Ge and Ga K-edge EXAFS analyses on the structure of Ge-Ga-S-CsBr glasses

The local structure of Ge and Ga ions in (1 − x)(Ge_0.25Ga_0.10S_0.65)-xCsBr glasses (x = 0.00, 0.05, 0.10 and 0.12) were investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. CsBr formed [GaS_3/2Br]⁻ structural units in glass while Ge ions remained in GeS_4/2 tetrahedra, unaffected by CsBr addition. Rare-earth ions can be surrounded by Br ions only when CsBr/Ga ratio in glass became larger than unity.     

RDF analysis, positron annihilation and Raman spectroscopy of xTiO2-(60 − x)SiO2-40Na2O non-linear optical glasses: III. Non-bridging oxygen bonds tracing and structure analysis

xTiO₂-(60 − x)SiO₂-40Na₂O glasses have proven an interesting linear and non-linear optical properties [M. Abdel-Baki, F. Abdel Wahab, F. El-Diasty, Mater. Chem. Phys. 96 (2006) 201]. The investigated glasses show one order of magnitude enhancement for the second-order index of refraction and third-order optical susceptibility over some TiO₂ silicate glasses. In this work, we continue studying these glasses using three different techniques to analyze the glass structures seeking to provide a deep insight for the relation between structure, compositions and the optical characteristics of these glasses. Radial Distribution Function analysis (RDF) combined Raman spectroscopy are used to study these glasses. Positron annihilation lifetime spectroscopy and Doppler broadening measurements are carried out to investigate the change in the glass structure as the incorporation of TiO₂ concentration into glass. The origin of the non-bridging oxygen (NBO) bonds has been traced to correlate their existence with the measured non-linear optical properties of the investigated glasses.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

Montmorillonite (MMT) effect on the structure of poly(oxyethylene) (PEO)–MMT nanocomposites and silica–PEO–MMT hybrid materials

In this work we report the effects of incorporation of variable amounts (0.5–25%w/w) of montmorillonite in poly(oxyethylene) based materials in order to decrease the polymer crystallinity. Two different classes of materials were studied: silica-poly(oxyethylene)-montmorillonite hybrids prepared by the sol–gel route and poly(oxyethylene)-montmorillonite nanocomposites prepared by mixing the dry clay or the clay aqueous suspension into the melt poly(oxyethylene). The effects of montemorillonite loading on the poly(oxyethylene) crystallization control and on the nanostructural features were investigated by X-ray powder diffraction, small-angle X-ray scattering and differential scanning calorimetry. Experimental results show that free montmorillonite layers coexist with open aggregates and tactoids in the poly(oxyethylene)-montmorillonite nanocomposites, with different features depending on the filler proportion and preparation route. The intercalation of polymer chains in montmorillonite galleries markedly hinders the crystallization of the poly(oxyethylene) matrix. For hybrids materials the silica phase favors the exfoliation of montmorillonite tactoids, so that samples are predominantly constituted by dispersed platelets.     

Preparation and the proton conductivity of chitosan/poly(vinyl phosphonic acid) complex polymer electrolytes

This study describes the preparation and proton conductivity of novel polymer complex electrolytes consisting of chitosan and poly(vinylphosphonic acid), PVPA. The materials were prepared via in situ polymerization of vinyl phosphonic acid in the presence of chitosan at various monomer feed ratios with respect to d-glucosamine repeat unit. Homogeneous materials were produced and they were extensively characterized for their compositions by elemental analysis (EA), thermal properties by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and the morphology by X-ray diffraction (XRD). The complexation of chitosan/PVPA via proton exchange reactions was confirmed by Fourier transform infrared (FT-IR). The methanol permeability of CHPVPA₅ was lower than Nafion^® 115. The water/methanol uptake was measured and the results showed that solvent absorption of the materials increased with increasing PVPA content in the matrix. The temperature dependence of the proton conductivity of the complex electrolytes follows VTF behavior at higher x. Proton conductivity of CHPVPA₅ was measured to be approximately 3 × 10⁻⁵ S/cm at 120 °C in the anhydrous state.     

Reactivity of polyiodide toward 1, 3-bis(4-pyridyl)propane (bpp): synthesis and structure of an organic-inorganic hybrid compound [(CuI2)2(N,N′-dimethyl-bpp)] n

The solution-phase reaction of a metal iodide with 1, 3-bis(4-pyridyl)propane (bpp) yields a new organic-inorganic hybrid framework, [(CuI2)2(N,N-dimethyl-bpp)] _n (1), which is made up of organic cations [dimethyl-bpp]²⁺ and an infinite [Cu₂I₄] _n ²⁻ anion chain; The cluster 1 obtained via the reaction between polyiodide complexes and organic N-donor ligand suggests that this simple synthetic approach is likely to be applicable to the construction of polymeric clusters.Supplementary material CCDC-253123 contains the crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk].     

Epitaxial films of metals from the platinum group (Ir,Rh, Pt and Ru) on YSZ-buffered Si(1 1 1)

In the present work we have grown twin-free single crystal metal films of iridium (Ir), rhodium (Rh), platinum (Pt) and ruthenium (Ru) on silicon (1 1 1) substrates via an yttria-stabilized zirconia (YSZ) buffer layer. A prerequisite for the realisation of heteroepitaxial metal films without additional texture components was the twin-free deposition of the YSZ films by pulsed laser deposition (PLD). For the metal films on top, a novel two-step growth process was applied with an extremely low deposition rate for the first 20 nm. For all metals, a drastic texture improvement by up to a factor of 9 could be observed compared to the oxide buffer layer. Minimum values were 0.18° (Ir) and 0.12° (Rh) for tilt and twist, respectively. For all four metals investigated, twin-free epitaxial films could be grown on YSZ/Si(1 1 1) whereas the twinning problem for platinum films was solved by decoupling the Pt-YSZ interface via an additional iridium interlayer. The grown metal/YSZ/Si(1 1 1) multilayer samples offer the possibility to integrate a variety of interesting nanostructures and functional materials on silicon. They are now available in 4 in wafer size.     

Silica-zirconia mixed oxide samples by an hybrid materials based innovative preparation procedure: Influence of preparation procedure and composition on active sites

In this work, the interaction of amorphous silica-zirconia mixed oxide samples obtained from inorganic-organic silica-based hybrid materials with pyridine and CO₂ was studied to investigate their acid/base character. Several silica-zirconia mixed oxide powders characterized by different [Zr/Si] atomic ratios were prepared and treated at increasing temperatures both in a conventional muffle and with microwave technology. The powder samples were characterized with Diffuse Reflectance Infrared Fourier Transform (DRIFT) and X-ray Photoelectron (XP) spectroscopies. The surface acidic and basic active sites were investigated (with DRIFT spectroscopy) by chemisorbing probe molecules (pyridine, carbon dioxide). The obtained results revealed the presence of both Lewis and Brønsted acidic sites on the amorphous silica-zirconia mixed oxide powder surfaces. Several acidic sites characterized by different strength were observed; the acidic sites distribution is markedly influenced by the sample composition and by the heat treatment: more numerous acidic sites form on the surface of the samples treated with microwaves with respect to the muffle treated ones; the increment of the temperature and the decrease of the zirconium content cause a significant decrement of the acidic sites. No basic sites were revealed.     

Influence of metal organic and inorganic precursors on spray pyrolyzed ceramic MgO (2 0 0) thin films for epitaxial over layers

The MgO (2 0 0) surface is widely used as a substrate for epitaxial growth of superconducting and ferro-electric films. Highly oriented, single crystalline, extremely flat and transparent MgO films have been successfully deposited on quartz substrates by the chemical spray pyrolysis technique using economically viable metal organic and inorganic precursors under optimized conditions at the substrate temperature of 600 °C. Thermal analysis (TGA/DTA) in the temperature range 30-600 °C with the heating rate of 10 °C/min revealed the decomposition behavior of the precursors and confirmed the suitable substrate temperature range for film processing. The heat of reaction, ΔH due to decomposition of metal organic precursor contributed additional heat energy to the substrate for better crystallization. The intensity of the (2 0 0) peak in X-ray diffraction (XRD) measurements and the smooth surface profiles revealed the dependency of precursor on film formation. The compositional purity and the metal-oxide bond formation were tested for all the films. UV-Vis-NIR optical absorption in the 200-1500 nm range revealed an optical transmittance above 80% and the absorption edge at about 238 nm corresponding to an optical band gap E_g = 5.25 eV. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) micrographs of MgO films confirmed better crystallinity with larger grain size (0.85 μm) and reduced surface roughness (26 nm), respectively.     

Spectral and barrier properties of heterocomposites based on poly (para-phenylene) disposed between the silicon films

ABSTRACT

Within the framework of the methods of the electron density functional and the ab initio pseudopotential, we have obtained the spatial distributions of the valence electrons density, the electron energy spectra and the Coulomb potential for heterocomposites based on poly (para-phenylene) and carbon nanotube disposed between the silicon films.

?It was revealed that the maximal value of the potential barrier was noticed in a composite material from polyparaphenylene filaments placed between silicon films and completed with carbon nanotubes perpendicularly to the surface of the silicon films and along them.     

Structural characterization of a lariat ether based on 4,10-diaza-2,3,11,12-dibenzo-18-crown-6 and of a coordination Pb(II) complex using picrate as co-ligand

A new lariat ether, N,N′-di(methylenecarboxyethoxy)-4,10-diaza-2,3,11,12-dibenzo-18-crown-6, C₂₈H₃₈N₂O₈, has been characterized and tested for Pb²⁺ extraction in a picrate medium. The isolated complex, [(picrato-O,O′)-(N,N′-di-methylenecarboxyethoxy-4,10-diaza-2,3,11,12-dibenzo-18-crown-6)]-lead(II) picrate, [C₃₄H₄₀N₅O₁₅Pb]⁺[C₆H₂N₃O₇]⁻, includes a 7-coordinated Pb²⁺ cation complex where sidearms of the macrocycle are involved in complexation, with Pb‐O bond lengths of 2.587(4) and 2.617(4) ?. Picrate ions serve both as co-ligand and as anion for charge balance. Both free ligand and Pb²⁺ complex have been X-ray characterized: a = 18.4723(16), b = 24.0057(19), c = 14.4309(14) ?, β = 112.962(7)°, P2₁/c, for the ligand; a = 12.7591(15), b = 21.951(2), c = 16.4389(15) ?, β = 97.144(8)°, P2₁/n, for the complex.     

Hydride-assisted growth of GaN nanowires on Au/Si(0 0 1) via the reaction of Ga with NH3 and H2

High quality, straight GaN nanowires (NWs) with diameters of 50 nm and lengths up to 3 μm have been grown on Si(0 0 1) using Au as a catalyst and the direct reaction of Ga with NH₃ and N₂:H₂ at 900 °C. These exhibited intense, near band edge photoluminescence at 3.42 eV in comparison to GaN NWs with non-uniform diameters obtained under a flow of Ar:NH₃, which showed much weaker band edge emission due to strong non-radiative recombination. A significantly higher yield of β-Ga₂O₃ NWs with diameters of ≤50 nm and lengths up to 10 μm were obtained, however, via the reaction of Ga with residual O₂ under a flow of Ar alone. The growth of GaN NWs depends critically on the temperature, pressure and flows in decreasing order of importance but also the availability of reactive species of Ga and N. A growth mechanism is proposed whereby H₂ dissociates on the Au nanoparticles and reacts with Ga giving Ga_xH_y thereby promoting one-dimensional (1D) growth via its reaction with dissociated NH₃ near or at the top of the GaN NWs while suppressing at the same time the formation of an underlying amorphous layer. The higher yield and longer β-Ga₂O₃ NWs grow by the vapor liquid solid mechanism that occurs much more efficiently than nitridation.     

Low-temperature growth of epitaxial (1 0 0) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor

Epitaxial (1 0 0) silicon layers were grown at temperatures ranging from 500 to 800 °C in a commercial cold-wall type UHV/CVD reactor at pressures less than 7×10⁻⁵ Torr. The substrates were 300 mm SIMOX SOI wafers and spectroscopic ellipsometry was used to assess growth rates and deposition uniformities. High-resolution atomic force microscopy (AFM) was employed to verify the atomic terrace configuration that resulted from epitaxial step-flow growth. Deposition from disilane exhibited a nearly perfect reaction limit for low temperatures and high precursor flow rates (partial pressures) with measured activation energies of ≈2.0 eV, while a linear dependence of growth rate on precursor gas flow was found for the massflow-controlled regime. A similar behavior was observed in the case of silane with substantially reduced deposition rates in the massflow-limited regime and nearly a factor of 2 reduced growth rates deep in the reaction limited regime. High growth rates of up to 50 μm/h and non-uniformities as low as 1σ=1.45% were obtained in the massflow-limited deposition regime. Silicon layers as thin as 0.6 nm (4.5 atomic layers ) were deposited continuously as determined using a unique wet chemical etching technique as well as cross-sectional high-resolution transmission electron microscopy (HRTEM). In contrast, epitaxial silicon deposited in RPCVD at 10 Torr using disilane within the same temperature range showed imperfect reaction limitation. While activation energies similar to that of UHV/CVD were found, no partial pressure limitation could be observed. Furthermore, layers deposited using disilane in RPCVD exhibited a large number of defects that appeared to form randomly during growth. We attribute this effect to gas phase reactions that create precursor fragments and radicals—an effect that is negligible in UHV/CVD.     

Two-dimensional crown ether complex. Synthesis and crystal structure of [Na(N15C5)]<Subscript>2</Subscript>[Cu(mnt)<Subscript>2</Subscript>]

A novel crown ether complex [Na(N15C5)]₂[Cu(mnt)₂] (1) (where N15C5 = 2,3-naphtho-15-crown-5 and mnt = maleonitriledithiolate, [C₂S₂(CN)₂]^2–) was synthesized and characterized by elemental analysis, FT-IR spectra, UV-visible spectra and single crystal X-ray diffraction. Structural analysis reveals that the complex 1 consists of two [Na(N15C5)]⁺ complex cations and one [Cu(mnt)₂]^2– complex anion and the complex molecules are assembled into a novel 2D network by Na–N interactions. In the network, there are 34-member macrocycles formed by four sodium ions of complex cations and four complex anions through Na–N bonds. In each macrocycle, – stacking interactions occurred between two parallel naphthylene moieties of N15C5. It is interesting that the 2D network displays an open-capped wave-like sheet viewed along the crystallographic a axis.     

A supramolecular structure of bis(1,3-diphenyl-1,3-propanedionato-O,O′) (1,10-phenanthroline-N,N′)cobalt(II) based on C–H…O, C–H…π and π…π interactions

The compound bis(1,3-diphenyl-1,3-propanedionato-O,O′) (1,10-phenanthroline-N, N′)cobalt(II) (Co(DBM)₂(1,10-phe)) was investigated as a potential host component for functional inclusion materials. The molecules of the complex in the crystal (a=21.015(5) ?, b=17.456(5) ?, c=9.503(3) ?, β=107.04(2)°, s.g. C 2/c) are centrosymmetric and assemble by way of C–H···O and C–H···π hydrogen bonds as well as π···π stacking interactions to provide a supramolecular ladder-like motif that is accompanied by a variant of the six phenyl-embrace synthon.     

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.