IR study of ultrathin plasma SiO2 and plasma treated thermal SiO2 films

The infrared absorption spectra both of ultrathin plasma SiO₂ and of thermal oxide (15–100 nm) treated in RF O₂ plasma are investigated in the wave number range from 400 to 4000 cm⁻¹. A conclusion is made that the plasma oxide consists of amorphous and different kinds of crystalline phases. The IR spectra of plasma oxide depend slightly on the discharge parameters. The structure of the as-grown thermal oxide, thinner than 40 nm, is shown to have defects and the presence of crystalline phases is observed too. The IR spectra of these oxides are changed significantly after plasma exposure. The influence of RF O₂ plasma on the thermal oxide, thicker than 40 nm, is negligible with respect to structural changes.     

Development of nanosized oxide composites for catalytic CO oxidation

Abstract

Effect of composition and morphology on the catalytic activity in CO oxidation of Co-, Cu-, Ce-containing oxide materials prepared by citrate method has been studied. It was shown that composites consist of well-dispersed oxides Со₃O₄, CuO and CeO₂ with average crystallite sizes 13–24?nm, 14–26?nm and 3–7?nm, respectively. It was established that Co-, Cu-, Ce-oxide nanocomposites demonstrated higher catalytic activity in CO oxidation in comparison with individual oxides. The ternary oxide system Со₃O₄–CuO–СеO₂ (Со:Cu:Се=10:1:1) was found to possess the highest catalytic activity due to formation finely dispersed particles of CuO and СеO₂ oxides simultaneously with nanosized Со₃O₄ species and formation of large quantity of mobile oxygen at the interfaces between the oxide nanoparticles.     

Investigation of the structure of nanocrystalline refractory oxides by X-ray diffraction,electron microscopy,and atomic force microscopy

The structures of nanocrystalline fibrous powders of refractory oxides have been investigated by different methods: determination of coherent-scattering regions, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic-force microscopy (AFM). The sizes of nanograins of different crystalline phases of refractory metal oxides have been determined during the formation of these nanograins and the dynamics of their growth during heat treatment in the temperature range 600–1600°C has been studied. The data on the structure of nanocrystalline refractory oxide powders, obtained by different methods, are in good agreement. According to the data on coherent-scattering regions, the sizes of the ZrO₂ (Y₂O₃) and Al₂O₃ grains formed are in the range 4–6 nm, and the particle sizes determined according to the TEM and AFM data are in the ranges 5–7 and 2–10 nm, respectively. SEM analysis made it possible to investigate the dynamics of nanoparticle growth at temperatures above 1000°C and establish the limiting temperatures of their consolidation in fibers.

     

Thermal decomposition and new luminescence bands in wet,dry, and additional oxygen implanted silica layers

Wet and dry silica oxide layers have been treated thermally up to T_a = 1300 °C and were investigated by cathodoluminescence (CL) spectroscopy. Whereas the dry oxides after high temperature treatment show an increase of the yellow–red spectra region, contrary, in wet oxides the UV–blue region is enhanced. Even a new strong band in the near-UV region (NV) at 330 nm (3.76 eV) is found for wet oxides at liquid nitrogen temperature (LNT), but much broader and with lower intensity for room temperature (RT) in a triple band structure UV: 290 nm, NV: 330 nm, and V: 400 nm. These violet bands should be associated with a thermally decomposed and rapidly cooled-down silica network in presence of OH groups or even dissociated oxygen. Additional oxygen implantation into dry silica with high doses up to 10¹⁷ ions/cm² and high thermal treatment T > 1100 °C leads as well to enhanced UV–NV–V luminescence emission bands supporting the fact that oxygen and structural decomposition play a decisive role in formation of near-UV luminescent defects in silica.     

Growth and characterization of KBi(WO4)2 single crystals

Potassium bismuth tungstate [KBi(WO₄)₂] single crystals with dimensions up to 20 mm × 15 mm × 15 mm have been successfully grown by using the top‐seeded solution growth technique and K₂W₂O₇ as solvent. Experiments show that this crystal is unstable in a strong acid or alkali environment and has a blue fluorescence emission. The density, hardness, melting point, absorption edge, transparency range, prominent Raman shift frequency are 7.57 g/cm³, 238 kg/mm², 800 °C, 380 nm, 400–5450 nm, 868 cm^–1 respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Matrix effect on absorption and infrared fluorescence properties of Bi ions in oxide glasses

Absorption and fluorescence spectra of Bi ions in multi-component oxide glasses melted in air were measured. Absorption bands in the range of 400–1000 nm and fluorescence bands at round 1210 nm were only detected in the silicate, borosilicate, borate, and germanate glasses with or without a small amount of alkali or alkaline-earth oxides. The relative IR fluorescence intensity increased with decreasing basicity of matrix glasses and with adding the Al₂O₃. The compositional dependence of the relative IR fluorescence intensity and the IR fluorescence mechanism are discussed in terms of redox reaction and energy diagram of bismuth-related species in oxide glasses.     

The effect of zirconia content on properties of Al2O3–ZrO2 (Y2O3) composite nanopowders synthesized by aqueous sol–gel method

Al₂O₃–ZrO₂ (Y₂O₃) nanopowders containing 5, 10 and 15 wt% ZrO₂ were synthesized by aqueous sol–gel method using aluminum sec-butoxide and zirconium butoxide as precursors. BET analysis shows that, increasing the zirconia content results in a decrease in surface area, 152, 125 and 121 m²/g, and an increase in pore size, 5.63, 9.79 and 11.05 nm for 5, 10 and 15 wt% ZrO₂, respectively. Furthermore, a shift toward higher temperatures is observed for transition of transitional aluminas to stable α-alumina phase through increasing the zirconia content. SEM micrograph of calcined nanopowders revealed nanosize spherical particles in the range of 15–75 nm.     

Broadband near-infrared emission from Bi–Er–Tm Co-doped germanate glasses

Bi–Er–Tm co-doped germanate glasses and Bi, Er, Tm singly doped glasses were prepared and characterized through absorption spectra, NIR emission spectra and decay lifetime. A super broadband near-infrared emission from 1000 nm to 1600 nm, covering the whole O, E, S, C, and L bands, was observed in the Bi–Er–Tm co-doped samples due to the result of the overlapping of the Bi related emission band (centered at 1300 nm), the emission from Er^{3+ 4}I_13/2 → ⁴I_15/2 transition (centered at 1534 nm) as well as the emission from Tm^{3+ 3}H₄ → ³F₄ transition (centered at 1440 nm), which is essential for broadly tunable laser sources and broadband optical amplifiers. The energy transfer process was also discussed at the end of the paper.     

Synthesis by sol–gel process,characterization and catalytic activity of vanadia–silica mixed oxides

This paper describes a new route for the preparation of V₂O₅–SiO₂ mixed oxides with high vanadium content and high surface area by the sol–gel processing method. Dry samples were characterized by powder X-ray diffraction, electron paramagnetic resonance, Fourier-transform infrared, and energy dispersive X-ray spectrometry. Our results show that the incorporation of vanadium pentoxide into the silica matrix was obtained by intimate mixing of the two different inorganic polymers (Si–O and V–O based polymers), and the overall structure is held together by a Si–O–Si network interpenetrated with V–O–V polymeric chains. The catalytic activity of the V₂O₅–SiO₂ mixed oxides was evaluated in the oxidation of the cyclooctene and styrene in liquid phase.     

Fabrication and characterization of Er3+-doped GeO2–PbO and GeO2–PbO–Bi2O3 glass fibers

Glass fibers were drawn from GeO₂–PbO–Bi₂O₃ and GeO₂–PbO melts previously doped with Er³⁺. From the differential thermal analysis curve, the glass transition temperature was determined to be 420 °C, and no crystallization peak was observed in the temperature range of that analysis, indicating stability with regard to devitrification. Raman spectroscopy was performed to characterize the structure of the glasses, which exhibited large transmission windows (0.5–5.0 μm) and large refractive indices (∼2.0). Infrared to visible upconversion of Er³⁺ was observed in the fibers. The visible emissions were related to the upconverted green emissions at about 530 nm (²H_11/2 → ⁴I_15/2) and 550 nm (⁴S_3/2 → ⁴I_15/2), and red emission at 668 nm (⁴F_9/2 → ⁴I_15/2) under 980 nm excitation. The infrared transition (⁴I_13/2 → ⁴I_15/2) was peaked at 1.53 μm. The results obtained suggest that the fibers exhibit the same structures as the parent glasses and can be used in upconversion fiber optical devices.     

Corrosion and crystallization at the inner surfaces of glass bricks

Glass bricks are important transparent building materials. They are produced by joining two halves of glass pressings at 600–700 °C. During this production process alkali oxides evaporate and are redeposited at the cooler inner front surfaces of the bricks. This surface layer reacts with H₂O and CO₂ from the residual brick atmosphere, leading to the formation of an alkali-rich silicate-hydrate layer of ?50 nm thickness, which could be evidenced leading to a reduced nano-hardness of similar thickness, and from which NaHCO₃ crystals can finally grow. Climate chamber experiments (repeated cooling between at ?8 and ?14 °C and reheating to 0 to 15 °C) resulted in reversible NaHCO₃ crystallization and redissolution, presumably influenced by water evaporation or condensation and driven by the NaHCO₃ supersaturation of the silicate-hydrate layer. Depending on the time–temperature schedule, different crystal morphologies became visible in this closed system, e.g. isolated spherical crystals, crystals arranged in chains and in double-chains, respectively, which can limit already the transmittance of the glass bricks. When a crack occurs or the brick is opened, the hygroscopic NaHCO₃ crystals take up more H₂O from the ambient, react irreversibly with the glass surface, finally leading to a total loss of transmittance.     

The crystallization of cadmium germanates in aqueous solutions of sodium hydroxide and sodium chloride under hydrothermal conditions

The crystallization in GeO₂-NaCl-H₂O, CdO-NaOH(NaCl)–H₂O. CdO–GeO₂–H₂O, CdO–GeO₂–NaOH (NaCl)–H₂O systems investigated under hydrothermal conditions depending on the initial ratio of CdO and GeO₂ oxides in starting material and solvent concentration. Single crystals of six Cd-germanates and five accessory compounds are obtained. The influence of the mineralizer on Cd-germanates crystallization and the chemism of the interaction of the charge components and the solvent are discussed.     

Doped-metal oxide nanoparticles for use as photocatalysts

Nano-sized doped-metal oxides such as Fe-doped TiO₂, WO₃-doped ZnO and Fe-doped CeO₂ were synthesized by the modified sol-gel/impregnation, flame spray pyrolysis and homogeneous precipitation/impregnation methods respectively. The crystalline phase, particles size, and crystallinity of nano-sized powder were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Specific surface area of the sample was examined by the Brunauer, Emmett and Teller (BET) adsorption–desorption of nitrogen gas. The photocatalytic activity of Fe-doped TiO₂, WO₃/ZnO, Fe-doped CeO₂ nanoparticles was examined by studying the mineralization of methanol, sucrose, glucose, oxalic acid and formic acid under UV and visible light irradiations in a pyrex spiral photoreactor. It was found that doped metal oxides could improve the photocatalytic activity of the pure metal oxides.     

Spatial and energetical profiles of defects extracted from ultra-low level trap-assisted leakage current in non-volatile floating thin tunnel oxide memory devices by using direct and floating gate technique measurements

Stress induced leakage currents (SILC) remain one of the main reliability problems preventing further SiO₂ tunnel oxide thickness reduction in floating thin oxide (FLOTOX) memory devices. In this work, we present ultra-low level SILC current–voltage (I–V) measurements performed by using the floating gate technique on 7–8 nm thick SiO₂ tunnel oxides and low-level measurements performed by direct technique measurements. Experimental characteristics obtained by the indirect measurement technique reached current levels as low as 2 × 10⁻¹⁷ A. They exhibit new phenomena such as negative differential resistance behavior and current threshold voltages. A physical one-step tunneling model (A-mode), taking into account the influence of defects located in the bulk SiO₂, is proposed. Both spatial and energetical defect profiles are extracted from experimental data. We show that the new phenomena experimentally observed can be interpreted as a one step trap-assisted tunneling mechanism via defects located near the middle oxide, even for highly stressed components.     

Mixed alkali effect in physical and optical properties of Li2O–Na2O–WO3–B2O3 glasses

Glasses with composition xLi₂O-(30 ? x)Na₂O–10WO₃–60B₂O₃ (where x = 0, 5, 10, 15, 20, 25 and 30 mol%) have been prepared using the melt quenching technique. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system through density and modulated DSC studies. The density and glass transition temperature of the present gasses varies non-linearly, the exhibiting the mixed alkali effect. From the optical absorption studies, the values of direct optical band gap, indirect optical band gap energy (E_o) and Urbach energy(ΔE) have been evaluated. The values of E_o and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The electronic polarizability of oxide ions, optical basicity and the Yamashita–Kurosawa's interaction parameter have been examined to check the correlation among them and bond character. Based on good correlation among electronic polarizability of oxide ions, optical basicity and the Yamashita–Kurosawa's interaction parameter, the present Li₂O–Na₂O–WO₃–B₂O₃ glasses were classified as normal ionic (basic) oxides.     

Synthesis and toxicity assay of ceramic nanophosphors for bioimaging with near-infrared excitation

Yttrium hydroxyl carbonate (Y(OH)CO₃) precursors were synthesized by the homogeneous co-precipitation method in the presence of polyacrylic acid (PAAc). Resultant precursor particle size is about 15–20 nm with narrow size distribution whereas the particle size is smaller than those acquired by the conventional homogeneous precipitation method. Effective decrease of Y(OH)CO₃ particle size was found to be higher for the presence of weak polyanionic ionomer such as PAAc than the presence of strong polyanionic ionomer such as sodium polystyrene sulfonate (PSS). It was observed that the morphology and size of the precursors are almost unchanged after the calcination process. Er³⁺ doped Y₂O₃ nanoparticles were synthesized by PAAc assisted homogeneous co-precipitation method showed bright green (550 nm) and red (660 nm) upconversion (UC) as well as near-infrared (NIR) fluorescence (1550 nm) under 980-nm excitation. UC and NIR fluorescence bioimaging and in-vitro cytotoxicity assay of Er³⁺ doped Y₂O₃ nanoparticles were successfully attempted with commercially available macrophages and B-cell hybridomas. Cellular uptake of nanoparticles is evidenced from bright field, UC and NIR fluorescence images of macrophages.     

Optical spectra of borate glasses containing Ti and Co in relation to their structure

The optical absorption spectra of borate glasses of the base composition (mol%) 35 (Na₂O+BaO)·65 B₂O₃, to which Ti and Co ions were introduced, have been measured in the range 190–900 nm. The addition of increasing amounts of TiO₂ up to 16 g/100 g glass, introduces an intense charge transfer band in the UV region that shifts towards longer wavelengths without imparting any coloration to the glass samples.Titanium ions are present as the Ti⁴ state and its addition produces glass of more coherent structure. The cobalt ions are found to be present in two symmetries; as CoO₆ units with absorption bands (denoted here as bands a and b) around 525–530 nm and 580–595 nm (respectively), and as CoO₄ units with absorption bands (denoted here as band-c) around 620–635 nm. Replacing Na ions by Ba ions or increasing the TiO₂ content favours the conversion of CoO₄ units into the CoO₆ ones.     

Structural heterogeneity in chalcogenide glass films prepared by thermal evaporation

GeSe₂ and Ge₂₈Sb₁₂Se₆₀ chalcogenide glass thin films have been deposited on single crystal silicon substrates by vacuum thermal evaporation. The surface morphology of these films has been investigated by field emission-scanning electron microscopy and atomic force microscopy, revealing heterogeneities in their microstructure consisting of granular regions ∼15–50 nm in size, which were coarser in the case of the GeSe₂ films. Typical RMS film surface roughness values were ∼0.9–1.3 nm.     

Tl ion-exchange borosilicate glass: Investigation of the Tl site by X-ray absorption spectroscopy

We present a first experimental investigation on the Tl site in Tl-doped borosilicate glass by Tl/Na ion-exchange process. The local order around Tl was investigated by the EXAFS spectroscopy on a set of different samples, obtained by varying the composition of the molten salt bath (TlNO₃/NaNO₃ molar ratio from 0.001 to 0.4) and the bath temperature (between 350 °C and 380 °C). We show that after a 1 h exchange, Tl is surrounded by O atoms in the first shell and that the Tl–O distance is sensibly shorter than both the Tl–O distance found in Tl oxides and the typical Na–O distance in a glass. When the exchange process is extended to 1 week, the measured Tl–O distance increases towards the values found in crystalline Tl oxides, suggesting a stress release during longer exchange duration.     

Study of fluorine losses in oxyfluoride glasses

Glasses in PbGeO₃–PbF₂–CdF₂ and GeO₂–PbO–PbF₂–CdF₂ systems were studied and the fluorine losses during synthesis were investigated. Samples were characterized by differential scanning calorimetry (DSC), X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and Raman scattering spectroscopy. The use of stoichiometric germanate glass, PbGeO₃, instead of introducing individual oxides (GeO₂ + PbO) lead to decreasing fluorine losses, as detected by a fluorine ion selective electrode. The main structural features obtained from vibrational spectroscopy could be described by a metagermanate basic structure permeating fluorine rich regions.