National Institute Of Materials Physics - Romania

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5781 articles found

3581. Trapping phenomena in silicon-based nanocrystalline semiconductors

Authors: Ciurea, ML; Iancu, V; Mitroi, MR

Published: OCT 2007, SOLID-STATE ELECTRONICS, 51, 1337, DOI: 10.1016/j.sse.2007.07.002

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.

3582. Stoichiometric arsenic sulphoselenides as testing probes for positron trapping in chalcogenide glasses

Authors: Hyla, M; Filipecki, J; Shpryuk, O; Popescuc, M; Balitska, V

Published: OCT 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3181, DOI:

Positron annihilation lifetime (PAL) measurements are performed for stoichiometric glassy arsenic sulphide g-AS(2)S(3) and selenide g-AS(2)Se(3) in order to probe the corresponding model of positron trapping. An ORTEC spectrometer with Na-22 source placed between two sandwiched glass samples was used in our experiments, the obtained spectra being treated with LT computer program. Two identical samples of g-A(2)S(3) and g-AS(2)Se(3) of about similar to 1.5 mm in thickness prepared from high-purity elemental constituents by conventional melt-quenching route and additionally annealed near glass transition to avoid the inner stressed regions were tested. It is shown that two-state positron trapping model is valid for both glasses, the process of positron trapping being saturated in extended free-volume defects like to atomic vacancies and their agglomerates. Despite difference in the character of nanovoid radius distribution estimated theoretically for g-AS(2)S(3) and g-AS(2)Se(3), they have very close average free volumes resulting in the observed similarity of PAL data. But, in contrast to g-AS(2)Se(3), which has at least three sets of nanovoids centered near similar to 1.5 angstrom, similar to 2.3 angstrom and similar to 2.9 angstrom, only two sets of nanovoids centered near similar to 1.3 angstrom and at similar to 2.8 angstrom are proper to g-AS(2)S(3) giving only one variant of fitting.

3583. Correlation between radiation processes in silicon and long-time degradation of detectors for high-energy physics experiments

Authors: Lazanu, S; Lazanu, I

Published: SEP 21 2007, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 580, 49, DOI: 10.1016/j.nima.2007.05.015

In this contribution, the correlation between fundamental interaction processes induced by radiation in silicon and observable effects which limit the use of silicon detectors in high-energy physics experiments is investigated in the frame of a phenomenological model which includes: generation of primary defects at irradiation, starting from elementary interactions in silicon; kinetics of defects, effects at the p-n junction detector level. The effects due to irradiating particles (pions, protons, neutrons), to their flux, to the anisotropy of the threshold energy in silicon, to the impurity concentrations and resistivity of the starting material are investigated as time, fluence and temperature dependences of detector characteristics. The expected degradation of the electrical parameters of detectors in the complex hadron background fields at LHC & SLHC is predicted. (C) 2007 Elsevier B.V. All rights reserved.

3584. Thickness-driven antiferroelectric-to-ferroelectric phase transition of thin PbZrO3 layers in epitaxial PbZrO3/Pb(Zr0.8Ti0.2)O-3 multilayers

Authors: Boldyreva, K; Pintilie, L; Lotnyk, A; Misirlioglu, IB; Alexe, M; Hesse, D

Published: SEP 17 2007, APPLIED PHYSICS LETTERS, 91, DOI: 10.1063/1.2789401

Epitaxial antiferroelectric/ferroelectric PbZrO3/PbZr0.8Ti0.2O3 multilayers were grown on SrRuO3-electroded SrTiO3(100) substrates by pulsed laser deposition. Polarization-field and switching current-voltage curves show a mixed antiferroelectric-ferroelectric behavior of the multilayers with an individual layer thickness above 10 nm, whereas below 10 nm the multilayers show only ferroelectric behavior. Clearly the PbZrO3 layers thinner than 10 nm experienced a transition into the ferroelectric state. X-ray diffraction reciprocal space mapping showed a corresponding orthorhombic-to-rhombohedral transition of the PbZrO3 layers. The observations are discussed in terms of the influence of strain.

3585. Low temperature two-dimensional behaviour of spin and orbital moments in Ni monolayers grown on Cu(001)

Authors: Teodorescu, CM

Published: SEP 15 2007, SURFACE SCIENCE, 601, 4296, DOI: 10.1016/j.susc.2007.04.233

The Bloch T-3/2 law is obtained for three-dimensional magnons at low temperatures. When decreasing the dimensionality of the magnetic system, the 3/2 exponent is expected to be equal to one for two-dimensional (2D) systems. I present a theoretical sketch of 2D spin wave theory, in which a long magnon wavelength cutoff is introduced, connected to the lateral size of the 2D system investigated. I report a study by X-ray magnetic circular dichroism at the Ni L-edges on the onset of ferromagnetic, order on 1.5 atomic Ni layers deposited on Cu(1 0 0), with spin and orbital moments' dependence with temperature. A linear dependence is found for both spin and orbital moments with the same initial slope. This temperature dependence can be used to probe the size of the 2D magnetic islands, which is particularly useful for embedded 2D objects. (C) 2007 Elsevier B.V. All rights reserved.

3586. On the hydrophilicity of nitrogen-doped TiO2 thin films

Authors: Mardare, D; Luca, D; Teodorescu, CM; Macovei, D

Published: SEP 15 2007, SURFACE SCIENCE, 601, 4520, DOI: 10.1016/j.susc.2007.04.156

Nitrogen doping was proposed recently as a low-cost means to shift the absorption edge of titania towards 450-500 run. Results are reported here on the preparation of nitrogen-doped 250 nm thick TiO2 films on glass substrates by RF magnetron sputtering. The nitrogen content in the films was modified by changing the N-2 partial pressure in the discharge from 0.00 to 0.27 Pa. An additional amount of oxygen was added in the deposition chamber (Po-2 = 0.11 Pa) to compensate for oxygen depletion in the films. Surface composition and structure were derived from XPS and XRD data, respectively. XRD data revealed a mixture of amorphous phase and rutile polymorph in the as-deposited films, while an anatase-rich phase occurs in the samples deposited under N2 pressure of 0.05 and 0.16 Pa. Above 0.16 Pa, a rutile-rich phase is present. Surface wettability was investigated from contact angle data of de-ionized water with solid surface. The films deposited under mid-range N2 pressure in the discharge are super-hydrophilic. The photo-induced super-hydrophilicity decays after 36 h. Film wettability is discussed in terms of the synergic effects of surface roughness (measured by AFM) and rutile-anatase TiO2 phase transition, induced by doping. The TiO2 surface super-hydrophilicity (and catalytic efficiency) are related to beneficial properties of anatase phase (E-g = 3.2 eV), associated with a low recombination rate of charge carriers in the surface. (C) 2007 Elsevier B.V. All rights reserved.

3587. Fe-doped TiO2 thin films

Authors: Mardare, D; Nica, V; Teodorescu, CM; Macovei, D

Published: SEP 15 2007, SURFACE SCIENCE, 601, 4483, DOI: 10.1016/j.susc.2007.04.139

The reactive sputtering technique was used to obtain undoped and Fe-doped TiO2 thin films deposited on glass substrates. At 250 degrees C substrate temperature, undoped TiO2 films crystallize in a mixed rutile/anatase phase, while Fe-doped films exhibit the rutile phase only. Presence of Fe3+ ions into the TiO2 lattice is suggested by the intensity variation of forbidden 1s -> 3d transitions between the Ti and Fe K-edges. Ti K-edge EXAFS data are assessed to a mixture of the two kinds of surroundings, a rutile-like crystalline phase, identified also by X-ray diffraction, and a nanosized or amorphous anatase-like surrounding. The local atomic order about Fe atoms is quite different and could be related also to an amorphous phase. The Swanepoel method is used to obtain the dispersion of the refractive index below the interband absorption edge. The dispersion energy, the single-oscillator energy and the coordination number of the Ti atoms are evaluated using the single-oscillator model (Wemple-DiDomenico). (C) 2007 Elsevier B.V. All rights reserved.

3588. Structure, morphology and magnetic properties of Fe-Au core-shell nanoparticles

Authors: Pana, O; Teodorescu, CM; Chauvet, O; Payen, C; Macovei, D; Turcu, R; Soran, ML; Aldea, N; Barbu, L

Published: SEP 15 2007, SURFACE SCIENCE, 601, 4357, DOI: 10.1016/j.susc.2007.06.024

The aim of the present work is the investigation of the core-shell Fe@Au nanoparticles obtained by the inverse micelles method. Also hybrid structures between Fe@Au nanoparticles and the well-known conducting polymer polypyrrole (PPY) were obtained and investigated. The properties of the composites were investigated by TEM, HRTEM, X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS) and magnetization measurements. The TEM, HRTEM measurements show that two categories of core-shell nanoparticles with mean diameters of 5 nm and 25 nm, respectively, are formed. The XPS investigations of the core-shell Fe@Au nanoparticles indicate that besides Fe-0 inside the cores, small amounts of Fen,111, located onto the gold surface were also formed during the samples preparation. The magnetization of the investigated nanocomposites measured versus increasing and decreasing magnetic field shows no hysteresis loop, this behavior being consistent with a superparamagnetic behavior. The behavior of the SQUID magnetization vs. temperature under field cooled (FC) and zero field cooled (ZFC) at 0.1 T magnetic field for Fe@Au nanoparticles confirmed the existence of superparamagnetism. The superparamagnetic model was used to fit the experimental magnetization vs. applied magnetic field as it was measured by SQUID up to 5 T. (C) 2007 Elsevier B.V. All rights reserved.

3589. Relaxation of remnant magnetisation in YBa2Cu3O7-delta films

Authors: Miu, D; Miu, L; Jakob, G; Adrian, H

Published: SEP 1 2007, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 460, 1244, DOI: 10.1016/j.physc.2007.04.069

The relaxation of the remnant magnetisation in optimally doped disk-shaped YBa2Cu3O7-delta films with the initially applied magnetic field H oriented along the c axis was measured using dc SQUID magnetometry. The temperature (7) dependence of the experimentally observed normalised magnetisation relaxation rate S exhibits three distinct regions. At high T, S(7) increases with increasing T, which can be explained in terms of plastic vortex creep. The well known plateau in the S(7) variation at intermediate T appears to be caused by collective (elastic) creep in a dynamically ordered vortex system. At low T, where S increases again with increasing T, the magnetisation relaxation is not in a pure thermally activated flux-creep regime, due to the occurrence of micro-flux jumps. This seems to be the origin of the recently proposed exponential current density dependence of the effective vortex pinning potential. (c) 2007 Elsevier B.V. All rights reserved.

3590. Discovery of Li-2(Pd,Pt)(3)B superconductors

Authors: Badica, P; Togano, K; Takeya, H; Hirata, K; Awaji, S; Watanabe, K

Published: SEP 1 2007, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 460, 94, DOI: 10.1016/j.physc.2007.03.163

Critical temperature T-c of the Li-2(Pd1-xPtx)(3) B was reported to be 7-8 K for x = 0 and 2.2-2.8 K for x = 1. In this article we present our preliminary results on behavior of magnetization-temperature curves with starting composition of Pd-B precursor, y-Li concentration in LiyPd3B and post-annealing of the Pd-end compound. Results suggest that to maximize T-c ratio Pd: B should be close to 3: 1, while y-Li has to be optimum. The lowest T-c for LiyPd3B was 4.4-4.6 K, while post-annealings at 560 degrees C allowed enhancement of T-c up to 8.2-8.4 K. Compositions Li(2)Z(3)B with Z = Ni, Ru, Rh, Re, Ag are not superconducting down to 1.8 K. Exception is composition with Re showing superconductivity due to Re3B compound. All samples were prepared by arc melting. (c) 2007 Elsevier B.V. All rights reserved.