Photoluminescence in MgxSr1−xAl2O4:Eu, Ndand electron-vibrational interaction in the Eu 5d states

Green phosphor compositions Mg_xSr_1−xAl₂O₄:Eu, Nd (with x=0.05-0.25) were prepared by solid state reaction method. The effect of Mg substitution on photoluminescence characteristics was investigated. The photoluminescence show intense green emission for MgSrAl₂O₄:Eu²⁺, Nd³⁺ with long persistence. This green emission corresponds to transitions from 4f⁶5d¹ to 4f⁷ of Eu²⁺ ion. Comparative analysis of the excitation and emission spectra were used to evaluate the crystal field splitting of the 5d states of Eu²⁺ and the parameters of electron-vibrational interaction, such as Huang-Rhys factor, effective phonon energy, and zero-phonon line position.     

Interference effects on the photoluminescence spectrum of GaN/InxGa1−xN single quantum well structures

Interference effects in the spectra of the III-nitride epilayers can produce ambiguities and misinterpretations. This problem has been addressed in the literature (e.g. [L. Siozade, J. Leymarie, P. Disseix, A. Vasson, M. Mihailovic, N. Grandjean, M. Leroux, J. Massies, Solid State Commun. 115 (2000) 575]) and a few approaches are given to produce an interference function with which the interference effects can be removed. However, the calculated interference functions may provide over or underestimated corrections because of scattering in the values of the optical parameters of the materials used in the calculation. This paper first describes a model that is developed to derive an interference function. Then, it presents the fit parameters required in the calculation of the interference function that are found experimentally. The experiments compare the corrected spectrum with an interference free spectrum, which can be obtained if the geometry of the light collection is adjusted to the Brewster angle. In this way, one avoids large estimations errors, since the validation data are derived experimentally from the same material, and under the same excitation conditions. The model has been applied to correct the low-temperature, low excitation density photoluminescence measurements of a set of five GaN/InGaN single quantum well samples with different indium concentrations. The effectiveness of this proposed technique is illustrated by the so found results.     

Luminescent properties of stabled hexagonal phase Sr1−xBaxAl2O4:Eu (x=0.37-0.70)

Stabled hexagonal phase Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) was prepared by solid-state method. Result revealed that the structure behavior of the SrAl₂O₄:Eu²⁺ calcined at 1350 °C in a reducing atmosphere for 5 h strongly depended on the Ba²⁺ concentration. With increasing Ba²⁺ concentration, a characteristic hexagonal phase can be observed. When 37-70% of the strontium is replaced by barium, the structure of the prepared sample is pure hexagonal. Photoluminescence and excitation spectra of the samples with different x and doped with 2% Eu²⁺ were investigated. Changes in the emission spectra were observed in the two different phases. The green emission at 505 nm from Eu²⁺ was found to be quite strong in the hexagonal phase. The intensity and peak position of the green luminescence from Eu²⁺ changed with increasing content of Ba²⁺. The strongest green emission was obtained from Sr_0.61Ba_0.37Al₂O₄:Eu²⁺. The decay characteristics of Sr_1−xBa_xAl₂O₄:Eu²⁺ (x=0.37-0.70) showed that the life times also varied with the value of x. Furthermore, the emission colors and decay times varying with x could be ascribed to the variation of crystal lattice.     

Potential PDP phosphors with strong absorption around 172 nm: Rare earth doped NaLaP2O7 and NaGdP2O7

NaLaP₂O₇ and NaGdP₂O₇ powder samples are prepared by solid-state reactions at 750 and 600 °C, respectively, and the VUV-excited luminescence properties of Ln³⁺ (Ln=Ce, Pr, Tb, Tm, Eu) in both diphosphates are studied. Ln³⁺ ions in both hosts show analogous luminescence. For Ce³⁺-doped samples, the five Ce³⁺ 5d levels can be clearly identified. As for Pr³⁺ and Tb³⁺-doped samples, strong 4f-5d absorption band around 172 nm is observed, which matches well with Xe-He excimer in plasma display panel (PDP) devices. As a result, Pr³⁺ can be utilized as sensitizer to absorb 172 nm VUV photon and transfer energy to appropriate activators, and Tb³⁺-doped NaREP₂O₇(RE=La, Gd) are potential 172 nm excited green PDP phosphors. For Tm³⁺ and Eu³⁺-doped samples, the Tm³⁺-O²⁻ charge transfer band (CTB) is observed to be at 177 nm, but the CTB of Eu³⁺ is observed at abnormally low energy position, which might originate from multi-position of Eu³⁺ ions. The similarity in luminescence properties of Ln³⁺ in both hosts indicates certain structural resemblance of coordination environment of Ln³⁺ in the two sodium rare earth diphosphates.     

Crystallinity and morphology dependent luminescence of Li-doped Y2−xGdxO3:Eu thin film phosphors

The influence of lithium doping on the crystallization, the surface morphology, and the luminescent properties of pulsed laser deposited Y_2−xGd_xO₃:Eu³⁺ thin film phosphors was investigated. The crystallinity, the surface morphology, and the photoluminescence (PL) of films depended highly on the Li-doping and the Gd content. The relationship between the crystalline and morphological structures and the luminescent properties was studied, and Li⁺ doping was found to effectively enhance not only the crystallinity but also the luminescent brightness of Y_2−xGd_xO₃:Eu³⁺ thin films. In particular, the incorporation of Li and Gd into the Y₂O₃ lattice could induce remarkable increase in the PL. The highest emission intensity was observed Li-doped Y_1.35Gd_0.6O₃:Eu³⁺ thin films whose brightness was increased by a factor of 4.6 in comparison with that of Li-doped Y₂O₃:Eu³⁺ thin films.     

New red phosphors BaZr(BO3)2 and SrAl2B2O7 doped with Eu for PDP applications

Vacuum ultraviolet (VUV) excitation and photoluminescence (PL) characteristics of Eu³⁺ ion doped borate phosphors; BaZr(BO₃)₂:Eu³⁺ and SrAl₂B₂O₇:Eu³⁺ are studied. The excitation spectra show strong absorption in the VUV region with the absorption band edge at ca. 200 nm for BaZr(BO₃)₂:Eu³⁺ and 183 nm for SrAl₂B₂O₇:Eu³⁺, respectively, which ensures the efficient absorption of the Xe plasma emission lines. In BaZr(BO₃)₂:Eu³⁺, the charge transfer band of Eu³⁺ does not appear strongly in the excitation spectrum, which can be enhanced by co-doping Al³⁺ ion into the BaZr(BO₃)₂ lattices. The luminescence intensity of BaZr(BO₃)₂:Eu³⁺ is also increased by Al³⁺ incorporation into the lattices. The PL spectra show the strongest emission at 615 nm corresponding to the electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺ in both BaZr(BO₃)₂ and SrAl₂B₂O₇, similar to that in YAl₃(BO₃)₄, which results in a good color purity for display applications.     

Photoluminescence at room temperature in polycrystalline BaxSr1−xTiO3 ceramics obtained by solid-state reaction method

Stoichiometric Ba_xSr_1−xTiO₃ ceramics (x=0.7, 0.8, and 0.9) have been prepared by solid-state reaction method. It has been observed in our experiments that the photoluminescence (PL) spectra in such samples at room temperature were centered around 800 nm; the results have been found to differ from observations in current literature. An explanation of this phenomenon was given, and the origin of such photoluminescence is ascribed to certain defects existing in these compounds, such as oxygen vacancies.     

Novel ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layers

ZnO/ZnGa₂O₄ composite layers were synthesized by simple thermal oxidation of ZnS substrates with gallium in the air. The continuous-wave and time-resolved photoluminescence measurements for the composites were performed at room temperature. It is found that the visible deep level emission from ZnO in ZnO/ZnGa₂O₄ composite layer was almost suppressed. In addition, the UV emission with long lifetime was also observed in comparison with that of pure ZnO layer without ZnGa₂O₄.     

Decay pathway and high-temperature luminescence of Eu in Ca2Gd8Si6O26

The temperature-dependent luminescence of Eu:Ca₂Gd₈Si₆O₂₆ and its decay pathways are investigated in order to assess the utility of the material as a thermometric phosphor. Non-radiative decays are found to compete with radiative processes even at room temperature. A decay pathway involving decay through charge-transfer states is proposed based on the decay profiles of emissions from ⁵D₁ and ⁵D₀ levels and on the temperature sensitivity of the spectra as observed after excitation by several wavelengths. The implications of this on solid-state lighting are also discussed.     

Luminescence properties of red-emitting praseodymium-activated BaTi4O9 phosphor

The photoluminescence and low-voltage cathodoluminescence characteristics of BaTi₄O₉:Pr³⁺ were investigated. The excitation band of intervalence charge transfer (IVCT) of BaTi₄O₉:Pr³⁺ emerged distinctly at 330 nm. The resultant emissions appeared at 606-643 nm corresponding to the ¹D₂→³H₄ transition. In BaTi₄O₉:Pr³⁺, the emission of ³P₀→³H₄ transition at 490 nm was not observed. The results were in a pure red color emission.     

Photoluminescence studies of Cd1−xZnxS nanocrystals

Cd_1−xZn_xS (0?x?0.5) nanocrystals have been synthesized using a simple chemical precipitation method. Morphological and crystallographic analyses have been done using transmission electron microscope (TEM) and X-ray diffraction (XRD). Room temperature energy and time resolved photoluminescence spectra of these synthesized nanophosphors have been studied using xenon lamp spectroflourometer and high peak power, pulsed N₂-laser excitation, respectively. Photoluminescence spectra are composed of broad peaks ranging from green to red region of the visible spectrum. Important optical parameters: excited state lifetime, trap-depth and decay constant values have been calculated from recorded luminescence decay curves. These nanophosphors show typical lifetime shortening and high quantum yield with increasing concentration of Zn.     

Dielectric and optical behaviors in relaxor ferroelectric Pb(In1/2Nb1/2)1−xTixO3 crystal

Dielectric permittivities (ε′,ε″) have been measured as functions of temperature (140-535 K) and frequency (500 Hz-2.0 MHz) in a (001)-cut Pb(In_1/2Nb_1/2)_0.7Ti_0.3O₃ (PINT30%) single crystal grown by the modified Bridgman method with Pb(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (PMNT29%) seed crystal. A diffused phase transition was observed in the temperature region of ∼430-460 K with strong frequency dispersion. Above the Burns temperature T_B≅510 K, the dielectric permittivity was found to follow the Curie-Weiss behavior, ε′=C/(T−T_C), with parameters C=3.9×10⁵ and T_C=472 K. Below T_B≅510 K, polar nanoclusters are considered to appear and are responsible for the diffused dielectric anomaly. Optical transmission, refractive indices, and the Cauchy equations were obtained as a function of wavelength at room temperature. The unpoled crystal shows almost no birefringence, indicating that the average structural symmetry is optically isotropic. The crystal exhibits a broad transparency in the wavelength range of ∼0.4-6.0 μm.     

Origin of PL intensity increase of CaMgSi2O6:Eu phosphor after baking process for PDPs application

We have synthesized blue-emitting CaMgSi₂O₆:Eu²⁺ (CMS) and evaluated its thermal stability after baking process. To evaluate its thermal stability, CMS was baked in air at 500 and 600 °C for 20 min, respectively, and compared with BaMgAl₁₀O₁₇:Eu²⁺ (BAM) treated in the same condition. After baking process, CMS showed somewhat increased photoluminescence (PL) intensity with baking temperature. To investigate the reasons behind the increase of PL intensity after baking process, vacuum ultraviolet (VUV)/PL, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) techniques were applied. From the ESR and the XPS analyses, it is noted that spectral intensity of Eu²⁺ ion somewhat increased. It was believed that due to charge balance Eu³⁺ ions reduced to Eu²⁺ ions during the baking process in air. It is clear that the concentration of Eu²⁺ increased after the baking process in air and it leads to slight increase of the VUV/PL intensity of CMS phosphor.     

Synthesis and photoluminescence of CeO2:Eu phosphor powders

The crystalline structure and photoluminescence (PL) properties of europium-doped cerium dioxide synthesized by the solid-state reaction method were analyzed. CeO₂:Eu³⁺ phosphor powders exhibit the pure cubic fluorite phase up to 10 mol% doping concentration of Eu³⁺. With indirect excitation of CeO₂ host at 373 nm, the PL intensity quickly increases with increasing Eu³⁺ concentration, up to about 1 mol%, and then decreases indicating the concentration quenching. While with direct excitation (467 nm), much more stronger PL emissions, especially the electric dipole emission ⁵D₀-⁷F₂ at 612 nm, are observed and no concentration quenching occurs up to 10 mol% doping concentration of Eu³⁺. The nature of this behavior and the cause of the concentration quenching were discussed.     

VUV spectroscopic properties of Ce and Pr-doped AREP2O7-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu)

Spectroscopic properties of Ce³⁺ and Pr³⁺-doped AREP₂O₇-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu) have been investigated using VUV spectroscopy technique. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 325-450 nm. The strong host absorption band starting at around 160 nm indicates that the optical band gap of AREP₂O₇ hosts is at least 7.7 eV, and the host→Ce³⁺ energy transfer process is rather efficient. However, AREP₂O₇:Pr³⁺ samples show less efficient host→Pr³⁺ energy transfer. The direct Pr³⁺ 4f²→4f¹5d¹ excitation, which are 12160±640 cm⁻¹ higher respect to that of Ce³⁺, leads to strong 4f¹5d¹→4f² emission bands in the range of 230-325 nm but no obvious 4f²→4f² emission lines.     

Highly enhanced photoluminescence and X-ray excited luminescence of Li doped Gd2O3:Eu thin films

Transparent Li-doped Gd₂O₃:Eu³⁺ thin-film phosphors were prepared by a modified sol-gel method. The effect of the Li⁺ ions on luminescent properties of the thin film was investigated. The results indicated that incorporation of Li⁺ ions into Gd₂O₃ lattice could result in a remarkable increase on photoluminescence or X-ray excited luminescence, and the strongest emission was observed from Gd_1.84Li_0.08Eu_0.08O_3−δ film, in which the intensity was increased by a factor of 1.9 or 2.3 in comparison with that of Gd_1.92Eu_0.08O₃ film. And it could be achieved the highest intensity for sintering the Gd_1.84Li_0.08Eu_0.08O_3−δ film at 700 °C. Such a temperature is much lower than the typical solid-state reaction temperature for its powder phosphors. This kind of transparent thin-film phosphors may promise for application to micro X-ray imaging system.     

Synthesis and optical properties of KZnLa0.99Nd0.01(VO4)2 triple vanadate(V)—New promising laser materials

In an attempt to find a neodymium-vanadate system with long lifetime of ⁴F_3/2 level and relatively strong ⁴F_3/2→⁴I_11/2 emission for laser applications, the optical properties of Nd³⁺ in a new KZnLa(VO₄)₂ host is reported. The crystalline samples were obtained at 900 °C in air. The samples were crystallized in monoclinic system and were isostructural with KZnLa(PO₄)₂. KZnLa_0.99Nd_0.01(VO₄)₂ strongly emits in the near infrared range with the maxima at 871.6 and 1057 nm upon excitation through the ⁴F_5/2 level (808 nm) or by the charge transfer bands of VO₄³⁻. The lifetime of ⁴F_3/2 level of Nd³⁺ ion is larger than that observed in other neodymium-vanadates systems.     

Influence of co-doping different rare earth ions on CaGa2S4: Eu, RE(RE=Ln) phosphors

Photoluminescent phosphors CaGa₂S₄: Eu²⁺, RE³⁺ (RE³⁺ including all rare earth ions except for Sc³⁺, Pm³⁺, Eu³⁺ and Lu³⁺) were prepared by sintering at high temperature in a reductive atmosphere, and their luminescent properties were studied intensively. The influences of co-doping rare earth ions on their luminescent properties were also investigated. No remarkable differences were found from excitation spectra of co-doped phosphors CaGa₂S₄: Eu²⁺, RE³⁺ in contrast with that of phosphor CaGa₂S₄: Eu²⁺, but there were a few differences in emission spectra of Ce³⁺, Pr³⁺ or Ho³⁺ co-doped phosphors. Phosphors CaGa₂S₄: Eu²⁺, RE³⁺ (RE=Ce, Pr, Gd, Tb, Ho and Y) had persistent afterglow, and very short afterglow was shown for Nd³⁺ or Er³⁺ co-doped phosphors, but no long afterglow appeared when auxiliary activator was La³⁺, Sm³⁺, Dy³⁺, Tm³⁺ or Yb³⁺. Among the phosphors with long-lasting phosphorescence, in our experiments, CaGa₂S₄: Eu²⁺, Ho³⁺ had the longest and the highest brightness long yellow afterglow. Thermo-luminescence of all co-doped phosphors was measured to find the answer of different influences from different rare earth auxiliary activators.     

Synthesis, microstructure and photoluminescence of Eu/Tb activated Y2SiO5 nanophosphors by new silicate sources

Y_2−xTb_xSiO₅ and Y_2−xEu_xSiO₅ nanophosphors with seven different kinds of silicate sources were synthesized by sol-gel method. The structures have been investigated to be composed of nanometer-size grains of 30-60 nm through X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used to compare the different morphology of patterns from seven different silicon sources. The photoluminescence of Y_2−xTb_xSiO₅ was investigated as a function of silicate sources and the results revealed that these nanometer materials showed the characteristic emission ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) of Tb ions. The characteristic emission ⁵D₀ → ⁷F_J (J = 1, 2, 4) of Eu ions was also found in the materials of Y_2−xEu_xSiO₅.