Identification of charge‐transfer transition between Gd3+ to O2− in gadolinium‐containing oxide phosphors in VUV region

Abstract— The broad bands at around 155 nm for GdAl₃(BO₃)₄:Eu, at 184 nm for Ca₄GdO(BO₃)₃:Eu, at 183 nm for Gd₂SiO₅:Eu, and at 170 nm for GdAlO₃:Eu were observed. These bands were assigned to the charge‐transfer (CT) transition of Gd³⁺‐O^2?. In the excitation spectrum of (Gd,Y)BO₃:Eu, a broadened excitation band was observed in VUV region. It could be considered that this band was composed of two bands at about 160 and 166 nm. The preceding band was assigned to the BO₃ group absorption. The later one at about 166 nm could be assigned to the CT transition of Gd³⁺‐O^2?, according to the result of GdAl₃(BO₃)₄:Eu, Ca₄GdO(BO₃)₃:Eu, Gd₂SiO₅:Eu, and GdAlO₃:Eu. The excitation spectra overlapped between the CT transition of Gd³⁺‐O^2? and BO₃ groups absorption. It caused the emission of Eu³⁺ to take place effectively in the trivalent europium‐doped (Gd,Y)BO₃ host lattice under 147‐nm excitation.     

Effect of alkali metal ions co-doping on the structure and luminescent properties of phosphor Zn3(BO3)2:Eu3+

A series of the Zn₃(BO₃)₂:Eu³⁺ without or with alkali metal ions doping at a low sintering temperature were synthesized by the solid-state reaction method. The XRD pattern shows that all samples exhibit Zn₃(BO₃)₂ crystalline phase. The samples co-doped with alkali metal ions have better crystallinity compared with the un-compensated ones. The different charge compensation approaches have no influence on the shape and position of the emission and excitation spectra. However, the luminescent intensity of samples has been obviously enhanced with different alkali metal ions co-doping. The introduction of Li⁺ can increase the red emission of Eu³⁺ compared with the others. Thus, the volume compensation and the equilibrium of mole number can be taken into consideration by charge compensated (CC) approaches.     

A study on the chromaticity shifts of blue phosphor for color plasma displays

The dependency of the chromaticity shifts on the concentration of Eu²⁺ doped in BaMgAl₁₀O₁₇ (BAM) was investigated under heat‐treatment and vacuum ultraviolet (VUV) irradiation. The Eu²⁺ ions in BAM show an asymmetrical broad emission band with a maximum at ～452 nm under excitation of VUV light at room temperature, showing that multiple crystalline cationic sites exist in the host. It was found that the chromaticity shifts greatly decrease with increasing heat‐treatment temperature. Regardless of the Eu²⁺ concentration, the chromaticity shifts caused by heat‐treatment are greater than that caused by VUV irradiation. Compared with conventional BAM, a solid solution of BAM with barium aluminate as a powder and film was also studied, and very few chromacity shifts were observed. It is suggested that the distribution of Eu²⁺ ions in different sites in a BAM lattice results in different chromaticity coordinates. By increasing the Eu²⁺ concentration in BAM, or under heat‐treatment and VUV irradiation, the emission band shifts towards longer wavelengths.     

Development of a new blue CaMgSi2O6:Eu2+ phosphor for PDPs

Abstract— A blue‐light‐emitting Eu²⁺‐doped CaMgSi₂O₆ phosphor having a long lifetime for a plasma‐display panel (PDP) was developed. The CaMgSi₂O₆:Eu²⁺(CMS:Eu) phosphors show no luminance degradation during the baking process, and an equivalent photoluminescence peak intensity compared to that of the conventional blue‐phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) after baking. CMS: Eu shows a poor luminescent characteristic for the Xe excimer band excitation due to the lack of absorption. To introduce the absorption center for the Xe excimer band, we performed Gd‐codoping of CMS: Eu as a sensitizer and found a new excitation band around 172 nm, which originated from Gd³⁺. The test PDPs panels using synthesized CMS: Eu phosphor and CMS: Eu, Gd phosphor were examined to investigate the luminescent and aging characteristics of a Xe‐discharge excitation source. The CMS: Eu panel shows an emission peak intensity comparable to that of the BAM panel (i.e., a comparable stimuli L/CIEy, 93% of BAM), while the CMS: Eu, Gd panel shows poorer blue emission intensity compared to the BAM panel (up to 53% of total stimuli of BAM). The CMS: Eu panel and the CMS: Eu, Gd panel show less luminance degradation than the BAM panel under the aging test, and the panel retains 90% of its luminance after 300 hours of driving. It was found that CMS: Eu appears to be a candidate for a new blue PDP phosphor because of its longevity in a Xe‐discharge plasma environment.     

Investigation on luminescence of Na3Ca6(PO4)5:Eu2+ phosphor for LEDs

In this paper, a series of Na₃Ca_6(1−x)(PO₄)₅:xEu²⁺ (NCP:xEu²⁺, 0 ≤ x ≤ 4%) phosphors were prepared by conventional solid-state reaction method, and their photoluminescence properties were studied. Upon 365 nm excitation, the typical NCP:2%Eu²⁺ phosphor shows an asymmetric bluish green emission band with the dominant peak at 498 nm which could be attributed to the 4f⁶5d¹-4f⁷ transition of Eu²⁺. By measuring the time-resolved photoluminescence spectra, it reveals more than one Eu²⁺ emission center in the Eu²⁺-activated NCP phosphors. By monitoring 498 nm, the excitation spectrum of NCP:2%Eu²⁺ demonstrates a broad excitation band ranging from 240 to 450 nm, which can match well with the emission wavelength of the NUV LED chip. The SEM image shows that the average particle size of NCP:2%Eu²⁺ is about 19.4 µm. The above results imply that the NCP:Eu²⁺ phosphor could have potential application in LEDs.     

Red phosphor Li2Mg2(WO4)3: Eu3+ with lyonsite structure for near ultraviolet light-emitting diodes

A series of Eu³⁺-activated Li₂Mg₂(WO₄)₃ (LMW) materials were synthesized by high temperature solid state reactions. The phosphor can be effectively excited by 394 nm near ultraviolet light and emit intense red light with high color purity. Prepared phosphors can be indexed to LMW with particular lyonsite structure. The occupation of Eu³⁺ in LMW is selective. Most of Eu³⁺ comes into ¹A sites without inversion symmetry. The present research suggests that LMW is a suitable host for luminescence applications and Eu³⁺-activated LMW is a promising phosphor for phosphor-converted white light-emitting diodes.     

Synthesis, characteristics and luminescent properties of a new europium(III) organic complex applied in near UV LED

A novel indane based β-diketone with trifluorobutane in the contraposition, 5-acetylindane-4,4,4-trifluorobutane-1,3-dione (HAITFBD) and its europium(III) ternary complex, Eu(AITFBD)₃phen, were designed and synthesized, where phen was 1,10-phenanthroline. The complex was characterized by IR, UV-visible, thermogravimetric analysis (TGA) and photoluminescence (PL) spectroscopy in details. The results show that the Eu(III) complex exhibits high thermal stability, wide and strong excitation bands from 300 nm to 425 nm when monitored at 611 nm, which matches well with the 380 nm-emitting InGaN chips. The complex exhibits intense red emission under excitation of near UV light due to the f-f transitions of the central Eu³⁺ ion. Based on the emission spectrum, the CIE chromaticity coordinates of the LED are calculated as x = 0.63 and y = 0.34, which is suitable to be used as an efficient red phosphor in fabrication of white LEDs. The fluorescence lifetime and the luminescence quantum yield were also measured. The lowest triplet state energy of the primary ligand AITFBD was measured to be 17,730 cm⁻¹, higher than that of the lowest excitation state energy level of the central Eu³⁺ ion, ⁵D₀, and this suggests that the photoluminescence of the complex is a ligand-sensitized luminescence process (antenna effect). Finally, a bright red light-emitting diode was fabricated by coating the Eu(AITFBD)₃phen complex onto a 380 nm-emitting InGaN chip. All the results indicate that Eu(AITFBD)₃phen can be applied as a red component for fabrication of near ultraviolet-based white light-emitting diodes.     

Red shift of the PSL spectra by Al3+ doping in BaFBr:Eu2+

Abstract— Photostimulated luminescent (PSL) materials are currently used for digital storage and display in radiography. The phosphor BaFBr:Eu²⁺ doped with Al³⁺ is shown to have a very high PSL intensity, and the peak of the PSL spectrum shifts to a longer wavelength. Compared with BaFBr:Eu²⁺ doped with Ca²⁺, Sr²⁺, or Mg²⁺, BaAlFBr:Eu²⁺ is better suited to the stimulated light wavelength emitted by a semiconductor laser. The red‐shift mechanism is considered to be a FAcenter for BaFBr:Eu²⁺ doped with Ca²⁺, Sr²⁺, or Mg²⁺ and a F^Z1 center for BaFBr:Eu²⁺ doped with Al³⁺.     

Investigation of spectroscopic properties of Sm-Eu codoped phosphate glasses

Phosphate glasses with chemical compositions of 74.5NaH₂PO₄–20ZnO–5Li₂O–0.5Sm₂O₃ and 74NaH₂PO₄–20ZnO–5Li₂O–0.5Sm₂O₃–0.5Eu₂O₃ were synthetized by melt quenching method. We investigated the influence of Sm³⁺/Eu³⁺ doping on the optical properties of phosphate glasses. X-ray Diffraction indicates that the samples have an amorphous structure. DSC measurements show a good thermal stability of phosphate glasses. Using the absorption spectra, Judd–Ofelt analysis was applied to absorption bands of Sm³⁺ (4f⁵) to carry out the three phenomenological parameters of Judd–Ofelt (JO). According to the obtained values of Ω₂, Ω₄ and Ω₆, some radiative properties were theoretically determined. We report both the photoluminescence (PL) and the PL lifetime measurements of a prominent emission transition ⁴G_5/2 → ⁶H_5/2 (604 nm) of Sm³⁺ both in absence and in presence of Eu³⁺. It is shown that Eu³⁺ ions act as sensitizers for Sm³⁺ ions and contribute largely to the improvement of the radiative properties of phosphate glasses. An improvement of the PL lifetime value after adding Eu³⁺ ions (4.58 ms) is reported. The predicted lifetime (τ_rad) calculated by Judd–Ofelt theory and the experimental lifetime (τ_meas) for the prepared phosphate glasses were compared with those of other works. Photoluminescence (PL) intensity of ⁴G_5/2 → ⁶H_5/2 (604 nm), ⁴G_5/2 → ⁶H_7/2 (567 nm), ⁴G_5/2 → ⁶H_9/2 (650 nm) and ⁴G_5/2 → ⁶H_11/2 (706 nm) and the quantum efficiency (η) for the excited ⁴G_5/2 level were enhanced after adding Eu³⁺. The radiative properties obtained for (Sm, Eu) codoped phosphate glasses suggest that the present material can be a potential candidate for the development of color display devices.     

Enhancement of red fluorescence and afterglow in CaWO4:Eu3+ by addition of MoO3

In this study, Eu³⁺ doped Ca(WO₄)_1?x(MoO₄)_x phosphors were synthesized via high temperature solid-state reaction. Compared with the Eu³⁺ activated CaWO₄ sample, an increment of MoO₃ doping concentration could improve the emission intensity. Improved red afterglow originating from the ⁵D₀ to ⁷F_J (J = 0, 1, 2, 3, 4) transitions of Eu³⁺ was observed after appropriate amount of MoO₃ was added, and the optimal MoO₃ doping concentration was experimentally determined to be 0.02. The proposed explanation for the afterglow property was also discussed.     

Red emitting MTiO3 (M = Ca or Sr) phosphors doped with Eu3+ or Pr3+ with some cations as co-dopants

Ca (or Sr)TiO₃:Eu³⁺, M (Li⁺ or Na⁺ or K⁺) and CaTiO₃:Pr³⁺, M (Li⁺ or Na⁺ or Ag⁺ or K⁺ or Gd³⁺ or La³⁺) powders were prepared by combustion synthesis method and the samples were further heated to ～1000 °C to improve the crystallinity. The structure and morphology of materials were examined by X-ray diffraction (XRD) and a scanning electron microscopy (SEM). The morphologies of SrTiO₃:Eu³⁺, CaTiO₃:Eu³⁺ or CaTiO₃:Pr³⁺ powders co-doped with other metal ions were very similar. Small and coagulated particles of nearly cubical shapes with small size distribution having smooth and regular surface were formed. Photo-luminescence spectra of CaTiO₃:Pr³⁺ and co-doped either with Li⁺, Na⁺, K⁺, Ag⁺, La³⁺ or Gd³⁺ ions showed red emissions at 613 nm due to the ¹D₂ → ³H₄ transition of Pr³⁺. The variation of intensity of emission peak with different co-doping follows the order: K⁺ > Ag⁺ > Na⁺ > Li⁺ > La³⁺ > Gd³⁺. The characteristic emissions of CaTiO₃:Eu³⁺ lattices had strong emission at 614 and 620 nm for ⁵D₀ → ⁷F₂ with other weak transitions observed at 580, 592, 654, 705 nm for ⁵D₀ → ⁷F_n transitions where n = 0, 1, 3, 4 respectively in all host lattices. Photoluminescence intensity in SrTiO₃:Eu³⁺ is more than CaTiO₃:Eu³⁺ lattices. A remarkable increase of photoluminescence intensity (in ⁵D₀ → ⁷F₂ transition) was observed if co-doped with Li⁺ ions in CaTiO₃:Eu³⁺ and SrTiO3:Eu³⁺.     

Design and characterization of new lanthanide fluorides and their optical properties

Three kinds of lanthanide phosphors (La_xLu_1−xF₃: Eu³⁺, LaF₃–CaF₂:Eu³⁺ and LaF₃: Eu³⁺) have been successfully synthesized based on three different ways such as molten salts, co-precipitation, supersonic and microwave irradiations. The as-prepared powder materials all exhibited red luminescence. Their crystal structures or morphologies were studied by means of X-ray powder diffraction and scanning electronic microscope. Eu³⁺-doped LaF₃–CaF₂ phosphor can be emissive under excitation at longer wavelengths (466 and 533 nm) excitations. Supersonic and microwave irradiations have shortened the reaction time of LaF₃: Eu³⁺ crystals in 40 min under very low temperature (50 °C).     

A new long-lasting phosphor Zr and Eu co-doped SrMg2(PO4)2

Zr⁴⁺- and Eu³⁺-codoped SrMg₂(PO₄)₂ phosphors were prepared by conventional solid-state reaction. Under the excitation of ultraviolet light, the emission spectra of Sr_0.95Eu_0.05Mg_2−2xZr_2xP₂O₈ (x = 0.0005-0.07) are composed of a broad emission band peaking at 500 nm from Zr⁴⁺-emission and the characteristic emission lines from the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3 and 4) transitions of Eu³⁺ ions. These phosphors show the long-lasting phosphorescence. The emission color varies from red to white with increasing Zr⁴⁺-content. The white-light emission is realized in single-phase phosphor of Sr_0.95Eu_0.05Mg_2−2xZr_2xP₂O₈ (x = 0.07) by combining the Zr⁴⁺- and Eu³⁺-emission. The duration of the persistent luminescence of Sr_0.95Eu_0.05Mg_2−2xZr_2xP₂O₈ (x = 0.07) reaches nearly 1.5 h. The time at which the long-lasting phosphorescence intensity is 50% of its original value (T_0.5) is 410 s. The afterglow decay curves and the thermoluminescence spectra were measured to discuss this long-lasting phosphorescence phenomenon. The co-doped Zr⁴⁺ ions act as both the luminescence centers and trap-creating ions.     

Study of the effect of water on thermal and operating degradation of BaMgAl10O17: Eu2+ (BAM) blue phosphor

Abstract— From the adsorption and desorption characteristics of water, we showed that water can intercalate into BaMgAl₁₀O₁₇: Eu²⁺ blue phosphor. ESR, XANES, and XPS analyses confirmed that oxidation by water causes thermal degradation of BAM. We also demonstrated that intercalated water accelerates luminance degradation under VUV irradiation and showed oxidation of Eu²⁺ during panel operation by means of μ‐XPS. We concluded that the cause of thermal and operating degradation of BAM is the oxidation of Eu²⁺ due to water.     

Synthesis and photoluminescent properties of orange-emitting Sm3+-activated KPb4(PO4)3 phosphor for LEDs

A series of Sm³⁺-doped KPb₄(PO₄)₃ phosphors have been successfully synthesized by high-temperature solid-state reaction method, and the structure, morphology and luminescent properties were investigated. The SEM images suggest that the prepared phosphor has an irregular morphology with a diameter of about 10 ~ 20 μm. Under near-ultraviolet (NUV) light (404 nm) excitation, all prepared phosphors KPb_4-xSm_xP₃O₁₂ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5) show the characteristic f → f emission bands of the Sm³⁺ activator. And the emission intensities have an upward trend with increasing the Sm³⁺ concentration when x is lower than 0.1. By monitoring 598 nm emission, the excitation spectrum of KPb_3.9Sm_0.1(PO₄)₃ contains a series of sharp bands in the range of 250 ~ 500 nm, which matches well with the NUV LED chip. The CIE coordinates of KPb_3.9Sm_0.1(PO₄)₃ phosphor was evaluated to be (0.5714, 0.4253), corresponding to orange color with a high color purity of about 90%.     

White and tunable light emission of Ho3+ and Pr3+ ions co-doped phosphate glasses

The Ho³⁺ and Pr³⁺ ions co-doped phosphate glasses were prepared by melt quenching procedure with the various composition of (70-x-y)P₂O₅ + 20SiO₂ + 10CaO + xHo₂O₃ + yPr₂O₃ (x = 0.4, 0.6, 0.8, 1.0 mol%, y = 0.6, 0.8, 1.0 mol%). The structural investigation (based on X-ray diffraction analysis) confirmed amorphous character of these glass materials. The optical properties were studied. The glass samples have strong absorption at 360 nm, and the excitation light at 360 nm can excite Ho³⁺ and Pr³⁺ ions very well, causing them to produce synergistic luminescence. The glass sample 68.8P₂O₅ + 20SiO₂ + 10CaO + 0.4Ho₂O₃ + 0.8Pr₂O₃ emits strong white light under 360 nm excitation. The chromaticity coordinate values are x = 0.3378, y = 0.3472 in white light region, and it has a moderate correlated color temperature (CCT) of 5277 K. Decay time data reveals that there is energy transfer from Pr³⁺ to Ho³⁺ ions. This glass will be a good material for white light and tunable light emitting.     

Orange‐red upconversion luminescence of Sm3+‐doped ZnO‐B2O3‐SiO2 glass by infrared femtosecond laser irradiation

Abstract— Near‐infrared‐to‐visible upconversion luminescence was observed in Sm³⁺‐doped ZnO‐B₂O₃‐SiO₂ glass under femtosecond laser irradiation. The luminescence spectra show that the upconversion luminescence originates from ⁴G^5/2 to ⁶H^j/2 (j = 5, 7, 9) transition of Sm³⁺. The dependence of the fluorescence intensity of Sm³⁺ on the pump power indicates that a two‐photon absorption process is dominant in the conversion of infrared radiation to the visible luminescence. The analysis of the upconversion mechanism reveals that the simultaneous absorption of two infrared photons produces the population of upper excited states, which leads to the characteristic orange‐red emission of Sm³⁺. A three‐dimensional display is demonstrated based on the multiphoton absorption upconversion luminescence.     

Spectroscopic and photoluminescence properties of MgO:Cr3+ nanosheets for WLEDs

This work explores the synthesis of nanocrystalline MgO:Cr³⁺ (1–9 mol%) nanophosphors via solution combustion route at 400 °C. The nanophosphors were well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infra-Red (FTIR) spectroscopy. PXRD results confirm cubic phase and SEM micrographs indicate that the particles are highly porous and agglomerated. The TEM images show that the powder consists of spherical particles of size ∼5–15 nm. Upon 356 nm excitation the emission profile of MgO:Cr³⁺ exhibits an emission peak at 677 nm due to ²E_g → ⁴A_2g transition. It was observed that PL intensity increases with increase in Cr³⁺ concentration and highest PL intensity was observed for 3 mol% doped sample and afterward it decreases, attributed to concentration quenching. The resultant CIE chromaticity co-ordinates in the white region make the present phosphor highly useful for display applications and also for white light-emitting diodes (WLEDs).     

Direct excitation of xenon by ballistic electrons emitted from nanocrystalline‐silicon planar cathode and vacuum‐ultraviolet light emission

Abstract— To verify the possible use of energetic electrons for direct excitation of inert gas molecules, a nanocrystalline‐silicon (nc‐Si) planar ballistic emitter is operated in a high‐pressure xenon gas ambience. Under the pulse drive, vacuum‐ultraviolet (VUV) light emission is detected without any signs of discharge. The transient behavior of the VUV light emission properly corresponds to that of the nc‐Si emitter. In accordance with quantitative analyses of electron‐emission characteristics and the VUV output, the electron‐to‐photon conversion efficiency reaches 81% in the relatively efficient emitter case. The VUV output power is mainly determined from the number of electrons with energies compatible the with internal excitation of xenon. The emission spectrum observed at a pressure of 10 kPa shows peaks at 152 and 172 nm, which are thought to be originated from metastable Xe²* states. In contrast to the case of conventional impact ionization, no near‐infrared (NIR) peaks are seen in the spectrum. These results strongly suggest that the incidence of energetic electrons causes direct excitation of xenon molecules followed by radiative relaxation through intermediate states. The generated VUV light can be easily converted to visible light using a phosphor screen. As a discharge‐free VUV light emission, this phenomenon is potentially applicable to mercury‐free, high‐efficacy, and high‐stability flat‐panel light‐emitting device.