M. J. M. Gomes

Effect of Nb doping on the microstructural and electrical properties of the PZT ceramics

Abstract The aim of this study is the determination of the effect of the aliovalent Nb 5+ doping in PZT (65/35) based bulk materials in their microstructural evolution and their electrical properties. The Nb content was chosen in the range 0–10 mol%. These materials were prepared using the conventional way. The calcined mixture was found to be monophasic perovskite powder. The characteristics of the as-formed powder were described and related to the microstructural ones in the sintered material, namely the grain size evolution and the porosity elimination. Nb addition was found to alter significantly the parameters of the crystal cell, and so, the dielectric and ferroelectric properties. The electrical properties were compared with those of common PZT and PLZT materials; As in the case of PLZT ( x /65/35), several Nb containing compositions show diffuse phase transition (DPT) characteristics. The Nb content is also leading to softening of the PZT. However, the ferroelectric and dielectric behaviour kept strongly dependent on the microstructural characteristics.

Competition between ferroelectric and semiconductor properties in Pb(Zr0.65Ti0.35)O3 thin films deposited by sol–gel

Asymmetric metal–ferroelectric–metal (MFM) structures were manufactured by sol–gel deposition of a lead zirconate-titanate (PZT with Zr/Ti ratio 65/35) film on Pt-coated Si, with a Au top electrode. The average remnant polarization of 9 μC/cm2 and the coercive field of 39 kV/cm were obtained from the hysteresis loop measurements. A detailed analysis of the polarization–electric field (P–E), capacitance–voltage (C–V), and current–voltage (I–V) measurement results allowed us to estimate the near-electrode space-charge region thickness (roughly half of the film thickness at zero voltage), net doping concentration (around 1018 cm−3), built-in potential (in the 0.4–0.8 V range, depending on the injecting electrode), and dynamic dielectric constant (5.2). The current logarithm–voltage dependence for the field-enhanced Schottky emission obeys a “1/4” law. The spectral distribution of the short circuit current measured under continuous light illumination in the 290–800 nm range exhibits a cutoff wavelength at 370...

Disorientation in Hypermedia Environments: Mechanisms To Support Navigation

The problem of navigation in hypertext is quite often associated with (dis)orientation. We begin this article by making a review of several mechanisms to support navigation after having done a brief presentation of the phenomena “getting lost in the hyperspace.” To simplify and clarify our explanation we have assembled the various proposals of support to navigation into the following: 1) solutions regarding the tools to support navigation, 2) solutions regarding interface and navigation metaphors, and 3) solutions regarding hardware and other approaches to deal with this problem. The analyses made of each mechanism to support navigation intend to offer different points of view about the pros and cons of their use.

Anti‐Stokes Photoluminescence in II–VI Colloidal Nanocrystals

For the first time highly efficient photon energy up-conversion (anti-Stokes photoluminescence) has been observed in colloids of CdTe nanocrystals (NCs) and CdSe NCs covered by ZnS shell. The maximum magnitude of the up-converted blue shift ΔE max UC = 335 meV was obtained for CdSe/ ZnS NCs under excitation by a low-power He-Ne laser. The maximum efficiency for energy conversion was estimated to be about η max = 2.7 x 10 -2 % at room temperature. The intensity of the anti-Stokes photoluminescence increases with temperature and shows a linear dependence on the excitation intensity. We suggest that subgap surface states are involved as intermediate states in the up-conversion process rather than nonlinear two-photon absorption or Auger processes.

Growth and characterisation of cadmium sulphide nanocrystals embedded in silicon dioxide films

Abstract In this communication we report on the growth and the optical characterisation of CdS quantum dots in the diameter range of 4–10 nm embedded in silicon dioxide glass films grown by magnetron rf-sputtering technique with post-deposition annealing. Optical transmission measurements display a marked blue shift of the absorption band edge due to the quantum confinement of the electrons and holes in the CdS nanocrystals. The mean diameter of the nanocrystals was estimated from absorption spectra using a standard theoretical model. A study of the effect of the nanocrystal size on the Raman spectra has been made from an analysis of the first order LO mode lineshape. The increase in the linewidth observed with decreasing crystal size is in agreement with the results of a calculation based on the phonon confinement model. The asymmetric lineshape is well reproduced by the model, but they fail to reproduce the large low-frequency tail. The origin of this tail is discussed considering several possible sources proposed in the literature and is under study. Room temperature infrared reflectance measurements have also been obtained showing a diffuse band at about 230–300 cm−1, which closely corresponds to CdS optical polar phonons observed in the bulk crystals. However, we are unable to fit the observed spectra by assuming just this contribution. Instead, by including an additional contribution due to CdO we are able to well simulate the experimental spectra using a model based on the Maxwell-Garnet theory.

Ancient Portuguese Ceramic Wall Tiles (“Azulejos”): Characterization of the Glaze and Ceramic Pigments

Ancient ceramic wall tiles, called “azulejo”, firstly used on Portuguese churches, monasteries and palaces (15-18th century) have progressively been used in particular houses till the last century. These tiles and its use in huge decorative panels can be considered as a precious but fragile cultural heritage from Brazil to India, in several countries influenced by Portuguese culture. Morphologically, these tiles are composed by a porous clay-based ceramic body, the terracotta, covered by a protective glassy phase, the glaze. As artistic paintings, these murals incorporated various kinds of pigments in the glaze layer to create a pictorial impact on the walls of rich palaces or churches, real and durable monumental works-of-art. In the 21st century, degradation marks are visible on these ceramic tiles because of their use under corrosive conditions (moisture, atmospheric cycles…) along centuries. In order to promote their conservation and enhance their restoration, the physical-chemical characterization of the azulejos is performed in the present work, using mainly non-destructive processes like micro-Raman spectroscopy or X-Ray diffraction. In particular, Raman spectroscopy allows the detection of some nano/microcrystals present in the amorphous glaze due to pigments or opacifying agents or related to the elaboration process of the azulejo. Based on the observation of various selected fragments, one states that very few pigments have been used as colouring agents in this ceramic art during 17-18th centuries. Thus, the relationship between the different colours, the introduced pigments and the structural aspects of the glass will be focused. Some features related with the ancient ceramic technology will also be discussed.

FIR absorption in CdSe quantum dot ensembles

Absorption of far-infrared (FIR) radiation in very small CdSe crystallites (quantum dots, QDs) via excitation of confined dipolar vibrational modes was studied, both theoretically and experimentally. Spatial quantisation of optical phonons was analysed in the framework of a continuum model, which couples the phonon amplitude to the electrostatic potential via the Frohlich interaction. The frequencies and oscillator strengths of the dipole-active optical vibration modes are presented calculated for a spherical QD, and for the first time for II-VI QDs these quantised modes were observed experimentally. Another effect studied in this work is the influence of the QD concentration on the FIR absorption of their ensemble. It is shown that the dipole-dipole interaction becomes very important in dense QD ensembles and leads to smearing of the structure in the CdSe phonon band as the QD concentration increases.

Simple model of polarization offset of graded ferroelectric structures

The abnormal polarization offset observed in case of graded ferroelectric structures is explained assuming the presence of a nonreversible part of polarization due to the imposed polarization gradient. It is shown that an upper limit for the offset should exist, fixed by the remnant polarization of the component materials considered as independent layers. This is valid for the structures in which polarization increases or decreases in steps from one layer to the other, but should also be valid for the structures with continuous gradient. However, large values of the charge offset could be achieved in the last case. The electric field dependence of the polarization offset is predicted, together with the possibility of obtaining large nonconventional pyroelectric coefficients. The simulation developed in the case of a bilayer ferroelectric structure and using data from the lead–lanthanum–zirconate–titanate system describes well the observed features of the graded ferroelectric structures.

Third-Order Optical Nonlinearities in Thin Films of CdS Nanocrystals

The non-resonant nonlinear optical properties of CdS nanocrystals (NCs) embedded in silica (i) and forming matrix-free (MF) almost close-packed films (ii) were studied using the Z-scan technique. Among the group (i), samples containing NCs covered with a HgS and CuS shell were investigated. The highest value of the nonlinear refractive index n2 1⁄4 1.85 10– 6 cm2/W was measured for the MF films. This number exceeds by far the value typical for bulk CdS and cannot be explained in terms of thermally induced nonlinearity. Strong enhancement of local fluctuations of the electro-magnetic field in MF films where NCs form aggregates or clusters is suggested to explain this result. Within the group (ii), an enhancement of the nonlinearity was observed under pulsed irradiation for both CdS/HgS and CdS/CuS nanoparticles, compared to CdS NCs without a shell.

One-phonon Raman scattering from arrays of semiconductor nano-crystals

Abstract We re-consider Raman scattering by LO-type phonons in a small semiconductor crystal and derive a simple formula taking into account the higher order confined phonon modes, to describe the asymmetric Raman lineshapes observed experimentally. For a single crystalline sphere, we explicitly calculate the relative contributions of the confined modes with different values of the radial quantum number. For arrays of such spheres, we argue that, apart from a superposition of the above contributions from individual scatterers, there exists a collective scattering by a coherent mode which involves many spheres, unless their concentration is too small. This collective scattering is determined by an effective dielectric function of the composite medium and peaks near the Frohlich frequency. We apply these principles to the analysis and modelling of Raman spectra taken from CdS doped SiO2 films.