Topotatic-like phase transformation of amorphous lead titanate to cubic lead titanate

The crystallization process of lead titanate (PT) prepared using the polymeric precursor method was investigated using X-ray diffractometry, Raman spectroscopy, electron microscopy, and X-ray absorption spectroscopy techniques. The results showed that amorphous PT was formed by an O-Ti-O structure composed of fivefold and sixfold oxygen-coordinated titanium. The local structure of the amorphous PT phase was similar to that of the cubic PT phase, i.e., similar coordination number and similar bond lengths, leading to a topotactic-like transformation during the phase transformation from amorphous to cubic perovskite PT. Because of the low crystallization temperature, every transformation observed during the crystallization process was associated with a short-range rearrangement process.

Predicting structural models for silicon clusters

This article introduces an efficient method to generate structural models for medium-sized silicon clusters. Geometrical information obtained from previous investigations of small clusters is initially sorted and then introduced into our predictor algorithm in order to generate structural models for large clusters. The method predicts geometries whose binding energies are close (95%) to the corresponding value for the ground-state with very low computational cost. These predictions can be used as a very good initial guess for any global optimization algorithm. As a test case, information from clusters up to 14 atoms was used to predict good models for silicon clusters up to 20 atoms. We believe that the new algorithm may enhance the performance of most optimization methods whenever some previous information is available. (C) 2003 Wiley Periodicals, Inc.

Micro-Raman analysis of cubic GaN layers grown by MBE on (001) GaAs substrate

Cubic GaN layers are grown by molecular beam epitaxy on (001) GaAs substrates. Optical micrographs of the GaN epilayers intentionally grown at Ga excess reveal the existence of surface irregularities such as bright rectangular structures, dark dots surrounded by rectangles and dark dots without rectangles. Micro-Raman spectroscopy is used to study the structural properties of these inclusions and of the epilayers in greater detail. We conclude that the observed irregularities are the result of a melting process due to the existence of a liquid Ga phase on the growing surface.

Photoreflectance studies of optical transitions in cubic GaN grown on GaAs(001) substrates

The optical properties of cubic GaN epitaxial layers were investigated by modulated photoreflectance (PR) and photoluminescence in the temperature interval from 5 to 300 K. The epilayers were grown on GaAs(001) substrates by molecular beam epitaxy using a nitrogen RIF-activated plasma source. The PR spectra show a transition which is well fitted using the third-derivative functional form of the unperturbed dielectric function, which we interpret as band-to-band transition. Our results allow determination of the temperature dependence of the main gap of c-GaN and give insights into the residual strain in the film, as well as allow us to estimate the binding energy of the complex formed by an exciton bound to a neutral acceptor. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Electrical conduction mechanism and phase transition studies using dielectric properties and Raman spectroscopy in ferroelectric Pb0.76Ca0.24TiO3 thin films

We have studied the phase transition behavior of Pb0.76Ca0.24TiO3 thin films using Raman scattering and dielectric measurement techniques. We also have studied the leakage current conduction mechanism as a function of temperature for these thin films on platinized silicon substrates. A Pb0.76Ca0.24TiO3 thin film was prepared using a soft chemical process, called the polymeric precursor method. The results showed that the dependence of the dielectric constant upon the frequency does not reveal any relaxor behavior. However, a diffuse character-type phase transition was observed upon transformation from a cubic paraelectric phase to a tetragonal ferroelectric phase. The temperature dependency of Raman scattering spectra was investigated through the ferroelectric phase transition. The soft mode showed a marked dependence on temperature and its disappearance at about 598 K. on the other hand, Raman modes persist above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive above the phase transition temperature. The origin of these modes must be interpreted in terms of a local breakdown of cubic symmetry by some kind of disorder. The lack of a well-defined transition temperature suggested a diffuse-type phase transition. This result corroborate the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in the thin film. The leakage current density of the PCT24 thin film was studied at elevated temperatures, and the data were well fitted by the Schottky emission model. The Schottky barrier height of the PCT24 thin film was estimated to be 1.49 eV. (C) 2003 American Institute of Physics.

Erbium- and ytterbium-doped sol-gel SiO2-HfO2 crack-free thick films onto silica on silicon substrate

Aware of the difficulties in applying sol-gel technology on the preparation of thin films suitable for optical devices, the present paper reports on the preparation of crack-free erbium- and ytterbium-doped silica: hafnia thick films onto silica on silicon. The film was obtained using a dispersion of silica-hafnia nanoparticles into a binder solution, spin-coating, regular thermal process and rapid thermal process. The used methodology has allowed a significant increase of the film thickness. Based on the presented results good optical-quality films with the required thickness for a fiber matching single mode waveguide were obtained using the erbium- and ytterbium-activated sol-gel silica:hafnia system. The prepared film supports two transversal electric modes at 1550 nm and the difference between the transversal electric mode and the transversal magnetic mode is very small, indicating low birefringence. Photoluminescence of the I-4(13/2) -> I-4(15/2) transition of erbium ions shows a broad band centered at 1.53 mu m with full width at a half maximum of 28 nm. Up-conversion emission was carried out under different pump laser powers, and just one transition at red region was observed. (c) 2006 Elsevier B.V. All rights reserved.

Non-isothermal kinetic of oxidation of tungsten carbide

Tungsten carbide, WC, has shown dissimilar thermal behavior when it is heated on changeable heating rate and flow of oxidant atmosphere. The oxidation of WC to WO3 tends to be in a single and slow kinetic step on slow heating rate and/or low flux of air. Kinetic parameters, on non-isothermal condition, could be evaluated to the oxidation of WC to heating rate below 15 degrees C min(-1) or low flow of air (10 mL min(-1)). The reaction is governed by nucleation and growth at 5 to 10 degrees C min(-1) then the tendency is to be autocatalytic, JMA and SB, respectively.

Low-temperature, self-nucleated growth of indium-tin oxide nanostructures by pulsed laser deposition on amorphous substrates

Indium-tin oxide nanostructures were deposited by excimer laser ablation in a nitrogen atmosphere using catalyst-free oxidized silicon substrates at 500 degrees C. Up to 1 mbar, nanowires grew by the vapor-liquid-solid (VLS) mechanism, with the amount of liquid material decreasing as the deposition pressure increased. The nanowires present the single-crystalline cubic bixbyite structure, oriented < 100 >. For the highest pressure used, pyramids were formed and no sign of liquid material could be observed, indicating that these structures grew by a vapor-solid mechanism. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Preparation, microstructural and electrical characterization of SrTiO3 thin films prepared by chemical route

Polycrystalline SrTiO3 thin films having a cubic perovskite structure were prepared at different temperatures by the polymeric precursor method on platinum-coated silicon substrate. Crystalline films with uniform composition and thickness were prepared by spin-coating and the post-deposition heat treatment was carried out at different temperatures. The film showed good structural, dielectric, and insulating properties, Scanning electron microscopy (SEM) micrographs showed no occurrence of interdiffusion between the bottom electrode (platinum) and the film during post-annealing, indicating a stable interface between the SrTiO3 and the bottom electrode. The dielectric constant and dissipation factor at a frequency of 100 kHz were 250 and 0.01, respectively, for a 360 nm thick film annealed at 600 degreesC. The capacitance versus applied voltage characteristics showed that the capacitance was almost independent of the applied voltage. The I-V characteristics were ohmic in low fields and a Schottky emission and/or Poole-Frenkel emission were postulated in high fields. Room temperature leakage current density was found to be in the order of 10(-7) A/cm(2) for a 360 nm thick film in an applied electric field of about 100 kV/cm. The charge storage density of 36 fC/mum(2) was obtained in an applied electric field of about 100 kV/cm. (C) 2001 Published by Elsevier B.V. Ltd. All rights reserved.

All electron fully relativistic Dirac-Fock calculation for darmstadtium carbide using prolapse free basis set

For the first time, ab inito all electron fully relativistic and correlated Dirac-Fock calculations with prolapse free basis set are reported for a Super Heavy Element. We investigated the relativistic effects on bonding and on some spectroscopic constants for the darmstadtium carbide and our results at DF/CCSD(T) with a prolapse free basis set suggest for R-e, omega(e) and D-e the values of 174 pm, 1114 cm(-1) and 7.29 eV, respectively. These values are very similar to the values for PtC found on literature. It was also found that prolapse free basis set may be important to estimate the dissociation energy using Relativistic 4-components correlated methods. (c) 2007 ELsevier B.V. All rights reserved.

Energy States of Phosphorous Donor in Silicon in Fields up to 18 T

The evolution of the energy states of the phosphorous donor in silicon with magnetic field has been the subject of previous experimental and theoretical studies to fields of 10 T. We now present experimental optical absorption data to 18 T in combination with theoretical data to the same field. We observe features that are not revealed in the earlier work, including additional interactions and anti-crossings between the different final states. For example, according to the theory, for the "1s -> 2p (+)" transition, there are anti-crossings at about 5, 10, 14, 16, and 18 T. In the experiments, we resolve at least the 5, 10, and 14 T anti-crossings, and our data at 16 and 18 T are consistent with the calculations.

Effect of processing conditions on the nucleation and growth of indium-tin-oxide nanowires made by pulsed laser ablation

Indium-tin oxide nanowires were deposited by excimer laser ablation onto catalyst-free oxidized silicon substrates at a low temperature of 500 degrees C in a nitrogen atmosphere. The nanowires have branches with spheres at the tips, indicating a vapor-liquid-solid (VLS) growth. The deposition time and pressure have a strong influence on the areal density and length of the nanowires. At the earlier stages of growth, lower pressures promote a larger number of nucleation centers. With the increase in deposition time, both the number and length of the wires increase up to an areal density of about 70 wires/mu m(2). After this point all the material arriving at the substrate is used for lengthening the existing wires and their branches. The nanowires present the single-crystalline cubic bixbyite structure of indium oxide, oriented in the [100] direction. These structures have potential applications in electrical and optical nanoscale devices.

Study of plasma immersion ion implantation into silicon substrate using magnetic mirror geometry

The effect of magnetic field enhanced plasma immersion ion implantation (PIII) in silicon substrate has been investigated at low and high pulsed bias voltages. The magnetic field in magnetic bottle configuration was generated by two magnetic coils installed outside the vacuum chamber. The presence of both, electric and magnetic field in PIII creates a system of crossed E x B fields, promoting plasma rotation around the target. The magnetized electrons drifting in crossed E x B fields provide electron-neutral collision. Consequently, the efficient background gas ionization augments the plasma density around the target where a magnetic confinement is achieved. As a result, the ion current density increases, promoting changes in the samples surface properties, especially in the surface roughness and wettability and also an increase of implantation dose and depth. (C) 2012 Elsevier B. V. All rights reserved.

Photocatalytic decomposition of methylene blue via Fenton mechanisms by silicon wafer doped with Au and Cu: a theoretical and experimental study

In this work, we studied the photocatalytic and the structural aspects of silicon wafers doped with Au and Cu submitted to thermal treatment. The materials were obtained by deposition of metals on Si using the sputtering method followed by fast heating method. The photocatalyst materials were characterized by synchrotron-grazing incidence X-ray fluorescence, ultraviolet-visible spectroscopy, X-ray diffraction, and assays of H(2)O(2) degradation. The doping process decreases the optical band gap of materials and the doping with Au causes structural changes. The best photocatalytic activity was found for thermally treated material doped with Au. Theoretical calculations at density functional theory level are in agreement with the experimental data.