KBaPO4:Ln (Ln=Eu, Tb, Sm) phosphors for UV excitable white light-emitting diodes

This letter reports the novel three emission bands based on phosphate host matrix, KBaPO₄ doped with Eu²⁺, Tb³⁺, and Sm³⁺ for white light-emitting diodes (LEDs). The phosphors were synthesized by solid-state reaction and thermal stability was elucidated by measuring photoluminescence at higher temperatures. Eu²⁺-doped KBaPO₄ phosphor emits blue luminescence with a peak wavelength at 420 nm under maximum near-ultraviolet excitation of 360 nm. Tb³⁺-doped KBaPO₄ phosphor emits green luminescence with a peak wavelength at 540 nm under maximum near-ultraviolet excitation of 370 nm. Sm³⁺-doped KBaPO₄ phosphor emits orange-red luminescence with a peak wavelength at 594 nm under maximum near-ultraviolet excitation of 400 nm. The thermal stabilities of KBaPO₄:Ln (Ln=Eu²⁺, Tb³⁺, Sm³⁺), in comparison to commercially available YAG:Ce³⁺ phosphor were found to be higher in a wide temperature range of 25-300 °C.     

Luminescence property of Ce-doped silica decorated with S and Cl anions

Ce³⁺-doped silica was synthesized by sol-gel technique and was further decorated with S²⁻ and Cl⁻ anions through chemical exchange in controlled ambient at elevated temperature. The structure and optical property of samples were examined by X-ray diffraction spectrum, XPS pattern, reflection pattern, and photoluminescence patterns. There is a broad luminescence band at 445 nm under the excitation at 320 nm in the Ce³⁺-doped silica heat-treated in air at 1000 °C. The heat-treatment of the sample in vacuum at 800 °C can increase the intensity of luminescence but have no effect on the wavelength of luminescence. The decoration of S²⁻ and Cl⁻ anions cannot only increase the luminescent intensity but also shift the luminescent wavelength to shorter wavelength.     

Preparation and characterization of Zn2SiO4: Mn phosphors with hydrothermal methods

Mn²⁺-doped Zn₂SiO₄ phosphors had been prepared by hydrothermal method in stainless-steel autoclaves. Effects of synthesized methods, reaction temperature, ambience of heat treatment on the structure and the luminescence properties of this silicate were studied with X-ray diffraction apparatus (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) and fluorescence spectrum. Results show that Zn₂SiO₄ nanocrystalline can be obtained by hydrothermal method at relatively low temperatures. The absorption pattern shows an absorption edge at about 380 nm originated from ZnO crystals and two absorption bands at about 215 and 260 nm. Mn²⁺-doped Zn₂SiO₄ has a luminescence band with the wavelength at about 522 nm under 255 nm excitation, and the luminescent intensity increases after being heat treated.     

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.     

Optical properties of Yb-doped fibers and fiber lasers at high temperature

Recent advances in power scaling of Yb^+ 3-doped fiber lasers to the kilowatt level suggest a need to examine the performance of Yb^+ 3-doped silica at temperatures well above ambient. We report experimental results for the absorption coefficient, emission cross-section, fluorescence lifetime, and slope efficiency of a Yb³⁺-doped large mode area (LMA) silica fiber for temperatures spanning 23 °C-977 °C. To the best of our knowledge these are the highest temperatures to date for which these optical properties have been measured. We find a sharp reduction in the energy storing capability and lasing performance of Yb^+ 3:SiO₂ above 500 °C that coincides with the onset of non-radiative transitions in the excited state manifold (thermal quenching). As the temperature increases from room temperature to 977 °C, absorption in the 1020-1120 nm operating band increases monotonically, concurrent with a reduction in absorption at the 920-nm and 977-nm pumping bands. Conversely, the spectral weight of the emission cross-section shifts from transitions above 1010 nm to those below, with the exception of the 977-nm emission band.     

Luminescence characteristics of Pb centres in undoped and Ce-doped Lu3Al5O12 single-crystalline films and Pb→Ce energy transfer processes

At 4.2-350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5-15 eV energy range for the undoped and Ce³⁺-doped Lu₃Al₅O₁₂ (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb²⁺-based centres were identified. The processes of energy transfer from the host lattice to Pb²⁺ and Ce³⁺ ions and from Pb²⁺ to Ce³⁺ ions were investigated. Competition between Pb²⁺ and Ce³⁺ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb²⁺ centres with the 3.6 eV absorption band of Ce³⁺ centres, an effective nonradiative energy transfer from Pb²⁺ ions to Ce³⁺ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce³⁺ centres and decrease of the Ce³⁺-related luminescence intensity.     

Luminescence of Ce and Pr doped Sr2Mg(BO3)2 under VUV-UV and X-ray excitation

This report presents the luminescence properties of Ce³⁺ and Pr³⁺ activated Sr₂Mg(BO₃)₂ under VUV-UV and X-ray excitation. The five excitation bands of crystal field split 5d states are observed at about 46 729, 44 643, 41 667, 38 314 and 29 762 cm⁻¹ (i.e. 214, 224, 240, 261 and 336 nm) for Ce³⁺ in the host lattice. The doublet Ce³⁺ 5d→4f emission bands were found at about 25 840 and 24 096 cm⁻¹ (387 and 415 nm). The influence of doping concentration and temperature on the emission characteristics and the decay time of Ce³⁺ in Sr₂Mg(BO₃)₂ were investigated. For Pr³⁺ doped samples, the lowest 5d excitation band was observed at about 42017 cm⁻¹ (238 nm), a dominant band at around 35714 cm⁻¹ (280 nm) and two shoulder bands were seen in the emission spectra. The excitation and emission spectra of Ce³⁺ and Pr³⁺ were compared and discussed. The X-ray excited luminescence studies show that the light yields are ∼3200±230 and ∼1400±100 photons/MeV of absorbed X-ray energy for the samples Sr_1.86Ce_0.07Na_0.07Mg(BO₃)₂ and Sr_1.82Pr_0.09Na_0.09Mg(BO₃)₂ at RT, respectively.     

Effect of Ce codoping on Er-doped bismuth-germanate glass and fiber under 980 nm excitation

Er³⁺/Ce³⁺ codoped bismuth-germanate glasses with the composition of Bi₂O₃-GeO₂-Ga₂O₃-Na₂O were prepared by the conventional melt-quenching method. The absorption spectra, fluorescence spectra, upconversion emission and lifetimes of Er³⁺ ions were measured, and the effects of Ce³⁺-doping on the spectroscopic properties of 1.53 μm band fluorescence of Er³⁺ ion were investigated based on the analysis of energy transfer between Er³⁺ and Ce³⁺ ions. The results indicate that the 1.53 μm band fluorescence intensity can be improved evidently with the Ce³⁺-doped concentration under the excitation of 980 nm. Meanwhile, the theoretical simulation based on the population rate equation and light power propagation equation indicates that the C + L band signal gain can also be improved dramatically by introducing Ce³⁺ ions into the Er³⁺-doped bismuth-germanate glass fiber. Therefore, it is necessary to introduce Ce³⁺ ions when Er³⁺-doped bismuth-germanate glass with low phonon energy is applied to the 1.53 μm band broad Er³⁺-doped fiber amplifier (EDFA).     

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.     

Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers

The objective of this study was to identify a material suitable to absorb radiation at the wavelength of neodymium-doped Yttrium Aluminum Garnet (Y₃Al₅O₁₂:YAG), 1064 nm. M-(M= Sm³⁺, Co²⁺, Co³⁺, Cr³⁺, and Cr⁴⁺) doped highly transparent YAG ceramics were fabricated, and their absorption spectra were measured. Unlike Co²⁺ and Cr³⁺-doped ceramic samples, Co³⁺ and Cr⁴⁺ and Sm³⁺-doped:YAG ceramics were found to have significant absorption at 1064 nm. However, the Sm³⁺-doped YAG clearly emerged as the best candidate because it is also transparent at 808 nm, the pumping wavelength laser diode (LD), and also at most absorption bands used for flash-lamp pumping.     

Luminescence spectroscopy of Ce3+-doped ABaPO4 (A=Li, Na, K) phosphors

Ce³⁺ doped ABaPO₄ (A=Li, Na, K) phosphors were prepared by conventional high temperature solid-state reaction. The phosphors were investigated by XRD, photoluminescence excitation and emission spectra, and luminescence decay curves. The five 5d levels corresponding to the 4f¹→4f⁰5d¹ transition of Ce³⁺ ions were identified. The spectroscopic parameters, e.g., the 5d barycenter, the crystal-field splitting, and the Stokes shift, were discussed. LiBaPO₄:Ce³⁺ phosphor could be efficiently excited by the near-UV lights (330–420 nm) and showed a broad emission band in the range of 430–620 nm with the maximum wavelength at 468 nm. In contrast, Ce³⁺-doped NaBaPO₄ and KBaPO₄ showed only excitation bands in a limited UV region (230–370 nm) and have blue emission at 385 nm and 416 nm, respectively. The temperature quenching of luminescence and the chromaticity coordinates were reported. The luminescence properties were discussed by analyzing the crystal structure and the local surroundings of Ce³⁺ ions on the Ba²⁺ sites.     

Stmmry talk on evaluation and theory

Abstract

Single crystal samples of Ce³⁺-doped fluorides CaNaYF₆ and BaMgF₄ have been grown in a reducing atmosphere, with the intent of studying the effects of symmetry and ligand coordination on the dopant energy levels. The optical absorption spectra of Ce³⁺ in these crystals consist, at most, of five overlapping bands. The luminescence is also a broad band due to strong electron—phonon interaction in the 5d excited state. The energies of the lowest absorption band and the luminescence band are shifted to low energy compared to those in LiCaAlF₆ and LiYF₄. This trend may be explained by lowering symmetry of the anion-coordinate polyhedra of Ce³⁺ in these crystals.     

Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce

Radioluminescence and thermally stimulated luminescence measurements on Lu₂O₃, Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce³⁺ (LSO:Ce) reveal the presence of intrinsic ultraviolet luminescence bands. Characteristic emission with maximum at 256 nm occurs in each specimen and is attributed to radiative recombination of self-trapped excitons. Thermal quenching of this band obeys the Mott-Seitz relation yielding quenching energies 24, 38 and 13 meV for Lu₂O₃, LSO and LSO:Ce, respectively. A second intrinsic band appears at 315 nm in LSO and LSO:Ce, and at 368 nm in Lu₂O₃. Quenching curves for these bands show an initial increase in peak intensity followed by a decrease. Similarity in spectral peak position and quenching behavior indicate that this band has a common origin in each of the samples and is attributed to radiative recombination of self-trapped holes, in agreement with previous work on similar specimens. Comparison of glow curves and emission spectra show that the lowest temperature glow peaks in each specimen are associated with thermal decay of self-trapped excitons and self-trapped holes. Interplay between the intrinsic defects and extrinsic Ce³⁺ emission in LSO:Ce is strongly indicated.     

Optical and gain properties of series of crystals LiF-YF3-LuF3 doped with Ce and Yb ions

Here, we present first results of systematic studies of host cation variation impact on spectral-kinetic, photochemical and gain properties of Ce³⁺-doped LiYF₄ (YLF), LiLuF₄ (LLF) and LiY_1−xLu_xF₄ family crystals. 5d-4f luminescence decay of Ce³⁺ ions studies, together with pump-probe experiments, indicate that previously reported twice higher luminescence quantum yield in LLF compared with that of YLF crystals is provided by more efficient upper lasing level feeding due to recombination and higher color center destruction rate in LLF against YLF crystals. Namely, it is responsible for higher energetic characteristics of laser based on Ce³⁺:LLF crystals. Strong and wide pump-induced absorption band centered at 310 nm is observed in Ce³⁺:YLF. This band is shifted to blue and its intensity goes down with Lu content. We have evaluated free charges recombination rate, excited state absorption cross-section for Ce³⁺ ions and some other photodynamic processes related microparameters. Fitting results indicate that pump-induced color centers lifetime decreases with Lu-content in LiYF₄-LiLuF₄ mixture and it can be associated with more efficient color center bleaching by Ce³⁺ ions 5d-4f fluorescence.     

Multi-peak behavior of photoluminescence of silica particles heat-treated in hydrogen at elevated temperature

Pure nano-partical silica was prepared by sol-gel method, and then was treated in a H₂ ambient at different temperatures. The surface structure and valence bonding of samples were analyzed with IR spectrum and X-ray induced photoelectron spectroscopy, respectively. Its photoluminescence property was studied with fluorescence spectroscopy. The results showed that only one luminescence band at 344 nm exists for silica heat-treated at 450 °C. Silica samples heat-treated at higher temperatures showed completely different luminescence comparing with the samples heat-treated at lower temperatures. Multi-peaks were found in the SiO₂ samples heat-treated at temperatures higher than 700 °C, in which the luminescence peaks at about 379 and 392 nm are similar with the β bands in silica glass originated from the same defect center of two-fold coordinated silicon atoms, and the luminescence bands at about 493 and 528 nm are few reported. The intensity of the luminescence bands increase with the increase of heat treatment temperature from 700 to 900 °C.     

Observation of green emission from Ce-doped gadolinium oxide nanoparticles

Green emission at around 500 nm is observed in Gd₂O₃:Ce³⁺ nanoparticles and the intensity is highly dependent on the concentration of Ce³⁺ in the nanoparticles. The luminescence of this emission displays both picosecond (ps) and millisecond (ms) lifetimes. The ms lifetime is over four orders of magnitude longer than typical luminescence lifetimes (10-40 ns) of Ce³⁺ in traditional Ce³⁺ doped phosphors and therefore likely originates from defect states. The picosecond lifetime is shorter than the typical Ce³⁺ value and is also likely due to defect or surface states. When the samples are annealed at 700 °C, this emission disappears possibly due to changes in the defect moieties or concentration. In addition, a blue emission at around 430 nm is observed in freshly prepared Gd₂O₃ undoped nanoparticles, which is attributed to the stabilizer, polyethylene glycol biscarboxymethyl ether. On aging, the undoped particles show similar emission to the doped particles with similar luminescence lifetimes. When Eu³⁺ ions are co-doped in Gd₂O₃:Ce nanoparticles, both the green emission and the emission at 612 nm from Eu³⁺ are observed.     

Temperature-dependent photoluminescence properties of (Y, Lu)3Al5O12:Ce phosphors for white LEDs applications

The effects of the excitation wavelength, Ce³⁺ concentration and chemical substitution on the thermal quenching of Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphors were investigated over a temperature range from 30 to 250 °C. The quenching behavior exhibits a complex dependence on the excitation wavelength and Ce³⁺ concentration, which can be attributed to temperature-dependent absorption strength of the different f-d absorption bands and thermally activated concentration quenching with or without energy migrations between Ce³⁺ ions, respectively. With increasing Lu³⁺content the luminescence of (Y, Lu)₃Al₅O₁₂:Ce³⁺ phosphors shows a pronounced blueshift, and simultaneously the temperature quenching is obviously improved due to a decrease in Stokes shift.     

The luminescent properties of CaYBO4:Ln(Ln=Eu, Tb) under UV-VUV range

The luminescent properties of CaYBO₄:Ln(Ln=Eu³⁺, Tb³⁺) were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) region. The CT band of Eu³⁺ at about 245 nm blue-shifted to 230 nm in VUV excitation spectrum; the band with the maximum at 183 nm was considered as the host lattice absorption. For the sample of CaYBO₄:0.08Tb³⁺, the bands at about 235 and 263 nm were assigned to the f-d transitions of Tb³⁺ and the CT band of Tb³⁺ was calculated according to Jφrgensen's theory. Under UV and VUV excitation, the main emission of Eu³⁺ corresponding to the ⁵D₀-⁷F₂ transition located at about 610 nm and two intense emission of Tb³⁺ from the ⁵D₄-⁷F₅ transition had been observed at about 542 and 552 nm, respectively. With the incorporation of Gd³⁺ into the host lattice of CaYBO₄, the luminescence of Tb³⁺ was enhanced while that of Eu³⁺ was decreased because of their different excitation mechanism.     

Photoluminescence characterization of Ce and Dy doped Li2CaGeO4 phosphors

Ce³⁺ and Dy³⁺ activated Li₂CaGeO₄ phosphors were prepared by a solid-state reaction method, and characterized by XRD (X-ray diffraction) and photoluminescence techniques. The characteristic emission bands of Dy³⁺ due to ⁴F_9/2→⁶H_15/2 (blue) and ⁴F_9/2→⁶H_13/2 (yellow) transitions were detected in the emission spectra of Li₂CaGeO₄:Dy³⁺. Ce³⁺ broad band emission was observed in Li₂CaGeO₄:Ce³⁺ phosphors at 372 and 400 nm due to 5d→4f transition when excited at 353 nm. Co-doping of Ce³⁺ enhanced the luminescence of Dy³⁺ significantly and concentration quenching occurs when Dy³⁺ is beyond 0.04 mol%. White-light with different hues can be realized by tuning Dy³⁺ concentration in the phosphors.     

Enhanced luminescent properties of Y3Al5O12:Tb,Ce phosphor prepared by spray pyrolysis

Yttrium aluminum garnet (YAG) particles doped with Tb³⁺ or double doped with Tb³⁺ and Ce³⁺ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce³⁺ activator into the line peaks of Tb³⁺ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d-f transition of Ce³⁺ and a reduction in the intensity of emission peaks due to ⁵D₃-⁷F_j (j=3, 4, 5, 6) transition of Tb³⁺ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb³⁺ to Ce³⁺ in YAG host. Codoping Ce³⁺ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb³⁺, which means that more lattice defects, involving in the energy absorption and transfer to Tb³⁺, are formed by the Ce³⁺ codoping. The finding gives a promising approach for enhancing the luminescence efficiency.