Photoluminescence studies on energy transfer processes in cerium-doped Gd3Al2Ga3O12 crystals

We have measured photoluminescence properties of cerium-doped Gd₃Al₂Ga₃O₁₂ (Ce:GAGG) crystals at low temperatures with use of synchrotron radiation. Excitation spectra for the Ce³⁺ 5d–4f emission exhibit prominent peaks at Gd³⁺ intra-4f absorption bands. The Gd³⁺ intra-4f emission band is observed at 3.91 eV, but is not in resonance with the lowest energy Gd³⁺ intra-4f absorption band at 3.95 eV. The temperature dependence of the Gd³⁺ emission intensity is not correlated with that of the Ce³⁺ emission intensity. Decay curves of the Ce³⁺ emission were also measured at 9 K under excitation at various photon energies. The decay curve is remarkably changed, depending on the excitation photon energies. The present results give us hints to understand the whole of energy transfer processes in Ce:GAGG crystals.     

Optical and scintillation properties of Nd-doped complex garnet

Nd 1% doped complex garnet scintillators were prepared by Furukawa and their optical and scintillation properties were investigated on a comparison with previously reported Nd-doped YAG. Chemical compositions of newly developed complex garnets were Lu₂Y₁Al₅O₁₂, Lu₂Y₁Ga₃Al₂O₁₂, Lu₂Gd₁Al₅O₁₂, Lu₂Gd₁Ga₃Al₂O₁₂, Gd₁Y₂Al₅O₁₂, Gd₁Y₂Ga₃Al₂O₁₂, and Gd₃Ga₃Al₂O₁₂. They all showed 50–80% transmittance from ultraviolet to near infrared wavelengths with several absorption bands due to Gd³⁺ or Nd³⁺ 4f–4f transition. In X-ray induced radioluminescence spectra, all samples exhibited intense lines at 310 nm due to Gd³⁺ or 400 nm due to Nd³⁺ depending on their chemical composition. Among them, the highest scintillation light yield was achieved by Lu₂Y₁Al₅O₁₂. Typical scintillation decay times of them resulted 1.5–3 μs. Thermally stimulated glow curve after 1 Gy exposure and X-ray induced afterglow were also investigated.     

Effect of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> concentration in Bi-containing high-temperature solutions on the luminescence of epitaxial Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> films

Single-crystal gadolinium gallium garnet films have been grown by liquid-phase epitaxy on (111) Gd₃Ga₅O₁₂ substrates from supercooled Bi₂O₃-B₂O₃ fluxed melts at different Gd₂O₃ concentrations. The luminescence spectra of the films have been measured at 10 and 300 K under unmonochromatized synchrotron X-ray excitation and selective UV synchrotron excitation. The Bi³⁺ luminescence is discussed.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃(Ga,Al)₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect––host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

Defect formation in Gd3Fe5O12-based garnets: a Mössbauer spectroscopy study

The Mössbauer spectroscopy of Gd_2.2Pr_0.8Fe₅O₁₂ garnet, which exhibits higher electronic conduction with respect to Gd₃Fe₅O₁₂ due to the presence of Pr⁴⁺ cations, showed that praseodymium doping decreases the coordination of Fe³⁺ in octahedral sites. Penta-coordinated Fe³⁺ ions, in combination with small quantities of Fe⁴⁺, are also formed in the lattice of Gd_2.5Ca_0.5Fe₅O₁₂ where the variations of ionic and electronic transport properties indicate charge compensation via generation of oxygen vacancies and electron holes. The mechanisms of garnet lattice disorder, induced by acceptor- and donor-type doping, appear thus quite similar; in all cases, the ionic defect formation requires substantial structural reconstruction, probably associated with direct linking of iron–oxygen tetrahedra. Due to the low concentration of charge carriers and the important role of lattice relaxation in the oxygen ion migration processes, this behavior results in similar activation energies for the ionic conductivity in all Gd₃Fe₅O₁₂-based garnets.     

Effect of Bismuth Ions on the Optical Absorption in Gd3Ga5O12〈Bi〉 Epitaxial Films

Single-crystal gadolinium gallium garnet films containing Bi impurity are grown on (111) Gd₃Ga₅O₁₂ substrates by liquid-phase epitaxy from Bi₂O₃–B₂O₃ fluxes. The absorption spectra of the films are measured from 0.2 to 1.5 m. The effect of Bi³⁺ impurity on the optical absorption in the films is examined.     

The development of Ce3+-activated (Gd,Lu)3Al5O12 garnet solid solutions as efficient yellow-emitting phosphors

Abstract

Ce³⁺-activated Gd₃Al₅O₁₂ garnet, effectively stabilized by Lu³⁺ doping, has been developed for new yellow-emitting phosphors. The powder processing of [(Gd_1?xLu_x)_1?yCe_y]₃Al₅O₁₂ solid solutions was achieved through precursor synthesis via carbonate precipitation, followed by annealing. The resultant (Gd,Lu)AG:Ce³⁺ phosphor particles exhibit typical yellow emission at ～570 nm (5d–4f transition of Ce³⁺) upon blue-light excitation at ～457 nm (the ²F_5/2–5d transition of Ce³⁺). The quenching concentration of Ce³⁺ was determined to be ～1.0 at% (y = 0.01) and the quenching mechanism was suggested to be driven by exchange interactions. The best luminescent [(Gd_0.9Lu_0.1)_0.99Ce_0.01]AG phosphor is comparative to the well-known YAG:Ce³⁺ in emission intensity but has a substantially red-shifted emission band that is desired for warm-white lighting. The effects of processing temperature (1000–1500 °C) on the spectroscopic properties of the phosphors, especially those of Lu³⁺/Ce³⁺, were thoroughly investigated and discussed from the centroid position and crystal field splitting of the Ce³⁺ 5d energy levels.     

Comparative study of ceramic and single crystal Ce:GAGG scintillator

Recent study revealed that single crystal Ce:Gd₃(Al,Ga)₅O₁₂ (Ce:GAGG) showed good scintillation response under γ-ray exposure. We discover here that ceramic Ce:GAGG scintillator exhibited better performance than the single crystal counterpart. We developed Ce 1% doped ceramic and single crystal GAGG scintillators with 1 mm thick and compared their properties. In radioluminescence spectra, they showed intense emission peaking at 530 nm due to Ce³⁺ 5d–4f transition. The ¹³⁷Cs γ-ray induced light yields of ceramic and single crystal resulted 70 000 ph/MeV and 46 000 ph/MeV with primary decay times of 165 and 143 ns, respectively. At present, the observed light yield was the brightest in oxide scintillators.     

Composition engineering of single crystalline films based on the multicomponent garnet compounds

The paper demonstrates our last achievement in development of the novel scintillating screens based on single crystalline films (SCF) of Ce doped multicomponent garnets using the Liquid Phase Epitaxy (LPE) method. We report in this work the optimized content and excellent scintillation properties of SCF of Lu_3-xGd_xAl_5-yGa_yO₁₂, Lu_3-xTb_xAl_5-yGa_yO₁₂ and Tb_xGd_xAl_5-yGa_yO₁₂ garnet compounds grown by the LPE method from PbOB₂O₃ based melt-solution onto Gd₃Al_2.5Ga_2.5O₁₂ and YAG substrates.We also show that the Tb_1.5Gd_1.5Al_2.5Ga_2.5O₁₂:Ce SCF possess the highest light yield (LY) in comparison with all ever grown garnet SCF scintillators. Namely, the LY of these SCF exceeds by 3.8 and 1.85 times the LY values of the best samples of YAG:Ce and LuAG:Ce SCF scintillators, respectively. The SCF samples of the mentioned compounds show low thermoluminescence in the above room temperature range and relatively fast scintillation decay time t_1/e in the 180–200 ns range.     

Optical spectroscopy of Ce3+ ions in Gd3(AlxGa1−x)5O12 epitaxial films

Epitaxial films of Ce-doped Gd₃(Al_xGa_1−x)₅O₁₂ with x = 0.00, 0.22, 0.31, 0.38 formula units were grown using liquid-phase epitaxy method, and their optical properties were studied. The emission of Ce³⁺ ions can be observed only when Al³⁺ ions are incorporated into the garnet structure, resulting in a shift of the 5d Ce³⁺ states from the conduction band to the bandgap. It is shown that the shift is caused by the cumulative effect of gradual low-energy shift of the lowest 5d level of Ce³⁺ and the raise of the garnet bandgap energy with increasing Al³⁺ concentration.     

Characteristics and synthesis mechanism of Gd2O2S:Tb phosphors prepared by vacuum firing method

Characteristics and synthesis mechanism of Gd₂O₂S:Tb phosphors prepared by vacuum firing were investigated by photoluminescence (PL) spectra, X-ray diffraction (XRD), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). The mixtures of raw materials were tested by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The firing temperature was reduced to about 725 °C by the vacuum firing method. The particle size decreased. Meanwhile the particle size distribution and morphology were improved and the luminescence of Gd₂O₂S:Tb phosphors was also enhanced as the degree of vacuum decreases. With the decrease of the degree of vacuum, the intensity of the excitation spectrum was strengthened and the band was widened being the particle size of the host lattice decreased to nano scales. The peak with high intensity around 272 nm in the excitation spectra (λ_Em = 545 nm) of Gd₂O₂S:Tb nanophosphor may be attributed to the 4d–5f transitions of Gd atoms, which may play a significant role in the energy transfer between Tb³⁺ and Gd³⁺ ions.⁵D₄–⁷F_J transitions of Tb³⁺ ion were mainly concentrated in the narrow green emission spectrum (535–555 nm) with its sharp peak at 545 nm. The synthesis mechanism of Gd₂O₂S:Tb phosphors prepared by vacuum firing was also studied.     

X-ray study of gadolinium gallium garnet epitaxial layers containing divalent Co ions

Structural perfection of gadolinium gallium garnet (GGG; Gd₃Ga₅O₁₂) epitaxial layers with incorporated divalent Co ions has been studied by means of high resolution X-ray diffraction, X-ray topography, transmission electron microscopy and optical spectroscopy. Epitaxial layers were grown by liquid phase epitaxy from super-cooled high temperature solution with different concentration of Co₃O₄ and GeO₂ on both sides of the polished < 111> oriented GGG substrates. In order to facilitate incorporation of Co²⁺ ions into the garnet lattice optically inert Ge⁴⁺ ions have to be introduced first. High structural perfection is a prerequisite to obtain absorption spectra required for the use GGG epitaxial layers as tunable infrared absorbers.     

Luminescent properties of Gd2Ti2O7: Eu phosphor films prepared by sol-gel process

Gd₂Ti₂O₇: Eu³⁺ thin film phosphors were fabricated by a sol-gel process. X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 800 °C and the crystallinity increased with the elevation of annealing temperatures. Uniform and crack free phosphor films were obtained, which mainly consisted of grains with an average size of 70 nm. The doped Eu³⁺ showed orange-red emission in crystalline Gd₂Ti₂O₇ phosphor films due to an energy transfer from Gd₂Ti₂O₇ host to them. Both the lifetimes and PL intensity of the Eu³⁺ increased with increasing the annealing temperature from 800 to 1000 °C, and the optimum concentrations for Eu³⁺ were determined to be 9 at.%. of Gd³⁺ in Gd₂Ti₂O₇ film host.     

Luminescence properties and gamma-ray response of the Ce and Ca co-doped (Gd,Y)F3 single crystals

The Ca0.5% and Ce1%, 3%, 7%, 10% co-doped Gd_0.5Y_0.5F₃ single crystals were grown by the μ-PD method. In the Ca0.5% and Ce3% co-doped sample, Ce³⁺-perturbed luminescence at 380 nm was observed with 32.4 ns photoluminescence decay time. The energy transfer in the sequence of the regular Ce³⁺→ (Gd³⁺)_n→ the perturbed Ce³⁺ sites was evidenced through observation of decay time shortening of the regular Ce³⁺ and Gd³⁺ centers and the change between the Gd³⁺ and Ce³⁺-perturbed emission intensity. The gamma-ray excited scintillation response of the Ca0.5%, Ce7% co-doped Gd_0.5Y_0.5F₃ sample was investigated with the help of the pulse height spectra and the light yield, energy resolution and non-proportionality was evaluated in the interval of energies of 59.4-1274 keV.     

Scintillation properties of Tm-doped Lu3Al5O12 single crystals

Using the micro-pulling-down (μ-PD) method, Tm-doped Lu₃Al₅O₁₂ (Tm:LuAG) single crystals were grown to examine their scintillation properties. In transmittance spectra, they exhibited about 80% transparency in the wavelengths longer than 320 nm and five absorption lines due to Tm³⁺ 4f–4f transitions were observed. ²⁴¹Am α-ray excited radioluminescence spectra were measured and intense 4f–4f emission peaks were observed with the host emission. When excited by ¹³⁷Cs γ-Ray to obtain pulse height spectra, Tm 1% doped LuAG showed the highest light yield coupled with a photomultiplier (PMT) or a silicon avalanche photodiode (Si-APD). The light yield was estimated to be 5800 and 7300 photons/MeV for PMT and Si-APD, respectively. Decay time profiles consist of two exponential components and the fast and slow components are considered to be attributed to the host and the combination of the host and Tm³⁺ 4f–4f emission, respectively.     

Synthesis,structural and optical studies of sol–gel Gd2O3:Eu3+, Tb3+ films

Eu³⁺ and Tb³⁺ co-doped Gd₂O₃ films were elaborated by sol–gel process and dip-coating technique. The films were synthesized by hydrolysis of gadolinium pentanedionate. A homogeneous and stable sol was obtained by the reaction with acetylacetone. Gd₂O₃:Eu³⁺, Tb³⁺ films were crystallized around 500 °C; at an increase of temperature up to 700 °C, oriented growth of (4 0 0) face was observed. The obtained transparent Gd₂O₃: 2.5 at.% Eu³+, 0.005 at.% Tb³⁺ waveguide films at 700 °C display significant optical properties. Different crystallographic properties can be obtained in Gd₂O₃:Eu³⁺, Tb³⁺ films with varying sintering temperatures.     

Luminescence properties and preparation of Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> powder doped with Ce and Pr ions

Lu₃Al₅O₁₂:Ce³⁺ phosphor powder, which exhibits green emission band, was synthesized by the high-temperature solid-state reaction method with a flux BaF₂. X-ray diffraction (XRD), photoluminescence (PL) spectra, and fluorescent lifetime spectra were used to characterize the structure and luminescent properties of the sample. The XRD patterns indicated that when prepared at 1550 °C for 3 h with 4 wt% flux, Lu₃Al₅O₁₂:Ce³⁺ phosphors powder is the garnet cubic crystal system structure. Photoluminescence (PL) spectra showed that the Lu₃Al₅O₁₂:Ce³⁺ phosphor powder can be effectively excited by near ultraviolet and blue light, emitting broad band peaking at 505 nm, which is attributed to ²F_5/2?→?²D_5/2 transition. The self-concentration quenching mechanism of Ce³⁺ is the dipole–dipole interaction. Small amount of Pr³⁺ increased red light emission at 610 nm. Photoluminescence (PL) spectra and fluorescent lifetime spectra indicated that there was an efficient energy transfer process between Ce³⁺ and Pr³⁺.     

Crystal growth and scintillation properties of Ce and Eu doped LiSrAlF6

Ce and Eu doped LiSrAlF₆ (LiSAF) single crystals for the neutron detection with different dopant concentrations were grown by the micro-pulling-down method (μ-PD). In Ce:LiSAF, intense emission peaks due to Ce³⁺ 5d-4f transitions were observed at approximately 315 and 335 nm in photo- and α-ray induced radio-luminescence spectra. In case of Eu:LiSAFs, an intense emission peak at 375 nm due to Eu²⁺ 5d-4f transition was observed in the radio-luminescence spectra. The pulse height spectra and decay time profiles were measured under ²⁵²Cf neutron irradiation to examine the neutron response. The Ce 3% and Eu 2% doped LiSAF showed the highest light yield of 2860 ph/n with 19 ns main decay time component and 24,000 ph/n with 1610 ns.     

Effective lattice stabilization of gadolinium aluminate garnet (GdAG) via Lu3+ doping and development of highly efficient (Gd,Lu)AG:Eu3+ red phosphors

Abstract

The metastable garnet lattice of Gd₃Al₅O₁₂ is stabilized by doping with smaller Lu³⁺, which then allows an effective incorporation of larger Eu³⁺ activators. The [(Gd_1?xLu_x)_1?yEu_y]₃Al₅O₁₂ (x = 0.1–0.5, y = 0.01–0.09) garnet solid solutions, calcined from their precursors synthesized via carbonate coprecipitation, exhibit strong luminescence at 591 nm (the ⁵D₀ → ⁷F₁ magnetic dipole transition of Eu³⁺) upon UV excitation into the charge transfer band (CTB) at ～239 nm, with CIE chromaticity coordinates of x = 0.620 and y = 0.380 (orange-red). The quenching concentration of Eu³⁺ was estimated at ～5 at.% (y = 0.05), and the quenching was attributed to exchange interactions. Partial replacement of Gd³⁺ with Lu³⁺ up to 50 at.% (x = 0.5) while keeping Eu³⁺ at the optimal content of 5 at.% does not significantly alter the peak positions of the CTB and ⁵D₀ → ⁷F₁ emission bands but slightly weakens both bands owing to the higher electronegativity of Lu³⁺. The effects of processing temperature (1000–1500 °C) and Lu/Eu contents on the intensity, quantum efficiency, lifetime and asymmetry factor of luminescence were thoroughly investigated. The [(Gd_0.7Lu_0.3)_0.95Eu_0.05]₃Al₅O₁₂ phosphor processed at 1500 °C exhibits a high internal quantum efficiency of ～83.2% under 239 nm excitation, which, in combination with the high theoretical density, favors its use as a new type of photoluminescent and scintillation material.     

Synthesis and Photoluminescence Properties of Eu3+‐Doped Silica@Coordination Polymer Core–Shell Structures and Their Calcinated Silica@Gd2O3:Eu and Hollow Gd2O3:Eu Microsphere Products

The conjugation of Eu³⁺‐doped coordination polymers constructed from Gd³⁺ and isophthalic acid (H₂IPA) with silica particles is investigated for the production of luminescent microspheres. A series of doping ratio‐controlled silica@coordination polymer core–shell spheres is easily synthesized by altering the amounts of metal nodes used in the reactions, where the ratios of Gd³⁺ and Eu³⁺ are 10:0 ( 1a ), 9:1 ( 1b ), 8:2 ( 1c ), 7:3 ( 1d ), 5:5 ( 1e ), and 0:10 ( 1f ). The formation of monodisperse uniform core–shell structures is achieved throughout the entirety of a series. Investigations of the photoluminescence property of the resulting series of silica@coordination polymer core–shell spheres reveal that 20% Eu³⁺‐doped product ( 1c ) has the strongest emission intensity. The subsequent calcination process on the silica@coordination polymer core–shell structures ( 1a ‐ f ) results in the formation of a series of doping ratio‐controlled silica@Gd₂O₃:Eu core–shell microspheres ( 2a ‐ f ) with uniform shell thickness. During the calcination step, the coordination polymers within silica@coordination polymer core–shells are transformed into metal oxides, resulting in silica@Gd₂O₃:Eu core–shell structures. The final etching process on the silica@Gd₂O₃:Eu core–shell microspheres ( 2a ‐ f ) produces a series of hollow Gd₂O₃:Eu microspheres ( 3a ‐ f ) as a result of the elimination of silica cores. The luminescence intensities of silica@Gd₂O₃:Eu core–shell ( 2a ‐ f ) and hollow Gd₂O₃:Eu microspheres ( 3a ‐ f ) also vary depending upon the doping ratio of Eu³⁺ ions.