Intrinsic- and Rare Earth Ion-Activated Luminescence in NbAlO4 with Wide Structure Channels.

Compounds with ordered and interconnected channels have versatile multifunctional applications in technological fields. In this work, we report the intrinsic- and Eu3+-activated luminescence in NbAlO4 with a wide channel structure. NbAlO4 is an n-type semiconductor with an indirect allowed transition and a band-gap energy of 3.26 eV. The conduction band and valence band are composed of Nb 3d and O 2p states, respectively. Unlike the common niobate oxide Nb2O5, NbAlO4 exhibits efficient self-activated luminescence with good thermal stability even at room temperature. The AlO4 tetrahedron effectively blocks the transfer/dispersion of excitation energy between NbO6 chains in NbAlO4, allowing for effective self-activated luminescence from NbO6 activation centers. Moreover, Eu3+-doped NbAlO4 displayed a bright red luminescence of 5D0 → 7F2 transition at 610 nm. The site-selective excitation and luminescence of Eu3+ ions in a spectroscopic probe were utilized to investigate the doping mechanism. It is evidenced that Eu3+ is doped in the structure channel in NbAlO4 lattices, not in the normal cation sites of Nb5+ or Al3+. The experimental findings are valuable in developing new luminescent materials and improving the understanding of the material's channel structure.

Photoenergy Conversion Behaviors of Photoluminescence and Photocatalysis in Silver-Coated LiBaPO4:Eu2.

LiBaPO4:Eu2+ phosphor and Ag-coated LiBaPO4:Eu2+ composites (Ag/LiBaPO4:Eu2+) were prepared via solid-state reaction and traditional photoreduction methods, respectively. The samples were characterized via XRD, SEM, and UV-vis optical absorption spectroscopy. Two photoenergy conversion processes, namely, photocatalysis and photoluminescence, were investigated in detail. In comparison with as-prepared LiBaPO4:Eu2+ phosphor, Ag-modified composites exhibited the enhanced photocatalytic effects together with the quenched Eu2+ luminescence. A Schottky barrier was created on the interface between Ag nanoparticles and LiBaPO4 host, thereby greatly delaying the recombination between the light-induced holes and electrons. A photoenergy conversion mechanism was suggested and discussed on the basis of the experiments. The Eu2+ ion luminescence centers directly participated in the photodegradation with the meditation of Ag nanoparticles on the surface. With the increase of the Ag coating level on the surfaces, some emission peaks corresponding to 5D0 → 7F0,1,2,3,4 transitions of Eu3+ ions were detected. Eu2+/Eu3+ couples also play an important role in improving photocatalysis. LiBaPO4:Eu2+ phosphor is a good candidate for the investigation of multimodal photoenergies of photoluminescence and photocatalysis.

Optical Thermometry Based on Vibration Sidebands in Y2MgTiO6:Mn4+ Double Perovskite.

Mn4+-doped Y2MgTiO6 phosphors are synthesized by the traditional solid-state method. Powder X-ray diffraction, scanning electron microscope, and energy-dispersive X-ray spectrometer are employed to characterize the samples. The Mn4+-doped Y2MgTiO6 phosphors show the far-red emission at ∼715 nm, which is assigned to the 2Eg → 4A2 spin-forbidden transition of Mn4+. The temperature-dependent luminescent dynamics of Mn4+ is described by a complete model associated with electron-lattice interaction and spin-orbit coupling. The noncontact optical thermometry of Y2MgTiO6:Mn4+ is discussed based on the fluorescence intensity ratio of thermally coupled anti-Stokes and Stokes sidebands of the efficient ∼715 nm far-red emission in the temperature range of 10-513 K. The maximum sensor sensitivity of Y2MgTiO6:Mn4+ is determined to be as high as 0.001 42 K-1 at 153 K, which demonstrates potential applications for the optical thermometry at low-temperature environments.

Excitation power dependent optical temperature behaviors in Mn4+ doped oxyfluoride Na2WO2F4.

A Mn4+ doped Na2WO2F4 phosphor was synthesized through a two-step wet chemical method. The relationship between crystal structure and luminescence properties is discussed and unusual strong intense zero phonon lines (ZPLs) have been found in a distorted octahedral environment. The power dependent luminescence spectra exhibit the existence of down conversion luminescence intensity saturation under a high pumping power limit. The fluorescence intensity ratios of anti-Stokes bands to the ZPL and Stokes bands reveal an obvious temperature dependent relationship based on thermal de-population from the low states to the upper states of an intrinsic Mn4+ 2Eg → 4A2g transition. The temperature dependent emission intensity of Mn4+ is investigated by changing the excitation power, and an optical temperature sensitivity as high as 0.00658 K-1 is achieved at 193 K with the intensity ratio of anti-Stokes bands to the ZPL under 488 nm excitation by a Xenon lamp. This work presents a new method to realize optical thermometry at low temperature by controlling the intensity ratio of the anti-Stokes bands to the ZPL.

Excitation powder dependent optical temperature behavior of Er3+ doped transparent Sr0.69La0.31F2.31 glass ceramics.

The knowledge of the pump power for which the population of thermally coupled energy levels (TCL) changes with power increase is of valuable importance for optical temperature sensors. In this paper, novel Er3+ doped transparent Sr0.69La0.31F2.31 glass ceramics was fabricated successfully, and its structure is studied by XRD, TEM and HRTEM analyses. The 2H11/2/4S3/2, 4F9/2(1)/4F9/2(2), and 4I9/2(1)/4I9/2(2) levels of Er3+ are proved as TCL by analyzing the temperature dependent fluorescence intensity ratios. The spectrum split, thermal quenching ratio, population stability, and temperature sensitivity from three TCL are observed to be dependent on the pump power. A new fitting method has been developed to establish the relation between fluorescence intensity ratios and temperature. It is found that the combined use of 2H11/2/4S3/2 and 4F9/2(1)/4F9/2(2) as thermally coupled energy levels will get a more precise temperature reading from 62.7 K to 800 K with the help of low excitation power at 66.8 mW/mm2.

Detecting the origin of luminescence in Er3+-doped hexagonal Na1.5Gd1.5F6 phosphors.

Understanding site-selective fluorescence is one of valuable importance for spectrum modulation. In this Letter, we observed the existence of two non-equivalent Gd-activated crystallographic sites in an Er3+-doped hexagonal Na1.5Gd1.5F6 phosphor. It is proved that two green emissions from the S3/24 level separately originate from the Gd1 (540 nm) and Na2/Gd2 (550-555 nm) crystallographic sites, and the 657 nm red emission from the F9/24 level only originates from Na2/Gd2 site through using the time-resolved luminescence spectra. The 142.2% absolute enhancement of the red emission is realized through the synergistic effect of ultraviolet downconversion and infrared upconversion induced by the 370 nm and 1.54 µm dual-mode excitation.

Photoluminescence properties of Eu(3+)-doped glaserite-type orthovanadates CsK(2)Gd[VO(4)](2).

Undoped and Eu(3+)-doped glaserite-type orthovanadates CsK2Gd1-xEux[VO4]2 with various Eu(3+) concentrations of x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 were synthesized via the solid-state reaction. The formation of a single phase compound was verified through the X-ray diffraction studies. The photoluminescence (PL) and PL excitation (PLE) spectra, PL decay curves, and absolute quantum efficiency (QE) were investigated. Unlike the conventional Eu(3+)-doped vanadates, these Eu(3+)-doped samples showed not only several sharp emission lines due to Eu(3+) but also a broad emission band with a maximum at 530 nm due to the [VO4](3-) host. The intensities of the host and Eu(3+) emissions increased when the Eu(3+) concentration was increased from x = 0 to x = 0.6 and decreased above x = 0.6. Similar concentration dependence was observed for QE. The host emission, even if in the Eu(3+)-condensed host of CsK2Eu(VO4), was never quenched indicating inefficient energy transfer from the host [VO4](3-) to Eu(3+). This inefficient energy transfer is understood by suppression of the energy transfer by the V-O-Eu bond angle deviated from 180° and the separation of Eu(3+) ions at the Gd(3+) site from [VO4](3-). Like the 530 nm charge transfer [VO4](3-) emission, two broad and intense PLE bands with maxima at 330 and 312 nm were observed for the Eu(3+) emission. A maximum QE of 38.5% was obtained from CsK2Gd1-xEux[VO4]2 (x = 0.6). A white-colored emission was obtained by the combination of the broad 530 nm emission band and the intense sharp lines due to Eu(3+) at 590-620 nm.

Abnormal reduction, Eu(3+) → Eu(2+), and defect centers in Eu(3+)-doped pollucite, CsAlSi2O6, prepared in an oxidizing atmosphere.

Eu-doped pollucite CsAlSi2O6 was synthesized by the sol-gel method and heated in an air atmosphere. The crystal structure and the microstructure of the phosphors were investigated by X-ray powder diffraction and SEM images, respectively. The photoluminescence spectra and temperature dependent decay curves were measured. An abnormal reduction phenomenon of Eu(3+) → Eu(2+) was reported when Eu(3+) ions were doped in alkaline metal cation sites in CsAlSi2O6 prepared in an oxidizing atmosphere. The abnormal mechanism was discussed on the basis of the charge compensation model and a rigid three-dimensional framework structure of CsAlSi2O6. The luminescence color centers were investigated by luminescence decay lifetimes and thermal stabilities of Eu(2+) ions. The defect complexes of [(Eu(3+)Cs)(••)-2VCs'] or [(Eu(3+)Cs)(••)-Oi″] induced by the substitution of Eu(3+) on Cs(+) were suggested in the lattices. Eu(2+) ions could be regarded as Eu(3+) ions combining with the released electrons from defects Oi″ or VCs' in close vicinity of Eu(3+) (Eu(3+) + e); the electrons cannot enter the atom track of Eu(2+) presenting luminescence of Eu(2+) ions. The results indicate that several defect traps can be attributed to the abnormal reduction mechanism of Eu(3+) to Eu(2+) ions in a matrix.

Theasaponin E1 as an effective ingredient for anti-angiogenesis and anti-obesity effects.

Theasaponin E1 (TSE1) has been suggested to have higher biological activity than other saponins present in tea seed. Saponins have recently been considered as a potential chemotherapeutic agent for treating cancer. We examined the anti-angiogenic and anti-obesity properties of TSE1 contributing to anti-cancer efficacy. Treating with a 10 µg/mL concentration of TSE1 completely inhibited tube formation in human umbilical vein endothelial cells (HUVECs). TSE1 showed toxicity toward cancer cells and inhibited in vivo growth of the tumor. The vascular endothelial growth factor (VEGF) receptor complex was suppressed, leading to the inhibition of protein kinase B (Akt) expression and down-regulation of nuclear factor-kappa B (NF-kB) activation. The differentiating 3T3-L1 cells treated with TSE1 had decreased lipid droplet formation measured by Oil Red O staining. Reduced weight was measured in mice fed with a TSE1 plus high-fat diet. The results taken together, and particularly the NF-kB inhibition, suggest that TSE1 may have multi-target action for treating cancer as a novel chemotherapeutic agent.

Immunomodulatory effect of tea saponin in immune T-cells and T-lymphoma cells via regulation of Th1, Th2 immune response and MAPK/ERK2 signaling pathway.

The anti-cancer activity of saponins and phenolic compounds present in green tea was previously reported. However, the immunomodulatory and adjuvanticity activity of tea saponin has never been studied. In this study, we investigated the immunomodulatory effect of tea saponin in T-lymphocytes and EL4 cells via regulation of cytokine response and mitogen-activated protein kinases (MAPK) signaling pathway. Quantitative analysis of mRNA expression level of cytokines were performed by reverse transcription polymerase chain reaction following stimulation with tea saponin, ovalbumin (OVA) alone or tea saponin in combination with OVA. Tea saponin inhibited the proliferation of EL4 cells measured in a dose-dependent manner. No cytotoxicity effect of tea saponin was detected in T-lymphocytes; rather, tea saponin enhanced the proliferation of T-lymphocytes. Tea saponin with OVA increased the expression of interleukin (IL)-1, IL-2, IL-12, interferon-γ and tumor necrosis factor (TNF)-α and decreased the expression level of IL-10 and IL-8 in T-lymphocytes. Furthermore, tea saponin, in the presence of OVA, downregulated the MAPK signaling pathway via inhibition of IL-4, IL-8 and nuclear factor kappaB (NF-κB) in EL4 cells. Th1 cytokines enhancer and Th2 cytokines and NF-κB inhibitor, tea saponin can markedly inhibit the proliferation and invasiveness of T-lymphoma (EL4) cells, possibly due to TNF-α- and NF-κB-mediated regulation of MAPK signaling pathway.

Triple-layered perovskite niobates CaRNb3O10 (R = La, Sm, Eu, Gd, Dy, Er, Yb, or Y): new self-activated oxides.

Niobates CaRNb3O10 (R = La, Sm, Eu, Gd, Dy, Er, Yb, or Y) were prepared by conventional high-temperature solid-state reaction. The formation of a single-phase compound with triple-layered perovskite-type structure was verified through X-ray diffraction (XRD) studies. The luminescence characteristics such as photoluminescence excitation and emission spectra, X-ray-excited luminescence (XEL), Stokes shift, decay curves, and color coordinates were investigated. The niobates can be efficiently excited by UV light and present luminescence behaviors with rich luminescence colors. Under excitation by ultraviolet radiation, CaRNb3O10 (R = La, Gd, Yb, or Y) exhibits strong blue luminescence due to the self-activation center of the octahedral NbO6 groups, even at room temperature. For the materials of composition CaRNb3O10 (R = Sm, Eu, Dy, or Er), the excitation at the host band produces a characteristic luminescence of rare earth ions, indicating a host-guest energy transfer process. CaRNb3O10 (R = Eu) has the strongest luminescence intensity, which can be efficiently excitated by near UV wavelength. It could be suggested to be a potential candidate for the application on near-UV excited white LEDs.

Near-infrared quantum cutting platform in thermally stable phosphate phosphors for solar cells.

This study investigated the photoluminescent properties of Tb(3+)-Yb(3+)-, Ce(3+)-Tb(3+)-Yb(3+)-, and Eu(2+)-Yb(3+)-doped KSrPO4. The samples were prepared by a solid-state reaction with various doping concentrations. Emission at near-infrared range was focused on the application of luminescent solar concentrator for solar cells. Quantum cutting (QC) energy transfer was confirmed by the lifetimes of the donor. Near-infrared QC involved emission of Yb(3+) ions was achieved by excitation of Ce(3+), Tb(3+), and Eu(2+) ions, where the energy transfer processes occurred from Ce(3+) to Tb(3+) to Yb(3+), Tb(3+) to Yb(3+), and Eu(2+) to Yb(3+), respectively. In addition, the concentration quenching effect of Yb(3+) ions was avoided by low doping concentrations. The overall quantum efficiencies were calculated, and the maximum efficiency reaches 139%. The energy diagrams for divalent and trivalent rare-earth ions in KSrPO4 host lattice were analyzed. Results of this study demonstrate that heat-stable phosphate phosphors are promising candidates for increasing the efficiency of silicon-based solar cells.

Significant effects of mixed cations on the morphology and photochemical activities of alkali-metal-antimony (Na,K)Sb3O7.

Oxide semiconductors with mixed-valence states generally exhibit excellent optoelectronic and photochemical properties due to facile charge transfer in redox reactions. In this work, we investigate the effects of mixed alkali on the optical absorption, luminescence spectra and photocatalytic abilities of (Na1-xKx)Sb3O7 nanoparticles. All the samples are fabricated using a simple one-step hydrothermal method. The structural studies show that the largest substitution of K+ ions in (Na1-xKx)Sb3O7 is at x = 0.3. In hydrothermal synthesis, the mixed arrangement of K+ and Na+ in (Na1-xKx)Sb3O7 has an influence on the crystal shape of particles. NaSb3O7 develops into a regular cube shape. With the increase of K+ ions in (Na1-xKx)Sb3O7, the edges and corners of the cube are further ground off, resulting in irregularly spherical particles. This mixed-alkali antimonite belongs to a p-type indirect allowed transition semiconductor, and the optical band gap is 2.71 eV (x = 0.3). The intrinsic luminescence of NaSb3O7 is detected at 540 nm, which is nearly quenched in Na0.7K0.3Sb3O7. It is demonstrated that the substitution of K+ in NaSb3O7 significantly increases the photodegradation of RhB solutions. There are two types of Sb cations, i.e., Sb5+ and Sb3+ mixed in the structure. The improved photocatalysis is attributed to the charge mediators between Sb5+/Sb3+ couples. The experiment shows that co-doping cations in antimonite oxides may be one of the strategies to improve photochemical properties.

Novel yellow-emitting phosphors of Ca5M4(VO4)6 (M=Mg, Zn) with isolated VO4 tetrahedra.

The self-activated yellow-emitting phosphors of vanadates Ca5M4(VO4)6 (M = Mg, Zn) were synthesized via the solid-state reaction route. The formation of single phase compound with garnet structure was verified through X-ray diffraction (XRD) studies. The excitation and emission spectra and the thermal quenching of luminescence intensities were measured. The different luminescence properties of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 phosphors were presented, e.g., the spectra shift, the luminescence lifetimes, the absolute quantum efficiency, the color coordinates and the Stokes shift. This deference was discussed on the base of the relationship between the micro-structure and the charge transfer transitions in [VO4]3- groups in the lattices. Ca5Mg4(VO4)6 could be suggested to be a potential yellow-emitting phosphor for the application on near-UV excited white LEDs.

Spectroscopic investigations of Er3+ :CdO-Bi2 O3-B2O3 glasses.

This article reports on the optical properties of Er3+ ions doped CdO-Bi2O3-B2O3 (CdBiB) glasses. The materials were characterized by optical absorption and emission spectra. By using Judd-Ofelt theory, the intensity parameters Ω(λ) (λ = 2, 4, 6) and also oscillatory strengths were calculated from the absorption spectra. The results were used to compute the radiative properties of Er3+ :CdBiB glasses. The concentration quenching and energy transfer from Yb3+ -Er3+ were explained. The stimulated emission cross-section, full width at half maximum (FWHM) and FWHM × σpE values are also calculated for all the Er3+ CdBiB glasses.

Immobile crystallization of radioactive iodide by redox transformation of a low crystalline copper phase.

Here we show an innovative way to effectively scavenge highly mobile radioiodide and to dramatically reduce its waste volume through a spontaneous phase transformation. Under an anaerobic condition, as metallic copper (II) was favorably associated with bicarbonate (HCO3-) in solution, a cupriferous carbonate compound (malachite) quickly formed, which was redox-sensitive and transformable to a compact crystal of CuI (marshite). The formation of CuI crystal was principally led by the spontaneous Cu-I redox reaction centering around the copper phase over the presence of sulfate (SO42-). The completely transformed CuI crystal was poorly soluble in water and grew to large microcrystals (∼µm) via a remarkable selectivity for I-. Interestingly, this redox-induced iodide crystallization was rather promoted over the existence of anionic competitors (e.g., HCO3- and SO42-), which usually exist in wastewater and natural water. Unlike the conventional methods, these competing anions positively behaved in our system by supporting that the initial malachite was more apt to be reactive to largely attract highly mobile I-. Under practical environments with various anions, such a selective I- uptake and fixation within a compact crystalline space will be a promising way to effectively remove I- in a great capacity.

Tunable white-light emission in single-phased K2Y(1-x)Eu(x)Zr(PO4)3 phosphor.

The single-phased K(2)Y(1-x)Eu(x)Zr(PO(4))(3) (x=0~1) phosphors were prepared by solid-state reaction. The greenish-blue Zr(4+)-emission due to the Zr(4+)-O(2-) charge transfer transition is observed in the K(2)YZr(PO(4))(3) (x=0) phosphor under UV excitation. Together with the Zr(4+)-emission, the red emission of Eu(3+) is progressively developed by replacement of Y(3+) to Eu(3+) over the full Eu-content (x=0~1) in K(2)Y(1-x)Eu(x)Zr(PO(4))(3). The emission color varies from greenish-blue to whitish with increasing Eu(3+)-content and the white-light emission is realized in single-phased phosphor of K(2)EuZr(PO(4))(3) (x=1) by combining the Zr(4+)-emission and the Eu(3+)-emission.

The new red-emitting phosphor of oxyfluoride Ca2RF4PO4:Eu3+ (R=Gd, Y) for solid state lighting applications.

The novel red-emitting phosphors of Eu3+-activated Ca2RF4PO4:Eu3+ (R=Gd, Y) prepared by a solid-state reaction have been evaluated as a candidate for white solid state lighting. The detailed luminescence properties, e.g., the excitation spectra, the luminescence spectra and quantum efficiency under the excitation of near-UV, and decay lifetimes were reported. The phosphors can be efficiently excited by near UV light and exhibit a dominant emission peaked at 611 nm (5D0-7F2) with CIE coordinates of (x=0.661, y=0.333). The thermal stabilities were investigated from the luminescence intensities, color purity and the decay curves by increasing temperature. The luminescence parameters related to white LEDs applications were compared to some red phosphors and discussed in details. The red-emitting Ca2RF4PO4:Eu3+ (R=Gd, Y) may be potentially useful in the fabrication of white LEDs.

White light generation in Dy(3+)-doped oxyfluoride glass and transparent glass-ceramics containing CaF2 nanocrystals.

The radiative emission properties of the Dy3+ ions in an oxyfluoride glass and glass-ceramics have been studied for the generation of white light. The x-ray diffraction pattern of the glass-ceramics shows the formation of CaF2 fluorite-type nanocrystals in the glass matrix after a suitable thermal treatment of the precursor glass, whereas time-resolved optical measurements show the incorporation of the Dy3+ ions in the CaF2 nanocrystals. Intense white light has been observed when the samples are excited with 451 nm laser light. From the visible emission spectra, yellow to blue intensity ratios and the chromaticity color coordinates have been determined. All the color coordinates are found to lie in the white light region of the chromaticity color diagram.

The thermal stabilities of luminescence and microstructures of Eu2+-doped KBaPO4 and NaSrPO4 with ß-K2SO4 type structure.

Eu(2+)-doped monophosphates NaSrPO(4) and KBaPO(4) with the ß-K(2)SO(4) structure were synthesized using the conventional high temperature solid state reaction. The X-ray powder diffraction, photoluminescence excitation, and emission spectra and decay curves were measured. The phosphors can be efficiently excited by UV-visible light from 220 to 430 nm to realize emission in the visible range. The natures of the Eu(2+) emission, e.g., the chromaticity coordinates, the Stokes shifts, and the luminescence absolute quantum efficiencies, were reported. The luminescence quenching temperatures and the thermal activation energies for NaSrPO(4):Eu(2+) and KBaPO(4):Eu(2+) were obtained from the temperature dependent (10-435 K) luminescence intensities and decay curves. KBaPO(4):Eu(2+) presents only one emission center; however, Eu(2+) ions have a "disordered environment" in NaSrPO(4) lattices. The relationship between the luminescence thermal stabilities and the crystal structures was discussed. The crystallographic occupations of rare earth ions doped in these hosts were analyzed by the site-selective emission spectra and the excitation spectra of Eu(3+) ions in the (7)F(0)→(5)D(0) transitions using a pulsed, tunable, and narrow-band dye laser. In KBaPO(4), the Eu(3+) ions could be distributed in the host with a high "ordered state" in only one site in the lattices. However, the multiple site structure of Eu(3+) ions with highly disordered distributions in NaSrPO(4) lattices was suggested.