Energy transfer between activators at different crystallographic sites in Sr3SiO5:Eu

A yellow phosphor, Sr₃SiO₅:Eu²⁺, was synthesized by a high temperature solid-state method. Sr₃SiO₅:Eu²⁺ exhibits a single yellow emission under the blue radiation excitation. However, Sr₃SiO₅:Eu²⁺ shows a two-peak emission under the ultraviolet radiation excitation when Eu²⁺ doping content is less than 0.01 mol. Moreover, the blue emission disappears and the yellow emission reaches the peak value when Eu²⁺ doping content is 0.01 mol. Namely, the energy transfer takes place between the Eu²⁺ activators, which is located at two different crystallographic sites in the Sr₃SiO₅. And the energy transfer mechanism is the dipole-dipole interaction.     

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.     

Luminescence characteristics of Eu3+ activated borate phosphor for white light emitting diode

In this paper, the Sr₃Y₂ (BO₃)₄:Eu³⁺ phosphor was synthesized by high temperature solid-state reaction method and the luminescence characteristics were investigated. The emission spectrum exhibits one strong red emission at 613nm corresponding to the electric dipole ⁵D₀--⁷F₂ transition of Eu³⁺ under 365nm excitation, this is because Eu³⁺ substituted for Y³⁺ occupied the non-centrosymmetric position in the crystal structure of Sr₃Y₂ (BO₃)₄. The excitation spectrum indicates that the phosphor can be effectively excited by ultraviolet (254nm, 365nm and 400nm) and blue (470nm) light. The effect of Eu³⁺ concentration on the red emission of Sr₃Y₂ (BO₃)₄:Eu³⁺ was measured, the result shows that the emission intensities increase with increasing Eu³⁺ concentration, then decrease. The Commission Internationale del'Eclairage chromaticity (x, y) of Sr₃Y₂(BO₃)₄:Eu³⁺ phosphor is (0.640,0.355) at 15 mol% Eu³⁺.     

Spherical Zn2SiO4:Eu@SiO2 phosphor particles in core-shell structure: Synthesis and characterization

Spherical SiO₂ particles have been coated with Zn₂SiO₄:Eu³⁺ phosphor layers by a Pechini sol-gel process. The microstructure and luminescent properties of the obtained Zn₂SiO₄:Eu³⁺@SiO₂ particles were well characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra, and lifetime. The results demonstrate that the Zn₂SiO₄:Eu³⁺@SiO₂ particles, which have regular and uniform spherical morphology, emitted an intensive red light emission at 613 nm under excitation at 395 nm. Besides, the effects of the Eu³⁺ concentration, annealing temperature and charge compensators of Li⁺ ions on the PL emission intensities were investigated in detail.     

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.     

White light-emitting diodes using blue and yellow-orange-emitting phosphors

A blue-emitting phosphor, BaMgAl₁₀O₁₇:Eu²⁺ (BAM) and a yellow-orange phosphor, Ba²⁺-codoped Sr₃SiO₅:Eu²⁺ were prepared by the solid-state reaction. Excitation and emission spectra results showed that BAM and Ba²⁺-codoped Sr₃SiO₅:Eu²⁺ can be efficiently excited by near-ultraviolet (n-UV)-visible light from 250 to 440 nm. The effects of the doped-Eu²⁺ concentration in BAM and Ba²⁺-codoped Sr₃SiO₅:Eu²⁺ on the photoluminescence were investigated in detail. White light-emitting diodes (LED) was obtained by combining n-UV LED chip (GaN-based 380 nm emitting) with BaMgAl₁₀O₁₇:0.09Eu²⁺ and 0.1Ba²⁺-codoped Sr₃SiO₅: 0.2Eu²⁺ phosphors with the characteristic of color-rendering index of 86, CIE chromaticity coordinates (x,y) of (0.3216,0.3096), and color temperature T_c of 5700 K. As the current increases, the relative intensity simultaneously increases. The CIE chromaticity coordinates (x,y) of the white LED tends to decrease. The correlated color temperature T_c increases from 4100 to 7500 K and the color-rendering index R_a increases from 82 to 87 simultaneously.     

Effects of dopant concentrations on phosphorescence properties of Eu/Dy-doped Sr3MgSi2O8

Long afterglow Sr₃MgSi₂O₈: Eu, Dy phosphor with high brightness was prepared by sintering at high temperature and weak reductive atmosphere. The luminescent properties of this photoluminescent pigment were studied systematically by investigating concentration effects. The analytical results indicated that the main emission peaks appear at 482 nm. The excitation and emission spectra of this phosphor show that both of them are broadband. This is ascribed to the 4f⁷→4f⁶5d¹ transition of Eu²⁺ in the pigment matrix, which is in good agreement with the calculated value of 470 nm, and implies that luminescent centers Eu²⁺ occupy the deca-coordinated Sr²⁺ sites with the host of Sr₃MgSi₂O₈.     

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.     

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.     

A red oxide phosphor, Sr2ScAlO5:Eu2+ with perovskite-type structure, for white light-emitting diodes

Sr2ScAlO5:Eu2+,a red oxide phosphor with a perovskite-type structure,has been synthesized through a solid-state reaction and its luminescence properties have been investigated.An absorption band centering at 450 nm is observed from the diffuse reflection spectra and the excitation spectra,indicating that the phosphor can match perfectly with the blue light of InGaN light-emitting diodes.A broad red emission band at 620 nm is found from the emission spectra,originating from the 4f 6 5d-4f 7 transition of the Eu 2+ ions.The best doping content of Eu in this material is about 5%.Sr2ScAlO5:Eu2+is a highly promising red phosphor for use in white light-emitting diodes.     

Characterization of the VUV excitation spectrum of BaZr(BO3)2:Eu

The excitation spectra of M (M=Si⁴⁺, Ti⁴⁺) and Eu³⁺ co-doped BaZr(BO₃)₂, BaZrO₃:Eu and La₂Zr₂O₇:Eu in the vacuum ultraviolet (VUV) regions of 110-300 nm are investigated and the host-lattice absorption are characterized. The result indicated that BaZr(BO₃)₂:Eu³⁺ phosphor has a strong absorption under the VUV excitation, and in the host-lattice excitation, the strong band at 130-160 nm could be due to the BO₃ atomic groups; the band at 160-180 nm is related to the excitation of Ba-O; 180-200 nm corresponds to the charge transfer (CT) transition of Zr-O. The band at 200-235 nm due to the CT band of Eu³⁺-O²⁻ and a bond valence study explained the observed weak CT band of Eu³⁺-O²⁻ in the excitation spectra of BaZr(BO₃)₂:Eu³⁺. The emission results show that Si⁴⁺ can sensitize luminescence in the host of BaZr(BO₃)₂:Eu but Ti⁴⁺ has no improvement effect on luminescence.     

Excitation energy dependent chromaticity of BaZr(BO3)2:Eu

The luminescence properties of BaZr(BO₃)₂:5% Eu were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and different luminescence behaviors were observed by different excitation energies. After the analyses of the luminescence spectra, the result indicates that Eu³⁺ occupying non-centrosymmetric sites Ba²⁺ can be excited preferentially under 254 nm excitation, while Eu³⁺ occupying centrosymmetric sites Zr⁴⁺ can be excited preferentially under 147 nm excitation.     

Synthesis and luminescent properties of Y6W2O15:Eu phosphors

In this paper, a novel phosphor, Y₆W₂O₁₅:Eu³⁺ was synthesized by thermal decomposition and phase transition of its decatungstate gel precursor. With stepwise increase of temperature to 750 °C, a crystalline phase of Y₆W₂O₁₅:Eu³⁺forms that gives intense red emission when excited at 466 nm, the emission is attributed to the Eu³⁺ ions transitions from ⁵D₀ excited states to ⁷F_J (J=0-4) ground states. The long excitation wavelength proves the Eu³⁺ transition follows the photoexcitation of the oxygen-metal (O→W lmct) charge transfer bands in yttrium tungstate. Some structural information regarding Y₆W₂O₁₅ provided by luminescence is in accord with that characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The long-wavelength excitation properties of this material may find application in the production of red phosphors for white light-emitting diodes (LEDs).     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

Highly water resistant surface coating by fluoride on long persistent Sr4Al14O25:Eu/Dy phosphor

A novel and efficient method of providing moisture resistance of inorganic particles such as divalent europium activated strontium aluminate phosphors (Sr₄Al₁₄O₂₅:Eu²⁺/Dy³⁺) was developed by firing the phosphor in the presence of appropriate amount of ammonium fluoride at a temperature of 600-700 °C. Scanning electron microscopy, X-ray diffraction, FT-IR, EDAX and Photoluminescence measurements were carried out to characterize the uncoated and coated samples. The pH measurements were carried out for the water resistivity measurements. The phosphor particles became coated with a moisture-impervious thin coating that did not suppress the luminescence of the phosphor and can withstand complete immersion in water for long periods of time, showing very high water resistivity.     

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.     

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.     

Luminescent properties of R doped Sr2SiO4:Eu(R=Li, Na and K) red-emitting phosphors for white LEDs

Sr₂SiO₄:Eu³⁺ and Sr₂SiO₄:Eu³⁺ doped with R⁺(R⁺=Li⁺, Na⁺ and K⁺) phosphors were prepared by conventional solid-state reaction and investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. XRD patterns and SEM reveal that the optimal firing condition for Sr₂SiO₄:Eu³⁺ was 1300 °C for 4 h. The excitation and emission spectra indicate that the phosphor can be effectively excited by ultraviolet (395 nm) and blue (466 nm) light and emits intense red light peaked at around 614 nm corresponding to the ⁵D₀→⁷F₂ transitions of Eu³⁺. In the research work, the effect of R⁺ contents on luminescence property and the Eu³⁺ concentration quenching process have also been investigated. The Eu³⁺ concentration quenching mechanism was verified to be a multipole-multipole interaction and the critical energy-transfer distance was calculated to be around 14.6 Å. The dopant R⁺(R⁺=Li⁺, Na⁺ and K⁺) as charge compensator in Sr₂SiO₄:Eu³⁺ can further enhance luminescence intensity, and the emission intensity of Sr₂SiO₄:Eu³⁺ doping Li⁺ is higher than that of Na⁺ or K⁺.     

Synthesis and luminescence properties of nanocrystalline Gd2O3:Eu by combustion process

The nanocrystalline Gd₂O₃:Eu³⁺ powders with cubic phase were prepared by a combustion method in the presence of urea and glycol. The effects of the annealing temperature on the crystallization and luminescence properties were studied. The results of XRD show pure phase can be obtained, the average crystallite size could be calculated as 7, 8, 15, and 23 nm for the precursor and samples annealed at 600, 700 and 800 °C, respectively, which coincided with the results from TEM images. The emission intensity, host absorption and charge transfer band intensity increased with increasing the temperature. The slightly broad emission peak at 610 nm for smaller particles can be observed. The ratio of host absorption to O²⁻-Eu³⁺ charge transfer band of smaller nanoparticles is much stronger compared with that for larger nanoparticles, furthermore, the luminescence lifetimes of nanoparticles increased with increasing particles size. The effects of doping concentration of Eu³⁺ on luminescence lifetimes and intensities were also discussed. The samples exhibited a higher quenching concentration of Eu³⁺, and luminescence lifetimes of nanoparticles are related to annealing temperature of samples and the doping concentration of Eu³⁺ ions.     

A potential reddish orange emitting phosphor Ca2BO3Cl:Eu for white light-emitting diodes

A series of phosphors Ca₂BO₃Cl:Eu³⁺ were synthesized by using a high-temperature solid-state reaction technique, and their UV–vis luminescence properties were investigated. The f–f transitions of Eu³⁺ in the host lattice were assigned and discussed. The excitation and emission spectra indicate that this phosphor can be effectively excited by ultraviolet (394 nm), and exhibit reddish orange emission corresponding to the ⁵D₀→⁷F_J (J=0, 1, 2) transitions of Eu³⁺. The influence of the doping concentration and charge compensators on the relative emission intensity of Eu³⁺ was investigated, and the optimum doping concentration is 0.04. The critical distance R_c was estimated to be 17.1 Å in terms of the concentration quenching data. The present study suggests that Ca₂BO₃Cl:Eu³⁺ can be a potential candidate as an UV-convertible phosphor for white light-emitting diodes (LEDs).