Li/Ag ratio dependent structure and upconversion photoluminescence of Li(x)Ag(1-x)Yb(0.99)(MoO4)(2):0.01Er(3+) phosphors.

A series of double molybdate scheelite-type phosphors LixAg1-xYb0.99(MoO4)2:0.01Er(3+) (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) were synthesized by the solid state reaction method, and their crystal structures and upconversion (UC) luminescence properties were investigated in detail. The phase structure evolution of this series samples was discussed and the selected Li0.5Ag0.5Yb0.99(MoO4)2:0.01Er(3+) was analyzed based on the Rietveld refinement. The UC emission properties and the related UC mechanism were also studied. With an increasing Li/Ag ratio in this host, the UC emission intensities of LixAg1-xYb0.99(MoO4)2:0.01Er(3+) increased obviously, and the enhancement could be attributed to the coupling effect and the nonradiative transition between two energy levels of LixAg1-xYb(MoO4)2 matrices and the activator Er(3+), which have also been analyzed based on the results of the ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS) and Raman spectroscopy.

Solid-State (29)Si NMR and neutron-diffraction studies of Sr(0.7)K(0.3)SiO(2.85) oxide ion conductors.

K/Na-doped SrSiO3-based oxide ion conductors were recently reported as promising candidates for low-temperature solid-oxide fuel cells. Sr0.7K0.3SiO2.85, close to the solid-solution limit of Sr1-xKxSiO3-0.5x, was characterized by solid-state (29)Si NMR spectroscopy and neutron powder diffraction (NPD). Differing with the average structure containing the vacancies stabilized within the isolated Si3O9 tetrahedral rings derived from the NPD study, the (29)Si NMR data provides new insight into the local defect structure in Sr0.7K0.3SiO2.85. The Q(1)-linked tetrahedral Si signal in the (29)Si NMR data suggests that the Si3O9 tetrahedral rings in the K-doped SrSiO3 materials were broken, forming Si3O8 chains. The Si3O8 chains can be stabilized by either bonding with the oxygen atoms of the absorbed lattice water molecules, leading to the Q(1)-linked tetrahedral Si, or sharing oxygen atoms with neighboring Si3O9 units, which is consistent with the Q(3)-linked tetrahedral Si signal detected in the (29)Si NMR spectra.

Promising oxonitridosilicate phosphor host Sr3Si2O4N2: synthesis, structure, and luminescence properties activated by Eu2+ and Ce3+/Li+ for pc-LEDs.

A novel oxonitridosilicate phosphor host Sr(3)Si(2)O(4)N(2) was synthesized in N(2)/H(2) (6%) atmosphere by solid state reaction at high temperature using SrCO(3), SiO(2), and Si(3)N(4) as starting materials. The crystal structure was determined by a Rietveld analysis on powder X-ray and neutron diffraction data. Sr(3)Si(2)O(4)N(2) crystallizes in cubic symmetry with space group Pa ̅3, Z = 24, and cell parameter a = 15.6593(1) Å. The structure of Sr(3)Si(2)O(4)N(2) is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO(2)N(2)] tetrahedra with bridging N and terminal O to form three-dimensional tunnels to accommodate the Sr(2+) ions. The calculated band structure shows that Sr(3)Si(2)O(4)N(2) is an indirect semiconductor with a band gap ≈ 2.84 eV, which is close to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr(3-x)Si(2)O(4)N(2):xEu(2+) shows a typical emission band peaking at ~600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce(3+)/Li(+)-codoped Sr(3)Si(2)O(4)N(2), one excitation band is in the UV range (280-350 nm) and the other in the UV blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr(3-2x)Si(2)O(4)N(2):xCe(3+),xLi(+) shows a asymmetric broad band peaking at ~520 nm. The long-wavelength excitation and emission of Eu(2+) and Ce(3+)/Li(+)-doped Sr(3)Si(2)O(4)N(2) make them attractive for applications in phosphor-converted white light-emitting diodes.

Preparation and electrical properties of inorganic electride [Y2Ti2O6]2+(2e-).

Oxygen-depleted samples [Y2Ti2O7-x ]2x+(2xe-) (0 ≤ x ≤ 1.0) were prepared by reducing Y2TiO7 powders at 500 °C to 650 °C using CaH2 as a reductive agent, where x represents the content of , which was determined by thermogravimetric analysis. Powder X-ray diffraction patterns illustrate that the pure pyrochlore phase is kept for the samples with x ≤ 1.0, whereas the apparent x values surpass 1.0, and the impurity phase Y2O3 appears. The electride [Y2Ti2O7-x ]2x+(2xe-) (x ≈ 1.0) can be obtained under a reductive condition, in which the concentration of VO is 7.75 × 1021 cm-3. The electron paramagnetic resonance measurements gave the concentration of unpaired electrons in the electride as 1.30 × 1021 cm-3, indicating that the degree of the ionization of is less than 10%. Conductivity measurements for a sintered pellet sample (relative density ∼ 70%) indicate that the electride has quite high conductivity (∼1.09 S cm-1 at 300 K). The conduction was interpreted by using the variable range hopping mechanisms.

Synthesis, structure, and thermally stable luminescence of Eu(2+)-doped Ba2Ln(BO3)2Cl (Ln = Y, Gd and Lu) host compounds.

A new family of chloroborate compounds, which was investigated from the viewpoint of rare earth ion activated phosphor materials, have been synthesized by a conventional high temperature solid-state reaction. The crystal structure and thermally stable luminescence of chloroborate phosphors Ba(2)Ln(BO(3))(2)Cl:Eu(2+) (Ln = Y, Gd, and Lu) have been reported in this paper. X-ray diffraction studies verify the successful isomorphic substitution for Ln(3+) sites in Ba(2)Ln(BO(3))(2)Cl by other smaller trivalent rare earth ions, such as Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb. The detailed structure information for Ba(2)Ln(BO(3))(2)Cl (Ln = Y, Gd, and Lu) by Rietveld analysis reveals that they all crystallize in a monoclinic P2(1)/m space group. These compounds display interesting and tunable photoluminescence (PL) properties after Eu(2+)-doping. Ba(2)Ln(BO(3))(2)Cl:Eu(2+) phosphors exhibit bluish-green/greenish-yellow light with peak wavelengths at 526, 548, and 511 nm under 365 UV light excitation for Ba(2)Y(BO(3))(2)Cl:Eu(2+), Ba(2)Gd(BO(3))(2)Cl:Eu(2+), and Ba(2)Lu(BO(3))(2)Cl:Eu(2+), respectively. Furthermore, they possess a high thermal quenching temperature. With the increase of temperature, the emission bands show blue shifts with broadening bandwidths and slightly decreasing emission intensities. It is expected that this series of chloroborate phosphors can be used in white-light UV-LEDs as a good wavelength-conversion phosphor.

Dipole-Orientation-Dependent Förster Resonance Energy Transfer from Aromatic Head Groups to MnBr42- Blocks in Organic-Inorganic Hybrids.

The understanding and visualization of dipole-dipole interaction on molecular scale are scientifically fundamental and extremely of interest. Herein, two new zero-dimensional (0D) Mn hybrids with aromatic head groups and alkyl tails as organic spacers are selected as models. It was found that the dipole interaction between head groups and Mn blocks could have a huge impact on their crystalline structures as well as the luminescent properties. The parallel-oriented dipoles of the head groups and MnBr42- blocks contribute to an efficient Förster Resonance Energy Transfer (FRET) in cetylpyridinium manganese bromide ([C16Py]2MnBr4), while the process is absent in 1-methyl-3-hexadecylimidazolium manganese bromide ([C16mim]2MnBr4) with perpendicular-oriented dipoles. This work gives insight into the influence of organic spacers on the geometry and the dipole interaction of Mn polyhedron in the hybrids, which could be of great interest in the future optical regulations and structural design.

Exploring reversible quenching of fluorescence from a pyrazolo[3,4-b]quinoline derivative by protonation.

Pyrazolo[3,4-b]quinoline derivatives are reported to be highly efficient organic fluorescent materials suitable for applications in light-emitting devices. Although their fluorescence remains stable in organic solvents or in aqueous solution even in the presence of H(2)O, halide salts (LiCl), alkali (NaOH) and weak acid (acetic acid), it suffers an efficient quenching process in the presence of protic acid (HCl) in aqueous or ethanolic solution. This quenching process is accompanied by a change in the UV spectrum, but it is reversible and can be fully recovered. Both steady-state and transient fluorescence spectra of 1-phenyl-3,4-dimethyl-1H-pyrazolo-[3,4-b]quinoline (PAQ5) during quenching are measured and analyzed. It is found that a combined dynamic and static quenching mechanism is responsible for the quenching processes. The ground-state proton-transfer complex [PAQ5H(+)] is responsible for static quenching. It changes linearly with proton concentration [H(+)] with a bimolecular association constant K(S)=1.95 M(-1) controlled by the equilibrium dissociation of HCl in ethanol. A dynamic quenching constant K(D)=22.4 M(-1) is obtained by fitting to the Stern-Volmer equation, with a bimolecular dynamic quenching rate constant k(d)=1.03x10(9) s(-1) M(-1) under ambient conditions. A change in electron distribution is simulated and explains the experiment results.

[Photoluminescence from bis-t-butylbenzoxazolylthiophene doped silica films].

Thin nano-porous silica films doped with high concentrations of fluorescent material, 2, 5-bis (5-tert-butyl-2-benzoxazolyl)-thiophene (BBOT) were prepared via a sol-gel process. Uniform and bright blue fluorescence was observed. Light emission properties of these organic molecule doped inorganic silica films, i.e., hybrid films, were measured using ultraviolet-visible (UV-Vis) absorption spectroscopy, steady and time-resolved fluorescence spectroscopy as well as optical microscopy. Features of these materials were revealed in this investigation: Firstly, photoluminescence intensity from BBOT doped silica films increased linearly as the concentrations of BBOT increased if the dopant concentration was relatively low and below 6 x 10(-3) mol x L(-1); Secondly, no molecular aggregation or phase separation was observed using optical microscopy when the BBOT concentration was below 6 x 10(-3) mol x L(-1) in BBOT doped silica films. Thirdly, the fluorescence lifetimes of BBOT in the doped silica films were longer than that in a dilute dioxane solution (1.957 ns), which was contradicted to our general understanding that the fluorescence lifetime may be reduced in a condensed matter due to molecular interactions or quenching. It was further found that the fluorescence lifetime also varied with the gelation conditions. Taking a BBOT concentration of 6 x 10(-3) mol x L(-1) for an example, the lifetime of BBOT in doped silica films was about 2.45 ns for a specimen polymerized at 50 degrees C; while the lifetime was increased to 3.04 ns for a specimen polymerized at 90 degrees C. This work demonstrates no concentration quenching when the BBOT dopant concentrations increased to as high as 6 x 10(-3) mol x L(-1) in the silica matrix. In comparison with the changes in time-resolved photoluminescence of BBOT in dioxane solution and that of the BBOT doped nano-porous silica in relation to their concentration dependence and the gelation conditions, it was found that concentration quenching can be effectively suppressed by the nano-porous silica matrix. A stable fluorescent organic-inorganic hybrid material is thus obtained.

Preparation of 1D ultrathin niobate nanobelts by liquid exfoliation as photocatalysts for hydrogen generation.

One-dimensional (1D) ultrathin nanobelts of protonated niobate HxRb1-xNbO3 (HRNO) were prepared through sonication-assisted liquid exfoliation from the parent material RbNbO3 (RNO) in water. The exfoliating mechanism from RNO to the ultrathin HRNO nanobelts was discussed, which would be related to Rb+ hydration and (NbO3)nn- hydrolysis. The ultrathin HRNO nanobelts have thicknesses less than 10 nm, and show high photocatalytic activity for hydrogen generation.

Reduced Local Symmetry in Lithium Compound Li2SrSiO4 Distinguished by an Eu3+ Spectroscopy Probe.

Research on lithium compounds has attracted much attention nowadays. However, to elucidate the precise structure of lithium compounds is a challenge, especially when considering the small ions that may be transferred between the interstitial voids. Here, the discovery of reduced local symmetry (symmetry breaking) in small domains of Li2SrSiO4 is reported by employing Eu3+ as a spectroscopic probe, for which X-ray, neutron, and electron diffraction have confirmed the average long-range structure with the space group P3121. However, luminescence shows a lower local symmetry, as confirmed by the extended X-ray absorption fine structure. By considering the reduced symmetry of the local structure, this work opens the door to a new class of understanding of the properties of materials.

Photochemical and photophysical properties of three carbon-bridged fullerene dimers: C121 (I, II, III).

The photochemical and photophysical properties of the three C121 isomers (I, II, III) were investigated with MADLI-TOF-MS, UV-vis spectra, fluorescence spectra, absorption spectra of their DMA complexes, and theoretical calculations. The three isomers of C121 (I, II, III) have different stabilities under laser irradiation, but isomer I and isomer II show good stability against the heat-induced conversion between different isomers: No conversion between the isomers was found after heating the mixture of isomer I and isomer II at 353 K for 12 h in Ar atmosphere. The results of UV-vis absorption and fluorescence spectra indicate that interactions between two C60 moieties of C60=C=C60 in the ground and singlet states are not significant, C121 (I, II, III) behaves as an electron-acceptor similar to C60. These indicate that the formation of the fullerene chain structure (e.g., C60=C=C60) does not disturb the photochemical and photophysical properties of the C60 monomer itself, even that the properties were enhanced by the formation of the polymer. This is significant for the C60 polymer in photochemical or photoelectronic applications in which C60=C=C60 can be an excellent basic unit of polymers.

Effects of composition modulation on the luminescence properties of Eu(3+) doped Li1-xAgxLu(MoO4)2 solid-solution phosphors.

Double molybdate scheelite-type solid-solution phosphors Li1-xAgxLu1-y(MoO4)2:yEu(3+) were synthesized by the solid state reaction method, and their crystal structures and luminescence properties were investigated in detail. The composition modulation and structural evolution of this series of samples were studied and the selected AgEu(MoO4)2, AgLu(MoO4)2, LiLu(MoO4)2 and LiEu(MoO4)2 phases were analyzed based on the Rietveld refinement. Depending on the variation of the Li/Ag ratio in Li1-xAgxLu1-y(MoO4)2:yEu(3+) phosphors, the difference in the luminescence properties of Li1-xAgxLu1-y(MoO4)2:yEu(3+) phosphors was ascribed to two factors, one reason could be assigned to the coupling effect and the nonradiative transition between the energy levels of LixAg1-xLu(MoO4)2 matrices and the activator Eu(3+), another could be due to the near ultraviolet energy absorption and transmission efficiency between the charge-transfer (CT) band of O(2-)-Mo(6+) and the 4f → 4f emissive transitions of Eu(3+). The ultraviolet-visible diffuse reflection spectra (UV-vis DRS) and Raman spectra analysis were also used to verify the above mechanism.

Nanometer-scale separation of d(10) Zn(2+)-layers and twin-shift competition in Ba8ZnNb6O24-based 8-layered hexagonal perovskites.

The 8-layered shifted hexagonal perovskite compound Ba8ZnNb6O24 was isolated via controlling the ZnO volatilization, which features long-range B-cation ordering with nanometer-scale separation by ∼1.9 nm of octahedral d(10) cationic (Zn(2+)) layers within the purely corner-sharing octahedral d(0) cationic (Nb(5+)) host. The long-range ordering of the B-site vacancy and out-of-center distortion of the highly-charged d(0) Nb(5+) that is assisted by the second-order Jahn-Teller effect contribute to this unusual B-cation ordering in Ba8ZnNb6O24. A small amount (∼15%) of d(10) Sb(5+) substitution for Nb(5+) in Ba8ZnNb6-xSbxO24 dramatically transformed the shifted structure to a twinned structure, in contrast with the Ba8ZnNb6-xTaxO24 case requiring 50% d(0) Ta(5+) substitution for Nb(5+) for such a shift-to-twin transformation. Multiple factors including B-cationic sizes, electrostatic repulsion forces, long-range ordering of B-site vacancies, and bonding preferences arising from a covalent contribution to the B-O bonding that includes out-of-center octahedral distortion and the B-O-B bonding angle could subtly contribute to the twin-shift phase competition of B-site deficient 8-layered hexagonal perovskites Ba8B7O24. The ceramics of new shifted Ba8ZnNb6O24 and twinned Ba8ZnNb5.1Sb0.9O24 compounds exhibited good microwave dielectric properties (εr ∼ 35, Qf ∼ 36 200-43 400 GHz and τf ∼ 38-44 ppm/°C).

A π-Extended Donor-Acceptor-Donor Triphenylene Twin Linked via a Pyrazine Bridge.

ß-Amino triphenylenes can be accessed via palladium catalyzed amination of the corresponding triflate using benzophenone imine. Transformation of amine 6 to benzoyl amide 18 is also straightforward, and its wide mesophase range demonstrates that the new linkage supports columnar liquid crystal formation. Amine 6 also undergoes clean aerobic oxidation to give a new twinned structure linked through an electron-poor pyrazine ring. The new discotic liquid crystal motif contains donor and acceptor fragments and is more oval in shape rather than disk-like. It forms a wide range columnar mesophase. Absorption spectra are strong and broad; emission is also broad and occurs with a Stokes shift of ca. 0.7 eV, indicative of charge-transfer character.

Tailored Near-Infrared Photoemission in Fluoride Perovskites through Activator Aggregation and Super-Exchange between Divalent Manganese Ions.

Biomedical imaging and labeling through luminescence microscopy requires materials that are active in the near-infrared spectral range, i.e., within the transparency window of biological tissue. For this purpose, tailoring of Mn2+-Mn2+ activator aggregation is demonstrated within the ABF3 fluoride perovskites. Such tailoring promotes distinct near-infrared photoluminescence through antiferromagnetic super-exchange across effective dimers. The crossover dopant concentrations for the occurrence of Mn2+ interaction within the first and second coordination shells comply well with experimental observations of concentration quenching of photoluminescence from isolated Mn2+ and from Mn2+-Mn2+ effective dimers, respectively. Tailoring of this procedure is achieved via adjusting the Mn-F-Mn angle and the Mn-F distance through substitution of the A+ and/or the B2+ species in the ABF3 compound. Computational simulation and X-ray absorption spectroscopy are employed to confirm this. The principle is applied to produce pure anti-Stokes near-infrared emission within the spectral range of ≈760-830 nm from codoped ABF3:Yb3+,Mn2+ upon excitation with a 976 nm laser diode, challenging the classical viewpoint where Mn2+ is used only for visible photoluminescence: in the present case, intense and tunable near-infrared emission is generated. This approach is highly promising for future applications in biomedical imaging and labeling.

Host composition dependent tunable multicolor emission in the single-phase Ba2(Ln(1-z)Tb(z))(BO3)2Cl:Eu phosphors.

A new strategy based on the host composition design has been adopted to obtain efficient color-tunable emission from Ba2Ln(0.97-z)Tb(z)(BO3)2Cl:0.03Eu (Ln = Y, Gd and Lu, z = 0-0.97) phosphors. This study reveals that the single-phase Ba2Ln(1-z)Tb(z)(BO3)2Cl compounds can be applied to use allowed Eu(2+) absorption transitions to sensitize Eu(3+) emission via the energy transfer Eu(2+) → (Tb(3+))n → Eu(3+). The powder X-ray diffraction (XRD) and Rietveld refinement analysis shows single-phase Ba2Ln(1-z)Tb(z)(BO3)2Cl. As-prepared Ba2Ln(0.97-z)Tb(z)(BO3)2Cl:0.03Eu phosphors show intense green, yellow, orange and red emission under 377 nm near ultraviolet (n-UV) excitation due to a variation in the relative intensities of the Eu(2+), Tb(3+) and Eu(3+) emission depending on the Tb content (z) in the host composition, allowing color tuning. The variation in emission color is explained by energy transfer and has been investigated by photoluminescence and lifetime measurements and is further characterized by the Commission Internationale de l'éclairage (CIE) chromaticity indexes. The quantum efficiencies of the phosphors are high, up to 74%, and show good thermal stabilities up to 150 °C. This investigation demonstrates the possibility to sensitize Eu(3+) line emission by Eu(2+)via energy migration over Tb(3+) resulting in efficient color tunable phosphors which are promising for use in solid-state white light-emitting diodes (w-LEDs).

A promising yellow phosphor of Ce3+/Li+ doped CaSiN(2-2δ/3)Oδ for pc-LEDs.

Single-phased orthorhombic nitridosilicate CaSiN2 powders were prepared at 1550 °C in a N2/H2 (6%) atmosphere by solid-state reaction using Ca3N2 and Si3N4 as the starting materials. The crystal structure refinements were carried out by the Rietveld method based on the X-ray and neutron powder diffraction data collected at 297 K, respectively. The CaSiN2 phase crystallizes in an orthorhombic unit cell with the space group Pbca (No. 61) and cell parameters a = 5.1334(3) Å, b = 10.3090(5) Å, c = 14.5756(5) Å, Z = 16 (XRD data). Instead of the ideal formula CaSiN2, small amounts of oxygen could be detected in the samples. The O/N composition of orthorhombic CaSiN2-2δ/3Oδ was analyzed by EDX and Rietveld refinement on the neutron powder diffraction data. SAED and HREM characterizations of the crystallites were conducted before the EDX analysis in order to select proper crystallites and exclude other minor second phases of (oxo)nitridosilicates such as cubic CaSiN2 and monoclinic CaSi2O2N2. The luminescence measurements indicate that Ce(3+)/Li(+) co-doped CaSiN2-2δ/3Oδ emits a typical shouldered yellow band peaking at 530 nm (FWHM ∼ 120 nm), which originates from the 5d → 4f transition of Ce(3+). The emission band shifts to longer wavelengths with an increment of Ce(3+) concentration. The strong excitation band appears in the range of 350-470 nm, which is very favorable for the applications in near-UV and blue phosphor-converted light-emitting diodes (pc-LEDs).

A mesogenic triphenylene-perylene-triphenylene triad.

A straightforward synthesis of triphenylene-perylene-triphenylene triad structures has been achieved by using versatile triphenylene intermediates bearing a single oxyalkyl amine side chain. Among these, PBITP(10) showed a stable columnar mesophase implying favorably matched core-core separations in the structure. Importantly, the triad can be used as a vehicle for doping columnar triphenylene matrices with functional but incompatible perylene units and a mixture of hexahexyloxytriphenylene matrix doped with 0.1% PBITP(10) is homogeneous and liquid crystalline.