Photocatalytic degradation of methylene blue by TiO(2)-Cu thin films: Theoretical and experimental study

In this work the effect of doping concentration and depth profile of Cu atoms on the photocatalytic and surface properties of TiO(2) films were studied. TiO(2) films of about 200 nn thickness were deposited on glass substrates on which a thin Cu layer (5 nm) was deposited. The films were annealed during 1 s to 100 degrees C and 400 degrees C, followed by chemical etching of the Cu film. The grazing incidence X-ray fluorescence measurements showed a thermal induced migration of Cu atoms to depths between 7 and 31 nm. The X-ray photoelectron spectroscopy analysis detected the presence of TiO(2), Cu(2)O and Cu(0) phases and an increasing Cu content with the annealing temperature. The change of the surface properties was monitored by the increasing red-shift and absorption of the ultraviolet-visible spectra. Contact angle measurements revealed the formation of a highly hydrophilic surface for the film having a medium Cu concentration. For this sample photocatalytic assays, performed by methylene blue discoloration, show the highest activity. The proposed mechanism of the catalytic effect, taking place on Ti/Cu sites, is supported by results obtained by theoretical calculations. (C) 2010 Elsevier B.V. All rights reserved.

Rare earth centers properties and electron trapping in SnO2 thin films produced by sol-gel route

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Photoluminescence of Eu(3+) ion in SnO(2) obtained by sol-gel

Photoluminescence data of Eu-doped SnO(2) xerogels are presented, yielding information on the symmetry of Eu(3+) luminescent centers, which can be related to their location in the matrix: at lattice sites, substituting to Sn(4+), or segregated at particles surface. Influence of doping concentration and/or particle size on the photoluminescence spectra obtained by energy transfer from the matrix to Eu(3+) sites is investigated. Results show that a better efficiency in the energy transfer processes is obtained for high symmetry Eu(3+) sites and low doping levels. Emission intensity from (5)D(0) -> (7)F(1) transition increases as the temperature is raised from 10 to 240 K, under excitation at 266 nm laser line, because in this transition the multiphonon emission becomes significant only above 240 K. As an extension of this result, we predict high effectiveness for room temperature operation of Eu-based optical communication devices. X-ray diffraction data show that the impurity excess inhibits particle growth, which may influence the asymmetry ratio of luminescence spectra.

Photocatalytic degradation of methylene blue by TiO2-Cu thin films: Theoretical and experimental study

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

Planar waveguides based on nanocrystalline and Er3+ doped SnO2

Luminescent SnO2: x%mol Er3+ (x=0.1-2.0) thin films have been spin coated on borosilicate and silica substrates from water colloidal suspensions that could be prepared containing up to 40% in weight SnO2 nanocrystalline powders. High Resolution Transmission Electron Microscopy results show the well known SnO2 cassiterite structure and nanocrystallites around 10 nm in diameter, corroborating results from X-ray diffraction. Mono and multi layers have been prepared from the stable colloidal suspensions and films thickness was observed to increase linearly, up to 200 nm, with the colloidal suspensions nanoparticles amount. Excitation and emission spectra have been measured and Er3+ ions were found to be essentially incorporated into the cassiterite structure, substituting for Sn4+, for doping concentration lower than 0.05 mol%. Er3+ ions also appear segregated at the grains surface for higher doping concentration. The optical parameters (refractive index, thickness and propagating modes) of a waveguide sample were measured at 632.8 and 543.4 nm by the prism coupling technique. A monomodal waveguide was obtained with attenuation loss of 3.5 dB/cm along a 2.5 cm optical path.

Sintering of ZnO doped SnO2

Dense SnO2 ceramics were obtained by doping with ZnO concentrations varies from 0.5 to 5.0 mel. The obtained powder was isostaticaly pressed to 210MPa in cylindrical shape and sintered from ambient to 1.500 degrees C using constant heating rate of 10 degrees C/min. Densities above 97% were obtained depending on the ZnO doping concentration. A maximum density was reached with the addition of 2 mol%. It can be concluded from shrinkage data and the observed microstructure obtained by SEM that the solid solution limit for ZnO in SnO2 is about 1.0 mol%. Above this concentration the formation of a second ceramic phase is observed.

Structure and electrical properties of potassium-doped siloxane-poly(oxypropylene) ormolytes

The structure and the ionic conduction properties of siloxane-poly(oxypropylene) (PPO) hybrids doped with different potassium salts (KCF3SO3, KI, KClO4 and KNO2) are reported for two polymer molecular weights (300 and 4000 g/mol), labelled PPO300 and PPO4000, respectively. The doping concentration, related to the concentration of the ether type oxygen of the PPO chain, is the same whatever the salt and verifies [O]/[K] = 20. Ionic room temperature conductivity shows the highest value for the KCF3SO3 doped PPO4000 hybrid (4 x 10(-7)Omega(-1).cm(-1)). The structure of these hybrids was investigated by X-ray powder diffraction (XRPD) and X-ray absorption spectroscopy (EXAFS and XANES) at the potassium K-edge (3607 eV). XRPD results show that the hybrid matrix is always amorphous and the formation of secondary potassium phases is observed for all the samples, except for the KCF3SO3 doped PPO4000 hybrid. EXAFS results evidence a good correlation between the ionic conductivity and the presence of oxygen atoms as first neighbours around potassium.

Rare Earth Doped SnO(2) Nanoscaled Powders and Coatings: Enhanced Photoluminescence in Water and Waveguiding Properties

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

Infrared-to-visible frequency upconversion in transparent glass ceramics containing trivalent-rare-earth-doped nanocrystals

In this work we report on visible upconversion emission in Er 3+-, and Ho3+-doped PbGeO3-PbF 2-CdF2-based transparent glass ceramics under 980 nm infrared excitation. In erbium-doped vitroceramic samples, blue(410 ran), green(530, and 550 nm) and red(660 nm) emission signals were generated, which were identified as due to the 2H9/2, 2H 11/2, 4S3/2, and 4F9/2 transitions to the 4I15/2 ground-state, respectively. Intense red(650 nm) upconversion emission corresponding to the 5F5 - 5I8 transition and very small blue(490 nm) and green(540 nm) signals assigned to the 5F 2,3 - 5I8 and 4S2, 5F4 - 5I8 transitions, respectively, were observed in the holmium-doped samples. The 540 nm is the dominant upconversion signal in Ho3+-doped vitroceramics under 850 nm excitation. The dependence of the upconversion processes upon pump power and doping concentration are also investigated, and the main routes for the upconversion excitation processes are also identified. The comparison of the upconversion process in transparent glass ceramics and the precursor glass was also examined and the results revealed that the former present higher upconversion efficiencies.

Correlation between local structure and ionic conductivity of potassium-doped siloxane poly(propyleneoxyde) ormolytes

The local order around K for K(CF3SO3) doped Siloxane-Poly(propyleneoxide) hybrids at different doping concentration was investigated by x-ray absorption spectroscopy (EXAFS and XANES) at the potassium K-edge. The results indicate that the use of HCl as hydrolytic catalyst for gelation induces the precipitation of KCl. The ionic conductivity is strongly related to the presence of oxygen atoms as first neighbours around potassium and to the amount of KCl precipitate. © Physica Scripta 2005.

Decay of photo-induced conductivity in Sb-doped SnO2 thin films, using monochromatic light of about bandgap energy

Doping tin dioxide (SnO2) with pentavalent Sb5+ ions leads to an enhancement in the electrical conductivity of this material, because Sb5+ substitutes Sn4+ in the matrix, promoting an electronic density increase in the conduction band, due to the donor-like nature of the doping atom. Results of computational simulation, based on the Density Functional Theory (DFT), of SnO2:4%Sb and SnO2:8%Sb show that the bandgap magnitude is strongly affected by the doping concentration, because the energy value found for 4 at%Sb and 8 at%Sb was 3.27 eV and 3.13 eV, respectively, whereas the well known value for undoped SnO2 is about 3.6 eV. Sb-doped SnO2 thin films were obtained by the sol-gel-dip-coating technique. The samples were submitted to excitation with below theoretical bandgap light (450 nm), as well as above bandgap light (266 nm) at low temperature, and a temperature-dependent increase in the conductivity is observed. Besides, an unusual temperature and time dependent decay when the illumination is removed is also observed, where the decay time is slower for higher temperatures. This decay is modeled by considering thermally activated cross section of trapping centers, and the hypothesis of grain boundary scattering as the dominant mechanism for electronic mobility. © 2012 Elsevier B.V. All rights reserved.

Caracterização de filmes finos de ZnO dopados com Al e Mn depositados em substrato vítreo pelo método de Spray Pirólise

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

A theoretical analysis of Sb5+ incorporation in highly doped SnO2 matrix

We have used the periodic quantum-mechanical method with density functional theory at the B3LYP hybrid functional level in order to study the doping of SnO2 with pentavalent Sb5+. The 72-atom 2x3x2 supercell SnO2 (Sn24O48) was employed in the calculations. For the SnO2:4%Sb , one atom of Sn was replaced by one Sb atom. For the SnO2:8%Sb, two atoms of Sn were replaced by two Sb atoms. The Sb doping leads to an enhancement in the electrical conductivity of this material, because these ions substitute Sn4+ in the SnO2 matrix, leading to an electronic density rise in the conduction band, due to the donor-like behavior of the doping atom. This result shows that the bandgap magnitude depends on the doping concentration, because the energy value found for SnO2:4%Sb was 2.8eV whereas for SnO2:8%Sb it was 2.7eV. It was also verified that the difference between the Fermi level and the bottom of the conduction band is directly related to the doping concentration. - See more at: http://www.eurekaselect.com/117255/article#sthash.Z5ezhCQD.dpuf

Theoretical luminescence spectra in p-type superlattices based on InGaAsN

In this work, we present a theoretical photoluminescence (PL) for p-doped GaAs/InGaAsN nanostructures arrays. We apply a self-consistent method in the framework of the effective mass theory. Solving a full 8 x 8 Kane's Hamiltonian, generalized to treat different materials in conjunction with the Poisson equation, we calculate the optical properties of these systems. The trends in the calculated PL spectra, due to many-body effects within the quasi-two-dimensional hole gas, are analyzed as a function of the acceptor doping concentration and the well width. Effects of temperature in the PL spectra are also investigated. This is the first attempt to show theoretical luminescence spectra for GaAs/InGaAsN nanostructures and can be used as a guide for the design of nanostructured devices such as optoelectronic devices, solar cells, and others.

Behavior of triple-gate Bulk FinFETs with and without DTMOS operation

In this paper, the combination of the Dynamic Threshold (DT) voltage technique with a non-planar structure is experimentally studied in triple-gate FinFETs. The drain current, transconductance, resistance, threshold voltage, subthreshold swing and Drain Induced Barrier Lowering (DIBL) will be analyzed in the DT mode and the standard biasing configuration. Moreover, for the first time, the important figures of merit for the analog performance such as transconductance-over-drain current, output conductance. Early voltage and intrinsic voltage gain will be studied experimentally and through three-dimensional (3-D) numerical simulations for different channel doping concentrations in triple-gate DTMOS FinFETs. The results indicate that the DTMOS FinFETs always yield superior characteristic; and larger transistor efficiency. In addition, DTMOS devices with a high channel doping concentration exhibit much better analog performance compared to the normal operation mode, which is desirable for high performance low-power/low-voltage applications. (C) 2011 Elsevier Ltd. All rights reserved.