National Institute Of Materials Physics - Romania

Piezoelectric and p type semiconductor ZnO thin films were deposited by radiofrequency beam assisted pulsed laser deposition. Optical, electrical and structural properties were found to depend on both deposition parameters and radiofrequency beam characteristics.

Defects in silicon are studied as function of the dimensionality of the investigated structures. Defects produced by strong irradiation in bulk crystals, like vacancies or interstitial defects, induce other defects, so that the irradiated devices are irreversibly damaged. Defects in nanostructures are shown to be produced mainly by surface/interface states and strains, and are therefore specific to the investigated structure. The modeling of the experimental methods allows the determination of defect parameters that are not directly measurable. The carriers capture on quantum confinement levels in OD systems has a similar behavior with the trapping phenomena.

Electrical properties of new Polymer-Iron nanocomposite materials obtained by laser pyrolysis were investigated, in order to find applications for the detection of reducing gases. The aim of this paper was to prove the necessity of complex experimental investigation in order to point out the contribution of different types of adsorption. Usually resistance measurement cannot fully describe the surface interactions. The electronaffinity changes are of real interest for in-field conditions which always involve humidity of surrounding gas atmosphere. On the other hand, the nanocomposite materials can be a solution for specific gas-sensing properties.

The attractive properties of zirconia, particularly fracture toughness, wear resistance and chemical stability, have led to vast research efforts designed to investigate, characterize and develop such materials. Applications vary from structural, mechanical, biomedical to electrical, especially the fuel cells field. The current study is aiming at preparation and structural characterization Of 10% Y2O3 stabilized ZrO2 nanometric powders. The sol-gel method is known to be easy to use for producing various kinds of materials, structured at nanometric level, so was chosen for the elaboration of powders. Different thermal treatments were applied to the obtained materials, which were consequently investigated through thermal analysis, X-ray diffraction (XRD) scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The characteristics Of Y2O3 doped zirconia powders are influenced by the processing parameters and proved promising for further applications in, for example, the Solid Oxides Fuel Cell field.

Water soluble gold nanoparticles, obtained by the reduction of the gold (III) chloride with sodium borohydride in the presence of citric acid or thioctic acid, were covered with a paramagnetic silica layer using the Stober method, yielding a hybrid metallic-inorganic nanomaterial (gold nanoparticles, with an average size of 5 nm, embedded into silica nanoparticles, with an average size of 100 nm). The paramagnetic silica layer was formed by copolymerization of a paramagnetic silica precursor (derived from 3-aminopropyltrimethoxysilane) with tetramethyorthosilicate. The paramagnetic silica precursor was obtained by coupling 3-aminopropyltrimethoxysilane with 3-carboxy-proxyl free radical. TEM pictures show that each silica nanoparticle of about 100 nm in size embedded about 10 gold nanoparticles. These hybrid nanoparticles are quite stable and exhibit the expected paramagnetic characteristics, as seen by electron paramagnetic resonance. The accessibility of methanol through the silica layer was also studied. Depending on the capping ligands of the gold nanoparticles (citric or thioctic acid), different silica networks are formed, as seen by the mobility of the spin-label inside the silica layer. The EPR spectra showed that the paramagnetic silica layer is very robust and the mobility of the spin-probe inside the silica layer is very little affected by methanol. However, if spin-labeled thioctic acid protected gold nanoparticles were used in the material synthesis, the mobility of the spins attached to the gold surface is quite high in the presence of methanol, while the spins embedded into the silica layer remains immobilized.

An x-ray spectroscopy study was performed for YbCU5-xAlx (x = 0.0-2.0) using high-resolution partial-fluorescence yield (PFY) and resonant inelastic x-ray scattering (RlXS) at the Yb L-3 absorption edge. From a careful analysis of the PFY and RlXS spectra, the mean-valence of Yb was estimated and is found to increase quite rapidly for compositions between 0 <= x <= 1.0, indicating strong intermediate-valence behavior. The mean valence continues to increase gradually with x and becomes nearly trivalent for x = 2.0. Our results, however, reveal that valence fluctuation occurs even at x = 1.5, for which non-Fermi liquid quantum critical behavior is known. The valence evolution cannot be explained solely in terms of chemical pressure effects. The study suggests the role of valence-coupled spin fluctuations for the quantum critical behavior.

Multilayers made of alternative layers of composition SnSe2 and tellurium (three pairs) have been prepared by pulsed laser deposition (PLD). The complex films have been investigated by X-ray diffraction. After a special annealing in oxygen fluence at various temperatures, up to 620 degrees C, the films were investigated both by X-ray diffraction and for gas sensing. Gas sensing properties were tested for water vapours, carbon monoxide (CO) and methane (CH4). After annealing at high temperature, the structure of the multilayer samples show oxide phases like TeO2, SnO2 and a mixed oxide ternary phase SnTe3O8. The annealed multilayers show good sensing properties to water vapours and CO gas.

A versatile method, that of sol-gel deposition was used to produce Si1-xGexO2 films on Si substrates. Samples after annealing at different temperatures are investigated by different techniques including high-resolution transmission electron microscopy and spectral ellipsometry. It was revealed that, by annealing at high temperatures, Ge accumulates in small clusters (5-10nm) inside of the oxide layer. By ellipsometric investigations, valuable data about the temperature evolution of the composition and optical constants were obtained.

In this paper, the trapping phenomena in silicon-based nanocrystalline semiconductors are studied. We propose a general and complete model for optical charging spectroscopy measurements, which takes into account the trapping-detrapping-retrapping processes and all the types of discharge currents that can occur in a nanocrystalline system. The model was applied to the measurements performed on multi-quantum well structures, (nc-Si/CaF2)(50), as well as on nanocrystalline porous silicon. This way, the trap parameters that are not directly measurable were determined. (C) 2007 Elsevier Ltd. All rights reserved.

Hydroxyapatite (HAP) thin films (0.2 pm, 0.3 pm and 1.2 mu m thickness) were grown by RF magnetron sputtering in argon atmosphere onto silicon substrates at high deposition rates (0.6 mu m/h). Crystalline HAP films were obtained using a low temperature (100 degrees C) followed by post-deposition annealing at 300 degrees C, 450 degrees C, 500 degrees C and 550 degrees C in environmental air for 1 hour. An important influence of the films thickness upon the crystallization degree was noticed at intermediate annealing temperatures, as obtained from XRD measurements. For low and high temperatures similar values were obtained with a better crystallization degree for the thinner films. FTIR absorption led to the same conclusion considering the shape of stretching and bending PO4 lines. This suggests that the crystallization process has a diffusion component besides usual thermal activation process.