Luminescence of La and Sc impurity centers in YAlO3 single-crystalline films

The luminescence of La_Y³⁺ and Sc_Y³⁺ and Sc_Al³⁺ centers created by lanthanum and scandium ions at Y³⁺ and Al³⁺ cation sites of YAlO₃ perovskite lattice was investigated. The features of emission of excitons localized at the mentioned centers in YAlO₃:La and YAlO₃:Sc single-crystalline films were analyzed by means of time-resolved emission spectroscopy and luminescence decay kinetics measurements under excitation by synchrotron radiation at 9 and 300 K.     

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.     

Scintillation and anomalous emission in elpasolite Cs2LiLuCl6:Ce

The luminescence and scintillation properties of Cs₂LiLuCl₆:0.5%Ce³⁺ are presented. Special attention is devoted to a 9.4 ns fast emission at 275 nm that can only be excited via the highest cubic field 5d_e state of Ce. Contrary to Cs₃LuCl₆ and Cs₂LiYCl₆, where the same type of fast emission was observed, the emission in Cs₂LiLuCl₆ is still observed at room temperature. Assuming that the 5d_e state is located inside the host conduction band (CB), we propose that the emission originates from a mixed state at or just below the bottom of the CB and ends at the 4f ground state of Ce³⁺. To proof this model we studied the thermal quenching of the anomalous luminescence and performed X-ray photoelectron spectroscopy. A model for a temperature-activated energy transfer from the anomalous state to the lowest 5d_t excited state of Ce³⁺ explains most of the results. Besides the 275 nm emission, the material shows 5d_t-4f Ce³⁺ emission at 370 and 406 nm and 2 ns fast core-valence luminescence when excited with 16-22 eV photons. The scintillation properties of Cs₂LiLuCl₆:Ce are briefly discussed.     

Observation of green emission from Ce-doped gadolinium oxide nanoparticles

Green emission at around 500 nm is observed in Gd₂O₃:Ce³⁺ nanoparticles and the intensity is highly dependent on the concentration of Ce³⁺ in the nanoparticles. The luminescence of this emission displays both picosecond (ps) and millisecond (ms) lifetimes. The ms lifetime is over four orders of magnitude longer than typical luminescence lifetimes (10-40 ns) of Ce³⁺ in traditional Ce³⁺ doped phosphors and therefore likely originates from defect states. The picosecond lifetime is shorter than the typical Ce³⁺ value and is also likely due to defect or surface states. When the samples are annealed at 700 °C, this emission disappears possibly due to changes in the defect moieties or concentration. In addition, a blue emission at around 430 nm is observed in freshly prepared Gd₂O₃ undoped nanoparticles, which is attributed to the stabilizer, polyethylene glycol biscarboxymethyl ether. On aging, the undoped particles show similar emission to the doped particles with similar luminescence lifetimes. When Eu³⁺ ions are co-doped in Gd₂O₃:Ce nanoparticles, both the green emission and the emission at 612 nm from Eu³⁺ are observed.     

Relocalization of Ce 5d electrons from host conduction band

Single crystal fibers of Ce³⁺ doped SrAl₂O₄ and CaAl₄O₇ were prepared through the laser heated pedestal growth method. Sites dependent Ce³⁺ emissions were found at 385 nm (427 nm) and 420 nm (325 nm) in SrAl₂O₄ and CaAl₄O₇ hosts, respectively. The Ce³⁺ emissions at 385 nm and 420 nm in the two hosts exhibited strong afterglows. They could persist for more than 10 h. The long persistence and sites dependence of Ce³⁺ emissions were originated from charge compensation of doping Ce³⁺ into divalent cation sites. The lifetimes of Ce³⁺ emissions in both hosts were found to depend on the laser excitation wavelengths. With 266 nm laser excitation, Ce³⁺ 5d electrons were delocalized into the host's conduction band, resulting in a prolonged decay time. The 355 nm laser excitation did not delocalize the 5d electrons and hence the measured lifetimes were the intrinsic Ce³⁺ emission lifetimes that were 17 and 35.5 ns in SrAl₂O₄ and CaAl₄O₇ hosts, respectively. The prolonged Ce³⁺ emission lifetime on 266 nm laser excitation was because of the relocalization of the 5d electrons from the host conduction band. The lifetimes of Ce³⁺ 5d electrons within the conduction band were found to be 34 and 44 ns in SrAl₂O₄ and CaAl₄O₇ hosts, respectively.     

Spectroscopic properties of Ce doped silica annealed at different temperatures

Ce³⁺-doped silica was synthesized by sol-gel technique. The absorption band at 252 nm of Ce³⁺-doped silica is close to the main absorption band of Ce(NO₃)₃ solution. Three different luminescence bands were observed in the samples annealed at temperatures from 100 to 1200 °C, and the intensity of these luminescence bands changed with the alteration of the heat-treating temperatures. In addition to two well-known main luminescence bands of 4f-5d transition of Ce³⁺ with the wavelength at 357 and 450 nm, a rarely reported luminescent band with the wavelength at 344 nm was also observed, which was attributed to some kind of oxygen-related defects of silica.     

Radiation and thermally induced effects in YAlO3:Mn crystals

We have studied effects induced by γ-radiation and temperature in Mn-doped YAlO₃ crystals. The studies have been performed by means of optical spectroscopy that include measuring of optical absorption changes induced by γ-radiation and elevated temperature as well as thermally stimulated luminescence (TSL). It has been shown that under γ-irradiation of YAlO₃:Mn crystals, along with the ionization of Mn_Al⁴⁺ ions (Mn_Al⁴⁺→Mn_Al⁵⁺+e⁻), some additional coloration processes take place. This additional coloration is characterized by a wide intense band centered at 26,000- that is ascribed to color centers intrinsic to YAlO₃ lattice. This coloration is removed by the way of crystal warming at , while the coloration caused by Mn_Al⁵⁺ ions is removed at higher temperature . The observed TSL glow of irradiated crystals reveals three peaks near 360, 400 and that correspond to three types of traps. Parameters of the traps have been determined. The TSL emission corresponds to intra-center luminescence of Mn_Al⁴⁺ and Mn_Y²⁺ ions. The possible ionization and trapping mechanisms in YAlO₃:Mn crystals are discussed.     

On the spectra luminescence properties of charoite silicate

Charoite is a hydrous alkali calcium silicate mineral [K₄NaCa₇Ba_0.75Mn_0.2Fe_0.05(Si₆O₁₅)₂(Si₂O₇)Si₄O₉(OH)·3(H₂O)] exhibiting an intense lilac colour related to Mn²⁺ and Fe³⁺ colour centres. These ions also contribute to a strong luminescence at ∼585 and 705 nm. This work studies the thermal dependence of these luminescent centres by (i) thermoluminescence (TL) of pre-heated and pre-irradiated charoite aliquots, (ii) by time-resolved cathodoluminescence (TRS-CL) at room and cryogenic temperatures (RT and CT), (iii) by spatially resolved spectra CL under scanning electron microscopy (SRS-CL-SEM) and (iv) by ion beam spectra luminescence (IBL) with H⁺, H₂⁺ and ⁴He⁺ ions at RT and LT. The main peak, ∼585 nm, is linked to a transition ⁴T_1,2 (G)→⁶A₇(S) in Mn²⁺ ions in distorted six-fold coordination and the emission at ∼705 nm with Fe²⁺→Fe³⁺ oxidation in Si⁴⁺ lattice sites. Less intense UV-blue emissions at 340 and 390 nm show multi-order kinetic TL glow curves involving continuous processes of electron trapping and de-trapping along with an irreversible phase transition of charoite by de-hydroxylation and lattice shortening of Δa=0.219 Å, Δb=0.182 Å; Δc=0.739 Å. The Si-O stressed lattice of charoite has non-bridging oxygen or silicon vacancy-hole centres, and Si-O bonding defects which seem to be responsible for the 340 nm emission. Extrinsic defects such as the alkali (or hydrogen)—compensated [AlO₄/M⁺] centres could be linked with the 390 nm emission. Large variations in 585 and 705 nm intensities are strongly temperature dependent, modifying local Fe-O and Mn-O bond distances, short-range-order luminescence centres being very resistant under the action of the heavy ion beam of ⁴He⁺. The SRS-CL demonstrates strong spatial heterogeneity in the luminescence of the charoite.     

In-situ XAS study on the Cu and Ce local structural changes in a CuO-CeO2/Al2O3 catalyst under propane reduction and re-oxidation

The redox behaviour of a CuO-CeO₂/Al₂O₃ catalyst is studied under propane reduction and re-oxidation. The evolution of the local Cu and Ce structure is studied with in-situ transmission X-ray absorption spectroscopy (XAS) at the Cu K and Ce L₃ absorption edges.CuO and CeO₂ structures are present in the catalyst as such. No structural effect on the local Cu structure is observed upon heating in He up to 873 K or after pre-oxidation at 423 K.Exposure to propane at reaction temperature (600-763 K) fully reduces the Cu²⁺ cations towards metallic Cu⁰. Quick EXAFS spectra taken during reduction show a small amount of intermediate Cu¹⁺ species. Parallel to the CuO reduction, CeO₂ is also reduced in the same temperature range. About 25% of the Ce⁴⁺ reduces rapidly to Ce³⁺ in the 610-640 K temperature interval, while beyond 640 K a further slower reduction of Ce⁴⁺ to Ce³⁺ occurs. At 763 K, Ce reduction is still incomplete with 32% of Ce³⁺.Re-oxidation of Cu and Ce is fast and brings back the original oxides.The propane reduction of the CuO-CeO₂/Al₂O₃ catalyst involves both CuO and CeO₂ reduction at similar temperatures, which is ascribed to an interaction between the two compounds.     

Spectral-kinetic studies of SrAlF5 doped by trivalent rare-earth ions

Photochemical properties of Ce³⁺:SrAlF₅ and Ce³⁺,Yb³⁺:SrAlF₅ single crystals together with spectroscopic and kinetic characteristics of several optically nonequivalent impurity centers and energy transfer between them are described. It is shown that co-activation by Yb³⁺ ions effectively suppresses color centers in Ce,Yb:SAF crystals. It was found out that in Ce,Yb:SAF crystals Yb ions exist simultaneously in 2+ and 3+ valent state. Three types of optically nonequivalent luminescent centers corresponding to the doublets in luminescence spectrum centered at 290, 305 and 370 nm (Ce^I, Ce^II, Ce^III, respectively) have been observed. Analysis of luminescence spectra and decays leads to the conclusion that there is no energy transfer between either cerium centers or from Ce³⁺ to Yb²⁺ apart from the Ce^III center which luminescence is slightly quenched by Yb²⁺.     

Luminescence characteristics of Pb centres in undoped and Ce-doped Lu3Al5O12 single-crystalline films and Pb→Ce energy transfer processes

At 4.2-350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5-15 eV energy range for the undoped and Ce³⁺-doped Lu₃Al₅O₁₂ (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb²⁺-based centres were identified. The processes of energy transfer from the host lattice to Pb²⁺ and Ce³⁺ ions and from Pb²⁺ to Ce³⁺ ions were investigated. Competition between Pb²⁺ and Ce³⁺ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb²⁺ centres with the 3.6 eV absorption band of Ce³⁺ centres, an effective nonradiative energy transfer from Pb²⁺ ions to Ce³⁺ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce³⁺ centres and decrease of the Ce³⁺-related luminescence intensity.     

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.     

Luminescence properties of europium-terbium double activated calcium tungstate phosphor

Double incorporation of Eu³⁺ and Tb³⁺ ions into a CaWO₄ crystalline lattice modifies the luminescence spectrum due to the formation of new emission centers. Depending on the activators concentration and nature, as well as on the interaction between the activators themselves, the luminescence color can be varied within the entire range of the visible spectrum. Variable luminescence was obtained when CaWO₄:Eu,Tb phosphors with 0-5 mol% activator ions were exposed to relatively low excitation energies as UV (365 and 254 nm). Under high energy excitation such as VUV (147 nm) radiation or electron beam, white light has been observed.This material with controlled properties seems to be promising for the applications in fluorescent lamps, colored lightning for advertisement industries, and other optoelectronic devices.     

Upconversion luminescence and kinetics in Er:YAlO3 under 652.2 nm excitation

Ultraviolet and visible upconversion properties of Er³⁺ in YAlO₃ were investigated following 652.2 nm excitation of the multiples ⁴F_9/2. The luminescence and excitation spectra were recorded. Ultraviolet (326-342 and 354-359 nm), violet (405-420 nm), blue (436-442 nm) and green (525-575 nm) upconversion and infrared downconversion luminescence were simultaneously observed. The intense green luminescence corresponds to the emissions from the thermal coupled ⁴S_3/2 and ²H_11/2 bands and ²G_9/2 level. Energy transfer upconversion processes were proposed to explain the upconversion phenomena. The luminescence kinetics was discussed in detail by the analyses of fluorescence decay curves.     

Preparation and characterization of nanosized (Y,Bi)VO4:Eu and Y(V,P)O4:Eu red phosphors

Nanosized luminescent (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ were synthesized at low temperatures either by a coprecipitation method or by a hydrothermal method from aqueous solutions. The effect of Bi³⁺ ion or P⁵⁺ ion content in the lattice, annealing temperature effects on the crystal structure and the particle size, and the luminescence property of (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ nanoparticles were examined with a field-enhanced scanning electron microscopy, XRD, and a spectrofluorometer. The pristine YVO₄:Eu³⁺, (Y,Bi)VO₄:Eu³⁺, or Y(V,P)O₄:Eu³⁺ nanoparticles are 35-50 nm in size. The luminescence spectrum of the Eu³⁺ ion was used to probe its position in the crystal lattice. The dopant ions enter the same lattice sites in the nanocrystalline as in the corresponding bulk material, resulting similar spectral features between them. Photoluminescence intensity is weak for the pristine nanoparticles. Annealing the nanoparticles at temperatures up to 1000 °C results in the increased luminescence intensity (>80% of micrometer-sized phosphors) with the minimal particle growth and the improved particle crystallinity.     

Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce

Radioluminescence and thermally stimulated luminescence measurements on Lu₂O₃, Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce³⁺ (LSO:Ce) reveal the presence of intrinsic ultraviolet luminescence bands. Characteristic emission with maximum at 256 nm occurs in each specimen and is attributed to radiative recombination of self-trapped excitons. Thermal quenching of this band obeys the Mott-Seitz relation yielding quenching energies 24, 38 and 13 meV for Lu₂O₃, LSO and LSO:Ce, respectively. A second intrinsic band appears at 315 nm in LSO and LSO:Ce, and at 368 nm in Lu₂O₃. Quenching curves for these bands show an initial increase in peak intensity followed by a decrease. Similarity in spectral peak position and quenching behavior indicate that this band has a common origin in each of the samples and is attributed to radiative recombination of self-trapped holes, in agreement with previous work on similar specimens. Comparison of glow curves and emission spectra show that the lowest temperature glow peaks in each specimen are associated with thermal decay of self-trapped excitons and self-trapped holes. Interplay between the intrinsic defects and extrinsic Ce³⁺ emission in LSO:Ce is strongly indicated.     

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.     

Cr-related centers in Gd3Ga5O12 polycrystals

The luminescence excitation spectra, emission spectra under photo- and X-ray excitation, luminescence decay kinetics and thermostimulated luminescence (TSL) of Gd₃Ga₅O₁₂ garnet (GGG) polycrystalline samples have been investigated. It was established that the spectrum of Cr³⁺ ion emission were present in all TSL peaks. The activation energies of traps that are responsible for appearance of TSL in the region 295-600 K were estimated. It is shown that delocalization of electrons from the Cr³⁺e traps leads to the appearance of thermoluminescence (TL) glow peak at 390 K. The nature of other TSL peaks is discussed. The influence of visible light on the TSL intensity of the preliminary X-ray-irradiated samples is shown.     

Photoluminescence and EPR studies on gamma irradiated Ce doped potassium halide single crystals

Electron Paramagnetic Resonance(EPR), Photoluminescence(PL), Thermoluminescence (TL) and other optical studies of γ-irradiated KBr, KCl:Ce³⁺ single crystals. Cerium when doped into the KBr, KCl is found to enter the host lattice in its trivalent state and act as electron trap during γ-irradiation, thereby partially converting itself to Ce²⁺. The Photoluminescence(PL) spectra of both KCl and KBr crystals doped with Ce exhibit the strong blue emissions of Ce corresponding to ⁵d(²D)→²F_5/2 and ⁵d(²D)→²F_7/2 transitions. The defect centers formed in the Ce³⁺ doped KBr and KCl. Crystals are studied using the technique of EPR. A dominant TL glow peak at 374, 422 K and KCl:Ce³⁺ at 466, 475 K is observed in the crystal. EPR studies indicate the presence at two centers at room temperature. Spectral distribution under the thermoluminescence emission(TLE) and optically stimulated emission(OSL) support the idea that defect annihilation process to be due to thermal release of F electron in KBr, KCl:Ce³⁺ crystals. Both Ce³⁺ and Ce²⁺ emissions were observed in the thermoluminescence emission of the crystals.     

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.