Blue-to-Orange Tunable Luminescence from Europium Doped Yttrium--Silicon--Oxide--Nitride Phosphors

Europium-doped yttrium-silicon-oxide-nitride phosphors are synthesized by carbothermal reduction and nitridation method. The crystal structure of the phosphors changed gradually from oxide Y2Si2O7 to nitride YSi3N5 state with increasing dosage of Si3N4 and carbon powder. The Y2Si2O7：Eu phosphor shows a blue emission at 465 nm with 300 nm excitation and a characteristic red emission of Eu^3＋ at 612 nm with 230 nm excitation. The YSi3N5：EU phosphor shows a broad emission band centred at 595nm with some sharp peaks of Eu^3＋ with 325nm excitation. The absorption of the studied phosphors increases from 450 to 700hm with an increment in nitrogen content. Blue-to-orange tunable luminescence is observed with 390 nm excitation.     

Influence of Ba-doping on structural and luminescence properties of Sr2SiO4:Eu phosphors

Ba²⁺-doped Sr₂SiO₄:Eu²⁺ phosphors were synthesized with the high-temperature solid-state reaction technique. The experimental results, summarized in the successful production of a single-phase powder with fine microstructure of spherical particles with smooth surface, suggest that Ba²⁺-doping favors the stabilization of α′-Sr₂SiO₄. Rietveld refinement of X-ray diffractograms suggests that Ba²⁺ and Eu²⁺ ions occupy the sites of Sr²⁺ in the lattice of α′-Sr₂SiO₄. The produced phosphors show two intense emission bands at green and yellow regions of spectrum, originated from Eu²⁺ ions accommodated at two different sites in the host crystal, whose peaks depend on the concentrations of Ba²⁺ and Eu²⁺. Intense and broad excitation spectra extend from ultraviolet to the blue region.     

Luminescence spectra of Eu ions and interstitial oxygen in PbWO4 crystal

The site-selective excitation and emission spectroscopy, and luminescence decay have been investigated under a pulsed, tunable, narrowband dye laser of the ⁵D₀→⁷F₀ region in the europium ions-doped lead tungstate PbWO₄ (PWO) in single crystal. In as-grown sample, the experimental results show that there is only one ⁷F₀→⁵D₀ excitation transition indicating the only one Eu³⁺ site in PbWO₄ lattices. The sequential annealing treatments were conducted to investigate the effects of oxygen components on the microstructure environments of Eu³⁺ in the lattices. The site distribution of Eu³⁺ was changed by the annealing in air atmosphere, which could create new sites in PWO lattices. Confirmation of interstitial oxygen and interpretations of charge compensation mechanism for the observed new sites were discussed in the context of site-selective excitation and emission spectra. The main Eu³⁺ site is related to the charge compensation by the [(Eu_Pb³⁺)-V″_Pb-(Eu_Pb³⁺)] complex; the other minor new sites after annealing are originated from [(Eu_Pb³⁺)-O″_i-(Eu_Pb³⁺)] defects. Emission spectra excited by 355-laser and RT-Raman spectra were also measured.     

Photoluminescence studies of γ-irradiated CaF2:Dy:Pb:Na single crystals

The optical absorption (OA) and photoluminescence (hereafter referred to as luminescence) studies were made on CaF₂:Dy:Pb:Na single crystals (Dy—0.005 at%, Pb—0.188 at% and Na—0.007 at%) before and after γ-irradiation. The unirradiated crystal exhibited a strong OA band around 6.36 eV attributed to the ‘A’ band absorption of Pb²⁺ ions. The γ-irradiated crystal exhibited OA bands around 2.06, 3.28, 3.75 (broad shoulder) and 2.48 eV. The first three bands could be tentatively attributed to M_Na centre when compared with that of the coloured CaF₂:Na. The origin of 2.48 eV band was not explicitly known. Luminescence emission and excitation of Pb²⁺ and Dy³⁺ ions were negligible in the unirradiated crystal. Irradiated crystal exhibited a strong excitation spectrum with overlapping bands, due to different colour centres, in the UV-vis region for the 2.15 eV emission characteristic of Dy³⁺ ion. When excited, the absorbed energy (may be a part) was transferred from a colour centre to nearby Dy³⁺ ions and Dy³⁺ characteristic emission was observed. Exciting the irradiated crystal around 3.28 eV yielded emission at 2.56, 2.15 and 1.76 eV. The first two emission bands were due to Dy³⁺ ions. The excitation spectrum for the 1.76 eV emission showed two prominent bands around 2.02 and 3.08 eV and hence the emission was attributed to the M_Na centre. The luminescence mechanism was described.     

Color tunability of nanophosphors by changing cations for solid-state lighting

X₃MgSi₂O₈: Eu²⁺, Mn²⁺ (X=Ba, Sr, Ca) phosphors with the mean particle size of 200 nm and the spherical shape are synthesized through combustion method. They show three emission colors under near-ultraviolet light: the blue and green colors from Eu²⁺ ions and the red color from Mn²⁺ ions. Three emission bands show the different emission colors with changing X²⁺ cations. These color shifts are discussed in terms of two competing factors of the crystal field strength and the covalency. These phosphors with maximum excitation of around 375 nm can be applied as color-tunable phosphors for white-light-emitting diode based on ultraviolet/phosphor technology.     

Photoluminescence Characterization of Silicon Nanoparticles Hybridized Europium Complex

A europium complex Eu (DBM)3 TPPO (Eu tris(benzoylmethide)-(triphenylphosphine oxide)) and silicon nanoparticles have been hybridized.The hybridization can evidently change the photoluminescence (PL) characteristics of the Eu complex in the following aspects:under an excitation of 390nm,the intensity of the PL peak at 611nm due to the ^5Du-^7F2 transition of the Eu^3 ions has been increased by 30%,and thc integrated PL intensity in the visible range has been increased by nearly 3 times;the PL excitation efficiency beyond 440nm has been improved cvidently;the peak in the PL excitation spectrum shifts from 408nm to 388nm,and the PL decay time decreases from 2.07 to 0.96μs,The experimental results indicatde that in the PL process,the photoexcited energy may transfer from the silicon nanoparticlcs to the Eu^3 ions.     

Upconversion luminescence in Er-doped germanate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics

Upconversion luminescence has been studied for Er³⁺ in a germanate-oxyfluoride and a tellurium-germanate-oxyfluoride transparent glass-ceramic using 800 nm excitation. Significantly increased upconversion luminescence was observed from transparent glass-ceramics compared with that from their corresponding as-prepared glasses. In addition to a strong green emission centered at 545 nm from ⁴S_3/2 state and a weaker red emission centered at 662 nm from ⁴F_9/2 state generally seen from Er³⁺-doped glasses, a violet emission centered at 410 nm from ²H_9/2 state and a near-ultra-violet emission centered at 379 nm from ⁴G_11/2 state were also observed from transparent glass-ceramics. The upconversion luminescence of Er³⁺ ions in transparent glass-ceramics revealed sharp Stark-splitting peaks generally seen in a crystal host. The increased upconversion efficiency is attributed to the decreased effective phonon energy and the increased energy transfer between excited ions when Er³⁺ ions were incorporated into the precipitated β-PbF₂ nanocrystals.     

Absorption and emission characteristics of Er3NbO7 phosphor: A comparison with ErNbO4 phosphor and Er:LiNbO3 single crystal

Er₃NbO₇ phosphor was synthesized by sintering a mixture of Er₂O₃ and Nb₂O₅ powder in a molar ratio of 3:1 at 1600 °C over 55 h. Optical absorption and emission characteristics of Er³⁺ ions in the calcined Er₃NbO₇ powder were investigated and discussed compared with ErNbO₄ phosphor and a Z-cut congruent Er (2 mol%):LiNbO₃ single crystal. The absorption and emission studies show that, due to different crystal structures, the spectroscopic properties of these niobates have some differences in spectral shape, peak position, and relative intensity, especially at 1.5 μm. The most obvious spectral feature of the Er₃NbO₇ is that the spectral structure of band instead of peak is observed in its absorption or emission spectrum due to the existence of local structural disorder and multiple Er³⁺ sites. The Er₃NbO₄ shows stronger upconversion emission than the single crystal but weaker than the ErNbO₄. Experimental results show that energy transfer upconversion and/or excited state absorption play a dominant role in the upconversion emissions, and, at higher pump level (>200 mW), the thermal effect becomes significant and results in drop of the upconversion intensity. The 1.5 μm lifetimes of Er³⁺ ion in the Er₃NbO₇, ErNbO₄ phosphor, and in the Er:LiNbO₃ crystal are measured to be ∼5.3, 2.0, and 2.4 ms, respectively. In combination with the measured Raman spectra, the quantum efficiency, multiphonon nonradiative decay rate, and theoretical radiative lifetime of the 1.5 μm emission of the two powder materials are expected. The differences in upconversion intensity and measured 1.5 μm lifetime between the three materials are explained qualitatively.     

Y2O3:Eu phosphor particles prepared by spray pyrolysis from a solution containing citric acid and polyethylene glycol

Y₂O₃:Eu phosphor particles were prepared by large-scale spray pyrolysis. The morphological control of Y₂O₃:Eu particles in spray pyrolysis was attempted by adding polymeric precursors to the spray solution. The effect of composition and amount of polymeric precursors on the morphology, crystallinity and photoluminescence characteristics of Y₂O₃:Eu particles was investigated. Particles prepared from a solution containing polyethylene glycol (PEG) with an average molecular weight of 200 had a hollow structure, while those prepared from solutions containing adequate amounts of citric acid (CA) and PEG had a spherical shape, filled morphology and clean surfaces after post-treatment at high temperature. Y₂O₃:Eu particles prepared from an aqueous solution with no polymeric precursors had a hollow structure and rough surfaces after post-treatment. The phosphor particles prepared from solutions with inadequate amounts of CA and/or PEG also had hollow and/or fragmented structures. The particles prepared from the solution containing 0.3 M  CA and 0.3 M  PEG had the highest photoluminescence emission intensity, which was 56% higher than that of the particles prepared from aqueous solution without polymeric precursors. Received: 22 March 2002 / Accepted: 26 March 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +82-42/861-4245, E-mail: yckang@pado.krict.re.kr     

Is Auger-free luminescence present in CeF3?

It is well known that Auger-free luminescence (AFL) is observable when the condition E_g>E_VC is satisfied, where E_g is the band-gap energy between the lowest unoccupied band and the highest occupied band and E_VC the energy difference between the top of the highest occupied band and the top of the next lower occupied band. From measurements of reflection and X-ray photoelectron spectra, CeF₃ is demonstrated to really satisfy this condition. No evidence for AFL is found, nevertheless. The absence of AFL in CeF₃ is related to a characteristic nature of its highest and next lower occupied bands, which are quite different from those of previously studied AFL-materials.     

Blue emission in Ti-sapphire laser crystals

The time resolved emission spectrum of the blue band of Ti:sapphire laser crystal has been investigated as a function of temperature (range 10–290 K) and UV (266 nm) laser excitation intensity. Two blue emission bands, centred at 420 nm and 460 nm, have been detected. The 420 nm band is attributed to Ti⁴⁺ centres whereas the 460 nm one is proposed to be due to Ti³⁺ ions. The evolution of the emission spectrum vs the UV excitation intensity has shown that the concentration of Ti⁴⁺ centres is increased under UV irradiation at the cost of the centres responsible for the 460 nm band.     

Tunneling recombination luminescence under excitation of PbWO4:Mo crystals in the defect-related absorption region

Time-resolved emission and excitation spectra and luminescence decay kinetics were studied at 150-300 K for the green emission of PbWO₄:Mo crystals. It was found that the slow (μs-ms) decay component observed under excitation in the defect-related absorption region (around 3.8-3.9 eV) arises from the G(II) emission which appears at the tunneling recombination of optically created electron and hole centers. The study of the emission decay kinetics at different temperatures and excitation intensities allowed concluding that both the monomolecular and the bimolecular tunneling recombination process can be stimulated in the mentioned energy range. The monomolecular process takes place in the isolated spatially correlated pairs of electron and hole centers produced without release of electrons into the conduction band. The bimolecular process takes place in the pairs of randomly distributed centers created at the trapping of free electrons from the conduction band. The formation of electron centers under irradiation in the defect-related absorption region was investigated by the electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. The possibility of various photo-thermally stimulated defects creation processes, which take place with and without release of free electrons into the conduction band, was confirmed.     

Efficient VUV sensitization of Eu3+ emission by Tb3+ in potassium rare‐earth double phosphate

The luminescence properties of K₃Tb(PO₄)₂ activated by Eu³⁺ were studied at excitation over the 120–300 nm wavelength range. It is demonstrated that Tb³⁺ ions, exhibiting a strong absorption band in the vacuum‐ultraviolet (VUV), can provide efficient sensitisation of Eu³⁺ emission in this wave length range, giving rise to intense red luminescence at 150 nm excitation. A proof is given for the concept of VUV sensitisation enabling the engineering of luminescence materials with enhanced conversion efficiency of VUV radiation into visible light. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectral properties and emission efficiencies of GdVO4 phosphors

GdVO₄ with activators Eu, Dy, Sm and Bi has been synthesised by a solid-state reaction. GdVO₄:Eu³⁺ (3%) yielded the highest quantum efficiency of 95%. Interesting energy-transfer properties have been revealed in the mixed-activator phosphor (GdVO₄:Eu³⁺, Sm³⁺) when excited in the 4f shell of Sm³⁺ at 408 nm. Bismuth-activated GdVO₄ gives rise to a broad-band emission peaking at 525 nm in comparison to YVO₄:Bi³⁺, which gives an emission peak at 570 nm under UV excitation. The quantum efficiency of GdVO₄:Bi³⁺ increases gradually with bismuth concentration and reaches a maximum of 80% for a bismuth concentration of ≈0.5%. There is a shift in the excitation band of GdVO₄:Bi³⁺ towards longer wavelengths with increasing concentration of bismuth, which can lead to energy transfer from bismuth to europium in a phosphor with both these activators. Heat treatment of GdVO₄:Bi³⁺ at 1500 °C for 3–3.5 h resulted in a large percentage of bismuth being lost from the lattice as evaluated by X-ray fluorescence. However, if a large percentage of bismuth (of the order of 3% or more) is initially added, a sufficient quantity of bismuth can still be retained after heat treatment, which can lead to the development of ceramic scintillators for X-ray tomographic applications. Addition of 3–5% boron gives a white GdVO₄ phosphor without any chemical treatment. Received: 27 Feruary 2001 / Accepted: 1 August 2001 / Published online: 30 October 2001     

Role of vitreous matrix on the optical activity of Ge-doped silica

We report an experimental study on the relationship between the optical activity of Ge-oxygen deficient centers and dynamic properties and conformational heterogeneity of vitreous matrix in silica. We focus our attention on the absorption band at ∼5.2 eV (B_2β) and on the two related emissions at ∼4.2 eV (α_E) and at ∼3.1 eV (β). From the temperature dependence of B_2β band we estimate a mean energy value of 26 meV for local vibrational modes coupled to the electronic transition, suggesting that the chromophore and its surrounding have access to low frequency dynamics. From the thermal behavior of the two emissions we distinguish the two competitive relaxation processes from the first singlet excited state S₁: the radiative one, giving rise the α_E band, and the thermally activated intersystem-crossing process between S₁ and the triplet state T₁, originating the β band. The intersystem-crossing rate increases on increasing the temperature, determining an opposite thermal behavior of the intensity of the two emissions. However, this temperature dependence cannot be rationalized by a simple Arrhenius law and the α_E decay kinetics at high temperatures do not follow a single exponential law, suggesting a complex landscape of configurational energies of the process.     

Application of point-group bases to f−fd transitions of lanthanide and actinide ions doped in crystals

The application of point-group coupling coefficients to the modeling of f^N↔f^N−1d transitions is discussed. There are several possible coupling schemes for the states of the f^N−1d configuration. Formulae for matrix elements of the Hamiltonian for the f^N−1d configuration and the relative line strengths for f^N↔f^N−1d transitions are derived. As an example, the f-d absorption spectrum of the crystal Yb²⁺: SrCl₂ is calculated using coupling coefficients based on the software developed by Butler and co-workers, which makes use of the Racah-Wigner calculus. The advantages and disadvantages of various coupling schemes are demonstrated. These coupling schemes are related to the simple model for f-d transitions.     

The role of hydrogen- and hydroxyl-doping in CsBr:Eu

Hydrogen- and hydroxyl-doped CsBr:Eu²⁺ storage phosphors were investigated by photostimulated luminescence (PSL) spectra, absorption spectra and PSL lifetime spectra. Hydrogen- and hydroxyl-doping plays an active role related to the trap centers in CsBr:Eu²⁺ storage phosphors. A sharp increase in the PSL yield was observed for non-hydroxyl-doped CsBr:Eu²⁺ annealed in 5%H₂+95%Ar atmosphere. For hydroxyl-doped CsBr:Eu²⁺, the PSL lifetime (361 ns) is obviously shorter than that of typical CsBr:Eu²⁺ storage phosphors.     

Temperature dependence of the photoluminescence and phosphorescence time decay of magnesia-stabilized zirconia

Temperature dependence of photoluminescence (PL) spectra and time decay ranging from 90 to 330 K are investigated in magnesia-stabilized zirconia single crystals. The emission PL spectra can be decomposed into two bands. The prominent one is centered in the blue-green region of the spectrum whereas the secondary one is centered in the yellow-orange region. The temperature dependence of these bands are analyzed in terms of the so-called configuration coordinate model. The Huang-Rhys parameter for the prominent band is found near 40 and the effective phonon at about 0.030 eV. Thermal quenching energy is determined to be 0.24 eV from the decreasing part of the I(T) curve. Luminescent decays were satisfactorily fitted by two exponentials over the whole temperature range investigated. Total lifetime temperature dependence can be accounted for by assuming a radiative decay from two metastable levels with a separation energy of 0.073 eV. Results are discussed on the basis of the major defects, oxygen vacancies and complex defects.     

Soft X-ray excited optical luminescence studies of gold(I) complex with diphosphine and 4,4′-bipyridyl ligands

The luminescence of gold(I) complex [{-Ph₂P(CH₂)₂Ph₂-AuNC₅H₄C₅H₄NAu-}_x]^2x+ [CF₃CO₂⁻]_2x, in the solid state has been studied by soft X-ray excited optical luminescence and near-edge X-ray absorption fine structures at the C K-edge and we find that the origin of the luminescence is primarily from the 4,4′-bipyridine ligand facilitated by gold(I)-ligand interaction.     

Fermi-level engineering of BaTiO3 by alkali codoping: increasing the near-infrared absorption by rhodium

Using electron paramagnetic resonance, optical absorption, and fast spectroscopy of light-induced absorption changes, it is shown that codoping BaTiO₃:Rh with Na_Ba acceptors raises the charge state of Rh³⁺ to Rh⁴⁺. Subsequent oxidation under high oxygen pressures can lower the Fermi level to Rh^4+/5+, leading to increased infrared absorption. The light-induced charge transfer in such specimens is characterised by “one center” behaviour. Received: 18 November 1998 / Revised version: 23 December 1998 / Published online: 7 April 1999