Luminescence and scintillation properties of Y<Subscript>3</Subscript>A<Subscript>l5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

Luminescence and scintillation properties of Y₃A_l5O₁₂:Ce single crystals grown from the melt by the Czochralski and horizontal directed crystallization methods in various gas media and Y₃A_l5O₁₂:Ce single-crystal films grown by liquid-phase epitaxy from a melt solution based on a PbO-B₂O₃ flux have been comparatively analyzed. The strong dependence of scintillation properties of Y₃A_l5O₁₂:Ce single crystals on their growth conditions and concentrations of Y_Al antisite defects and vacancy defects has been established. Vacancy defects are involved in Ce³⁺ ion emission excitation as the centers of intrinsic UV luminescence and trapping centers. It has been shown that Y₃A_l5O₁₂:Ce single-crystal films are characterized by faster scintillation decay kinetics than single crystals and a lower content of slow components in Ce³⁺ ion luminescence decay during high-energy excitation due to the absence of Y_Al antisite defects in them and low concentration of vacancy defects. At the same time, the light yield of Y₃A_l5O₁₂:Ce single-crystal films is comparable to that of single crystals grown by directed crystallization due to the quenching effect of the Pb²⁺ ion impurity as a flux component and is slightly lower (∼25%) than the light yield of single crystals grown by the Czochralski method.     

Luminescence of excitons and antisite defects in Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single crystals and single-crystal films

The luminescence of excitons and antisite defects (ADs) was investigated, as well as the specific features of the excitation energy transfer from excitons and ADs to the activator (Ce³⁺ ion) in phosphors based on Lu₃Al₅O₁₂:Ce (LuAG:Ce) single crystals and single-crystalline films, which are characterized by significantly different concentrations of ADs of the Lu _Al ³⁺ type and vacancy-type defects. The luminescence band with λ_max = 249 nm in LuAG:Ce single-crystal films is due to the luminescence of self-trapped excitons (STEs) at regular sites of the garnet lattice. The excited state of STEs is characterized by the presence of two radiative levels with significantly different transition probabilities, which is responsible for the presence of two excitation bands with λ_max = 160 and 167 nm and two components (fast and slow) in the decay kinetics of the STE luminescence. In LuAG:Ce single crystals, in contrast to single-crystal films, the radiative relaxation of STEs in the band with λ_max = 253.5 nm occurs predominantly near Lu _Al ³⁺ ADs. The intrinsic luminescence of LuAG:Ce single crystals at 300 K in the band with λ_max = 325 nm (τ = 540 ns), which is excited in the band with λ_max = 175 nm, is due to the radiative recombination of electrons with holes localized near Lu _Al ³⁺ ADs. In LuAG:Ce single crystals, the excitation of the luminescence of Ce³⁺ ions occurs to a large extent with the participation of ADs. As a result, slow components are present in the luminescence decay of Ce³⁺ ions in LuAG:Ce single crystals due to both the reabsorption of the UV AD luminescence in the 4f-5d absorption band of Ce³⁺ ions with λ_max = 340 nm and the intermediate localization of charge carriers at ADs and vacancy-type defects. In contrast to single crystals, in phosphors based on LuAG:Ce single-crystal films, the contribution of slow components to the luminescence of Ce³⁺ ions is significantly smaller due to a low concentration of these types of defects.     

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 properties of transparent ceramics Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Yb

We present the results of studying the luminescence properties of transparent ceramics Y₃Al₅O₁₂:Yb obtained by the vacuum sintering and nanocrystalline technology. In the course of research, we measured the luminescence and luminescence excitation spectra, as well as the temperature and kinetic behavior of luminescence. Our results are analyzed in comparison with the characteristics of corresponding single crystals. We revealed that processes of generation and relaxation of electronic excitations that occur in ceramics, in particular, in the charge transfer state, are similar to processes occurring in crystals. The behavior of two charge-transfer luminescence bands at 340 and 490 nm is studied. In the range 300–600 nm, we revealed a broad emission band of radiation of other type, which is also observed in spectra of undoped ceramics. This broad band is attributed to F⁺ centers. Emission and excitation spectra of charge transfer luminescence at a maximum of the temperature dependence of 100 K are measured for the first time. We found that, upon excitation in the charge transfer band, luminescence in ceramics is more intense than in single crystals with similar concentrations of Yb and has a higher quenching temperature.     

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.     

The influence of the specific surface of grains on the luminescence properties of Nd<Superscript>3+</Superscript>-doped Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> nanopowders

Nd³⁺:Y₃Al₅O₁₂ (Nd:YAG) powders were prepared by the Pechini method in the temperature range of 800 to 1400 °C. The pure garnet phase of the obtained materials was confirmed by XRD studies. The size of the grains was controlled by the annealing temperature of the samples. Their morphologies were investigated by TEM and porosity measurements (BET). The effect of annealing temperature on the morphology and luminescence properties of Nd:YAG nanocrystallites was studied, and the results were compared to the properties of a Nd:YAG single crystal. A significant enhancement of the ⁴F_3/2→⁴I_9/2/⁴F_3/2→⁴I_11/2 intensity ratio with decreasing grain size was observed. It was found that the decay times of the Nd³⁺ luminescence depends on the specific surface and is significantly longer for well crystallized nanocrystalline grains than for single crystals having the same concentration of Nd³⁺ ions. The role of crystallinity and specific surface on the radiative processes is analyzed. PACS 78.55.-m; 78.20.Ci; 78.67.Bf; 78.68.+m     

Laser performance of holmium-doped Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and (Lu,Y)<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> crystals pumped by Tm:fiber laser

Optical properties of Ho³⁺-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ crystals were investigated and compared. Substitution of Y for Lu in the host garnet Lu₃Al₅O₁₂ results in broad absorption and emission spectra, and improvements in the laser behavior of Ho³⁺. Pumped by Tm:fiber laser, a maximum output power of 5.02 and 5.73 W of Ho-doped Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂ have been obtained, respectively. The center lasing wavelength are 2124.5 and 2123.0 nm for Lu₃Al₅O₁₂ and (Lu,Y)₃Al₅O₁₂, respectively.     

Various types of photoactive behavior of the Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> ferrimagnetic insulator

The photomagnetic behavior of single-crystal yttrium iron garnet Y₃Fe₅O₁₂ doped with iridium, substituting the cation of iron in the octahedron, is investigated upon illumination at room temperature. It is shown that the photomagnetic properties of Y₃Fe_4.97Ir_0.03O₁₂ samples are to a large degree related to the impurity distortion of the sublattice of iron atoms in octahedral coordination, rather than solely to the possible presence of Fe⁴⁺ cations, which are inactive at room temperature and may even be lacking in single crystals doped with iridium. It is concluded that the photoinduced change in the magnetic parameters of this material is determined by the location of impurity cations and increased surface imperfection of the material. The reasons for the different photoactive behavior of this promising material for spintronics, that is, a singlecrystal yttrium iron garnet, are summarized.     

Morphology-dependent luminescence behavior of Y<Subscript>2-x</Subscript>Gd<Subscript>x</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> thin-film phosphors grown by a laser ablation

Y_2-xGd_xO₃:Eu³⁺ luminescent thin films have been grown on Al₂O₃(0001) substrates using pulsed laser deposition. Films grown under different deposition conditions have been characterized using microstructural and luminescence measurements. The crystallinity, surface morphology and photoluminescence (PL) of the films are highly dependent on the amount of Gd present. The photoluminescence (PL) brightness data obtained from Y_2-xGd_xO₃:Eu³⁺ films grown under optimized conditions have indicated that Al₂O₃(0001) is one of the most promising substrates for the growth of high-quality Y_2-xGd_xO₃:Eu³⁺ thin-film red phosphors. In particular, the incorporation of Gd into the Y₂O₃ lattice could induce a remarkable increase of PL. The highest emission intensity was observed with Y_1.35Gd_0.60Eu_0.05O₃, whose brightness was increased by a factor of 3.1 in comparison with that of Y₂O₃:Eu³⁺ films. This phosphor may be promising for application in flat-panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Luminescence of excitons in single-crystal garnets

Comparative studies of the luminescence of Y₃Al₅O₁₂:Ce and Lu₃Al₅O₁₂:Ce single-crystal films and their volume analogues—Y₃Al₅O₁₂ and Y₃Al₅O₁₂:Ce single crystals, excited by synchrotron radiation with energy E=120–150 eV, have been performed. The films were grown from melt-solution by liquid-phase epitaxy and the crystals were grown from melt. The single-crystal films and single crystals studied are characterized by different degrees of structural order, in particular, different concentrations of substitutional defects of the Y _Al ³⁺ and LU _Al ³⁺ types. It was ascertained that the bands at 260 and 250 nm in the intrinsic luminescence spectra of Y₃Al₅O₁₂:Ce and Lu₃Al₅O₁₂:Ce single-crystal films and single crystals are due to the emission of self-trapped excitons. The luminescence band with λ_max=300 nm and τ=0.36 μs, which is present in the luminescence spectrum of single crystals and absent in the spectra of single-crystal films, is due to the recombination of electrons with holes localized at Y _Al ³⁺ centers. It is shown that an efficient energy transfer by excitons to activator ions occurs in Y₃Al₅O₁₂ and Lu₃Al₅O₂ single-crystal films doped with Ce³⁺ ions.     

Spectral properties of Nd<Superscript>3+</Superscript> ions in samples of transparent Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramics

The spectral and luminescent characteristics of samples of Y₂O₃:Nd³⁺ ceramics obtained from different precursors under different preparation conditions (the concentration of an HfO₂ compacting additive, the temperature and time of synthesis) are studied at 300 and 77 K. It is shown that the spectral positions of absorption and luminescence lines of ceramics correspond to those of a Y₂O₃:Nd³⁺ single crystal. At the same time, the absorption and luminescence spectra show an inhomogeneous broadening, characteristic of disordered crystals and glass. The energies of the ⁴ I _9/2 and ⁴ F _3/2 Stark states of the Nd³⁺ ion are calculated. The calculation results nearly coincide with the data from the literature for the Y₂O₃:Nd³⁺ single crystal and transparent ceramics. Samples containing the compacting additive show additional lines, whose intensities correlate with its concentration and the method of preparation of Y₂O₃:Nd³⁺ ultradispersed powders. It is assumed that these lines are related to the fact that either Nd³⁺ ions enter the composition of the HfO₂ compacting additive or Hf⁴⁺ ions are present in the nearest environment of Nd³⁺ ions at the boundaries of granules enriched with HfO₂.     

Effect of sintering temperature of Ce<Superscript>3+</Superscript>-doped Lu<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> phosphors on light emission and properties of crystal structure for white-light-emitting diodes

The effect of the sintering temperature of Ce³⁺-doped Lu₃Al₅O₁₂ (Ce-LuAG) phosphors on the emission and properties of the crystal structure was studied. A cathodoluminescence peak at 317 nm, which was assigned to lattice defects, was exhibited in addition to emission peaks at 508 and 540 nm for the Ce-LuAG phosphors. The intensities of the 317 nm emission peak for the phosphors with mean particle diameters of 5.0 and 10.0 µm formed at a low sintering temperature of 1430 °C were higher than those for the phosphors with mean particle diameters of 18.0 and 20.5 µm formed at a high sintering temperature of 1550 °C. In contrast, the electroluminescence spectra for fabricated white-light-emitting diodes (LEDs) using the phosphors revealed that the intensity of the peak at 540 nm was strong for the mean particle diameters of 18.0 and 20.5 µm. The intensity of the 540 nm peak, which is attributed to the 4f→5d transition of the Ce³⁺ activator, showed a dependence on the sintering temperature. The relationship between the optical properties and the lattice defects is discussed.     

Spontaneous luminescence of Eu<Superscript>3+</Superscript> ions in porous Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanospheres

A study of the luminescence of Eu³⁺ ions in Y₂O₃ nanospheres indicates a significant influence of the porous structure of nanoparticles on the luminescence of dopant ions. It is shown that filling the nanopores of initially porous Y₂O₃ nanospheres shortens the decay time of the spontaneous luminescence of doping europium ions. The change in the decay time is associated with the change in the effective refractive index of the porous nanospheres.     

Electronic excitations and defects in nanostructural Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al₂O₃ (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al₂O₃ nanopowders probed before and after Fe⁺ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ～17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al₂O₃ nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ₁ = 0.5 ns, τ₂ = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P^? centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F⁺ center (F _S ⁺ -type centers). In addition to the fast luminescence, nano-Al₂O₃ was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al₂O₃ suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.     

Luminescence excitation in reactive molecular CO + O<Subscript>2</Subscript> collisions on the surface of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Eu

It is shown that the CO + O₂ → CO₂ catalytic reaction on the surface of Y₂O₃-Eu may lead to electronic excitation of Eu³⁺ luminescence centers due to the chemical energy release. The luminescence observed allows one to study the interaction between molecular particles of ultralow (thermal) energies with surface by optical methods.     

Sensitizer-dependent up-conversion of Ho<Superscript>3+</Superscript> in nanocrystalline Y<Subscript>2</Subscript>O<Subscript>3</Subscript>

Ho³⁺–Yb³⁺ co-doped Y₂O₃ nanocrystals were synthesized by firing hydroxy carbonate precursors. Yb³⁺-concentration-dependent up-conversion properties of Ho³⁺ in Y₂O₃ nanocrystals have been investigated. The relative intensity of up-converted red emission increases more quickly than that of the green and the near-infrared ones with the enhancement of the concentration of Yb³⁺. It is believed that the energy process ⁵ S ₂ (⁵F₄) (Ho) + ⁵ I ₇ (Ho) →⁵ I ₆ (Ho)+⁵ F ₅ (Ho) plays an important role in the population of the ⁵ F ₅ level of Ho³⁺. The result indicates that the intensity ratio of the green emission to the red one can be tuned by changing the sensitizer concentration. PACS 78.55.-m     

Spectral-kinetic characteristics of crystals and nanoceramics based on BaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript>: Ce

Optical characteristics of BaF₂ and BaF₂: Ce single crystals and nanoceramic materials prepared from these single crystals by uniaxial hot pressing have been studied. It has been shown that the introduction of Ce₃₊ ions into BaF₂ hardly affects the ultrafast (∼0.9 ns) luminescence component. The integrated luminescence intensity of the BaF₂: Ce nanoceramics is higher than that of the corresponding single crystal and considerably higher that the intensity of the undoped BaF₂ crystal. It has been demonstrated that the slow (several hundred nanoseconds) component of the luminescence decay of BaF₂: Ce is due to the energy transfer from excitons to Ce³⁺ ions.