X-ray absorption study of rare earth ions in Sr2MgSi2O7:Eu,R persistent luminescence materials

The valence of the europium dopant and selected rare earth co-dopants (Ce³⁺, Dy³⁺, and Yb³⁺) in the Sr₂MgSi₂O₇:Eu²⁺,R³⁺ persistent luminescence materials were studied by room temperature XANES measurements. The results indicated the co-existence of both divalent and trivalent europium in all the studied materials. The relative amount of Eu³⁺ was observed to increase upon increasing exposure to X-rays, as expected by the persistent luminescence mechanism. This suggests a simultaneous filling of oxygen vacancies initially created by the reducing preparation conditions. For the Dy and Yb co-dopants, only trivalent species were observed. On the other hand, traces of tetravalent cerium were present in the Eu,Ce co-doped materials.     

Spectroscopic properties of CuO, SnO, and Dy2O3 co-doped phosphate glass: from luminescent material to plasmonic nanocomposite

Prospective applications of noble metal and rare-earth co-doped dielectrics in optical devices demand for a comprehensive understanding of the influence of material composition and processing on resulting properties. In this study, we report on the spectroscopic properties of a 50P₂O₅:50BaO glass matrix containing copper, tin, and dysprosium prepared by melting and subsequently subjected to heat treatment (HT). An achievement in terms of material preparation is that addition of stoichiometric amounts of CuO and SnO dopants along with the source of Dy³⁺ ions (Dy₂O₃) is shown effective for the precipitation of Cu nanoparticles (NPs) during HT. Optical absorption and photoluminescence (PL) spectroscopy including emission decay dynamics are employed in the characterization of the co-doped material as prepared, and as a function of HT. The basic structure of the phosphate host is assessed by ³¹P nuclear magnetic resonance spectroscopy. The optical data suggests the presence of both Cu²⁺ and Cu⁺ ions in the melt-quenched co-doped glass together with the Dy³⁺ ions. Thermal processing is indicated to result in the chemical reduction of ionic copper species via Sn²⁺ and ultimately produces the non-luminescent plasmonic Cu particles. The presence of such NPs is also observed to produce a quenching effect on Dy³⁺ PL, interpreted in terms of an ion-to-particle excitation energy transfer operating via interband transitions in the nanoscale metal. Thus, the glass may act as either a luminescent material or a plasmonic nanocomposite desirable for nonlinear optics dependent upon its thermal history.     

The influence of different alkaline earth oxides on the structural and optical properties of undoped,Ce-doped,Sm-doped,and Sm/Ce co-doped lithium alumino-phosphate glasses

Undoped, singly Sm doped, Ce doped, and Sm/Ce co-doped lithium alumino-phosphate glasses with different alkaline earth modifiers were prepared by melt quenching. The structure of the prepared glasses was investigated by FT-IR and Raman, as well as by optical spectroscopy. The effect of the optical basicity of the host glass matrix on the added active dopants was studied, as was the effect doping had on the phosphate structural units. The optical edge shifts toward higher wavelengths with an increase in the optical basicity due to the increased polarizability of the glass matrix, but also with increasing CeO₂ concentration as a result of Ce³⁺/Ce⁴⁺ inter valence charge transfer (IV-CT) absorption. The optical band gap for direct and indirect allowed transitions was calculated for the undoped glasses. The glass sample containing Mg²⁺ modifier ions is found to have the highest value (4.16 eV) for the optical band gap while Ba²⁺ has the lowest value (3.61 eV). The change in the optical band gap arises from the structural changes and the overall polarizability (optical basicity). Refractive index, molar refractivity R_m and molar polarizability α_m values increase with increasing optical basicity of the glasses. The characteristic absorption peaks of Sm³⁺ were also investigated. For Sm/Ce co-doped glasses, especially at high concentration of CeO₂, the absorption of Ce³⁺ hinders the high energy absorption of Sm³⁺ and this effect becomes more obvious with increasing optical basicity.     

New Strategy: Molten Salt-Assisted Synthesis to Enhance Lanthanide Upconversion Luminescence

Lanthanide-doped upconversion luminescent materials (LUCMs) have attracted much attention in diverse practical applications because of their superior features. However, the relatively weak luminescence intensity and low efficiency of LUCMs are the bottleneck problems that seriously limit their development. Unfortunately, most of the current major strategies of luminescence enhancement have some inherent shortcomings in their implementation. Here, a new and simple strategy of molten salt-assisted synthesis is proposed to enhance lanthanide upconversion luminescence for the first time. As a proof-of-concept, a series of rare earth oxides with obvious luminescence enhancement are prepared by a one-step method, utilizing molten NaCl as the high-temperature reaction media and rare earth chlorides as the precursors. The enhancement factors at different reaction temperatures are systematically investigated by taking Yb³⁺/Er³⁺ co-doped Y₂O₃ as an example, which can be enhanced up to more than six times. In addition, the molten salts are extended to all alkali chlorides, indicating that it is a universal strategy. Finally, the potential application of obtained UCL materials is demonstrated in near-infrared excited upconversion white light-emitting diodes (WLEDs) and other monochromatic LEDs.     

Brannerite,UTi<Subscript>2</Subscript>O<Subscript>6</Subscript>: Crystal chemistry,synthesis, properties,and use for actinide waste immobilization

The host materials suggested for immobilization of actinide waste of military or civil origin often contain the secondary (U,Pu)Ti2O6 phase of brannerite structure. For example, the materials for incorporation of excess plutonium, mainly consisting of pyrochlore, contain up to 30% brannerite. This is a usual phase in titanate host materials for isolating spent nuclear fuel (SNF) and products of its reprocessing, including waste from production of ^99mТс for medical purposes and other kinds of waste with high U and Pu content. Despite simple ideal stoichiometry, brannerite can contain large amounts of rare earths. This feature is due to the presence of uranium not only in the 4+ oxidation state, but also in the 5+ and 6+ states, which favors the exchange of rare earth elements (REE), e.g., in accordance with the scheme 2U⁴⁺ ? U⁵⁺ + REE³⁺. The REE amount in brannerite reaches 0.5–0.7 atom per formula unit. Therefore, brannerite is of interest as a host material for the rare earth–actinide fraction of high-level waste (HLW). To evaluate the prospects for such use of brannerite, data on the radiation resistance of brannerite and its behavior in aqueous solutions are analyzed. In these properties, brannerite is inferior to pyrochlore and zirconolite. The rate of actinide leaching from brannerite is higher by an order of magnitude than from these phases, but lower by 3–4 orders of magnitude than from glass host materials. Natural brannerite is stable in media with weakly alkaline and reducing waters. Therefore, brannerite seems suitable for immobilization of rare earth–actinide waste. This host material can be synthesized by sintering or cold crucible induction melting followed by crystallization.     

Comparative investigation on the effect of alkaline earth oxides on the intensity of absorption bands due to Cu<Superscript>2+</Superscript>, Mn<Superscript>3+</Superscript> and Cr<Superscript>3+</Superscript> ions in ternary silicate glasses

Absorption characteristics of Cu²⁺, Mn³⁺ and Cr³⁺ ions in ternary silicate (20Na₂O·10RO·70SiO₂, where R=Ca, Sr, Ba) glasses were investigated. The intensities of absorption bands due to Cu²⁺ ion was found to increase with increasing ionic radii of the alkaline earth ions whereas it was found to decrease in case of Mn³⁺ and Cr³⁺ ions with increasing ionic radii of the alkaline earth ions. The results were discussed in the light of relation between linear extinction coefficients of these ions and coulombic force of alkaline earth ions. The change in intensities of Cu²⁺, Mn³⁺ and Cr³⁺ ion is attributed due to change in silicate glass compositions.     

Sol–gel synthesis of long-lasting phosphors CdSiO3: Mn2+, RE3+ (RE = Tb,Eu, Nd) and luminescence mechanism research

Mn²⁺ and RE³⁺ (RE = Tb, Eu, Nd) co-doped CdSiO₃ orange phosphors were prepared at 1050 °C by a sol–gel method. The phase and crystallinity of the synthesized materials were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The luminescence characteristics were analyzed using photoluminescence (PL) spectra, afterglow decay curves, long-lasting phosphorescence spectra, and thermoluminescence (TL) spectra. Due to the difference in co-doped rare earth ionic radii, it varied greatly in trap density and trap depth caused by the different defects deriving from RE³⁺ ions co-doping into the CdSiO₃: Mn²⁺ host. The afterglow intensity and time for these samples increased as follows: CdSiO₃: Mn²⁺0.2%, Nd³⁺0.8% < CdSiO₃: Mn²⁺0.4%, Tb³⁺0.8% < CdSiO₃: Mn²⁺0.4%, Eu³⁺0.3%. CdSiO₃: Mn²⁺0.4%, Eu³⁺0.3% had the best afterglow properties, which could be due to the proper traps formed by Eu³⁺ ions co-doping into the host. The role of RE³⁺ co-doped into the CdSiO₃: Mn²⁺ matrix and the possible long-lasting phosphorescence process was also discussed in this paper.     

Efficient Bi → Nd energy transfer in Gd2O3:Bi,Nd

Bi³⁺,Nd³⁺ co-doped Gd₂O₃ were prepared by solid state reaction and the optical properties were investigated. The results show that the near-infrared emission of Nd³⁺ ions is significantly enhanced by the introducing of Bi³⁺ in co-doped samples. An efficient energy transfer from Bi³⁺ to Nd³⁺ corresponds to the near-infrared emission enhancement. The energy transfer efficiency reaches 64.1% for the sample with the strongest near-infrared emission, which has the optimized doping concentrations of 0.5% for Bi³⁺ and 2% for Nd³⁺. The interesting optical properties make Bi³⁺,Nd³⁺ co-doped Gd₂O₃ promising as the luminescent down-conversion layers in front of c-Si solar cells to enhance the performance of the solar cells.     

Towards a Green Synthesis of LAB's: Effect of Rare Earth Metal Ions on the Benzene Alkylation with 1-Dodecene over NaFAU-Y Zeolites

Chemically modified microporous materials can be prepared as robust catalysts suitable for application in vapor phase processes such as Friedel-Crafts alkylation. In the present paper we have investigated the use of rare earth metal (Ce³⁺, La³⁺, RE³⁺, and Sm³⁺) exchanged Na-Y zeolites as catalysts for the alkylation of benzene with long chain linear 1-olefin; 1-dodecene. Thermodesorption studies of 2,6-dimethylpyridine adsorbed catalysts (in the temperature range 573 to 873 K) show that the rare earth zeolites are highly Brönsted acidic in nature. A perfect correlation between catalyst selectivity towards the desired product (2-phenyldodecane) and Brönsted acid sites amount has been observed.     

Optical downconversion in rare earth (Tb and Yb) doped CdS nanocrystals

CdS host nanocrystals (4.2-5.5 nm in diameter) have been prepared by a low-cost synthetic chemical route using air stable precursors and doped with rare earth (RE) ions, terbium (Tb³⁺) and ytterbium (Yb³⁺). The resulting RE³⁺-doped nanocrystals show a complete broadband absorption below their band edge (400-460 nm) to the deep UV region. CdS host nanocrystals act as an energy receptor and sensitizer for the RE³⁺-ions. A broadband emission in the visible region is observed for the RE³⁺-doped CdS nanocrystals by room temperature photoluminescence (PL) measurements. Our results show that these novel RE³⁺-doped nanocrystals can be used as an optical down-conversion material for solar spectral matching in solar cells or as a wavelength shifting down-converter for nanoscale optoelectronic applications.     

Tunable color emission and afterglow in CaGa2S4: Eu, Ho phosphor

The tunable color emission and persistent luminescence lifetime in phosphor CaGa₂S₄: Eu²⁺, Ho³⁺ were achieved through the introduction of alkaline earth elements Mg, Sr and Ba, in which the Ca was partially replaced. The duration of the persistent luminescence of the material CaGa₂S₄: Eu²⁺, Ho³⁺ was remarkably shortened as Al was introduced, substituting for Ga. The luminescent properties were investigated via thermo-luminescence (TL) glow curves, phosphorescence spectra and decay time curves. These results show that vast changes in trap levels and charge density takes place with introduction of other alkaline earth elements or Al. Trap depths and the trap density were also evaluated by simple methodologies.     

Synthesis and luminescence properties of Na+, Li+ compensated Er3+ doped CaAl4O7 phosphors

Here the green-emitting highly luminescent Er³⁺ doped, Er³⁺-Li⁺ co-doped, Er³⁺-Na⁺ co-doped CaAl₄O₇ is synthesized by Pechini method at 1000^°C. Photoluminescence (PL) of CaAl₄O₇: Er³⁺ studies have been compared with Li⁺ co-doped CaAl₄O₇: Er³⁺ and Na⁺ co-doped CaAl₄O₇: Er³⁺. Na⁺ co-doped CaAl₄O₇:Er³⁺ shows increases in luminescence intensity compared to Li⁺ co-doped CaAl₄O₇: Er³⁺ and Er³⁺ doped CaAl₄O_7. The results suggest that CaAl₄O₇:Er³⁺ phosphor can be used as efficient green-emitting phosphor in white LED. The resultant phosphor emits green color peaking at 549 nm upon 378 nm excitation. Powder X-ray diffraction (PXRD) and photoluminescence (PL) techniques have been studied to characterize the synthesized microparticles. Further, this phosphor has good thermal stability that implies its potential to act as green phosphor in white light-emitting diodes. The effect of activator (Er³⁺), Na⁺ co-doped CaAl₄O₇:Er³⁺, and Li⁺ co-doped CaAl₄O₇:Er³⁺ phosphors luminescence spectra as well as photoluminescence life time studies were studied in detail. The results show that as the concentration of Er³⁺ in CaAl₄O₇ increases, the symmetry around the Er³⁺ ion decreases due to the creation of lattice defects in the crystal. Addition of Na⁺ and Li⁺ ions in CaAl₄O₇: Er³⁺leads to a small distortion in the local symmetry of Er³⁺ ions, thereby significantly enhancing its luminescence property. Analysis of photoluminescence life time studies of the prepared samples shows a smaller concentration quenching of Er³⁺ luminescence in charge compensated Na⁺ and Li⁺ CaAl₄O₇ phosphor.

     

Infrared to visible up conversion energy transfer confined at ordered micro-ring structures

Infrared to visible up-conversion energy transfer processes are demonstrated in two dimensional ordered arrays of fluorescent Er³⁺:Yb³⁺:CaNb₂O₆ micro-rings embedded in LiNbO₃ optical substrate. Spatially resolved confocal spectroscopy reveals that Yb³⁺ → Er³⁺ energy transfer process only takes place at these confined ring spatial regions. Thus, the formation of rare earth based co-doped micro-rings can be used to generate green and red visible fluorescent ring emitters upon infrared diode laser excitation on wide spatial areas. The results allow to envisage diode pumped multi-color integrated photonic devices for high quality shadow-free phosphor displays.     

Irradiation induced defects in glasses resulting in the photoionization of polyvalent dopants

The extent of solarization is strongly influenced by many factors such as glass matrix, dopants, or irradiation source. Typical irradiation induced effects are illustrated in a series of glasses doped with a selection of polyvalent ions. Intrinsic and extrinsic defects, consisting of electron and hole centers, are characterized with optical and EPR spectroscopy. Used in combination, both spectroscopic methods allow to detect a wide range of defects. UV-lamp irradiation causes in fluoride-phosphate glasses the photoreduction of Ni²⁺, Pb²⁺ and Ag⁺ while Co²⁺, Ce³⁺, Fe²⁺ and Mn²⁺ are photooxidized. The influence of the glass matrix is evident for Ni²⁺ which is photooxidized in borosilicate, but reduced in fluoride-phosphate glasses. Iron ions also show different redox reactions depending on the glass matrix, as well as on the melting conditions, which will be shown for low alkaline borosilicate glasses doped with iron or tin. Changes in the nature and the rate of defect formation could be observed within the irradiation process of a fluoride-phosphate glass co-doped with Mn²⁺ and Fe²⁺ and during thermal annealing of a lamp irradiated phosphate glass containing Co²⁺. The series Mn²⁺, Fe²⁺, Co²⁺ and Ni²⁺ shows a trend of increased photooxidation with increasing electro-negativity or decreasing mass of the ions.     

Compositional and structural dependence of up-converting rare earth fluorides obtained through EDTA assisted hydro/solvothermal synthesis

In this feature article, we highlight our works on compositional and structural dependence of up-converting rare earth (RE) fluorides obtained through ethylenediamine tetraacetic acid (EDTA) assisted hydrothermal synthesis. Various nanostructures were obtained by tuning of experimental conditions, such as precursor’s concentration, degree of doping, reaction time and solvent used during synthesis. We correlated in detail the structural, morphological and optical properties of YF₃ and NaYF₄ compounds co-doped with Yb³⁺ and Er³⁺ (introduced in total mol% of 8 and 20). For this purpose, X-ray powder diffraction, scanning and transmission electron microscopy, energy dispersive X-ray and Furrier transform infrared spectroscopy, as well as, the photoluminescence spectra and decay times were recorded and analyzed. The particle size and phase content were found to be dependent on the nucleation rate, which, in turn, was governed by the precursor concentration, degree of doping and solvent type. The transformation from cubic to hexagonal NaYF₄:Yb³⁺/Er³⁺ phase was found to be sensitive to the reaction time and precursors concentration, while the crystallization of orthorhombic YF₃:Yb³⁺/Er³⁺ phase is achieved through lowering of dopants concentration or by changing of solvent during hydrothermal treatment. The up-conversion photoluminescence demonstrated morphology and crystal phase dependence and is found to be superior in microcrystalline samples, independent on their phase composition.     

Bifunction in Er3+/Yb3+ co-doped BaTi2O5–Gd2O3 glasses prepared by aerodynamic levitation method

Novel Er³⁺/Yb³⁺ co-doped BaTi₂O₅–Gd₂O₃ spherical glasses have been fabricated by aerodynamic levitation method. The thermal stability, upconversion luminescence, and magnetic properties of the present glass have been studied. The glasses show high thermal stability with 763.3 °C of the onset temperature of the glass transition. Red and green emissions centered at 671 nm, 548 nm and 535 nm are obtained at 980 nm excitation. The upconversion is based on a two-photon process by energy transfer, excited-state absorption, and energy back transfer. Yb³⁺ ions are more than Er³⁺ ions in the glass, resulting in efficient energy back transfer from Er³⁺ to Yb³⁺. So the red emission is stronger than the green emissions. Magnetization curves indicate that magnetic rare earth ions are paramagnetic and the distribution is homogeneous and random in the glass matrix. Aerodynamic levitation method is an efficient way to prepare glasses with homogeneous rare earth ions.     

Facile synthesis of water-soluble YF3 and YF3: Ln nanocrystals

Water-soluble lanthanide-doped YF₃ nanocrystals with cubic structure were successfully synthesized for the first time using a simple solvothermal method with ethanol as solvent at 160 °C for 12 h. SEM and TEM results demonstrated that the obtained nanocrystals have an irregular shape and an average size of below 30 nm. Tb³⁺, Tb³⁺/Ce³⁺ co-doped YF₃ nanocrystals were also prepared and their photoluminescence properties were investigated. The luminescent intensity of the Tb³⁺ for the Tb³⁺/Ce³⁺ co-doped YF₃ nanocrystals is about ten times higher than that of the Tb³⁺ doped YF₃ nanocrystals. The products were characterized by XRD, SEM and TEM. Owing to their water-soluble properties and high processability, extensive applications may be found.     

Color control of emissions from rare earth-co-doped La2O3:Bi phosphor thin films prepared by magnetron sputtering

Multicolor photoluminescence (PL) and electroluminescence (EL) were observed from developed Bi- and rare earth (RE)-co-doped La₂O₃ (La₂O₃:Bi,RE) phosphor thin films. The phosphor thin films were prepared with various contents of co-doped RE, such as Dy, Er, Eu, Tb and Tm, by a combinatorial radio frequency magnetron sputtering deposition method. The obtainable luminance in EL and PL intensities changed considerably as the kind and content of RE were varied. Color changes from blue and blue-green to various colors in PL and EL emissions, respectively, were obtained in postannealed La₂O₃:Bi,RE phosphor thin films: films prepared by co-doping Bi at a constant content with various REs at varying levels of content. However, all of the observed emission peaks in PL and EL from La₂O₃:Bi,RE phosphor thin films were assigned to either the broad emission originating from the transition in Bi³⁺ or the visible emission peaks originating from the transition in the co-doped trivalent RE ion. The difference between PL and EL characteristics in La₂O₃:Bi,RE phosphor thin films is mainly attributed to the difference in the excitation mechanism.     

Shape-controlled synthesis of octahedral α-NaYF4 and its rare earth doped submicrometer particles in acetic acid

Submicrometer sized pure cubic phase, Eu³⁺ doped, and Yb³⁺/Er³⁺ co-doped α-NaYF₄ particles with octahedral morphology have been prepared in acetic acid. The acetate anion plays a critical role in the formation of such symmetric octahedral structures through its selective adsorption on the (111) faces of the products. The size of the as-prepared octahedra can be tuned by varying the amount of sodium acetate added to the acetic acid. A possible formation mechanism for these octahedra has been proposed. The doped α-NaYF₄ octahedral submicrometer particles show down-conversion and up-conversion photoluminescence typical of these materials. This article is published with open access at Springerlink.com     

Polyspectral white light emission from Eu3+, Tb3+, Dy3+, Tm3+ co-doped GdAl3(BO3)4 phosphors obtained by combustion synthesis

Polycrystalline sub-micron-sized GdAl₃(BO₃)₄ phosphors co-doped with Eu³⁺, Tb³⁺, Dy³⁺ and Tm³⁺ have been prepared by combustion synthesis with urea. The phosphors have been characterised by X-ray diffraction, scanning electron microscopy, excitation and emission spectroscopy. The chromaticity co-ordinates and the colour temperatures of the fluorescence of the materials presented have been calculated and analysed with Commission Internationale l’Eclairage (CIE) programs and diagrams. Depending on the excitation wavelength, different colour temperatures of the light emitted can be achieved. Due to its polyspectral nature, the emitted light reveals a high colour rendering index.