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.     

Photoluminescence and phosphorescence properties of MAl2O4:Eu, Dy (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C

Eu²⁺ and Dy³⁺ co-doped calcium aluminate, barium aluminate and strontium aluminate phosphors were synthesized at an initiating combustion temperature of 500 °C using urea as an organic fuel. The crystallinity of the phosphors was investigated by using X-ray diffraction (XRD) and the morphology was determined by a scanning electron microscope (SEM). The low temperature monoclinic structure for both CaAl₂O₄ and SrAl₂O₄ and the hexagonal structure of BaAl₂O₄ were observed. The effect of the host materials on the photoluminescence (PL) and phosphorescence properties were investigated by using a He-Cd Laser and a Cary Eclipse fluorescence spectrophotometer, respectively. The broad band emission spectra observed at 449 nm for CaAl₂O₄:Eu²⁺, Dy³⁺, 450 nm (with a shoulder-peak at 500 nm) for BaAl₂O₄:Eu²⁺, Dy³⁺ and 528 nm for SrAl₂O₄:Eu²⁺, Dy³⁺ are attributed to the 4f⁶5d¹ to 4f⁷ transition in the Eu²⁺ ion in the different hosts.     

The effects of substrate temperature on the structure, morphology and photoluminescence properties of pulsed laser deposited SrAl2O4:Eu,Dy thin films

In this study, SrAl₂O₄:Eu²⁺,Dy³⁺ thin film phosphors were deposited on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique. The films were deposited at different substrate temperatures in the range of 40-700 °C. The structure, morphology and topography of the films were determined by using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) data was collected in air at room temperature using a 325 nm He-Cd laser as an excitation source. The PL spectra of all the films were characterized by green phosphorescent photoluminescence at ∼530 nm. This emission was attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺. The highest PL intensity was observed from the films deposited at a substrate temperature of 400 °C. The effects of varying substrate temperature on the PL intensity were discussed.     

Sr2CeO4:Eu and Sr2CeO4:5 mol% Eu, 3 mol% Dy microphosphors: Wet chemistry synthesis from hybrid precursor and photoluminescence properties

In this article, Sr₂CeO₄:x mol% Eu³⁺ and Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors were synthesized from assembling hybrid precursors by wet chemical method. As-prepared samples present uniform grain-like morphology and the particle size is about 0.2 μm. The luminescence spectra of Sr₂CeO₄:x mol% Eu³⁺ have been measured to examine the influence of the intensity of red emission lines for Eu³⁺ on the concentration of Eu³⁺, showing that the intensity of the red emission increases with an increase of the concentration from 1 to 5 mol%. Additionally, from the emission spectra of Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors, the characteristic lines of Dy³⁺ have also been observed. This result indicates that there also exists an energy transfer process between Sr₂CeO₄ and Dy³⁺.     

Radioluminescence properties of rare earths doped SrAl2O4 nanopowders

Nanopowders of SrAl₂O₄ pure and doped with rare earths were prepared via a proteic sol-gel methodology. The prepared materials presented a single crystalline phase, confirmed by XRD measurements. AFM results indicate that the average particle size is about 53 nm for SrAl₂O₄ powders. The radioluminescence spectrum of SrAl₂O₄: Eu²⁺, Dy³⁺ is composed by two intense peaks around 520 and 570 nm followed by a weaker emission peaking at 615 nm. It was observed that the intensity of RL emission during irradiation with X-rays decreased as a function of the irradiation time, indicating the build up of radiation damage in the nanopowders. The irradiated samples exhibited a persistent radiation damage that changes the colour of the sample, and also influenced the reduction in the scintillation efficiency. The saturation level of SrAl₂O₄: Eu²⁺ is 96%, exhibiting good resistance to radiation damage.     

Synthesis and luminescence properties of SrAl2O4:Eu,Dy nanosheets

We report an effective method to synthesize SrAl₂O₄:Eu²⁺,Dy³⁺ nanosheets. Sheet-like precursors were firstly synthesized by the solvothermal method, and acetate was used as the raw material. Then the final products were obtained by calcinating the precursor in a weak reductive atmosphere of H₂. The crystal structure and particle morphology were investigated by X-ray diffraction (XRD) patterns, field-emission scanning electron microscopy and transmission electron microscopy (FE-SEM and TEM) , respectively. A possible growth mechanism was proposed to reveal the formation process. Luminescence properties of the SrAl₂O₄:Eu²⁺,Dy³⁺ long-lasting phosphor were analyzed by measuring the excitation spectra, the emission spectra, the afterglow decay curve and the thermoluminescence curve. Both the photoluminescence (PL) spectra and luminance decay revealed that the phosphors had efficient luminescent and long lasting properties compared with the phosphors prepared by using nitrates as the raw material. Furthermore, the photoluminescence intensity was even a little higher than commercial phosphors.     

The effect of different gas atmospheres on luminescent properties of pulsed laser ablated SrAl2O4:Eu,Dy thinfilms

SrAl₂O₄:Eu²⁺,Dy³⁺ thin films were grown on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique to investigate the effect of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological, photoluminescence (PL) and cathodoluminescence (CL) properties of the films. The films were ablated using a 248 nm KrF excimer laser. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and fluorescence spectrophotometry were used to characterize the thin films. Auger electron spectroscopy (AES) combined with CL spectroscopy were employed for the surface characterization and electron-beam induced degradation of the films. Better PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres with respect to those prepared in vacuum. A stable green emission peak at 515 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions were obtained with less intense peaks at 619 nm, which were attributed to transitions in Eu³⁺. 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 CL intensity increased under prolonged electron bombardment during the removal of C due to electron stimulated surface chemical reactions (ESSCRs) on the surface of the SrAl₂O₄:Eu²⁺, Dy³⁺ thin films. The CL stabilized and stayed constant thereafter.     

Persistent luminescence dosimetric properties of UV-irradiated SrAl2O4:Eu, Dy phosphor

Current radiation dosimetry methods involve the release of trapped charge carriers in the form of electrons-holes pairs generated by irradiation exposure of the dosimetric materials. Thermal and optical stimulations of the irradiated material freed the trapped charges that eventually recombine with interband centers producing the emission of light. The integrated intensity of the emitted light is proportional to the radiation dose exposure. In this work, we present an UV radiation dosimetry technique based on the characteristic persistence luminescence (PLUM) 4f⁶5d¹→4f⁷ electronic transition of Eu²⁺ ions in SrAl₂O₄:Eu²⁺, Dy³⁺. The dose assessment is carried out by measuring the PLUM signal integrated during a certain time. The PLUM performance of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor exhibited a linear behavior for the first 50 s of UV irradiation. For higher UV time exposure the behavior is sublinear with no apparent saturation during a 10 min period. The PLUM dosimetry response was performed at 400 nm that corresponds to the main band component of the PLUM excitation spectrum in the 250-500 nm range. The main advantage of a dosimeter device based on the PLUM of SrAl₂O₄:Eu²⁺, Dy³⁺ is that neither thermal nor optical stimulation is required, avoiding the need of cumbersome electronic photo/thermal stimulation equipment. Due to the highly efficient 250-500 nm PLUM response of SrAl₂O₄:Eu²⁺, Dy³⁺, it could have potential application as UV radiation dosimeter in the UV range of grate human health concerns caused by UV solar radiation.     

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.     

Synthesis and characterization of BaAl2O4:Eu co-doped with different rare earth ions

Combustion method was used in this study to prepare BaAl₂O₄:Eu²⁺ phosphors co-doped with different trivalent rare-earths (Re³⁺=Dy³⁺, Nd³⁺, Gd³⁺, Sm³⁺, Ce³⁺, Er³⁺, Pr³⁺ and Tb³⁺) ions at an initiating temperature of 600 °C. The phosphors were annealed at 1000 °C for 3 h. As confirmed from the X-ray diffraction (XRD) data, both as prepared and post annealed samples crystallized in the well known hexagonal structure of BaAl₂O₄. All samples exhibited bluish-green emission associated with the 4f⁶5d¹→4f⁷ transitions of Eu²⁺ at ∼500 nm. Although the highest intensity was observed from Er³⁺ co-doping, the longest afterglow (due to trapping and detrapping of charge carriers) was observed from Nd³⁺ followed by Dy³⁺ co-doping. The traps responsible for the long afterglow were studied using thermoluminescence (TL) spectroscopy.     

Luminescence studies on SrMgAl10O17:Eu, Dy phosphor crystals

Using urea as fuel, SrMgAl₁₀O₁₇:Eu, Dy phosphor was prepared by a combustion method. Its luminescence properties under ultraviolet (UV) excitation were investigated. Pure SrMgAl₁₀O₁₇ phase was formed by urea-nitrate solution combustion synthesis at 550 °C. The results indicated that the emission spectra of SrMgAl₁₀O₁₇:Eu, Dy has one main peak at 460 nm and one shoulder peak near 516 nm, which are ascribed to two different types of luminescent Eu²⁺ centers existing in the SrMgAl₁₀O₁₇ matrix crystal. The blue luminescence emission of SrMgAl₁₀O₁₇:Eu phosphors was improved under UV excitation by codoping Dy³⁺ ions. The SrMgAl₁₀O₁₇:Eu phosphors showed green afterglow (λ=516 nm) when Dy³⁺ ions were doped. Dy³⁺ ions not only successfully play the role of sensitizer for energy transfer in the system, but also act as trap levels and capture the free holes in the spinel blocks.     

Photoluminescence and afterglow behavior of Eu, Dy and Eu, Dy in Sr3Al2O6 matrix

This paper reports the photoluminescence and afterglow behavior of Eu²⁺ and Eu³⁺ in Sr₃Al₂O₆ matrix co-doped with Dy³⁺. The samples containing Eu²⁺ and Eu³⁺ were prepared via solid-state reaction. X-ray diffraction (XRD), photo luminescent spectroscope (PLS) and thermal luminescent spectroscope (TLS) were employed to characterize the phosphors. The comparison between the emission spectra revealed that Sr₃Al₂O₆ phosphors doped with Eu²⁺, Dy³⁺ and Eu³⁺, Dy³⁺ showed different photoluminescence. The phosphor doped with Eu³⁺, Dy³⁺ showed an intrinsic f-f transition generated from Eu³⁺, with two significant emissions at 591 and 610 nm. However, the phosphor doped with Eu²⁺, Dy³⁺ revealed a broad d-f emission centering around 512 nm. After the UV source was turned off, Eu²⁺, Dy³⁺ activated Sr₃Al₂O₆ phosphor showed excellent afterglow while Eu³⁺, Dy³⁺ activated phosphor almost showed no afterglow. Thermal simulated luminescence study indicated that the persistent afterglow of Sr₃Al₂O₆: Eu²⁺, Dy³⁺ phosphor was generated by suitable electron traps formed by the co-doped rare-earth ions (Dy³⁺) within the host.     

Temperature dependence of persistent luminescence in β-irradiated SrAl2O4:Eu, Dy phosphor

SrAl₂O₄:Eu²⁺, Dy³⁺ is a phosphor characterized by a long persistent luminescence (PLUM) when excited with UV-vis light and ionizing radiation exhibiting intensity variation in the 10-320 K temperature range and maximum intensity around 320 K. In this work, we study the PLUM behavior of SrAl₂O₄:Eu²⁺, Dy³⁺ as a function of temperature from room temperature to 670 K in samples exposed to β irradiation. The room-temperature irradiation followed by PLUM readout revealed an integrated PLUM maximum at 323 K decreasing later. In contrast, irradiation and PLUM readout at temperatures above room temperatures produced integrated PLUM intensities maxima around 425 and 625 K. Successive cycles of preheating followed by irradiation and PLUM readout produced an increasing of the PLUM intensity as a function of cycle number. The observed phenomenon was ascribed to trapped electrons at the multiple trapping states related to the 425 and 625 K defects levels and electron transfer from one trap to another (electron hopping). Eventually, there is a return to the 5d level of Eu³⁺ cations with the characteristic PLUM emission by thermal energy supplied at room temperature (lattice vibrations) or by a preheating-irradiation-readout cycle. This property may allow keeping up the PLUM properties of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors through background radiation self exposure and adequate heating processes.     

The influence of the number of pulses on the morphological and photoluminescence properties of SrAl2O4:Eu,Dy thin films prepared by pulsed laser deposition

The current work reports on the influence of the number of laser pulses on the morphological and photoluminescence properties of SrAl₂O₄:Eu²⁺,Dy³⁺ thin films prepared by the pulsed laser deposition (PLD) technique. Atomic force microscopy (AFM) was used to study the surface topography and morphology of the films. The AFM data showed that the film deposited using a higher number of laser pulses was packed with a uniform layer of coarse grains. In addition, the surface of this film was shown to be relatively rougher than the films deposited at a lower number of pulses. Photoluminescence (PL) data were collected using the Cary Eclipse fluorescence spectrophotometer equipped with a monochromatic xenon lamp. An intense green photoluminescence was observed at 517 nm from the films prepared using a higher number of laser pulses. Consistent with the PL data, the decay time of the film deposited using a higher number of pulses was characteristically longer than those of the other films. The effects of laser pulses on morphology, topography and photoluminescence intensity of the SrAl₂O₄:Eu²⁺,Dy³⁺ thin films are discussed.     

Eu/Tb ions co-doped white light luminescence Y2O3 phosphors

Y₂O₃:Eu³⁺, Tb³⁺ phosphors with white emission are prepared with different doping concentration of Eu³⁺ and Tb³⁺ ions and synthesizing temperatures from 750 to 950 °C by the co-precipitation method. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the crystallinity of the synthesized samples increases with enhancing the firing temperature. The photoluminescence spectra indicate the Eu³⁺ and Tb³⁺ co-doped Y₂O₃ phosphors show five main emission peaks: three at 590, 611 and 629 nm originate from Eu³⁺ and two at 481 and 541 nm originate from Tb³⁺, under excitation of 250-320 nm irradition. The white light luminescence color could be changed by varying the excitation wavelength. Different concentrations of Eu³⁺ and Tb³⁺ ions were induced into the Y₂O₃ lattice and the energy transfer from Tb³⁺→Eu³⁺ ions in these phosphors was found. The Commission International de l’Eclairage (CIE) chromaticity shows that the Y₂O₃:Eu³⁺, Tb³⁺ phosphors can obtain an intense white emission.     

Red emission in B- and Li-doped SrAl2O4:Eu phosphor under UV excitation

Samples of SrAl₂O₄:Eu³⁺ doped with B³⁺ and SrAl₂O₄:Eu³⁺ co-doped with B³⁺ and Li⁺ have been prepared by the solid-reaction method. The influence of B³⁺ and Li⁺ contents on luminescence property has been investigated. It is found that the substitution of B³⁺ for Al³⁺ greatly improves red emission intensity at 591, 615 and 701 nm. The dopant Li⁺ as charge compensator in SrAl₂O₄:Eu³⁺, B³⁺ can further enhance luminescence intensity. The strongest red emission is obtained in the Sr(Al_1.9, B_0.1)O₄:Eu_0.02³⁺, Li⁺_0.02 sample. The developed phosphors can be efficiently excited by ultraviolet (UV) light from 350 to 480 nm, which indicates that B³⁺ and Li⁺ co-doped SrAl₂O₄:Eu³⁺ is a good candidate phosphor applied in solid-state lighting in conjunction with white UV light-emitting diodes (LEDs).     

Luminescence in trivalent rare earth activated Sr4Al2O7 phosphor

In this paper we report the combustion synthesis of trivalent rare-earth (RE³⁺ = Dy, Eu and Ce) activated Sr₄Al₂O₇ phosphor. The prepared phosphors were characterized by the X-ray powder diffraction (XRD) and photoluminescence (PL) techniques. Photoluminescence emission peaks of Sr₄Al₂O₇:Dy³⁺ phosphor at 474 nm and 578 nm in the blue and yellow region of the spectrum. The prepared Eu³⁺ doped phosphors were excited by 395 nm then we found that the characteristics emission of europium ions at 615 nm (⁵D₀?⁷F₂) and 592 nm (⁵D₀?⁷F₁). Photoluminescence (PL) peaks situated at wavelengths of 363 and 378 nm in the UV region under excitation at around 326 nm in the Sr₄Al₂O₇:Ce³⁺ phosphor.     

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.     

Photoluminescence and gamma-ray irradiation of SrAl2O4:Euand Y2O3:Eu phosphors

Y₂O₃:Eu³⁺ phosphor is a very attractive material for use as a red phosphor in many fields. SrAl₂O₄:Eu²⁺ belongs to long lasting phosphor (LLP) and it is a useful bluish-green luminescence material, which can also be a promising candidate as a simple and easy-to-use radiation detection element for visual display of two dimensional radiation distributions. In the present study, both these two kinds of phosphors were synthesized using high temperature solid state reactions. In our work, the influence of gamma-ray irradiation on the properties of these two kinds of phosphors was studied by comparing photoluminescence, brightness and the decay curve of unirradiated and gamma-ray-irradiated samples. Conclusions from the present work can be briefly summarized as follows. In irradiated samples, the brightness is decreased without sensible change in the wavelength distribution of the luminescence spectrum and in the decay kinetic upon gamma exposure. Moreover, the emission due to Eu³⁺→Eu²⁺ conversion in Y₂O₃:Eu³⁺ phosphors was not observed in our sample after irradiation to high exposure. Also the brightness of SrAl₂O₄:Eu²⁺ phosphor turned out to decrease after the exposition to ionizing radiation while the luminescence wavelength distribution remained unchanged. The reason for the effect of gamma-ray irradiation on the properties of phosphors is also discussed in the paper.     

Self-assembled CaMoO4 and CaMoO4:Eu hierarchical superstructures: Facile sonochemical route synthesis and tunable luminescent properties

Tetragonal CaMoO₄ and CaMoO₄:Eu³⁺ with various novel three-dimensional (3D) hierarchical architectures were successfully synthesized via a facile, efficient sonochemistry process in the absence of any surfactant or template. XRD, EDS, FE-SEM, and photoluminescence (PL) were employed to characterize the as-obtained products. It was found that morphology modulation could be easily realized by changing pH value of the precursor. The pH value of the precursor not only affected the substructures of the hierarchical structures, but also determined the size distributions of the final products. The formation mechanism for different hierarchical architectures was proposed on the basis of time-dependent experiments. The luminescence spectra showed that CaMoO₄:Eu³⁺ phosphors can be effectively excited by the near ultraviolet (UV) (396 nm) and blue (466 nm) light, and exhibited strong red emission around 615 nm, which was attributed to the Eu³⁺⁵D₀→⁷F₂ transition. Compared with Y₂O₃:Eu³⁺ phosphor, CaMoO₄:Eu³⁺ is much more stable, efficient and suitable, therefore, this phosphors could be a promising red component for possible applications in the field of LEDs.