Uniform Ln (Eu, Tb) doped NaLa(WO4)2 nanocrystals: Synthesis, characterization, and optical properties

Uniform shuttle-like Ln³⁺ (Eu³⁺, Tb³⁺) doped NaLa(WO₄)₂ nanocrystals have been solvothermally synthesized, and the size of the nanocrystals could be easily controlled by adjusting the volume ratio of ethylene glycol (EG) to water. Doped with 5 mol% Eu³⁺ and Tb³⁺ ions, the NaLa(WO₄)₂ nanocrystals showed strong red and green emissions with lifetimes of 0.8 and 1.40 ms, respectively. A high quenching concentration of 15 mol% was observed in Eu³⁺-doped NaLa(WO₄)₂ nanocrystals and 35 mol% in Tb³⁺-doped NaLa(WO₄)₂ nanocrystals. The emission intensity measurements of Eu³⁺-doped NaLa(WO₄)₂ with different sizes indicated that the emission intensity of shuttles with length of 300 nm in average was stronger than that of shuttles with length of 900 nm in average, but was weaker than that of needles with length of 4 and 9 μm in average.     

A new promising phosphor, Na3La2(BO3)3:Ln (Ln=Eu, Tb)

We present an efficient way to search a host for ultraviolet (UV) phosphor from UV nonlinear optical (NLO) materials. With the guidance, Na₃La₂(BO₃)₃ (NLBO), as a promising NLO material with a broad transparency range and high damage threshold, was adopted as a host material for the first time. The lanthanide ions (Tb³⁺ and Eu³⁺)-doped NLBO phosphors have been synthesized by solid-state reaction. Luminescent properties of the Ln-doped (Ln=Tb³⁺, Eu³⁺) sodium lanthanum borate were investigated under UV ray excitation. The emission spectrum was employed to probe the local environments of Eu³⁺ ions in NLBO crystal. For red phosphor, NLBO:Eu, the measured dominating emission peak was at 613 nm, which is attributed to ⁵D₀-⁷F₂ transition of Eu³⁺. The luminescence indicates that the local symmetry of Eu³⁺ in NLBO crystal lattice has no inversion center. Optimum Eu³⁺ concentration of NLBO:Eu³⁺ under UV excitation with 395 nm wavelength is about 30 mol%. The green phosphor, NLBO:Tb, showed bright green emission at 543 with 252 nm excited light. The measured concentration quenching curve demonstrated that the maximum concentration of Tb³⁺ in NLBO was about 20%. The luminescence mechanism of Ln-doped NLBO (Tb³⁺ and Eu³⁺) was analyzed. The relative high quenching concentration was also discussed.     

A novel red emitting phosphor Ca2SnO4: Eu

A novel red emitting phosphor, Eu³⁺-doped Ca₂SnO₄, was prepared by the solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the formation of Ca₂SnO₄: Eu³⁺. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 500 nm for the particles with spherical shape. Photoluminescence measurements indicated that the phosphor exhibits bright red emission at about 615 nm under UV excitation. The excellent luminescence properties make it possible as a good candidate for plasma display panels (PDP) application. Splitting of the ⁵D₀-⁷F_J transitions of Ca₂SnO₄: Eu³⁺ suggests that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite. The luminescence lifetime measurement showed a bi-exponential decay, providing other evidence for the existence of two different environments for Eu³⁺ ions.     

The crystal structure, vibrational and luminescence properties of the nanocrystalline KEu(WO4)2 and KGd(WO4)2:Eu obtained by the Pechini method

The luminescent nanocrystalline KEu(WO₄)₂ and KGd_0.98Eu_0.02(WO₄)₂ have been prepared by the Pechini method. X-ray diffraction, infrared and Raman spectroscopy as well as optical spectroscopy were used to characterise the obtained materials. The crystal structure of KEu(WO₄)₂ was refined in I2/c space group indicating the isostructurality to KGd(WO₄)₂. The size of the crystalline grains depended on the annealing temperature, increasing with the increase of the temperature. The average size of crystallites of both crystals formed at 540 °C was about 50 nm. Vibrational spectra showed noticeable changes as a function of size due to, among others, phonon confinement effect. Luminescence studies did not reveal significant changes for the nanocrystallites with the lowest grain size in comparison with the bulk material. The differences observed in luminescence spectra in form of slight inhomogeneous broadening of the spectral lines and increase of the hypersensitive I_0-2/I_0-1 ratio point to very low symmetry of Eu³⁺ ions and change of the polarisation of the local vicinities of Eu³⁺. X-ray diffraction, vibrational and optical studies showed that the structure of the synthesised nanocrystalline KEu(WO₄)₂ and KGd(WO₄)₂:Eu is nearly the same as that found for the bulk material. The size-driven phase transitions were established for both compounds.     

Photoluminescence of Eu-doped triple phosphate Ca8MgR(PO4)7 (R=La, Gd, Y)

Eu³⁺-doped triple phosphate Ca₈MgR(PO₄)₇ (R=La, Gd, Y) was synthesized by the general high-temperature solid-state reaction. Excitation and emission spectra as well as luminescence decay were used to characterize the phosphors. Photoluminescence excitation and emission spectra showed that the phosphor could be efficiently excited by UV-vis light from 260 to 450 nm to give bright red emission assigned to the transition (⁵D₀→⁷F₂) at 612 nm. The richness of the red color has been verified by determining their color coordinates (X, Y) from the CIE standard.     

Visible and infrared luminescence properties of Er-doped Y2Ti2O7 nanocrystals

Er³⁺-doped Y₂Ti₂O₇ nanocrystals were fabricated by the sol-gel method. While the annealing temperature exceeds 757 °C, amorphous pyrochlore phase Er_xY_2−xTi₂O₇ transfers to well-crystallized nanocrystals, and the average crystal size increases from ∼70 to ∼180 nm under 800-1000 °C/1 h annealing. Er_xY_2−xTi₂O₇ nanocrystals absorbing 980 nm photons can produce the upconversion (526, 547, and 660 nm; ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively) and Stokes (1528 nm; ⁴I_13/2→⁴I_15/2) photoluminescence (PL). The infrared PL decay curve is single-exponential for Er³⁺ (5 mol%)-doped Y₂Ti₂O₇ nanocrystals but slightly nonexponential for Er³⁺ (10 mol%)-doped Y₂Ti₂O₇ nanocrystals. For 5 and 10 mol% doping concentrations, the mechanism of up-converted green light is the two-photon excited-state absorption. Much stronger intensity of red light relative to green light was observed for the sample with 10 mol% dopant. This phenomenon can be attributed to the reduced distance between Er³⁺-Er³⁺ ions, resulting in the enhancement of the energy-transfer upconversion and cross-relaxation mechanisms.     

Synthesis and photoluminescence properties of the high-brightness Eu-doped M2Gd4(MoO4)7 (M=Li, Na) red phosphors

A series of red-emitting phosphors Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) have been successfully synthesized at 850 °C by solid state reaction. The excitation spectra of the two phosphors reveal two strong excitation bands at 396 nm and 466 nm, respectively, which match well with the two popular emissions from near-UV and blue light-emitting diode chips. The intensity of the emission from ⁵D₀ to ⁷F₂ of M₂(Gd_1−xEu_x)₄(MoO₄)₇ phosphors with the optimal compositions of x=0.85 for Li or x=0.70 for Na is about five times higher than that of Y₂O₃:Eu³⁺. The quantum efficiencies of the entitled phosphors excited under 396 nm and 466 nm are also investigated and compared with commercial phosphors Sr₂Si₅N₈:Eu²⁺ and Y₃A₅O₁₂:Ce³⁺. The experimental results indicate that the Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) phosphors are promising red-emitting phosphors pumped by near-UV and blue light.     

Synthesis and luminescence properties of Yb and Er doped KLa(WO4)2 nanoparticles

The luminescent nanocrystalline Yb³⁺ and Er³⁺ codoped KLa(WO₄)₂ has been prepared by Pechini method. X-ray diffraction and transmission electron microscope were used to study the structure of the obtained samples. The average grain size of these samples depended on the annealing temperature, increasing with the increase of the temperature. The cell parameters and the crystallite size of KYb_xEr_0.02La_0.98−x(WO₄)₂ nanocrystalline decreased with the increase of x value. Luminescence studies showed that the intensity of upconversion emission of the Yb³⁺ and Er³⁺ codoped samples was much stronger than that of the Er³⁺ single doped samples (pumped by 980 nm LD). The upconversion emission mechanisms suggested that all the three bands of upconversion emissions were two-photon process.     

Growth and spectroscopic properties of Nd-doped Sr3Y2 (BO3)4 crystal

A new crystal of Nd³⁺:Sr₃Y₂ (BO₃)₄ with a dimension of Φ 15×30 mm³ was grown by the Czochralski method. The grown crystal was characterized using X-ray diffraction. The absorption and emission spectra of Nd³⁺:Sr₃Y₂ (BO₃)₄ were investigated. The absorption transition at 807 nm has an FWHM of 16 nm. The absorption and emission cross sections are 6.32×10⁻²⁰ cm² at 807 nm and 1.07×10⁻¹⁹ cm² at 1065 nm, respectively. The luminescence lifetime τ_f is 51.7 μs at room temperature.     

Fabrication and luminescent properties of rare earths-doped Gd2(WO4)3 thin film phosphors by Pechini sol-gel process

Rare earth ions (Eu³⁺ and Dy³⁺)-doped Gd₂(WO₄)₃ phosphor films were prepared by a Pechini sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting powders and films. The results of XRD indicate that the films begin to crystallize at 600°C and the crystallinity increases with the elevation of annealing temperatures. The film is uniform and crack-free, mainly consists of closely packed fine particles with an average grain size of 80 nm. Owing to an energy transfer from WO₄²⁻ groups, the rare earth ions show their characteristic emissions in crystalline Gd₂(WO₄)₃ phosphor films, i.e., (J=0, 1, 2, 3; J′=0, 1, 2, 3, 4, not in all cases) transitions for Eu³⁺ and (J=13/2, 15/2) transitions for Dy³⁺, with the hypersensitive transitions (Eu³⁺) and (Dy³⁺) being the most prominent groups, respectively. Both the lifetimes and PL intensity of the Eu³⁺ () and Dy³⁺ () increase with increasing the annealing temperature from 500°C to 800°C, and the optimum doping concentrations for Eu³⁺ and Dy³⁺ are determined to be 30 and 6 at% of Gd³⁺ in Gd₂(WO₄)₃ film host lattices, respectively.     

Determining the structure of tetragonal Y2WO6 and the site occupation of Eu dopant

The compound Y₂WO₆ is prepared by solid state reaction at 750 °C using sodium chloride as mineralizer. Its structure is solved by ab-initio methods from X-ray powder diffraction data. This low temperature phase of yttrium tungstate crystallizes in tetragonal space group P4/nmm (No. 129), Z=2, a=5.2596(2) Å, c=8.4158(4) Å. The tungsten atoms in the structure adopt an unusual [WO₆] distorted cubes coordination, connecting [YO₆] distorted cubes with oxygen vacancies at the O₂ layers while other yttrium ions Y₂ form [YO₈] cube coordination. Y³⁺ ions occupy two crystallographic sites of 2c (C_4v symmetry) and 2a (D_2d symmetry) in the Y₂WO₆ host lattice. With Eu³⁺ ions doped, the high resolution emission spectrum of Y₂WO₆:Eu³⁺ suggests that Eu³⁺ partly substituted for Y³⁺ in these two sites. The result of the Rietveld structure refinement shows that the Eu³⁺ dopants preferentially enter the 2a site. The uniform cube coordination environment of Eu³⁺ ions with the identical eight Eu-O bond lengths is proposed to be responsible for the intense excitation of long wavelength ultraviolet at 466-535 nm.     

The effect of replacement of Sr by Ca on the structural and luminescence properties of the red-emitting Sr2Si5N8:Eu LED conversion phosphor

The influence of the replacement of Sr by Ca on structural and luminescence properties of Eu²⁺-doped Sr₂Si₅N₈ is reported. The Rietveld refinement of the powder X-ray diffraction data shows that the Ca²⁺ ion preferentially occupies the larger Sr site in Sr₂Si₅N₈:Eu²⁺. Although the excitation spectrum is hardly modified, the position of the emission band of Eu²⁺ can be tailored through partial replacement of Sr by Ca in Sr₂Si₅N₈:Eu²⁺, resulting in red-emission shifting from 620 to 643 nm. Furthermore, (Sr, Ca)₂Si₅N₈:Eu²⁺ shows high potential as a conversion phosphor for white-light LED applications due to similar absorption, conversion efficiency and thermal quenching behaviour for 465 nm excitation after the introduction of the Ca ion.     

Enhanced photoluminescence of Ba2GdNbO6: Eu/Dy phosphors by Li doping

The Ba₂GdNbO₆: Eu³⁺/Dy³⁺ and Li⁺-doped Ba₂GdNbO₆: Eu³⁺/Dy³⁺ phosphors were prepared by solid-state reaction process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and photoluminescence (PL) as well as lifetimes, was utilized to characterize the resulting phosphors. Under the excitation of ultraviolet light, the Ba₂GdNbO₆: Eu³⁺/Dy³⁺ and Li⁺-doped Ba₂GdNbO₆: Eu³⁺/Dy³⁺ show the characteristic emissions of Eu³⁺ (⁵D₀-⁷F_1,2,3 transitions dominated by ⁵D₀-⁷F₁ at 593 nm) and Dy³⁺ (⁴F_9/2-⁶H_15/2,_13/2 transitions dominated by ⁴F_9/2-⁶H_15/2 at 494 nm), respectively. The incorporation of Li⁺ ions into the Ba₂GdNbO₆: Eu³⁺/Dy³⁺ phosphors has enhanced the PL intensities depending on the doping concentration of Li⁺, and the highest emission was obtained in Ba₂Gd_0.9NbO₆: 0.10Eu³⁺, 0.01Li⁺ and Ba₂Gd_0.95NbO₆: 0.05Dy³⁺, 0.07Li⁺, respectively. An energy level diagram was proposed to explain the luminescence process in the phosphors.     

Synthesis and spectral properties of Eu-doped YF3 nanobundles

YF₃:Eu³⁺ nanobundles were synthesized by a facile microemulsion method. Analysis of X-ray diffraction, scanning electron microscope, and transmission electron microscopy reveals that each nanobundle consists of numerous nanowhiskers with a mean length of ∼500 nm and a mean diameter of ∼2 nm. Under 393-nm excitation, the luminescence was dominated by ⁵D₀ → ⁷F₁ transition, indicating the inversion symmetry of Eu³⁺ site. The luminescence intensity increased with increasing Eu³⁺ concentration, up to about 30 mol%, and then decreased abruptly. The peak positions and spectral shapes of emissions were independent of Eu³⁺ concentration. Finally, the critical distance of energy transfer was calculated.     

Valence and environment of rare earth ions in CaAl2O4:Eu,R persistent luminescence materials

The existence of the different R²⁺/R³⁺/R^IV (R: rare earth) ions as well as the modifications in the structural environment around the dopant and co-dopants in CaAl₂O₄:Eu²⁺,R³⁺ persistent luminescence materials was studied by L_III edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements at Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY) (Hamburg, Germany). The measurements were carried out at 10 and 296 K for selected rare earth (co-)dopants (Eu²⁺; Ce³⁺, Nd³⁺, Sm³⁺, and Yb³⁺).     

Luminescence in trilanthanumtrichlorotungstate (La3WO6Cl3)

The luminescence properties of La₃WO₆Cl₃ are reported and discussed. The tungstate group occurs as a trigonal prismatic WO^6?₆ complex. The blue luminescence is, for the greater part, quenched at room temperature. No energy migration occurs in this lattice. The decay times are discussed in terms of a simple molecular-orbital (MO) scheme. The luminescence of the following activating ions was studied: Mo⁶⁺, Bi³⁺, Eu³⁺, Sm³⁺, Ce³⁺, and Tb³⁺. The molybdate group produces a red emission with low efficiency. The Bi³⁺ ion induces a narrow band emission with small Stokes shift. This is interpreted using a Bi³⁺O^2?W⁶⁺ charge-transfer state. Except for Ce³⁺, the rare earth activators show luminescence, but the total transfer efficiency from tungstate to the rare-earth ions is low. This is not due to the one-step tungstate-rare-earth transfer (which is efficient), but to the localized nature of the tungstate excitation. The Eu³⁺ charge-transfer band is at very low energies.     

Preparation, structure and photoluminescence properties of Eu and Ce-doped SrYSi4N7

Undoped and Eu²⁺ or Ce³⁺-doped SrYSi₄N₇ were synthesized by solid-state reaction method at 1400-1660 °C under nitrogen/hydrogen atmosphere. The crystal structure was refined from the X-ray powder diffraction data by the Rietveld method. SrYSi₄N₇ and EuYSi₄N₇, being isotypic with the family of compounds MYbSi₄N₇ (M=Sr, Eu, Ba) and BaYSi₄N_7, crystallize with the hexagonal symmetry: space group P6₃mc (No. 186), Z=2, a=6.0160 (1) Å, c=9.7894 (1) Å, V=306.83(3) Å³; and a=6.0123 (1) Å, c=9.7869 (1) Å, V=306.37(1) Å³, respectively. Photoluminescence properties have been studied for Sr_1−xEu_xYSi₄N₇ (x=0-1) and SrY_1−xCe_xSi₄N₇ (x=0-0.03) at room temperature. Eu²⁺-doped SrYSi₄N₇ shows a broad yellow emission band peaking around 548-570 nm, while Ce³⁺-doped SrYSi₄N₇ exhibits a blue emission band with a maximum at about 450 nm. SrYSi₄N₇:Eu²⁺ can be very well excited by 390 nm radiation, which makes this material attractive as conversion phosphor for LED lighting applications.     

Eu luminescence—a structural probe in BiCa4(PO4)3O, an apatite related phosphate

Eu³⁺ luminescence is studied in apatite-related phosphate BiCa₄(PO₄)₃O. Compositions of the formula Bi_1−xEu_xCa₄(PO₄)₃O [x=0.05, 0.1, 0.3, 0.5, 0.8 and 1.0] are synthesized and they are isostructural with parent BiCa₄(PO₄)₃O. Room temperature photoluminescence shows the various transitions ⁵D₀→⁷F_J(=0,1,2) of Eu³⁺. The emission results of compositions with different Eu³⁺ content show the difference in site occupancy of Eu³⁺ in Bi_1−xEu_xCa₄(PO₄)₃O. The intense ⁵D₀-⁷F₀ line at 574 nm for higher Eu³⁺ content is attributed to the presence of strongly covalent Eu-O bond that is possible by substituting Bi³⁺ in the Ca(2) site. This shows the preferential occupancy of Bi³⁺ in Ca(2) site and this has been attributed to the 6s² lone pair electrons of Bi³⁺. This is further confirmed by comparing the emission results with La_0.95Eu_0.05Ca₄(PO₄)₃O.     

Growth and spectroscopic characterization of Nd:Sr6GdSc(BO3)6 crystal

A crystal of Nd³⁺:Sr₆GdSc(BO₃)₆ with the dimension of φ20×30 mm³ was grown by Czochralski method. The grown crystal was characterized by X-ray diffraction and DSC analysis. The DSC analysis showed that the crystal congruently melt at 1306.7°C. The absorption and emission spectra of Nd³⁺:Sr₆GdSc(BO₃)₆ were investigated. The absorption band at 806 nm has a FWHM of 13 nm. The absorption and emission cross-sections are 2.33×10⁻²⁰ cm² at 806 nm and 1.58×10⁻¹⁹ cm² at 1062 nm, respectively. The luminescence lifetime τ_f is 75 μs at room temperature.     

Crystal, electronic structures and photoluminescence properties of rare-earth doped LiSi2N3

The crystal and electronic structures, and luminescence properties of Eu²⁺, Ce³⁺ and Tb³⁺ activated LiSi₂N₃ are reported. LiSi₂N₃ is an insulator with an indirect band gap of about 5.0 eV (experimental value ∼6.4 eV) and the Li 2s, 2p states are positioned on the top of the valence band close to the Fermi level and the bottom of the conduction band. The solubility of Eu²⁺ is significantly higher than Ce³⁺ and Tb³⁺ in LiSi₂N₃ which may be strongly related to the valence difference between Li⁺ and rare-earth ions. LiSi₂N₃:Eu²⁺ shows yellow emission at about 580 nm due to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. Double substitution is found to be the effective ways to improve the luminescence efficiency of LiSi₂N₃:Eu²⁺, especially for the partial replacement of (LiSi)⁵⁺ with (CaAl)⁵⁺, which gives red emission at 620 nm, showing highly promising applications in white LEDs. LiSi₂N₃:Ce³⁺ emits blue light at about 450 nm arising from the 5d¹→4f¹5d⁰ transition of Ce³⁺ upon excitation at 320 nm. LiSi₂N₃:Tb³⁺ gives strong green line emission with a maximum peak at about 542 nm attributed to the ⁵D₄→⁷F_J (J=3-6) transition of Tb³⁺, which is caused by highly efficient energy transfer from the LiSi₂N₃ host to the Tb³⁺ ions.