Photoluminescence and optical properties of Eu3+/Eu2+-doped transparent Al2O3 ceramics

Transparent Eu³⁺-doped (0.05–0.15 at. %) alumina ceramics with fine-grained microstructure were prepared and studied in terms of optical properties and photoluminescence (PL). The light transmission through ceramics up to dopant concentrations 0.125 at. % is dominated by birefringence scattering at grain boundaries. As confirmed by HRTEM/EDS element mapping, high photoluminescence intensity was achieved as the result of the dopant segregation at grain boundaries. The PL emission spectra of Al₂O₃:Eu³⁺ ceramics exhibited red light emissions with the highest intensity (394 nm excitation) for material containing 0.125 at. % of Eu³⁺. The luminescence decay was single-exponential with a lifetime ～1.5 ms. The post-sintering reduction of Eu³⁺→Eu²⁺ under an H₂ atmosphere (at 1300 °C) was difficult. Two simultaneously coexisting Eu²⁺ emitting PL centers were identified, one emitting blue light with average decay constant of 150 ns, and the other green light (more intense) with average decay constant of 1.3 μs.     

Effects of Er3+ and/or Cr3+ doping on crystallization activation energy and fluorescence properties of transparent ZnGa2O4 glass-ceramics

Er³⁺ and/or Cr³⁺ doped transparent ZnGa₂O₄ glass-ceramics were successfully obtained by one-step heat treatment. The results showed that Er³⁺ ions can enrich around ZnGa₂O₄ crystal to reduce the crystallization activation energy and promote the growth of ZnGa₂O₄ crystal. Cr³⁺ ions may successfully occupy the Ga³⁺ sites in the ZnGa₂O₄ lattice but will increase crystallization activation energy and inhibit the growth of the ZnGa₂O₄ crystal. Before and after crystallization, the coordination-field intensity of Cr³⁺ ions increased from 2.17 to 2.86, resulting in the peak position of its emission spectra moving from 850 to 688 nm. By excitation at 378 nm, the precursor glass co-doped with Er³⁺ and Cr³⁺ ions only showed the characteristic emission peaks belonging to Er³⁺ ions. After heat treatment, the characteristic emission peaks belonging to Er³⁺ and Cr³⁺ ions existed simultaneously, and the emission color changed from green to yellow. By excitation at 980 nm, there were only characteristic emission peaks belonging to Er³⁺ ions of the Er³⁺/Cr³⁺ co-doped glasses before and after heat treatment. The results showed that the Er³⁺ and/or Cr³⁺ doped ZnGa₂O₄ glass-ceramics have adjustable luminescence ability and show potential application value in the field of luminescence display.     

Microstructure and optical properties of Pr3+-doped hafnium silicate films

In this study, we report on the evolution of the microstructure and photoluminescence properties of Pr³⁺-doped hafnium silicate thin films as a function of annealing temperature (T_A). The composition and microstructure of the films were characterized by means of Rutherford backscattering spectrometry, spectroscopic ellipsometry, Fourier transform infrared absorption, and X-ray diffraction, while the emission properties have been studied by means of photoluminescence (PL) and PL excitation (PLE) spectroscopies. It was observed that a post-annealing treatment favors the phase separation in hafnium silicate matrix being more evident at 950°C. The HfO₂ phase demonstrates a pronounced crystallization in tetragonal phase upon 950°C annealing. Pr³⁺ emission appeared at T_A = 950°C, and the highest efficiency of Pr³⁺ ion emission was detected upon a thermal treatment at 1,000°C. Analysis of the PLE spectra reveals an efficient energy transfer from matrix defects towards Pr³⁺ ions. It is considered that oxygen vacancies act as effective Pr³⁺ sensitizer. Finally, a PL study of undoped HfO₂ and HfSiO_x matrices is performed to evidence the energy transfer.     

Synthesis and photoluminescence properties of the high-brightness Eu3+-doped Sr3Y(PO4)3 red phosphors

A series of red-emitting phosphors Eu³⁺-doped Sr₃Y(PO₄)₃ have been successfully synthesized by conventional solid-state reaction, and its photoluminescence properties have been investigated. The excitation spectra reveal strong excitation bands at 392 nm, which match well with the popular emissions from near-UV light-emitting diode chips. The emission spectra of Sr₃Y(PO₄)₃:Eu³⁺ phosphors exhibit peaks associated with the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺ and have dominating emission peak at 612 nm under 392 nm excitation. The integral intensity of the emission spectra of Sr₃Y_0.94(PO₄)₃:0.06Eu³⁺ phosphors excited at 392 nm is about 3.4 times higher than that of Y₂O₃:Eu³⁺ commercial red phosphor. The Commission Internationale de l’Eclairage chromaticity coordinates, the quantum efficiencies and decay times of the phosphors excited under 392 nm are also investigated. The experimental results indicate that the Eu³⁺-doped Sr₃Y(PO₄)₃ phosphors are promising red-emitting phosphors pumped by near-UV light.     

Synthesis,structure, luminescence and photocatalytic properties of CaTiO(SiO4):Eu3+

Titanite CaTiO(SiO₄):Eu³⁺ were synthesized by a solvothermal-calcination method and an improved high-temperature solid-state synthesis method. Their structure, luminescence properties and photocatalytic propoties were comparatively studied in detail. It was found that CaTiO(SiO₄):Eu³⁺ synthesized by the two methods belong to two different types of crystalline and each of them has its own advantages and disadvantages. Titanite synthesized by solvothermal-calcination method has rough and porous surface morphology, larger BET, and smaller crystallite size, which result in a weaker luminescence intensity than the one synthesized by high-temperature solid-state method. But in the experiment of photocatalysis, all these disadvantages are turned into advantages. In addition, the high temperature process also increases the symmetry of Eu³⁺ occupation and leads to many surface traps, which result in a less red color and a shorter fluorescence life than solvothermal synthesized titanite. We hope that this comparative study will extend the applications and research ideas of titanite.     

Optical,thermal properties,and hydrolysis resistance of transparent YF3-doped CaO ceramics fabricated by vacuum sintering

Transparent calcium oxide (CaO) ceramic was successfully fabricated by pressureless vacuum sintering technique with 0.5 at% YF₃ as the sintering additive. In consideration of the hydrolysis of CaO, the precursor powders were calcined at 600 °C. Within the sintering temperature range of 1150–1450 °C, the effect of YF₃ additive on the phase structure, relative density, transmittance, and microstructure of CaO ceramics was studied. Benefiting from the assisted liquid-phase sintering mechanism and vacancy diffusion mechanism of YF₃, full-dense (3.35 g/cm³) YF₃-doped CaO transparent ceramic was fabricated at 1350 °C with high thermal conductivity of 15.42 W/(m·K). The in-line transmittance attained 47.10% at the wavelength of 1200 nm. Moreover, the hydrolysis resistance was extremely enhanced due to the pore-free structure. Compared to the CaO ceramic with low relative density of 78.66%, the weight gain of full-dense CaO ceramic greatly decreased from 22% to 1.2% within 35 days.     

A novel Nd3+-doped MgO-Al2O3-SiO2-based transparent glass-ceramics: Toward excellent fluorescence properties

Generally, glass-ceramics have superior properties compared to their parent glasses. Here, we prepared a novel Nd³⁺-doped MgO-Al₂O₃-SiO₂-based transparent glass-ceramics with excellent fluorescence properties. The effects of Nd₂O₃ content on the structure and properties of glass-ceramics were studied, aiming to provide a key guidance for preparing this transparent glass-ceramics. The results revealed that the glass stability increased originally and then decreased with increasing Nd₂O₃ content, so did the variation of wavenumbers in infrared spectra. And these glass-ceramics are mainly composed of cordierite with residual glassy phase. The three phenomenological intensity parameters (Ω_2,4,6) and radiative properties were estimated by Judd-Ofelt theory, and the values of Ω₂ first decreased and then increased with increasing Nd₂O₃ content. Three main emission peaks ascribed to the transitions from ⁴F_3/2 to ⁴I_9/2, ⁴I_11/2, ⁴I_13/2 at 898, 1057, 1330  nm were observed, respectively. The branching ratios for ⁴F_3/2→⁴I_11/2 transition increased as the Nd₂O₃ content raised, and the fluorescence lifetimes of the ⁴F_3/2 level were found to increase first and then decrease with Nd₂O₃ content (from 181 to 726 μs). The excellent fluorescence properties indicate that this novel glass-ceramics can be used as a potential solid-state optical functional material for 1.06 μm laser emission.     

Improving white light emission and quantum yield of Ce@Eu co-doped silicate glass by reducing Eu3+ to Eu2+ with Si3N4

Rare-earth-doped transparent glass shows great potential in white light-emitting diodes (wLEDs) application due to its excellent optical and luminous properties. Currently reported commercial wLEDs have a drawback in red emission missing, which leads to a relatively low color rendering index (CRI) and a relatively high correlated color temperature (CCT). In this work, Ce@Eu Sr–Si–O glass is fabricated using a high-temperature quenching method. The white light is available when the ratio of Ce³⁺/Eu³⁺ equals 1, and the emitting color can be adjusted from blue to red by controlling the ratio of Ce³⁺/Eu³⁺. To further optimize the white light, Eu³⁺ ions can be reduced to Eu²⁺ according to the reaction of 6Eu³⁺ + 2N³⁻ → 6Eu²⁺ + N₂↑ by introducing Si₃N₄. As a result, the standard white light emission can be achieved in the Ce@Eu silicate glass contributed by the blue light from Ce³⁺, red light from Eu³⁺, and yellow–green light from Eu²⁺ (two elements, three emission). This glass shows excellent luminous properties, such as a color coordinate is (0.3651, 0.3269) in CIE 1931 color coordinate diagram, a CRI is over 70, a high quantum yield of 36.02%, and a CCT of 4117 K.     

Synthesis and photoluminescence properties of CaGd2(MoO4)4:Eu3+ red phosphors

The novel red-emitting Eu³⁺ ions activated CaGd₂(MoO₄)₄ phosphors were prepared by a citrate sol–gel method. The X-ray diffraction patterns confirmed their tetragonal structure when the samples were annealed above 600 °C. The photoluminescence excitation spectra of CaGd₂(MoO₄)₄:Eu³⁺ phosphors exhibited the charge transfer band (CTB) and intense f–f transitions of Eu³⁺ ion. The optimized annealing temperature and Eu³⁺ ion concentration were analyzed for CaGd₂(MoO₄)₄:Eu³⁺ phosphors based on the dominant red (⁵D₀→⁷F₂) emission intensity under NUV (394 nm) excitation. All decay curves were well fitted by the single exponential function. These luminescent powders are expected to find potential applications such as WLEDs and optical display systems.     

Fabrication and optical properties of Tb3+-doped NaCaPO4 glass-ceramics and their radiation detection applications

Tb³⁺-doped 25Na₂O-23CaO-6P₂O₅-44B₂O₃-2ZrO₂ glass was fabricated by conventional melt-quenching technique. Glass-ceramics containing NaCaPO₄ crystals were then obtained by heating the as-prepared glasses. Their optical and luminescence properties were studied by FT-IR spectroscopy, photoluminescence (PL), absorption spectra, thermoluminescence (TL), and optically stimulated luminescence in continuous wave modality (CW-OSL). The glasses were composed of [PO₄], [BO₃], and [BO₄] basic structural units. The PL excitation and emission spectra exhibited Tb³⁺-related transitions, as well as the strongest excitation and emission wavelengths at 370 and 454 nm, respectively. We further investigated the CW-OSL properties as a function of dopant concentration and time elapsed after irradiation (signal fading). Results indicated that the CW-OSL intensity reached a maximum when the Tb₄O₇ concentration was 0.25 mol%. The fading of the OSL signal showed that the OSL signal of Tb³⁺-doped NaCaPO₄ glass-ceramics was approximately 65% in 8 days, after which the intensity remained stable. The TL glow curves had a broad peak feature peaking at 180 ± 5ºC. The samples also exhibited good signal reusability and a broad linear dose-response range (0.03-1000 Gy). The excellent luminescent and dosimetric properties of these Tb³⁺-doped NaCaPO₄ glass-ceramics indicated their potential applications in radiation dosimetry.     

Fabrication,microstructure, mechanical and luminescence properties of transparent Yb3+-doped Sr5(PO4)3F nanostructured ceramics

The Sr₅(PO₄)₃F (S-FAP) crystal material is regarded as one of the most ideal optical materials for diode pumping owing to its huge absorption and emission cross sections and long fluorescence lifespan. In this investigation, S-FAP powders with varying Yb concentrations (0.1–5%) were produced using the coprecipitation method. Then a variety of S-FAP transparent ceramics with varying Yb content were fabricated using hot-pressing sintering. The crystalline phase structure of hexagonal Sr₅(PO₄)₃F was verified by XRD analysis of the precursor powder and the final ceramics. According to the powder SEM, the average grain size and the long axial-radial ratio of powders are decreasing as the Yb³⁺ concentration increases. Thermal-etched surface SEM reveals nanostructured S-FAP transparent ceramics with an average grain size of less than 200 nm were synthesized. The highest transmittances of the 3% ceramics at 500 and 1100 nm wavelengths are 51% and 79.78%, respectively. The ceramic cross-sectional SEM demonstrated that porosity is the primary scattering source influencing the enhancement of optical characteristics. The absorption, emission, and fluorescence lifetimes of S-FAP transparent ceramics with varying Yb concentrations were tested and discussed, and the absorption and emission cross sections corresponding to the major peak were reported. Some physical parameters of this set of ceramic samples were shown, including thermal diffusivity, specific heat capacity, and thermal diffusivity data, as well as micro-hardness.     

白光LED用红色发光材料CaLa_2(MoO_4)_4:Eu~(3+)的微波辅助溶胶–凝胶法合成及发光特性

以柠檬酸为络合剂,采用微波辅助溶胶–凝胶法制备了CaLa2(MoO4)4:Eu3+红色荧光粉。研究了前驱体的热分解历程,分析表征了样品的结构、形貌和发光性能。探讨了焙烧温度、Eu3+掺杂量、柠檬酸与乙二醇摩尔比和硼酸用量等对样品发光性能的影响。结果表明:前驱体经700~900℃焙烧均能得到目标产物CaLa2–x(MoO4)4:x Eu3+,样品具有白钨矿结构,属于四方晶系。样品的激发光谱在250~350 nm处有一宽吸收带,对应于Mo–O,Eu–O电荷迁移带;在395和464 nm处存在很强的吸收峰,归属于Eu3+的4f–4f跃迁。发射光谱主峰位于616 nm处,归属于Eu3+的5D0→7F2电偶极跃迁发射。前驱体经800℃焙烧所得样品发光强度最大,且发光强度随着Eu3+掺杂量的增加而增大,在x=0.2~1.0范围内未出现猝灭现象。体系中加入适量乙二醇,可以起到细化晶粒、提高粉体分散性的作用,但浓度过高则会降低样品的发光强度;助熔剂硼酸的用量对样品发光强度影响较大,当用量为3%时,样品的发光性能较好。     

A new family of Cu-doped lanthanum silicate apatites as electrolyte materials for SOFCs: Synthesis,structural and electrical properties

La_9.67Si_6-xCu_xO_26.5-x (LSC, x = 0, 0.1, 0.3 and 0.5) are synthesized by a citric-nitrate method. Substitution Si with Cu promotes the densification process of silicate apatite. Unit cell parameters and volume increase linearly with Cu content. The Rietveld refinement reveals a much more distorted (Si,Cu)O₄ tetrahedra in the oxygen stoichiometric La_9.67Si_5.5Cu_0.5O₂₆ sample. The structural observation from high temperature XRD implies a second-order phase transition in La_9.67Si_5.5Cu_0.5O₂₆. Cu-doping decreases the activation energy of oxygen ion conduction and increases the conductivity of LSC materials in the temperature range of 550–800 °C. La_9.67Si_5.5Cu_0.5O₂₆ shows the conductivity values of 29.3 and 12.3 mS cm⁻¹ at 800 °C and 650 °C, respectively. The oxygen ion transference number of La_9.67Si_5.5Cu_0.5O₂₆ is higher than 0.99. These attractive properties make the La_9.67Si_5.5Cu_0.5O₂₆ a promising oxygen ion conducting electrolyte for applications of solid oxide fuel cells, oxygen sensors, oxygen separation membranes, etc.     

Fluorescence intensity ratio (FIR) analysis of the temperature sensing properties in transparent ferroelectric PMN-PT:Pr3+ ceramic

A transparent ferroelectric 0.75Pb(Mg_1/3Nb_2/3)O₃-0.25PbTiO₃:0.015Pr³⁺ ceramic was synthesized and its temperature-sensing ability was investigated based on the fluorescence intensity ratio (FIR) method. The transparency was found to be of the order of 68% at 900 nm for a sample thickness of 0.7 mm, comparable to the theoretical value of ~71%, benefiting the photoluminescence of the Pr³⁺ ions inside the ceramic. Instead of the traditional Boltzmann exponential style and varying sensitivity, a highly linear temperature response was obtained for the studied ceramic. Further, a constant FIR sensitivity of 0.70 %K^-1 was achieved over the temperature range of -50–40 °C, making the ceramic suitable for thermometry at room temperature and below.     

Fabrication and optical properties of transparent fine-grained Zn1.1Ga1.8Ge0.1O4 and Ni2+ (or Cr3+)-doped Zn1.1Ga1.8Ge0.1O4 spinel ceramics

For the first time, a Zn_1.1Ga_1.8Ge_0.1O₄ transparent spinel ceramic has been fully densified by spark plasma sintering. XRD measurements show that this ceramic is composed of a pure cubic spinel phase. SEM analysis revealed a homogeneous and dense microstructure with the average grain size being 200 ± 100 nm. The transmittance of these fine-grained ceramics reached 70 % in the visible range and is very close to 80 % at 2 µm, thus close to the T_max value deduced from the measurement of the refractive index. The ceramics exhibit excellent mechanical properties with a Young modulus of 222 GPa, a Vickers hardness of 14.25 GPa and a thermal conductivity of 7.3 W.m⁻¹. K⁻¹. By doping with Cr³⁺ ions, transparent Zn_1.1Ga_1.8Ge_0.1O₄ ceramics present both a red luminescence and a long-lasting afterglow during several minutes. Moreover, a near infrared broadband emission at 1.3 µm is also achieved with Ni²⁺ ions.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Effects of ZrO2 on the photoluminescence of 0.5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics

Zr-codoped 0.5 at% Eu: (Y_0.9La_0.1)₂O₃ ceramics sintered in H₂-reducing atomsphere, together with the ceramics with annealing treatment, were fabricated by solid-state reactions and the effects of Zr codoping on these materials’ photoluminescence examined. The obtained emission spectra showed that Zr codoping adjust the materials’ photoluminescence with UV excitation and a logical explanation was proposed. The results suggested that an Eu-doped, yttrium-lanthanum oxide transparent ceramic with Zr in low concentration appeared to have promising potential in modern lighting applications.     

Fabrication and optical thermometry of transparent glass-ceramics containing Ag@NaGdF4: Er3+ core-shell nanocrystals

Novel transparent glass-ceramics containing Ag@NaGdF₄:Er³⁺ core-shell nanocrystals were fabricated successfully by a melt-quenching method and subsequent heating. X-ray diffraction and transmission electron microscope images show that precious metal Ag is successfully encapsulated by the NaGdF₄:Er³⁺ nanoparticles to form an Ag@NaGdF₄:Er³⁺ core-shell structure in glass matrix. Compared with the NaGdF₄:Er³⁺ glass-ceramics, Ag@NaGdF₄:Er³⁺ core-shell glass-ceramics shows the great enhancement of emission intensity. The thermometric parameters such as fluorescence emission intensity, fluorescence intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2), and temperature sensitivity can be effectively controlled by changing the Ag concentration. When 0.15 mol% Ag is co-doped, the sensitivity of S_R in Ag@NaGdF₄:Er³⁺ core-shell glass-ceramics reaches a maximum value. This work presents a new method to enhance emission intensity and optical thermometry ability of NaGdF₄:Er³⁺ through constructing Ag@NaGdF₄:Er³⁺ core-shell structure.     

Enhanced up-conversion luminescence and optical thermometry characteristics of Er3+/Yb3+ co-doped Sr10(PO4)6O transparent glass-ceramics

In this paper, the Yb³⁺/Er³⁺ co-doped parent glass (PG) with composition (in mol%) of 30P₂O₅-10B₂O₃-38SrO-22K₂O and transparent glass-ceramics (GCs) containing hexagonal Sr₁₀(PO₄)₆O nanocrystals (NCs) were synthesized for the first time by melt-quenching method and subsequent heating treatment in air. Under 980 nm laser prompting, the GCs samples showed intense red and green up-conversion emissions compared to those characteristics for the PG sample. The emission intensities varied with Er³⁺ concentration and heat treatment conditions. Furthermore, in Yb³⁺/Er³⁺ co-doped GCs specimens, the optical thermometry was researched by means of fluorescence intensity ratio (FIR) of ⁴S_3/2 and ²H_11/2 levels. The GC sample heated at 620°C for 5 hours possessed a high relative temperature sensitivity (S_r) of 0.769% K⁻¹ at 303 K and the maximal absolute temperature sensitivity (S_a) of 5.951 × 10⁻³ K⁻¹ at 663 K, respectively. It is expected that the as-fabricated GC materials with Sr₁₀(PO₄)₆O NCs are promising efficient up-conversion materials for optical temperature sensor.     

Ca3(VO4)2:Eu3+的燃烧合成及光致发光

以稀土氧化铕、偏钒酸铵、硝酸钙为原料,以尿素作燃烧剂,采用燃烧法合成了Ca3(VO4)2:Eu3+.利用XRD,SEM,荧光分光光度计对其结构、形貌和发光性能进行了表征,并探讨其实际应用的可能性.结果表明,燃烧法合成的稀土Eu3+掺杂ca3(VO4)2荧光粉颗粒规则、均一,发射主峰位于614.0 nm,是现有PDP商品(Y,Gd)BO3:Eu3+红色荧光粉的良好替代品.