Thermoluminescence and thermal properties of rare-earth-ion-activated YAG crystals

The paper contains results of studies of repeated thermoluminescence of yttrium-aluminum garnet (YAG) crystals (Y_3−xLn_xAl₅O₁₂, x=0, 2) activated by rare earth ions (Pr³⁺, Nd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺) previously exposed to⁶⁰Co γ-radiation at 77 K and subjected to many cooling-heating cycles. A possible mechanism of repeated thermoluminescence is considered from the viewpoint of a dynamic evolutionary approach. The thermal conductivity of YAG-TR³⁺ crystals (TR³⁺: Gd³⁺, Tb³⁺, Dy³⁺, Er³⁺, Tm³⁺, and Lu³⁺) is studied to establish its relation with repeated thermoluminescence. Presented at the National Conference on Molecular Spectroscopy, Samarkand (Uzbekistan), September 25–27, 1996. Samarkand State University, 15, University Blvd., 703004, Samarkand, Republic of Uzbekistan. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 137–140, January–February, 1998.     

Luminescent properties and energy transfer of Y3Al5O12:Ce3+, Ln3+ (Ln=Tb,Pr) prepared by polymer-assisted sol–gel method

Y₃Al₅O₁₂:Ce³⁺, Pr³⁺ and Y₃Al₅O₁₂:Ce³⁺, Tb³⁺ nano-particles have been synthesized by polymer-assisted sol–gel method. Crystal structure, luminescent properties and energy transfer of the phosphors are analyzed. XRD study of polycrystalline powders shows that all the samples are of YAG phase without impurity. Photoluminescence (PL) emission and excitation spectra illustrate that in YAG:Ce, Pr phosphors, energy transfer occurs mutually between Ce³⁺ and Pr³⁺, while in YAG:Ce, Tb systems, only one-way path energy transfer of Tb³⁺→Ce³⁺ is observed.     

GSAG and YSAG: a study on isomorphism and crystal growth

The composition of gadolinium-scandium-aluminium (GSAG) and yttrium-scandium-aluminium (YSAG) garnet crystals grown by the Czochralski method and by floating zone melting was analysed. It is shown that Sc³⁺ and Al³⁺ ions in GSAG and Y³⁺, Sc³⁺ and Al³⁺ ions in YSAG can simultaneously occupy two types of sites in the lattice. The congruent melting composition of GSAG is {Gd_2.88Sc_0.12} [Sc_1.77Al_0.23] (Al₃)O₁₂. The homogeneity range of GSAG is rather narrow. In the Y₂O₃-Sc₂O₃-Al₂O₃ system a wide range of garnet-structured solid solutions exist. However, there is no ternary garnet that melts congruently. Distribution coefficients of components in YSAG are slightly varied from unity in the whole concentration range, and as the melt composition approaches YAG they tend to unity. Cr³⁺ and Nd³⁺ distribution coefficients in GSAG and YSAG are presented.

     

Luminescence properties of Y3Al5O12:Ce nanoceramics

Comparative analysis of the luminescent properties of Y₃Al₅O₁₂:Ce (YAG:Ce) transparent optical ceramics (OС) with those of single crystal (SC) and single crystalline film (SCF) analogues has been performed under excitation by pulsed synchrotron radiation in the fundamental absorption range of YAG host. It has been shown that the properties of YAG:Ce OC are closer to the properties of the SCF counterpart, where Y_Al antisite defects are completely absent, rather than to the properties of SC of this garnet with large concentration of Y_Al antisite defects. At the same time, the luminescence spectra of YAG:Ce OC show weak emission bands in the 200-470 nm range related to Y_Al antisite defects and charged oxygen vacancies (F⁺ and F centers). YAG:Ce ОС also possesses significantly larger contribution of slow components in the Ce³⁺ luminescence decay under high-energy excitation in comparison with SC and SCF of this garnet due to the involvement of antisite defects, charged oxygen vacancies as well as boundaries of grains in the energy transfer processes from the host to the Ce³⁺ ions.     

Red, green and blue (RGB) emission doped Y3Al5O12 (YAG) phosphors prepared by non-hydrolytic sol-gel route

An evolutionary optimization process involving combination chemistry was employed in an attempt to develop Y₃Al₅O₁₂ (YAG). The combination chemistry process utilized here consisted the doping of the YAG host with appropriate amounts of red (R), green (G), and blue (B) dopants in a single layer, for use in tricolor white light. The doped YAG was acieved by means of the non-hydrolytic sol-gel route. Four samples were prepared, three of which were mono-doped samples containing 1.0% of a certain lanthanide (Eu³⁺, Tb³⁺, or Tm³⁺) ion, while the fourth contained the three ions. The samples were characterized by X-ray diffractometry and photoluminescence. The diffraction pattern of the mono-doped samples synthered at 800 °C for 16 h displayed peaks corresponding to the Y₃Al₅O₁₂ (YAG) phase, while the sample doped with the three ions revealed the presence of a mixture of Y₃Al₅O₁₂ (YAG) and Y₄Al₂O₉ (YAM) phases. The emission spectra of the three mono-doped YAG samples displayed the typical bands of the blue, green, and red emission of the corresponding lanthanide ions. As for the sample doped with the three lanthanide ions; it simultaneously emitted R, G and B lights. The green emission (546 nm) was more intense and narrow in relation to the red and blue emissions, which may be due to differences in the size of the three incorporated ions.     

Disorder and the shape of the R-lines in Cr3+-doped garnets

This paper describes measurements of the R-line emission from Cr³⁺ ions in Y₃Ga₅O₁₂, Gd₃Sc₂Ga₃O₁₂, and Gd₃Sc₂Al₃O₁₂. Discrete splittings of the lines are interpreted as due to disorder on the cation sublattices. Assuming statistical distributions of the cations on the different cation sublattices enables estimates to be made of the degree of non-stoichiometry in the crystals.     

Enhanced photoluminescence of Sm/Bi co-doped Gd2O3 phosphors by combustion synthesis

Gd₂O₃:Sm³⁺ and Gd₂O₃:Sm³⁺,Bi³⁺ powders were prepared by a combustion method. Their structures were determined using X-ray diffraction. UV-visible absorption and photoluminescence spectra were investigated for Gd₂O₃:Sm³⁺ and Gd₂O₃:Sm³⁺,Bi³⁺ at different annealing temperatures and different doping concentrations. The emission spectra of all samples presented the characteristic emission narrow lines arising from the ⁴G_5/2→⁶H_J transitions (J=5/2, 7/2, and 9/2) of Sm³⁺ ions upon excitation with UV irradiation. The emission intensity of Sm³⁺ ions was largely enhanced with introducing Bi³⁺ ions into Gd₂O₃:Sm³⁺ and the maximum occurred at a Bi³⁺ concentration of 0.5 mol%. The relevant mechanisms were discussed with the sensitization theory by Dexter and the aggregation behavior of Bi³⁺ ions.     

Defects in Ce-doped LuAG and YAG scintillation layers grown by liquid phase epitaxy

The single crystalline Lu₃Al₅O₁₂ (LuAG) and Y₃Al₅O₁₂ (YAG) garnet layers doped by Ce³⁺ ions were grown by the liquid phase epitaxy from the flux. The effect of the flux composition, growth conditions, and substrate polishing on the layer morphology, creation of defects, and on optical and emission properties of layers was studied. The defects typical of the epitaxial growth are discussed.     

Synthesis and characterization of mono-dispersed Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Er<Superscript>3+</Superscript>-coated SiO<Subscript>2</Subscript> nanoparticles by co-precipitation process

In this paper, a facile co-precipitation process for preparing mono-dispersed core–shell structure nanoparticles is reported. The 110 nm SiO₂ cores coated with an yttrium aluminum garnet (Y₃Al₅O₁₂) layer doped with Er³⁺ were synthesized and the influence of the concentration ratio of [urea]/[metal ions] on the final product was investigated. The structure and morphology of samples were characterized by the X-ray powder diffraction, Fourier transform IR spectroscopy and transmission electron microscopy, respectively. The results indicate that a layer of well-crystallized garnet Y₃Al₅O₁₂:Er³⁺ were successfully coated on the silica particles with the thickness of 20 nm. The near infrared and upconversion luminescent spectra of the SiO₂@Y₃Al₅O₁₂:Er³⁺ powders further confirm that a Y₃Al₅O₁₂:Er³⁺ coating layer has formed on the surface of silica spherical particles.     

Energy transfer and photoextinction from Ln to Tb and Eu in aqueous chloride solutions

Energy transfer and photoextinction from Ln³⁺ to Tb³⁺ and Eu³⁺ in aqueous chloride solutions were studied by absorption, emission and excitation spectra. The energy gaps below the luminescent terms of Gd³⁺, Tb³⁺ and Eu³⁺ are spanned by 10, 5 and 4 quanta, respectively, of the highest energy vibration in aqueous solution, and luminescence is observed in each case. This is not so for other Ln³⁺ where nonradiative deactivation dominates over luminescence. It was verified that only Gd³⁺ could transfer energy to Tb³⁺ and Eu³⁺ ions in aqueous or acid solutions. The ions Pr³⁺, Nd³⁺, Sm³⁺, Dy³⁺, Ho³⁺ and Er³⁺ exhibited strong competitive absorption at certain wavelengths, resulting in the photoextinction of Tb³⁺ and Eu³⁺ emission.     

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.     

Photophysics of the triple state of metal complexes of 9-anthracenecarboxylic acid

The triplet-triplet (T-T) absorption spectra and the T-T absorption decay kinetics are measured for solutions of 9-anthracenecarboxylic acid (ACA) and its complexes with metal ions (Cd³⁺ and Ln³⁺=Y³⁺, La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, and Tb³⁺) in dimethylsulfoxide (DMSO) by the methods of flashlamp and laser pulse photolysis. The rate constants k _T of intracomplex quenching of the triplet state are measured for ACA complexes with ions Gd³⁺, Ce³⁺, Tb³⁺, and Eu³⁺. Larger values of k _T in complexes of ACA with paramagnetic ions Ce³⁺, Tb³⁺, and Eu³⁺, which have low-lying energy levels, compared to the values of k _T for complexes with other ligands (pyrene-3-sulfonate, pyrene-1,3,6,8-tetrasulfonate, and benzo[ghi]perylene-1,2-dicarboxylate) were explained by the lower energy of the triplet state of ACA (14400 cm^?1). For a complex with a paramagnetic ion Gd³⁺, which has no low-lying energy levels, the value of k _T is close to that measured by us earlier for the inner-sphere complex of pyrene-1,3,6,8-tetrasulfonate with the same ion. These results confirm our earlier assumption about the inner-sphere complexing of ACA with Ln³⁺ ions in DMSO.     

Size-effect on concentration quenching in Yb-doped Y3Al5O12 nano-crystals

Ytterbium-doped nano-crystalline powders of yttrium aluminium garnet Y₃Al₅O₁₂ (Yb³⁺-doped YAG) have been prepared by a modified Pechini method. The morphology of the annealed materials was observed using SEM images. The structure and the average crystalline grain sizes have been determined by X-ray powder diffraction measurements. An increase in elongated spherical crystalline grain sizes from 20 to 50 nm has been observed with annealing temperature increasing from 800 to 1200 °C. The Yb³⁺ concentration dependence of both luminescence spectra and lifetimes has been investigated and compared with those reported for the Yb³⁺-doped YAG bulk single crystals. Decays of Yb^{3+ 2}F_5/2 excited state, much longer in nano-crystalline powders than in bulk single crystals, are discussed in terms of refractive index dependence.     

Effect of Eu,Tb codoping on the luminescent properties of Y2O3 nanorods

Red-emitting Y₂O₃:Eu³⁺ and green-emitting Y₂O₃:Tb³⁺ and Y₂O₃:Eu³⁺, Tb³⁺ nanorods were synthesized by hydrothermal method. Their structure and micromorphology have been analyzed by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The photoluminescence (PL) property of Y₂O₃:Eu³⁺,Tb³⁺ phosphor was investigated. In the same host (Y₂O₃), upon excitation with ultraviolet (UV) irradiation, it is shown that there are strong emissions at around 610 and 545 nm corresponding to the forced electric dipole ⁵D₀-⁷F₂ transition of Eu³⁺ and ⁵D₄-⁷F₅ transition of Tb³⁺, respectively. Different qualities of Eu³⁺and Tb³⁺ ions are induced into the Y₂O₃ lattice. From the excitation spectrum, we speculate that there exists energy transfer from Tb³⁺ to Eu³⁺ ions .The emission color of powders reveals regular change in the separation of light emission. These powders can meet with the request of optical display material for different colors or can be potentially used as labels for biological molecules.     

Energy levels of rare-earth ions in Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S

The energies of the ground 4f ⁿ levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd₂O₂S crystal has been determined. It is shown that the Pr³⁺, Tb³⁺, and Eu³⁺ ions can be luminescence centers in Gd₂O₂S. The levels of the Nd³⁺, Dy³⁺, Er³⁺, Tm³⁺, Sm³⁺, and Ho³⁺ ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce³⁺ ion is near the midgap, due to which Ce³⁺ effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd₂O₂S:Pr and Gd₂O₂S:Tb phosphors.     

Luminescent and scintillation properties of the Pr3+ doped single crystalline films of Lu3Al5−xGaxO12 garnet

The Pr³⁺ d–f luminescence was investigated in the single crystalline films (SCF) of Lu₃Al_5−xGa_xO₁₂:Pr garnet solid solution at x = 1–3, grown by the liquid phase epitaxy (LPE) method from the melt-solution based on the PbO–B₂O₃ flux. The shape of CL spectra and decay kinetics of Pr³⁺ ions in Lu₃Al_5−xGa_xO₁₂ SCFs strongly depend on the total gallium concentration x and distribution of Ga³⁺ ions between the tetrahedral and octahedral position of the garnet host. The best scintillation properties of Lu₃Al_5−xGa_xO₁₂:Pr SCF are achieved at the nominal Ga content in melt-solution in the x = 2–2.5 range.     

Green luminescence and EPR studies on Mn-activated yttrium aluminum garnet phosphor

Yttrium aluminum garnet (Y₃Al₅O₁₂) and Mn activated Y₃Al₅O₁₂ phosphors have been prepared by urea combustion route in less than 5 min. The phosphors are well characterized by powder X-ray diffraction, Scanning electron microscopy and Fourier-transform infrared spectroscopic techniques. Photoluminescence tests on the pure Y₃Al₅O₁₂ showed a strong green emission at 525 nm (2.36 eV) attributed to the strongly allowed transition of F⁺ center whereas in Mn²⁺ activated YAG the green emission at 519 nm is due to the ⁴T₁ (G)→⁶A₁ (S) transition of Mn²⁺ ions. EPR studies have been carried out on Mn²⁺ activated Y₃Al₅O₁₂ phosphor at 300 and 110 K. From EPR spectra the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting (A) indicates that the Mn²⁺ ions are in a moderately ionic environment. The spin concentration (N) and paramagnetic susceptibility (χ) have been evaluated and discussed.     

Tailored photoluminescence of YAG:Ce phosphor through various methods

Trivalent cerium Ce³⁺ (Ce) activated yttrium aluminum garnet Y₃Al₅O₁₂ (YAG) phosphor was synthesized by two methods: solid state reaction (SS), and combustion (CB) with urea, respectively. The crystallization and luminescent properties of the phosphors were studied. Factors influencing on the intensity of luminescence and the location of emission band of YAG:Ce, such as the type of flux used in SS, the reaction atmosphere, the concentration of activator, were investigated in detail. We accomplished red or blue shift of Ce emission band by a number of techniques in order to match with the variable emission wavelength of blue light emitting diodes. The change of emission in color coordinates was illustrated by chromaticity. Co-doping other rare earth ions with Ce³⁺ ions into YAG was attempted to increase the color rendering index.     

Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers

The objective of this study was to identify a material suitable to absorb radiation at the wavelength of neodymium-doped Yttrium Aluminum Garnet (Y₃Al₅O₁₂:YAG), 1064 nm. M-(M= Sm³⁺, Co²⁺, Co³⁺, Cr³⁺, and Cr⁴⁺) doped highly transparent YAG ceramics were fabricated, and their absorption spectra were measured. Unlike Co²⁺ and Cr³⁺-doped ceramic samples, Co³⁺ and Cr⁴⁺ and Sm³⁺-doped:YAG ceramics were found to have significant absorption at 1064 nm. However, the Sm³⁺-doped YAG clearly emerged as the best candidate because it is also transparent at 808 nm, the pumping wavelength laser diode (LD), and also at most absorption bands used for flash-lamp pumping.