National Institute Of Materials Physics - Romania

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5781 articles found

4101. The influence of Cu on the morphological and chemical properties of nanostructured TiO2 films

Authors: Gartner, M; Ghita, A; Anastasescu, M; Osiceanu, P; Dobrescu, G; Zaharescu, M; Macovei, D; Modreanu, M; Trapalis, C; Kordas, G

Published: FEB 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 405, DOI:

TiO2 multilayer films were deposited by the sol-gel method on glass and SiO2/glass substrates. 10% of Cu was added to the coating solution, to study its effect on the amorphous to anatase phase transition, with a view to improving the photocatalytic activity. Densification of the films was obtained by thermal treatment at 500 degrees C for 1 hour in oxidative and reductive conditions. In order to check the correlation between the influence of copper, substrate and annealing temperatures on the structural, morphological and optical properties, the samples were characterized by FTIR, XPS, SEM and fractal analysis. Transformation of CuO from as-prepared samples into Cu2O and metallic Cu, depending on the treatment conditions, was shown. The composition and stoichiometry of the oxide films in the first atomic layers were quantitatively determined as chemical state relative concentrations.

4102. High frequency investigation of graded gap injectors for GaAs Gunn diodes

Authors: Montanari, S; Forster, A; Lepsa, MI; Luth, H

Published: FEB 2005, SOLID-STATE ELECTRONICS, 49, 250, DOI: 10.1016/j.sse.2004.08.014

In this work, we present a detailed high frequency characterization of conventional and optimized Gunn-diodes containing a graded gap injector. It is shown that the Al-concentration in the hot electron injector influences the occupation of the Gamma- and L-valleys and hence the electron drift velocity in the active Gunn layer. Quantitative values for the occupation of the L-valley can be extracted from the S-parameter measurements. (C) 2004 Elsevier Ltd. All rights reserved.

4103. Interface trap density in amorphous La2Hf2O7/SiO2 high-kappa gate stacks on Si

Authors: Mereu, B; Dimoulas, A; Vellianitis, G; Apostolopoulos, G; Scholz, R; Alexe, M

Published: FEB 2005, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 80, 257, DOI: 10.1007/s00339-004-2910-9

The present paper investigates the interface trap density of a new high-kappa gate dielectric stack, La2Hf2O7/SiO2 on Silicon. Amorphous La2Hf2O7 thin films are deposited by metal evaporation in the presence of atomic oxygen beams on an ultra-thin SiO2 layer (1.5 nm) grown by rapid thermal oxidation on a p-type Si substrate. A combination of electrical (C-V) and cross sectional TEM measurements indicates a value of the dielectric constant kappa of about 19 +/- 2.2. The interface states density (D-it) was measured using the conductance method for different La2Hf2O7 thicknesses ranging from 3 nm to 14 nm. Dit ranges from 3.4 x 10(10) eV(-1) cm(-2) to 4.8 x 10(12) eV(-1) cm(-2) and shows a quasi-linear dependence on the La2Hf2O7 layer thickness. The interface state density increase with film thickness could have different explanations, such as oxygen de-passivation and/or defect generation at the Si-SiO2 interface, formation of a new Si-SiO2 interface by Si oxidation or alternatively, the SiO2 interfacial layer reduction by oxygen vacancies.

4104. Photoluminescence of manganese- and copper-doped CdS nanowires

Authors: Ghiordanescu, V; Sima, M; Enculescu, I; Grecu, MN; Mihut, L; Secu, M; Neumann, R

Published: FEB 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 454, DOI: 10.1002/pssa.200406927

Arrays of US: Mn2+ : Cu+ micro- and nanowires grown in polycarbonate ion-track templates exhibit photoluminescence in the spectral domain ranging from 500 to 800 mn at room temperature. A comparison with similar US and CdS:Mn2+ wire arrays is presented. The individual contributions to the emission spectra of Cu+ and Mn2+ ions in the US matrix are explained using their energy level schemes. Also SEM, EDX and EPR data are given or these wires. (c) 2005 WILEY-VCH Verlag GmbH A Co. KGaA, Weinheim.

4105. Effect of UV radiation on the semi-interpenetrating polymer networks based on polyurethane and epoxy maleate of bisphenol A

Authors: Rosu, L; Cascaval, CN; Ciobanu, C; Rosu, D; Ion, ED; Morosanu, C; Enachescu, M

Published: JAN 15 2005, JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, 169, 185, DOI: 10.1016/j.jphotochem.2004.06.008

The behavior of two semi-interpenetrating polymer networks (S-IPNs) based on polyurethane (PU) and epoxy maleate of bisphenol A (EMBA) under exposure to UV radiation for long period of times (up to 200 h) was investigated. The networks obtained by the specific mixture under certain thermal treatment included in their structures crosslinked EMBA and linear PU. Advanced photooxidative degradation took place for long time of UV irradiation (over 160 h, especially), with almost a total destruction of the raw structures. The important modification of both the surface and of the morphology of the tested samples with the time of irradiation was also analyzed. Simultaneously, as compared to results obtained for short time of UV radiations (up to 10 h), where the mechanical properties of the tested samples improved, long time of UV radiations led to deterioration of many of the mechanical properties. (C) 2004 Elsevier B.V. All rights reserved.

4106. Local interactions and electronic phenomena in substituted LaFeO3 perovskites

Authors: Russo, U; Nodari, L; Faticanti, M; Kuncser, V; Filoti, G

Published: JAN 14 2005, SOLID STATE IONICS, 176, 102, DOI: 10.1016/j.ssi.2004.06.001

Effects of electronic charge disproportionation were observed by Fe-57 Mossbauer spectroscopy at low temperatures in substituted LaFeO3 perovskites as (La1-xCax)FeO3 and La(Fe1-xMgx)O-3 (xless than or equal to0.5). The presence of both Fe3+ and Fe5+ species was evidenced in both series of compounds. Direct evidence for disproportionation was supported by correlating the Mossbauer results with the chemical determination of tetravalent iron at RT. The evolution of hyperfine parameters vs. substitution degree was comparatively analysed for Fe3+ and Fe5+ ions in relation to their electronic microscopic origin. Electron delocalisation processes are discussed in terms of ion distribution and valence state. (C) 2004 Elsevier B.V. All rights reserved.

4107. Structural phase transition induced in Fe50Rh50 alloys by high pressure

Authors: Kuncser, V; Nicula, R; Ponkratz, U; Jianu, A; Stir, M; Burkel, E; Filoti, G

Published: JAN 11 2005, JOURNAL OF ALLOYS AND COMPOUNDS, 386, 11, DOI: 10.1016/j.jallcom.2004.04.139

The equiatomic composition of the FeRh alloy was studied by in situ high pressure energy dispersive X-ray experiments. Pressures as high as 19 GPa were generated using a diamond anvil cell. An incomplete bcc --> fcc phase transition was evidenced, both bcc and fcc phases coexisting above 10.0 GPa. The transition is accompanied by a decrease of the lattice parameter of the remaining bcc phase by approx. 1.5%. A substantial low lattice parameter was observed for the newly formed fee phase, reflecting a rather compressed structure. (C) 2004 Elsevier B.V. All rights reserved.

4108. The role of non-collinear spins on the magnetic properties of uncoupled nanometer-size particles

Authors: Zavaliche, F; Bensebaa, F; L'Ecuyer, P; Veres, T; Cochrane, RW

Published: JAN 1 2005, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 285, 209, DOI: 10.1016/j.jmmm.2004.07.042

A study of the magnetic properties of an assembly of non-interacting iron oxide-based nano-particles is presented. The measured average particle size is found to approach the expected thickness of the spin disordered layer. In this regime, the magnetization is characterized by two parameters: T-b above which the particle acts as a single domain, and T-E below which intra-particle coupling between pinned and unpinned spins become important. (C) 2004 Elsevier B.V. All rights reserved.

4109. Recoilless fraction of tin-doped hematite nanoparticles obtained by hydrothermal synthesis

Authors: Sorescu, M; Diamandescu, L; Tarabasanu-Mihaila, D

Published: JAN 2005, MATERIALS LETTERS, 59, 25, DOI: 10.1016/j.matlet.2004.09.009

Tin-doped hematite nanoparticles (1-x)alpha-Fe2O3-xSnO(2) were synthesized using the hydrothermal method. The samples were characterized using Mossbauer spectroscopy as a function of the substitution level x. The recoilless fraction was determined using the dual absorber method introduced by us [M. Sorescu, Mater. Lett. 54 (2002) 256]. It has been observed that the recoilless fraction of the hematite samples first decreases with increasing the amount x of tin substitution, then increases as iron enters the SnO2 lattice and, finally, it slightly decreases again as the particle size of the nanoparticles decreases. (C) 2004 Elsevier B.V. All rights reserved.

4110. X-ray diffraction and magnetic measurements of itinerant electron magnetism in the Y3Ni13-xCoxB2 system

Authors: Plugaru, N; Rubin, J; Bartolome, J; Pop, V

Published: JAN 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.024433

The crystallographic and magnetic properties of the Y3Ni13-xCoxB2 series of compounds, with the Nd3Ni13B2-type structure and xless than or equal to5, were investigated by powder x-ray diffraction and magnetic measurements. The susceptibility and magnetization data show that Y3Ni13B2 is a weak itinerant antiferromagnet with a Neel temperature T-N=68 K. A weak ferromagnetic component appears at T-N, suggesting uncompensated antiferromagnetic structure. The compounds with x=0.5 and 1, also order antiferromagnetically, with T-N=94 and 106 K, respectively. The intrinsic weak ferromagnetic component increases with the cobalt content and a field-induced transition, from the antiferromagnetic state to a ferromagnetic state, is observed at the critical transition fields 5.6 and 1.6 T, at 5 K for x=0.5 and 1, respectively. A heterogeneous ferromagnetic state and in-plane magnetic anisotropy are determined for the compounds in the 2less than or equal toxless than or equal to5 composition range. The Curie temperature and spontaneous magnetization increase linearly with the cobalt content. Based on the experimental data, the T-x phase diagram is constructed for xless than or equal to5. The effect of spin fluctuations is highlighted in the nickel-rich compounds in the 3:13:2 and related systems.