Publications

Phosphosilicate films with 90%SiO2-10%P2O5 molar composition, derived from tetraethoxysilane as SiO2 precursor and triethylphosphate, triethylphosphite or phosphoric acid as P2O5 precursors were prepared using the sol-gel method. The films were deposited on glass and ITO coated glass supports. The influence of the type of P2O5 precursor, type of substrate and of the thermal treatment (200, 300 and 500 degrees C) on their structure and properties was studied. By spectroellipsometric and XPS measurements the high vaporization of the phosphorous during the densification of the films by thermal treatment was noticed when alkoxide were used, underlying that the mentioned precursors are not recommended for thin phosphosilicate films preparation. The phosphoric acid that forms chemical bond with silica network during the sol-gel process lead to better incorporation of P in the silica network as compared to the P-alkoxides.

Thin films having ZnO nanocrystallites, with a minimum average size of 2.2 nm, embedded in an amorphous gold oxide matrix have been investigated. These structures do not present a blue-shift of the absorption spectra relative to those corresponding to large ZnO crystallites, a shift which would be expectable for this range of particle size. It was found that the hydrated gold oxide has a much narrower energy gap than ZnO and does not creates quantum wells in the ZnO crystallite region. The Raman spectra present only the confined vibration modes of ZnO. These peculiarities of the mixed system were analyzed by means of an adequate energy band diagram. A Raman resonant effect, determined by interface transitions, has also been detected.

The magnetic nanocomposite materials represent an important class of nanomaterials extensively studied nowadays due to their varied applications from medical diagnostic to storage information. The iron oxides in silica matrix systems are highly investigated. The sol-gel method is a suitable way of preparation of Fe3O4-SiO2 nanocomposite materials, since this method allowed the preparation of nanocomposite materials with narrow size distribution of magnetite in silica matrix. In the present work, nanocomposite materials in the Fe3O4-SiO2 system were prepared by sol-gel method via alkoxide and aqueous route. As SiO2 sources, tetraethoxysilan (TEOS) for the alkoxide route, as well as silica sol Ludox (30%) for the aqueous route, were used. This study shows the influence of the type of silica matrix on the structure, size, and distribution of the Fe3O4 nanoparticles in the Fe3O4-SiO2 systems. The gels were annealed at 550 degrees C in order to consolidate the matrices. The structural characterization of the obtained materials via the two preparation routes was performed by DTA/TGA analysis, X-ray diffraction, IR and Mossbauer spectroscopy, Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED).

It is shown that self-organization in non-crystalline networks of amorphous semiconductors, as well as in non-crystalline chalcogenide materials is rather a rule than an exception. The fundamental principles are stated on the basis of few examples. The most extended order in glasses is given by the uniform distribution of large stochastic clusters.

The paper presents a quantum confinement model for the electrical transport, the phototransport and the photoluminescence phenomena in nanocrystalline silicon. The infinite rectangular quantum well was proved to be the best choice for the investigated systems - nanocrystalline porous silicon and silicon nanodots embedded in an amorphous silicon dioxide matrix. Previous microstructure investigations have shown that the nanocrystalline porous silicon is formed by a nanowire network, so that the electron Hamiltonian is the sum of a one-dimensional Bloch-like Hamiltonian and a two-dimensional infinite rectangular quantum well. In the case of the silicon nanodots, the quantum well is three-dimensional. In both cases, the quantum well introduces quantum confinement levels in the band gap, the investigated phenomena being related with transitions between these levels.

Diodes processed on n-type epitaxial silicon with a thickness of 25, 50 and 75 mu m had been irradiated with reactor neutrons and high-energy protons (24 GeV/c) up to integrated fluences of Phi(eq) = 10(16) cm(-2). Systematic experiments on radiation-induced damage effects revealed the following results: in contrast to standard and oxygen-enriched float zone (FZ) silicon devices no space charge sign inversion was observed after irradiation. It is shown that the radiation-generated concentration of deep acceptors, dominating the behavior of n-type FZ diodes, is compensated by creation of shallow donors. Thus a positive space charge is maintained throughout the irradiation up to the highest fluence and even during prolonged elevated-temperature annealing cycles. Defect analysis studies using thermally stimulated current measurements attribute the effect to a damage-induced shallow donor at E-C-0.23 eV. It is argued that, as in the case of thermal donors, oxygen dimers, out diffusing from the Cz substrate during the diode processing, are responsible precursers. Results from extensive annealing experiments at elevated temperatures are verified by comparison with prolonged room-temperature annealing. These results showed that in contrast to FZ detectors, which always have to be cooled, room-temperature storage during beam off periods of future elementary particle physics experiments would even be beneficial for n-type epi-silicon detectors. A dedicated experiment at CERN-PS had successfully proven this expectation. It was verified, that in such a scenario the depletion voltage for the epi-detector could always be kept at a moderate level throughout the full S-LHC operation (foreseen upgrade of the large hadron collider). Practically no difference with respect to FZ-silicon devices was found in the damage-induced bulk generation current. The charge trapping measured with Sr-90 electrons (mip's) is also almost identical to what was expected. A charge collection efficiency of 60% (2500 e) in 50 mu m n-type epi-diodes after 24 GeV/c proton irradiation with Phi(eq) = 6.2 x 10(15) cm(-2) was reported recently, independent of the operating temperature down to -20 degrees C. (c) 2006 Elsevier B.V. All rights reserved.

Microstructural characterization by transmission electron microscopy of the {111} planar defects induced in Si by treatment in hydrogen plasma is discussed. The {111} defects are analyzed by conventional ( TEM) and high-resolution transmission electron microscopy (HRTEM). Quantitative image processing by the geometrical phase method is applied to the experimental high-resolution image of an edge-on oriented {111} defect to measure the local displacements and strain field around it. Using these data, a structural model of the defect is derived. The validity of the structural model is checked by high-resolution image simulation and comparison with experimental images.

Theoretical studies of the coherent electronic transport in a system of coupled quantum wires show that switching on the conducting channel in one wire can be manifested in the other coupled wires. Interference processes and electronic correlations are taken into account in our studies on the same footing. The conductance changes depend on the interference conditions of a transmitted wave with that one reflected from the wires that indirectly influence the transport. We show that electronic correlations lead to a dynamical Coulomb blockade effect, which changes the conductance response quantitatively but its shape is still kept the same. Our results are discussed in correspondence with an experiment recently performed by Morimoto [Appl. Phys. Lett. 82, 3952 (2003)] on a system of coupled quantum wires.

Surface enhanced Raman scattering (SERS) performed with 676.4 and 1064 nm excitations were used to investigate single-walled carbon nanotubes (SWNTs) thin films prepared from platelets obtained by nonhydrostatic compression at 0.58 GPa. SWNTs were compressed alone or dispersed into chemical reactive and non-reactive host matrices. SERS spectra indicate that by compression, SWNTs break into fragments of different sizes, which in turn can react or not with the host matrix. In inorganic hosts (KI, Ag) donor-acceptor complexes are formed. The appearance of short fragments of SWNTs with a closed-shell fullerenes behaviour is revealed in SERS spectra. This typical signature appears in the Raman spectrum as a line at ca. 1460 cm(-1) associated with a pentagonal pinch mode. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fabrication by colloidal chemistry and an extended structural and magnetic investigation are reported for AgCo bimetallic colloidal nanoparticles using X-ray diffraction, transmission electron microscopy and superconducting quantum interference device magnetometry. The AgCo nanoparticles exhibit a core-shell structure, with a face-centred cubic Ag core and a hexagonal close-packed Co (either complete or partial) shell. A bimodal size distribution of superparamagnetic (SPNI) nanoparticles together with a small fraction of nanoparticles ferrornagnetic at room temperature has been determined and their influence on magnetic properties is discussed. The arrays of self-assembled nanoparticles exhibit a lack of saturation of magnetisation and typical SPM behaviour. The annealing of the arrays greatly enhances the ferromagnetic character of the samples. Finally, the observed magnetic behaviour of the AgCo nanoparticles is correlated with surface spin disorder induced by the particular structural features of the sample, high fraction of magnetic atoms in surface states and finite-size effects. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.