Energy storage in Ce-doped LiCaAlF6 and LiSrAlF6 crystals

Absorption, emission and thermoluminescence (TL) of as-grown and X-irradiated pure and Ce-doped LiCaAlF₆ (LiCAF) and LiSrAlF₆ (LiSAF) crystals have been analyzed. It is shown that the energy storage is defined by the dissimilarity of basic matrix properties, intrinsic defect structure, preferred Ce³⁺ ion disposition and charge compensation defect type. This is the reason of higher colorability and TL efficiency of as-grown Ce:LiSAF compared to Ce:LiCAF. Pre-filling of ultra-deep traps leads to enormous increases in the TL response. Due to these properties Ce:LiSAF and Ce:LiCAF are promising thermoluminescent dosimetric materials.     

Luminescence properties of novel NaBa4(BO3)3:Ce, Eu phosphors

Novel Eu³⁺, Ce³⁺ activated NaBa₄(BO₃)₃ phosphors were synthesized by solid-state reactions. The excitation spectrum of NaBa₄(BO₃)₃:Ce³⁺ consists of an intense band peaking at 350 nm and a weak band in the higher energy side, and the emission spectrum exhibits a blue band with a maximum at about 420 nm. The Eu³⁺ emission in NaBa₄(BO₃)₃ consists of the transitions from ⁵D₀ to ⁷F_J, and the excitation spectrum consists of broad excitation band peaking at 270 nm and some intense narrow lines. The optimum doped concentration, the critical distance of the concentration quenching, and the fluorescence lifetime have also been investigated.     

Optical absorption, fluorescence and decay properties of Pr-doped PbO-H3BO3-TiO2-AlF3 glasses

Spectroscopic and laser properties of PbO-H₃BO₃-TiO₂-AlF₃ glasses doped with 0.1, 0.5, 1.0 and 2.0 mol% of Pr₆O₁₁ have been studied. Optical absorption spectra were recorded in the UV-vis-NIR regions and the observed absorption bands were assigned to different electronic transitions from ³H₄ ground state of 4f² configuration. The three phenomenological Judd-Ofelt (J-O) parameters Ω_t (t=2, 4, 6) were determined from the measured oscillator strengths by including as well as excluding the ³H₄→³P₂ hypersensitive transition in J-O analysis. The emission characteristics such as stimulated emission cross-sections (σ_e), measured branching ratios (β_m), measured lifetimes (τ_m), quantum efficiencies (η) and gain parameters (σ_e×τ_m) have been evaluated for the principal intermanifold transitions of Pr³⁺ from the ³P₀ and ¹D₂ states to the lower lying manifolds in the visible region. From the emission and decay measurements, the effect of Pr³⁺ ion concentration on the quenching of the ¹D₂ measured lifetimes has been discussed.     

Luminescence and energy transfer in Eu, Mn co-doped Li4SrCa(SiO4)2 for white light-emitting-diodes

Li₄(Sr_0.96Eu_0.04)(Ca_1 − xMn_x)(SiO₄)₂ phosphors were synthesized by solid-state reactions and photoluminescence (PL) properties were investigated. These phosphors have intense absorption in n-UV region, which is suitable for excitation of UV LEDs. The orange-reddish emission of Mn²⁺ can be adjusted by changing the Mn²⁺/Eu²⁺ ratio. Energy transfer from Eu²⁺ to Mn²⁺ is observed. Li₄(Sr_0.96Eu_0.04)(Ca_1 − xMn_x)(SiO₄)₂ phosphors could be used in white LEDs.     

Excited-state dynamics of Yb in LiCaAlF6 single crystal

Single crystals of Yb²⁺-doped LiCaAlF₆ were grown by the Czochralski technique under CF₄ atmosphere. Photoluminescence, thermally stimulated luminescence and decay kinetics of Yb²⁺ centre in the LiCaAlF₆ host were measured in the 4–300 K temperature interval. Phenomenological two excited-state-level model is introduced to obtain quantitative characteristics of the excited state dynamics of Yb²⁺. The role of Yb²⁺ centre in trapping processes is discussed. Moreover, defect centres related to LiCaAlF₆ host were found.     

Fluorescence emission spectrum and energy transfer in Eu and Mn co-doped Ba2Ca(BO3)2 phosphors

Eu²⁺ and Mn²⁺ co-doped Ba₂Ca(BO₃)₂ phosphors yield two emission bands consisting of green and red components under the excitation of 360 nm, which shows a great potential for white LEDs. Effective energy transfer occurs in Eu²⁺/Mn²⁺ co-doped Ba₂Ca(BO₃)₂ host due to the large spectral overlap between the emission of Eu²⁺ and the excitation of Mn²⁺. The energy transfer from Eu²⁺ to Mn²⁺ is thoroughly investigated by their excitation, emission and photoluminescence decay behaviors, and is demonstrated to be via the dipole–quadrupole interaction.     

Scintillation properties of PbWO4 crystals doped with the rare-earth ions

In PbWO₄(PWO) crystals grown by Czochralski method the influence of atmosphere of the growth (O₂, air) and doping with the rare-earth ions of different types (A³⁺=Lu³⁺, Gd³⁺,Tb³⁺,Eu³⁺ as well as doubly doped A³⁺–Li⁺) on light yield and luminescence decay were analyzed. PWO scintillator with the ultra-fast (τ=0.5 ns) main component of luminescence decay (87% of total light yield) was obtained using the O₂-growth atmosphere and doping by Eu₂O₃ at a concentration of 5000 ppm. It is concluded that the decrease of decay constant of the main scintillation component is the result of the resonant energy transfer between the centers of “blue” PWO luminescence (λ_max=420 nm) and the 4f–4f-transitions of Eu³⁺ ions in this spectral region.     

Photoluminescence studies on Eu and Ce-doped Li2SrSiO4

The luminescence of Li₂SrSiO₄: 0.01Eu, xCe (x=0.0025, 0.005, 0.0075, and 0.01) is studied as a potential ultraviolet light-emitting diode (UV-LED) phosphor that is capable of converting the ultraviolet emission of a UV-LED into white light with good luminosity. There are broad blue and yellow emissions peaked at 413 and 575 nm, respectively. The two emissions come from d-f transitions of Ce³⁺ and Eu²⁺, respectively. The emission intensity of Li₂SrSiO₄: 0.01Eu, xCe reaches its maximum at x=0.0075. The energy transfer from Ce³⁺ to Eu²⁺ is demonstrated to be the type of electric dipole-dipole interaction with considerable spectral overlap and nonradiative transition is calculated to dominate. The Commission International de I’Eclairage (CIE) chromaticity coordinates of Ce³⁺/Eu²⁺ substituted compounds is also discussed.     

Luminescence study of pure and Fe- or Mo-doped ZnWO4 crystals

ZnWO₄, ZnWO₄:Fe and ZnWO₄:Mo crystals were investigated by the methods of time-resolved spectroscopy in the temperature range of 4.2–300 K. It is shown that the Mo and Fe impurities significantly reduce the light yield of ZnWO₄. The main 2.5 eV emission of ZnWO₄ and the 1.77 eV emission band of ZnWO₄:Mo are shown to originate from the triplet excited state of the WO₆ and MoO₆ complex, respectively. In ZnWO₄:Fe,Mo the MoO₆ emission band is shifted to lower energies due to the perturbing influence of the iron impurity. No perturbing effect of Fe or Mo ions was observed for the main emission of ZnWO₄:Fe and ZnWO₄:Mo. The creation spectrum of self-trapped holes was measured for ZnWO₄, ZnWO₄:Fe and ZnWO₄:Mo crystals in the energy region of 4–30 eV.     

Luminescence and energy transfer of Eu/Mn co-doped SiO2–Al2O3–ZnO–K2O glass ceramics for white LEDs

Transparent Eu²⁺/Mn²⁺ co-doped new glass ceramics (GC) containing β-Zn₂SiO₄ nanocrystals were prepared under a reduced atmosphere. The optical properties of these samples have been investigated. The emission spectra of Eu²⁺/Mn²⁺ co-doped glass ceramics show two broadband peakings at 458 and 560 nm under ultraviolet radiation, which can be attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺ and ⁴T₁(⁴G)→⁶A₁(⁶S) transition of Mn²⁺, respectively. Energy transfer (ET) from Eu²⁺ to Mn²⁺ is discovered by directly observing significant overlap of the excitation spectrum of Mn²⁺ and the emission spectrum of Eu²⁺. ET from Eu²⁺ to Mn²⁺ in glass ceramics is further confirmed by fluorescence studies performed on the samples with various activator (Mn²⁺) concentrations. The optimal composition generates white light with chromaticity coordinates (0.291, 0.344). The results indicate that Eu²⁺/Mn²⁺ co-doped glass ceramics is potential material for white light-emitting diodes (LEDs).     

Luminescence studies on SrMgAl10O17:Eu, Dy phosphor crystals

Using urea as fuel, SrMgAl₁₀O₁₇:Eu, Dy phosphor was prepared by a combustion method. Its luminescence properties under ultraviolet (UV) excitation were investigated. Pure SrMgAl₁₀O₁₇ phase was formed by urea-nitrate solution combustion synthesis at 550 °C. The results indicated that the emission spectra of SrMgAl₁₀O₁₇:Eu, Dy has one main peak at 460 nm and one shoulder peak near 516 nm, which are ascribed to two different types of luminescent Eu²⁺ centers existing in the SrMgAl₁₀O₁₇ matrix crystal. The blue luminescence emission of SrMgAl₁₀O₁₇:Eu phosphors was improved under UV excitation by codoping Dy³⁺ ions. The SrMgAl₁₀O₁₇:Eu phosphors showed green afterglow (λ=516 nm) when Dy³⁺ ions were doped. Dy³⁺ ions not only successfully play the role of sensitizer for energy transfer in the system, but also act as trap levels and capture the free holes in the spinel blocks.     

Luminescence of BaAl2O4:Mn,Ce phosphor

Powder samples of barium aluminate doped with Mn²⁺ and Ce³⁺ were prepared by solid-state reaction method and their photoluminescence and thermoluminescence properties were studied. Substitution of Ca/Sr in place of Ba resulted in enhanced emission from Ce³⁺ ions without changing the spectral profile. Cerium efficiently sensitized the manganese luminescence in barium aluminate. Photoluminescence and thermo luminescence observations have indicated the presence of V_k³⁺ defects in undoped barium aluminate. However, Barium aluminate (either undoped or doped with manganese) did not exhibit long afterglow.     

Ir(ppy)3掺杂PVK体系发光特性的研究

对常温下磷光染料Ir(ppy)3掺杂PVK薄膜的光致发光(PL)和电致发光(EL)特性进行了研究。器件结构为ITO/PEDOT:PSS/PVK:Ir(ppy)3/BCP/Alq3/Al。实验发现随磷光材料掺杂浓度的不同,器件的发光性能发生变化。当浓度适宜时,主体材料PVK的发光很弱,主要为Ir(ppy)3的磷光发射。通过L-I-V特性曲线的比较,掺杂浓度为5%的光电性能最好,说明器件在掺杂浓度为5%时效果最佳。     

Luminescence properties of LiPrxCe1−xP4O12

LiPr_1−xCe_xP₄O₁₂ (x=0, 0.002, 0.02; 0.1) powder samples were prepared using the melt solution technique. Luminescent parameters of LiPr_1−xCe_xP₄O₁₂ phosphors have been investigated under ultraviolet-vacuum ultraviolet (3-12 eV) synchrotron radiation and X-rays excitation at room and near liquid He temperatures. Excitation luminescence spectra of Ce³⁺ emission, luminescent spectra and decay curves from the lower excited state levels of the 4f¹5d¹ and 5d¹ electronic configuration of the Pr³⁺ and Ce³⁺, respectively, clearly indicate energy transfer from Pr³⁺ to Ce³⁺. Energy migration proceeds via the Pr-sublattice followed by nonradiation transfer from Pr³⁺ to Ce³⁺ ions.     

Luminescence of Mn ions in Tb3Al5O12 garnet

The paper is dedicated to investigation of the Mn²⁺ luminescence in Tb₃Al₅O₁₂ (TbAG) garnet, as well as the processes of excitation energy transfer between host cations (Tb³⁺ ions) and activators (Mn²⁺ and Mn²⁺-Ce³⁺ pair ions) in single crystalline films of TbAG:Mn and TbAG:Mn,Ce garnets which can be considered as promising luminescent materials for conversion of LED's radiation. Due to the effective energy transfer between TbAG host and activator, Mn²⁺ ions in TbAG possess the bright orange luminescence in the bands peaked at 595 nm with a lifetime of 0.64 ms which are caused by the ⁴T₁→⁶A₁ radiative transitions. The simultaneous process of energy transfer is realized in TbAG:Mn,Ce: (i) from Tb³⁺ to Mn²⁺ ions; (ii) from Tb³⁺ cations to Ce³⁺ ions and then partly to Mn²⁺ ions through Tb³⁺ ion sublattice and Ce-Mn dipole-dipole interaction.     

Luminescence properties of Ce3+ doped gadolinium-calcium-silicaborate glass scintillator

In this work, the Ce³⁺ doped gadolinium-calcium-silicaborate glass scintillators of the composition ratio 25Gd₂O₃:10CaO:10SiO₂:(55−x)B₂O₃:xCeF₃, have been fabricated by using the melt-quenching technique. The doping concentration of the Ce³⁺ was varied from 0.05 mol% to 2.5 mol%. The 4f-5d transition of the Ce³⁺ allowed scintillation with a fast decay time. The absorption spectrum, X-ray induced emission spectrum, photo luminescence spectrum, laser luminescence spectrum and decay time of the scintillators were measured for studying the luminescence properties. From the X-ray induced emission spectrum result, we checked the trend between doping concentration and light yield. The laser induced luminescence spectrum was measured while changing the temperature from 300 K to 10 K. We also measured the decay time by using the laser excitation of the 0.15 mol% Ce³⁺ doped glass scintillator.     

Effects of the surface coating of BaMgAl10O17:Eu phosphor with SiO2 nano-particles

BaMgAl₁₀O₁₇:Eu²⁺(BAM) phosphors were coated with SiO₂ nano-particles by a modified sol-gel method. In order to elucidate the effects of SiO₂ coating onto BAM phosphor, the luminescent decay time is measured before and after SiO₂ coating. It was revealed that surface coating of BAM phosphors with SiO₂ leads to an increase in luminescent decay time and quantum efficiency, due to the suppression of nonradiative decay via surface defects. The experimental results suggest that the nonradiative decay via surface defects is an important relaxation process, related to the optical properties of display devices such as plasma display panels (PDP) and lamps.     

Red emission of PbWO4 crystals

Luminescence characteristics of a large number of undoped and doped PbWO₄ crystals, grown by the Czochralski or Bridgman method, as-grown or annealed in the nitrogen atmosphere or in air, were studied in the 4.2–300 K temperature range. Two types of red emission centres were found. The centres with the emission band, peaking at 4.2 K at 1.57 eV, were observed in most of the crystals studied. The centres with the emission band, peaking at 4.2 K at 1.48 eV, were observed only in the PbWO₄ : Mo⁶⁺, Y³⁺ crystal. It is suggested that incompletely compensated lead vacancies are responsible for the appearance of the red emission.     

The luminescence properties of the ZnO@ZnWO4:Eu core–shell composites

The ZnO@ZnWO₄:Eu³⁺ core–shell composites were prepared by a two-step hydrothermal method and the photoluminescence properties of the composites were studied in contrast to the corresponding hexagonal ZnO and monoclinic ZnWO₄: Eu³⁺ nanocrystals prepared by the one-step hydrothermal method. The results demonstrate that in the nanocomposites the Eu³⁺ ions in the ZnWO₄ phase occupy two symmetry sites, one a well-crystalline inner site and one a disordered surface site; while in the ZnWO₄: Eu³⁺ nanocrystals, the local environments surrounding Eu³⁺ ions are relatively disordered for both the inner and the surface sites. This indicates that in the composites, the crystallinity of the ZnWO₄ becomes better, which have positive influence on the improvement of photoluminescence. The temperature stabilities for both the emissions of ZnO and Eu³⁺ ions are improved in contrast to the pure ZnO or ZnWO₄:Eu³⁺ nanocrystals.     

两种磷光材料在聚合物掺杂体系中的发光特性研究

磷光材料由于可以利用电致激发所形成的单重态和三重态激子,因而可以得到接近100%的内量子效率。文章对常温下基于磷光材料Ir(ppy)₃及Ir(piq)₃掺杂PVK薄膜为发光层的器件的光学和电学特性进行了研究。光致发光的结果显示相同掺杂质量比下由PVK到Ir(piq)₃的能量传递比到Ir(ppy)₃更加困难。通过研究两种掺杂体系不同质量比的电致发光特性,可以认为这两种磷光器件的发光主要来自于磷光客体分子直接俘获载流子发光而非主体的能量传递。Ir(piq)₃掺杂体系对掺杂比例的依赖更为明显,从能级结构分析,认为是由于Ir(piq)₃的更低的HOMO及高的LUMO能级,而比Ir(ppy)₃具有更好的载流子俘获和传输特性。