Materials Science Forum Vols. 636-637

Abstract: With the study of composite materials based on the In2O3-SnO2 pair, we look for better sensitivity and selectivity to the gases, than those of the sensors made out of only one of those metal oxides. This would be due to the fact that some of the interstitial positions that were initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if the single covalent/ionic adsorption is decisive in the observed changes in the materials conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity and selectivity. We will present the results obtained for a film obtained by sol-gel method and deposited using a slow spin coating process, of the In2O3-SnO2 pair, with a 2:3 mol ratio of Indium and Tin, respectively. The rather involved behaviour of our sample is understood by measuring their complex impedance subjected to an external sinusoidal varying electric field, which is being applied in the presence of different relative humidities, at various working temperatures. The main goal of this work here described is the study of the relative humidity influence on the sensing properties of the composite thick film.

Abstract: The actual interest on polymer light emitting diodes (PLEDs) is based on the fact that they are easy to process, which reduces the cost of fabrication and thus opening a new branch in the electronic market – the low-cost electronics. However, these devices present a limited efficiency compared to their inorganic counterparts mainly due to the unbalanced charge injection, which reduces the fluorescence emission. One of the first strategies to improve PLEDs efficiency was using a bilayer structure composed by two polymers to improve charge injection and transport, and at the same time tune charge recombination zone to reduce the effect of the electrodes on exciton quenching. Although this is a very ingenious device architecture some of these bilayer devices showed a lower efficiency than it was expected. The reason for that is attributed to the dissolution of the first polymer layer by the solvent used for the deposition of the second polymer layer, which do not allow to create a define polymer/polymer interface. Although cross-linking the first polymer layer can solve this problem, there is not a clear understanding why the presence of a graded interface between both polymer layers can lead to a change on PLED efficiency. In order to clarify the effect of a graded polymer/polymer interface as compared to a sharp one on the functioning of a PLED, we performed computer experiments using a mesoscopic model of a bilayer PLED developed by us that considers the morphology of both polymers at nanoscale and their properties at molecular scale. The results present in this work show clearly a significant change on the charge recombination profile within the polymer device depending on the type of interface formed between the two polymers, which can be a plausible explanation for the loss of efficiency in the bilayer 7-CN-PPV/PPV LED.

Abstract: Semiconductor polymers are successfully implemented in a broad range of applications such as light emitting diodes, field effect transistors and photovoltaic devices. Most of the achievements reached in the development of these devices were obtained at experimental level, being difficult to identify individually the influence of each factor that limits and controls these devices efficiency. One of the factors that strongly influence the performance of polymer-based devices is the presence of chemical defects in the polymer strands that change their molecular properties. As a result, these polymer strands can work like traps or deep energetic states for charge transport, leading, for instance, to a decrease on charge mobility. At experimental level it is a difficult task to isolate the influence of each type of chemical defects individually on the molecular properties of the polymer strands. It is in this context that theoretical modelling seems to be the most suitable approach to get a deep understanding of the influence of chemical defects on the molecular properties of semiconductor polymers. By performing quantum molecular dynamics calculations we study the influence of chemical defects on the molecular properties of poly(para-phenylenevinylene) (PPV). Our results show clearly a significant difference on the electronic properties of defective polymer strands (e.g. charge carrier localization, ionization potential, electron affinity and electric-field threshold for charge carrier mobility along the polymer backbone) as compared with defect-free strands.

Abstract: This work investigates the influence of initial compound synthesis prehistory on the phase sequence during formation of single-phase Sr2FeMoO6 (SFMO). Analytical-grade SrCO3, Fe2O3 and MoO3 (sample No.1) and partially reduced precursors of SrFeO3-x (SFO) and SrMoO4-y (SMO) (sample No.2) were used as initial reagents. In the latter case, kinetic limitations of SFMO phase formation are resolved by increasing the diffusivity of both of Fe3+ and Mo5+ and decreasing diffusion lengths to the reaction zone. This enhances the double-perovskite growth rate, lowers synthesis temperature and increases the intensity of X-ray reflections of (011) and (013) planes suggesting a superstructural ordering of Fe3+ and Mo5+ cations. Samples No.1 and No.2 have both a Тс ~ 420K while the magnetization value at 77 K in the sample No.2 is higher by a factor 2.3 compared to that of sample No.1. A decrease of the oxygen vacancy concentration by annealing Sr2FeMoO5.82 lowered magnetization of the samples and promoted the formation of a second magnetic phase with Тс = 700 К. We suppose that an increase of oxygen partial pressure during annealing causes formation of clusters with antiferromagnetic coupling in Fe3+-О2--Fe3+ chains. In order to increase the magnetoresistive effect at temperatures relevant for technical application, weak intergrain bonds should be formed.

Abstract: UV emitting luminescent materials are of strong interest for UV emitting fluorescent lamps driven by a Hg low-pressure or a by Xe excimer discharge. Pr3+ doped host lattices exhibit efficient UV emission upon deep UV excitation, if the site of the host lattice, where Pr3+ is located, is suitable for this purpose. This work deals with Pr3+ activated VUV ceramic luminophores, i.e. materials, which show efficient luminescence upon 160 nm excitation. As host lattices for the potentially UV emitting Pr3+ ion aluminates and silicates have been studied. All samples were prepared by conventional mix and fire synthesis techniques with metal oxides as starting materials. Firstly, powder samples were annealed between 1000 and 1700 °C and secondly, ceramic samples were repeatedly thermally treated at similar temperatures after pressing. It is demonstrated that translucent ceramics show mainly UV luminescence upon deep UV excitation, while excitation by a blue 450 nm LED results in green to red luminescence with a similar decay time as observed for single crystals.

Abstract: Ferroelectric transparent variant ceramics can be used as a base material for photovoltaic driven piezoelectric transformers. This work reports on the preparation conditions of x/65/35 PLZT ceramics for x = 0, 2, 4, 8, 10, 16, 32 at.% La, via conventional mixed oxide method. The study gives a detailed account of the relationships between technology conditions and basic physical properties of obtained PLZT samples as potential material for transformers application.

Abstract: The multiferroic ceramic samples of Co substituted Bi0.9Nd0.1Fe1-xCoxO3 ( x = 0.00, 0.03, 0.05, 0.07, 0.09, 0.10, 0.20 and 0.30) were prepared by a rapid liquid phase sintering technique. As Co content increases, the Bi0.9Nd0.1Fe1-xCoxO3 samples have a structural transformation from rhombohedral (space group R3c) to cubic structure (space group I23). A small amount of Co substitution maintains the ferroelectric properties while high doping content deteriorates the ferroelectric behavior due to the attendant high leakage current caused by the valence fluctuation of Co ions. It is found that the remnant magnetization (Mr) is enhanced greatly from a value of 4.87  10-6 emu/g at x = 0.00 to 0.054 emu/g at x = 0.30.

Abstract: This research reports the development of high homogeneity silica glass for photonic components synthesized by the VAD (Vapor-phase Axial Deposition) method. Structural radial homogeneity of silica soot were characterized by scanning electron microscopy (SEM) and small angle X-ray scattering (SAXS). It was established a relationship with the refractive index homogeneity, n, and the birefringence of consolidated boules characterized through interferometry, optical spectrometry, and polarization spectrophotometry. By controlling the silica soot nanostructure during the deposition stage, a material with high radial homogeneity of refractive index of 1 ppm and birefringence of 2 nm/cm can be synthesized.

Abstract: Lanthanum-modified lead zirconate titanate (PLZT) ceramic materials have gained considerable attention due to their photostriction, which is the superposition of photovoltaic and piezoelectric effects. Functionally Graded Materials (FGM) implemented in construction of Piezoelectric Transformer (PT) can be used for direct converting photonic energy to electrical one by implementing photostrictive actuators with piezoelectric generator in one graded structure of piezoelectric transformer. Possible application in electronic industry needs thoroughly electrical and mechanical characterisation of this new FGM structure constructed from the PLZT/PZT material. Measurements presented in this work reveal high electrical anisotropy of this graded structure and the Impedance Spectroscopy (IS) proved to be a method capable to present such inequality in form of well separated semicircles. Additionally, dielectric measurements demonstrated that the characteristics of the graded structure are not a simply addition of starting materials parameters but are deeply influenced by a predominantly diffusion direction. Finally, additional nano-mechanical and -electrical methods are used for clarifying the complexity of the integration process of FGMs.

Abstract: CdTe nanocrystals have been synthesized in aqueous solution at 92oC under open-air conditions. During the reaction, aliquots of the samples were taken at different growth times and used to obtain their UV-Vis absorbance and photoluminescence spectra in order to estimate the nanocrystal size. The absorption peaks are located around 459 nm for 1 h, 478 nm for 2 h, 491 nm for 4 h, 532 nm for 7 h and 610 nm for 94 h of growth time. The mean nanocrystal size for these samples is 2 nm, 2.2 nm, 2.3 nm, 2.6 nm and 3.4 nm, respectively, according to the theoretical calculations of 1s1/2 – 1s3/2 excitonic transition. Finally, CdTe nanocrystals were assembled using layer-by-layer technique on glass substrates, using PDDA as cationic polyelectrolyte and negatively charged CdTe nanocrystals. The Raman spectroscopy shows that CdTe nanocrystals preserve the nanoparticle properties after being assembled.