Up‐conversion luminescence of ytterbium and thulium codoped potassium yttrium double tungstate crystal

Thulium and ytterbium co‐doped double tungstate Yb³⁺,Tm³⁺: NaY(WO₄)₂ single crystals were prepared by using RF‐heating Czochralski (CZ) pulling method. Its polarized transmittance spectra have been recorded in the region of 290‐2000 nm at room temperature. The energy levels transitions were assigned to the corresponding absorption line. The up‐conversion luminescences at 793 nm and 475 nm were measured when the sample were pumped by 972 nm LD and the energy transfer mechanism between Yb³⁺ and Tm³⁺ ions was analyzed.     

Growth process and optical investigations of Nd:NaGd (MoO4)2 crystals with varying content of Nd and Gd

In this paper, [NaGd_1‐xNdx](MoO₄)₂ crystals with x = 0.005–0.05 were grown by Czochralski method at 950 ℃ for 10 hours and subsequently characterized by absorption spectroscopy and fluorescence spectroscopy. The intensity parameters of Nd³⁺ were calculated by Judd‐Ofelt theory, and the relationship between the parameters and Nd³⁺ concentration was analyzed. The results show that with the increasing Nd³⁺ concentration, the oscillator strength, stimulated emission cross‐section, Ω_t and spontaneous transition probability decrease slightly, while the fluorescence lifetime decreases significantly. However, the fluorescence branching ratio is almost unchanged with the increasing Nd³⁺concentration. The [NaGd_1‐xNd_x](MoO₄)₂ crystal with (x = 0.01) possesses the highest quantum efficiency of 88.98%, a good fluorescence line‐width of 23 nm, a large value of σ_em·τ_f up to 0.55 × 10⁻²² cm²·s and a stimulated emission cross‐section up to 3.747 × 10⁻¹⁹ cm² for the transition from ⁴F_3/2 to⁴I_11/2 at 1064 nm. Test results indicate that [NaGd_1‐xNd_x](MoO₄)₂ crystals are an excellent gain media for the solid state laser system.     

Hydrothermal synthesis of MoO3 nanobelt‐graphene composites

A composite of graphene sheets decorated with molybdenum trioxide (MoO₃) nanobelts has been fabricated via a facile and efficient hydrothermal route in the presence of NaCl. The structure, morphology of these promising composites were investigated by means of field‐emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), Raman spectroscopy and thermogravimetric (TG) analysis. FESEM and TEM studies suggest the presence of uniform crystalline MoO₃ nanobelts and graphene sheets in as‐prepared hybrid materials. XRD and Raman results confirm the reduction of graphite oxide (GO) sheets to graphene sheets accompanying by the formation of MoO₃ nanobelts. Moreover, thermal properties of GO and MoO₃ nanobelt‐graphene composites reveal that thermal stability of the obtained MoO₃ nanobelt‐graphene composites is obviously higher than that of GO due to the transformation of GO sheets to highly stable graphene sheets in the hybrids. This work could provide new insights into the fabrication of high quality MoO₃‐graphene hybrid nanomaterials and facilitate their potential applications in different fields. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Continuous-wave laser operation of disordered double tungstate and molybdate crystals doped with ytterbium

Several ytterbium doped disordered doubles tungstate and molybdate crystals, NaGd(WO₄)₂, NaLa(WO₄)₂, NaLa(MoO₄)₂ and LiGd(MoO₄)₂, were grown with sufficient Yb-doping and optical quality. The crystal field splitting of the Yb³⁺ multiplets and the lifetimes are similar to the biaxial monoclinic double tungstates but the peak absorption cross sections are roughly one order of magnitude lower. Room temperature cw laser operation was achieved under Ti:sapphire and diode laser pumping. The polarization dependence of the laser emission and possible power scaling were studied. The slope efficiency achieved with the best sample, Yb:LiGd(MoO₄)₂, was 64.5%. The maximum output power obtained was 900 mW for Yb:NaLa(MoO₄)₂ pumped in the absorption maximum. The laser tunability was studied with an intracavity Lyot filter and the tuning ranges (FWHM up to 33 nm) achieved indicate interesting potential for mode-locked femtosecond operation.     

Structural analysis of bis{(N,N′–dimethylformamide) (μ‐formato)[μ‐bis(salicylidene)–1,3‐propanediaminato] nickel(II)} copper(II) and zinc(II) monohydrate hetero‐trinuclear complexes

The two linear hetero‐trinuclear metallic complexes with Cu²⁺, Ni²⁺ and Zn²⁺ ions, [CuNi₂ {(CHO₂)(SALPD)[(CH₃)₂NCHO]}₂],(I) and [ZnNi₂{(CHO₂)(SALPD)[(CH₃)₂NCHO]}₂],(II), form crystals which belong to the monoclinic system, space group P2₁/c, with unit cell dimensions a=10.537(4), b=16.612(5), c=13.837(3) Å, β=111.09(5)°, V=2259.8(12) ų in (I) and a=10.525(4), b=16.658(5), c=13.826(3) Å, β=111.11(5) °, V=2261.4(8) ų in (II), respectively. The coordinations around the M (Ni²⁺, Cu²⁺, Zn²⁺) ions in the title complexes are distorted octahedrals. The stereochemistries of the bridge ligands, linking to the metal ions each other, are changed between Ni…M (Cu²⁺, Zn²⁺) distances. The Ni…M (Cu²⁺, Zn²⁺) distances are 3.0469(13) and 3.0645(14), respectively. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Luminescence characteristics of undoped and Eu‐doped GdCa4O(BO3)3 single crystals and nanopowders

Single crystals of GdCa₄O(BO₃)₃ (GdCOB) pure and doped with Eu concentration of 1 and 4 at% were grown by the Czochralski and micropulling‐down methods. The distribution of Eu ions in GdCOB crystals was uniform. The substitutions of Eu³⁺ in Gd, Ca(1) and Ca(2) cation sites and eventually formation Eu²⁺ have been investigated. The spectroscopic properties of crystals are compared with the properties of nanopowders obtained by sol‐gel method. Radioluminescence spectra of undoped GdCOB crystal show the characteristic emission of Gd³⁺ at about 312 nm, whereas this emission dramatically decreases in Eu‐doped crystals upon X‐ray excitation, as well as in Eu‐doped nanopowders excited in vacuum ultraviolet (VUV) region. The VUV excitation in the range 125‐333 nm for Eu‐doped samples leads to strong emission in red coming from the ⁵D₀ multiplet of Eu³⁺, only. In the photoluminescence decay kinetics of 312 nm emissions substantial shortening and departure for single exponential decay in Eu‐doped samples is clearly observed. Higher Eu doping results in further acceleration of the decay. In undoped GdCOB crystal, the lifetime of the Gd^{3+ 6}P_7/2 multiplet is 2.79 ms. The Eu^{3+ 5}D₀ decay kinetics monitored at 613 nm are rather constant. Numerical fitting of fully exponential curves, reveals lifetimes 2.7 ms for nanopowder and 2.5 ms for single crystal. The results suggest that this material may be used as a red phosphor in plasma display panels in nanopowder form because of strong excitation band of Eu³⁺ luminescence in the 160‐200 nm regions. Contrary to nanopowder sample, such an excitation band, attributed to the Gd³⁺–O^2– charge transfer was not observed in crystal obtained by the micropulling‐down method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

UV‐visible emission of (O2‐ ‐ F+) centres in KBr

Single crystals of KBr containing the (O^2‐ ‐ F⁺) color centres and elaborated in atmospheric air by the Czochralski method are studied. The measure of the UV‐visible emission spectra at 78 K present several emission bands situated between 340 and 800 nm. The photoluminescence excitation shows that these emissions are attributed to O^‐₂ and (O^2‐ ‐ F⁺) centres. This study show the new emission bands of (O^2‐ ‐ F⁺) centres which are situated in the UV‐visible spectral domain. The comparison between the emission spectra before and after the thermal treatment of samples shows clearly the transformation of O^‐₂ centers at high temperature to (O^2‐ ‐ F⁺) centres. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Well‐defined Sb2S3 nanostructures: citric acid‐assisted synthesis,electrochemical hydrogen storage properties

Well‐defined (three‐dimensional) 3‐D dandelion‐like Sb₂S₃ nanostructures consisted of numerous nanorods have been achieved via a facile citric acid‐assisted solvothermal process. The as‐prepared products were characterized by X‐ray diffraction (XRD), field‐emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high‐resolution TEM (HRTEM), respectively. The influence factors of the formation of the hierarchical Sb₂S₃ nanostructures are discussed in details based on FESEM characterizations. By simply controlling the quantity of citric acid, the nucleation and growth process can be readily tuned, which brings the different morphologies and nanostructures of the final products. On the basis of a series of contrastive experiments, the aggregation‐based process and anisotropic growth mechanism are reasonably proposed to understand the formation mechanism of Sb₂S₃ hierarchical architectures with distinctive morphologies including nanorods, and dandelion‐like nanostructures. Charge‐discharge curves of the obtained Sb₂S₃ nanostructures were measured to investigate their electrochemical hydrogen storage behaviors. It revealed that the morphology played a key role on the hydrogen storage capacity of Sb₂S₃ nanostructure. The dandelion‐like Sb₂S₃ nanostructures exhibited higher hydrogen storage capacity (108 mAh g⁻¹) than that of Sb₂S₃ nanorods (95 mAh g⁻¹) at room temperature.     

Spin‐Hamiltonian parameters for Dy3+ ion at the trigonal 12‐fold coordinated La3+ site of La2Mg3(NO3)12·24H2O

The spin‐Hamiltonian parameters (g factors g_‖, g_⊥ and hyperfine structure constants ¹⁶¹A_‖, ¹⁶¹A_⊥, ¹⁶³A_‖, ¹⁶³A_⊥) for ¹⁶¹Dy³⁺ and ¹⁶¹Dy³⁺ isotopes in the trigonal 12‐fold coordinated La³⁺ site of La₂Mg₃(NO₃)₁₂·24H₂O crystal are calculated from a diagonalization (of energy matrix) method. In the method, the Zeeman and hyperfine interaction terms are added to the conventional Hamiltonian used in the studies of crystal‐field energy levels, and a 66×66 energy matrix concerning the ground multiplet ⁶H_15/2 and the first to fifth excited multiplets ⁶H_13/2, ⁶H_11/2, ⁶H_9/2, ⁶H_7/2 and ⁶H_5/2 are applied. The calculated results are discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

synthesis of high‐quality crystalline α‐MoO3 nanobelts

High‐quality crystalline MoO₃ nanobelts were successfully fabricated with a facile hydrothermal route by using common and inexpensive NaCl as a capping agent. The products are thoroughly characterized by the combination of different techniques. The results indicate that as‐prepared MoO₃ nanobelts have an orthorhombic crystal structure (α‐MoO₃) with growth preferential in the [001] direction, the size of the obtained MoO₃ nanobelts ranges from 200 nm to 300 nm in width and micrometers in length. The effects of the amount of NaCl on the morphology and size of the resultant MoO₃ were also investigated, it is clearly shown that the presence of appropriate amount of NaCl plays a crucial role in controlling the size and morphology of the obtained MoO₃. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of ferroelectric‐ferroelastic Tb2(MoO4)3 crystals

Transparent and nearly colorless ferroelectric‐ferroelastic β′‐Tb₂(MoO₄)₃ (TMO) single crystals have been grown by the Czochralski (CZ) method. The single crystal structure was investigated by X‐ray powder diffraction and was shown to be a single phase with the structure similar as the β′‐Gd₂(MoO₄)₃ crystal. The optical transparency of the TMO crystal has been measured and the crystal is almost transparent in the visible and near infrared regions. The defects of TMO crystal were evaluated by etching technique and the ferroelectric domain structures were observed by an optical microscope. A high‐resolution X‐ray diffraction analysis demonstrates that the as‐grown TMO crystal possesses relatively high optical quality. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of synthesis conditions on the yields and properties of HZSM‐5

In the SiO₂‐Al₂O₃‐TPAOH‐NH₄OH system through a two‐step procedure, the effects of synthesis parameters on the yields and properties of HZSM‐5 were studied. As the feeding n(SiO₂)/n(Al₂O₃) ratio increased, the yields of synthesized samples were between 11% and 14% while the crystallinity tended to decrease at the ratio > 300. At the ratio of 150, the yield was 13.7% and the ratio of actual yield to theoretical yield was 93%. The sample shaped ellipse‐like particles of 3 × 10⁻⁶ m. At the n(NH₃)/n(SiO₂) ratio of 1.5, the yield was 13.1% and the sample showed regular ellipse‐like particles of 2 × 10⁻⁶ m. Too high dosage of NH₄OH lowered the yield and the crystallinity. The addition of seed crystal favored the HZSM‐5 synthesis in high crystallinity. At the addition amount of 4%, the yield reached 13.9% and the sample shaped in spherical particles of 0.5 × 10⁻⁶ m. Synthesized HZSM‐5 with a surface area of 455 m²·g⁻¹ and intrinsic acidity showed highly catalytic activity toward the methanol conversion to olefins reaction.     

Influence of Zn2+ ions concentration on the optical properties of Zn/Er:LiNbO3 crystals

Congruent Er³⁺(3 mol%):LiNbO₃ crystals codoped with ZnO (X mol %, X=0, 3, 6 and 7) were grown by the Czochralski technique. The Er contents in the crystals were measured by an inductively coupled plasma atomic emission spectrometer (ICP‐AES). Under 800 nm excitation, the upconversion emission spectra reveal an enhancement of the green emission with respect to the red emission when the Zn²⁺ ions are introduced into Er:LiNbO₃ crystal. The effect of Zn²⁺ ions concentration on the intensity ratio of the green to red emission has been investigated. Two cross‐relaxation processes (²H_11/2 + ⁴I_13/2 → ⁴I_11/2 + ⁴F_9/2 and ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2) are involved in populating the ⁴F_9/2 state, which bypass the green‐emitting states. The OH^‐ absorption spectra indicate that the Zn²⁺ codoping leads to a decreased concentration of Er³⁺ cluster sites contributing to the enhancement of the green emission. The studies on UV‐vis absorption spectra show that the heavily codoped with Zn²⁺ results in the reformation of the Er³⁺ cluster sites in Er:LiNbO₃. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure and electrochemical behavior of a novel polyoxometalate [Ni(bpy)3]2[W4V2O19] with Lindqvist‐type structure

A novel polyoxometalate [Ni(bpy)₃]₂[W₄V₂O₁₉] (Ni₂V₂W₄) has been synthesized by the hydrothermal method and the structure determined by X‐ray crystallography. Ni₂V₂W₄ crystallizes in a trigonal system with space group R ‐3c (a = 15.8984 (5) Å, b = 15.8984 (5) Å, c = 43.855 (3) Å). In the structure of the compound, terminal and bridging oxygen atoms are bond to the metal centers by σ or π bonds. The W⁶⁺ and V⁵⁺ ions in isopolyanion [W₄V₂O₁₉]^4‐ statistically distribute over the six metal centers. Using cathodic adsorptive voltammetric method with a carbon paste electrode, the redox property and the electron transferring process were studied. The results show that electrochemical behavior about W(VI) and V(V) atoms give one‐electron, three‐electron and two‐electron reduction waves. Three successive oxidation waves are observed too. The compound was also characterized by thermal gravimetric analysis and IR spectra. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase‐manipulable synthesis of Cu‐based nanomaterials using ionic liquid 1‐butyl‐3‐methyl‐imidazole tetrafluoroborate

Using the ionic liquid (IL), 1‐butyl‐3‐methyl‐imidazole tetrafluoroborate, and the precursor Cu₇Cl₄(OH)₁₀·H₂O, series of phase‐manipulable Cu‐based nanomaterials were synthesized by hydrothermal and microwave assisted routes, respectively. The structural characters of the as‐prepared CuO, CuO/Cu₂O composites and pure Cu nanoparticles were investigated by XRD, SEM, TEM and HRTEM, and their surface photovoltaic properties were studied by surface photovoltage spectra. Via hydrothermal route Cu²⁺ ions were found to be reduced gradually into Cu⁺ and subsequently Cu⁰ with increasing the IL, and various phase ratio of CuO, Cu₂O and Cu composite nanosheets and pure Cu nanoparticles were obtained. This implies that the IL could function as both a reductant in the oxygen‐starved condition and a template for the nanosheet products. The ¹H‐NMR result of the IL supports it being a reductant. In microwave assisted route, however, only monoclinic single crystalline CuO nanosheets were obtained, which indicates the IL being a template only in oxygen‐rich condition. Therefore, the crystal phase, composition and morphology of the Cu‐based products could be controlled by simply adjusting the quantity of the IL and oxygen in solution routes. The molecular structure of the IL after oxidation reactions was investigated by ¹H‐NMR and a possible reaction mechanism was proposed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionic conductivity of KMgCr(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> molybdate

The ionic conductivity σ of KMgCr(MoO₄)₃ crystal has been investigated in a temperature range of 575–932 K by impedance spectroscopy in the frequency range of (5–5) × 10⁵ Hz. Ternary molybdate was obtained from the initial MgMoO₄ and KCr(MoO₄)₂ reagents by solid-phase technique in air at 923–973 K for 200 h. The temperature dependence σ(T) of a ceramic sample exhibits a jump of σ by a factor of about 4 at 833 ± 5 K, which is caused by the first-order phase transition. The σ value above the phase-transition temperature reaches 6 × 10^–4 S/cm (932 K) at an ion-transport activation enthalpy of 0.84 ± 0.05 eV. The most likely carriers in KMgCr(MoO₄)₃ are K⁺ cations.     

pH‐Controlled topologies of two Co(II) complexes based on 5‐Nitro‐1,2,3‐benzenetricarboxylic acid and 1,6‐Bis(1,2,4‐triazole‐1‐yl)hexane

Two new Co(II) coordination polymers, [Co(Hnbta)(bth)]_n ( 1 ) and {[Co₃(nbta)₂(bth)₃(H₂O)]·H₂O}_n ( 2 ), (H₃nbta = 5‐nitro‐1,2,3‐benzenetricarboxylic acid and bth = 1,6‐bis(1,2,4‐triazole‐1‐yl)hexane), have been synthesized under different pH values through hydrothermal reactions. Both complexes were characterized by elemental analysis, IR spectra, thermogravimetric analysis (TGA), and single‐crystal X‐ray crystallography. Complex 1 exhibits a 3D polythreaded network based on 4‐connected sql 2D layer. Complex 2 has a (4,4,5)‐connected trinodal 3D pillar‐layered network with a (4²·6⁴)₂(4²·6⁷·8)₂(6⁴·8²) topology. Magnetic susceptibility measurements indicate that complexes 1 and 2 show weak antiferromagnetic interactions between the adjacent Co(II) ions.     

Growth and downconversion luminescence of Ho3+/Yb3+ codoped α‐NaYF4 single crystals by the Bridgman method using a KF flux

Downconversion (DC) luminescence with emission at about 1000 nm under excitation of 448‐nm light in Ho³⁺/Yb³⁺ codoped α‐NaYF₄ single crystal is realized. The crystal was grown by the Bridgman method using KF as an assisting flux in a NaF‐YF₃ system. The energy‐transfer process and quantum cutting (QC) mechanisms are presented through the analysis of the spectra. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of the emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process. When the Ho³⁺ concentration is fixed at about 0.8 mol%, the optimal concentration for ∼1000 nm emission is 3.02 mol% Yb³⁺ in the current research. The energy‐transfer efficiency and the total quantum efficiency are analyzed through the luminescence decay curves. The maximum quantum cutting efficiency approaches to 184.4% in α‐NaYF₄ single crystals of 0.799 mol% Ho³⁺ and 15.15 mol% Yb³⁺. However, the emission intensity at 1000 nm decreases while the energy‐transfer efficiency from Ho³⁺ to Yb³⁺ increases, which may result from the fluorescence quenching between Ho³⁺ and Yb³⁺ ions, Yb³⁺ and Yb³⁺ ions.     

Laser emission cross-section and threshold power for laser operation at 1077 nm and 1370 nm; chalcogenide mini-lasers doped by Nd3+

Laser emission cross-sections of the 1077 nm and 1370 nm emissions of Nd³⁺ in 3Ga₂S₃ (0.85)La₂S₃0.15Nd₂S₃ (GLS) and 3Al₂S₃ (0.872)La₂S₃0.128Nd₂S₃ (ALS) were obtained from the calculated matrix elements of Nd³⁺ in the glasses, experimentally measured intensity parameters, emission half-widths and lifetimes. The laser threshold power for side pumping in the chalcogenide glasses is much lower than for ED-2 glass.     

Flux growth and characterization of Ti‐ and Ni‐doped forsterite single crystals

Forsterite monocrystals doped with Ti and Ni were grown by the flux growth technique. A suitable mixture of flux (MoO₃, V₂O₅, Li₂CO₃) and nutrient was slowly cooled down to 750 °C from 1250 °C or 1350 °C. The crystals were then characterized by powder and single‐crystal X‐ray diffraction, scanning electron microscopy and differential scanning calorimetry (DSC). Variations observed in crystal size were attributed by both the varying experimental conditions in which they had been obtained, and to the amount of Ni substituted for Mg in the structure. High abundances of doped forsterite required a cooling rate of 1.8 K h^‐1. These synthetic, well‐characterized Ti and Ni doped forsterite crystals may have potential for exploitation in industrial fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)