Photoluminescence studies of red-emitting NaEu(WO4)2 as a near-UV or blue convertible phosphor

Sodium europium double tungstate [NaEu(WO₄)₂] phosphor was prepared by the solid-state reaction method. Its crystal structure, photoluminescence properties and thermal quenching characteristics were investigated aiming at the potential application in the field of white light-emitting diodes (LEDs). The influences of Sm doping on the photoluminescence properties of this phosphor were also studied. It is found that this phosphor can be effectively excited by 394 or 464 nm light, which nicely match the output wavelengths of near-ultraviolet (UV) or blue LED chips. Under 394 or 464 nm light excitation, this phosphor exhibits stronger emission intensity than the Y₂O₂S:Eu³⁺ or Eu²⁺-activated sulfide phosphor. The introduction of Sm³⁺ ions can broaden the excitation peaks at 394 and 464 nm of the NaEu(WO₄)₂ phosphor and significantly enhance its relative luminance under 400 and 460 nm LEDs excitation. Furthermore, the relative luminance of NaEu(WO₄)₂ phosphor shows a superior thermal stability compared with the commercially used sulfide or oxysulfide phosphor, and make it a promising red phosphor for solid-state lighting devices based on near-UV or blue LED chips.     

Role of Ti in the luminescence process of Al2O3:Si,Ti

Al₂O₃:Si,Ti, prepared under oxidizing condition at high temperature, gives PL emission around 430 nm when excited with 240 nm. The Al₂O₃:C, TL/OSL phosphor, also shows emission around 430 nm, which corresponds to characteristic emission of F-center. Thus, to identify the exact nature of luminescent center in Al₂O₃:Si,Ti, fluorescence lifetime measurement studies were carried out along with the PL,TL and OSL studies. The PL and TL in Al₂O₃:Si,Ti show emission around 430 nm and the time-resolved fluorescence studies show lifetime of about 43 μs for the 430 nm emission, which is much smaller than the reported lifetime of ∼35 ms for the 430 nm emission (F-center emission) in Al₂O₃:C phosphor. Therefore, the emission observed in Al₂O₃:Si,Ti phosphor was assigned to Ti⁴⁺ charge transfer transition. Fluorescence studies of Al₂O₃:Si,Ti do not show any traces of F and F⁺ centers. Also, Ti⁴⁺ does not show any change in the charge state after gamma-irradiation. On the basis of the above studies, a mechanism for TSL/OSL process in Al₂O₃:Si,Ti is proposed.     

Preparation and luminescent properties of (Ca1-x,Srx)S:Eu red-emitting phosphor for white LED

The (Ca_1−x,Sr_x)S:Eu²⁺ red-emitting phosphors were prepared by using sulfide and sulfate in CO-reductive atmosphere, respectively, and their luminescent properties have been investigated. The XRD data showed that the doping of divalent Europium ion (Eu²⁺) enlarges the lattice parameters, and Sr/Ca ratio not only affects the lattice parameters, but also influences the emission peak. The excitation and emission spectra indicated that this phosphor can be effectively excited by visible light from 430 to 500 nm, and exhibits a satisfactory red performance. The annealing treatment process was also discussed in detail. Compared with the commercial afterglow sulfide phosphor, the present synthesized phosphor has higher emission efficiency and is a favorable choice for white light-emitting diode (LED). The white LED implanted by present phosphor shows desirable luminescence and chromaticity properties.     

A simple procedure for electroluminescent phosphor assessment

A simple assessment procedure is described which facilitates optimal selection of an electroluminescent phosphor based on the measurement of two optically deduced energies—absorption and emission. These two energies are used as input for an appraisal analysis based on a classical configuration-coordinate diagram model of the phosphor. A phosphor performance figure-of-merit is proposed as a consequence of the formulation of this assessment procedure. This procedure is then used to rationalize the relative performance of five electroluminescent phosphors (ZnS:Mn, BaAl₂S₄:Eu, SrS:Ce, SrGa₂S₄:Ce, CaGa₂S₄:Ce). Additionally, two cathodoluminescent phosphors (ZnS:Ag and ZnS:Cu) are appraised using this methodology, suggesting that this procedure may be of some utility in the evaluation of other types of phosphors.     

Red electroluminescent process excited by hot holes in SrGa2S4:Ce, Mn thin film

This paper reports the first observation of red electroluminescence (EL) in SrGa₂S₄:Ce, Mn thin film. The EL spectrum consists of single broad emission band having a peak wavelength of 665 nm. The dominant EL decay time was 31 μs. The relationship between the applied voltage and the EL waveform was measured in single insulating thin film electroluminescent (TFEL) devices. An asymmetric EL waveform was observed in SrGa₂S₄:Ce, Mn TFEL devices under a rectangular applied voltage. The polarity of the EL waveform in these devices was different from the waveform in manganese-activated zinc sulfide ZnS:Mn devices. This indicates that hot holes excite the Mn²⁺ ions to cause the red EL.     

Sm-activated gadolinium molybdate: an intense red-emitting phosphor for solid-state lighting based on InGaN LEDs

Sm³⁺-activated gadolinium molybdate, Gd₂(MoO₄)₃:Sm³⁺ red-emitting phosphor was prepared by conventional solid-state method. The structure, morphology, and luminescent properties of these powder samples have been investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and fluorescent spectrophotometry, respectively. The as-obtained phosphor has a monoclinic structure with single crystalline phase. Its mean particle size is about 6-8 μm with pseudo-pompon shape and large surface area, which is suitable for manufacture of white LEDs. The phosphor can be efficiently excited by incident light of 348-445 nm, well matched with the output wavelength of a near-UV InGaN-based chip, and re-emits an intense red light peaking at 650 nm. By combing this phosphor with a 405 nm-emitting InGaN chip, a red LED was fabricated, so that the applicability of this novel phosphor to white LEDs was confirmed. It is considered to be an efficient red-emitting conversion phosphor for solid-state lighting based on InGaN LEDs.     

High luminance of new green emitting phosphor, Mg2SnO4:Mn

Mg₂SnO₄, which has an inverse spinel structure, was adopted as the host material of a new green emitting phosphor. Luminescence properties of the manganese-doped magnesium tin oxide prepared by the solid state reaction were investigated under vacuum ultraviolet (VUV) ray and low-voltage electron excitation. The Mg₂SnO₄:Mn phosphor exhibited green luminescence with the emission spectrum centered at 500 nm due to spin flip transition of the d-orbital electron associated with the Mn²⁺ ion. Optimum Mn concentration of Mg₂SnO₄:Mn under VUV excitation with 147 nm wavelength and electron beam excitation with 800 V excitation voltage are 0.25 and 0.6 mol%, respectively. The emission intensities of Mg₂SnO₄:Mn phosphors under the two excitation sources are higher than those of Zn₂SiO₄:Mn and ZnGa₂O₄:Mn phosphors. At 0.25 mol% of Mn concentration, on the other hand, the decay time is shorter than 10 ms.     

Characterization and luminescent properties of SiO2:ZnS:Mn and ZnS:Mn nanophosphors synthesized by a sol-gel method

ZnS and SiO₂-ZnS nanophosphors, with or without different concentration of Mn²⁺ activator ions, were synthesized by using a sol-gel method. Dried gels were annealed at 600 °C for 2 h. Structure, morphology and particle sizes of the samples were determined by using X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM). The diffraction peaks associated with the zincblende and the wurtzite structures of ZnS were detected from as prepared ZnS powders and additional diffraction peaks associated with ZnO were detected from the annealed powders. The particle sizes of the ZnS powders were shown to increase from 3 to 50 nm when the powders were annealed at 600 °C. An UV-Vis spectrophotometer and a 325 nm He-Cd laser were used to investigate luminescent properties of the samples in air at room temperature. The bandgap of ZnS nanoparticles estimated from the UV-Vis data was 4.1 eV. Enhanced orange photoluminescence (PL) associated with ⁴T₁→⁶A₁ transitions of Mn²⁺ was observed from as prepared ZnS:Mn²⁺and SiO₂-ZnS:Mn²⁺ powders at 600 nm when the concentration of Mn²⁺ was varied from 2-20 mol%. This emission was suppressed when the powders were annealed at 600 °C resulting in two emission peaks at 450 and 560 nm, which can be ascribed to defects emission in SiO₂ and ZnO respectively. The mechanism of light emission from Mn²⁺, the effect of varying the concentration on the PL intensity, and the effect of annealing are discussed.     

Emission characteristics of inorganic/organic hybrid white-light phosphor

A white-light light-emitting diode (LED) was successfully fabricated by converting near-UV LED emission (390–420 nm) with a new inorganic/organic hybrid phosphor. This new white-light phosphor consisted of three fluorescence materials; two strontium aluminates based with lanthanide oxides, SrAl₂O₄:Eu²⁺ (for green (520 nm) emission) and Sr₄Al₁₄O₂₅:Eu²⁺ (for blue (490 nm) emission) and a new organic Eu metal complex, Eu(BTFA)₃phen (for red (614 nm) emission). These materials have good absorption in the 300–500-nm range and have high quantum efficiencies (4.7–24.5%). The white phosphor has a CIE value of (0.321, 0.365). PACS 42.70.-aj; 78.40.-q; 78.55.-m; 81.05.Lg     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

Preparation and luminescent properties of phosphor MGd2(MoO4)4: Eu (M=Ca, Sr and Ba)

Intense red emitting phosphors MGd₂(MoO₄)₄: Eu³⁺ (M=Ca, Sr and Ba) have been synthesized by the simple sol-gel technique. The formation processes and the phase impurity of phosphors are characterized by thermogravimetry-differential thermal analysis (TG-DTA) and power X-ray diffraction (XRD). The narrower size distribution and the regular shape of the phosphor particles are also measured by Field emission scanning electronic microscopy (FE-SEM). Photo-luminescent properties of the phosphors are performed at room temperature. Their excitation spectra present strong absorption at 395 nm near-UV light and 465 nm blue light, which match well with commercial LED chips. The phosphors exhibit satisfactory and excellent red light dominated by 616 nm and their photoluminescence intensity is about 3-4 times stronger than that of phosphor YAG under the 465 nm excitation. In addition, the optimal concentrations of Eu³⁺ for phosphors MGd₂(MoO₄)₄ (M=Ca, Sr and Ba) have also been determined.     

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.     

Improved moisture resistance of SrS:Eu phosphors with nanoscale SiO2 coating

Orange-emitting SrS:Eu²⁺ phosphors were coated with nanoscale SiO₂ and their photoluminescence (PL) degradation behavior in moist air was investigated. The SiO₂ coating was obtained by sol-gel process using diethoxydimethylsilane (DEDMS) and the coating content was varied from 0.5 to 2 wt%. The coatings were composed of a uniform, continuous, and amorphous SiO₂ layer of 30-50 nm thickness and the coating thickness was not varied significantly with the coating content. No peak shift and no decrease of PL intensity were observed after coating. The PL intensity of the coated phosphors decreased to ∼75% of the original value after 10 h exposure to moist air, while the uncoated phosphor decreased to ∼33%, which indicates the improved moisture resistance of the nanoscale SiO₂ coated SrS:Eu²⁺ phosphors.     

Charge compensation on the luminescence properties of ZnWO4:Tb phosphors via hydrothermal synthesis

A phosphor Tb³⁺-doped ZnWO₄ (ZWO:Tb) phosphors were prepared by a hydrothermal method. X-ray powder diffraction (XRD) analysis revealed that the as-obtained sample is pure ZnWO₄ phase. The excitation and emission spectra indicated that the phosphor could be well excited by ultraviolet light (272 nm) and emit blue light at about 491 nm and green light at about 545 nm. Significant energy transfer from WO₄²⁻ groups to Tb³⁺ ions has been observed. Two approaches to charge compensation are investigated: (a) 2Zn²⁺ = Tb³⁺ + M⁺, where M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ acting as a charge compensator; (b) 3Zn²⁺ = 2Tb³⁺ + vacancy. Compared with two charge compensation patterns in the ZnWO₄:Tb³⁺, it has been found that ZnWO₄:Tb³⁺ phosphors used Li⁺ as charge compensation show greatly enhanced bluish-green emission under 272 nm excitation.     

Photophysical and photoluminescence properties of co-activated ZnS:Cu,Mn phosphors

ZnS:Cu,Mn phosphors were prepared by conventional solid state reaction with the aid of NaCl-MgCl₂ flux at 900 °C. The samples were characterized by X-ray powder diffraction, UV-vis absorbance spectra and photoluminescence spectra. All samples possess cubic structure. Cu has a much stronger effect on the absorption property of ZnS than Mn. Incorporation of Mn into ZnS host only slightly enhances the light absorption, while addition of Cu remarkably increases the ability of absorption due to ground state Cu⁺ absorption. The emission spectra of the ZnS:Cu,Mn phosphors consist of three bands centered at about 452, 520 and 580 nm, respectively. Introduction of Mn significantly quenches the green luminescence of ZnS:Cu. The excitation energy absorbed by Cu is efficiently transferred to Mn activators non-radiatively and the Mn luminescence can be sensitized by Cu behaving as a sensitizer (energy donor).     

Luminescent impurity doping trends in alternating-current thin-film electroluminescent phosphors

The doping properties of three alternating-current thin-film electroluminescent (ACTFEL) phosphor host/luminescent impurity systems, ZnS:Mn, SrS:Ce, and SrS:Cu, are elucidated, and the ACTFEL device implications of these properties are assessed. Mn is isovalent, Ce is a donor, and Cu is an acceptor. Moreover, Ce is readily ionized in SrS, so that it behaves as a double donor. The distinctly different doping nature of these three luminescent impurities leads to dramatically disparate defect and device physics trends. The donor/acceptor nature of Ce/Cu in SrS results in charge neutrality being achieved in SrS:Ce and SrS:Cu via self-compensation-induced vacancy creation; subsequent defect complexing between oppositely charged luminescent impurities and self-compensation-induced vacancies results in more complex ACTFEL device behaviors such as dynamic space charge, trailing-edge emission, charge collapse, color tuning, and electroluminescence (EL) thermal quenching. In contrast, the isovalent nature of ZnS:Mn leads to more ideal ACTFEL device operation. This suggests that the optimal ACTFEL phosphor luminescent impurity is isovalent.     

Synthesis and optimum luminescence of monodispersed spheres for BaWO4-based green phosphors with doping of Tb

Monodispersed spheres for Tb³⁺-doped BaWO₄ (BWO:Tb) phosphors were prepared by a hydrothermal method. X-ray diffraction (XRD) and field-emission scanning electron microscopy were used to characterize the resulting samples. Emission and excitation spectra were studied using xenon excited spectroscopic experiments at room temperature. Because 12 at% BWO:Tb phosphor exhibits intensive green emission under 254 nm excitation in comparison with the commercial green fluorescent lamp phosphor (LaPO₄:Ce,Tb), it is considered to be a new promising green phosphor for fluorescent lamps application.     

Scintillation and anomalous emission in elpasolite Cs2LiLuCl6:Ce

The luminescence and scintillation properties of Cs₂LiLuCl₆:0.5%Ce³⁺ are presented. Special attention is devoted to a 9.4 ns fast emission at 275 nm that can only be excited via the highest cubic field 5d_e state of Ce. Contrary to Cs₃LuCl₆ and Cs₂LiYCl₆, where the same type of fast emission was observed, the emission in Cs₂LiLuCl₆ is still observed at room temperature. Assuming that the 5d_e state is located inside the host conduction band (CB), we propose that the emission originates from a mixed state at or just below the bottom of the CB and ends at the 4f ground state of Ce³⁺. To proof this model we studied the thermal quenching of the anomalous luminescence and performed X-ray photoelectron spectroscopy. A model for a temperature-activated energy transfer from the anomalous state to the lowest 5d_t excited state of Ce³⁺ explains most of the results. Besides the 275 nm emission, the material shows 5d_t-4f Ce³⁺ emission at 370 and 406 nm and 2 ns fast core-valence luminescence when excited with 16-22 eV photons. The scintillation properties of Cs₂LiLuCl₆:Ce are briefly discussed.     

Luminescence properties of CaS:Ce, Sm nanophosphors

CaS:Ce, Sm nanophosphors were synthesized via solid state diffusion method. X-Ray diffraction confirmed the cubic crystalline phase of CaS:Ce, Sm nanoparticles. The particle size calculated using Debye-Scherrer formula was found to be 52 nm. The morphological investigations of the nanoparticles were made using TEM and found to have nearly spherical morphology with diameter 45-50 nm, which is in close agreement with the XRD result. The PL emission characteristics of CaS:Ce, Sm as a function of cerium and samarium concentrations have been studied and CaS:Ce_0.6Sm_0.4 system has maximum emission intensity, hence it was opted for further studies. The CaS:Ce_0.6Sm_0.4 system showed independent emission of Sm and Ce when excited at 330 and 450 nm, respectively. To study the energy transfer between cerium and samarium, the CaS:Ce_0.6Sm_0.4 was excited at wavelengths other than the excitation wavelengths of Ce (450 nm) and Sm (330 nm). The existence of Ce emission (at an excitation of 390 nm) even in the absence of Ce excitation band and Sm emission at an excitation of 405 nm, which is the excitation band of Ce, indicates the energy transfer at these two wavelengths. Thermoluminescence characteristics of ⁶⁰Co irradiated CaS:Ce_0.6Sm_0.4 have been investigated for different doses of 0.14-125 Gy. All the glow curves show a single peak at 475 K. With increasing dose, the intensity of this peak increases and a shoulder is formed on the lower temperature side at 415 K at 21 Gy of exposure. CaS:Ce_0.6Sm_0.4 shows almost linear dose dependence up to 125 Gy.     

Synthesis and photoluminescence of water-soluble Mn-doped ZnS quantum dots

The water-soluble Mn²⁺-doped ZnS quantum dots (Mn:ZnS d-dots) were synthesized by using thioglycolic acid (TGA) as stabilizer in aqueous solutions in air, and characterized by X-ray powder diffraction (XRD), UV-vis absorption spectra and photoluminescence (PL) emission spectroscopy. The sizes of Mn:ZnS d-dots were determined to be about 2 nm using XRD measurements and the UV-vis absorption spectra. It was found that the Mn²⁺⁴T₁ → ⁶A₁ emission intensity of Mn:ZnS d-dots significantly increased with the increase of Mn²⁺ concentration, and showed a maximum when Mn²⁺ doping content was 1.5%. If Mn²⁺ concentration continued to increase, namely more than 1.5%, the Mn²⁺⁴T₁ → ⁶A₁ emission intensity would decrease. In addition, the effects of TGA/(Zn + Mn) molar ratio on PL were investigated. It was found that the peak intensity ratio of Mn²⁺⁴T₁ → ⁶A₁ emission to defect-states emission showed a maximum when the TGA/(Zn + Mn) molar ratio was equal to 1.8.