Luminescence properties of phosphors based on Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> (TbAG) terbium-aluminum garnet

The processes of excitation energy transfer in phosphors based on single-crystal Tb₃Al₅O₁₂:Ce (TbAG:Ce) and Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet films have been investigated. These films are considered to be promising materials for screens for X-ray images and luminescence converters of blue LED radiation. The conditions for excitation energy transfer from the matrix (Tb³⁺ cations) to Ce³⁺ and Eu³⁺ ions in TbAG:Ce and TbAG:Ce,Eu phosphors have been analyzed in detail. It is established that a cascade process of excitation energy transfer from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions and from Ce³⁺ ions to Eu³⁺ ions is implemented in TbAG:Ce,Eu via dipole-dipole interaction and through the Tb³⁺ cation sublattice.     

Luminescence of Mn ions in Tb3Al5O12 garnet

The paper is dedicated to investigation of the Mn²⁺ luminescence in Tb₃Al₅O₁₂ (TbAG) garnet, as well as the processes of excitation energy transfer between host cations (Tb³⁺ ions) and activators (Mn²⁺ and Mn²⁺-Ce³⁺ pair ions) in single crystalline films of TbAG:Mn and TbAG:Mn,Ce garnets which can be considered as promising luminescent materials for conversion of LED's radiation. Due to the effective energy transfer between TbAG host and activator, Mn²⁺ ions in TbAG possess the bright orange luminescence in the bands peaked at 595 nm with a lifetime of 0.64 ms which are caused by the ⁴T₁→⁶A₁ radiative transitions. The simultaneous process of energy transfer is realized in TbAG:Mn,Ce: (i) from Tb³⁺ to Mn²⁺ ions; (ii) from Tb³⁺ cations to Ce³⁺ ions and then partly to Mn²⁺ ions through Tb³⁺ ion sublattice and Ce-Mn dipole-dipole interaction.     

VUV-UV excited luminescent properties of LnCa4O(BO3)3:RE (Ln=Y, La, Gd; Re=Eu, Tb, Dy, Ce)

Calcium lanthanide oxyborate doped with rare-earth ions LnCa₄O(BO₃)₃:RE³⁺ (LnCOB:RE, Ln=Y, La, Gd, RE=Eu, Tb, Dy, Ce) was synthesized by the method of solid-state reaction at high temperature. Their fluorescent spectra were measured from vacuum ultraviolet (VUV) to visible region at room temperature. Their excitation spectra all have a broadband center at about 188 nm, which is ascribed to host absorption. Using Dorenbos’ and Jφrgensen's work [P. Dorenbos, J. Lumin. 91 (2000) 91, R. Resfeld, C.K. Jφrgensen, Lasers and Excite States of Rare Earth [M], Springer, Berlin, 1977, p. 45], the position of the lowest 5d levels E(Ln,A) and charge transfer band E_ct were calculated and compared with their excitation spectra.Eu³⁺ and Tb³⁺ ions doped into LnCOB show efficient luminescence under VUV and UV irradiation. In this system, Ce³⁺ ions do not show efficient luminescence and quench the luminescence of Tb³⁺ ions when Tb³⁺ and Ce³⁺ ions are co-doped into LnCOB. GdCOB doped with Dy³⁺ shows yellowish white light under irradiation of 254 nm light for the reason that Gd³⁺ ions transfer the energy from itself to Dy³⁺. Because of the existence of Gd³⁺, the samples of GdCOB:RE³⁺ show higher excitation efficiency than LaCOB:RE³⁺ and YCOB:RE³⁺, around 188 nm, which indicates that the Gd³⁺ ions have an effect on the host absorption and can transfer the excitation energy to the luminescent center such as Tb³⁺, Dy³⁺ and Eu³⁺.     

Effect of Tb3+ concentration on luminescence properties of Ce/Tb/Eu co-doped borosilicate glasses for white LEDs

Ce, Tb and Eu single and ternary doped borosilicate glasses were prepared and effect of Tb³⁺ concentration on luminescence properties of ternary co-doped glasses were analyzed by utilizing emission spectra, excitation spectra, the Commission International de I’Eclairage (CIE) colorimetric system and fluorescence decay curves. The results show that Tb³⁺ concentration significantly affects spectral intensities of ternary co-doped glasses when excited by near ultraviolet (NUV). With the increasing of Tb³⁺ concentration, the blue emission of Ce³⁺ is weakened and the red emission of Eu³⁺ is slightly enhanced. Both the color coordinates and correlated color temperatures (CCTs) can be adjusted by Tb³⁺ concentration. Besides, the energy transfers from Ce³⁺ to Tb³⁺ and from Tb³⁺ to Eu³⁺ were observed. Measured characteristic lifetimes of Tb³⁺ indicate that the energy transfer from Ce³⁺ to Tb³⁺ tends to predominate in whole process. The studies show that Ce/Tb/Eu ternary doped borosilicate glasses might be promising luminescence materials for NUV pumped white LEDs.     

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.     

Cerium and terbium luminescence in LaMgAl11O19

Under 254 nm excitation LaMgAl₁₁O₁₉ : Ce³⁺ shows emission peaking between 340 and 360 nm with a quantum efficiency up to 65%, which is almost independent of the Ce³⁺ concentration. Energy transfer between the Ce³⁺ ions is not efficient. In CeMgAl₁₁O₁₉ : Tb³⁺ efficient energy transfer occurs from Ce³⁺ to Tb³⁺, resulting in a green emission with a quantum efficiency up to 65%. The Ce³⁺ - Tb³⁺ energy transfer is ascribed to electric dipole-quadrupole interaction.     

The luminescence of CaYBO4:RE (REEu, Gd, Tb, Ce) in VUV-visible region

Phosphors CaYBO₄:RE³⁺ (RE=Eu, Gd, Tb, Ce) were synthesized with the method of solid-state reaction at high temperature, and their vacuum ultraviolet (VUV)-visible luminescent properties in VUV-visible region were studied at 20 K. In CaYBO₄, it is confirmed that there are two types of lattice sites that can be substituted by rare-earth ions. The host excitation and emission peaks of undoped CaYBO₄ are very weak, which locate at about 175 and 350-360 nm, respectively. The existence of Gd³⁺ can efficiently enhance the utilization of host absorption energy and result in a strong emission line at 314 nm. In CaYBO₄, Eu³⁺ has typical red emission with the strongest peak at 610 nm; Tb³⁺ shows characteristic green emission, of which the maximum emission peak is located at 542 nm. The charge transfer band of CaYBO₄:Eu³⁺ was observed at 228 nm; the co-doping of Gd³⁺ and Eu³⁺ can obviously sensitize the red emission of Eu³⁺. The fluorescent spectra of CaYBO₄:Ce³⁺ is very weak due to photoionization; the co-addition of Ce³⁺-Tb³⁺ can obviously quench the luminescence of Tb³⁺.     

Quenching of thermo-stimulated photo-ionization by energy transfer in CaAl4O7: Tb, Ce

Crystal fibers of Ce³⁺ and Tb³⁺ singly doped and co-doped CaAl₄O₇ were grown by the LHPG method. Photoluminescence, excitation spectra and photoconduction were measured. Thermo-stimulated photo-ionization (delocalization) of electrons from the lowest field component of the 5d excited state of Ce³⁺ was observed in the Ce³⁺ singly doped sample under excitation at 355 nm. The 5d sublevel was found to locate at 0.3 eV below the conduction band of the host. However, the thermo-stimulated photo-ionization was greatly quenched due to the fast energy transfer from the 5d sublevel to Tb³⁺ ions in the Ce³⁺/Tb³⁺ double doped sample.     

Luminescent properties of SrAl2B2O7: Ce, Tb

By using metal nitrates as starting materials, SrAl₂B₂O₇: Tb³⁺ and SrAl₂B₂O₇: Ce³⁺, Tb³⁺ powder phosphors were prepared by sol-gel method. X-ray diffraction (XRD), photoluminescence excitation and emission, as well as kinetic decays were employed to characterize the resulting samples. The results show that energy transfers from Ce³⁺ to Tb³⁺ ions. The emission intensity of Tb³⁺ ions in SrAl₂B₂O₇ could be greatly intensified when Ce³⁺ ions are doped into SrAl₂B₂O₇: Tb³⁺. The decay times of SrAl₂B₂O₇: Tb³⁺ were prolonged when Ce³⁺ ions were doped. The doping of Ce³⁺ ions not only improved the luminescent intensity, but also made the materials gets stable luminescent properties.     

Photoluminescence studies on Eu and Ce-doped Li2SrSiO4

The luminescence of Li₂SrSiO₄: 0.01Eu, xCe (x=0.0025, 0.005, 0.0075, and 0.01) is studied as a potential ultraviolet light-emitting diode (UV-LED) phosphor that is capable of converting the ultraviolet emission of a UV-LED into white light with good luminosity. There are broad blue and yellow emissions peaked at 413 and 575 nm, respectively. The two emissions come from d-f transitions of Ce³⁺ and Eu²⁺, respectively. The emission intensity of Li₂SrSiO₄: 0.01Eu, xCe reaches its maximum at x=0.0075. The energy transfer from Ce³⁺ to Eu²⁺ is demonstrated to be the type of electric dipole-dipole interaction with considerable spectral overlap and nonradiative transition is calculated to dominate. The Commission International de I’Eclairage (CIE) chromaticity coordinates of Ce³⁺/Eu²⁺ substituted compounds is also discussed.     

New promising phosphors Ba3InB9O18 activated by Eu/Tb

Borate Ba₃InB₉O₁₈ (BIBO) has been adopted as a host material for phosphors for the first time. Lanthanide ions (Eu³⁺/Tb³⁺)-doped BIBO phosphors have been synthesized by solid-state reaction and luminescent properties investigated under ultravoilet (UV) excitation. For red phosphor BIBO:Eu, dominant emission peaking at 590 nm was attributed to ⁵D₀→⁷F₁ transition of Eu³⁺, which confirmed that the local site of Eu³⁺ occupied by In³⁺ ion in BIBO crystal lattice is at inversion symmetry center. Optimum Eu³⁺ concentration of BIBO:Eu under UV excitation with 227 nm wavelength is around 40%. The green phosphor BIBO:Tb showed bright green emission at 550 with 232 nm light excited and optimal of Tb³⁺ concentration measured in BIBO is about 8%. The corresponding luminescence mechanisms of Ln-doped BIBO (Ln=Eu³⁺/Tb³⁺) were analyzed. The luminescent intensity of Tb³⁺ can be significantly improved by co-doping of Bi³⁺ in the BIBO:Tb lattice. The likely reason was proposed in terms of the different interactions of the host lattice with these ions, and of these ions with each other.     

Eu/Tb ions co-doped white light luminescence Y2O3 phosphors

Y₂O₃:Eu³⁺, Tb³⁺ phosphors with white emission are prepared with different doping concentration of Eu³⁺ and Tb³⁺ ions and synthesizing temperatures from 750 to 950 °C by the co-precipitation method. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the crystallinity of the synthesized samples increases with enhancing the firing temperature. The photoluminescence spectra indicate the Eu³⁺ and Tb³⁺ co-doped Y₂O₃ phosphors show five main emission peaks: three at 590, 611 and 629 nm originate from Eu³⁺ and two at 481 and 541 nm originate from Tb³⁺, under excitation of 250-320 nm irradition. The white light luminescence color could be changed by varying the excitation wavelength. Different concentrations of Eu³⁺ and Tb³⁺ ions were induced into the Y₂O₃ lattice and the energy transfer from Tb³⁺→Eu³⁺ ions in these phosphors was found. The Commission International de l’Eclairage (CIE) chromaticity shows that the Y₂O₃:Eu³⁺, Tb³⁺ phosphors can obtain an intense white emission.     

Multiplication of electronic excitations in nanophosphors Lu2O3:Eu and Lu2O3:Tb

Luminescence properties of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanocrystalline powders with the particle size varying from 46 to 6 nm were studied under excitation by synchrotron radiation in the photon energy range (up to ∼22.5 eV) covering the region where the processes of multiplication of electronic excitation occur. It was found that the excitation spectra of Tb³⁺ emission from all Lu₂O₃:Tb³⁺ nanopowders have similar behavior, whereas the shape of the excitation spectra of Eu³⁺ emission from Lu₂O₃:Eu³⁺ nanopowders strongly depends on the particle size. The difference in the behavior of Lu₂O₃:Eu³⁺ and Lu₂O₃:Tb³⁺ nanophosphor systems was explained by different mechanisms of the energy transfer from the host to Eu³⁺ or Tb³⁺ ions (either the hole or electron recombination mechanism, respectively), which are differently influenced by losses of electronic excitations near the particle surface.     

Luminescence studies on lanthanide ions (Eu, Dy and Tb) doped YAG:Ce nano-phosphors

Yttrium aluminum garnet nanoparticles both undoped and doped with lanthanide ions (Ce³⁺, Eu³⁺, Dy³⁺ and Tb³⁺) having average size around 30 (±3 nm) nm were prepared by glycine nitrate combustion method followed by annealing at a relatively low temperature of 800 °C. Increase in the annealing temperature has been found to improve the luminescence intensity and for 1200 °C heated samples there exists strong energy transfer from Tb³⁺ to Ce³⁺ ions in YAG:Ce(2%),Tb(2%) nanoparticles as revealed by luminescence studies. Co-doping the YAG:Ce nanoparticles with Eu³⁺ results in significant decrease in the emission intensity of both Ce³⁺ and Eu³⁺ ions and this has been attributed to the oxidation of Ce³⁺ to Ce⁴⁺ and reduction of Eu³⁺ to Eu²⁺ ions. Dy³⁺ co-doping did not have any effect on the Ce³⁺ emission as there is no energy transfer between Dy³⁺ and Ce³⁺ ions.     

Luminescence enhancement by energy transfer from Ce ions in Ba1.6Ca0.4P2O7:Ce:Tb phosphor

The optical properties of Ba_1.6Ca_0.4P₂O₇ doped with Ce³⁺ and Tb³⁺ are investigated. Under excitation at 280 nm the emission spectrum of Ba_1.6Ca_0.4P₂O₇:Ce³⁺ consists of a peak at 370 nm and a shoulder at the longer wavelength side. The emission spectra of Ba_1.6Ca_0.4P₂O₇:Tb³⁺ shows the well-known emission lines due to ⁵D₄-⁷F_J transitions of Tb³⁺. The green emissions of Tb³⁺ ions are enhanced upon UV excitation through energy transfer from Ce³⁺ to Tb³⁺ ions. The efficiency of such an energy transfer is estimated based on spectroscopic data. The dependence of photoluminescence (PL) intensities of Ce³⁺ and Tb³⁺ emissions on Ce³⁺ or Tb³⁺ concentrations in the systems (Ba_1.6Ca_0.4P₂O₇:0.04Ce³⁺,xTb³⁺ and Ba_1.6Ca_0.4P₂O₇:xCe³⁺,0.04Tb³⁺) and the temperature dependence of PL emission spectra of Ba_1.6Ca_0.4P₂O₇:0.06Ce³⁺,0.04Tb³⁺ is also investigated.     

Enhanced luminescence of Tb by efficient energy transfer from Ce in Sr2B5O9Cl host

Ce³⁺ and Tb³⁺ co-doped Sr₂B₅O₉Cl phosphors with intense green emission were prepared by the conventional high-temperature solid-state reaction technique. A broad band centered at about 315 nm was found in phosphor Sr₂B₅O₉Cl: Ce³⁺, Tb³⁺ excitation spectrum, which was attributed to the 4f-5d transition of Ce³⁺. The typical sharp line emissions ranging from 450 to 650 nm were originated from the ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions of Tb³⁺ ions. The photoluminescence (PL) intensity of green emission from Tb³⁺ was enhanced remarkably by co-doping Ce³⁺ in the Tb³⁺ solely doped Sr₂B₅O₉Cl phosphor because of the dipole-dipole mechanism resonant energy transfer from Ce³⁺ to Tb³⁺ ions. The energy transfer process was investigated in detail. In light of the energy transfer principles, the optimal composition of phosphor with the maximum green light output was established to be Sr_1.64Ce_0.08Tb_0.1Li_0.18B₅O₉Cl by the appropriate adjustment of dopant concentrations. The PL intensity of Tb³⁺ in the phosphor was enhanced about 40 times than that of the Tb³⁺ single doped phosphor under the excitation of their optimal excitation wavelengths.     

Energy transfer from benzoic acid to lanthanide ions in benzoic acid-functionalized lanthanide-doped CaF2 nanoparticles

The preparation of benzoic acid-functionalized CaF₂:Ln³⁺ (Ln = Eu or Tb) nanoparticles and their sensitized luminescence are described in this report. First, to achieve sufficient proof for energy transfer from benzoic acid (BA) to lanthanide ions doped in nanoparticles, we employ Eu³⁺ as the microscopic probe and investigate the luminescent spectra of benzoic acid-functionalized CaF₂:Eu³⁺ (BA-CaF₂:Eu³⁺) nanoparticles. Next, to further reveal the difference between sensitized luminescence and common luminescence for Eu³⁺ doped in CaF₂ nanoparticles, we study the emission spectra of BA-CaF₂:Eu³⁺ nanoparticles excited at 286 nm and 397 nm, respectively. Finally, we analyze and compare the luminescent spectra of BA-CaF₂:Tb³⁺ and CaF₂:Ce³⁺, Tb³⁺ nanoparticles in detail. Our results indicate that both Eu³⁺ and Tb³⁺ doped in CaF₂ nanoparticles can be efficiently sensitized through benzoic acid.     

Enhanced luminescence and degradation of SiO2:Ce,Tb powder phosphors prepared by a sol-gel process

Enhanced green photoluminescence and cathodoluminescence (CL) from Tb³⁺ ions due to co-doping with Ce³⁺ ions were observed from SiO₂:Ce,Tb powder phosphors prepared by a sol-gel technique. Blue emission from the Ce³⁺ ions was completely suppressed by Tb co-doping, presumably due to energy transfer from Ce³⁺ to Tb³⁺. In addition, the green CL intensity from SiO₂:Ce,Tb degraded by ∼50% when the powders were irradiated for 10 h with a 2 keV, 54 mA/cm² beam of electrons in an ultra-high vacuum chamber containing either 1×10⁻⁸ or 1×10⁻⁷ Torr O₂. Desorption of oxygen from the surface was observed during the decrease of CL intensity. The mechanisms for energy transfer from Ce³⁺ ions to Tb³⁺ ions to enhance the green luminescence, and mechanisms for desorption of oxygen from the phosphor surface that would result in decreased CL intensity are discussed.     

Luminescent Properties of a Polymer Photoluminescent Composite Containing the Binuclear (EU,TB) Complex as an Emitter

Uninuclear europium (Eu), as well as binuclear Eu and terbium (Tb), complexes were synthesized using acrylic acid (AA) as the first ligand and 1,10-phenanthroline (Phen) as the second ligand. The relative weight ratio of the europium (III) (Eu³⁺) to terbium (III) (Tb³⁺) ions of the binuclear complex was 1:1 as determined via energy dispersive X-ray analysis. The structures of the Eu(AA)₃Phen and Eu_0.5Tb_0.5(AA)₃Phen complexes were characterized by Fourier transform infrared spectroscopy. A series of tri-cellulose acetate (TCA)/ the Eu(AA)₃Phen and TCA/Eu_0.5Tb_0.5(AA)₃Phen composites were prepared by solution blending, and their luminescent properties were investigated by fluorescence spectrophotometry. The excitation spectra of all composites indicated that the TCA matrix probably affected the energy absorption and transfer of organic ligands. In TCA/Eu_0.5Tb_0.5(AA)₃Phen composites the introduced Tb³⁺ ions had some influence on energy absorption and transfer of organic ligands; the energy transfer process of the complex is suggested to be as follows: Phen→AA→Tb³⁺ion→Eu³⁺ion. The emission spectra indicated that the luminescent intensity of the TCA/Eu_0.5Tb_0.5(AA)₃Phen composites was noticeably stronger than that of the TCA/Eu(AA)₃Phen composites, suggesting that the comparatively stable and high-efficiency energy transfer process was only slightly influenced by the TCA matrix. In summary, the TCA/Eu_0.5Tb_0.5(AA)₃Phen (90/10) composite possesses fine luminescent properties for potential usage as red fluorescent materials.     

Enhanced luminescent properties of Y3Al5O12:Tb,Ce phosphor prepared by spray pyrolysis

Yttrium aluminum garnet (YAG) particles doped with Tb³⁺ or double doped with Tb³⁺ and Ce³⁺ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce³⁺ activator into the line peaks of Tb³⁺ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d-f transition of Ce³⁺ and a reduction in the intensity of emission peaks due to ⁵D₃-⁷F_j (j=3, 4, 5, 6) transition of Tb³⁺ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb³⁺ to Ce³⁺ in YAG host. Codoping Ce³⁺ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb³⁺, which means that more lattice defects, involving in the energy absorption and transfer to Tb³⁺, are formed by the Ce³⁺ codoping. The finding gives a promising approach for enhancing the luminescence efficiency.