Photoluminescence in disordered Zn2TiO4

In this work, the polymeric precursor method Was used to obtain disordered Zn2TiO4 powders, either Undoped or doped with Sn4+, Cr3+ and V5+, to be applied its photoluininescent material. The characterization was undertaken by means of thermal analysis (TG and DTA), X-ray diffraction (XRD), infrared spectroscopy (IR) and photoluminescence (PL). Previous works stated that titanate octahedra containing a short Ti-O distance show efficient luminescence at roorn temperature if these octahedra are isolated from each other. In the present work, the phenomenon was observed in condensed octahedra, sharing edges. The room temperature PL noticed in undoped Zn2TiO4 had its intensity increased by the dopant addition-the increase was of about 300% for V5+ doping 400% for Cr3+ and 800% for Sn4+. (c) 2005 Elsevier B.V. All rights reserved.

Photoluminescence temperature dependence of doped parabolic quantum wells

Parabolic quantum wells (PQWs) have been studied by temperature dependent photoluminescence (PL). Two kind of samples have been studied. Concerning the undoped sample, the dominant luminescences were the bulk GaAs and the fundamental transition of the PQW. The evolution on temperature of the energy position of both PL emissions follows the well known Varshing formula. For the doped samples strong radiative recombination of the electron gas with photogenerated holes was observed. At low temperature strong Fermi level enhancement occurs in the luminescence as a result of the multi-electron-hole scattering, which is smear out increasing the temperature.

Synthesis and luminescence properties of water dispersible Eu3+-doped boehmite nanoparticles

Nanocrystallized boehmite gamma-AlOOH center dot nH(2)O had been synthesized by spray-drying (SD) of a solution of aluminium tri-sec-butoxide peptized by nitric acid. The sub-micronic spherical particles obtained had an average diameter of 500 nm and were built of 100 nm or less platelet-like sub-particles. The average crystallite size calculated from XRD was 1.6 nm following the b axis (i.e. one unit cell) and 3-4 nm perpendicular to b. As a result of the nanometric sizes of crystallites, there was a large surface free for water adsorption and it was found to be n = 1.18 +/- 0.24H(2)O per AlOOH. The SD spheres spontaneously dispersed in water at room temperature and formed stable-over months-suspensions with nanometre-size particles (25-85 nm). Luminescent europium-doped nanocrystallized boehmites AlOOH: Eu (Al0.98Eu0.02OOH center dot nH(2)O) were synthesized the same way by SD and demonstrated the same crystallization properties and morphologies as the undoped powders. It is inferred from the Eu3+ luminescence spectroscopy that partly hydrated europium species are immobilized on the boehmite nanocrystals where they are directly bonded to alpha(OH) groups of the AlOOH surface. The europium coordination is schematically written [Eu3+(OH)(alpha)(H2O)(7-alpha/2)]. The europium-doped boehmite from SD spontaneously dispersed in water: the luminescence spectroscopy proves that most of the Eu3+ ions were detached from the NPs during water dispersion. The AlOOH: Eu nanoparticles were modified by the amine acid asparagine (ASN). The modification aimed to render the NPs compatible for further bio-functionalization. After surface modification, the NPs easily dispersed in water; the luminescence spectra after dispersion prove that the Eu3+ ions were held at the boehmite surface.

Drude's model calculation rule on electrical transport in Sb-doped SnO2 thin films, deposited via sol-gel

The evaluation of free carrier concentration based on Drude's theory can be performed by the use of optical transmittance in the range 800-2000 nm (near infrared) for Sb-doped SnO2 thin films. In this article, we estimate the free carrier concentration for these films, which are deposited via sol-gel dip-coating. At approximately 900 mn, there is a separation among transmittance curves of doped and undoped samples. The plasma resonance phenomena approach leads to free carrier concentration of about 5 x 1020 cm(-3). The increase in the Sb concentration increases the film conductivity; however, the magnitude of measured resistivity is still very high. The only way to combine such a high free carrier concentration with a rather low conductivity is to have a very low mobility. It becomes possible when the crystallite dimensions are taken into account. We obtain grains with 5 nm of average size by estimating the grain size from X-ray diffraction data, and by using line broadening in the diffraction pattern. The low conductivity is due to very intense scattering at the grain boundary, which is created by the presence of a large amount of nanoscopic crystallites. Such a result is in accordance with X-ray photoemission spectroscopy data that pointed to Sb incorporation proportional to the free electron concentration, evaluated according to Drude's model. (c) 2006 Elsevier Ltd. All rights reserved.

Poole-Frenkel effect in Er doped SnO2 thin films deposited by sol-gel-dip-coating

Electrical properties of Er-doped SnO2 thin films obtained by sol-gel-dip-coating technique were measured. When compared to undoped tin dioxide, rare-earth doped films present much higher resistivity, indicating that Er3+ presents an acceptor-like character into the matrix, which leads to a high degree of electric charge compensation. Current-voltage characteristics, measured above room temperature for Er-doped films, lead to non-linear behavior and two conduction regimes. In the lower electric field range the conduction is dominated by Schottky emission over the grain boundary potential barrier, which presents an average value of 0.85 eV. Increasing the applied bias, a second regime of conduction is observed, since the Poole-Frenkel coulombic barrier lowering becomes a significant effect. The obtained activation energy for ionization is 0.67 eV. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Charge-orbital ordering and phase separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions

The main properties of realistic models for manganites are studied using analytic mean-field approximations and computational numerical methods, focusing on the two-orbital model with electrons interacting through Jahn-Teller (JT) phonons and/or Coulombic repulsions. Analyzing the model including both interactions by the combination of the mean-field approximation and the exact diagonalization method, it is argued that the spin-charge-orbital structure in the insulating phase of the purely JT-phononic model with a large Hund couphng J(H) is not qualitatively changed by the inclusion of the Coulomb interactions. As an important application of the present mean-held approximation, the CE-type antiferromagnetic state, the charge-stacked structure along the z axis, and (3x(2) - r(2))/(3y(2) - r(2))-type orbital ordering are successfully reproduced based on the JT-phononic model with large JH for the half-doped manganite, in agreement with recent Monte Carlo simulation results. Topological arguments and the relevance of the Heisenberg exchange among localized t(2g) spins explains why the inclusion of the nearest-neighbor Coulomb interaction does not destroy the charge stacking structure. It is also verified that the phase-separation tendency is observed both in purely JT-phononic (large JH) and purely Coulombic models in the vicinity of the hole undoped region, as long as realistic hopping matrices are used. This highlights the qualitative similarities of both approaches and the relevance of mixed-phase tendencies in the context of manganites. In addition, the rich and complex phase diagram of the two-orbital Coulombic model in one dimension is presented. Our results provide robust evidence that Coulombic and JT-phononic approaches to manganites are not qualitatively different ways to carry out theoretical calculations, but they share a variety of common features.

Thermal decomposition and rehydration of strontium oxalate: morphological evolution

Undoped and Eu3+ doped monohydrate strontium oxalate samples were precipitated under ultrasound and conventional stirring and were heated at different temperatures. All samples were characterized by X-ray powder diffraction (XDR), infrared spectra (IR) and scanning electron microscopy (SEM). Monohydrate, dehydrate oxalates and carbonate particles are ellipsoids indicating a topotatic process. Particle size decrease is observed when ultrasound stirring and europium. doping are used and rehydration of strontium oxalate results in uniform hexagonal particle shape. An oxide and carbonate mixture is obtained from oxalates treated at 1050 degreesC and its suspension in water undergoes incomplete hydrolysis. The products from this incomplete hydrolysis present dendrite shape particles only when the former is precipitated under ultrasound stirring. In this process, surface energy is important for particle dispersion and ultrasound supplies activation energy to oxalate precursor. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Electronic structure of Pb1-xLaxTiO3 ferroelectric materials from Ti 2p and O 1s soft x-ray absorption spectroscopy

The electronic structure of Pb1-xLaxTiO3 (PLT) compounds for x ranging from 0 to 30 at. % of La is investigated by means of soft x-ray absorption near edge structure (XANES) at the Ti L-3,L-2 and O K edges. The greatest modification in the structure of the Ti 2p XANES spectra of the PLT compounds is observed in the region of the high energy peak of the L-3 edge (e(g) states), which exhibits a splitting in the undoped sample. As the amount of lanthanum increases, this splitting becomes less pronounced. This modification is interpreted as a decrease in the degree of disorder of titanium atoms, which is correlated to the substitution of Pb by La atoms. The structural changes observed at the low energy peaks of the O K-edge XANES spectra of the PLT compounds may be interpreted in terms of hybridization between O 2p, Ti 3d, and Pb 6p orbitals. A decrease in the degree of hybridization observed as Pb atoms are replaced by La atoms may be related to the differences in the ferroelectric properties observed between x=0.0 and x=0.30 compounds. (c) 2006 American Institute of Physics.

A new method to control particle size and particle size distribution of SnO2 nanoparticles for gas sensor applications

The controlled growth of SnO2 nanoparticles for gas sensor applications is reported by these authors. Nb2O5 additive is used to control nucleation and growth of the SnO2 (see Figure), which is synthesized by the polymeric precursor method. Preliminary gas sensing measurements are performed and it is demonstrated that the response time of the Nb2O5-doped SnO2 is faster than that of the undoped material.

Electron trapping of laser-induced carriers in Er-doped SnO2 thin films

In order to investigate optically excited electronic transport in Er-doped SnO2, thin films are excited with the fourth harmonic of an Nd:YAG laser (266nm) at low temperature, yielding conductivity decay when the illumination is removed. Inspection of these electrical characteristics aims knowledge for electroluminescent devices operation. Based on a proposed model where trapping defects present thermally activated cross section, the capture barrier is evaluated as 140, 108, 100 and 148 meV for doped SnO2, thin films with 0.0, 0.05, 0. 10 and 4.0 at% of Er, respectively. The undoped film has vacancy levels as dominating, whereas for doped films. there are two distinct trapping centers: Er3+ substitutional at Sn lattice sites and Er3+ located at grain boundary. (C) 2007 Elsevier Ltd. All rights reserved.

The influence of cation segregation on the methanol decomposition on nanostructured SnO2

Here we describe a new route to synthesize ultrafine rare earth doped and undoped tin oxide particles for catalytic applications. The catalytic behavior observed in SnO2 samples suggests the control of the catalytic activity and the selectivity of the products by the segregation of a layer of a rare earth compound with the increase of the heat-treatment temperature. The ultrafine particles were characterized by means of BET, XPS, TEM, XRD and Rietveld refinement. It was demonstrated that the effects of the dopant on the methanol decomposition reaction and on the H-2 selectivity were correlated with the segregation of a rare earth layer on the tin oxide samples. (C) 2002 Published by Elsevier B.V. B.V.

Increasing the dielectric breakdown strength of poly(ethylene terephthalate) films using a coated polyaniline layer

The dielectric strength of films made from poly(ethylene terephthalate) (PET) coated with a thin layer of polyaniline (PANI) was studied. The PANI layer was deposited on the PET films by the 'in situ' chemical polymerization method. The PANI layer of the PANI/PET films was undoped in NH4OH 0.1 M solution and re-doped with aqueous HCl solution under different pH values varying from 1 to 10. Electric breakdown measurements were performed by applying a voltage ramp and the results showed a dependence of the dielectric strength on the pH of the doping solution due to the changes in the electrical conductivity of the PANI layer. The dielectric strength of PET/PANI films treated under higher pH conditions showed an electric strength about 30% larger than the PET films, since it leads to a non-conductive PANI layer.

Nanocrystalline Ga1-xMnxN films grown by reactive sputtering

The growth of nanocrystalline Ga1-zMnxN (0.00 <= x <= 0.18) films grown by reactive RF-magnetron sputtering is focused here for the first time. The films were grown in a N-2 atmosphere by co-sputtering technique using a Ga target covered with small pieces of Mn onto c-GaAs (10 0), c-Si (10 0) and amorphous SiO2 substrates maintained at 500 K. Scanning electron microscopy and X-ray diffraction (XRD) experiments did not show any evidence for Mn segregation within the studied composition range. EDX measurements show that the Mn concentration is increased monotonically with the fraction of the target area covered by Mn. The XRD characterization show that the films are nanocrystalline, the crystallites having mean grain sizes in the 15-19 nm range and wurtzite structure with preferential growth orientation along the c-axis direction. The lattice parameters of alpha-GaN (a and c) increase practically linearly with the increase of Mn incorporation. The changes in the structural properties of our films due to the Mn incorporation are similar to those that occur in ferromagnetic GaMnN single-crystal films. (c) 2006 Elsevier B.V. All rights reserved.

Influence of the network former on the properties of magnesium spinels

Undoped and/or doped with 1 mol% of Co2+ Mg2TiO4 and Mg2SnO4 powders were synthesized by the polymeric precursor method. The influence of the network former (Sn4+ or Ti4+) on the thermal, structural and optical properties was investigated. The recorded mass losses are due to the escape of water and adsorbed gases and to the elimination of the organic matter. Mg2TiO4 crystallizes at lower temperatures and also presents more ordered structure with a smaller unit call and having more intense green color than Mg2SnO4 has.

Luminescence of europium(III) and manganese(II) in barium and zinc orthosilicate

The aim of this work is to report on the luminescence properties of BaZnSiO4 activated by Eu3+ and Mn2+ ions. Doped and undoped powder samples were prepared by solid-state reaction starting from oxides and carbonates or Ba2SiO4:Eu3+ and Zn2SiO4:Mn2+ precursors. X-ray diffraction powder data, IR vibrational, and UV-vis luminescence spectroscopies were carried out. Results showed that doped and undoped samples from both types of precursors have the same structure and crystallize with a superstructure of hexagonal kalsilite. Vibrational spectroscopy has confirmed the formation of a silicate group, which outlines differences between products and silicate precursors. The observed luminescence assigned to Eu3+ and Mn2+ transitions covered most parts of the visible spectrum, an important requirement for phosphors in fluorescent low-pressure mercury vapor lamps.