Growth, thermal and spectroscopic characterizations of BaNd2(MoO4)4 crystal

A new crystal, BaNd₂(MoO₄)₄, has been grown from the flux melt based on Li₂Mo₃O₁₀ by a spontaneous nucleation method. The phase structure of the obtained crystals was determined by X-ray powder diffraction. The result shows that the as-grown crystals are well crystallized and indexed in a monoclinic crystal system with space group B2/b. The specific heat of BaNd₂(MoO₄)₄ crystal at 20 °C is 0.485 J/g K. Absorption and fluorescence spectra were also measured at room temperature. There are several strong and broad absorption peaks from 200 to 1200 nm and three emission transition bands located at 890, 1060, and 1334 nm are detected.     

Direct current and impedance spectroscopic studies on MoO3 modified ZnPc/ITO Schottky diodes

We use experimental results of direct current and low signal impedance spectroscopy to investigate the conduction mechanism in organic semiconductor ZnPc. The experimental results demonstrate an increase in current and holes mobility by the introduction of a thin MoO₃ film at the ITO/ZnPc interface. This significantly improves the device performance. The improvement is explained in terms of the reduction in the effective barrier for charge transfer from ZnPc to ITO.     

In situ pressure-induced solid-state amorphization in Sm2(MoO4)3, Eu2(MoO4)3 and Gd2(MoO4)3 crystals: chemical decomposition scenario

The effect of pressure on the phase transformations in Sm₂(MoO₄)₃, Gd₂(MoO₄)₃ and Eu₂(MoO₄)₃ crystals has been studied in situ using synchrotron radiation. All three isostructural compounds undergo a structural phase transition at 2.2-2.8 GPa to a new phase, which is interpreted as a possible precursor of amorphization. Amorphization in these crystals occurs irreversibly over a wide pressure range, and its mechanism, interpreted as a chemical decomposition, is found to be weakly affected by the degree of hydrostaticity.     

Structural evolution of the MoO3(010) surface during lithium intercalation

Atomic force microscopy (AFM) has been used to characterize the structural evolution of the MoO₃(010) surface during the initial stage of Li⁺ intercalation. Lithiation was observed in situ using model cells comprised of a single crystal MoO₃ cathode, a dilute propylene carbonate (PC)-based electrolyte, and an Li metal anode. The insertion of Li⁺ into MoO₃ results in the topotactic nucleation and growth of acicular Li_xMoO₃ (x0.25) precipitates at the (010) surface. Because the interlayer spacing of Li_xMoO₃ (d=7.88 Å) is greater than that of MoO₃ (d=b/2=6.93 Å), the Li_xMoO₃ precipitates expand out of the (010) surface as they grow into the MoO₃ crystal along 010. The local strain associated with this expansion causes the Li_xMoO₃ precipitates to crack parallel to the MoO₃001 and 010 axes once their height exceeds approximately 20 nm. With continued Li⁺ intercalation, cracking becomes more prominent, and the (010) surface begins to fragment and disintegrate. Anisotropies in Li⁺ ion uptake and the influence of surface morphology on Li_xMoO₃ precipitation and crack propagation are described.     

The optical properties of Dy-doped NaY(MoO4)2 crystal

The Dy³⁺-doped NaY(MoO₄)₂ single crystals were grown successfully by the Czochralski technique. The main spectroscopic properties (absorption, luminescence, decay curve) of Dy³⁺-doped NaY(MoO₄)₂ have been determined for both the σ and π polarizations. By using the Judd-Ofelt theory, the measured room temperature absorption spectra were applied to determine the intensity parameters, spontaneous transition probabilities, branching ratios, and radiative lifetimes of Dy³⁺ transitions. The results show that the Dy³⁺-doped NaY(MoO₄)₂ crystal may realize the yellow laser operation.     

Stoichiometric and oxygen deficient MoO3(010) surfaces

The (010) surface of single crystal MoO₃ has been prepared and examined using LEED, XPS, UPS, and ELS. Three methods yield the stoichiometric surface: scraping in UHV and annealing, ion etching followed by reoxidation (770 K, 10² Pa O₂), or oxygen treatment to remove carbon contamination. LEED shows the surface periodicity is the same as that of the bulk (010). The MoO₃ valence band is 7 eV wide with density of states maxima at 1.5, 3.6, and 5.6 eV below the top of the valence band. Heating MoO₃ in vacuum reduces the surface region. XPS indicates the O/Mo atomic ratio decreases to 2.85 ± 0.12 on heating to 600 K. Ar ion bombardment disorders the surface and reduces the surface O/Mo atomic ratio to 1.6. Annealing of reduced surfaces at > 770 K incompletely reoxidizes them by diffusion of oxygen from the bulk. UPS of reduced and annealed MoO₃ exhibits two new emission features in the bandgap at 0.9 and 2.0 eV above the top of the valence band. These features originate from Mo derived states of a defect involving two or more Mo atoms, such as crystallographic shear planes. Because of the insulating nature of MoO₃, surface charging and electron beam induced damage were substantial hindrances to electron spectroscopic examination.     

Spectroscopic properties of Pr:KY(MoO4)2 crystal as a visible laser gain medium

A Pr³⁺-doped KY(MoO₄)₂ single crystal was grown by the Czochralski method. The polarized absorption and fluorescence spectra of the Pr³⁺:KY(MoO₄)₂ crystal were measured at room temperature. The stimulated emission cross-sections for the transitions from the ³P₀ multiplet were estimated from the fluorescence spectra. The fluorescence lifetime of the ³P₀ multiplet was estimated from the fluorescence decay curve at room temperature. The analysis of spectral properties shows that the Pr³⁺:KY(MoO₄)₂ crystal is a promising gain medium for visible lasers.     

Hyperfine interaction of d-electrons in the amorphous semiconductors MoO3-TeO2 and MoO3-P2O5

ESR spectra of the amorphous semiconducting systems MoO₃-TeO₂ and MoO₃-P₂O₅ are presented. The occurence of a hyperfine splitting is confirmed by numerical simulations of the observed spectra and by enriching with the isotope Mo⁹⁵. The isotropic hyperfine field at the molybdenum nucleus amounts to (113±10) kG at high MoO₃ concentration. This value is compared with results of measurements on crystalline MoO₃.     

花状NaY(MoO4)2的合成和(MoO4)2:Eu3+的光学性质

用微波辅助水热-煅烧法成功合成了花状NaY(MoO₄)₂颗粒,用XRD、XPS、FESEM进行了表征,提出了花状NaY(MoO₄)₂颗粒可能的形成机理. 采用相同的方法合成了NaY(MoO₄)₂:Eu³⁺荧光体,该荧光材料在612 nm处有一个强的发射峰,可用作白色发光二极管的红色磷光剂. 此外,微波辅助水热-煅烧法可能发展成为制备其他花状稀土钼酸盐的有效途径.     

Low driving voltage in organic light-emitting diode using MoO3/NPB multiple quantum well structure in hole transport layer

Driving voltage of organic light-emitting diode (OLED) is lowered by employing molybdenum trioxide (MoO₃)/N, N'-bis(naphthalene-1-yl)-N,N'-bis(phe-nyl)-benzidine (NPB) multiple quantum well (MQW) structure in hole transport layer. For the device with double quantum well (DQW) structure of ITO/ [MoO₃ (2.5 nm)/NPB (20 nm)]₂/Alq₃(50 nm)/LiF (0.8 nm)/Al (120 nm)], the turn-on voltage is reduced to 2.8 V, which is lowered by 0.4 V compared with that of the control device (without MQW structures), the driving voltage is 5.6 V, which is reduced by 1 V compared with that of the control device at the 1000 cd/m². In this work, the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure, which is attributed not only to the reducing energy barrier between ITO and NPB, but also to the forming charge transfer complex between MoO₃ and NPB induced by the interfacial doping effect of MoO₃.     

Spectroscopic characteristic and energy levels of Cr in Cr:KAl(MoO4)2 crystal

This paper reports polarized spectral properties and energy levels of Cr³⁺ in KAl(MoO₄)₂ crystal. The absorption and emission cross sections are estimated as 3.72×10^-20 cm² at 669 nm and 2.74×10^-20 cm^-2 at 823 nm for σ-polarization, respectively. The energy levels of Cr³⁺ ion in KAl(MoO₄)₂ crystal were calculated based on the Tanabe-Sugano theory. It is suggested that Cr³⁺ ions occupy at an intermediate crystal field site in Cr³⁺:KAl(MoO₄)₂.     

Raman spectra of CrO2 and MoO2 single Crystals

The Raman spectra of single crystals of CrO₂ and MoO₂ have been studied from 77°K to 500°K for a comparison with the heavily damped phonon features in metallic VO₂. The spectrum of MoO₂ is very similar to that of monoclinic VO₂ whereas CrO₂ exhibits phonon spectra similar to rutile with no appreciable change at the ferromagnetic c curie temperature. The results are discussed with respect to the various models proposed for the semiconductor-to-metal transition in VO₂.     

Spectral characterization and energy levels of Cr:Sc2(MoO4)3 crystal

This paper reports the spectral properties and energy levels of Cr³⁺:Sc₂(MoO₄)₃ crystal. The crystal field strength Dq, Racah parameter B and C were calculated to be 1408 cm⁻¹, 608 cm⁻¹ and 3054 cm⁻¹, respectively. The absorption cross sections σ_α of ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ transitions were 3.74×10⁻¹⁹ cm² at 499 nm and 3.21×10⁻¹⁹ cm² at 710 nm, respectively. The emission cross section σ_e was 375×10⁻²⁰ cm² at 880 nm. Cr³⁺:Sc₂(MoO₄)₃ crystal has a broad emission band with a broad FWHM of 176 nm (2179 cm⁻¹). Therefore, Cr³⁺:Sc₂(MoO₄)₃ crystal may be regarded as a potential tunable laser gain medium.     

Magnetic properties of large-area one-dimensional WO2 and MoO2 nanorods

Large-area one-dimensional (1D) monoclinic WO₂ and MoO₂ nanorods in the space group P2₁/c were synthesized by reactive thermal evaporation. The as-synthesized 1D WO₂ and MoO₂ nanorods become soft magnetic materials at 10 K, implying that structural or magnetic transitions occur. There are large differences in saturation magnetization, the coercive field, and remanence between the 1D WO₂ and MoO₂ nanorods, although both 1D nanorods have a similar shape.     

Synthesis and characterization of spherical core-shell particles SiO2@AgEu(MoO4)2

Submicron spherical SiO₂ particles have been coated with AgEu(MoO₄)₂ phosphor layers by a sol-gel process, followed by surface reaction at high temperature, to get core/shell structured SiO₂@AgEu(MoO₄)₂ particles. X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) were used to characterize the structure and morphology of the resulted core-shell phosphors. The luminescent properties of the core-shell structured phosphors have also been measured at room temperature, and their photoluminescence (PL) spectra are similar to the pure AgEu(MoO₄)₂ phosphor prepared by the same sol-gel method exhibiting red emission.     

Electronic properties of Bi2O3 and MoO3 and relationships to oxidation catalysis

A semiempirical atom superposition and electron delocalization molecular orbital analysis of the bonding and electronic structure of MoO₃, oxygen deficient MoO₃, and the α, β, and δ phases of Bi₂O₃ has been made. It is found that both small — e.g. MoO₆ — and large — e.g. Mo₆O₂₄ — clusters are useful models for cation electronic structure within the theory used. From the calculations, an interpretation is given for all available optical and photoemission data for the oxides. The color, conductivity, and new photoemission peak of oxygen-deficient MoO₃ conducting bronzes are found to be due to the addition of electrons to the lowest of three Mo 5d bands which are empty in MoO₃. Weakly allowed d ← d transitions in the red are responsible for the color. Strongly allowed Mo 5d ← O 2p charge transfer excitations are responsible for the optical absorption above 3.2 eV. For the bismuth oxides, three occupied bands are found showing strong Bi 6s, 6p, 6d and O 2p hybridization. These bands have been seen experimentally. The highest band surprisingly has Bi 6p lone-pair character which is explained in terms of the relative Bi 6s and 6p and O 2p ionization potentials using perturbation theory. Rather similar electronic structures are found for the three phases despite their varying cation coordinations and structures. A charge transfer optical absorption edge at ～ 2.6 eV for the β form agrees well with observations reported in the literature, and similar edges should occur for the other phases. The cubic δ form has an unusual low-lying band suggesting absorption in the infrared. Our results provide insight into the surface properties of these oxides.     

Investigation of monolayer dispersion of MoO3 supported on titanate nanotubes

The monolayer dispersion of MoO₃ supported on the surface of titanate nanotubes (TNT) were prepared by heating mechanical mixture of molybdate (HMA) and TNT. The result shows that MoO₃ can disperse spontaneously onto the surface of TNT, and the dispersion capacity is ca. 27 mg MoO₃/g TNT by X-ray diffraction (XRD). On the basis of thermogravimetric (TG) and X-ray photoelectron spectroscopy (XPS) analysis, it was found that the HMA as precursor could not decompose completely into MoO₃ crystal on the surface of TNT around the threshold above decomposed temperature due to the strong interaction between HMA and the surface of TNT.     

Ferroelectric and paramagnetic properties of Li2Gd4 (MoO4)7 single crystals

Thermal behavior of such fundamental physical properties as polarization, pyroelectric current, dielectric constant and paramagnetic susceptibility are reported for dilithium heptamolybdotetragadolinate, Li₂Gd₄ (MoO₄)₇. The ferroelectric transition point has been determined by various methods and the results compared. The most reliable value of the Curie point has been obtained by the measurement of differential magnetic susceptibility as a function of temperature and is found to be 52±2°C. The room temperature values for the relative dielectric constant and paramagnetic susceptibility are 51.5 and 59.8 x 10^-6 cm³. g^-1, respectively. From the susceptibility measurements the values obtained for the Curie constant, C, and the paramagnetic Curie point, θp, are 1.79 x 10^-2 cm³ . g^-1 . deg and 247°K, respectively. It is believed that Li₂Gd₄ (MoO₄)₇ could be antiferromagnetic between 273 and 325°K.     

Gas sensing properties of nanostructured MoO3 thin films prepared by spray pyrolysis

Thin films of molybdenum trioxide (MoO₃) were deposited on common glass using the chemical spray pyrolysis technique. A (NH₄)₆Mo₇O₂₄4H₂0 solution 0.1 M was used as the precursor one. The influence of substrate temperature on the crystallographic structure, surface morphology and electrical behavior of MoO₃ thin films was studied. MoO₃ can exist in two crystalline forms, the thermodynamically stable orthorhombic α-MoO₃ and the metastable monoclinic β-MoO₃ phase. XRD-spectra showed a growth of α-MoO₃ phase percentage as substrate temperature increases from 420 K up to 670 K. Films deposited in the 500–600 K range have a clearly porous surface structure of nanometer order as can be seen in SEM images. Changes up to six magnitude orders were observed in MoO₃ thin films electrical resistance when films temperature varied from 100 K up to 500 K. The sensing property of these MoO₃ films was also studied. The sensitivity was investigated in the temperature range 160 and 360 K for H₂O and CO gases, respectively. Both of them are of reducing nature. In all studied cases sensitivity decreases slowly as film temperature is raised. At room temperature the sensitivity changes from 12 up to 75% depending on substrate temperature. The sensitivity for CO gas was found to be lower than that of H₂O.     

Electrochemical lithium incorporation into WO3 and MoO3 thin films

The generally accepted mechanism of electrochromic phenomena into WO₃ thin films involves the simultaneous injection of cations and electrons to form a tungsten bronze M_xWO₃, e.g. H_xWO₃,Li_xWO₃. Electrochromic cells of the type ITO MO₃/LiClO₄(M) in propylene carbonate/Pt, where M = W or MO and MO₃ is an amorphous thin film have been used. Different amounts of charges have been injected through these cells by electrochemical means at room temperature. The variations of the MO₃ electrode potential with the injected charge are in good agreement with the assumption of the formation of Li_xMO₃ compounds. The free enthalpies of formation of these bronzes have been calculated. The behavior of thin film electrodes seems to be intermediate between amorphous and crystallized bulky materials. Some measurements of electrode potential have also been carried out with crystallized thin films of sputtered WO₃.