Electronic energy relaxation and luminescence decay dynamics of Eu in Zn2SiO4:Eu phosphors

Electronic energy relaxation and decay dynamics of Eu³⁺ in Zn₂SiO₄:Eu³⁺ phosphors display evidence of intra-ion energy transfer from the ⁵D₁ to the ⁵D₀ manifold. The energy transfer timescale does not depend on Eu³⁺ concentration, or the addition of Mn²⁺ as a co-dopant and is estimated to be about 11 μs in Zn₂SiO₄. Evidence for intra-ion Eu³⁺ electronic energy transfer has also been observed in Eu-doped MgS as well as Eu³⁺ encapsulated in zeolite-Y. The energy transfer timescale in these other materials is shorter than in Zn₂SiO₄, most likely due to differences in Eu³⁺ surroundings or site symmetry.     

Combustion synthesis of Sr3MgSi2O8:Eu and Sr2MgSi2O7:Eu phosphors

Sr₃MgSi₂O₈:Eu²⁺ and Sr₂MgSi₂O₇:Eu²⁺ phosphors find uses in applications such as plasma display panel (PDP), solid-state lighting, longafter glow. Preparation of these phosphors by a modified combustion synthesis is described in this paper. As-prepared samples did not show photoluminescence. After reducing the samples at 900 °C, characteristic Eu²⁺ emission was observed. Preparation of these phosphors by using similar methods helped clarifying various results obtained for Sr₃MgSi₂O₈:Eu²⁺ by different investigators.     

Synthesis and properties of submicron range CaSO4:Eu particles

CaSO₄:Eu with particle size in submicron range was synthesized. Radiation induced Eu³⁺↔Eu²⁺ conversion as well as thermal conversion was studied. The samples showed thermal conversion above 400 °C. However, no radiation induced conversion in submicron range particles was observed. Particles heated above 400 °C coalesce and when heated at 925 °C bigger particles of 20 μm size were formed. Optical microscopy of these particles reveals red inclusion of about 5 μm inside CaSO₄ particle. It is speculated that the red inclusion is CaS:Eu²⁺.     

Emission properties of Eu, Mn in MAl2Si2O8 (M=Sr, Ba)

The emission properties of Eu²⁺ and Mn²⁺ in monoclinic SrAl₂Si₂O₈ (M-SAS) and hexagonal BaAl₂Si₂O₈ (H-BAS), both of which have only one alkaline-earth site, were studied. The emission peaks of both Eu²⁺ (405 nm) and Mn²⁺ (564 nm) in SrAl₂Si₂O₈, are located at longer wavelengths, compared with those in H-BAS (373 nm for Eu²⁺ and 518 nm for Mn²⁺), because of the stronger crystal field strength at the Sr site. EPR spectra showed that the g values of Mn²⁺ are 4.5065 in M-SAS:Mn and 2.0247 in H-BAS:Mn. Magnetic measurements proved that Mn²⁺ was at high-spin state in both hosts. The large g value of Mn²⁺ in M-SAS was ascribed to the mixing of the first excitation state to the ground state, both of which have lower d orbital degeneracy due to the lower symmetry of Mn²⁺ site. The transfer efficiency from Eu²⁺ to Mn²⁺was about 10% in M-SAS, higher than that in H-BAS (5%). This was probably because Eu²⁺ emission overlaps the relatively low excitation level of Mn²⁺ in M-SAS. In order to obtain high transfer efficiency, it was necessary for the Eu²⁺ emission to overlap the lowest excitation level of Mn²⁺. The results obtained in this work may be helpful to design the new white or red phosphors for white-light emitting diode (w-LED) applications.     

Luminescence spectroscopy of Eu in Ca3Sc2Si3O12

Ca₃Sc₂Si₃O₁₂ doped with 1 mol% Eu³⁺ and having a cubic garnet structure was prepared by a solid state reaction. The low temperature luminescence spectrum shows no measurable ⁵D₀→⁷F₀ band, in agreement with the location of the lanthanide dopant in a site of D₂ symmetry, i.e. with a Ca²⁺ substitution. On the other hand, the spectrum is clearly dominated by the ⁵D₀→⁷F₄ band, which is significantly stronger than that for the other transitions originating from the ⁵D₀ level. This unusual behavior is explained on the basis of a model describing the distortion of the EuO₈ coordination polyhedron from a cubic geometry to the actual D₂ one.     

Luminescence of BaBr2:Eu under hydrostatic pressure

Luminescence spectra of BaBr₂:Eu²⁺ have been measured under pressures up to 27 GPa at room temperature. In the low-pressure range a red-shift of the 5d-4f transition of −225 cm⁻¹/GPa is observed. From 3 to 10 GPa a phase mixture of the original orthorhombic phase and the high-pressure monoclinic phase gives rise to two luminescence bands. Above 10 GPa the crystal is completely transformed to its high-pressure phase where two different Eu²⁺ sites exist of which, however, only one 5d-4f transition is detected. This transition exhibits a red-shift of −200 cm⁻¹/GPa. The shifts are compared with other literature data.     

Effect of cation mixing on luminescence of Eu in SrCl2

SrCl₂ forms solid solutions of various compositions with BaCl₂ and CaCl₂. Orthorhombic and tetragonal phases are also known to occur in the SrCl₂-CaCl₂ system. Though efficient emission of Eu²⁺ in SrCl₂ and CaCl₂ is known, there are no reports available on Eu²⁺ luminescence in the solid solutions and other phases. Investigations on these lines were carried out and the results are presented in this paper. Very intense emission comparable to that of the commercial lamp phosphors was observed for the solid solutions while the intensities for the orthorhombic and tetragonal phases are an order of magnitude smaller. Emission wavelengths do not vary much while excitation spectra for the solid solutions are much enhanced in the near UV region (350-400 nm). It is suggested that these results will be significant for developing phosphors with near UV excitations needed in applications such as solid state lighting.     

Luminescence and EPR studies of Eu doped BaAl12O19 blue light emitting phosphors

Europium doped BaAl₁₂O₁₉ powder phosphors have been synthesized by combustion process within few minutes. The phosphors have been characterized by XRD, SEM, FT-IR, EPR and PL techniques. The EPR spectrum exhibits an intense resonance signal at g=1.96 characteristic of Eu²⁺ ions. In addition to this two weak resonance signals have been observed at g=2.28 and g=4.86. The population of the spin levels (N) for the resonance signal at g=1.96 is calculated as a function of temperature. By post-treating the phosphor at 1350 °C under a reducing atmosphere, it is observed that the population of spin levels has been increased five times. The excitation spectrum shows a peak at 326 nm with a shoulder at 290 nm. Upon excitation at 326 nm, the emission spectrum exhibits a well defined broad band with maximum at 444 nm emitting a blue light corresponding to 4f⁶5d→4f⁷ transition. The luminescence intensity also has been enhanced to 60% by post-treating the phosphor at 1350 °C under a reducing atmosphere.     

Novel green-emitting Na2CaPO4F:Eu phosphors for near-ultraviolet white light-emitting diodes

In this study, green-emitting Na₂CaPO₄F:Eu²⁺ phosphors were synthesized by solid-state reactions. The excitation spectra of the phosphors showed a broad hump between 250 and 450 nm; the spectra match well with the near-ultraviolet (NUV) emission spectra of light-emitting diodes (LEDs). The emission spectrum showed an intense broad emission band centered at 506 nm. White LEDs were fabricated by integrating a 390 nm NUV chip comprising blue-emitting BaMgAl₁₀O₁₇:Eu²⁺, green-emitting Na₂CaPO₄F:0.02 Eu²⁺, and red-emitting CaAlSiN₃:Eu²⁺ phosphors into a single package; the white LEDs exhibited white light with a correlated color temperature of 5540 K, a color-rendering index of 90.75, and color coordinates (0.332, 0.365) close to those of ideal white light.     

Thermoluminescence spectroscopy of Eu and Mn doped BaMgAl10O17

Thermoluminescence (TL) studies of Eu²⁺ and Mn²⁺ doped BaMgAl₁₀O₁₇ (BAM) are reported and discussed. The TL spectra that are measured after irradiation with ultraviolet (120-) show a series of TL peaks between 100 and . The TL spectra are similar for BAM with the two dopants, which suggest that the shallow traps are typical for the BAM host lattice. Using the Hoogstraaten analysis trap depths between 0.1 and are determined. A model is proposed based on thermally activated recombination in local TL centres (not via the conduction band). Further support for this model is obtained from the observation that the TL signal is strongest for excitation around the band edge of BAM . Upon heating the samples in air all low temperature TL peaks decrease in intensity. In addition a new peak appears in the TL spectrum, which is connected with a deeper trap and also a partial oxidation of Eu²⁺ to Eu³⁺ is observed. The luminescence efficiency is lower and the UV induced degradation is faster after annealing in air. These results indicate that the shallow traps are related to oxygen vacancies. The shallow traps do not have a negative influence on performance (efficiency and degradation) of BAM as a lighting phosphor. The luminescence efficiency and stability are strongly influenced by the formation of Eu³⁺ and a deeper trap during annealing in air. Subsequent annealing in a reducing atmosphere restores the original properties.     

Investigation of ageing characteristics and identification of surface chemical changes on SrGa2S4:Ce display phosphor under electron beam bombardment

Cathodoluminescent ageing characteristics of SrGa₂S₄:Ce³⁺ under prolonged electron beam bombardment was studied and the data are presented. The cathodoluminescent intensity with an increasing Coulomb loading was observed to degrade under different primary electron beam voltages. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a loss in S and C and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of a SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs).     

Synthesis and luminescence properties of Eu, Ce and Tb-activated Sr3La2(BO3)4 under UV excitation

The spectroscopic properties in UV-excitable range for the phosphors of Sr₃La₂(BO₃)₄:RE³⁺ (RE³⁺=Eu³⁺, Ce³⁺, Tb³⁺) were investigated. The phosphors were synthesized by conventional solid-state reactions. The photoluminescence (PL) spectra and commission international de I'Eclairage (CIE) coordinates of Sr₃La₂(BO₃)₄:RE³⁺ were investigated. The f-d transitions of Eu³⁺, Ce³⁺ and Tb³⁺ in the host lattices are assumed and corroborated. The PL and PL excitation (PLE) spectra indicate that the main emission wavelength of Sr₃La₂(BO₃)₄:Eu³⁺ is 611 nm, and Sr₃La₂(BO₃)₄:Ce³⁺ shows dominating emission peak at 425 nm, while Sr₃La₂(BO₃)₄:Tb³⁺ displays green emission at 487, 542, 582 and 620 nm. These phosphors were prepared by simple solid-state reaction at 1000 °C. There are lower reactive temperature and more convenient than commercial phosphors. The Sr₃La₂(BO₃)₄:Tb³⁺ applied to cold cathode fluorescent lamp was found to emit green light and have a major peak wavelength at around 542 nm. These phosphors may provide a new kind of luminescent materials under ultraviolet excitation.     

Synthesis and properties of colloidal ternary ZnGa2O4:Eu nanocrystals

A nonhydrolytic hot solution synthesis technique was used to grow monodisperse ternary oxide nanocrystals of ZnGa₂O₄:Eu³⁺. The shape of ZnGa₂O₄:Eu³⁺ nanocrystals was a function of the type of precursor, and their size was controlled by changing the concentration ratio of Zn precursor to surfactant. The crystal structure of synthesized ZnGa₂O₄ nanocrystals was a cubic spinel with no detectable secondary phases. Photoluminescence of red-emitting ZnGa₂O₄:Eu³⁺ nanocrystals resulted in a high (⁵D₀-⁷F₂)/(⁵D₀-⁷F₁) intensity ratio, suggesting that the Eu³⁺ ions occupy tetrahedral Zn²⁺ sites or distorted octahedral Ga³⁺ sites with no inversion symmetry in ZnGa₂O₄ nanocrystals.     

Sol-gel growth of Gd2MoO6:Eu nanocrystalline layers on SiO2 spheres (SiO2@Gd2MoO6:Eu) and their luminescent properties

SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors were prepared by the sol-gel process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra as well as kinetic decays were used to characterize the resulting SiO₂@Gd₂MoO₆:Eu³⁺ core-shell phosphors. The XRD results demonstrate that the Gd₂MoO₆:Eu³⁺ layers on the SiO₂ spheres begin to crystallize after annealing at 600 °C and the crystallinity increases with raising the annealing temperature. The obtained core-shell phosphors have a near perfect spherical shape with narrow size distribution (average size ca. 600 nm), are not agglomerated, and have a smooth surface. The thickness of the Gd₂MoO₆:Eu³⁺ shells on the SiO₂ cores could be easily tailored by varying the number of deposition cycles (50 nm for four deposition cycles). The Eu³⁺ shows a strong PL luminescence (dominated by ⁵D₀-⁷F₂ red emission at 613 nm) under the excitation of 307 nm UV light. The PL intensity of Eu³⁺ increases with increasing the annealing temperature and the number of coating cycles.     

Energy transfers between Sm and Eu in YPO4, LaP5O14 and LaP3O9 phosphates. Potential quantum cutters for red emitting phosphors

This work concerns the studies of energy transfers between Sm³⁺ and Eu³⁺ ions in some phosphates as new luminescent materials emitting in the orange-red color. The choose of ions is based on the possibility of quantum cutting process and the matrices are selected according to the 5d bands position of Sm³⁺ ion. The Sm³⁺ and Eu³⁺ doped YPO₄, LaP₅O₁₄ and LaP₃O₉ are synthesized and spectroscopic studies in ultraviolet and vacuum ultraviolet ranges have been achieved.     

A white light emitting phosphor Sr1.5Ca0.5SiO4:Eu, Tb, Eu for LED-based near-UV chip: Preparation, characterization and luminescent mechanism

In this work, we report preparation, characterization and luminescent mechanism of a phosphor Sr_1.5Ca_0.5SiO₄:Eu³⁺,Tb³⁺,Eu²⁺ (SCS:ETE) for white-light emitting diode (W-LED)-based near-UV chip. Co-doped rare earth cations Eu³⁺, Tb³⁺ and Eu²⁺ as aggregated luminescent centers within the orthosilicate host in a controlled manner resulted in the white-light phosphors with tunable emission properties. Under the excitation of near-UV light (394 nm), the emission spectra of these phosphors exhibited three emission bands: one broad band in the blue area, a second band with sharp lines peaked in green (about 548 nm) and the third band in the orange-red region (588-720 nm). These bands originated from Eu²⁺ 5d→4f, Tb³⁺⁵D₄→⁷F_J and Eu³⁺⁵D₀→⁷F_J transitions, respectively, with comparable intensities, which in return resulted in white light emission. With anincrease of Tb³⁺ content, both broad Eu²⁺ emission and sharp Eu³⁺ emission increase. The former may be understood by the reduction mechanism due to the charge transfer process from Eu³⁺ to Tb³⁺, whereas the latter is attributed to the energy transfer process from Eu²⁺ to Tb³⁺. Tunable white-light emission resulted from the system of SCS:ETE as a result of the competition between these two processes when the Tb³⁺ concentration varies. It was found that the nominal composition Sr_1.5Ca_0.5SiO₄:1.0%Eu³⁺, 0.07%Tb³⁺ is the optimal composition for single-phased white-light phosphor. The CIE chromaticity calculation demonstrated its potential as white LED-based near-UV chip.     

Thermoluminescent and stuctural properties of BaAl2O4:Eu,Nd,Gdphosphors prepared by combustion method

BaAl₂O₄:Eu²⁺,Nd³⁺,Gd³⁺ phosphors were prepared by a combustion method at different initiating temperatures (400–1200 °C), using urea as a comburent. The powders were annealed at different temperatures in the range of 400–1100 °C for 3 h. X-ray diffraction data show that the crystallinity of the BaAl₂O₄ structure greatly improved with increasing annealing temperature. Blue-green photoluminescence, with persistent/long afterglow, was observed at 498 nm. This emission was attributed to the 4f⁶5d¹–4f⁷ transitions of Eu²⁺ ions. The phosphorescence decay curves were obtained by irradiating the samples with a 365 nm UV light. The glow curves of the as-prepared and the annealed samples were investigated in this study. The thermoluminescent (TL) glow peaks of the samples prepared at 600 °C and 1200 °C were both stable at ∼72 °C suggesting that the traps responsible for the bands were fixed at this position irrespective of annealing temperature. These bands are at a similar position, which suggests that the traps responsible for these bands are similar. The rate of decay of the sample annealed at 600 °C was faster than that of the sample prepared at 1200 °C.     

PL and CL degradation and characteristics of SrAl2O4: Eu,Dy phosphors

Strontium aluminate phosphors are ideal for luminescent infrastructure materials. Their brightness and persistent glow time are much higher than previously used sulphide phosphors. Strontium aluminates prepared by the sol–gel and combustion methods are compared with commercially available strontium aluminate. High luminescent efficient SrAl₂O₄:Eu²⁺,Dy³⁺ pulsed laser deposited (PLD) thin films were also produced using the commercially available powder. Photoluminescence (PL) degradation studies showed that the phosphor intensity decreased about 20% over a period of 2 weeks under ultraviolet (UV) irradiation. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) showed that cathodoluminescence (CL) degradation is due to the formation of SrO due to electron stimulated surface reactions. The light output mechanism of the phosphor is also discussed in more detail.     

Surface characterization and luminescent properties of SrAl2O4:Eu,Dy nano thin films

SrAl₂O₄:Eu²⁺, Dy³⁺ thin films were grown on Si (1 0 0) substrates in different atmospheres using the pulsed laser deposition (PLD) technique. The effects of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological and photoluminescence (PL) properties of the films were investigated. The films were ablated using a 248 nm KrF excimer laser. Improved PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres compared to those prepared in vacuum. A stable green emission peak at 520 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions was obtained. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The diffusion of adventitious C into the nanostructured layers deposited in the Ar and O₂ atmospheres was most probably responsible for the quenching of the PL intensity after annealing.     

Phosphorescent and thermoluminescent properties of SrAl2O4:Eu,Dy phosphors prepared by solid state reaction method

Long persistent SrAl₂O₄:Eu²⁺ phosphors co-doped with Dy³⁺ were prepared by the solid state reaction method. The main diffraction peaks of the monoclinic structure of SrAl₂O₄ were observed in all the samples. The broad band emission spectra at 497 nm for SrAl₂O₄:Eu²⁺, Dy³⁺ were observed and the emission is attributed to the 4f⁶5d¹ to 4f⁷ transition of Eu²⁺ ions. The samples annealed at 1100–1200 °C showed similar broad TL glow curves centered at 120 °C. The similar TL glow curves suggest that the traps responsible for them are similar. The long afterglow displayed by the phosphors annealed at different temperatures, may be attributed to the Dy³⁺ ions acting as the hole trap levels, which play an important role in prolonging the duration of luminescence.