Sinterização de filmes finos de LiNbO3 em forno microondas: estudo da influência da direção do fluxo de calor

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

Epitaxially grown LiNbO3 thin films by polymeric precursor method

LiNbO3 thin films were grown on (0001) sapphire substrates by a chemical route, using the polymeric precursor method. The overall process consists of preparing a coating solution from the Pechini process, based on metallic citrate polymerization, the precursor films, deposited by dip coating, are then heat treated to eliminate the organic material and to synthesize the phase. In this work, we studied the influence of the heat treatment on the structural and optical properties of single-layered films. Two routes were also investigated to increase the film thickness: increasing the viscosity of the coating solution and/or increasing the number of successively deposited layers. The x-ray diffraction theta -2 theta scans revealed the c-axis orientation of the single- and multilayered films and showed that efficient crystallization can be obtained at temperatures as low as 400 degreesC, the phi-scan diffraction evidenced the epitaxial growth with two in-plane variants, A microstructural study revealed that the films were crack free, homogeneous, and relatively dense. Finally, the investigation of the optical properties (optical transmittance and refractive index) confirmed the good quality of the films. These results indicate that the polymeric precursor method is a promising process to develop lithium niobate waveguides.

Study on the orientation degree of Pb1-xLaxTiO3 thin films by the rocking curve technique and its morphological aspects

Thin films of perovskite-type materials such as PbTiO3, BaTiO3, (Pb,La)TiO3, (Pb, La)(ZrTi)O-3, KNbO3, and Pb(Mg,Nb)03 have been attracting great interest for applications like non-volatile memories, ultrasonic sensors and optical devices. Thin film should be epitaxially grown or at least highly textured since the properties of this anisotropic material depend on the crystallographic orientation. For optical devices, in particular, an epitaxial thin film without defects are essential to reduce optical propagation losses. Pb1-xLaxTiO3 (PLT) where x=0, 13 and 27% thin films were prepared by a chemical method (polymeric precursors method), and deposited by the spin coating technique onto substrates of SrTiO3 (STO) and LaAlO3 (LAO). The films were then beat treated at 500 degrees C in a controlled atmosphere of 0,. The orientation degree of the thin films was obtained from rocking curve technique, by means of X-ray difftaction analysis. A microstructural study revealed that the films were crack-free, homogeneous and have low roughness. (c) 2006 Elsevier B.V. All rights reserved.

Epitaxial growth of LiNbO3 thin films in a microwave oven

Oriented LiNbO3 thin films were prepared using a polymeric precursor solution deposited on (0001) sapphire substrate by spin coating and crystallized in a microwave oven. Crystallization of the films was carried out in a domestic microwave oven. The influence of this type of heat treatment on the film orientation was analyzed by X-ray diffraction and electron channeling patterns, which revealed epitaxial growth of films crystallized at 550 and 650 degreesC for 10 min. A microstructural study indicated that the films treated at temperatures below 600 degreesC were homogeneous and dense, and the optical properties confirmed the good quality of these films. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Raman investigation of structural photoinduced irreversible changes of Ga10Ge25S65 chalcogenide glasses

The influence of time exposure, when exposed to above band gap light (3,52 eV) and annealing, on Ga10Ge25S65 glasses has been studied through their effects on the structure and optical properties. To evaluate the photostructural change infrared and Raman spectra for bulk Ga10Ge25S65 glasses have been measured before and after exposure. The Raman spectra are interpreted in terms of models in which the Ge atoms are fourfold coordinated and the S atoms are two fold coordinated. The observed changes in the spectral region of (S-S) stretching vibration (470-490 cm (-1)) is a direct evidence for the occurrence of important structural changes in local bonding configuration caused by optical irradiation. It is shown that the dominant photostrucural changes are chain formation tendency of the chalcogenide atoms under the laser irradiation rather than rings.

Electron scattering and effects of sources of light on photoconductivity of SnO2 coatings prepared by sol-gel

Electro-optical properties of sol-gel derived 2 mol% antimony or niobium doped tin dioxide films have been measured. The electron density has been calculated considering all the relevant scattering mechanisms and experimental conductivity data measured in the range -197 to 25 degrees C. The results support the hypothesis that both ionised impurity scattering and grain boundary scattering have comparable effects in the resistivity of coatings, for free electron density congruent to 5 x 10(18) cm(-3). We have measured variation of photoconductivity excitation with wavelength using xenon and deuterium lamp as light sources. Results show that the main band in the photoconductivity spectrum is dependent on the spectral light source emission, the excitation peak reaching 5 eV (deuterium lamp). This band is due to the recombination process involving oxygen species and photogenerated electron-hole pairs. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Photoluminescence of nanostructured PbTiO3 processed by high-energy mechanical milling

This letter reports on a process to prepare nanostructured PbTiO3 (PT) at room temperature with photoluminescence (PL) emission in the visible range. This process is based on the high-energy mechanical milling of ultrafine PbTiO3 powder. The results suggest that high-energy mechanical milling modifies the particle's structure, resulting in localized states in an interfacial region between the crystalline PT and the amorphous PT. These localized states are believed to be responsible for the PL obtained with short milling times. When long milling times are employed, the amorphous phase that is formed causes PL behavior. An alternative method to process nanostructured wide-band-gap semiconductors with active optical properties such as PL is described in this letter. (C) 2001 American Institute of Physics.

Multi-layered LiNbO3 films prepared by a polymeric precursor method

The polymeric precursor method was used to prepare multi-layered LiNbO3 films. The overall process consists of preparing a coating solution from the Pechini process and the deposited film is subsequently heat-treated. Two-layered films were prepared by this process, onto (0001) sapphire substrates. Two different routes were investigated for the heat-treatment. The amorphous route consisted of performing, after each deposition, a pre-treatment at low temperature to eliminate the organic material. In this case, the crystallization heat-treatment was performed only after the two layers had been deposited. on the other hand, a process layer-after-layer crystallization was used. Both routes led to (0001) LiNbO3 oriented films. However, only the film prepared by the layer-after-layer crystallization presented an epitaxial growth and a crack-free morphology. Moreover, the layer-after-layer crystallization process led to a film exhibiting the best optical properties. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

New tungstate fluorophosphate glasses

\A new class of tungstate fluorophosphate glasses was identified in the NaPO3-BaF2-WO3 ternary system. The variation of several physical properties was determined with respect to chemical composition. Characteristic temperatures, density and refractive index increase as tungsten oxide content increases. The optical transmission range and specially the energy bandgap depend of the WO3 amount. No crystallization could be observed for the Most WO3 concentrated vitreous samples (greater than or equal to20% molar). Color and optical properties of the glasses depend of the melting time because of the presence of reduced tungsten species like W5+ and W4+. In addition, photodarkening is observed in tungsten rich glass samples under UV laser illumination and this phenomenon can be reversible by heat treatment near the glass transition temperature. (C) 2004 Published by Elsevier B.V.

Theoretical and experimental study of the relation between photoluminescence and structural disorder in barium and strontium titanate thin films

Thin films of barium and strontium titanate (BST), synthesized by the polymeric precursor solution and spin coated on [Pt (140nm)/Ti (10 nM)/SiO2(1000 nm)/Si] substrates were found to be photoluminescent at room temperature when heat treated below 973 K, i.e. before their crystallization. First principles quantum mechanical techniques, based on density functional theory (DFT) were employed to study the electronic structure of two periodic models: one is standing for the crystalline BST thin film and the other one for the structurally disordered thin film. The aim is to compare the photoluminescence (PL) spectra of the crystalline and disordered thin films with their UV-vis spectra and with their computed electronic structures. The calculations show that new localized states are created inside the band gap of the crystalline model, as predicted by the UV-vis spectra. The study of the charge repartition in the structure before and after deformation of the periodic model shows that a charge gradient appears among the titanate clusters. This charge gradient, together with the new localized levels, gives favorable conditions for the trapping of holes and electrons in the structure, and thus to a radiative recombination process. Our models are not only consistent with the experimental data, they also allow to explain the relations between structural disorder and photoluminescence at room temperature. (c) 2005 Elsevier Ltd. All rights reserved.

Photoluminescent BaMoO4 nanopowders prepared by complex polymerization method (CPM)

The BaMoO4 nanopowders were prepared by the Complex Polymerization Method (CPM). The structure properties of the BaMoO4 powders were characterized by FTIR transmittance spectra, X-ray diffraction (XRD), Raman spectra, photoluminescence spectra (PL) and high-resolution scanning electron microscopy (HR-SEM). The XRD, FTIR and Raman data showed that BaMoO4 at 300 degrees C was disordered. At 400 degrees C and higher temperature, BaMoO4 crystalline scheelite-type phases could be identified, without the presence of additional phases, according to the XRD, FTIR and Raman data. The calculated average crystallite sizes, calculated by XRD, around 40 nm, showed the tendency to increase with the temperature. The crystallite sizes, obtained by HR-SEM, were around of 40-50 nm. The sample that presented the highest intensity of the red emission band was the one heat treated at 400 degrees C for 2 h, and the sample that displayed the 'highest intensity of the green emission band was the one heat treated at 700 degrees C for 2 h. The CPM was shown to be a low cost route for the production of BaMoO4 nanopowders, with the advantages of lower temperature, smaller time and reduced cost. The optical properties observed for BaMoO4 nanopowders suggested that this material is a highly promising candidate for photoluminescent applications. (C) 2005 Elsevier B.V. All rights reserved.

Low temperature annealing of amorphous gallium arsenide films

This work reports the changes in the optical properties produced by annealing of amorphous GaAs at temperatures smaller than or just sufficient to produce crystallization of the material. The films were grown by the flash evaporation technique on glass substrates at room temperature. Optical and structural changes of our samples were monitored through photothermal deflection spectroscopy, optical transmittance and reflectance and X-ray diffraction (XRD). The structural results from XRD detected no crystallization of the films for temperatures up to 240 degreesC. We have observed consistent changes in the optical gap and Urbach energy of the annealed film. The optical gap increases with increasing annealing temperature from 1.17 to 1.32 eV. The Urbach energy decrease from 120 meV (as-grown film) to 105 meV (anneal at 200 degreesC). We propose that these changes are due to a diminution of the tail state defects and/or the relaxation of strained bonds. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Totally visible transparent chloro-sulphide glasses based on Ga2S3-GeS2-CsCI

In this paper, the influence on optical properties of alkali halides such as CsCl in a covalent glassy matrix has been investigated. Chalcogenide glasses belonging to the (GeS2)-(Ga2S3)-CsCI system with high ratio of CsCl present an entire transparency in the visible range. These glasses maintain good transmission up to 12 mu m. Furthermore, the thermo-mechanical properties and the glass hygroscopicity have been investigated as function of the CsCl amount. This new generation of glasses presents a great interest for optical application. They could be used both for passive applications (multi-spectral imaging) and active applications for rare-earth doping due to their good transmission in the visible range, increasing optical pumping possibilities.

Effects of self-assembly process of latex spheres on the final topology of macroporous silica

This paper surveys the topology of macroporous silica prepared using latex templates covering the submicrometric range (0.1-0.7 mu m). The behavior of latex spheres in aqueous dispersion has been analyzed by dynamic light scattering (DLS) measurement indicating the most appropriate conditions to form well-defined cubic arrays. The optical behavior of latex spheres has been analyzed by transmittance and reflectance measurements in order to determine their diameter and filling factor when they were assembled in bidimensional arrays. Macroscopic templates have been obtained by a centrifugation process and their crystalline ordering has been confirmed by porosimetry and scanning electron microscopy. These self-assembled structures have been used to produce macroporous silica, whose final topology depends on the pore size distribution of the original template. It has been seen that latex spheres are ordered in a predominant fcc arrangement with slipping of tetragonal pores due to the action of attractive electrostatic interactions. The main effect is to change the spherical shape of voids in macroporous silica into a hexagonal configuration with possible applications to fabricate photonic devices with novel optical properties. (c) 2005 Elsevier B.V. All rights reserved.

Effects of dispersion by Gd3+ upon europium diphenylphosphinate luminescence

A series of binary diphenylphosphinates with Eu3+ and Gd3+ were prepared. The compounds have the general formula Gd1-xEux(DPP)(3), where x ranges from 0 to 1. The spectroscopic measurements show interesting behavior. The intensity of the D-5(0) --> F-7(2) transitions decreases relative to D-5(0) --> F-7(1) With an increase in europium dispersion. Inside the temporal domain, the same decrease is observed with increasing delay after excitation. The lifetimes are also affected, which can be seen in the x = 1 compound, where the decay is a first-order process and lifetime values are 4.81 ms. In the binary compound, as an effect of dispersion, the lifetime of the D-5(0) level measured at the D-5(0) --> F-7(1) transition increases with europium dispersion, and the average along the series is 6.25 ms. The decay measured at the D-5(0) --> F-7(2) transitions reveals a second-order process with lifetimes ranging from 1.90 to 6.00 ms. (C) 2001 Elsevier B.V. B.V. All rights reserved.