Papers by Author: M.J.M. Gomes

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Authors: Anatoli Khodorov, M.J.M. Gomes
Abstract: Lanthanum modified lead zirconate titanate (PLZT) thin films were fabricated on indium doped tin oxide (ITO) - coated glass substrate by sol-gel method. The structure of the films was characterized with XRD and SEM. In the case of PLZT the dielectric function was modelled as a sum of Lorentzian oscillators and found by fitting the transmittance and reflectance spectra measured at normal incidence in the wavelength range of 220-2400 nm. The anomalous behaviour of dielectric function was observed below the absorption edge that was suggested to be due to formation of some defect states. The evolution of the absorption edge as well as dielectric function with film thickness was observed and discussed.
Authors: M. Pereira, T. de Lacerda-Arôso, M.J.M. Gomes, A. Mata, L.C. Alves, Philippe Colomban
Abstract: 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.
Authors: Julia T. Gonçalves, Victor I. Boev, Alexey Solovyev, Carlos J.R. Silva, M.J.M. Gomes
Abstract: Transparent hybrid organic-inorganic ureasil xerogels with embedded PbS nanoparticles were prepared by exposing the xerogels doped with lead (II) nitrate to hydrogen sulphide atmosphere. Absorbance spectrum of the nonocomposites showed well defined peaks at 560 nm and 370 nm attributed to 1s(v) - 1s(c) and 1s(v) - 2p(c) exciton transitions, respectively. After four days of thermal treatment at 50 °C under argon atmosphere the obtained materials showed enhanced photoluminescence response in the near infra-red region.
Authors: Victor I. Boev, Rute A. Sá Ferreira, Sonia S. Nobre, Carlos J.R. Silva, M.J.M. Gomes, Luís M. Liz-Marzán, Luís D. Carlos
Abstract: Photoluminescence of di-ureasil hybrids containing homogeneously dispersed Ag nanoparticles, covered with silica shells was investigated at room temperature and compared with emission of undoped hybrids. Metal-doped di-ureasils show a broad emission band in the blue spectral region similar to what has been observed in pure di-ureasils but with lower relative intensity. A broad minimum arises around 438 nm for Ag@SiO2 - containing di-ureasils which coincides with the peak of plasmon resonance band in the absorption spectrum. This behaviour can be attributed to inner filtering effect, which manifests itself as absorption by metal nanoparticles of light emitted from the matrix. Low-temperature time-resolved spectroscopy and lifetime measurements demonstrate that changes are induced in the time-scale behind the di-ureasil host emission when the nanoparticles are incorporated. In particular, the lifetime values for Ag@SiO2- doped di-ureasils are smaller than those measured for the undoped host, suggesting that energy transfer might exist between the hybrid's emitting centres (NH groups and siliceous nanodomains) and the nanoparticles.
Authors: J.P.B. Silva, S.A.S. Rodrigues, Anatoli Khodorov, J. Martín-Sánchez, M. Pereira, E. Alves, M.J.M. Gomes, Philippe Colomban
Abstract: Barium Strontium Titanate Ba0.8Sr0.2TiO3 (BST) thin films have been deposited on Pt/Ti/SiO2/Si substrates by pulsed laser deposition technique. The X-ray diffraction (XRD) shows that the films crystallize in a perovskite phase. XRD and Atomic Force Microscopy (AFM) characterization reveal that the grains are nano-sized. Rutherford Backscattering Spectrometry (RBS) analysis shows the stoichiometry of the films to be close to the stoichiometry of the target. The Raman spectroscopy shows that the films exhibit the tetragonal structure by the presence of the Raman active modes at 301 cm-1 and 729 cm-1, at room temperature. Leakage current measurements of Au/ Ba0.8Sr0.2TiO3/Pt capacitors have been done, at room temperature, to investigate the conduction mechanisms of the films. We found that there are two different conduction regions in the capacitors, namely, an ohmic behavior at low voltages and a Schottky emission mechanism at high voltages. The Schottky barrier height has been estimated to be 0.99 eV.
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