Luminescence and energy transfer processes in Lu2SiO5:Ce scintillator

The energy transfer processes in Lu₂SiO₅:Ce³⁺ luminescence was investigated through the temperature dependent luminescence under excitation with VUV-UV. Ce1 center emission peaking at 393 and 422 nm and Ce2 center emission peaking at 462 nm were observed. Ce2 center emission is enhanced with the temperature, which can be explained by the rate of energy transfer from Ce1 center increases when the temperature rises. The Ce1 emission shows the thermal quenching effect under the direct excitation of Ce³⁺ at 262 nm. However, under the interband excitation of 183 nm, the Ce1 center emission exhibits undulating temperature dependence. This is because the emission is governed by thermal quenching and possible thermal enhancement of the transport of free carriers with the rising temperature.     

Photoluminescence characteristics of energy transfer between Er and Bi in Gd2O3: Er, Bi

The phosphors, Bi³⁺- activated Gd₂O₃:Er³⁺, were prepared by sol-gel combustion method, and their photoluminescent properties were investigated under ultraviolet light excitation. The emission spectrum exhibited sharp peaks at about 520, 535, 545, 550 and 559 nm due to (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺ ions. The luminescent intensity was remarkably improved by the incorporation of Bi³⁺ ions under 340 nm light excitation, which suggested very efficient energy transfer from Bi³⁺ ions to Er³⁺ions. The introducing of Bi³⁺ ions broadened the excitation band of the phosphor, of which a new strong peak occurred ranging from 320 to 360 nm due to the 6s²→6s6p transition of Bi³⁺ ions. There is significant energy overlap between the emission band of Bi³⁺ ions and the excitation band of Er³⁺ ions. Under 340 nm light excitation, Bi³⁺ absorbed most of the energy and transferred it to Er³⁺. The energy transfer probability from Bi³⁺ to Er³⁺ is strongly dependent on the Bi³⁺ ion concentration. Also, the sensitization effectiveness was studied and discussed in this paper.     

Enhancement of red spectral emission intensity of Y3Al5O12:Ce phosphor via Pr co-doping and Tb substitution for the application to white LEDs

We have enhanced color-rendering property of a blue light emitting diode (LED) pumped white LED with yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor using addition of Pr and Tb as a co-activator and host lattice element, respectively. Pr³⁺ addition to YAG:Ce phosphor resulted in sharp emission peak at about 610 nm through ¹D₂→³H₄ transition. And when Tb³⁺ substituted Y³⁺ sites, Ce³⁺ emission band shifted to a longer wavelength due to larger crystal field splitting. Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and (Y_1−xTb_x)₃Al₅O₁₂:Ce³⁺ phosphors were coated on blue LEDs to fabricate white LEDs, respectively, and their color-rendering indices (CRIs, R_a) were measured. As a consequence of the addition of Pr³⁺ or Tb³⁺, CRI of the white LEDs improved to be R_a=83 and 80, respectively. Especially, blue LED pumped (Y_0.2Tb_0.8)₃Al₅O₁₂:Ce³⁺ white LED showed both strong luminescence and high color-rendering property.     

Luminescent characterization of CaAl2S4:Eu powder

The photoluminescent (PL) emission and excitation behaviour of green-emitting CaAl₂S₄:Eu²⁺ powder phosphor is reported in detail. CaAl₂S₄:Eu²⁺ emission provides good CIE colour coordinates (x=0.141; y=0.721) for the green component in display applications. Powder with a dopant concentration of 8.5 mol% shows the highest luminescence efficiency. Temperature dependence of the radiative properties, such as luminescence intensity and decay time, was investigated. In particular, the Stokes shift, the mean phonon energy, the redshift, the energy of the f→d and d→f transition and the crystal field splitting of the CaAl₂S₄:Eu²⁺ emission were determined. The thermal quenching of the emission was examined.     

Laser-excited spectra of Lu2SiO5:Ce scintillator

The emission spectra of Lu₂SiO₅:Ce single crystal under the excitation of 266 nm laser were investigated. The emission spectra of LSO single crystal show no temperature quenching from 20 to 300 K, under the excitation of 266 nm laser with 2 mJ pulse energy. With rising temperature, the Ce1 emission is slightly decreased, while the Ce2 emission is slightly increased. These results show the emissions of Ce1 and Ce2 is not only dependent on the concentration ratio but also influenced by the possible energy transfer processes, including Ce1 to Ce2, intrinsic STHs to Ce2 and the phonon-assisted transfer processes. The spectral thermal broadening and the spectral overlap become evident at high temperature, leading to the enhancement of energy transfer. When the excitation power lowers, the ratio of Ce1 and Ce2 emission increases, and is close to the Xe lamp ultraviolet (UV) excitation, suggesting that the energy transfer from Ce1 center to Ce2 center may be also dependent on the excitation power.     

Potential PDP phosphors with strong absorption around 172 nm: Rare earth doped NaLaP2O7 and NaGdP2O7

NaLaP₂O₇ and NaGdP₂O₇ powder samples are prepared by solid-state reactions at 750 and 600 °C, respectively, and the VUV-excited luminescence properties of Ln³⁺ (Ln=Ce, Pr, Tb, Tm, Eu) in both diphosphates are studied. Ln³⁺ ions in both hosts show analogous luminescence. For Ce³⁺-doped samples, the five Ce³⁺ 5d levels can be clearly identified. As for Pr³⁺ and Tb³⁺-doped samples, strong 4f-5d absorption band around 172 nm is observed, which matches well with Xe-He excimer in plasma display panel (PDP) devices. As a result, Pr³⁺ can be utilized as sensitizer to absorb 172 nm VUV photon and transfer energy to appropriate activators, and Tb³⁺-doped NaREP₂O₇(RE=La, Gd) are potential 172 nm excited green PDP phosphors. For Tm³⁺ and Eu³⁺-doped samples, the Tm³⁺-O²⁻ charge transfer band (CTB) is observed to be at 177 nm, but the CTB of Eu³⁺ is observed at abnormally low energy position, which might originate from multi-position of Eu³⁺ ions. The similarity in luminescence properties of Ln³⁺ in both hosts indicates certain structural resemblance of coordination environment of Ln³⁺ in the two sodium rare earth diphosphates.     

The luminescent properties of CaYBO4:Ln(Ln=Eu, Tb) under UV-VUV range

The luminescent properties of CaYBO₄:Ln(Ln=Eu³⁺, Tb³⁺) were investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) region. The CT band of Eu³⁺ at about 245 nm blue-shifted to 230 nm in VUV excitation spectrum; the band with the maximum at 183 nm was considered as the host lattice absorption. For the sample of CaYBO₄:0.08Tb³⁺, the bands at about 235 and 263 nm were assigned to the f-d transitions of Tb³⁺ and the CT band of Tb³⁺ was calculated according to Jφrgensen's theory. Under UV and VUV excitation, the main emission of Eu³⁺ corresponding to the ⁵D₀-⁷F₂ transition located at about 610 nm and two intense emission of Tb³⁺ from the ⁵D₄-⁷F₅ transition had been observed at about 542 and 552 nm, respectively. With the incorporation of Gd³⁺ into the host lattice of CaYBO₄, the luminescence of Tb³⁺ was enhanced while that of Eu³⁺ was decreased because of their different excitation mechanism.     

New red phosphors BaZr(BO3)2 and SrAl2B2O7 doped with Eu for PDP applications

Vacuum ultraviolet (VUV) excitation and photoluminescence (PL) characteristics of Eu³⁺ ion doped borate phosphors; BaZr(BO₃)₂:Eu³⁺ and SrAl₂B₂O₇:Eu³⁺ are studied. The excitation spectra show strong absorption in the VUV region with the absorption band edge at ca. 200 nm for BaZr(BO₃)₂:Eu³⁺ and 183 nm for SrAl₂B₂O₇:Eu³⁺, respectively, which ensures the efficient absorption of the Xe plasma emission lines. In BaZr(BO₃)₂:Eu³⁺, the charge transfer band of Eu³⁺ does not appear strongly in the excitation spectrum, which can be enhanced by co-doping Al³⁺ ion into the BaZr(BO₃)₂ lattices. The luminescence intensity of BaZr(BO₃)₂:Eu³⁺ is also increased by Al³⁺ incorporation into the lattices. The PL spectra show the strongest emission at 615 nm corresponding to the electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺ in both BaZr(BO₃)₂ and SrAl₂B₂O₇, similar to that in YAl₃(BO₃)₄, which results in a good color purity for display applications.     

Luminescence properties of red-emitting praseodymium-activated BaTi4O9 phosphor

The photoluminescence and low-voltage cathodoluminescence characteristics of BaTi₄O₉:Pr³⁺ were investigated. The excitation band of intervalence charge transfer (IVCT) of BaTi₄O₉:Pr³⁺ emerged distinctly at 330 nm. The resultant emissions appeared at 606-643 nm corresponding to the ¹D₂→³H₄ transition. In BaTi₄O₉:Pr³⁺, the emission of ³P₀→³H₄ transition at 490 nm was not observed. The results were in a pure red color emission.     

Hydrothermal synthesis and luminescent properties of LnPO4:Tb,Bi (Ln=La,Gd) phosphors under UV/VUV excitation

Monoclinic LnPO₄:Tb,Bi (Ln=La,Gd) phosphors were prepared by hydrothermal reaction and their luminescent properties under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation were investigated. LaPO₄:Tb,Bi phosphor and GdPO₄:Tb phosphor showed the strongest emission intensity under 254 and 147 nm excitation, respectively, because of the different energy transfer models. In UV region, Bi³⁺ absorbed most energy then transferred to Tb³⁺, but in VUV region it was the host which absorbed most energy and transferred to Tb³⁺.     

Remarkable improvement of brightness for the green emissions in Ce and Tb co-activated LaPO4 nanowires

Ce³⁺ and Tb³⁺ co-activated LaPO₄ nanowires (NWs) were synthesized by the hydrothermal method and studied in contrast to corresponding micrometer rods (MRs). The results indicate that electronic transition rate of Ce³⁺ and Tb³⁺ in NWs had only a little variation in comparison with that in MRs, and energy transfer (ET) rate and efficiency of Ce³⁺→Tb³⁺ in NWs reduced. It is interesting to observe that the brightness for ⁵D₄-⁷F₅ of Tb³⁺ via ET of Ce³⁺→Tb³⁺ in NWs increased several times than that in MRs. This was attributed to the decreased energy loss in excited states being higher than ⁵D₄ of Tb³⁺ ions due to hindrance of boundary.     

Synthesis and luminescence properties of Tb:NaGd(WO4)2 novel green phosphors

Tb³⁺:NaGd(WO₄)₂ (Tb:NGW) phosphors with different Tb³⁺ concentrations have been synthesized by a mild hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize the Tb:NGW phosphors. XRD analysis confirmed the formation of NGW with scheelite structure. SEM study showed that the obtained Tb:NGW phosphors appeared to be nearly spherical and their sizes ranged from 1 to 1.5 μm. The excitation spectra of these systems showed an intense broad band with maximum at 270 nm related to the O→W ligand-to-metal charge-transfer state. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under UV light excitation. Analysis of the photoluminescence spectra with different Tb³⁺ concentrations revealed that the optimum dopant concentration for Tb³⁺ is about 15 at% of Tb³⁺ ions in Tb:NGW phosphors.     

Novel ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layers

ZnO/ZnGa₂O₄ composite layers were synthesized by simple thermal oxidation of ZnS substrates with gallium in the air. The continuous-wave and time-resolved photoluminescence measurements for the composites were performed at room temperature. It is found that the visible deep level emission from ZnO in ZnO/ZnGa₂O₄ composite layer was almost suppressed. In addition, the UV emission with long lifetime was also observed in comparison with that of pure ZnO layer without ZnGa₂O₄.     

Luminescence and Tb-Ce-Eu ion energy transfer in single-crystalline films of Tb3Al5O12:Ce,Eu garnet

The paper is devoted to investigation of the processes of excitation energy transfer between the host cations (Tb³⁺ ions) and the activators (Ce³⁺ and Eu³⁺ ions) in single-crystalline films of Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet which is considered as a promising luminescent material for the conversion of LED's radiation. The cascade process of excitation energy transfer is shown to be realized in TbAG:Ce,Eu: (i) from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions; (ii) from Ce³⁺ ions to Eu³⁺ ions by means of dipole-dipole interaction and through Tb³⁺ ion sublattice.     

Synthesis, microstructure and photoluminescence of Eu/Tb activated Y2SiO5 nanophosphors by new silicate sources

Y_2−xTb_xSiO₅ and Y_2−xEu_xSiO₅ nanophosphors with seven different kinds of silicate sources were synthesized by sol-gel method. The structures have been investigated to be composed of nanometer-size grains of 30-60 nm through X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used to compare the different morphology of patterns from seven different silicon sources. The photoluminescence of Y_2−xTb_xSiO₅ was investigated as a function of silicate sources and the results revealed that these nanometer materials showed the characteristic emission ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) of Tb ions. The characteristic emission ⁵D₀ → ⁷F_J (J = 1, 2, 4) of Eu ions was also found in the materials of Y_2−xEu_xSiO₅.     

Origin of PL intensity increase of CaMgSi2O6:Eu phosphor after baking process for PDPs application

We have synthesized blue-emitting CaMgSi₂O₆:Eu²⁺ (CMS) and evaluated its thermal stability after baking process. To evaluate its thermal stability, CMS was baked in air at 500 and 600 °C for 20 min, respectively, and compared with BaMgAl₁₀O₁₇:Eu²⁺ (BAM) treated in the same condition. After baking process, CMS showed somewhat increased photoluminescence (PL) intensity with baking temperature. To investigate the reasons behind the increase of PL intensity after baking process, vacuum ultraviolet (VUV)/PL, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) techniques were applied. From the ESR and the XPS analyses, it is noted that spectral intensity of Eu²⁺ ion somewhat increased. It was believed that due to charge balance Eu³⁺ ions reduced to Eu²⁺ ions during the baking process in air. It is clear that the concentration of Eu²⁺ increased after the baking process in air and it leads to slight increase of the VUV/PL intensity of CMS phosphor.     

Synthesis and luminescence investigation of RE (Eu, Tb and Ce)-doped lithium silicate (Li2SiO3)

Synthesis and photoluminescence (PL) investigations of lithium metasilicate doped with Eu³⁺, Tb³⁺ and Ce³⁺ were carried out. PL spectra of Eu-doped sample showed peaks corresponding to the ⁵D₀→⁷F_j (j=1, 2, 3 and 4) transitions under ultraviolet excitation. Strong red emission coming from the hypersensitive ⁵D₀→⁷F₂ transition of Eu³⁺ ion suggested the presence of the dopant ion in structurally disordered environment. Tb³⁺-doped silicate sample showed blue-green emission corresponding to the ⁵D₄→⁷F_j (j=6, 5 and 4) transitions. Ce-doped sample under excitation from UV, showed a broad emission band in the region 350-370 nm with shoulders around 410 nm. The fluorescence lifetimes of Eu³⁺ and Tb³⁺ ions were found out to be 790 and 600 μs, respectively. For Ce³⁺, the lifetime was of the order of 45 ns. PL spectra of the europium- and terbium-doped samples were compared with commercial red (Y₂O₃:Eu³⁺) and green (LaPO₄:Tb³⁺) phosphors, respectively. It was found that the emission from the doped silicate sample was 37% of the commercial phosphor in case of the Tb-doped sample and 8% of the commercial phosphor in case of the Eu-doped sample.     

Preparation and spectroscopic properties of Nb2O5 nanorods

Nb₂O₅ nanorods have been prepared using water/ethanol media. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible absorption and photoluminescence spectroscopy. The as-prepared Nb₂O₅ nanorods appeared to be single pseudohexagonal (TT-Nb₂O₅) phase. From the photoluminescence spectrum, two emission bands at 407 and 496 nm, respectively, were observed. The origin of the luminescence was discussed in detail.     

In situ sol-gel composition of multicomponent hybrid precursor to hexagon-like Zn2SiO4:Tb microcrystalline phosphors with different silicate sources

Zn₂SiO₄ doped with Tb³⁺ were in situ synthesized by a modified sol-gel technology with the assembly hybrid precursor employed four different silicate sources, i.e. 3-aminopropyl-trimethoxysilane (APMS), 3-aminopropyl-triethoxysilane (APES), 3-aminopropyl-methyl-diethoxysilane (APMES) and tetraethoxysilane (TEOS), respectively. The SEM result shows that there exist some novel unexpected micromorphological structures of hexagon-like with the dimension of 0.5-1.0 μm. The photoluminescent properties of Zn₂SiO₄:Tb³⁺ phosphors have been studied as a function of Tb³⁺ doping concentration. Cross-relaxation process between identical Tb³⁺ ions results in the quenching of the ⁵D₃ emission for high concentration sample.     

Eu red long afterglow in Y2O2S:Ti, Eu phosphor through afterglow energy transfer

Eu,Ti co-doped Y₂O₂S:0.03Ti,0.03Eu phosphors and single Eu or Ti doped Y₂O₂S phosphors were prepared and their luminescent properties were investigated in detail by photoluminescence (PL) spectra, long afterglow spectra and thermoluminescence spectra measurements. The results showed that Y₂O₂S:Ti,Eu phosphors possess orange-red afterglow color with afterglow time above 5 h. The reddish afterglow color, which corresponds to a set of linear Eu³⁺ emissions at low-energy range (540-630 nm), was demonstrated to come from the energy transfer process from yellow Ti afterglow emissions, the proposed energy transfer mechanism may well explain the Eu³⁺ afterglow emission.