by BaI₂ ? 2Н₂О (sp. gr. C2/c), and that the Eu²⁺: Eu³⁺ ratio in the powders is determined by the morphology of the precursor.     

Synthesis and luminescence properties of LiBaPO<Subscript>4</Subscript>:Bi<Superscript>3+</Superscript> yellow-emitting phosphor for solid-state lighting

Novel LiBaPO₄:Bi³⁺ yellow-emitting phosphor is synthesized by high temperature solid-state reaction method in air. With excitation 260 nm, LiBaPO₄:Bi³⁺ phosphor emits yellow light with the chromaticity coordinate (0.4272, 0.4657) and color rendering index 77.7. Emission band peaking at ~?588 nm of LiBaPO₄:Bi³⁺ phosphor in the range of 400–790 nm is attributed to the ³P₁ → ¹S₀ electron transition of Bi³⁺ ion. Excitation band monitored at 588 nm in the range of 220–300 nm is assigned to the ¹S₀ → ³P₁ electron transition of Bi³⁺ ion. The optimal Bi³⁺ ion concentration in LiBaPO₄:Bi³⁺ phosphor is ~?1.0 mol%. Time resolved spectra and fluorescence lifetime data confirm that there is only Bi³⁺ ion luminous center in LiBaPO₄:Bi³⁺ phosphor. The luminous mechanism is analyzed by configurational coordinate diagram of Bi³⁺ ion. The experiment results are helpful to develop other new Bi³⁺-doped optical materials for solid-state lighting.     

New NaSrPO<Subscript>4</Subscript>:Sm<Superscript>3+</Superscript> phosphor as orange-red emitting material

Sm³⁺-activated NaSrPO₄ phosphors could be efficiently excited at 403 nm, and exhibited a bright red emission mainly including four wavelength peaks of 565, 600, 646 and 710 nm. The highest emission intensity was found for NaSr _1?x PO₄: xSm³⁺ with a composition of x = 0.007. Concentration quenching was observed as the composition of x exceeds 0.007. The decay time values of NaSr_1?x PO ₄ : xSm³⁺ phosphors range from around 2.55 to 3.49 ms. NaSr_1?x PO₄: xSm³⁺ phosphor shows a higher thermally stable luminescence and its thermal quenching temperature T ₅₀ was found to be 350°C, which is higher than that of commercial YAG:Ce³⁺ phosphor and ZnS:(Al, Ag) phosphor. Because NaSr_1?x PO₄: xSm³⁺ phosphor features a high colour-rendering index and chemical stability, it is potentially useful as a new scintillation material for white light-emitting diodes.     

NaF induced enhancement of luminous efficiency in narrow-band red-emitting K<Subscript>2</Subscript>TiF<Subscript>6</Subscript>:Mn<Superscript>4+</Superscript>@NaF phosphors

A series of K₂TiF₆:xMn⁴⁺ @NaF samples were prepared by the cation exchange method in HF solution. Coating effects of NaF on the fluorescent properties of the samples were discussed. It is interesting that NaF has induced enhancement of luminous efficiency for the samples. Mechanism of NaF induced enhanced luminescence effect was suggested. That is that the enhancement effect of NaF coating is mainly attributed to a suitable local distortion of the crystal field surrounding the Mn⁴⁺ activator through doping with NaF. The results indicate that the optimal conditions are x?=?0.07 and w_NaNO3?=?2.5 g. Decay lifetime and the photoluminescence quantum yield of the optimal sample are 5.25 ms and 99.19?±?0.03%, respectively. The chromaticity coordinates of the optimal sample are x?=?0.6926, y?=?0.3073. So, the phosphor emits deep red light, which can be applied for blue light-based white LED.     

A magnetooptical study of the odd crystal field component in a terbium-yttrium aluminum garnet

We have studied the photoluminescence spectra and the luminescence magnetic circular polarization (LMCP) spectra in the region of the 4f-4f radiative transition ⁵D₄→⁷F₆ in the rare earth Tb³⁺ ion in a Y₃Al₅O₁₂ garnet matrix. A comparison of the experimental and theoretically calculated LMCP spectra allowed parameters of the odd crystal field component to be determined that removes the prohibition with respect to parity from the 4f-4f transitions in Tb³⁺ ion in the garnet structure. The energy spectra and wave functions of ⁵D₄ and ⁷F₆ multiplets of Tb³⁺ ion in a crystal field with the D₂ symmetry have been calculated.     

Tunable luminescence properties and energy transfer of single-phase Ca<Subscript>4</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>O: Dy<Superscript>3+</Superscript>, Eu<Superscript>2+</Superscript> multi-color phosphors for warm white light

The novel Ca_4?x(PO₄)₂O: xDy³⁺ and Ca_4?x?y(PO₄)₂O: xDy³⁺, yEu²⁺ multi-color phosphors were synthesized by traditional solid-state reaction. The crystal structure, particle morphology, photoluminescence properties and energy transfer process were investigated in detail. The X-ray diffraction (XRD) results demonstrate that the products showed pure monoclinic phase of Ca₄(PO₄)₂O when x < 0.1. The scanning electron microscopy (SEM) indicated that the phosphors were grain-like morphologies with diameters of ~ 3.7–7.0 μm. Under excitation of 345 nm, Dy³⁺-doped Ca₄(PO₄)₂O phosphors showed multi-color emission bands at 410, 481 and 580 nm originated from oxygen vacancies and Dy³⁺. Interestingly, Ca₄(PO₄)₂O: Dy³⁺, Eu²⁺ phosphors exhibited blue emission band at 481 nm and broad emission band from 530 to 670 nm covering green to red regions. The energy transfer process from Dy³⁺ to Eu²⁺ was observed for the co-doped samples, and the energy transfer efficiency reached to 60% when Eu²⁺ molar concentration was 8%. In particular, warm/cool/day white light with adjustable CCT (2800–6700 K) and high CRI (R_a > 85) can be obtained by changing the Eu²⁺ co-doping contents in Ca₄(PO₄)₂O: Dy³⁺, Eu²⁺ phosphors. The optimized Ca_3.952(PO₄)₂O: 0.04Dy³⁺, 0.008Eu²⁺ phosphor can achieve the typical white light with CCT of 4735 K and CRI of 87.     

Recombination luminescence of activated CaI<Subscript>2</Subscript> crystals

Data are presented on the recombination luminescence of CaI₂:Eu²⁺, CaI₂:Gd²⁺, CaI₂:Tl⁺, CaI₂:Pb²⁺, and CaI₂:Mn²⁺ crystals grown by the Bridgman-Stockbarger method. It follows from their photostimulated luminescence, roentgenoluminescence, and thermostimulated luminescence spectra that the activation of CaI₂ with Gd²⁺, Eu²⁺, Tl⁺, Pb²⁺, and Mn²⁺ cations, which produce anion defects, leads to the formation of defect complexes which act as electron traps and determine the 90-K photostimulation spectra of the crystals. The observed effect of the nature of the dopant on the photostimulation spectrum indicates that the doped calcium iodide crystals contain near-activator F- and F _A -type electron centers. Under x-ray and optical excitation, the trapping levels in the crystals are filled mainly by charge carriers delocalized from hydrogen- and oxygen-containing centers. The activation increases the decay probability of impurity-bound excitons.