Luminescence investigation of Eu activated molybdates red phosphors and their applications in near-UV light-emitting diodes

The red phosphors, Na₅La_1−xSm_x(MoO₄)₄ and Na₅Eu_1−xSm_x(MoO₄)₄, were prepared by solid-state reaction technique at high temperature. Their structure and photo-luminescent properties were investigated. The excitation bands around 400 nm of these phosphors are broadened by Sm³⁺-Eu³⁺ co-doping. And the possible energy transfer process from Sm³⁺ to Eu³⁺ in these compounds is also discussed briefly. The phosphor Na₅Eu_0.90Sm_0.10(MoO₄)₄ exhibits efficient red-emitting with broadened absorption around 400 nm and appropriate CIE chromaticity coordinates (x = 0.66, y = 0.34), bright red light can be observed from the red LED based Na₅Eu_0.90Sm_0.10(MoO₄)₄.     

The VUV–vis spectroscopic properties of phosphors Ca3Gd2(1−x)Ln2x(BO3)4 (Ln3+ = Ce,Sm, Eu,Tb)

The spectroscopic properties in VUV–vis range for phosphors calcium and gadolinium double borate Ca₃Gd₂(BO₃)₄ doped with rare-earth ions Ce³⁺, Sm³⁺, Eu³⁺ and Tb³⁺ were investigated. The host-related absorption, the f–d transitions of Ce³⁺ and Tb³⁺, as well as the charge transfer transitions of Sm³⁺ and Eu³⁺ in the host lattice are assigned and discussed. The CIE chromaticity coordinates for Eu³⁺- and Tb³⁺-activated phosphors are calculated.     

Intense red-emitting phosphors for LED solid-state lighting

The phosphors Gd_2−xEu_x(MoO₄)₃ (x = 0.20, 0.40, 0.60, 0.80, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0), Gd_0.8−xY_xEu_1.2(MoO₄)₃ (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8) and Gd_0.2Y_0.6−xEu_1.2Sm_x(MoO₄)₃ (x = 0.02, 0.024, 0.028, 0.032, 0.036, 0.04) were prepared by solid-state reaction technique at 950 °C. The presence of the Y³⁺ and Sm³⁺ ions strengthen and broaden the absorption of the phosphors at ∼400 nm. The intense red-emitting phosphor Gd_0.2Y_0.572Eu_1.2Sm_0.028(MoO₄)₃ with orthorhombic structure was obtained. Both Eu³⁺ and Sm³⁺ f-f transition absorptions are observed in the excitation spectra, the main emission line is at 616 nm (⁵D₀ → ⁷F₂ transition of Eu³⁺) and the chromaticity coordinates (x = 0.66, y = 0.33) is very close to the NTSC standard values (x = 0.67, y = 0.33). It is considered to be an efficient red-emitting phosphor for GaN-based light emitting diode (LED).     

Synthesis and luminescence properties of Eu3+ and Sm3+ doped and co-doped BiPO4 powders by a hydrothermal route

BiPO₄:Eu³⁺, BiPO₄:Sm³⁺, and BiPO₄:(Eu³⁺, Sm³⁺) powders were synthesized by a hydrothermal route. The X-ray diffraction and Fourier-transform infrared spectroscopy results show that the samples are pure hexagonal phases. The excitation spectrum of BiPO₄:Eu³⁺ sample shows the characteristic absorption peaks of Eu³⁺ corresponding to the direct excitation from the ground state to the higher excited states of the europium f-electrons. The excitation spectrum of BiPO₄:Sm³⁺ sample consists of several narrow excitation lines due to the characteristic f–f transition of Sm³⁺. The emission spectrum of BiPO₄:Eu³⁺ specimen excited by 395 nm consists of lines mainly locating in the red area. The emission spectrum of BiPO₄:Sm³⁺ sample is composed of four emission bands, which are attributed to transitions from ⁴G_5/2 excited state to ⁶H_J/2 (J = 5, 7, 9, and 11) ground states of Sm³⁺. The excitation and emission spectra of BiPO₄:(Eu³⁺, Sm³⁺) samples are different from single doped BiPO₄ samples. The introduction of Sm³⁺ can broaden and enhance the excitation intensity of Eu³⁺ and improve the emission dominance of Eu³⁺ at 617 nm.     

Luminescence properties of Ca4Y6(SiO4)6O:RE (RE = Eu, Tb, Dy, Sm and Tm) under vacuum ultraviolet excitation

The vacuum ultraviolet excited luminescent properties of Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺ and Tm³⁺ in the matrices of Ca₄Y₆(SiO₄)₆O were investigated. The bands at about 173 nm in the vacuum ultraviolet excited spectra were attributed to host lattice absorption of the matrix Ca₄Y₆(SiO₄)₆O. For Eu³⁺-doped samples, the O²⁻ → Eu³⁺ CTB was identified at 258 nm. Typical 4f-5d absorption bands in the region of 195-300 nm were observed in Tb³⁺-doped samples. For Dy³⁺-doped and Sm³⁺-doped samples, the broad excitation bands consisted of host absorptions, CTB and f-d transition. For Tm³⁺-doped samples, the O²⁻ → Tm³⁺ CTB was located at 191 nm. About the color purity and emission intensity, Ca₄Y₆(SiO₄)₆O:Tb³⁺ is an attractive candidate of green light PDP phosphor, and Ca₄Y₆(SiO₄)₆O:Dy³⁺ has potential application in the field of mercury-free lamps.     

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.     

NaY(MoO4)2:Eu3+ and NaY0.9Bi0.1(MoO4)2:Eu3+ submicrometer phosphors: Hydrothermal synthesis assisted by room temperature-solid state reaction,microstructure and photoluminescence

NaY(MoO₄)₂:Eu³⁺ and NaY_0.9Bi_0.1(MoO₄)₂:Eu³⁺ submicrometer phosphors have been synthesized by a composite technology involving hydrothermal process assisted solid state reaction at room temperature. It is revealed that crystalline water is necessary for the solid phase reaction at room temperature. The XRD patterns indicate that both NaY(MoO₄)₂:Eu³⁺ and NaY_0.9Bi_0.1(MoO₄)₂:Eu³⁺ submicrometer phosphors crystallize well with the scheelite structure. Both SEM and TEM images illustrate that the average grain size of NaY(MoO₄)₂:Eu³⁺ and NaY_0.9Bi_0.1(MoO₄)₂:Eu³⁺ is about 200 nm without conglomeration. The luminescent lifetimes and quantum efficiencies for NaY(MoO₄)₂:Eu³⁺ and NaY_0.9Bi_0.1(MoO₄)₂:Eu³⁺ are determined, indicating that the introduction of Bi³⁺ is favorable for the luminescence of Eu³⁺.     

Synthesis and luminescent properties of BaLn2(1−x)ZnO5:2xTb3+ (Ln = Y, Gd) nanophosphors

The synthesis of BaLn_2(1?x)ZnO₅:2xTb³⁺ (Ln = Y, Gd) nanophosphors using solution combustion method with an aim to study the effect of sintering temperature and Tb³⁺ ions concentration on the luminescent properties has been investigated. Under UV excitation, BaY_2(1?x)ZnO₅ and BaGd_2(1?x)ZnO₅ nanoparticles exhibit apparent characteristic green emission from ⁵D₄ state to ⁷F_6?3 states of Tb³⁺ ions with the strongest at 544 nm. Luminescence concentration quenching could be observed when the Tb³⁺ ions contents were more than 4 mol% in both nanophosphors. The luminescence decay curves suggest monoexponential behavior of both nanophosphors. X-ray diffraction results confirmed the single-phased orthorhombic structure of both powders belonging to space group Pbnm. TEM analysis indicates the spherical morphology of nanoparticles with average grain size in the range of 85–95 nm. BaY_2(1?x)Tb_2x ZnO₅ and BaGd_2(1?x)Tb_2x ZnO₅ nanophosphors may have potential applications in field emission displays based on their uniform shape, low-cost synthetic route, and diverse luminescent properties.     

The optical spectroscopic properties of rare earth-activated barium orthophosphate in VUV-Vis range

The optical properties of rare earth ions-activated barium orthophosphate phosphors, Ba₃(PO₄)₂:RE (RE=Ce³⁺, Sm³⁺, Eu³⁺, Eu²⁺, and Tb³⁺), were investigated in vacuum ultraviolet (VUV)-Vis range. A band-band transition of PO₄³⁻ in Ba₃(PO₄)₂ is observed in the region of 150-170 nm. The partial reduction of Eu³⁺ to Eu²⁺ was observed and confirmed by luminescent spectra under the VUV-UV excitation. It is proposed that the electronegative defects that formed by aliovalent substitution of Eu³⁺ on the Ba²⁺ site in the host are responsible for the reduction process.     

Photoluminescence investigation of rare-earth activated GdCa3(GaO)3(BO3)4 phosphors under UV excitation

A borate compound was adopted as a new host material of Eu³⁺ and Tb³⁺ activators to fabricate efficient luminescence materials. The phosphor compositions, Gd_1−xEu_xCa₃(GaO)₃(BO₃)₄ and Gd_1−xTb_xCa₃(GaO)₃(BO₃)₄, were synthesized by conventional solid-state reactions. The crystalline phases of the resulting powders were identified using an X-ray diffraction system. Their photoluminescence properties were investigated under long-wavelength UV excitation. The Eu³⁺-doped and Tb³⁺-doped GdCa₃(GaO)₃(BO₃)₄ phosphors efficiently emitted red and green light, respectively. The temperature dependency of emission intensity was measured in a range from room temperature to 150 °C. The emission intensities of the red and green phosphors at 150 °C are 87% and 91% of those at room temperature, respectively. In addition, the decay times of both the red and green phosphors are shorter than 3 ms.     

Enhanced novel orange red emission in LiSr4−x (BO3)3:xSm3+ by K+

LiSr_4?x (BO₃)₃:xSm³⁺ and LiSr_3.985?x (BO₃)₃:0.015Sm³⁺, xK⁺ phosphors were prepared by solid-state reactions. The phases and luminescent properties of the obtained phosphors were characterized. The results demonstrate that the phosphors particles emit an intensive reddish orange light emission under excitation at 403 nm. LiSr_4?x (BO₃)₃:xSm³⁺ phosphor can be efficiently excited by ultraviolet and blue light, and the emission spectrum consists of three emission peaks at 564, 601 and 647 nm. The introduction of the charge compensator K⁺ into the LiSr_4?x (BO₃)₃:xSm³⁺ phosphor matrix promotes the increase of the emission intensity, as well as the decrease of the E _g value. Results suggest that LiSr_3.97(BO₃)₃:0.015Sm³⁺, 0.015 K⁺ is a promising orange–red emitting phosphor for UV LED applications.     

Synthesis and luminescence properties of ternary complexes of Sm x Tb1−x (TTA)3Phen nanoparticles and their surface modification

A series of Sm _x Tb_1?x (TTA)₃Phen (x = 1.0, 0.9, 0.7, 0.5, 0.3, 0.1, 0) complexes was synthesized by wet chemical method and characterized using Fourier transform infrared spectroscopy and fluorescence spectroscopy. The IR absorption spectra indicate that α-thenoyltrifluoroacetone was coordinated to the rare earth ions and that chemical bonds were formed between rare earth ions and the nitrogen atoms in 1,10-Phenanthroline (Phen). The fluorescence spectra of the complexes indicate that the emission intensity of Sm³⁺ was enhanced by the addition of substitutive Tb³⁺. These data show that not only the ligands (TTA and Phen) but also Tb³⁺ could absorb and transmit energy to Sm³⁺ in the complexes. Formation of dual-core complexes appears to be responsible for co-fluorescence, which can greatly promote the illuminant ability of Sm (TTA)₃Phen complex. After further encapsulation by SiO₂, the Sm _x Tb_1?x (TTA)₃Phen@SiO₂ core–shell structure was very stable to UV light.     

Luminescent properties of GdAl3(BO3)4:Ln3+ (Ln3+:Eu3+, Tb3+, Dy3+) nano-phosphors

GdAl₃(BO₃)₄:Ln³⁺ (Ln³⁺:Eu³⁺, Tb³⁺, Dy³⁺) nano-phosphors were prepared by sol–gel method. The structure properties of the phosphors are characterized by XRD, and GdAl₃(BO₃)₄:Ln³⁺ nano-phosphors have average sizes around 40 nm. The doping concentrations of Eu³⁺, Tb³⁺ and Dy³⁺ ions in GdAl₃(BO₃)₄ nano-phosphors are from 1 to 9 mol% for Eu³⁺ ions, from 2 to 12 mol% for Tb³⁺ ions and from 1 to 5 mol% for Dy³⁺ ions, respectively. The luminescent properties of rare-earth ions doped GdAl₃(BO₃)₄ nano-phosphors are analyzed by the photoluminescence spectra, which prime doping concentration of Eu³⁺, Tb³⁺, and Dy³⁺ ions are at 5, 12 and 3 mol%, respectively. The energy transfers in the luminescent processes of rare-earth ions doped GdAl₃(BO₃)₄ nano-phosphors are discussed.     

Layered rare-earth hydroxide and oxide nanoplates of the Y/Tb/Eu system: phase-controlled processing,structure characterization and color-tunable photoluminescence via selective excitation and efficient energy transfer

Abstract

Well-crystallized (Y_0.97?xTb_0.03Eu_x)₂(OH)₅NO₃·nH₂O (x = 0–0.03) layered rare-earth hydroxide (LRH) nanoflakes of a pure high-hydration phase have been produced by autoclaving from the nitrate/NH₄OH reaction system under the optimized conditions of 100 °C and pH ～7.0. The flakes were then converted into (Y_0.97?xTb_0.03Eu_x)₂O₃ phosphor nanoplates with color-tunable photoluminescence. Detailed structural characterizations confirmed that LRH solid solutions contained NO₃^? anions intercalated between the layers. Characteristic Tb³⁺ and Eu³⁺ emissions were detected in the ternary LRHs by selectively exciting the two types of activators, and the energy transfer from Tb³⁺ to Eu³⁺ was observed. Annealing the LRHs at 1100 °C produced cubic-lattice (Y_0.97?xTb_0.03Eu_x)₂O₃ solid-solution nanoplates with exposed 222 facets. Multicolor, intensity-adjustable luminescence was attained by varying the excitation wavelength from ～249 nm (the charge transfer excitation band of Eu³⁺) to 278 nm (the 4f⁸–4f⁷5d¹ transition of Tb³⁺). Unitizing the efficient Tb³⁺ to Eu³⁺ energy transfer, the emission color of (Y_0.97?xTb_0.03Eu_x)₂O₃ was tuned from approximately green to yellowish-orange by varying the Eu³⁺/Tb³⁺ ratio. At the optimal Eu³⁺ content of x = 0.01, the efficiency of energy transfer was ～91% and the transfer mechanism was suggested to be electric multipole interactions. The phosphor nanoplates developed in this work may be incorporated in luminescent films and find various lighting and display applications.     

Luminescent properties of Eu3+ activated MLa2(MoO4)4 based (M = Ba,Sr and Ca) novel red-emitting phosphors

Eu³⁺-activated novel red phosphors, MLa₂(MoO₄)₄ (M = Ba, Sr and Ca) were synthesized by the conventional solid state method. The excitation and emission spectra indicate that these phosphors can be effectively excited by UV (395 nm) and blue (466 nm) light, and exhibit a satisfactory red performance at 614 nm. Upon excitation with a 466 nm light, our synthesized phosphors have stronger emission intensity than the sulfide red phosphors used in white LEDs. Due to high emission intensity and a good excitation profile, the Eu³⁺-doped CaLa₂(MoO₄)₄ phosphor may be a promising candidate in solid-state lighting applications.     

Luminescence of SrGdGa3O7:RE(RE = Eu, Tb) phosphors and energy transfer from Gd to RE

Eu³⁺- and Tb³⁺-activated SrGdGa₃O₇ phosphors were synthesized by the solid-state reaction and their luminescence properties were investigated. Sr(Gd_1 − xEu_x)Ga₃O₇ and Sr(Gd_1 − xTb_x)Ga₃O₇ formed continuous solid solution in the range of x = 0-1.0. Unactivated SrGdGa₃O₇ exhibited a typical characteristic excitation and emission of Gd ion. The SrGdGa₃O₇:xEu³⁺ and SrGdGa₃O₇:xTb³⁺ phosphors also showed the well-known Eu³⁺ and Tb³⁺ excitation and emission. The energy transfer from Gd³⁺ to Eu³⁺ and Tb³⁺ were verified by photoluminescence spectra. The dependence of photoluminescence intensity on Eu³⁺ and Tb³⁺ concentration were also studied in detail and the photoluminescence (PL) intensity of SrGdGa₃O₇:Eu and SrGdGa₃O₇:Tb were compared with commercial phosphors, Y₂O₃:Eu and LaPO₄:Ce,Tb. The luminescence decay measurements showed that the lifetimes of Eu³⁺ and Tb³⁺ were in the range of microsecond. The energy transfer from Gd³⁺ to Tb³⁺ was also observed in decay curve.     

Phosphors based on NaZr2(PO4)3-type calcium and strontium phosphates activated with Eu2+ and Sm3+

Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sr_0.5Zr₂(PO₄)₃:Eu²⁺, and Ca_0.5Zr₂(PO₄)₃:Eu²⁺, Sm³⁺ orthophosphates prepared through precipitation using sol-gel processes are analogs of NaZr₂(PO₄)₃ (NZP) and crystallize in space group R $\bar 3$ . Their crystallographic parameters determined by X-ray diffraction are consistent with the interatomic distances extracted from EXAFS data. Their luminescence spectra obtained under excitation in the range 300?C400 nm contain emission bands between 425 and 525 nm. Substitution of the larger sized cations Eu²⁺ and Sm³⁺ for Ca²⁺ shifts the emission bands to shorter wavelengths and reduces their width because of the decrease in the effect of the crystal field. Analysis of the spectra indicates that Eu²⁺ occupies two types of crystallographic sites (independent interstitial sites of different sizes and shapes in the NZP framework structure). Codoping with Eu and Sm has ensured luminescence with chromaticity coordinates approaching those of white light: (x = 0.27, y = 0.34).     

Properties of Gd2O3:Eu, Tb nanopowders obtained by sol-gel process

A significant practical application for nanostructured materials is X-ray medical imagery, because it is necessary to use dense materials in order to enable absorption of high energy photons. An important requirement of these materials is UV-vis range emission produced by X-ray excitation, which can be influenced by the particle size. Europium doped gadolinium oxide is a well known red phosphor. Moreover, nanophosphors of Gd₂O₃ codoped with Tb³⁺, Eu³⁺ increase their light yield by energy transfer between Tb³⁺ and Eu³⁺. In this study, Gd₂O₃ nanopowders codoped with Eu³⁺ and Tb³⁺ (2.5 at.% Eu³⁺, and 0.005 and 0.01 at.% Tb³⁺) were obtained via a sol-gel process using gadolinium pentanedionate as precursor and europium and terbium nitrates as doping sources. In this paper, we report the influence of annealing temperature on the structure, morphology and luminescent properties of Gd₂O₃:Eu³⁺, Tb³⁺ by means of TGA, XRD, TEM and X-ray emission measurements.     

Investigation of energy absorption and transfer process of Tb3+ or Eu3+ excited Na3Gd(PO4)2 in the VUV region

Na₃Gd(PO₄)₂, Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ and Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ are prepared by solid-state reaction and their photoluminescence (PL) properties are investigated in the ultraviolet (UV) and vacuum ultraviolet (VUV) region. The obtained results show that Na₃Gd_0.94(PO₄)₂:0.06Tb³⁺ has an efficient emission under 147 nm excitation, but the emission efficiency of Na₃Gd_0.94(PO₄)₂:0.06Eu³⁺ is low under 147 nm excitation. We discuss the energy absorption and transfer process in the VUV region to solve the special phenomenon.     

Color-tunable luminescence properties of Sm<Superscript>3+</Superscript>/Dy<Superscript>3+</Superscript> co-doped NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> phosphors and their energy transfer mechanism

The Sm³⁺, Dy³⁺ doped and Sm³⁺/Dy³⁺ co-doped NaLa(MoO₄)₂ spherical phosphors were hydrothermally synthesized by the EDTA-2Na mediated method. Under the excitation of 297 nm, the quenching concentration of Sm³⁺ in NaLa(MoO₄)₂ host was determined to be 13%, and the concentration quenching mechanism was discussed to be the electric quadrupole–quadrupole interaction. After Sm³⁺ and Dy³⁺ ions were co-doped into the NaLa(MoO₄)₂ host, the energy transfer behaviors resulted from Dy³⁺ to Sm³⁺ ions were investigated by the help of the luminescent spectra of the obtained phosphors. By varying co-doping concentrations of Sm³⁺/Dy³⁺ ions, the emission color of NaLa(MoO₄)₂:Sm³⁺/Dy³⁺ can be tuned from reddish-orange, pink and white to bluish-green. The CIE chromaticity coordinate, the correlated color temperature and the quantum efficiency of NaLa_0.87(MoO₄)₂:1%Sm³⁺, 12%Dy³⁺ were calculated to be (0.356, 0.320), 4353 K and 20%, respectively. Furthermore, in the temperature-dependent analysis, it presented good thermal stability, which can become a promising single-phased white-emitting phosphor for white LEDs devices. Based on these results, the possible energy transfer mechanism between Dy³⁺ and Sm³⁺ in NaLa(MoO₄)₂:Sm³⁺/Dy³⁺ was also proposed.