Luminescent features of sol–gel derived rare-earth multi-doped oxyfluoride nano-structured phosphors for white LED application

Rare-earth doped oxyfluoride 75SiO₂:25PbF₂ nano-structured phosphors for white-light-emitting diodes were synthesized by thermal treatment of precursor sol–gel derived glasses. Room temperature luminescence features of Eu³⁺, Sm³⁺, Tb³⁺, Eu³⁺/Tb³⁺, and Sm³⁺/Tb³⁺ ions incorporated into low-phonon-energy PbF₂ nanocrystals dispersed in the aluminosilicate glass matrix and excited with UV light emitting diode were investigated. The luminescence spectra exhibited strong emission signals in the red (600, 610, 625, and 646 nm), green (548 and 560 nm), and blue (485 nm) wavelength regions. White-light emission was observed in Sm/Tb and Eu/Tb double-doped activated phosphors employing UV-LED excitation at 395 nm. The dependence of the luminescence emission intensities upon annealing temperature and rare-earth concentration was also examined. The results indicated that there exist optimum annealing temperature and activator ion concentration in order to obtain intense visible emission light with high color rendering index. The study suggests that the nanocomposite phosphor based upon 75SiO₂:25PbF₂ host herein reported is a promising contender for white-light LED applications.     

A potential red-emitting phosphor with high color-purity for near-UV light emitting diodes

New red tungstates phosphors, Na₅La_1?xLn_x(WO₄)₄ (Ln = Eu, Sm) and Na₅Eu_1?xSm_x(WO₄)₄, were prepared by solid-state reaction technique. And their structure and photo-luminescent properties were investigated. The introduction of Sm³⁺ broadened the excitation band around 400 nm of the phosphors, and strengthened the red emission. And the possible energy transfer process from Sm³⁺ to Eu³⁺ is discussed. The single red LED was fabricated by combining InGaN chip with Na₅Eu_0.94Sm_0.06(WO₄)₄ as red phosphor, intense red light can be observed by naked eyes. Then the phosphor Na₅Eu_0.94Sm_0.06(WO₄)₄ may be a good candidate for red component of near-UV InGaN-based W-LEDs, because of efficient red-emitting with broadened absorption around 400 nm and appropriate CIE chromaticity coordinates (x = 0.65, y = 0.34).     

Novel UV-emitting single crystalline film phosphors grown by LPE method

This work reports the development of new types of UV-emitting phosphors based on single crystalline films (SCF) of aluminum garnet and perovskite compounds grown by the liquid phase epitaxy method. We consider peculiarities of the growth and the luminescent and scintillation properties of the following four types of UV SCF phosphors: i) Ce-doped SCF of Y–Lu–Al-perovskites with the Ce³⁺ emission in the 300-450 nm range of the decay time of 16–17 ns; ii) Pr-doped SCF of Y–Lu–Al garnets and perovskites with the Pr³⁺ emission in the 300–400 nm and 235–330 nm ranges with the decay time of 13–19 and 7–8 ns, respectively; iii) La³⁺ or Sc³⁺ doped SCF of Y–Lu–Al-garnets, emitting in the 280-400 nm range due to formation of the La_Y,Lu, Sc_Y,Lu and Sc_Al centers with decay time of the order of several hundreds of nanoseconds; iv) Bi³⁺ doped SCF of garnets with Bi³⁺ emission in 275–350 nm with decay time of about 1.9 μs.     

Fluorescent properties of a green- to red-emitting Eu3+, Tb3+ codoped amorphous calcium silicate phosphor

A Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor was prepared by heating a Eu³⁺, Tb³⁺ codoped calcium silicate hydrate phosphor formed by liquid-phase reaction for 30 min at 900 °C. The excitation peak wavelength of the resulting phosphor was 379 nm and the emission peak wavelengths were at 542 nm, attributed to the ⁵D₄→⁷F₅ transition of Tb³⁺, and at 613 mm, attributed to the ⁵D₀→⁷F₁ transition of Eu³⁺. The intensity ratio of the two peaks could be freely controlled by varying the Eu/Tb atomic ratio of the Eu³⁺, Tb³⁺ codoped amorphous calcium silicate phosphor, allowing light to be emitted over a wide range from green to red. It was clarified that electron transfer from Tb³⁺ to Eu³⁺ is occurring.     

Luminescent properties of Eu3+ and Sm3+ activated M2SiO4 (M=Ba,Sr and Ca) red-emitting phosphors for WLEDs

Eu³⁺ and Sm³⁺ activated M₂SiO₄ (M=Ba, Sr and Ca) red-emitting phosphors were synthesized by a solid state reaction. The results of XRD and SEM measurements show that the samples are single phase and have irregular shape. The excitation and emission spectra indicate that these phosphors were effectively excited by ultraviolet (395 nm) and blue (466 nm) light and exhibited red performance. The charge compensator R⁺ (R⁺=Li⁺, Na⁺ and K⁺) injecting into the host efficiently enhanced the luminescence intensity of the M₂SiO₄: Eu³⁺ and M₂SiO₄: Sm³⁺ phosphors. The emission intensity of M₂SiO₄: Eu³⁺ and Sm³⁺ doping Li⁺ were higher than that of Na⁺ or K⁺.     

Enhanced photoluminescence intensity of Y2O3: Eu3+ Sheets by codoping Pr3+ and involved energy transfer mechanisms

Y₂O₃: Eu³⁺ has been widely applied as red phosphors in the fields of displaying and illumination. Here, we report the enhanced luminescence intensity of Y₂O₃: Eu³⁺ by codoping Pr³⁺ ion. The Pr³⁺ and Eu³⁺ doped Y₂O₃ microsheets with high aspect ratio were synthesized by a simple route combining chemical precipitation and pyrolysis, which could emit intense red light centered at 610 nm under the 254 and 365 nm UV excitation. The fluorescence measurement indicated that the luminescence intensity of Y₂O₃: Eu³⁺, Pr³⁺ did not increase monotonously with increasing Pr³⁺ concentration. The highest improvement of the photoluminescence intensity of Y₂O₃:Eu³⁺ was realized in the sample doped with 2 mol% Pr³⁺, which was of 17.8% higher than the whole intensity of only Eu³⁺ doped Y₂O₃.The mechanism analysis based on SEM, XRD, fluorescence spectra, and simplified energy level diagram indicated that (1) energy transfer process between Pr³⁺ and Eu³⁺, (2) crystallinity, and (3) symmetry should respond for this nonmonotonous variation phenomenon by competition with each other. For energy transfer process between Pr³⁺ and Eu³⁺, it was suggested that the cross relaxation of ⁵D₀ + ⁷F₁(Eu³⁺)?³P₀ + ³H₆(Pr³⁺) and the efficient energy transfer from ³P₀ state of Pr³⁺ to ⁵D₁ energy level of Eu³⁺ lead to the improvement of the population of the ⁵D₀ state of Eu³⁺ so that the 610 red emission of Eu³⁺ ion was accordingly enhanced.     

Energy transfer induced Eu3+ photoluminescence enhancement in tellurite glass

In this work, structural, thermal and optical properties of Eu³⁺ doped TeO₂–La₂O₃–TiO₂ glass were investigated. The differential scanning calorimetry (DSC) measurements reveal an important stability factor ΔT=143.52 K, which indicates the good thermal and mechanical stabilities of tellurite glass. From the absorption spectrum, the optical band gap was found to be direct with E_g=3.23 eV. The temperature dependences of photoluminescence (PL) properties of Eu-doped and Eu–Tb codoped tellurite glass are investigated. As the temperature increases from 7 to 300 K, both the PL intensity and the PL lifetime relative to the ⁵D₂→⁷F₀ are nearly constant below 230 K and then an enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from charged intrinsic defects states to Eu³⁺ energy levels.By co-doping tellurite glasses with Eu and Tb, a strong Eu³⁺ PL enhancement is shown due to excitation transfer from Tb³⁺ and intrinsic defects to Eu ions.     

Luminescent properties and energy transfer of double-emitting NaSrPO4:Eu2+,Tb3+ phosphor for near-UV white LEDs

Novel blue/green NaSrPO₄ phosphors co-doped with Eu²⁺ and Tb³⁺ were synthesized by a conventional solid-state reaction. Their luminescent properties were characterized by using powder X-ray diffraction, photoluminescence excitation and emission spectra, lifetime, and temperature dependent emission spectra, respectively. The NaSrPO₄:Eu²⁺,Tb³⁺,Na⁺ phosphor showed an intense broad excitation band between 250 and 430 nm, which was in agreement with the near-UV chip (350–420 nm), and it exhibited two dominating emission bands at 445 and 545 nm, corresponding to the allowed 4f⁶5d¹→4f⁷(⁸S_7/2) transition of Eu²⁺ ion and the ⁵D₄→⁷F₅ transition of Tb³⁺ ion, respectively. The emission intensity and lifetime of Eu²⁺ ion decreased with the increasing concentration of Tb³⁺ ion, which strongly indicated that an effective energy transfer occurred from Eu²⁺ to Tb³⁺ in NaSrPO₄ host. The principle of the energy transfer should be the combined effect of the non-radiative resonant energy transfer and the phonon-assisted non-radiative process.     

Novel EGCG assisted ultrasound synthesis of self-assembled Ca2SiO4:Eu3+ hierarchical superstructures: Photometric characteristics and LED applications

This paper reports for the first time ultrasound, EGCG assisted synthesis of pure and Eu³⁺ (1–5 mol%) activated Ca₂SiO₄ nanophosphors having self-assembled superstructures with high purity. The shape, size and morphology of the product were tuned by controlling influential parameters. It was found that morphology was highly dependent on EGCG concentration, sonication time, pH and sonication power. The probable formation mechanism for various hierarchical superstructures was proposed. The PL studies of Ca₂SiO₄:Eu³⁺ phosphors can be effectively excited by the near ultraviolet (UV) (396 nm) light and exhibited strong red emission around 613 nm, which was attributed to the Eu³⁺ (⁵D₀ → ⁷F₂) transition. The concentration quenching phenomenon was explained based on energy transfer between defect and Eu³⁺ ions, electron–phonon coupling and Eu³⁺–Eu³⁺ interaction. The Judd–Ofelt intensity parameters and radiative properties were estimated by using PL emission spectra. The photometric studies indicate that the obtained phosphors could be a promising red component for possible applications in the field of white light emitting diodes.     

Self-assembled 3D sphere-like SrMoO4 and SrMoO4:Ln3+ (Ln = Eu,Sm, Tb,Dy) microarchitectures: Facile sonochemical synthesis and optical properties

Three-dimensional (3D) well-defined SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures of obvious sphere-like shape have been successfully synthesized using a large-scale and facile sonochemical route without using any catalysts or templates. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and photoluminescence (PL) spectra were used to characterize the samples. The intrinsic structural feature of SrMoO₄ and external factor, namely the ultrasonic time and the pH value, are responsible for the ultimate shape evolutions of the product. The possible formation mechanism for the product is presented. Additionally, the PL properties of SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures were investigated in detail. The Ln³⁺ ions doped SrMoO₄ samples exhibit respective bright red–orange, yellow, green and white light of Eu³⁺, Sm³⁺, Tb³⁺ and Dy³⁺ under ultraviolet excitation, and have potential application in the field of color display. Simultaneously, this novel and efficient pathway could open new opportunities for further investigating about the properties of molybdate materials.     

Fabrication and luminescence properties of Al2O3:Tb3+ microspheres via a microwave solvothermal route

Al₂O₃:Tb³⁺ green phosphors were synthesized via a microwave solvothermal and thermal decomposition route, and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra, and decay curves. XRD results indicate that Tb³⁺ doped samples are γ-Al₂O₃ after being calcined at 773 K. SEM results show that the particles of Al₂O₃:Tb³⁺ are hierarchically nanostructured microspheres assembled from nanosheets. The PL spectra indicate that the ⁵D₄→⁷F₅ (545 nm) electric dipole transition is the most intensive when excited at 235 nm. It is shown that 0.7 mol% of doping concentration of Tb³⁺ ions in γ-Al₂O₃:Tb³⁺ is optimum. According to Dexter's theory, the critical distance between Tb³⁺ ions for energy transfer was determined to be 18.4 Å. It is found that the curve followed the single-exponential decay. The excellent chromaticity coordinates of Al₂O₃:Tb³⁺ phosphors, as defined by the International Commission on Illumination (CIE), indicate that it is a good candidate for use in light display systems and optoelectronic devices.     

Synthesis,characterization and studies of radiative properties on Eu3+-doped ZnAl2O4

Eu³⁺-doped ZnAl₂O₄ phosphors were successfully synthesized in air atmosphere at 900 °C. The phosphors were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermally stimulated luminescence (TSL) and photoluminescence (PL) techniques. The average particle size of the system as determined from SEM was found to be 100–150 nm (for samples annealed at 900 °C). PL spectra of the doped phosphors showed emission peaks corresponding to Eu³⁺ ions. Lifetime studies revealed Eu³⁺ ions to be in two different sites. The asymmetric ratio (I₆₁₆/I₅₉₂) was observed to be about 3.75. This suggested that Eu³⁺ ion entered the host mainly substituting Al³⁺ site distorting the local environment and is partly located on surface of the phosphors. A prominent glow peak at 430 K was observed in the TSL of γ-irradiated Eu³⁺-doped ZnAl₂O₄ phosphors. Trap parameters for this peak have been determined and the probable mechanism for the glow peak is proposed. CIE chromaticity coordinates for the system were evaluated. It was observed that, the system could be employed as a potential red emitting phosphor. Commercial utility of the phosphor was investigated by comparing it with commercial red phosphor. The PL intensity of the as prepared phosphors was 63% of that of the commercial phosphor. Apart from this, various radiative properties such as the Judd–Ofelt intensity parameters, spontaneous emission probabilities, luminescence branching ratios, radiative lifetimes and quantum efficiency were evaluated for the system.     

Luminescence and red long afterglow investigation of Eu3+–Sm3+ CO-doped CaWO4 phosphor

A series of Eu³⁺–Sm³⁺ co-doped CaWO₄ phosphors were synthesized by the high temperature solid-state method. The crystal structure of the obtained samples was identified by XRD, and the results showed that all the phases were indexed to scheelite structure. The effect of the doping concentration of Sm³⁺ on the luminescent properties of the obtained products was investigated, and the optimal Sm³⁺ concentration was experimentally determined to 0.5%. The photoluminescence properties indicate that there is an efficient energy transfers from Sm³⁺ to Eu³⁺. The energy-transfer process between Sm³⁺ and Eu³⁺ was also given. Red long afterglow originating from the ⁵D₀–⁷F_J (J=0, 1, 2, 3, 4) transitions of Eu³⁺ was observed after samples were excited by 254 nm, and the duration of the optimal sample can last more than 35 min in dark with naked eyes. The proposed explanation for the afterglow property was also discussed.     

Improved photoluminescence of BaMgAl10O17 blue phosphor prepared by spray pyrolysis

BaMgAl₁₀O₁₇:Eu²⁺ (BAM) blue phosphor particles with improved photoluminescence (PL) intensity under vacuum ultraviolet (VUV) excitation were prepared by a spray pyrolysis process. In order to improve the PL intensity, Er³⁺ and Nd³⁺ ions were used as co-doping elements. The VUV characteristics of BAM:Eu²⁺, M⁺ (M=Er, Nd) were monitored with varying the Er³⁺ and Nd³⁺ content in order to find the optimal co-doping concentration when they were prepared by spray pyrolysis. It was found that doping Er³⁺ or Nd³⁺ enhances the PL intensity of BAM:Eu²⁺ blue phosphor particles. In particular, the M³⁺ doping effect on the PL intensity was pronounced when the prepared BAM:Eu²⁺, M³⁺ particles were excited by 172 nm VUV. The maximum intensity was obtained when the M³⁺ content was 1.0 at% with respect to Ba element. The PL intensity of BAM:Eu²⁺, M⁺ (M=Er³⁺, Nd³⁺) particles was also further improved by producing them in a spherical shape, which was successfully achieved by controlling the spray solution. The optimized BAM:Eu²⁺, M⁺ particles had about 10% higher PL intensity than that of the commercial particles, which are made by a conventional solid-state reaction.     

Synthesis and luminescence of SrZnO2 phosphors

SrZnO₂ phosphors have been synthesized by two new methods viz. carbonate decomposition at 1000 °C and combustion synthesis. Phosphors activated with Pb²⁺, Sm³⁺, Tb³⁺, Bi³⁺ and Pr³⁺ could be prepared in one step using the combustion synthesis. Characteristic emission and excitation were observed for Bi³⁺. For the remaining activators excitation spectra always contained a band at 283 nm. Presence of this band for all these different types of activators was interpreted as host sensitization.     

Preparation and photoluminescence properties of the Sr1.56Ba0.4SiO4:0.04Eu2+ phosphor

The Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ phosphors were prepared via a combustion reaction and following the calcination method at low temperature. The influences of the amount of the uncommonly used SrCl₂ flux, different calcination temperatures and time on the structure and the photoluminescence (PL) properties of the phosphors were investigated. Under the excitation of 450 nm blue light, the phosphor shows the intense broad emission band from 490 nm to 650 nm, and the emission peak is centered at 553 nm. The luminescence intensity of Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ was very sensitive to the crystallinity and morphology characteristics of the phosphor. The phosphor calcined at 950 °C for 3 h in 20%H₂/80%Ar atmosphere exhibits improved PL properties due to its high crystallinity and excellent morphology characteristics. The use of the SrCl₂ flux provides a novel way to improve the crystallinity of the silicates phosphors at low preparation temperature.     

5D0→7F1 and 5D0→7F2 transition in europium doped halosulphates for mercury-free lamps

Polycrystalline Na₃SO₄F:Eu and NaMgSO₄F:Eu halosulphate phosphors prepared by a wet chemical method have been studied for its photoluminescence (PL) and thermoluminescence (TL) characteristics. Two well resolved peaks are observed at 593 nm and 614 nm, which are assigned to due to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺ ions. TL is observed at temperatures between 100 °C and 300 °C. In this paper, we report PL emission spectra of Eu³⁺ and TL glow curves, which are more sensitive than the standard TLD-CaSO₄:Dy. The presented phosphors are applicable for the mercury free lamps and solid state lighting devices.     

Synthesis and photoluminescence properties of the high-brightness Eu3+-doped Sr3Bi(PO4)3 phosphors

A series of orange reddish emitting phosphors Eu³⁺-doped Sr₃Bi(PO₄)₃ have been successfully synthesized by conventional solid-state reaction, and its photoluminescence (PL) properties have been investigated. The excitation spectra reveal strong excitation bands at 392 nm, which match well with the popular emissions from near-UV light-emitting diode chips. The emission spectra of Sr₃Bi(PO₄)₃:Eu³⁺ phosphors invariably exhibit five peaks assigned to the ⁵D₀→⁷F_J (J=0, 1, 2, 3, 4) transitions of Eu³⁺ and have dominating emission peak at 612 nm under 392 nm excitation. The luminescence intensity was enhanced with increasing Eu³⁺ content and the emission reached the maximum intensity at x=0.05 in Sr₃Bi(PO₄)₃:xEu³⁺. The energy transfer behavior in the phosphors was discussed. The Commission Internationale de lEclairage (CIE) chromaticity coordinates, the quantum efficiencies, and the decay curves of the entitled phosphors excited under 392 nm are also investigated. The experimental results indicate that the Eu³⁺-doped Sr₃Bi(PO₄)₃ phosphors are promising orange reddish-emitting phosphors pumped by near-UV light.     

Photoluminescence properties of Eu2+–Mn2+ codoped Ca-α-SiAlON phosphors

Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors, Ca_0.736?ySi_9.6Al_2.4O_0.8N_15.2:0.064 Eu²⁺, yMn²⁺, were firstly synthesized by the high temperature solid state reaction method. The effects of doped Eu²⁺ and Eu²⁺–Mn²⁺ concentrations on the photoluminescence properties of the as-prepared phosphors were investigated systematically. Powder X-ray diffraction shows that pure Ca-α-SiAlON phase is synthesized after sintering at 1700 °C for 2 h under 0.5 MPa N₂ atmosphere. The excitation spectra of Eu²⁺-doped Ca-α-SiAlON phosphors are characterized by two dominant bands centered at 286 nm and 395 nm, respectively. The photoluminescent spectrum of Eu²⁺-doped Ca-α-SiAlON phosphor exhibits an intense emission band centered at 580 nm due to the allowed 4f ⁶5d→4f ⁷ transition of Eu²⁺, showing that the phosphor is a good candidate for creating white light when coupled to a blue LED chip. The intensities of both excitation and emission spectra monotonously decrease with the increment of codoped Mn²⁺ content (i.e. y value), indicating that energy transfer between Eu²⁺ and Mn²⁺ is inefficient in the case of Eu²⁺–Mn²⁺ codoped Ca-α-SiAlON phosphors.     

Preparation and luminescence properties of terbium-doped lanthanum oxide nanofibers by electrospinning

Terbium-doped lanthanum oxide (La₂O₃:Tb³⁺) nanofibers were prepared by electrospinning followed by calcination at high temperature. Thermogravimetric analyzer (TGA), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence (PL) were used to characterize the obtained fibers. The results reveal that the nanofibers have an average diameter of ca. 95±25 nm and are composed of pure La₂O₃ phase. Under the excitation of 274 nm light, the La₂O₃:Tb³⁺ nanofibers exhibit the characteristic emission resulting from the ⁵D₄→⁷F_J (J=3, 4, 5, 6) transitions of Tb³⁺ ions. And the PL emission intensity is stronger than that of their nanoparticle counterparts.