Search Results (7)

Gd³⁺ and Sm³⁺ co-activation, the effect of cation substitutions and the creation of cation vacancies in the scheelite-type framework are investigated as factors influencing luminescence properties. Ag_xGd_{((2−x)/3)−0.3−y}Sm_yEu³⁺_0.3☐_(1−2x)/3WO₄ (x = 0.50, 0.286, 0.20; y = 0.01, 0.02, 0.03, 0.3) scheelite-type phases (A_xGS_yE) have been synthesized by a solid-state method. A powder X-ray diffraction study of A_xGS_yE (x = 0.286, 0.2; y = 0.01, 0.02, 0.03) shows that the crystal structures have an incommensurately modulated character similar to other cation-deficient scheelite-related phases. Luminescence properties have been evaluated under near-ultraviolet (n–UV) light. The photoluminescence excitation spectra of A_xGS_yE demonstrate the strongest absorption at 395 nm, which matches well with commercially available UV-emitting GaN-based LED chips. Gd³⁺ and Sm³⁺ co-activation leads to a notable decreasing intensity of the charge transfer band in comparison with Gd³⁺ single-doped phases. The main absorption is the ⁷F₀ → ⁵L₆ transition of Eu³⁺ at 395 nm and the ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺ at 405 nm. The photoluminescence emission spectra of all the samples indicate intense red emission due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺. The intensity of the ⁵D₀ → ⁷F₂ emission increases from ~2 times (x = 0.2, y = 0.01 and x = 0.286, y = 0.02) to ~4 times (x = 0.5, y = 0.01) in the Gd³⁺ and Sm³⁺ co-doped samples. The integral emission intensity of Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄ in the red visible spectral range (the ⁵D₀ → ⁷F₂ transition) is higher by ~20% than that of the commercially used red phosphor of Gd₂O₂S:Eu³⁺. A thermal quenching study of the luminescence of the Eu³⁺ emission reveals the influence of the structure of compounds and the Sm³⁺ concentration on the temperature dependence and behavior of the synthesized crystals. Ag_0.286Gd_0.252Sm_0.02Eu_0.30WO₄ and Ag_0.20Gd_0.29Sm_0.01Eu_0.30WO₄, with the incommensurately modulated (3 + 1)D monoclinic structure, are very attractive as near-UV converting phosphors applied as red-emitting phosphors for LEDs. Full article

Tungstate and molybdate phosphors have received great attention for their excellent photoluminescent properties and thermal stabilities. In the article, Tb³⁺-activated tungstate and molybdate green phosphors were prepared by a solid-state reaction method at different caline temperatures and were compared and studied. The crystal structures and the morphologies of samples were characterized by X-ray diffraction (XRD) patterns and field emission scanning electron microscopy (FE-SEM) images. The energy-dispersive spectra (EDS) proved the compositions of the prepared samples. The photoluminescence (PL) spectra showed that the PL excitation spectra of Tb³⁺-doped Lu₂W₃O₁₂ and Lu₂Mo₃O₁₂ green phosphors consisted of a broad and strong charge transfer band (CTB) and 4f–5d transitions of Tb³⁺ in the ultraviolet (UV) wavelength range and some narrowed excitation peaks from the 4f–4f transition of Tb³⁺ in the near ultraviolet (NUV) wavelength region. The PL emission spectra of the phosphors exhibited the characteristic green emissions owing to the ⁵D₄→⁷F₅ transition of Tb³⁺ located at about 547 nm. The values of energy gap E_g were calculated based on the diffuse reflection spectra (DRS). The measuring temperature-dependent PL spectra illustrated the thermal stabilities of phosphors. The Tb³⁺-doped Lu₂Mo₃O₁₂ phosphor presented normal thermal quenching phenomena and the values of the thermal activation energy E_a were calculated based on the measuring temperature dependent PL emission spectra. The Tb³⁺-doped Lu₂W₃O₁₂ phosphor exhibited abnormal thermal enhancing CTB excitation intensity at about 170 °C. Furthermore, the PL decay curves suggested that the lifetime corresponding to the ⁵D₄ level of Tb³⁺ in the Lu₂W₃O₁₂ host lattice was longer than that in the Lu₂Mo₃O₁₂ host lattice. Compared the Tb³⁺-doped Lu₂Mo₃O₁₂ phosphor, the Tb³⁺-doped Lu₂W₃O₁₂ phosphor has shown potential as an application in temperature sensors. Full article

Red LEDs with a high color purity and high color rendering index are often used to compensate for the lack of red-light components in current white LEDs. Therefore, the new type of garnet-structured high color purity red phosphor Y_2−xSrAl₄SiO₁₂: xEu³⁺ was synthesized by the solid-state method. The band gap structure of the host matrix was studied through the DFT calculation and found that the matrix belongs to a direct band gap structure with a band gap size of 4.535 ev. The phosphor exhibits a wide excitation spectrum under the monitoring of 710 nm. The strongest excitation wavelength is 393 nm, and it exhibits bright red light under the excitation of 393 nm, and the emission peak positions are located at 570 nm, 597 nm, 613 nm, 650 nm, 710 nm and 748 nm, respectively, which are attributed to the ⁵D₀→⁷F_j of Eu³⁺ (j = 0–5) electronic transitions. In the crystal structure of Y₂SrAl₄SiO₁₂, Eu³⁺ occupies a symmetry site. The compositional changes and thermal studies found favorable at 20% mol. At this concentration, the luminescence intensity gradually weakened due to the Eu³⁺ electric multi-level interaction. It is worth noting that the emission intensity of Y₂SrAl₄SiO₁₂: 20%Eu³⁺ at 433 K can be maintained to 92% of that at 293 K. Finally, we combined it with the NUV chip and packaged it into a red LED with a color purity of up to 90% and a correlated color temperature of 1492 K. The high purity, low color temperature and thermal stability indicate that it has a place in LED applications. Full article

In this work, the series of Tb³⁺/Eu³⁺ co-doped xerogels and derivative glass-ceramics containing CaF₂ nanocrystals were prepared and characterized. The in situ formation of fluoride crystals was verified by an X-ray diffraction technique (XRD) and transmission electron microscopy (TEM). The studies of the Tb³⁺/Eu³⁺ energy transfer (ET) process were performed based on excitation and emission spectra along with luminescence decay analysis. According to emission spectra recorded under near-ultraviolet (NUV) excitation (351 nm, ⁷F₆ → ⁵L₉ transition of Tb³⁺), the mutual coexistence of the ⁵D₄ → ⁷F_J (J = 6–3) (Tb³⁺) and the ⁵D₀ → ⁷F_J (J = 0–4) (Eu³⁺) luminescence bands was clearly observed. The co-doping also resulted in gradual shortening of a lifetime from the ⁵D₄ state of Tb³⁺ ions, and the ET efficiencies were varied from η_ET = 11.9% (Tb³⁺:Eu³⁺ = 1:0.5) to η_ET = 22.9% (Tb³⁺:Eu³⁺ = 1:2) for xerogels, and from η_ET = 25.7% (Tb³⁺:Eu³⁺ = 1:0.5) up to η_ET = 67.4% (Tb³⁺:Eu³⁺ = 1:2) for glass-ceramics. Performed decay analysis from the ⁵D₀ (Eu³⁺) and the ⁵D₄ (Tb³⁺) state revealed a correlation with the change in Tb³⁺–Eu³⁺ and Eu³⁺–Eu³⁺ interionic distances resulting from both the variable Tb³⁺:Eu³⁺ molar ratio and their partial segregation in CaF₂ nanophase. Full article

In the present work, the Tb³⁺/Eu³⁺ co-activated sol-gel glass-ceramic materials (GCs) containing MF₃ (M = Y, La) nanocrystals were fabricated during controlled heat-treatment of silicate xerogels at 350 °C. The studies of Tb³⁺ → Eu³⁺ energy transfer process (ET) were performed by excitation and emission spectra along with luminescence decay analysis. The co-activated xerogels and GCs exhibit multicolor emission originated from 4fⁿ–4fⁿ optical transitions of Tb³⁺ (⁵D₄ → ⁷F_J, J = 6–3) as well as Eu³⁺ ions (⁵D₀ → ⁷F_J, J = 0–4). Based on recorded decay curves, it was found that there is a significant prolongation in luminescence lifetimes of the ⁵D₄ (Tb³⁺) and the ⁵D₀ (Eu³⁺) levels after the controlled heat-treatment of xerogels. Moreover, for both types of prepared GCs, an increase in ET efficiency was also observed (from η_ET ≈ 16% for xerogels up to η_ET = 37.3% for SiO₂-YF₃ GCs and η_ET = 60.8% for SiO₂-LaF₃ GCs). The changes in photoluminescence behavior of rare-earth (RE³⁺) dopants clearly evidenced their partial segregation inside low-phonon energy fluoride environment. The obtained results suggest that prepared SiO₂-MF₃:Tb³⁺, Eu³⁺ GC materials could be considered for use as optical elements in RGB-lighting optoelectronic devices operating under near-ultraviolet (NUV) excitation. Full article

This paper investigates the reliability of blue-emitting phosphors for Near-UV (NUV) laser excitation. By means of a series of thermal stress experiments, and of stress under high levels of optical excitation, we have been able to identify the physical process responsible for the degradation of Eu²⁺-activated alkaline-earth halophosphate phosphors under typical and extreme operating conditions. In particular, for temperatures equal to or greater than 450 °C the material exhibited a time-dependent drop in the Photo-Luminescence (PL), which was attributed to the thermally induced ionization of the Eu²⁺ optically active centers. Several analytical techniques, including spatially and spectrally resolved PL, Electron Paramagnetic Resonance (EPR) and X-ray Photo-emission Spectroscopy (XPS) were used to support this hypothesis and to gain insight on the degradation process. By means of further tests, evidence of this degradation process was also found on samples stressed under a relatively low power density of 3 W/mm² at 405 nm. This indicated that the optically (and thermally) induced ionization of the optically active species is the most critical degradation process for this family of phosphorescent material. The operating limits of a second-generation Eu-doped halophosphate phosphor were also investigated by means of short-term stress under optical excitation. The experimental data showed that a threshold excitation intensity for continuous pumping exists. Above this threshold, decay of the steady-state PL performance and non-recoverable degradation of the material were found to take place. This behavior is a consequence of the extremely harsh excitation regime, mainly due to the thermal management capabilities of the substrate material employed for our experimental purposes rather than from intrinsic properties of the phosphors. Full article

White light-emitting diodes (WLEDs) have matched the emission efficiency of florescent lights and will rapidly spread as light source for homes and offices in the next 5 to 10 years. WLEDs provide a light element having a semiconductor light emitting layer (blue or near-ultraviolet (nUV) LEDs) and photoluminescence phosphors. These solid-state LED lamps, rather than organic light emitting diode (OLED) or polymer light-emitting diode (PLED), have a number of advantages over conventional incandescent bulbs and halogen lamps, such as high efficiency to convert electrical energy into light, reliability and long operating lifetime. To meet with the further requirement of high color rendering index, warm light with low color temperature, high thermal stability and higher energy efficiency for WLEDs, new phosphors that can absorb excitation energy from blue or nUV LEDs and generate visible emissions efficiently are desired. The criteria of choosing the best phosphors, for blue (450-480 nm) and nUV (380-400 nm) LEDs, strongly depends on the absorption and emission of the phosphors. Moreover, the balance of light between the emission from blue-nUV LEDs and the emissions from phosphors (such as yellow from Y₃Al₅O₁₂:Ce³⁺) is important to obtain white light with proper color rendering index and color temperature. Here, we will review the status of phosphors for LEDs and prospect the future development. Full article