REVO4‐Based Nanomaterials (RE = Y,La, Gd,and Lu) as Hosts for Yb3+/Ho3+, Yb3+/Er3+, and Yb3+/Tm3+ Ions: Structural and Up‐Conversion Luminescence Studies

Rare‐earth vanadates of the form REVO₄ (RE = Y, La, Gd, and Lu) doped by Yb³⁺/Ho³⁺, Yb³⁺/Er³⁺_, or Yb³⁺/Tm³⁺ lanthanide ions were successfully synthesized using the sol–gel method and annealing at 600°C in an air atmosphere. The structure and morphology of the prepared nanocrystals were investigated by X‐ray diffraction, thermogravimetric analysis, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. All prepared materials were homogenous and had nanosized dimensions. Their elemental compositions were confirmed by optical emission spectrometry. Spectroscopic analysis of the materials was carried out by measuring excitation and emission spectra, luminescence decays, and dependence between the intensity of the luminescence and the laser energy. Following effective excitation by NIR radiation, Ln³⁺ co‐doped vanadate matrices exhibited a strong up‐conversion (UC) luminescence. Differences in spectroscopic properties between monoclinic LaVO₄ and tetragonal YVO₄, GdVO₄, or LuVO₄ doped by Ln³⁺ ions were observed, indicating the influence of the crystal structure on the UC emission. Drawing conclusions from these spectroscopic investigations, the UC mechanisms were proposed, including energy‐transfer processes between Yb³⁺ ions and emitting ions.     

Dy3+-doped Y3Al5O12 transparent ceramic for high efficiency ultraviolet excited single-phase white-emitting phosphor

A kind of Dy-doped yttrium aluminum garnet (YAG) transparent ceramic for ultraviolet excited single-phase white light-emitting phosphor was investigated, which has high-quantum efficiency (45%). The temperature field of Dy:YAG transparent ceramic was calculated by steady-state thermal simulation. Moreover, by combining with 365 nm light-emitting diodes (LED) chip directly, the Commission Internationale de l’Éclairage coordinate (x = 0.33, y = 0.35) is close to the standard equal energy white light illumination. The Dy:YAG transparent ceramic, possessing of good optical and thermal properties, is promising for applications in high-power LEDs devices.     

Deep red upconversion photoluminescence in Er3+-doped Yb3Ga5O12 nanocrystalline garnet

The nanocrystalline single-phase Er³⁺-doped Yb₃Ga₅O₁₂ garnets have been prepared by the sol-gel combustion technique with a crystallite size of ≈30 nm. The presence of Yb³⁺ in garnet hosts allows their efficient excitation at the ≈977 nm wavelength. The Er³⁺ doping of Yb₃Ga₅O₁₂ garnet host results in deep red Er³⁺: ⁴F_9/2 → ⁴I_15/2 upconversion photoluminescence (UCPL) emission. The dominance of the red UCPL emission over the green Er³⁺: ⁴F_7/2/²H_11/2/⁴S_3/2 → ⁴I_15/2 component was investigated using the measurement of the steady-state and time-dependent Er³⁺ and Yb³⁺ emission spectra in combination with the power-dependent UCPL emission intensity. The proposed upconversion mechanism is discussed in terms of the Er³⁺ → Yb³⁺ energy back transfer process as well as Yb³⁺(Er³⁺) → Er³⁺ energy transfer and Er³⁺ ↔ Er³⁺ cross-relaxation processes. The studied Er³⁺-doped Yb₃Ga₅O₁₂ garnet may be utilized as a red upconversion emitting phosphor.     

Optical and Judd-Ofelt spectroscopic study of Er3+-doped strontium gadolinium gallium garnet single-crystal

Er³⁺-doped strontium gadolinium gallium garnet (SrGdGa₃O₇) single-crystal was grown by Czochralski method. The Er³⁺ concentration in the crystal was determined as 4.2 × 10²¹ ions/cm³ by inductively coupled plasma atomic absorption spectroscopy. Refractive index of the crystal was measured at the wavelengths of 633, 1311 and 1553 nm by prism coupling technique. The results show that the crystal is a positive uniaxial crystal with a birefringence of ~0.01, and the Sellmeier equation reported previously for the crystal doped with Nd³⁺ is also valid for the one doped with Er³⁺. Unpolarized absorption spectrum of the crystal was measured at room temperature and the Er³⁺ absorption cross-section spectrum was calibrated from it. The Er³⁺ spectroscopic properties were studied by Judd-Ofelt theory. Some fundamental spectroscopic parameters were obtained that include absorption coefficient and cross-section spectral distributions, electronic transition oscillator strength, Judd-Ofelt parameters, fluorescence branch ratio, transition probability, radiative and fluorescence lifetimes, and quantum efficiency.     

Tb~(3+)在Lu_3Ga_5O_(12)中的光致发光性质及浓度猝灭机制

采用高温固相合成法在还原气氛中制备了(Lu1–xTbx)3Ga5O12荧光粉,研究了其晶体结构、光致发光性能以及Tb3+掺杂浓度对荧光粉性能的影响规律与机制。研究表明:荧光粉具有石榴石型晶体结构,当x增加时,晶格膨胀。荧光粉的激发光谱由归属于Tb3+的4f 8→4f 7 5d1跃迁的A、B两个宽激发带以及归属于4f 8→4f 8跃迁的一些窄谱峰构成;随x的增加,A、B带发生红移,带间距缩小。在紫外光激发下,荧光粉发射绿光;发射光谱对应于Tb3+的5D4→7FJ和5D3→7FJ跃迁,其中5D4→7F5跃迁发射最强。5D4→7FJ各跃迁发射的浓度猝灭以交互作用为主,猝灭浓度xm=0.08。与5D4→7FJ跃迁相比,5D3→7FJ猝灭浓度低,猝灭机制以电偶极–电偶极相互作用下交叉弛豫为主。     

Electro‐Optic Property of Mg2+/Er3+‐Codoped LiNbO3 Crystal: Mg2+ Concentration Threshold Effect

A series of Mg²⁺/Er³⁺‐codoped congruent LiNbO₃ crystals were grown by Czochralski method from the growth melts containing 0.5 mol% Er₂O₃ while varied MgO content from 0.0 to 7.0 mol%. The unclamped electro‐optic coefficients γ₁₃ and γ₃₃ of these crystals were measured by Mach–Zehnder interferometry. Two different voltage‐applying schemes were adopted: one is the DC voltage applied to the crystal via Al films coated onto crystal surfaces and another is via a pair of external Cu slab electrodes. The coefficients measured by the two schemes show similar strong dependence on Mg²⁺ concentration. The dependence is non‐monotonous, dramatic, and unusual, and reveals the features of two Mg²⁺ concentration thresholds of optical damage: one in the Mg²⁺ concentration range of 1.2–2.0 mol% (in crystal) and another in 4.5–5.0 mol%. Around the threshold the electro‐optic coefficient decreases abruptly at first and then recovers quickly, and the coefficient drops by >20% (12%) at the first (second) threshold, which exceeds the error 3% considerably. The dramatic behavior is qualitatively explained on the basis of the EO coefficient model of LiNbO₃ and the defect structure model for Mg²⁺‐doped LiNbO₃.     

Laser excitation-activated self-propagating sintering of NaYbF4:Pr3+/Gd3+ white light microcrystal phosphors

We demonstrate that self-propagating sintering reaction could be activated and dramatically enhanced by laser excitation of ion dopants in the solid-state reactants. Near-resonant laser absorption and subsequent nonradiative decays make the solid-state reactants be sintered efficiently while ionic excitations catalyze self-propagating solid-state reactions. As a prototype demo, we synthesized white light upconversion phosphors NaYbF₄:Pr³⁺/Gd³⁺. A continuous-wave laser at 980 nm was used to populate Yb³⁺ ions in YbF₃ to excited level, which react with NaF to preform NaYbF₄ nuclei. The preformed nuclei enhanced laser excitation and energy transfer to those ions that could not be directly excited by the pump laser and thus enabled self-propagating solid-state sintering synthesis of NaYbF₄ microcrystals at quite low laser powers. Laser excitation of Yb³⁺ ions could also benefit facile rare-earth ion doping through activated self-propagating reactions. Gd³⁺ and Pr³⁺ ions were doped in NaYbF₄ by simply adding Gd³⁺ and Pr³⁺ ionic oxides or fluorides in the raw materials. In addition, Gd³⁺ ions doping in F⁻ anions ambient could transform the NaYbF₄ microcrystal phase from cubic to hexagonal and tune upconversion photoluminescence. This synthetic method can be widely applied to synthesize many other solid-state compounds, perovskite solar cells, photocatalysts, solid oxide fuel cells, and so forth.     

铒镱共掺钆镓石榴石激光晶体生长及荧光光谱分析

采用提拉法生长了掺10%(摩尔分数,下同)Yb3+、掺Er3+分别为3%,5%和10%的Er3+:Yb3+:Gd3Ga5O12(Er:Yb:GGG)晶体。分析了Er:Yb:GGG晶体的结构和荧光光谱。结果表明:所生长的晶体属于立方晶系,Ia3d空间群。在980nm激光激发下,晶体样品在1000～1600nm范围内存在3个较强的发射带,相应的发射峰分别位于1030,1471nm和1534nm附近。由于Yb3+对Er3+的敏化作用,随着Er3+掺量的递增,1030nm处的发射峰强度逐渐减弱,1471nm和1534nm处的发射峰强度逐渐增强。计算了各个发射峰的受激发射截面积,铒和镱离子掺量为10%的晶体(10%Er:10%Yb:GGG)的受激发射截面高达2.0073×10–18cm2,可以产生较强的1534nm人眼安全波段的激光。     

A broadband-sensitive upconverter: Garnet-type Ca3Ga2Ge3O12 codoped with Er3+, Y3+, Li+, Ni2+, and Nb5+

We have developed a new broadband-sensitive photon upconversion (UC) material that can be used for transparent ceramic plates mounted on the rear faces of crystalline silicon solar cells. We selected the host material of a cubic crystal structure codoped with Er³⁺ and Ni²⁺ so that the Ni²⁺ dopants were fully activated to sensitize the Er³⁺ emitters. In garnet-type Ca₃Ga₂Ge₃O₁₂ with additional codopants of Nb⁵⁺ and Li⁺ for charge compensation, all the Ni²⁺ dopants occupied the six-coordinated Ga³⁺ sites, leading to highly efficient energy transfer from the Ni²⁺ to the Er³⁺. Formation of four-coordinated Ni²⁺ that quenches the UC emission of the Er³⁺ was prevented, because Ni²⁺ cannot substitute the four-coordinated Ge⁴⁺ much smaller than Ni²⁺. Consequently, energy dissipation from the Er³⁺ to the Ni²⁺ was well reduced compared with the previously developed Gd₃Ga₅O₁₂:Er,Ni,Nb in which the Ni²⁺ dopants partially occupied the four-coordinated Ga³⁺ sites. Additional introduction of Y³⁺ and Li⁺ enhanced optical transitions and improved the UC performance, owing to more enhanced lattice distortion, along with eliminating different phases. The optimal composition (Ca_0.6Er_0.1Y_0.1Li_0.2)₃(Ga_0.98Ni_0.01Nb_0.01)₂Ge₃O₁₂ exhibited a broadband sensitivity ranging from 1.1 μm (the absorption edge of silicon) to 1.6 μm for the UC emission at 0.98 μm.     

Modeling and Monte Carlo simulation on photothermal effect in Gd3Al3Ga2O12:Ce3+/Y3Al5O12:Cr3+ layered composite ceramic

Layered composite ceramics have wide application in solid-state lasers. However, the photothermal effect in the layered ceramics has not been clarified, due to the interface effect between layers. In this work, the model of photon propagation and thermal distribution in the Gd₃Al₃Ga₂O₁₂:Ce³⁺/Y₃Al₅O₁₂:Cr³⁺ layered ceramics are established. The property of photon absorption, reflection, transmission, and thermal distribution are studied by combining the Monte Carlo method and the convolution method. It is found that the photon absorption distribution and thermal distribution of this layered ceramics show the gradient change. Furthermore, this change is strongly dependent on the type, beam width, and power of laser. The temperature of layered ceramics induced by Gaussian beam is higher than that induced by flat circular beam. This work provides a useful research method for the design of layered ceramic materials with excellent scintillation performance.     

掺钕钆镓石榴石晶体的激光性能

采用提拉法生长了Nd：Gd3Ga3O12（Nd：GGG）晶体。切割后的样品经过端面抛光,测试了荧光光谱、吸收光谱和激光性能。荧光光谱表明：晶体的最强的荧光发射峰位于1062nm,是Nd^3＋的4^F3/2-4^I11/2谱项导致的荧光发射。由吸收光谱发现：Nd：GGG晶体的最强吸收峰位于808nm,表明该晶体适合于激光二极管泵浦．并且吸收峰强度随掺杂离子浓度的增加而增加。激光性能测试结果表明：激光二极管泵浦时光-光转换效率为33.＋8％,斜效率为57.8％。     

Effect of Ho-Dy co-doping on the electrical properties and reliability of BaTiO3-based nanoceramics for Base Metal Electrode Multilayer Ceramic Capacitor

Fine-grained BaTiO₃-based ceramics with an average grain size of 120−140 nm were prepared by a chemical coating method. The effect of Ho-Dy concentration on the microstructure, dielectric properties, and reliability of BaTiO₃-based nanoceramics was investigated. Results showed that appropriate doping contents of Ho and Dy provide the highest dielectric constant of 2323 at room temperature and the temperature stability satisfied EIA X7R specification. Higher doping concentration specimens exhibit lower dielectric constant but gentler temperature stability, indicating thicker grain shells. The failure time under the highly accelerated lifetime test increased along with the increase of Ho-Dy and was consistent with the impedance analysis results. Doping elements are mainly distributed in grain shells, leading to an increase of resistance and activation energy. The compositions for the highest dielectric constant and best reliability were not the same, providing support for the composition design of dielectric layer material for different BME-MLCC applications.     

Layered Y3Al5O12:Pr/Gd3(Ga,Al)5O12:Ce optical ceramics: Synthesis and photo-physical properties

We report a study of composite scintillating ceramics based on coupled layers of two different garnets, namely Ce-doped gadolinium gallium aluminium (GGAG:Ce) and Pr-doped yttrium aluminium (YAG:Pr), fabricated by hot isostatic pressing. Two samples were prepared, with different GGAG:Ce layer thickness, 120 µm and 690 µm respectively, but with a comparable overall thickness of 1.4 mm. The key finding is that the material architecture strongly determines the scintillation response. The radioluminescence is that expected from the irradiated material when a thick layer of ceramic is exposed to X-rays. Conversely, exposing a thin layer allows a non-null probability —about 0.3% for 120 µm of GGAG— of finding an X-ray photon in the underlying layer, and thus radioluminescence from both materials is recorded. We believe these results can extend the potential of layered optical ceramics for advanced devices, such as energy- and direction-sensitive X-ray detectors.     

Tunable dual-mode emission with excellent thermal stability in Ca4ZrGe3O12:Eu phosphors prepared in air for NUV-LEDs

Novel dual valence Eu-doped Ca₄ZrGe₃O₁₂ (CZGO) phosphors were successfully fabricated in air atmosphere through a solid-state route. Their crystal structure, photoluminescence properties as well as thermal quenching performance were investigated systematically. The spectra show that part of Eu³⁺ were reduced to Eu²⁺ and the mechanism is interpreted by the charge compensation. By altering Eu concentration, multi-color luminescence covering from blue to red is realized when irradiated by 370 nm light, which perfectly matches with the near ultraviolet (NUV) light-emitting diode (LED) chips. More importantly, under NUV excitation, luminescent intensities are almost unchanged even up to 423 K. And chromaticity exhibits only a tiny shift with growing temperature. Such suitable luminescent spectra and superior thermal stability indicate that CZGO:Eu phosphors are promising candidates for blue-red components in NUV pumped W-LEDs. Finally, the fabricated W-LED based on the combination of CZGO:Eu phosphors, Ba₂SiO₄:Eu²⁺ and a 365 nm NUV-LED chip gives a high color rendering index, a low correlated color temperature and suitable CIE chromaticity coordinates.     

Effect of Gd3+/Ga3+ on Yb3+ emission in mixed YAG at cryogenic temperature

We investigated the effect of Gd³⁺ and Ga³⁺ on Yb:YAG emission at cryogenic temperatures by preparing ceramic pellets by solid state reaction method with different Gd³⁺ and Ga³⁺ content. Incorporation of Gd³⁺ shows only a small effect on spectral broadening whereas incorporation of Ga³⁺ in Yb:YAG results in significant broadening. Such an inhomogeneous broadening is due to the replacement of larger Ga³⁺ ion in two different cationic sites of Al³⁺ which causes a change in local crystal field on the Yb³⁺ site.     

Tunable white light and energy transfer of Eu2+-Tb3+-Eu3+ tri-activated glasses synthesized in air

An ever increasing demand for white light-emitting diodes (W-LEDs) results in the gradual growth of research on functionalized luminescent glasses. In this paper, single-composition tunable white-emitting Eu²⁺-Tb³⁺-Eu³⁺ tri-activated glasses were synthesized by melt quenching method without additional reducing atmosphere. The coexistence of Eu²⁺ and Eu³⁺ was confirmed by ultraviolet-visible transmission spectra, photoluminescent spectra, fluorescence decay curves, and X-ray photoelectron spectroscopy. Tb³⁺ can act as bridge to connect Eu²⁺-Eu³⁺ luminescent centers by energy transfer. Tone-tunable white light can be achieved by coupling the emission centered at 412, 541, and 612 nm contributed from Eu²⁺, Tb³⁺, and Eu³⁺, respectively. By adjusting the relative content of Eu²⁺/Tb³⁺/Eu³⁺, ideal chromaticity coordinates of (0.33, 0.33) can be achieved under excitation of ultraviolet light. High thermal stability and tiny chromaticity shift were exhibited in samples. These results suggest that Eu²⁺-Tb³⁺-Eu³⁺ tri-activated glasses have great potential application in ultraviolet-driven W-LEDs.     

Synthesis and luminescence characterization of Pr3+, Gd3+ co-doped SrF2 transparent ceramics

Pr³⁺, Gd³⁺ co-doped SrF₂ transparent ceramic, as the potential material for visible luminescent applications, was prepared by hot-pressing of precursor nanopowders. The microstructure, phase compositions, and in-line transmittance, as well as the photoluminescence properties were investigated systematically. Highly optical quality Pr,Gd:SrF₂ transparent ceramic with nearly pore-free microstructure was obtained at 800°C for 1.5 hours. The average in-line transmittance of the x at.% Pr, 6 at.% Gd:SrF₂ (x = 0.2, 0.5, 1.0, 2.0) transparent ceramics reached to 87.3 % in the infrared region. The photoluminescence spectra presented intense visible light emissions under the excitation of 444 nm, the main intrinsic emission bands located at 483 and 605 nm, which were attributed to the transitions of Pr³⁺: ³P₀ → ³H₄ and ¹D₂ → ³H₄, respectively. With the co-doping of Gd³⁺ ions, the emission intensity of the Pr:SrF₂ transparent ceramic was greatly enhanced. All the emission bands of x at.% Pr, 6 at.% Gd:SrF₂ transparent ceramics exhibited the highest luminescence intensity with the 1.0 at.% Pr³⁺ doping concentrations, whereas the lifetimes decreased dramatically with the Pr³⁺ doping contents increasing from 0.2 to 2.0 at.% due to its intense concentration quenching effect. The 1 at.% Pr, 6 at.% Gd:SrF₂ transparent ceramic is a promising material for visible luminescent device applications.     

Tunable ultra-wideband NIR emission and irradiation resistance in novel Er3+/Tm3+/Pr3+ tri-doped La2O3- Ga2O3-Ta2O5 glasses synthesized using aerodynamic levitation method

Glasses with ultra-wideband near-infrared emission and superior irradiation resistance are important for the potential applications in optical communications under harsh environments. Here, transparent 35La₂O₃-(65-x)Ga₂O₃-xTa₂O₅ (LGT) and Er³⁺/Tm³⁺/Pr³⁺ tri-doped LGT glasses are fabricated using the levitation method. LGT glasses exhibit a wide glass-formation region, low largest vibration energy, high refractive indices, and excellent mechanical properties. Additionally, Er³⁺/Tm³⁺/Pr³⁺ tri-doped LGT samples with varying Pr³⁺ contents are characterized by possessing good thermal stability (T_g>849°C), wide transparent optical window, strong radiation resistance, excellent compatibility between low wavelength dispersion (v_d>31.2), and large refractive index (n_d>2.048). By optimizing the doping content of Er³⁺, Tm³⁺, and Pr³⁺ in an appropriate ratio, the ultra-wideband near-infrared luminescence ranging from 1250 to 1640 nm (FWHM = 251 nm) has been acquired under 808 nm pumping. Furthermore, decay curves are measured to reveal the fluorescence dynamics, and then the related emission mechanism is elaborated systematically. Meanwhile, the effects of gamma irradiation doses on microstructure, transmittance spectra, and fluorescence characteristics are studied. This work may offer a valuable reference for doping optimization and new design strategy of multifunctional materials.     

Synthesis and luminescent properties of Sr4−xSi3O8Cl4: xEu3+ red phosphor by hydrothermal method

Sr_4‐xSi₃O₈Cl₄:xEu³⁺ (SSOC:Eu³⁺) phosphors were successfully synthesized by hydrothermal method. The crystallization of this phosphor was analyzed by means of X‐ray diffraction patterns. The size and morphology were recorded using SEM patterns of samples. And the PLE and PL spectra were characterized by a PL spectrophotometer. Excited by 394 nm UV light, the intense red emission is recognized in SSOC:Eu³⁺ phosphor and the main emission peak located at 620 nm. The influences of Eu³⁺ concentration, pH value of reaction solution, and charge compensator on PL spectra of SSOC:Eu³⁺ phosphors were investigated. The results revealed that this red phosphor had potential applications for white LEDs.     

Yb3+:Gd3Ga5O12激光晶体原料合成与生长的研究

采用溶胶-凝胶法合成了Yb：GGG多晶料.以XRD分析方法对合成料的前驱体及烧成的粉体的相结构进行了研究,XRD分析结果表明,950 ℃煅烧的粉体已是Yb:GGG纯相.采用提拉法,通过设计合理而稳定的温场、选择最佳工艺参数等生长了Yb:GGG晶体.荧光光谱测试结果表明,Yb:GGG晶体主要发射峰位于1030 nm,该荧光是由Yb3+激发态2F5/2向基态2F7/2能级跃迁产生的.