Publications

Cobalt-doped vanadium oxide thin layers prepared by pulsed laser ablation are investigated by (i) X-ray photoelectron spectroscopy, (ii) the local atomic order by X-ray absorption near-edge structure (XANES), (iii) the morphology of the films was investigated by atomic force microscopy (AFM), and (iv) magnetic properties were quantified by magneto-optical Kerr effect (MOKE). In most cases, the chemical composition of the host matrix was found to be the vanadium (5+) oxide V2O5 at the sample surface and lower ionization states (+2 and +3) in the bulk. Co ions are found either in high ionization state Co5+ (for samples synthesized in high vacuum condition, denoted by VO1), or with Co in lower ionization states Co4+ (for samples synthesized in a mixture of argon and oxygen atmosphere, denoted by VO2). Consistent information was obtained from chemical shifts from individual core level scans in XPS, compared with existent data in litterature, and the amplitude of the pre-edge peak in XANES, which is a sign of quadrupole 1s -> 3d dipole-forbidden transition and whose amplitude is proportional to the number of 3d vacancies per atom. AFM revealed big particles with sizes > 100 mu m for VO1 samples, whereas smaller nanoparticles with sizes ranging between 20 and 30 mu m were observed for VO2 samples. VO1 samples presented very high coercitive fields with a relatively low saturation magnetisation at room temperature, whereas VO2 samples presented double-loop hysteresis curves, indicating the coexistence of exchange bias between two kinds of magnetic moieties with strong anisotropy.

An important tool for the development of devices (detectors, cell solar, electronic circuit components) for high energy accelerator facilities or for space utilization, where new missions and experiments will be operated, is to find new materials with harder radiation properties. The radiation fields in these environments are extremely complex and the tests of the behaviour of different materials and devices for concrete situations are difficult to realise and very expensive. Thus, scaling of degradation effects would represent a useful tool and it is the main aim of the present contribution. Some analytical expressions for NIEL that suggest possible scaling formula are given.

The paper presents the modeling of trap discharging processes in Multiple Quantum Well nanostructures. The coupling between trapping and detrapping phenomena, due to the CaF2 buffer layers is discussed. The relative role of tunneling and displacement currents is also analyzed. The model allows the determination of trap parameters that are not directly measurable. The results are in good agreement with the experimental data.

Fe clusters have been synthesised in ultrahigh-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.

The fullerenated poly vinyl chloride was synthesized by the addition reaction of azido-substituted poly vinyl chloride with C-60. Structural characterization carried out using Raman and FTIR spectroscopy confirm the presence Of C-60 chemically bound in the new compound. Also, the photoluminescence properties of fullerenated poly vinyl chloride are considerably changed from those Of C-60.

Optical properties of amorphous Strontium Barium Niobate (a-SBN) thin film are investigated using spectroscopic ellipsometry. Since the SBN can be applied to optoelectronic devices, the dispersion of refractive index is desirable. The films are obtained by Pulsed Laser Deposition (PLD) onto MgO and Si substrates by targeting a SBN:60 monocrystal: The dielectric function of a-SBN is approximated using a single Tauc-Lorentz oscillator model. Thicknesses of the films and of their rough layer are in agreement with SEM and AFM results. The dispersion of the refractive index is presented in the 1 - 5 eV range.

Design, simulation fabrication and measurement of fractal antennas for wireless communications are presented in this paper Two types of geometries, Le. Sierpinski gasket and Koch curve, were investigated The proposed antennas are compact, can operate for multiple frequency bands and exhibit small mutual coupling.

The paper presents a few examples relevant for the application of radiofrequency plasmas for processing of materials and surfaces. The examples have been chosen in order to match the trends in plasma processing science, namely processing at atmospheric pressure, growth of nanostructured materials and design of filtration systems responsive to external stimuli.

This paper deals with preparation of PEM, based on commercial block copolymer of the styrene-butadiene. The copolymer was structurally changed by sulfonation followed by cross linking, in order to design a Proton Exchange Membrane for Fuel Cells. The membranes were structural tested by FTIR Spectroscopy and Scanning Electron Microscopy. Ionic Exchange Capacity (IEC) and thermal behavior by Differential Scanning Calorimetry (DSC) were measured too.

A high quality PZT type material was designed and produced in order to be used as sensor for a high internsity siren. Two disks from this materials were assembled into a parallel bimorph type transducer with an intermediate metallic plate. This assembly behaves like an acoustic resonator in the high frequency audio range. The fundamental of the flexural mode for a free resonator is taken into account Its frequency dependence on the geometry of the system and on its material properties is computed using the results for a circular plate with clamped edges and an observation on the equivalence between the movement of a particular part of a resonator in a stationary state and the movement of that pari separated from the resonator but with the same border conditions. Acoustical and impedance measurements were made in order to appreciate the influence of the metallic plate on the frequency. In order to increase the acoustic emission a frontal cardboard horn was added to the resonator.