Eu and Ce emission in sulphate based phosphors

Polycrystalline KMgSO₄Cl:Eu and Na₅(PO₄)SO₄:Ce phosphors prepared by a wet chemical method have been studied for its photoluminescence (PL) and thermoluminescence (TL) characteristics. The TL glow curve of the compound has a prominent peak at 200 °C and may be useful for TL study. TL sensitivity of the KMgSO₄Cl:Eu phosphor is found to be 1.7 times less than that of TLD—CaSO₄:Dy. The presence of bands at around 420, 435 and 445 nm in the PL emission spectra of the phosphor suggests the presence of Eu²⁺ in the host compound. Moreover a TL glow curve of the Na₅(PO₄)SO₄:Ce gives a better understanding of the TL mechanism (peaks at 271 and 310 °C) involved in the concerned phosphor. The PL emission spectra are observed at 382 nm for the various concentrations. In this paper we report PL and TL characteristics of KMgSO₄Cl:Eu halosulphate and Na₅(PO₄)SO₄:Ce phosphate sulphate phosphors first time.     

Photoluminescence and thermoluminescence characterization of Eu- and Dy -activated Ca3(PO4)2 phosphor

Rare-earth-doped polycrystalline Ca₃(PO₄)₂:Eu, Ca₃(PO₄)₂:Dy and Ca₃(PO₄)₂:Eu,Dy phosphors prepared by a modified solid-state synthesis has been studied for its X-ray diffraction, thermoluminescence (TL) and photoluminescence (PL) characteristics. The PL emission spectra of the phosphor suggest the presence of Eu³⁺ ion in Ca₃(PO₄)₂:Eu and Dy³⁺ ion in Ca₃(PO₄)₂:Dy lattice sites. The TL glow curve of the Ca₃(PO₄)₂:Eu compounds has a simple structure with a prominent peak at 228 °C, while Ca₃(PO₄)₂:Dy peaking at 146 and 230 °C. TL sensitivity of phosphors are compared with CaSO₄: Dy and found 1.52 and 1.20 times less in Ca₃(PO₄)₂:Eu and Ca₃(PO₄)₂:Dy phosphors, respectively. The Ca₃(PO₄)₂:Eu,Dy phosphors shows switching behavior under two different excitation wavelengths and enhancement in PL intensity of Dy³⁺ ions were reported. The paper discusses the photoluminescence and thermoluminescence behavior of Eu³⁺ and Dy³⁺ ion in Ca₃(PO₄)₂ hosts, it may be applicable to solid-state lighting as well as thermoluminescence dosimetry applications.     

Dosimetric properties of rare earth doped LiCaBO3 thermoluminescence phosphors

Lithium Calcium borate (LiCaBO₃) polycrystalline thermoluminescence (TL) phosphor doped with rare earth (RE³⁺) elements has been synthesized by high temperature solid state diffusion reaction. The reaction has produced a very stable crystalline LiCaBO₃:RE³⁺ phosphors. Among these RE³⁺ doped phosphors thulium doped material showed maximum TL sensitivity with favorable glow curve shape. TL glow curve of gamma irradiated LiCaBO₃:Tm³⁺ samples had shown two major well-separated glow peaks at 230 and 430 °C. The glow peak at 430 °C is almost thrice the intensity of the glow peak at 230 °C. The TL sensitivity of the phosphor to gamma radiation was about eight times that of TLD-100 (LiF). Photoluminescence and TL emission spectra showed the characteristic Tm³⁺ peaks. TL response to gamma radiation dose was linear up to 10³ Gy. Post-irradiation TL fading on storage in room temperature and elevated temperatures was studied in LiCaBO₃:Tm³⁺ phosphor.     

Luminescence investigations on sulfate apatite Na6(SO4)2FCl:RE (RE=Dy, Ce or Eu) phosphors

A new phosphor in the Cl-F system doped with Dy, Ce and Eu has been reported. Characterization of this phosphor using XRD, PL and TL techniques is described. Polycrystalline Na₆(SO₄)₂FCl:Dy; Na₆(SO₄)₂FCl:Ce and Na₆(SO₄)₂FCl:Eu phosphors prepared by a solid state diffusion method have been studied for their X-ray diffraction, photoluminescence (PL) and thermoluminescence (TL)characteristics. The PL excitation and emission spectra of phosphors were obtained. Dy³⁺ emission in the host at 475 and 570 nm is observed due to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transition, respectively, whereas the PL emission spectra of Na₆(SO₄)₂FCl:Ce phosphor shows the Ce³⁺ emission at 322 nm due to 5d→4f transition of Ce³⁺ ion. In Na₆(SO₄)₂FCl:Eu lattice, Eu²⁺ as well as Eu³⁺ emissions are observed. The emission of europium ion in this compound exhibits the blue as well as red emission. The TL glow curves of the same compounds have the simple structure with a prominent peak at 150, 175 and 200 °C. TL response, fading, reusability and trapping parameters of the phosphors are also studied. The TL glow curves of γ-irradiated Na₆(SO₄)₂FCl sample show one glow peak indicating that only one set of traps is being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). The trapping parameters associated with the prominent glow peak are calculated using Chen’s half width method. The release of hole/electron from defect centers at the characteristic trap site initiates the luminescence process in these materials. The intensity of the TL glow peaks increases with increase of the added γ-ray dose to the samples.     

Excitation of thermoluminescence in Eu doped Ba0.12Sr0.88SO4 nanophosphor by low energy argon ions

In the present paper thermoluminescence properties of argon ions irradiated barium strontium mixed sulphate phosphor are reported. The Ba_0.12Sr_0.88SO₄ phosphor was prepared by chemical co-precipitation method. The X-ray diffraction study of prepared sample suggests orthorhombic structure with average grain size of 37 nm. The samples were irradiated with 1.2 MeV Argon ions at fluences varying between 10¹¹-10¹⁵ ions/cm². The argon ions penetrate to the depth of 1.89 μm and lose their energy mainly via electronic stopping. Due to ion irradiation, a large number of defects in the sample are formed. Thermoluminescence (TL) glow curves were recorded for each of the ion fluence. These curves exhibit one broad peak with maximum intensity at 498 K composed of three overlapping peaks. This indicates that different sets of traps are being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). The peaks were observed due to formation of trap levels by ion irradiation and subsequently activation of traps on thermal stimulation. The TL response of the nanophosphor is linear in the dose range 59 kGy-590 MGy. Kinetic parameters associated with the prominent peaks were calculated using glow curve deconvolution (GCD) and verified by different glow curve shape and sample heating rate methods.     

A novel blue-emitting long-lasting proyphosphate phosphor Sr2P2O7:Eu,Y

By introducing the Y³⁺ into Sr₂P₂O₇:Eu²⁺, we successfully prepared a kind of new phosphor with blue long-lasting phosphorescence by the high-temperature solid-state reaction method. In this paper, the properties of Sr₂P₂O₇:Eu²⁺,Y³⁺ were investigated utilizing XRD, photoluminescence, luminescence decay, long-lasting phosphorescence and thermoluminescence (TL) spectra. The phosphor emitted blue light that was related to the 4f⁶5d¹-⁸S_7/2 transition of Eu²⁺. The bright blue phosphorescence could be observed by naked eyes even 8 h after the excitation source was removed. Two TL peaks at 317 and 378 K related to two types of defects appeared in the TL spectrum. By analyzing the TL curve the depths of traps were calculated to be 0.61 and 0.66 eV. Also, the mechanism of LLP was discussed in this report.     

Blue-emitting long-lasting phosphor, Sr3Al10SiO20:Eu,Ho

A novel blue-emitting long-lasting phosphor Sr₃Al₁₀SiO₂₀:Eu²⁺,Ho³⁺ is prepared by the conventional high-temperature solid-state technique and their luminescent properties are investigated. XRD, photoluminescence (PL) and thermoluminescence (TL) are used to characterize the synthesized phosphors. These phosphors are well crystallized by calcinations at 1500-1600 °C for 3 h. The phosphor emits blue light and shows long-lasting phosphorescence after it is excited with 254/365 nm ultraviolet light. TL curves reveal the introduction of Ho³⁺ ions into the Sr₃Al₁₀SiO₂₀:Eu²⁺ host produces a highly dense trap level at appropriate depth, which is the origin of the long-lasting phosphorescence in this kind of material. The long-lasting phosphorescence lasts for nearly 6 h in the light perception of the dark-adapted human eye (0.32 mcd/m²). All the results indicate that this phosphor has promising potential practical applications.     

Thermoluminescence of rare earth activated CdSO4, SrSO4 and BaSO4

The thermoluminescence (TL) of rare earth (RE) activated sulfates of Cd, Sr and Ba was studied above room temperature. Many of the phosphors prepared exhibit an extremely bright TL following X-irradiation (most notably with Sm, Eu, Tb, Dy and Tm dopants), having an efficiency comparable to that of the highest sensitivity phosphors available for TL dosimetry, and exhibiting activator-induced glow peaks between 405 and 480°K. In a given lattice, the RE³⁺ ions produce a characteristic glow peak at the same temperature (independent of the particular RE ion), whereas Eu²⁺ produces a single glow peak at a different temperature. A decrease in glow peak temperature with increasing interatomic spacing was observed in the homologous SrSO₄-BaSO₄ system - this shift being most pronounced in the Eu²⁺ -doped materials. TL emission spectra were obtained for trivalent Sm, Tb, Dy and Tm and for divalent Eu in these sulfates (and also in CaSO₄).     

Argon ions induced thermoluminescence properties of Ba0.12Sr0.88SO4 phosphor

Thermoluminescence properties of barium strontium mixed sulfate have been studied by irradiation with Argon ions. The sample was recrystallized by chemical co-precipitation techniques using H₂SO₄. The X-ray diffraction study of prepared sample suggests the orthorhombic structure with average grain size of 60 nm. The samples were irradiated with 1.2 MeV Argon ions at fluences varying between 10¹¹ and 10¹⁵ ions/cm². The argon ions penetrate to the depth of 1.89 μm and lose their energy mainly via electronic stopping. Due to ion irradiation, a large number of defects in the sample are formed. Thermally stimulated luminescence (TSL) glow curves of ion irradiated Ba_0.12Sr_0.88SO₄ phosphor exhibit broad peak with maximum intensity at 495 K composed of four overlapping peaks. This indicates that different sets of traps are being activated within the particular temperature range each with its own value of activation energy (E) and frequency factor (s). Thermoluminescence (TL) glow curves were recorded for each of the ion fluences. A linear increase in intensity of TL glow peaks was found with the increase in ion dose from 59 kGy to 5.9 MGy. The kinetic parameters associated with the prominent glow peaks were calculated using glow curve deconvolution (GCD), different glow curve shape and sample heating rate methods.     

Lattice site dependent cathodoluminescence behavior and surface chemical changes in a Sr5(PO4)3F host

Eu activated Sr₅(PO₄)₃F phosphor powders have been subjected to the electron bombardment at 2 keV (10 μA) at an oxygen pressure of 1×10⁻⁶ Torr. The synthesized Sr₅(PO₄)₃F phosphor was identical to the hexagonal apatite structure, with the Sr present at two different sites C_s (S1) and C₃ (S2) in the Sr₅(PO₄)₃F host, as inferred from the crystallographic study. Cathodoluminescence (CL) and Auger electron spectroscopy of the phosphor excited by the same electron beam were used to monitor changes in the surface state during prolonged electron bombardment. A direct correlation between the surface reactions and the degradation of the CL brightness was observed. Both C and F were depleted from the surface during electron bombardment. The postulated mechanism for the electron stimulated chemical reactions on the phosphor surface is electron beam dissociation of molecular species to atomic species, which subsequently react with C to form volatile compounds CO₂, CH₄, etc. and with Sr₅(PO₄)₃F to form a non luminescence layer of metal oxides of Sr and P.     

Blue and red long lasting phosphorescence (LLP) in β-Zn3(PO4)2:Mn,Zr

Multi-color long lasting phosphorescent (LLP) phenomenon in β-Zn₃(PO₄)₂:Mn²⁺,Zr⁴⁺ was systematically investigated. It is found that the red (λ_Em=616 nm) LLP performance of Mn²⁺ such as brightness and duration is largely improved, and that the blue (λ_Em=475 nm) LLP of Zr⁴⁺ with lower intensity appears when Zr⁴⁺ ions are co-doped into the matrix. The fluorescence, phosphorescence and thermoluminescence (TL) spectra show that Mn²⁺ ion is solely expected as a luminescent center, while Zr⁴⁺ ion not only acts as a luminescent center, but also induces an electron trap (Trap_Zr) associated with a TL peak at 344 K. The trap depth for Trap_Zr is 0.25 eV, while that for the intrinsic trap is 0.38 eV, associated with a dominant peak at 385 K for Zn₃(PO₄)₂:Mn²⁺. The Zr⁴⁺-induced trap with suitable depth is responsible for the improvement of the red LLP of Mn²⁺ ion and the appearance of the blue LLP of Zr⁴⁺ ion. The LLP mechanism is also investigated.     

Low voltage electron induced cathodoluminescence degradation and surface characterization of Sr3(PO4)2:Tb phosphor

Tb³⁺-doped Sr₃(PO₄)₂ phosphor was prepared by a sol-gel combustion method. A trigonal structure having Sr and O atoms occupying two different lattice sites were obtained. Scanning Auger nanoprobe was used to analyze the morphology of the particles. Photoluminescence (PL) and cathodoluminescence (CL) properties of Sr₃(PO₄)₂:Tb powder phosphors were evaluated and compared. In addition, the CL intensity degradation of Sr₃(PO₄)₂:Tb was evaluated when the powders were irradiated with a beam of electrons in a vacuum chamber maintained at an O₂ pressure of 1 × 10⁻⁶ Torr or a background pressure of 1 × 10⁻⁸ Torr O₂. The surface chemical composition of the degraded powders, analyzed by X-ray photoelectron spectroscopy (XPS), suggests that new compounds (metal oxides) of strontium and phosphorous were formed on the surface. It is most likely that these compounds contributed to the CL intensity degradation of the Sr₃(PO₄)₂:Tb phosphors. The CL properties and possible mechanism by which the new metal oxides were formed on the surface due to a prolonged electron beam irradiation are discussed.     

Structural and luminescence characterization of synthetic Cr-doped Ni3(PO4)2

Ni_3–xCr_2x/3(PO₄)₂ (x=0 and 0.02) microcrystalline powders were obtained as single phases via a modified sol–gel Pechini-type in situ polymerizable complex method. The samples were characterized using scanning electron microscopy, X-ray diffraction, cathodoluminescence (CL), and thermoluminescence (TL) techniques. We found that Cr³⁺ doping modified the average particle and distribution. The mean particle size was 0.441 μm for Ni₃(PO₄)₂ and 0.267 μm for Ni_2.98Cr_0.013(PO₄)₂. The results also reveal that Cr³⁺ doping notably enhanced the CL and TL UV-blue emission.     

A rapid combustion process for the preparation of MgSrAl10O17:Eu phosphor and related luminescence and defect investigations

Blue-emitting europium-ion-doped MgSrAl₁₀O₁₇ phosphor, prepared using the combustion method, is described. An efficient phosphor can be prepared by this method in a muffle furnace maintained at 500 °C in a very short time of few minutes. The phosphor is characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET surface area measurements. Photoluminescence (PL) spectra revealed that europium ions were present in divalent oxidation state. The thermoluminescence (TL) glow curve shows two peaks at around 178 and at 354 °C. The defect centres formed in the phosphor are studied using electron spin resonance (ESR). The ESR spectrum indicates the presence of Fe³⁺ ions in the non-irradiated system. Irradiated MgSrAl₁₀O₁₇:Eu exhibits lines due to radiation-sensitive Fe³⁺ ion and a defect centre. The centre is characterized by an isotropic g-value of 2.0012 and is assigned to a F⁺ centre. The radiation-sensitive Fe³⁺ ion appears to correlate with the main TL peak at 178 °C. During irradiation an electron is released from Fe²⁺ and is trapped at an anion vacancy to form F⁺ centre. During heating, an electron is liberated from the defect centre and recombines with Fe³⁺ emitting light.     

Luminescence properties of Tm co-doped Sr2Si5N8:Eu red phosphor

The Sr₂Si₅N₈:Eu²⁺ phosphors, both undoped and doped with Tm³⁺, were synthesized by high temperature solid-state method. The XRD pattern shows that only Sr₂Si₅N₈ phase is formed whatever Tm³⁺ was doped or not. The peak positions of both phosphors are centered at 612 nm which is assigned to the 4f⁶5d→4f⁷ transition of Eu²⁺. It implies that the crystal field, which affects the 5d electron states of Eu²⁺, is not changed dramatically after the phosphor is doped with Tm³⁺. The afterglow time is about 10 min after Tm³⁺ ion is introduced into the phosphor. The concentration of Tm³⁺ has little influence on the afterglow time of the phosphor. The depths of trap energy level of the two phosphors were calculated based on the TL spectra. The depths of Sr₂Si₅N₈:Eu²⁺ and Sr₂Si₅N₈:Eu²⁺, Tm³⁺ are 1.75 and 1.01 eV, respectively.     

Low-temperature thermoluminescence in CaGa2S4:Eu

In the last years many insulating and semiconducting materials activated with rare-earth elements were found to exhibit phosphorescence and thermoluminescence properties, and are attracting increasing interest due to the variety of application of long-lasting phosphors. In this work we studied the phosphorescence decay and thermoluminescence properties of CaGa₂S₄:Eu²⁺ as a function of temperature in the 9-325 K range. The comparison between spectra recorded as a function of time delay from the excitation pulse at different temperatures indicates that long-lasting emissions peaked at about 2.2 eV occurs at Eu²⁺ sites. Thermoluminescence glow curve is characterized by five components at 69, 98, 145, 185 and 244 K. Experimental data are discussed in the framework of generalized order of kinetic model and allow to estimate the activation energies of trapping defects. The origin of glow components at 69, 98, 145 and 244 K is correlated to trapping defects induced by Eu²⁺ doping, while the component at 185 K is attributed to a continuous distribution of defects.     

Thermoluminescence studies on Cu-doped Li2B4O7 single crystals

Lithium tetraborate (Li₂B₄O₇) is a tissue equivalent material and single crystals of this material doped with Cu are promising for dosimetric applications. In the present study highly transparent single crystals of lithium tetraborate (Li₂B₄O₇) doped with Cu (0.5 wt%) have been grown using the Czochralski technique. The Li₂B₄O₇:Cu crystals were studied using photoluminescence, X-ray diffraction (XRD), UV-vis transmission, time resolved fluorescence and thermoluminescence (TL) techniques. The TL readout of Li₂B₄O₇:Cu crystals showed two well-defined glow peaks at 402 K (peak-1) and 513 K (peak-2) for a 4 K/s heating rate. While the low temperature TL peak-1 fades completely within 24 h at room temperatures, the main dosimetric peak-2 remains the same. The TL sensitivity of the grown single crystal is found to be 3.3 times that of a conventional TL phosphor, TLD-100. The Li₂B₄O₇:Cu crystals showed a linear TL dose-response in the range from 1 mGy to 1 kGy. The TL analysis using a variable dose method revealed first order kinetics for both the peaks. Trap depth and frequency factor for peak-1 were found to be 0.81 eV and 5.2×10⁹ s⁻¹, whereas for peak-2 the values were 1.7 eV and 1.7×10¹⁶ s⁻¹, respectively.     

Improved luminescent properties of red-emitting Ca0.54Sr0.16Eu0.08Gd0.12(MoO4)0.2(WO4)0.8 phosphor for LED application by charge compensation

The red-emitting Ca_0.54Sr_0.16Eu_0.08Gd_0.12(MoO₄)_0.2(WO₄)_0.8 phosphor is improved in the emission charateristics by charge compensation, of which chromaticity coordinates (CIE) are x=0.66 and y=0.33. Three approaches to charge compensation are investigated, namely (a) 3Ca²⁺/Sr²⁺→2Eu³⁺/Gd³⁺+vacancy, (b) 2Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+M⁺(M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ employed as a charge compensator) and (c) Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+N⁻ (N⁻ is a monovalent anion like F⁻, Cl⁻, Br⁻ and I⁻ employed as charge compensation ions). Through photoluminescent spectra analyzing the radiative and non-radiative relaxation mechanisms of luminescent system are obtained. Under 20 mA forward-bias current, one red-emitting LED is made by combining 390-405 nm-emitting LED chip and the phosphor. The red-emitting phosphor has broad prospects in LED application field.     

Photoluminescence of double-color-emitting phosphor Ca5(PO4)3Cl:Eu, Mn for near-UV LED

Eu²⁺ activated Ca₅(PO₄)₃Cl blue-emitting phosphors were prepared by the conventional solid state method using CaCl₂ as the chlorine source and H₃BO₃ as flux. The structure and luminescent properties of phosphors depend on the concentrations of Eu²⁺, the amount of CaCl₂ and the usage of the H₃BO₃ flux were investigated systematically. Eu²⁺ and Mn²⁺ Co-doped Ca₅(PO₄)₃Cl with blue and orange double-band emissions were also researched based on the optimal composition and synthesis conditions. The energy transfer between Eu²⁺ and Mn²⁺ was found in the phosphor Ca₅(PO₄)₃Cl:Eu²⁺, Mn²⁺, and the Co-doped phosphor can be efficiently excited by near-UV light, indicating that the phoshor is a potentional candidate for n-UV LED used phosphor.     

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.