M.R.N. Soares

Phase coexistence region and dielectric properties of PZT ceramics

In PZT ceramics it is commonly observed that the tetragonal and the rhombohedral phases may coexist around the morphotropic phase boundary (MPB). Some controversy still exists concerning the causes of the real occurrence of the phase coexistence, the distribution of the coexisting phases and their chemical and structural properties. In a previous work we found a relation between the width of the coexistence region and the grain size of the ceramic that could be explained by the statistical distribution model, as long as the elementary phase volumes were considered as the ferroelectric domains inside the grain. In the present work the structural parameters of the phases and the dielectric permittivity of PZT in a compositional range covering the phase coexistence region are determined and analysed. It is observed that in both tetragonal and rhombohedral phases the permittivity increases as the lattice distortion relative to the cubic symmetry decreases. The dielectric permittivities of PZT inside the phase coexistence region were calculated considering that the phase coexistence corresponds to a statistical distribution of phases with the same composition. This model provides dielectric results consistent with the experimental ones. It was also shown that the maximum of the dielectric properties in the MPB does not result from the phase coexistence, but it is a consequence of the approach to a minimum structure distortion.

Phase coexistence in PZT ceramics

Abstract In lead zirconate titanate ceramics, the low temperature tetragonal and rhombohedral ferroelectric phases coexist for compositions near the morphotropic phase boundary (MPB), and the explanation of this fact remains a controversial issue. A model of a statistical distribution of the phases can describe the coexistence of the phases, assuming that it results from the thermal fluctuation quenching during the cooling from the paraelectric to the ferroelectric phases. In this model the width of the phase coexistence region (Δx) is related to the grain size, G, and it should be observed that Δx∝G−3. In this work we checked the applicability of this relation on PZT powers and ceramics. Chemical equilibrium is not attained in calcined powders at 900, 1000 and 1100xa0°C, even after long time heat treatments, and therefore we considered the results from sintered samples. In these samples, instead of the Δx∝G−3 predicted relation, we found that Δx∝G−3/2. To explain this relation, we assumed that the elementary volume in the model is the ferroelectric domain instead of the grain of the material, based in a parabolic relation between the domain size and the grain size. Calculation of the elementary volumes using thermodynamic data is in good agreement with the volumes of the domains. ©

Structural and optical characterization of Mg-doped GaAs nanowires grown on GaAs and Si substrates

We report an investigation on the morphological, structural, and optical properties of large size wurtzite GaAs nanowires, low doped with Mg, grown on GaAs(111)B and Si(111) substrates. A higher density of vertical nanowires was observed when grown upon GaAs(111)B. Very thin zinc-blende segments are observed along the axis of the nanowires with a slightly higher linear density being found on the nanowires grown on Si(111). Low temperature cathodoluminescence and photoluminescence measurements reveal an emission in the range 1.40–1.52 eV related with the spatial localization of the charge carriers at the interfaces of the two crystalline phases. Mg related emission is evidenced by cathodoluminescence performed on the GaAs epilayer. However, no direct evidence for a Mg related emission is found for the nanowires. The excitation power dependency on both peak energy and intensity of the photoluminescence gives a clear evidence for the type II nature of the radiative transitions. From the temperature dependence on the photoluminescence intensity, non-radiative de-excitation channels with different activation energies were found. The fact that the estimated energies for the escape of the electron are higher in the nanowires grown on Si(111) suggests the presence of wider zinc-blende segments.

Tunable green to red ZrO2:Er nanophosphors

Pulsed laser ablation in water was validated as an effective method to produce highly crystalline erbium doped ZrO2 nanoparticles. Different concentrations of erbium doped ZrO2 ceramic precursor targets were used in the ablation, to study the efficiency of erbium incorporation in the zirconia lattice during nanoparticle synthesis by this method. The spherical nanoparticles produced, with a diameter of up to 200 nm, preserve the crystallinity and optical properties of the precursor target, even for the higher dopant amounts. The optical activation of Er3+ ions was achieved without the need for any additional thermal annealing, usually required for particles produced by several chemical routes. A tunable green to red color of the ZrO2:Er3+ nanoparticles is accomplished through the manipulation of the erbium ion concentration. Particularly, through the sequential absorption of two infrared photons, intense visible up conversion luminescence was observed at room temperature highlighting the doped nanoparticles as promising alternative imaging agents.

ZnO Nano/Microstructures Grown by Laser Assisted Flow Deposition

Nano/microstructures of zinc oxide (ZnO) were grown by the laser assisted flow deposition (LAFD) method. This new process has proved to be very efficient, allowing high yield ZnO deposits at high-rate applicable to large-scale substrates. Laser local heating promotes fast ZnO decomposition and recombination under a self-catalytic vapour–liquid-solid mechanism for the nucleation and growth. Three types of ZnO morphologies were obtained according to the temperature/oxygen availability inside the growth chamber. The morphology can also be controlled adding rare-earth elements to the initial composition. Particularly, tetrapod morphology was obtained by europium oxide addition to the precursors. The structural and microstructural characterizations confirm the good crystallinity of the wurtzite structure. The photoluminescence spectroscopy revealed high optical quality of the as-grown ZnO. Specifically, the free exciton recombination and a strong near band edge recombination due to donor bound exciton transitions can be clearly recognized, although deep level emission in the green spectral region is present.

Structural and optical characterization of Gd_2SiO_5 crystalline fibres obtained by laser floating zone

Gadolinium oxyorthosilicate single crystals (Gd2(SiO4)O (GSO)) were obtained by the laser floating zone (LFZ) technique. The fibres were pulled at faster growth rates (10 mm/h) than those produced by the conventional Czochralski method and the growth was performed in air, which did not require environmental control. The structural characterization of as-grown fibres made by X-ray diffraction (XRD) and Raman spectroscopy puts in evidence the high degree of crystallinity and its monoclinic nature. The optical characterization, accomplished by photoluminescence and photoluminescence excitation, suggests that GSO fibres are a promising candidate to be doped in order to develop new high optical efficient laser materials.

Structural, optical, and electrical properties of SmNbO4

Rare-earth orthoniobates constitute a class of materials that has been exploited due to their interesting physical properties depending on the lanthanide element. Besides paramagnetism, ferroelasticity, and negative compressibility, these materials are known by their interesting optical properties and mixed types of conduction processes (protonic, ionic, and electronic). In this work, two types of SmNbO4 samples were studied: polycrystalline samples, prepared by a sol-gel route using the Pechini method, and single crystalline fibres grown by the Laser Floating Zone technique. These samples were structurally characterized based on powder and single-crystal X-ray diffraction studies. A metastable tetragonal phase, stabilized by grain size, was identified in the synthesized powders. After a sintering process of such powders, a single monoclinic phase was obtained. Complementarily, scanning electron microscopy and Raman spectroscopy analyses were performed to these samples. Photoluminescence and photoluminesc...

Prospects on laser processed wide band gap oxides optical materials

Wide band gap oxide media including 4fn or 3dn ions attracts a considerable attention in the context of photonics and bio-photonics applications due to the electromagnetic widespread spectral range covered by the intraionic radiative relaxation of the charged lanthanide and transition metal ions. Converting ultraviolet commercial light into visible luminescence continues to raise interest for the solid state light market, justifying the demand for new and efficient phosphors with wide spectrum coverage and improved thermal quenching behavior. New materials and methods have been thoroughly investigated for the desired purposes. In this work, we report on laser processing for the growth of oxides media such as ZrO2, ZnO among other oxide hosts. The transparent crystalline materials in-situ doped with different amounts of lanthanide or transition metal ions are explored in order to enhance the room temperature ions luminescence by pumping the samples with ultraviolet photons. Spectroscopic studies of the undoped and doped oxide hosts were performed using Raman spectroscopy, photoluminescence (PL) and photoluminescence excitation (PLE).

Laser assisted flow deposition: a new method to grow ZnO

In the present work, nano and micro crystals of ZnO were grown by the laser assisted flow deposition method at atmospheric pressure. In this new process, laser radiation impinges on the top of an extruded rod precursor, producing ZnO crystals by a vapour/solid (VS) mechanism. This method has proved to be very efficient, allowing high yield ZnO deposits at high growth rates applicable to largescale substrates [1]. ZnO structures were grown with and without the presence of silver under a power laser of 35 W during 3 minutes.

Fluctuating potentials in GaAs:Si nanowires: critical reduction of the influence of polytypism on the electronic structure

In this work, the effects of Si doping in GaAs nanowires (NWs) grown on GaAs (111)B by molecular beam epitaxy with different Si doping levels (nominal free carrier concentrations of 1 × 1016, 8 × 1016, 1 × 1018 and 5 × 1018 cm-3) are deeply investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), grazing incidence X-ray diffraction (GID), photoluminescence (PL) and cathadoluminescence (CL). TEM results reveal a mixture of wurtzite (WZ) and zinc-blende (ZB) segments along the NW axis independently of the Si doping levels. GID measurements suggest a slight increase of the ZB fraction with the Si doping. Low temperature PL and CL spectra exhibit sharp lines in the energy range 1.41-1.48 eV, for the samples with lower Si doping levels. However, the emission intensity increases and is accompanied by a clear broadening of the observed lines for the samples with higher Si doping levels. The staggered type-II band alignment only determines the optical properties of the lower doping levels in GaAs:Si NWs. For the higher Si doping levels, the electronic energy level structure of the NWs is determined by electrostatic fluctuating potentials intimately related to the amphoteric behavior of the Si dopant in GaAs. For the heavily doped NWs, the estimated depth of the potential wells is ∼96-117 meV. Our results reveal that the occurrence of the fluctuating potentials is not dependent on the crystalline phase and shows that the limitation imposed by the polytypism can be overcome.