Thermal stability and spectroscopic properties of Er-doped lead fluorogermanate glasses

The thermal characterization and spectroscopic properties of Er³⁺-doped 0.6GeO₂-(0.4-x)PbO-xPbF₂ glasses were investigated experimentally. With the replacement of PbO by PbF₂ the thermal stability of glasses is improved and the infrared fluorescence intensity at 1530 nm is increased. The Judd-Ofelt intensity parameters, radiative transition rates, and fluorescence lifetimes of the excited ⁴I_13/2 level of Er³⁺ ions were calculated from Judd-Ofelt theory. The asymmetric ligand field around Er³⁺ ions resulted from the incorporation of PbF₂ into germanate glasses, broadens the infrared emission spectra at 1530 nm. Upconversion luminescence in the investigated glasses was observed at room temperature under the excitation of 976 nm laser diode. The glass 0.6GeO₂-0.3PbO-0.1PbF₂ exhibits the maximum upconversion emission intensity, while no frequency upconversion luminescence was observed in the 0.6GeO₂-0.4PbO glass. The quadratic dependence of the green and red emissions on excitation power indicates that two-photon absorption contributes to the visible emission under the 976-nm excitation.     

Spectroscopic investigations of Nd-, Er-, Er/Yb-, and Tm-ions doped SiO2-Al2O3-CaF2-GdF3 glasses

This paper reports on the absorption, visible and near-infrared luminescence properties of Nd³⁺, Er³⁺, Er³⁺/2Yb³⁺, and Tm³⁺ doped oxyfluoride aluminosilicate glasses. From the measured absorption spectra, Judd-Ofelt (J-O) intensity parameters (Ω₂, Ω₄ and Ω₆) have been calculated for all the studied ions. Decay lifetime curves were measured for the visible emissions of Er³⁺ (558 nm, green), and Tm³⁺ (650 and 795 nm), respectively. The near infrared emission spectrum of Nd³⁺ doped glass has shown full width at half maximum (FWHM) around 45 nm (for the ⁴F_3/2→⁴I_9/2 transition), 45 nm (for the ⁴F_3/2→⁴I_11/2 transition), and 60 nm (for the ⁴F_3/2→⁴I_13/2 transition), respectively, with 800 nm laser diode (LD) excitation. For Er³⁺, and Er³⁺/2Yb³⁺ co-doped glasses, the characteristic near infrared emission bands were spectrally centered at 1532 and 1544 nm, respectively, with 980 nm laser diode excitation, exhibiting full width at half maximum around 50 and 90 nm for the erbium ⁴I_13/2→⁴I_15/2 transition. The measured maximum decay times of ⁴I_13/2→⁴I_15/2 transition (at wavelength 1532 and 1544 nm) are about 5.280 and 5.719 ms for 1Er³⁺ and 1Er³⁺/2Yb³⁺ (mol%) co-doped glasses, respectively. The maximum stimulated emission cross sections for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ and Er³⁺/Yb³⁺ are 10.81×10⁻²¹ and 5.723×10^-21 cm². These glasses with better thermal stability, bright visible emissions and broad near-infrared emissions should have potential applications in broadly tunable laser sources, interesting optical luminescent materials and broadband optical amplification at low-loss telecommunication windows.     

Photoluminescence characteristics of energy transfer between Er and Bi in Gd2O3: Er, Bi

The phosphors, Bi³⁺- activated Gd₂O₃:Er³⁺, were prepared by sol-gel combustion method, and their photoluminescent properties were investigated under ultraviolet light excitation. The emission spectrum exhibited sharp peaks at about 520, 535, 545, 550 and 559 nm due to (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺ ions. The luminescent intensity was remarkably improved by the incorporation of Bi³⁺ ions under 340 nm light excitation, which suggested very efficient energy transfer from Bi³⁺ ions to Er³⁺ions. The introducing of Bi³⁺ ions broadened the excitation band of the phosphor, of which a new strong peak occurred ranging from 320 to 360 nm due to the 6s²→6s6p transition of Bi³⁺ ions. There is significant energy overlap between the emission band of Bi³⁺ ions and the excitation band of Er³⁺ ions. Under 340 nm light excitation, Bi³⁺ absorbed most of the energy and transferred it to Er³⁺. The energy transfer probability from Bi³⁺ to Er³⁺ is strongly dependent on the Bi³⁺ ion concentration. Also, the sensitization effectiveness was studied and discussed in this paper.     

Temperature-dependent fluorescence characteristics of an ytterbium-sensitized erbium-doped tellurite glass

In this article, the 1.5 μm emission spectra corresponding to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ in tellurite glass are studied within the temperature from 8 to 300 K. The emission spectra of Er³⁺: ⁴I_13/2→⁴I_15/2 transition are also analyzed using a peak-fit routine, and an equivalent four-level system is proposed to estimate the stark splitting for the ⁴I_15/2 and ⁴I_13/2 levels of Er³⁺ in the tellurite glass. The results indicate that the ⁴I_13/2→⁴I_15/2 emission of Er³⁺ can exhibit a considerable broadening due to a significant enhance the peak a′, and b′ change, respectively, and the peaks of which are located at about 1507 and 1556 nm. A detailed study of temperature-dependent 1.5 μm emission spectra involving the change of the corresponding sub-bands shows that as the temperature decreases from 300 to 8 K, its line-shape becomes sharper and more intense (the full-width at half-maximum decreases from 59 to 38 nm). Temperature-dependent fluorescence intensities and the experimentally determined lifetimes are investigated; the results show that a decrease behavior of fluorescence intensities and lifetimes are observed for temperature from 8 to 300 K.     

Unusual Ln substitutional defects: The local chemical environment of Eu and Er in nanocrystalline Nb2O5 by Ln-K edge EXAFS

The local chemical environment of the trivalent lanthanide cations in Nb₂O₅ nanopowders doped with 1 mol% of Eu³⁺ and Er³⁺, prepared via a Pechini approach, has been studied by means of EXAFS at the Ln-K edge. It can be demonstrated that the lanthanide ions enter the Nb₂O₅ structure as substitutional defects with respect to Nb, giving rise to a very large amount of disorder: both Eu³⁺ and Er³⁺ ions substitute Nb in the nine-fold coordinated site, with clustering of oxygen vacancies around the substitutional defects. Valence bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. The Er³⁺-doped nanocrystalline Nb₂O₅ sample shows efficient luminescence in the near infrared region around 1.5 μm. The emission and excitation spectra are affected by significant inhomogeneous broadening, in agreement with the presence of strong disorder around the dopant ions in nanosized Nb₂O₅.     

Water-Soluble Ln<Superscript>3+</Superscript>-Doped LaF<Subscript>3</Subscript> Nanoparticles: Retention of Strong Luminescence and Potential as Biolabels

The use of optically robust, luminescent lanthanide-based particles is becoming an area of interest for biolabel-related chemistry, due to their long lifetimes and range of non-overlapping absorption and emission lines from the visible to the near-infrared. We report the synthesis and optical properties of water-soluble, luminescent Ln³⁺-doped nanoparticles (NPs) coordinated with a hydrophilic (RO)PO₃²⁻ ligand that facilitates the stabilization of the NPs in aqueous conditions, and that regulates particle growth to the nanometer range. The use of lanthanide ions as dopants, in particular Eu³⁺ and Er³⁺ ions, yields optically robust particles with narrow emission lines in the visible (591 nm) and in the near-infrared (1530 nm), respectively. Luminescent lifetimes range from the microsecond to the millisecond for Er³⁺ and Eu³⁺ ions, respectively, and the NPs are not expected to be susceptible to photo-bleaching due to the fact that the emissions arise from intra-4f transitions of the lanthanide ions.     

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.     

Energy transfer in Er:Eu:Yb co-doped tellurite glasses: Yb as enhancer and quencher

Energy transfer has been studied from Er³⁺ to Eu³⁺ ions on excitation with NIR photons (796 and 980 nm) with and without Yb³⁺ ions. It is found that in one case the presence of Yb³⁺ enhances the fluorescence yield (980 nm excitation) whereas in the other case it quenches (796 nm excitation). Energy transfer from Er³⁺ ion's levels ⁴S_3/2 and ²H_11/2 is verified by decay curve analysis in both the cases. The nature of interaction between the donor (Er) and the acceptor (Eu) ions is found to be dipole-dipole. The energy transfer parameters viz. transfer probability, critical distance etc. have been calculated.     

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.     

Temperature-dependent photoluminescence spectra of Er-Tm-codoped Al2O3 thin film

Er-Tm-codoped Al₂O₃ thin films with different Tm to Er concentration ratios were synthesized by cosputtering from separated Er, Tm, Si, and Al₂O₃ targets. The temperature dependence of photoluminescence (PL) spectra was studied. A flat and broad emission band was achieved in the 1.4-1.7 μm and the observed 1470, 1533 and 1800 nm emission bands were attributed to the transitions of Tm³⁺: ³H₄ → ³F₄, Er³⁺: ⁴I_13/2 → ⁴I_15/2 and Tm³⁺: ³F₄ → ³H₆, respectively. The temperature dependence is rather complicated. With increasing measuring temperature, the peak intensity related to Er³⁺ ions increases by a factor of five, while the Tm³⁺ PL intensity at 1800 nm decreases by one order of magnitude. This phenomenon is attributed to a complicated energy transfer (ET) processes involving both Er³⁺ and Tm³⁺ and increase of phonon-assisted ET rate with temperature as well. It should be helpful to fully understand ET processes between Er and Tm and achieve flat and broad emission band at different operating temperatures.     

Synthesis and luminescence investigation of RE (Eu, Tb and Ce)-doped lithium silicate (Li2SiO3)

Synthesis and photoluminescence (PL) investigations of lithium metasilicate doped with Eu³⁺, Tb³⁺ and Ce³⁺ were carried out. PL spectra of Eu-doped sample showed peaks corresponding to the ⁵D₀→⁷F_j (j=1, 2, 3 and 4) transitions under ultraviolet excitation. Strong red emission coming from the hypersensitive ⁵D₀→⁷F₂ transition of Eu³⁺ ion suggested the presence of the dopant ion in structurally disordered environment. Tb³⁺-doped silicate sample showed blue-green emission corresponding to the ⁵D₄→⁷F_j (j=6, 5 and 4) transitions. Ce-doped sample under excitation from UV, showed a broad emission band in the region 350-370 nm with shoulders around 410 nm. The fluorescence lifetimes of Eu³⁺ and Tb³⁺ ions were found out to be 790 and 600 μs, respectively. For Ce³⁺, the lifetime was of the order of 45 ns. PL spectra of the europium- and terbium-doped samples were compared with commercial red (Y₂O₃:Eu³⁺) and green (LaPO₄:Tb³⁺) phosphors, respectively. It was found that the emission from the doped silicate sample was 37% of the commercial phosphor in case of the Tb-doped sample and 8% of the commercial phosphor in case of the Eu-doped sample.     

Investigation of spectroscopic properties and thermal stability of Er/Yb-co-doped Bi2O3-B2O3-Ga2O3 glasses

A series of Er³⁺/Yb³⁺-co-doped 60Bi₂O₃-(40−x) B₂O₃ -xGa₂O₃ (BBGA x=0, 4, 8, 12, 16 mol%) glasses have been prepared. The absorption spectra, emission spectra, fluorescence lifetime of Er³⁺:⁴I_13/2 level and thermal stability were measured and investigated. Three Judd-Ofelt intensity parameters Ω_t (t=2,4,6) (Ω₂=(4.67-5.93)×10⁻²⁰ cm², Ω₄=(1.50-1.81)×10⁻²⁰ cm², Ω₆=(0.92-1.17)×10⁻²⁰ cm²) of Er³⁺ ions were calculated by Judd-Ofelt theory. It is found that the Ω₆ first increases with the increase of Ga₂O₃ content from 0 to 8 mol% and then decreases, which is mainly affected by the number of non-bridging oxygen ions of the glass network. The high peak of stimulated emission cross-section () of Er³⁺: ⁴I_13/2→⁴I_15/2 transition were obtained according to McCumber theory and broad full width at half maximum (FWHM=69-76 nm) of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions were measured. The results indicate that these new BBGA glasses can be used as a candidate host material for potential broadband optical amplifiers.     

Photoluminescence investigations of Zn2SiO4 co-doped with Eu and Tb ions

Zinc silicate phosphors co-doped with Eu³⁺ ions and also with both Eu³⁺ and Tb³⁺ ions were prepared by high temperature solid state reaction in air or reducing atmosphere. The luminescence characteristics of the prepared phosphors were investigated. While in the samples prepared in air, Eu³⁺ emission was found to be dominant over Tb³⁺ emission, in the samples prepared in reducing atmosphere, intense Eu²⁺ emission at 448 nm was found to be predominant over narrow Tb³⁺ emission. Luminescence studies showed that Eu³⁺ ions occupy asymmetric sites in Zn₂SiO₄ lattice. The intense f-f absorption peak of Eu³⁺ at 395 nm observed in these phosphors suggests their potential as red emitting phosphors for near ultra-violet light emitting diodes.     

Eu luminescence in tellurite glasses with gold nanostructures

Luminescence properties of Eu³⁺ doped TeO₂-PbO-GeO₂ glasses containing gold nanoparticles (NPs) were investigated. The emission spectra of the samples exhibited enhancement of Eu³⁺ luminescence due to the presence of gold NPs. The emission at 614 nm, due to the Eu³⁺ hypersensitive transition ⁵D₀-⁷F₂, is much influenced by the gold NPs and increases by ≈100% for samples heat-treated at 350 °C during 41 h.     

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.     

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.     

Sensitized red luminescence from Bi co-doped Eu: ZnO-B2O3 glasses

Photoluminescence properties of Bi³⁺ co-doped Eu³⁺ containing zinc borate glasses have been investigated and the results are reported here. Bright red emission due to a dominant electric dipole transition ⁵D₀→⁷F₂ of the Eu³⁺ ions has been observed from these glasses. The nature of Stark components from the measured fluorescence transitions of Eu³⁺ ions reveal that the rare earth ions could take the lattice sites of C_s or lower point symmetry in the zinc borate glass hosts. The significant enhancement of Eu³⁺ emission intensity by 346 nm excitation (¹S₀→³P₁ of Bi³⁺ ions) elucidates the sensitization effect of co-dopant. The energy transfer mechanism between sensitizer (Bi³⁺) and activator (Eu³⁺) ions has been explained.     

New erbium complexes emitting in infrared region based on oligothiophene and thiophenefluorene carboxylate

Lanthanide ions emitting in the near-infrared (NIR) region possess an intrinsically small molar absorption coefficient in the ultraviolet (UV)-vis-NIR spectrum, which is unfavourable for pumping efficiency. On the contrary, using organic lanthanide complexes it is possible to populate the excited state levels of the emitting ion through an efficient intramolecular energy transfer from the optically excited ligands, which act as light-harvesting antennae.With the aim of studying and maximizing the transfer to lanthanide metals, we have synthesized oligothiophene and thiophenefluorene ligands bearing carboxylate clamps able to complex erbium and other lanthanide 3⁺ ions. The complexes of {4′-(hydroxycarbonyl)methyl-[2,2′;5′,2″]terthiophen-3′-yl}acetic acid and 9-(hydroxycarbonyl)-methyl-2,7-dithien-2-yl-[fluoren-9-yl-]acetic acid with Er³⁺ and different ancillary ligands have been prepared and their optical properties were carefully studied. Moreover, relaxation dynamics measurements have been carried out on all complexes in order to determine emission lifetimes, which result to be of the order of magnitude 2 μs. Quantum chemical calculations have been performed to explain optical absorption data in terms of different coordination types. The complexes containing phenanthroline/pyridine are modelled by adding to the dianion of the ligand one univalent/divalent counterion. The absorption spectra computed in this way are in close agreement with experiment, and the univalent→divalent theoretical wavelength shift goes in the right direction. The addition of a counterion has an even bigger effect on the triplet states, and hence on their matching with the emitting states of the ion.     

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.     

Luminescence properties of europium-terbium double activated calcium tungstate phosphor

Double incorporation of Eu³⁺ and Tb³⁺ ions into a CaWO₄ crystalline lattice modifies the luminescence spectrum due to the formation of new emission centers. Depending on the activators concentration and nature, as well as on the interaction between the activators themselves, the luminescence color can be varied within the entire range of the visible spectrum. Variable luminescence was obtained when CaWO₄:Eu,Tb phosphors with 0-5 mol% activator ions were exposed to relatively low excitation energies as UV (365 and 254 nm). Under high energy excitation such as VUV (147 nm) radiation or electron beam, white light has been observed.This material with controlled properties seems to be promising for the applications in fluorescent lamps, colored lightning for advertisement industries, and other optoelectronic devices.