Green,blue, red,near-infrared up-conversion luminescence of Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript>/Tm<Superscript>3+</Superscript> co-doped YVO<Subscript>4</Subscript> nanoparticles

YVO₄:Yb³⁺,Er³⁺; YVO₄:Yb³⁺,Tm³⁺; and YVO₄:Yb³⁺,Er³⁺,Tm³⁺ were all synthesized via sol-gel method with a subsequent thermal treatment. Specifically, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ phosphors were prepared with different annealing temperatures to study the influence of temperature. The transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffractometer (XRD), and photoluminescent (PL) spectrofluorometer were used to investigate the morphology, crystal structure, and up-conversion luminescent properties of all samples. In summary, all samples were granular-like nanoparticles and well crystallized with the same tetragonal phase as YVO₄. Under the irradiation at 980 nm, YVO₄:Yb³⁺,Er³⁺ phosphors can generate green emission at 525 and 553 nm and red emission at 657 nm, while YVO₄:Yb³⁺,Tm³⁺ phosphors can generate blue emission at 476 nm, red emission at 648 nm, and near-infrared emission at 800 nm. Notably, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ samples can exhibit green emission, blue emission, red emission, and near-infrared emission at the same time, which might endow the as-prepared samples with potential applications in many fields, such as luminous paint, infrared detection, and biological label.     

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.     

Synthesis and characterization of mono-dispersed Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Er<Superscript>3+</Superscript>-coated SiO<Subscript>2</Subscript> nanoparticles by co-precipitation process

In this paper, a facile co-precipitation process for preparing mono-dispersed core–shell structure nanoparticles is reported. The 110 nm SiO₂ cores coated with an yttrium aluminum garnet (Y₃Al₅O₁₂) layer doped with Er³⁺ were synthesized and the influence of the concentration ratio of [urea]/[metal ions] on the final product was investigated. The structure and morphology of samples were characterized by the X-ray powder diffraction, Fourier transform IR spectroscopy and transmission electron microscopy, respectively. The results indicate that a layer of well-crystallized garnet Y₃Al₅O₁₂:Er³⁺ were successfully coated on the silica particles with the thickness of 20 nm. The near infrared and upconversion luminescent spectra of the SiO₂@Y₃Al₅O₁₂:Er³⁺ powders further confirm that a Y₃Al₅O₁₂:Er³⁺ coating layer has formed on the surface of silica spherical particles.     

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.     

Quantum cutting in Gd<Subscript>2</Subscript>SiO<Subscript>5</Subscript>: Eu<Superscript>3+</Superscript> by VUV excitation

The emission and excitation spectra of Gd₂SiO₅∶Eu³⁺ were investigated using the VUV beam line of the Beijing Synchrotron Radiation Facility (BSRF). The experimental results were discussed in the frame of visible quantum cutting process involved in Gd³⁺−Eu³⁺ system. Upon direct excitation into the⁶G _J states of Gd³⁺, two visible photon emissions from Eu³⁺ were observed. Cursory evaluation proved that Gd₂SiO₅∶Eu³⁺ is an efficient visible quantum cutter.     

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).     

Time-Resolved Vacuum Ultraviolet Spectroscopy of Er<Superscript>3+</Superscript> Ions in the SrF<Subscript>2</Subscript> Crystal

The photoluminescence and photoexcitation spectra as well as the luminescence decay kinetics of Er³⁺ ions in the visible ultraviolet and vacuum ultraviolet (VUV) regions have been studied by the method of low-temperature, time-resolved VUV-spectroscopy on excitation by synchrotron radiation. In the VUV spectral region of the luminescence of SrF₂:1% Er³⁺, the 146.5-nm band with a time of decay of less than 0.6 nsec was revealed together with the well-known emission band at 164.3 nm (decay constant in the microsecond range). Its possible nature is discussed. The specific features of the formation of photoexcitation spectra of the f-f and f-d transitions in the Er³⁺ ion are considered. Competition between the processes of excitation of f-f and d-f luminescence has been revealed. It manifests itself in the inverse relationship of their photoexcitation spectra in a range of energies of incident photons that are close to the position of the 4fⁿ⁻¹5d configuration levels. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 519–523, July–August, 2005.     

Luminescent properties of SrMg<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>2+</Superscript>, and Mn<Superscript>2+</Superscript> as a potential phosphor for ultraviolet light-emitting diodes

Eu²⁺ and Mn²⁺ co-doped SrMg₂(PO₄)₂ phosphors with blue and red two emission bands were prepared by the high temperature solid state method and their luminescent properties have been investigated as a function of activator and co-activator concentrations. Resonance-type energy transfers from Eu²⁺ to Mn²⁺ were discovered by directly overlapping the Eu²⁺ emission spectrum and the excitation spectrum of Mn²⁺. Efficiencies of energy transfer were also calculated according to the changes of relative intensities of Eu²⁺ and Mn²⁺ emission. According to the principle of energy transfer, we demonstrated that the phosphor SrMg₂(PO₄)₂:Eu²⁺,Mn²⁺ with double emission bands exhibited a great potential as a phosphor for ultraviolet light-emitting diodes and the relative intensities of blue and red emission could be tuned by adjusting the contents of Eu²⁺ and Mn²⁺. PACS 78.55.-m     

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.     

Polarized spectral properties of Er<Superscript>3+</Superscript> ions in NaGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

Polarized spectral properties of Er³⁺:NaGd(WO₄)₂ single crystal are reported. The crystal was grown by the Czochralski method. The Judd–Ofelt theory was applied to analyze the polarized absorption spectra and then calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios. Fluorescence decay curves of the ⁴ I _13/2, ⁴ I _11/2, and ⁴ S _3/2 multiplets for the Er³⁺ ions were measured. Stimulated emission cross-sections of the ⁴ I _13/2→⁴ I _15/2 transition obtained by the Fuchtbauer–Ladenberg formula and the reciprocity method were compared. Multi-phonon relaxation rates of the crystal were estimated. Green up-conversion fluorescence around 531 and 552 nm was observed, and the possible up-conversion mechanisms were proposed. PACS 78.20.-e; 42.70.Hj     

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.     

Synthesis and Study of the Photophysical Properties of a New Eu<Superscript>3+</Superscript> Complex with 3-Hydroxypicolinamide

This work reports on the synthesis and characterization of a new complex of Eu³⁺ with the 3-hydroxypicolinamide ligand (Hhpa). Here we present an approach for obtaining bis[2-carbamoyl(κO)pyridin-3-olato(κO’)] lanthanide complexes, which were characterized through elemental analysis, thermal analysis, infrared and photoluminescence spectroscopies (emission, excitation, luminescence lifetimes, quantum efficiencies, Judd-Ofelt parameters and quantum yields). Although hpa can act as a bidentate ligand in different conformations, the results attest for the occurrence of a unique coordination site of low symmetry for the Eu³⁺ ions, in which two anionic hpa ligands coordinate the cations through an O/O chelating system. The phosphorescence of the synthesized gadolinium complex provides the energy of the triplet state, which is determined to be at 20,830 cm^-1 over the ground state. This makes the Hhpa ligand very adequate for sensitizing the Eu³⁺ luminescence, which leads to a very efficient antenna effect and opens a wide range of applications for the complex in light emitting organic-inorganic devices.     

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 %.     

Enhanced Up-Conversion Emission in Al<Superscript>3+</Superscript> Co-Doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript>,Tm<Superscript>3+</Superscript> Powder Phosphors

Yb³⁺-Tm³⁺ co-doped up-conversion powder phosphors using Zn(Al_xGa_1-x)₂O₄ (ZAGO) as the host materials were synthesized via solid-state reaction successfully. In addition, the morphology, structural characterization and up-conversion luminescent properties were all investigated by scanning electron microscope (SEM), x-ray diffraction (XRD) and fluorescence spectrophotometer (F-7000), respectively. Under the excitation of a 980 nm laser, all as-prepared powders can carry out blue emission at about 477 nm (corresponding to ¹G₄ → ³H₆ transition of Tm³⁺ ions), and red emission at about 691 nm (attributed to ³F₃ → ³H₆ transition of Tm³⁺ ions). Also, the influence of doping Al³⁺ ions were investigated. In brief, the doping of Al³⁺ ions has no effect on the position of emission peak. Howbeit the up-conversion efficiency and intensity of ZAGO:Yb,Tm phosphors are stronger than ZGO:Yb,Tm and ZAO:Yb,Tm phosphors, while the crystallinity is the opposite. More particularly, all as-prepared powder phosphors emit strong luminescence, which is observable by the naked eye, demonstrating the potential applications in luminous paint, luminescent dye, etc.     

Photon avalanche upconversion and pump power studies in LaF<Subscript>3</Subscript>:Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> phosphor

Hexagonal LaF₃:Er³⁺/Yb³⁺ phosphor material has been synthesized by chemical precipitation method to obtain high near-infrared to green upconversion (UC) efficiency. Its thermal, structural and fluorescence properties have been studied. UC emission bands have been observed up to 315 nm in UV region. The effect of input pump power on the intensities of various emission bands has been studied in detail and photon avalanche UC mechanism has been identified. On increasing the excitation power, some bands have shown saturation in intensity. Also, at higher pump intensities two new UC bands were observed and their origin has been discussed. The phosphor has also been tested for possible UC-based fingerprint detection.     

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.     

Study of frequency upconversion in Yb<Superscript>3+</Superscript>/Eu<Superscript>3+</Superscript> by cooperative energy transfer in oxyfluoroborate glass matrix

Oxyfluoroborate glass co-doped with Eu and Yb ions has been prepared and characterized for its optical properties through photoluminescence, absorption and lifetime measurements. An intense red upconversion is observed from the ⁵D₀ level of Eu³⁺ ions through energy transfer from Yb³⁺ to Eu³⁺ ion when excited with 980 nm. The Judd–Ofelt parameters have been evaluated to estimate the local site symmetry around the Eu³⁺ ions. These parameters have been used to derive radiative properties such as transition probabilities, branching ratios, radiative lifetimes and stimulated emission cross-sections for the ⁵D₀–⁷F_J transitions. Decay of excitation of the ⁵D₀ level has been measured by monitoring the ⁵D₀→⁷F₂ transition (613 nm) at room temperature. Quantum efficiency for this transition is found to be 73%. We also monitored the effect of temperature on the fluorescence emission of Eu³⁺. PACS 42.70.Ce; 42.70.Hj     

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.     

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.     

Photolumiscent Properties of Nanorods and Nanoplates Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript>

Nanorods and nanoplates of Y₂O₃:Eu³⁺ powders were synthesized through the thermal decomposition of the Y(OH)₃ precursors using a microwave-hydrothermal method in a very short reaction time. These powders were analyzed by X-ray diffraction, field emission scanning electron microscopy, Fourrier transform Raman, as well as photoluminescence measurements. Based on these results, these materials presented nanoplates and nanorods morphologies. The broad emission band between 300 and 440 nm ascribed to the photoluminescence of Y₂O₃ matrix shifts as the procedure used in the microwave-hydrothermal assisted method changes in the Y₂O₃:Eu³⁺ samples. The presence of Eu³⁺ and the hydrothermal treatment time are responsible for the band shifts in Y₂O₃:Eu³⁺ powders, since in the pure Y₂O₃ matrix this behavior was not observed. Y₂O₃:Eu³⁺ powders also show the characteristic Eu³⁺ emission lines at 580, 591, 610, 651 and 695 nm, when excited at 393 nm. The most intense band at 610 nm is responsible for the Eu³⁺ red emission in these materials, and the Eu³⁺ lifetime for this transition presented a slight increase as the time used in the microwave-hydrothermal assisted method increases.