EXAFS investigation on Sb incorporation effects to electrical transport in SnO2 thin films deposited by sot-gel

The effect of Sb doping in SnO2 thin films prepared by the sol-gel dip-coating (SGDC) process is investigated. Electronic and structural properties are evaluated through synchrotron radiation measurements by EXAFS and XANES. These data indicate that antimony is in the oxidation state W, and replaces tin atoms (Sn4+), at a grain surface site. Although the substitution yields net free carrier concentration, the electrical conductivity is increased only slightly, because it is reduced by the high grain boundary scattering. The overall picture leads to a shortening of the grain boundary potential, where oxygen vacancies compensate for oxygen adsorbed species, decreasing the trapped charge at grain boundary. (c) 2007 Elsevier Ltd. All rights reserved.

CeO2 nanoparticles synthesized by a microwave-assisted hydrothermal method: evolution from nanospheres to nanorods

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Strong violet-blue light photoluminescence emission at room temperature in SrZrO3: Joint experimental and theoretical study

Ultrafine ordered and disordered SrZrO3 powders were prepared by the polymeric precursor method. The structural evolution from structural disorder to order was monitored by X-ray diffraction and X-ray absorption near-edge spectroscopy. Complex cluster vacancies [ZrO5 center dot V-O(Z)] and [SrOII center dot V-O(Z)] (where V-O(Z) = V-O(X), V-O(center dot) and V-O(center dot center dot)) were proposed for disordered powders. The intense violet-blue light photoluminescence emission measured at room temperature in the disordered powders was attributed to complex cluster vacancies. High-level quantum mechanical calculations within the density functional theory framework were used to interpret the experimental results. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Structure and optical properties of [Ba1-xY2x/3](Zr0.25Ti0.75)O-3 powders

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Shape controlled synthesis of CaMoO4 thin films and their photoluminescence property

CaMoO4 (CMO) disordered and ordered thin films were prepared by the complex polymerization method (CPM). The films were annealed at different temperatures and time in a conventional resistive furnace (RF) and in a microwave (MW) oven. The microstructure and surface morphology of the structure were monitored by atomic force microscopy (AFM) and high-resolution scanning electron microscopy (HRSEM). Order and disorder were characterized by X-ray diffraction (XRD) and optical reflectance. A strong photoluminescence (PL) emission was observed in the disordered thin films and was attributed to complex cluster vacancies. The experimental results were compared with density functional and Hartree-Fock calculations. (C) 2008 Elsevier B.V. All rights reserved.

Influence of different solvents on the structural, optical and morphological properties of CdS nanoparticles

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structural and dielectric properties of Ba0.5Sr0.5(SnxTi1-x)O-3 ceramics obtained by the soft chemical method

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Electronic Perspective on the Electrochemistry of Prussian Blue Films

The derivative of the voltabsommetric scans, together with previous nano-electrogravimetric and X-ray diffraction results, allow different electrochemical processes to be distinguished during the Prussian blue (PB) voltammetric scan. Potassium, proton, and hydrated proton counterions involved in PB electrochemistry are related here to the electrochemical reactions of specific Fe sites. Potassium counterions show two different sites for their insertion: one located in the crystalline framework and another in ferrocyanide vacancies. From the monitoring of electroactive Fe sites, the covalent-exchange model is suggested as one of the first approaches to explain the origin of the PB magnetic ordering observed at room temperature during voltammetric scanning.

Structural conditions that leads to photoluminescence emission in SrTiO3: An experimental and theoretical approach

Complex cluster [TiO5 center dot V-O(z)] and [SrO11 center dot V-O(z)] (where V-O(z)=V-O(X), V-O(center dot), V-O(center dot center dot)) vacancies were identified in disordered SrTiO3 powders prepared by the polymeric precursor method, based on experimental measurements by x-ray absorption near edge structure spectroscopy. The paramagnetic complex states of [TiO5 center dot V-O(center dot)] and [SrO11 center dot V-O(center dot)] with unpaired electrons were confirmed by electron paramagnetic resonance spectroscopy. The disordered powders showed strong photoluminescence at room temperature. Structural defects of disordered powders, in terms of band diagram, density of states, and electronic charges, were interpreted using high-level quantum mechanical calculations in the density functional framework. The four periodic models used here were consistent with the experimental data and explained the presence of photoluminescence. (C) 2008 American Institute of Physics.

The role of oxygen vacancy in the photoluminescence property at room temperature of the CaTiO3

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Evolution of photoluminescence as a function of the structural order or disorder in CaMoO4 nanopowders

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

A new interpretation for the degradation phenomenon of ZnO varistors

The electrical degradation phenomena of zinc oxide-based varistors were studied using a high-energy current pulse and a.c. polarization at different temperatures. Activation energy measurements during the degradation process showed that these phenomena are associated with diffusion and that the diffusion-controlling species are slower than Zn., For degradation promoted by current pulses of 8×20 μs, the Schottky potential barrier deformation was measured. A decrease in height and width of the potential barrier due to the reduction of surface states density, N s, without a significant change in donor density, N d, was observed. To explain these results, a modification of the unstable components model is proposed for the potential barrier in which the degradation is due to oxi-reduction reactions between atomic defects. These reactions promote the elimination of zinc vacancies and/or adsorbed oxygen on the grain boundaries. © 1992 Chapman & Hall.

Effect of cobalt(II) oxide and manganese(IV) oxide on sintering of tin(IV) oxide

Additions of 0.5 to 2.0 mol% of CoO or MnO2 onto SnO, promote densification of this oxide up to 99% of theoretical density. The temperature of the maximum shrinkage rate (TM) and the relative density in the maximum densification rate (p*) during constant sintering heating rate depend on the dopant concentration. Thus, dopant concentration controls the densifying and nondensifying mechanisms during sintering. The densification of SnO2 witih addition of CoO or MnO, is explained in terms of the creation of oxygen vacancies.

Light and thermal excitation of depolarization current in indirect bandgap Alx Ga1-xAs

Monochromatic light excitation in conjunction with thermally stimulated depolarization current measurements are applied to indirect bandgap AlxGa1-xAs. The obtained average activation energy for dipole relaxation is in very close agreement with the DX center binding energy. Monochromatic light induces state transition in the defect and makes possible the identification of dipoles observed in the dark. Charge relaxation currents are destroyed by photoionization of Al0.5Ga0.5As using either 647 nm Kr+ or 488 nm Ar+ laser lines, which are above the DX center threshold photoionization energy. It suggests that correlation may exist among charged donor states DX--d+. Sample resistance as a function of temperature is also measured in the dark and under illumination and shows the probable X valley effective mass state participation in the electron trapping. Ionization with energies of 0.8 eV and 1.24 eV leads to striking current peak shifts in the thermally stimulated depolarization bands. Since vacancies are present in this material, they may be responsible for the secondary band observed in the dark as well as participation in the light induced recombination process.

Effect of atmosphere and dopants on sintering of SnO2

Tin oxide is an n-type semiconductor material with a high covalent behavior. Mass transport in this oxide depends on the surface state promoted by atmosphere or by the solid solution of a non-isovalent oxide doping The sintering and grain growth of this type of oxide powder is then controlled by atmosphere and by extrinsic oxygen vacancy formation. For pure SnO2 powder the surface state depends only on the interaction of atmosphere molecules with the SnO2 surface. Inert atmosphere like argon or helium promotes oxygen vacancy formation at the surface due to reduction of SnO2 to SnO at the surface and liberation of oxygen molecules forming oxygen vacancies. As consequence surface diffusion is enhanced leading to grain coarsening but no densification. Oxygen atmosphere inhibits the SnO2 reduction decreasing the surface oxygen vacancy concentration. Addition of dopants with lower valence at sintering temperature creates extrinsic charged oxygen vacancies that promote mass transport at grain boundary leading to densification and grain growth of this polycrystalline oxide.