Publications

Very compact planar resonators were developed for the 1950 MHz frequency band. Resonators are of the stepped-impedance (SIR) type and they are folded, in order to occupy a very small surface area. On a 6.635 mm height commercial substrate with a 10.8 dielectric constant, a resonator can be fitted into a square with an edge size of 5.8 m. Small-size filters were designed by cross-coupling the compact resonators. The four-pole filters have an improved response due to the cross-couplings, which introduces transmission zeros on each side of the pass-band. The filters were manufactured and characterized. The measurement data are in good agreement with the electromagnetic field simulations. The developed filters are small-size and cost-effective and their technology is simple,. without any via holes, air-bridges or lumped elements.

Wireless communications systems require new materials with special properties in specific frequency bands. The investigations on ZST type ceramics, (Zr0.8Sn0.2)TiO4, presented in this paper, recommend this materials for applications in microwaves and millimeter waves. The ZST materials were prepared using a standard solid-state reaction technology. The samples morphology, phase-composition and microstructure investigations were performed by using the scanning electron microscopy (SEM), and energy-disperse X-ray spectrometry (EDX). The crystalline phases were identified by X-ray diffractometry (XRD). The electromagnetic properties were investigated on ZST resonators by using a Computer Aided Measurement (CAM) in microwaves, combining a HP 8757C network analyzer and a HP 8350B sweep oscillator. The dielectric characteristics at millimeter waves were analyzed by investigation of the Whispering Gallery Modes on ZST disks. The low level NiO doping provides ZST materials with temperature coefficient tau(f) in the range (-2 divided by +4) ppm/degrees C and decreases the dielectric loss. Materials with high values of the Q f product up to 50,000 and a dielectric constant about 36 at microwave frequencies were obtained. ZST dielectric resonators and substrates for hybrid integrated circuits with dimensions 1" x 1" and thickness in the range 0.6 divided by 1 mm were manufactured.

Surface-enhanced Raman scattering (SERS) was used to investigate C-60 self-assembling in solvents like pyrrolidine (Py) and Nmethyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C-60 aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C-60 and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at similar to342cm(-1). In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I-h C-60 symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800cm(-1) is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C-60 in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C-60 cage. These complexes are noticed in the SERS spectrum by the replacement of the original H-g(l) band at similar to269cm(-1) with two others at similar to255 and similar to246cm(-1). In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118cm(-1), which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C60 clusters of different size. (C) 2004 Published by Elsevier Ltd.

The structure and magnetic properties of films of iron-modified amorphous carbon (a-C:Fe) prepared by magnetron cosputtering of iron and graphite targets are studied. X-ray diffraction measurements show that iron enters the samples in the form of Fe nanocrystals that are typically about 20 nm in size and also forms nanocrystals of hexagonal iron carbide. The temperature dependences of the magnetization, measured under cooling in zero and nonzero magnetic fields, are studied. At temperatures T less than or similar to 8 K, a magnetic transition, which provides evidence for the onset of magnetic ordering in the material, is observed to occur. The magnetization isotherms obtained in the 8- to 20-K temperature range are in agreement with this observation. It is shown that a modified version of Langevin's formalism adequately describes the observed features of a-C:Fe film magnetization. (c) 2005 Pleiades Publishing, Inc.

Cubic boron nitride (c-BN) is a synthetic material which exhibits exceptional physicochemical properties such as: hardness, thermal conductivity, thermo-chemical stability, semiconducting properties and radiations resistance. Such outstanding properties make it a promising multifunctional material for applications in extreme conditions, as those found in the outer space environment. Its further use for such applications requires, however, a much better understanding of the lattice defects and radiation damage properties. Here we present the results of multifrequency ESR studies concerning the native and radiation induced point defects in crystalline c-BN under irradiation with high intensity 1MeV electron beams.

Lead zirconate-titanate (PZT) epitaxial thin films are considered wide-gap p-type semiconductors with standard Schottky contacts. Capacitance-voltage, current-voltage, as well as the dielectric hysteresis are analyzed using a model based on the conventional metal-semiconductor Schottky contact in which the effect of the ferroelectric polarization on band-bending was considered. In addition, presence of the deep traps and its effect on the measured quantities were considered and discussed. For a single-crystal like PZT films with a remnant polarization of 40 mu C/cm(2) the free carrier concentration is estimated to about 3 x 10(18) cm(-3), whereas the effective density of the fixed charges in the depletion region is of about 1.8 x 10(19) cm(-3) .

The goal of this work is to prepare doped PZT-type materials with improved radial piezoelectric properties. Ceramic materials were obtained by solid-state reaction at different sintering temperatures. SAED, bright field TEM and HRTEM methods were used for the microstructure studies. The influence of the composition and nanostructure on the dielectric and piezoelectric properties of the materials is discussed. One of the materials with high coupling constant was used in the construction of a miniature flexural ventilator. Designated to function at an emf of 220 V/50 Hz, this device is characterized by high efficiency, reliability and low energy consumption. (c) 2005 Elsevier Ltd. All rights reserved.

We review our experimental data obtained by Raman spectroscopy on single-walled carbon nanotubes (SWNTs) using surface-enhanced Raman scattering (SERS). We focused on the study of the dependence of the SERS spectra of SWNTs on both the rough metallic substrate type and the film thickness. In particular, we show that degradation of the nanotubes, mainly metallic, occurs for very thin films with the formation of graphitic, fullerene-like or amorphous carbon particles. We show also that SWNTs reacting with H2SO4 form a salt of the hydrogensulfate type similar to that resulting from graphite-H2SO4 interaction. SWNTs exhibit a complicated spectroelectrochemical behavior in aqueous and non-aqueous H2SO4 solutions that depends also on the sweep potential range. We demonstrate that the oxidation-reduction processes have a quasi-reversible or irreversible character. Finally, SERS studies on compressed SWNTs in different host matrices, inorganic and organic, reveal nanotube breaking and that tube fragments may react or not with the host matrix used. Copyright (c) 2005 John Wiley & Sons, Ltd.

The paper presents the photoluminescence of nanocrystalline porous silicon. Two maxima were observed for rather fresh samples, one located at the infrared edge of the visible range and the other one in red. After ageing, the first maximum vanishes, suggesting its relation with the surface states, while the red one undergoes a significant blue shift. A simple quantum confinement model allows to correlate the photon energy of the red maximum with a transition between two confinement levels and to interpret the blue shift in terms of size reduction by oxidation. These results are in good agreement with previous microstructure measurements.