Spontaneous luminescence of Eu<Superscript>3+</Superscript> ions in porous Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanospheres

A study of the luminescence of Eu³⁺ ions in Y₂O₃ nanospheres indicates a significant influence of the porous structure of nanoparticles on the luminescence of dopant ions. It is shown that filling the nanopores of initially porous Y₂O₃ nanospheres shortens the decay time of the spontaneous luminescence of doping europium ions. The change in the decay time is associated with the change in the effective refractive index of the porous nanospheres.     

Synthesis and luminescent properties of Eu<Superscript>3+</Superscript>- and Eu<Superscript>2+</Superscript>-activated sodium pyrophosphate Na<Subscript>4</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

The luminescent properties of Eu³⁺ and Eu²⁺ ions in sodium pyrophosphate, Na₄P₂O₇, have been studied. The excitation spectrum of the Eu³⁺ emission in Na₄P₂O₇ consists of several sets of bands in the range 280–535 nm due to 4f–4f transitions of Eu³⁺ ions and a broad band with a maximum at about 240 nm interpreted to be due to a charge transfer (CT) transition from oxygen 2p states to empty states of the Eu³⁺ 4f⁶-configuration. Although the CT band energy is large enough, the quantum efficiency (η) of the Eu³⁺ emission in Na₄P₂O₇ under CT excitation was estimated to be very low (η ≤ 0.01). In terms of a configurational coordinate model, this fact is interpreted as a result of the high efficiency of a radiationless relaxation from the CT state to the ⁷F₀ ground state of Eu³⁺ ions occupying sodium sites in Na₄P₂O₇. A strong reducing agent is required in order to stabilize Eu²⁺ ions in Na₄P₂O₇ during the synthesis. Several nonequivalent Eu²⁺ luminescence centers in Na₄P₂O₇ were found.     

Luminescence and energy transfer mechanism in Eu<Superscript>3+</Superscript>/Tb<Superscript>3+</Superscript>-co-doped ZrO<Subscript>2</Subscript> nanocrystal rods

Nanocrystal rods of Eu³⁺/Tb³⁺-co-doped ZrO₂ were synthesized using a simple chemical precipitation technique. Both ions were successfully doped into the Zr⁴⁺ ion site in a mixed structure containing both monoclinic and tetragonal phases. The Eu³⁺ or Tb³⁺ singly doped zirconia produced red and green luminescence which are characteristics of Eu³⁺ and Tb³⁺ ions, respectively. The co-doped zirconia samples produced blue emission from defect states transitions in the host ZrO₂, red and green luminescence from dopant ions giving cool to warm white light emissions. The phosphors were efficiently excited by ultraviolet and near-ultraviolet/blue radiations giving white and red light, respectively. The decay lifetime was found to increase with increasing donor ion concentration contrary to conventional observations reported by previous researchers. Weak quadrupole–quatdrupole multipolar process was responsible for energy transfer from Tb³⁺ (donor) ion to Eu³⁺ ion. No energy back-transfer from Eu³⁺ to Tb³⁺ ion was observed from the excitation spectra. Temperature-dependent photoluminescence shows the presence of defects at low temperature, but these defects vanished at room temperature and beyond. The Eu³⁺/Tb³⁺-co-doped ZrO₂ nanocrystal rod is a potential phosphor for white light application using UV as an excitation source. Thermoluminescence measurements show that the inclusion of Tb³⁺ ion increases trap depths in the host zirconia.     

PL Properties of Sr<Subscript>2</Subscript>CeO<Subscript>4</Subscript> With Eu<Superscript>3+</Superscript> and Dy<Superscript>3+</Superscript> for Solid State Lighting Prepared by Precipitation Method

Photoluminescence studies of pure and Dy³⁺, Eu³⁺ doped Sr₂CeO₄ compounds are presented by oxalate precipitation method for solid state lighting. The prepared samples also characterized by XRD, SEM (EDS) and FTIR spectroscopy. The pure Sr₂CeO₄ compound displays a broad band in its emission spectrum when excited with 280 nm wavelength, which peaks centered at 488 nm, which is due to the energy transfer between the molecular orbital of the ligand and charge transfer state of the Ce⁴⁺ ions. Emission spectra of Sr₂CeO₄ with different concentration of Dy³⁺ ions under near UV radiation excitation, shows that intensity of luminescence spectra is found to be affected by Dy³⁺ ions, and it increases with adding some percentages of Dy³⁺ ions. The maximum doping concentration for quenching is found to be Dy³⁺?=?0.2 mol % to Sr²⁺ions. The observed broad spectrum from 400 to 560 nm is mainly due to CT transitions in Sr₂CeO₄ matrix and some fractional contribution of transitions between ⁴F_9/2 → ⁶H_15/2 of Dy³⁺ ions. Secondly the effect of Eu³⁺ doping at the Sr²⁺ site in Sr₂CeO₄, have been studied. The results obtained by doping Eu³⁺ concentrations (0.2 mol% to 1.5 mol%), the observed excitation and emission spectra reveal excellent energy transfer between Ce⁴⁺ and Eu³⁺. The phenomena of concentration quenching are explained on the basis of electron phonon coupling and multipolar interaction. This energy transfer generates white light with a color tuning from blue to red, the tuning being dependent on the Eu³⁺ concentration. The results establish that the compound Sr₂CeO₄ with Eu³⁺?=?1 mol% is an efficient “single host lattice” for the generation of white lights under near UV-LED and blue LED irradiation. The commission internationale de I’Eclairage (CIE) coordinates were calculated by Spectrophotometric method using the spectral energy distribution of prepared phosphors.     

Spectral-luminescent properties of Eu<Superscript>3+</Superscript> ions in inorganic D<Subscript>2</Subscript>O and POCl<Subscript>3</Subscript>-SnCl<Subscript>4</Subscript> solvents

Absorption and luminescence spectra of Eu³⁺ ions in D₂O and POCl₃-SnCl₄ inorganic solvents were measured. Oscillator strengths and spontaneous radiation transition probabilities for the most intense electron transitions in Eu³⁺ were calculated. Judd-Ofelt parameters, the radiative lifetime of the ⁵ D ₀ metastable level of Eu³⁺ in D₂O-Eu³⁺ and POCl₃-SnCl₄-Eu³⁺ solutions, and matrix elements for radiative transitions from the ⁵ D ₀ level were determined. The luminescence lifetime of the ⁵ D ₀ level of Eu³⁺ in these solutions was measured. The photoluminescence quantum yield for transitions from the ⁵ D ₀ level of Eu³⁺ in ⁵ D ₀ and POCl₃-SnCl₄-Eu³⁺ solutions was found to be 0.32 and 0.88, respectively.     

luminescence of BaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals activated by Eu<Superscript>2+</Superscript> and Ce<Superscript>3+</Superscript> ions

We have studied the effect of doping with Eu²⁺ and Ce³⁺ ions on the photoluminescence (PL) of BaGa₂Se₄ crystals in the temperature range 77–300 K. We have established that the broad bands with maxima at wavelengths 456 nm and 506 nm observed in the photoluminescence spectra of BaGa₂Se₄:Ce³⁺ crystals are due to intracenter transitions 5d → ²F_7/2 and 5d →²F_5/2 of the Ce³⁺ ions, while the broad photoluminescence band with maximum at 521 nm in the spectrum of BaGa₂Se₄:Eu²⁺ is associated with 4f⁶ 5d → 4f⁷ (⁸S_7/2) transitions of the Eu²⁺ ion. We show that in BaGa₂Se₄:Eu²⁺,Ce³⁺ crystals, excitation energy is transferred from the Ce³⁺ ions to the Eu²⁺ ions.     

On the Luminescence of Ce<Superscript>3+</Superscript>, Eu<Superscript>3+</Superscript>, and Tb<Superscript>3+</Superscript> in Novel Borate LiSr<Subscript>4</Subscript>(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>

This paper reports on the photoluminescence (PL) and time-resolved properties of Ce³⁺, Eu³⁺, and Tb³⁺ in novel LiSr₄(BO₃)₃ powder phosphors. Ce³⁺ shows an emission band peaking at 420 nm under 350-nm UV excitation. Energy transfer from Ce³⁺ to Mn²⁺ takes place in the co-doped samples. Eu³⁺ shows red emission under near UV excitation. LiSr₄(BO₃)₃:Eu³⁺ phosphor could be a suitable candidate for phosphor-converted solid state lighting. The luminescence lifetime is 2.13 ms for Eu³⁺ in LiSr₄(BO₃)₃:0.001Eu³⁺. As Eu³⁺ concentration increasing, the decay curves deviate from exponential behavior. Tb³⁺ shows the strongest ⁵D₄→⁷ F₅ emission line at 540 nm. Decay curves of ⁵D₄→⁷ F₅ and ⁵D₃→⁷ F₅ emission with different Tb³⁺ concentrations were also measured. Cross-relaxation process is discussed based on the decay curves.     

Luminescence properties of phosphors based on Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> (TbAG) terbium-aluminum garnet

The processes of excitation energy transfer in phosphors based on single-crystal Tb₃Al₅O₁₂:Ce (TbAG:Ce) and Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb³⁺ cations) to Ce³⁺ and Eu³⁺ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions and from Ce³⁺ ions to Eu³⁺ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb³⁺ cation sublattice.     

Formation of Eu<Superscript>2+</Superscript> and Eu<Superscript>3+</Superscript> centers in synthesis of CaF<Subscript>2</Subscript>:Eu luminophores

We have studied the effect of CaF₂:Eu luminophore synthesis methods on the charge state of europium. We have shown that Eu³⁺ predominates over Eu²⁺ in samples obtained by coprecipitation of europium with calcium fluoride, and the ratio Eu³⁺/Eu²⁺ grows as the total amount of europium increases. Partial charge conversion of the europium occurs during calcination of the samples, due to changes in the excess fluorine balance. We studied the luminescence, magnetic susceptibility, and EPR of the synthesized samples. We have shown that in a solid solution, europium forms large ordered clusters, determining both the luminescent and the magnetic properties of the material. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 773–779, November–December, 2007.     

Possibility of adjusting photoluminescence spectra of Ca scheelites (CaMoO<Subscript>4</Subscript>: Eu<Superscript>3+</Superscript> and CaWO<Subscript>4</Subscript>: Eu<Superscript>3+</Superscript>) to the emission spectrum of incandescent lamps

The peculiarities of the photoluminescence of compounds CaMoO₄: Eu³⁺ and CaWO₄: Eu³⁺ with the scheelite structure associated with a change in the short- and long-range orders of the crystal lattice upon a change in the activator (Eu³⁺) of the photoluminescence range in the interval 1–4 mol %, in which the photoluminescence of the matrix is preserved in the range 484–557 nm, are investigated using X-ray phase analysis as well as photoluminescence, Raman, and diffuse reflection spectroscopies. The introduction of Eu³⁺ ions leads to the reconstruction of the lattice so that up to 10% of these ions stimulate the formation of centrosymmetric localization upon the substitution of Ca²⁺ ions in the noncentrosymmetric positions. It is found that the spectral radiant emittance of the more effective luminophore CaMoO₄: Eu³⁺ can be adjusted to this parameter for an incandescent lamp for the Eu³⁺ concentration of 1–2 mol %.     

Concentration quenching anomalies of activated Y<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Pr<Superscript>3+</Superscript> nanocrystal luminescence

For the fist time in Y₂SiO₅:Pr³⁺ nanocrystals, the ordered stage in the ¹ D ₂ luminescence decay curves for Pr³⁺ ions has been observed at anomalously low doped ion concentration (0.5 at %). This effect is caused by preferred location of the activator ions in the near-surface layer of the nanocrystal that provides the relaxation of elastic tension arising due to the difference of ionic radii of Pr³⁺ and Y³⁺ ions. Concentration quenching of Pr³⁺ luminescence is caused by the cooperative cross-relaxation.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Red,Yellow, Blue and Green Emission from Eu<Superscript>3+</Superscript>, Dy<Superscript>3+</Superscript> and Bi<Superscript>3+</Superscript> Doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript>nano-Phosphors

Rare earth elements (RE = Eu³⁺& Dy³⁺)and Bi³⁺ doped Y₂O₃ nanoparticles were synthesized by urea hydrolysis method in ethylene glycol, which acts as reaction medium as well as a capping agent, at a low temperature of 140 °C,followed by calcination of the obtained product. Transmission electron microscope (TEM) images reveals that ovoid shaped Y₂O₃ nanoparticles of around 22–24 nm size range were obtained in this method. The respective RE and Bi³⁺ doped Y₂O₃ precursor nanoparticles when heated at 600 and 750 °C, retains the same shape as that of the as-synthesized Y₂O₃ precursor samples. From EDAX spectra, the incorporation of RE ions into the host has been studied. XRD pattern reveals the crystalline nature of the heated nanoparticles and indicate the absence of any impurity phase other than cubic Y₂O₃.However, the as-synthesized nanoparticles were highly amorphous without the presence of any sharp XRD peaks. Photoluminescence study suggests that the synthesized samples could be used as red (Eu³⁺), yellow (Dy³⁺), blue and green (Bi³⁺)emitting phosphors.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.     

Thermal quenching of luminescence of BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals activated with Er<Superscript>3+</Superscript> and Tm<Superscript>3+</Superscript> ions

Thermal quenching of interconfigurational 5d-4f luminescence of Er³⁺ and Tm³⁺ ions in BaY₂F₈ crystals is studied in the temperature range of 330–790 K. The quenching temperatures are ~575 and ~550 K for Er³⁺ and Tm³⁺, respectively. It is shown that quenching of 5d-4f luminescence of Tm³⁺ ions is caused by thermally stimulated ionization of 5d electrons to the conduction band.     

Growth and spectral-luminescent study of SrMoO<Subscript>4</Subscript> crystals doped with Tm<Superscript>3+</Superscript> ions

SrMoO₄ crystals doped with Tm³⁺ ions have been produced from a melt using the Czochralski method; their spectral-luminescent characteristics have been studied, and laser radiation has been generated at the wavelength of 1.94 μm using laser-diode excitation. The high absorption section at the wavelength of 795 nm, the fairly high luminescence section, the long lifetime at the upper laser level ³F4 of 1.5 ms, and a wide luminescence band allow one to hope for developing efficient tunable Tm³⁺: SrMoO₄ crystal lasers with diode pumping in the range of 1.7–2.0 μm, which are capable of implementing SRS self-transformation of radiation into the middle IR band.     

Luminescence of Eu<Superscript>3+</Superscript> ions in ultradisperse diamond powders

The luminescence of Eu³⁺ ions implanted in ultradisperse diamond powders, activated by impregnating with a solution of Eu(NO₃)₃·6H₂O and heat-treated at various temperatures, is studied. A multiple increase in the efficiency of excitation in the charge-transfer band is observed for the ⁵D₀ state of Eu³⁺ ions as compared to europium nitrate heat-treated similarly. This effect is explained by an increase in the degree of Eu-O bond covalency and a change in the activator coordination polyhedron due to the formation of chemical bonds Eu-O-C.     

Intense White Luminescence from Combustion Synthesized Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript>: Yb<Superscript>3+</Superscript>/Yb<Superscript>2+</Superscript> Single Phase Phosphor

The Ca₁₂Al₁₄O₃₃: Yb³⁺/Yb²⁺ single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb³⁺ and Yb²⁺ states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb³⁺ state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb²⁺ ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb²⁺ state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb²⁺ to Yb³⁺ has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb³⁺ and Yb²⁺ states have been examined and discussed. Quantum yield of emission has also been determined for different samples.     

Eu<Superscript>3+</Superscript> As a Luminescent Probe for Studying the Structure of R<Subscript>2</Subscript>O<Subscript>3</Subscript> Materials (R = Y,Eu, and Gd)

Spectra of Eu³⁺ in various dielectric matrices (Gd₂O₃:Eu³⁺, Y₂O₃:Eu³⁺, Eu₂O₃, and mSiO₂/Gd₂O₃:Eu³⁺ mesoporous particles) are studied by local cathodoluminescence. The results allowed identification of the local environment of Er³⁺ ions in amorphous samples and detection of the monoclinic Eu₂O₃ phase impurity in samples with yttrium oxide. The cathodoluminescence spectra of chemically pure Y₂O₃, Eu₂O₃, and Gd₂O₃ are recorded. Conclusions about the structural features of the materials are made and confirmed by other methods (XRD and EPMA).     

Effects of Li<Superscript>+</Superscript> co-doping on the concentration quenching threshold and luminescence of GdVO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> nanophosphors

Europium ions (Eu³⁺) and Lithium ions (Li⁺) codoped gadolinium orthovanadate with a tetragonal phase had been successfully synthesized by an efficient hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) were utilized to characterize the microstructure, morphology, and luminescent properties of as-prepared samples. The various concentrations (0～14 at.%) of Li ions were applied to investigate the effect of Li⁺ co-doping concentration on the crystalline structure, microstructure, and emission intensity of GdVO₄:Eu³⁺, Li⁺ nanophosphors. The results demonstrated that Li⁺ ion co-doping changes the lattice parameters in two different ways. Moreover, the optical photoluminescent property was obtained when the Li⁺ co-doping concentration is 10 at.%. The influence of Li⁺ co-doping on the concentration quenching effect of Eu³⁺ was discussed as well. The concentration quenching threshold of Eu³⁺ was increased distinguishably. The potential mechanism was proposed in this paper.