National Institute Of Materials Physics - Romania

Publications

847 articles found

751. Non-equilibrium electronic processes in nanocrystalline silicon

Authors: Ciurea, ML; Iancu, V; Draghici, M; Jdira, L

Published: 2002, ADVANCED TOPICS IN OPTOELECTRONICS, MICROELECTRONICS, AND NANOTECHNOLOGIES, 5227, 273, DOI:

Nanocrystalline silicon is studied with a view to obtaining a new photonic material. Non-equilibrium electronic processes in such materials play a significant role. We have studied trapping phenomena in nanocrystalline porous silicon and nanocrystalline silicon-based Multi-Quantum Wells structures by means of Optical Charging Spectroscopy method, which is a very good and sensitive method. We have also analyzed the modeling of the processes that occur during our measurements. This modeling allows us to separate the relative contribution of the different types of discharge currents that can appear: ohmic conduction currents of either equilibrium or non-equilibrium carriers, displacement currents, diffusion currents and tunneling currents through insulating layers (in Multi-Quantum Wells structures). It also allows us to increase the accuracy of the determination of the experimental trap parameters and to determine parameters that are not directly measurable. The model can be applied to other nanocrystalline semiconductors and can be easily generalized for other high resistivity materials.

752. Tl-0 and Tl2+ centers as paramagnetic probes for the 74K phase transition in Rb2ZnCl4

Authors: Stefan, M; Nistor, SV; Schoemaker, D

Published: 2002, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 157, 697, DOI: 10.1080/10420150215809

Tl-0 (6 p(1)) irradiation centers are proposed as new paramagnetic probes for structural phase transition investigations. The P2(1)cn --> C1 c1 phase transition in Rb-2 ZnCl4 single crystals is monitored using the Tl-0 and the well-known Tl2+ (6 s(1)) paramagnetic probes. The anomalous temperature dependence exhibited by the EPR spectra of both types of centers in the C1 c1 phase was analyzed considering a soft mode contribution.

753. The stability of cadmium chalcogenide pigments in the process of laser cleaning of painted surfaces

Authors: Radvan, R; Popovici, N; Savastru, R; Ghica, C; Sava, F; Popescu, M

Published: DEC 2001, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 3, 902, DOI:

Pigments based on cadmium selenide sulphides and cadmium sulfides were analysed as regarding their structure and stability against the laser irradiation, a new technique for cleaning the painted surfaces in artwork restoration, The pulsed Nd:YAG laser radiation determines the effusion of some sulphur from the red pigment Cd(S,Se) and significant decrease of the average size of the crystallites. The color turns to dark red. For yellow pigment (CdS) no change of crystallite size was detected after laser irradiation.

754. PbSe1-xTex thick thermoelectric films obtained by electrochemical deposition from aqueous solutions

Authors: Nedelcu, M; Sima, M; Stoica, T; Manea, AS; Lazarescu, MF; Visan, T

Published: DEC 2001, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 3, 914, DOI:

It is presented a study of PbSe1-xTex thick films electrodeposition on glassy carbon and ITO electrodes in acid solutions (0.1M HNO3), using cyclic voltammetry and rotating-disk techniques. Electrochemical impedance spectroscopy study of PbSe1-xTex film in 0.1M HNO3 is reported.

755. Influence of the deposition configuration on the composition, structure and morphology of La0.6Y0.07Ca0.33Mn3-delta thin films obtained by pulsed laser deposition

Authors: Ghica, C; Valeanu, M; Nistor, LC; Teodorescu, V; Sandu, C; Ristoscu, C; Mihailescu, IN; Werckmann, J; Deville, JP

Published: DEC 2001, INTERNATIONAL JOURNAL OF INORGANIC MATERIALS, 3, 1256, DOI: 10.1016/S1466-6049(01)00153-2

We present a structural and morphological study on La0.6Y0.07Ca0.33MnO3-delta (LYCMO) thin films deposited by laser ablation onto MgO-buffered silicon wafers using two deposition configurations named on-axis and off-axis. All thin films have been deposited at high temperatures in an oxygen atmosphere. A comparative study between the on-axis and off-axis samples is performed by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The films are polycrystalline and show a columnar morphology. The off-axis sample shows a better crystalline texture, although slight compositional variations have been found. (C) 2001 Elsevier Science Ltd. All rights reserved.

756. Trapping levels in (nc-Si/CaF2)(n) multi-quantum wells

Authors: Ioannou-Sougleridis, V; Nassiopoulou, AG; Ciurea, ML; Bassani, F; d'Avitaya, FA

Published: AUG 20 2001, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 15, 47, DOI: 10.1016/S0928-4931(01)00215-6

Trapping levels which govern the vertical transport in (nc-Si/CaF2)(n) multi-quantum wells were investigated using two different techniques: (a) the temperature dependence of the dark current and (b) the thermally stimulated depolarization current technique (TSDC). Measurements by the first technique showed that the conduction mechanism was thermally activated above 200 K with activation energies of 0.35-0.7 eV, These activation energies were found to increase with increasing electric field. TSDC measurements showed also the existence of at least one broad peak above 200 K with estimated activation energies in the range of 0.4-0.45 eV. Analysis of the peak by the fractional heating method showed a continuous distribution of defect states from 0.3 to 0.83 eV. (C) 2001 Elsevier Science B.V. All rights reserved.

757. Magnetic resonance study of the Fe+(I) center in SrCl2 single crystals

Authors: Vrielinck, H; Callens, F; Matthys, P; Nistor, SV; Ghica, D; Schoemaker, D

Published: JUL 1 2001, PHYSICAL REVIEW B, 64, DOI: 10.1103/PhysRevB.64.024405

An Fe+-type center with axial (001) symmetry has been studied by electron nuclear double resonance (ENDOR) in the microwave X and Q bands. The Fe+(I) center is produced only after irradiation with x or gamma rays at 80 K of SrCl2:Fe2+ crystals grown in a Cl-2 atmosphere. It has a 3d(7) F-4 Gamma (8) ground state with spin S =3/2. As shown by the correlated analysis of the ENDOR data and electron paramagnetic resonance superhyperfine structure, the Fe+ ion is situated in the center of a tetragonally compressed cube of eight nearest Cl- ions. The simplest model of the Fe+(I) center, compatible with the magnetic resonance results, consists of an interstitial Fe+ ion with two substitutional monovalent cations, very likely K+ ions, in two opposite nearest-neighbor Sr2+ sites along the tetragonal axis of the center.

758. Single-crystal high-frequency electron paramagnetic resonance investigation of a tetranuclear iron(III) single-molecule magnet

Authors: Bouwen, A; Caneschi, A; Gatteschi, D; Goovaerts, E; Schoemaker, D; Sorace, L; Stefan, M

Published: APR 5 2001, JOURNAL OF PHYSICAL CHEMISTRY B, 105, 2663, DOI: 10.1021/jp003441d

A high-frequency (95 GHz) electron paramagnetic resonance (EPR) study is reported on single crystals of the planar tetranuclear complex Fe-4(OCH3)(6)(dpm)(6) (where Hdpm = dipivaloylmethane), which has been previously shown to present typical single-molecule magnet behavior. The spectra, all originating from the S = 5 ground state, possess quasi-axial symmetry along the normal to the plane defined by the four Fe(m) ions. The measured spectra are shown to belong to three different structural variations of the compound, resulting from disorder in the ligands around two of the Fe(III) ions. Accurate values could be obtained for the second- and fourth-order crystal field parameters related to the parallel EPR spectra, while the other parameters could be determined only for the dominant species. The separation between individual lines is decreasing and vanishing with increasing temperature. This effect is attributed to the contribution of fast relaxing excited states, whose population is varying with temperature.

759. In situ transmission electron microscopy study of the silicidation process in Co thin films on patterned (001) Si substrates

Authors: Ghica, C; Nistor, L; Bender, H; Steegen, A; Lauwers, A; Maex, K; Van Landuyt, J

Published: MAR 2001, JOURNAL OF MATERIALS RESEARCH, 16, 708, DOI: 10.1557/JMR.2001.0121

The results of an in situ transmission electron microscopy study of the formation of Co-silicides on patterned (001) Si substrates are discussed. It is shown that the results of the in situ heating experiments agreed very well with the data based on standard rapid thermal annealing experiments. Fast heating rates resulted in better definition of the silicide lines. Also, better lines were obtained for samples that received already a low-temperature ex situ anneal. A Ti cap layer gave rise to a higher degree of epitaxy in the CoSi2 silicide.

760. Fourth-order zero-field splitting parameters of [Mn(cyclam)Br-2]Br determined by single-crystal W-band EPR

Authors: Mossin, S; Stefan, M; ter Heerdt, P; Bouwen, A; Goovaerts, E; Weihe, H

Published: 2001, APPLIED MAGNETIC RESONANCE, 21, 596, DOI: 10.1007/BF03162431

Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values B-4(0) = 0.0009(3) cm(-1), B2 = 0.0006(2) cm(-1) and 4 4 B-4(4) = 0.0017(3) cm(-1), to be considered together with the second-order parameters D = -1.1677(7) cm(-1) and E = -0.0135(6) cm(-1).