Luminescence properties of lanthanide doped alkaline earth chlorides under (V)UV and X-ray excitation

The photoluminescence and radioluminescence of Ce³⁺, Pr³⁺ and Nd³⁺ in SrCl₂ and BaCl₂ are reported and discussed in relation to application as a (fast) scintillator material. The Ce³⁺ doped materials exhibit the typical fast cerium d-f emission (358, 382 nm in SrCl₂ and 350, 375 nm in BaCl₂) both under synchrotron and ionizing radiation excitations. A weak afterglow is observed. For Pr³⁺ a very fast d-f emission is observed in SrCl₂ (250, 263, 300 and 328 nm, τ = 13 ns) and BaCl₂ (260, 288 and 315 nm, τ = 10 ns), but only under direct f-d excitation. No d-f luminescence is observed for Pr³⁺ under X-ray excitation. The absence of d-f emission is explained by the energy of the host lattice emission which is too low for energy transfer to the high energy fd states of Pr³⁺ (230 nm in SrCl₂ and 235 nm in BaCl₂). The neodymium doped chlorides do not show d-f luminescence.     

Luminescence properties of a new blue-emitting phosphor Ce-doped CaLaGa3S7

A new series of blue-emitting Ce³⁺-doped CaLaGa₃S₇ thiogallate chalcogenide phosphors were synthesized by a solid-state reaction method. Their luminescence properties were investigated by photoluminescence excitation, emission spectra and lifetime. The critical dopant concentration was found to be 0.15 mol of Ce³⁺ (R_c = 15 Å) and the fluorescence lifetime of Ce³⁺ in CaLaGa₃S₇:0.15 Ce³⁺ was 12.9 ns. The blue-emitting LED was fabricated by combining an InGaN chip (395 nm) with a CaLaGa₃S₇:Ce³⁺ phosphor. The CIE chromaticity coordinates of the blue LED were calculated to be (0.14, 0.23).     

Preparation of Dy-activated strontium orthosilicate (Sr2SiO4:Dy) phosphors and its photoluminescent properties

Dy³⁺-activated β/α′-Sr₂SiO₄ phosphors were successfully prepared by solid-state reaction method with ammonium chloride (NH₄Cl) as the flux. The influences of calcination temperatures, amounts of NH₄Cl and the concentrations of Dy³⁺ on phase composition, morphology and the photoluminescent properties of as-prepared powders were investigated in detail. The β and α′ phases of Sr₂SiO₄ were obtained with 1 wt% and 2-5 wt% NH₄Cl, respectively, as the sintered condition was at 1000 °C for 4 h. With increasing the amount of NH₄Cl, the morphology of phosphors changed from needlelike to regular polyhedron shape and the colors of the Sr₂SiO₄:Dy³⁺ phosphors changed from blue-green to white. The luminescence intensity of ⁴F_9/2 → ⁶H_15/2 transition was slightly higher than that of ⁴F_9/2 → ⁶H_13/2 (ΔL = 2, ΔJ = 2) transition owing to the low-symmetry around Dy³⁺ ions. The optimum concentration of Dy³⁺ was 2.0 mol% and the concentration quenching were caused by the d-d interaction and a cross relaxation. The yellow-to-blue intensity ratio (Y/B) of Dy³⁺ emission did not to change with varying the Dy³⁺ concentration using Li⁺ ions for charge compensation. These indicate that this phosphor can be used as a potential candidate for the phosphor-converted white LEDs with a UV chip.     

Photoluminescence properties and energy-transfer of thermal-stable Ce, Mn-codoped barium strontium lithium silicate red phosphors

A series of thermal-stable Ce³⁺, Mn²⁺-codoped barium strontium lithium silicate (BSLS) phosphors was synthesized by a high-temperature solid-state reaction. The XRD patterns of this phosphor seem to be a new phase that has not been reported before. BSLS:Ce³⁺, Mn²⁺ showed two emission bands under 365 nm excitation: one observed at 421 nm was attributed to Ce³⁺ emission, and the other found in red region was assigned to Mn²⁺ emission through Ce³⁺-Mn²⁺ efficient energy transfer. The Mn²⁺ emission shifted red along with the replacement of barium by strontium, which was due to the change of crystal field. A composition-optimized phosphor, BSLS:0.10Ce³⁺, 0.05Mn²⁺ (Ba = 65), exhibited strong and broad red-emitting and supreme thermal stability. The results suggest that this phosphor is suitable as a red component for NUV LED or high pressure Hg vapor (HPMV) lamp.     

Reddish orange long afterglow phosphor Ca2SnO4:Smprepared by sol-gel method

A reddish orange light emissive long afterglow phosphor, Ca₂SnO₄:Sm³⁺ was prepared by sol-gel method at lower temperature. The synthesized phosphors were characterized by X-ray diffraction, scanning electron micrograph images, photoluminescence spectra, afterglow decay curves and thermoluminescence spectra. Three emission peaks locate at 565 nm, 609 nm and 655 nm corresponding to CIE chromaticity coordinates of x = 0.53 and y = 0.47, which indicates the reddish orange light emitting. The fluorescent intensity and the afterglow characteristic depends on the concentration of Sm³⁺ and the optimized concentration is 1.5 mol%. The afterglow decay curves are well fitted with triple-exponential decay models. The thermoluminescence glow curves show that the Sm³⁺ induces suitable trap depth and result in the long afterglow phenomenon, and the corresponding increase or decrease in afterglow is associated with trap concentration, nearly no change in trap depth. The 1.5 mol% Sm³⁺-doped Ca₂SnO₄ sample has the biggest trap concentration and exhibit the best afterglow characteristic, its’ afterglow time is about 1 h. The phosphorescence mechanism of this long afterglow phosphor was discussed.     

Synthesis and photoluminescence of a novel Sr-SiAlON:Eu blue-green phosphor (Sr14Si68−sAl6+sOsN106−s:Eu (s ≈ 7))

A novel Eu²⁺ activated Sr-SiAlON oxynitride phosphor, with the chemical composition of Sr₁₄Si_68−sAl_6+sO_sN_106−s:Eu²⁺ (s ≈ 7), was synthesized by firing the powder mixture of SrO, SrSi₂, α-Si₃N₄, AlN and Eu₂O₃ at 1900 °C for 6 h under 1 MPa nitrogen atmosphere. The structure has a typical feature of SiAlON consisting of the host framework which is constructed by a three-dimensional MX₄ tetrahedral (M: Si or Al; X: O or N) network, and Sr or Eu²⁺ ions as the guest ions. It has been shown that the Sr-SiAlON:Eu²⁺ phosphor has the excitation band covering the range of the ultraviolet light region to 500 nm, and exhibits an intense blue-green color with the emission band centered at about 508 nm. The temperature dependent emission intensity of the Sr-SiAlON:Eu²⁺ phosphor is better than that of a typical blue-green Ba₂SiO₄:Eu²⁺ phosphor. It is demonstrated that Sr-SiAlON:Eu²⁺ phosphor is very promising for use in white -LEDs.     

Synthesis, photoluminescence, thermoluminescence and dosimetry properties of novel phosphor KSr4(BO3)3:Ce

Polycrystalline powder sample of KSr₄(BO₃)₃ was synthesized by high-temperature solid-state reaction. The influence of different rare earth dopants, i.e. Tb³⁺, Tm³⁺ and Ce³⁺, on thermoluminescence (TL) of KSr₄(BO₃)₃ phosphor was discussed. The TL, photoluminescence (PL) and some dosimetric properties of Ce³⁺-activated KSr₄(BO₃)₃ phosphor were studied. The effect of the concentration of Ce³⁺ on TL intensity was investigated and the result showed that the optimum Ce³⁺ concentration was 0.2 mol%. The TL kinetic parameters of KSr₄(BO₃)₃:0.002 Ce³⁺ phosphor were calculated by computer glow curve deconvolution (CGCD) method. Characteristic emission peaking at about 407 and 383 nm due to the 4f⁰5d¹ → ²F_{(5/2, 7/2)} transitions of Ce³⁺ ion were observed both in PL and three-dimensional (3D) TL spectra. The dose–response of KSr₄(BO₃)₃:0.002 Ce³⁺ to γ-ray was linear in the range from 1 to 1000 mGy. In addition, the decay of the TL intensity of KSr₄(BO₃)₃:0.002 Ce³⁺ was also investigated.     

An open aperture z-scan study of Sr2CeO4 blue phosphor

Sr₂CeO₄ blue phosphor has been prepared by the solid-state reaction method. The X-ray diffraction (XRD) study confirms the structure of the system to be orthorhombic. High resolution electron transmission microscopy reveals that Sr₂CeO₄ prepared by the solid state reaction method is composed of elongated spherical structures of length ∼0.2-0.6 μm and width ∼90-150 nm. The excitation spectrum shows a broad band which peaks at 275 nm. The emission spectrum shows a broad band which peaks at 467 nm when excited at 275 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer (CT) state of the Ce⁴⁺ ion. The Commission International de l’Eclairage (CIE) co-ordinates are x = 0.15, and y = 0.23. The nonlinear absorption behavior of Sr₂CeO₄ has been investigated using the open aperture z-scan technique. The calculated effective two-photon absorption coefficient shows that the Sr₂CeO₄ blue phosphor is a promising optical limiting material.     

Luminescent properties of UV excitable blue emitting phosphors MSr4(BO3)3:Ce (M = Li and Na)

A series of Ce³⁺ doped novel borate phosphors MSr₄(BO₃)₃ (M = Li or Na) were successfully synthesized by traditional solid-state reaction. The crystal structures and the phase purities of samples were characterized by powder X-ray diffraction. The optimal concentrations of dopant Ce³⁺ ions in compound MSr₄(BO₃)₃ (M = Li or Na) were determined through the measurements of photoluminescence spectra of phosphors. Ce³⁺ doped phosphors MSr₄(BO₃)₃ (M = Li or Na) show strong broad band absorption in UV spectral region and bright blue emission under the excitation of 345 nm light. In addition, the temperature dependences of emission spectra of M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors with optimal composition x = 0.05 for Li and x = 0.09 for Na excited under 355 nm pulse laser were also investigated. The experimental results indicate that the M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors are promising blue emitting phosphors pumped by UV light.     

Fabrication and properties of Ba(Zr1−xCex)0.9Y0.1O2.95/NaCl composite electrolyte materials

Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95/NaCl (x = 0.1, 0.2 and 0.3) composite electrolyte materials were fabricated with ZnO as sintering aid. The effect of ZnO on the properties of Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 matrix were investigated. The phase composition and microstructure of samples were characterized by XRD and SEM, respectively. The electrochemical performances were studied by three-probe conductivity measurement and AC impedance spectroscopy. XRD results showed that Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 with 2 mol% of ZnO was perovskite structure. The relative density of this sample was above 95% when sintered at 1450 °C for 6 h. By adding 10 mol% of NaCl to Ba(Zr_1−xCe_x)_0.9Y_0.1O_2.95 with 2 mol% of ZnO that was sintered at 1400 °C for 6 h, the conductivity was increased. The electrical conductivity of 1.26 × 10⁻² S/cm and activation energy of 0.23 eV were obtained when tested at 800 °C in wet hydrogen.     

Photoluminescence and energy transfer studies on Eu and Ce co-doped SrCaSiO4 for white light-emitting-diodes

The luminescence of SrCaSiO₄:Eu²⁺, Ce³⁺ is studied as a potential ultraviolet light-emitting diodes (UV-LEDs) phosphor that is capable of converting the ultraviolet emission of a UV-LED into green light with good luminosity. There are two emissions peaks peaking at 420 and 500 nm, respectively. The two emissions come from d-f transitions of Ce³⁺ and Eu²⁺, respectively. Effective energy transfer occurs in Ce³⁺/Eu²⁺ co-doped SrCaSiO₄ due to a part of spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺. The Ce³⁺/Eu²⁺ energy transfer, thoroughly investigated by the diffuse reflection emission and excitation spectra, photoluminescence decay curves, is demonstrated to be in the mechanism of electric dipole-dipole interaction.     

Impact of Ce ion on microstructure and luminescence character of Ho/Yb co-doped ZrO2 nanocrystal

By solid-state synthesis method, Ho³⁺/Yb³⁺ co-doped CeO₂-ZrO₂ nano-powders have been prepared. The concentration of Ce⁴⁺ ions has greater effect to the oxygen lattice structure. When the concentration of Ce⁴⁺ ions is 30 mol%, the oxygen lattice is a tetrahedral space group and the luminescence intensity of the sample is strongest. The results show that the lattice structure can be changed by inducting the Ce⁴⁺ ions into Ho³⁺/Yb³⁺ co-doped ZrO₂. And the emission character can be improved.     

Preparation and its luminescent properties of AlPO4:Eu phosphor for w-LED applications

Orange-reddish-emitting phosphor AlPO₄:Eu³⁺ were fabricated by solid-state reactions at high temperature. X-ray diffraction analysis revealed that AlPO₄ doped with 3 mol% of Eu³⁺ (AlPO₄:0.03Eu³⁺) was pure orthorhombic phase. The photoluminescence study shows that the intensity of magnetic dipole transition (⁵D₀ → ⁷F₁) at 594 nm dominates over that of electric dipole transition (⁵D₀ → ⁷F₂) at 613 nm. The optimum concentration of Eu³⁺ for the highest luminescence is found to be 3 mol%. The PL excitation spectrum is composed of CTB of Eu-O and excitation lines of Eu³⁺ ions. The strongest excitation lines appeared at 392 nm. The color coordinates, quantum yield and lifetime for AlPO₄:0.03Eu³⁺ were measured. All the spectrum features indicate that AlPO₄:Eu³⁺ might be a promising phosphor for display devices or w-LEDs.     

Photoluminescent properties of LiSrxBa1−xPO4:RE (RE = Sm, Eu) f-f transition phosphors

Rare-earth ions (Sm³⁺ or Eu³⁺) doped LiSr_xBa_1−xPO₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) f-f transition phosphor powders were prepared by a high temperature solid-state reaction. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the phase structure of the sample changes from LiBaPO₄ to LiSrPO₄ when x changes from 0 to 1.0. The excitation spectra indicate that only direct excitation of rare earth ions (Sm³⁺ or Eu³⁺) can be observed. The doped rare earth ions show their characteristic emission in LiSr_xBa_1−xPO₄, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Sm³⁺⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2), respectively. The dependence of the emission intensities of the LiSr_xBa_1−xPO₄:Sm³⁺ and LiSr_xBa_1−xPO₄:Eu³⁺ phosphors on the x value and Ln³⁺ (Ln³⁺ = Sm³⁺, Eu³⁺) concentration is also investigated.     

Photoluminescence and energy transfer of Ce and Tb doped oxyfluoride aluminosilicate glasses

The transmission and photoluminescence (PL) properties of Ce³⁺ or Tb³⁺ doped and Tb³⁺/Ce³⁺ codoped oxyfluoride aluminosilicate glasses were reported. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were applied to confirm the structure and thermal stability of samples. PL spectra revealed a bright and broad violet-blue emission derived from Ce³⁺ [5d (²D) → ²F_5/2,7/2] and an intense sharp green emission (543 nm) derived from Tb³⁺ (⁵D₄ → ⁷F₅) in the Ce³⁺ and Tb³⁺ doped glasses, respectively. Concentration quenching is not observed even the mole ratio of Tb³⁺ is up to 8% in Tb³⁺ doped glass. This indicates that the as-made host glass provides a good distribution of Tb³⁺ activators in glass matrix. For Tb³⁺/Ce³⁺ codoped glasses, a strong green emission corresponding to Tb³⁺ (⁵D₄ → ⁷F₅) and an energy transfer phenomenon from Ce³⁺ to Tb³⁺ were observed upon excitation with an UV wavelength (289 nm). It was also observed that their PL intensity depends on the concentration of Ce³⁺ when the concentration of Tb³⁺ is fixed. The mechanism involved in the energy transfer between Ce³⁺ and Tb³⁺ was explained with an energy level diagram.     

Effect of Bi doping on the quenching concentration of H11/2/S3/2 level of Er

Bi³⁺ and Er³⁺ codoped Y₂O₃ was prepared by sol-gel method. The upconversion emission was investigated under 980 nm excitation. For samples without Bi³⁺, the quenching concentration of ²H_11/2/⁴S_3/2 level of Er³⁺ is 3.0 mol%. However, by 1.5 mol% Bi³⁺ doping the quenching concentration increases to 5.0 mol%; meanwhile, the green emission is enhanced 1.9 times. The results indicate that both the quenching concentration and the emission intensity of ²H_11/2/⁴S_3/2 level can be increased by Bi³⁺ doping.     

Structural, absorption and fluorescence spectral analysis of Pr ions doped zinc bismuth borate glasses

Glasses having composition 20ZnO·xBi₂O₃·(79.5 − x)B₂O₃·0.5Pr₆O₁₁ were prepared by melt quench technique. The amorphous nature of the prepared glass samples was confirmed by X-ray diffraction. The spectroscopic properties of these glasses were investigated by recording optical absorption and fluorescence spectra. The structural investigations of the glasses were carried out by recording the IR spectra. The optical properties of Pr³⁺ ions doped zinc borate glasses with varying concentration of bismuth oxide have been studied. The Judd-Ofelt intensity parameters Ω_λ (λ = 2, 4, 6) and other radiative properties like radiative transition probability, radiative life time, branching ratio and stimulated emission cross-section of the prepared glasses have been calculated. The variation of Ω₂ with Bi₂O₃ content has been attributed to changes in the asymmetry of the ligand field at the rare earth ion site and to the changes in their rare earth oxygen (RE-O) covalency. The branching ratio for ³P₀ → ³F₂ transition is 42% and the predicted spontaneous radiative transition probability rates are fairly high (14,347-14,607 s⁻¹). This is beneficial for lasing emission. The stimulated emission cross-section for all the emission bands has been calculated.     

Photoluminescence and thermoluminescence studies of Mg2SiO4:Eu nano phosphor

Nanoparticles of Eu³⁺ doped Mg₂SiO₄ are prepared using low temperature solution combustion technique with metal nitrate as precursor and urea as fuel. The synthesized samples are calcined at 800 °C for 3 h. The Powder X-ray diffraction (PXRD) patterns of the sample reveled orthorhombic structure with α-phase. The crystallite size using Scherer's formula is found to be in the range 50-60 nm. The effect of Eu³⁺ on the luminescence characteristics of Mg₂SiO₄ is studied and the results are presented here. These phosphors exhibit bright red color upon excitation by 256 nm light and showed the characteristic emission of the Eu³⁺ ions. The electronic transition corresponding to ⁵D₀ → ⁷F₂ of Eu³⁺ ions (612 nm) is stronger than the magnetic dipole transition corresponding to ⁵D₀ → ⁷F₁ of Eu³⁺ ions (590 nm). Thermoluminescence (TL) characteristics of γ-rayed Mg₂SiO₄:Eu³⁺ phosphors are studied. Two prominent and well-resolved TL glows with peaks at 202 °C and 345 °C besides a shoulder with peak at ∼240 °C are observed. The trapping parameters-activation energy (E), order of kinetics (b) and frequency factor (s) are calculated using glow curve shape method and the results obtained are discussed.     

Spectroscopic properties of Pr:SrMoO4 crystal

A Pr³⁺-doped SrMoO₄ single crystal has been grown successfully by the Czochralski technique. The absorption spectra, fluorescence spectra, and fluorescence decay curves have been measured at room temperature. The spectroscopic parameters, including spectroscopic parameters Ω_t (t = 2,4,6), radiative transition probabilities, radiative lifetimes and branching ratios were determined according to the standard and modified Judd-Ofelt theories. The stimulated emission cross-sections, the fluorescence lifetimes and the quantum efficiency of Pr³⁺ ions in SrMoO₄ were obtained. This study implies that Pr³⁺:SrMoO₄ is an attractive candidate for solid-state laser medium.