National Institute Of Materials Physics - Romania

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860 articles found

751. Tin nanoclusters obtained in potassium chloride by thermal annealing

Authors: Apostol, E; Enculescu, M; Polosan, S; Vasile, E; Topa, V

Published: 2007, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 3, 4, +, DOI: 10.1002/pssc.200673734

Tin nanoclusters were obtained in potassium chloride crystals following the electrolytic coloration of KCl:Sn2+ and KCl:Sn2++Ca2+ crystals and thermal annealing at various temperatures and times. Electrolytic coloration of KCl:Sn2+ crystals produces electronic centers inside of samples, presumably negative metal ions of tin. Optical absorption measurements were performed on as grown sample, colored and thermal annealed samples. The behavior of the samples with Sn colloids was compared with theoretical predictions according to Mie's theory. Tin nanoclusters in KCl exhibit an absorption band with a peak at 215 nm. The structure of the nanoclusters was studied by optical absorption and transmission electron microscopy means. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

752. Fe3O4-SiO2 nanocomposites obtained via alkoxide and colloidal route

Authors: Jitianu, A; Raileanu, M; Crisan, M; Predoi, D; Jitianu, M; Stanciu, L; Zaharescu, M

Published: DEC 2006, JOURNAL OF SOL-GEL SCIENCE AND TECHNOLOGY, 40, 323, DOI: 10.1007/s10971-006-9321-7

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).

753. A distinctive signature in the Raman and photoluminescence spectra of intercalated PbI2

Authors: Preda, N; Mihut, L; Baibarac, M; Baltog, I; Lefrant, S

Published: OCT 4 2006, JOURNAL OF PHYSICS-CONDENSED MATTER, 18, 8912, DOI: 10.1088/0953-8984/18/39/020

Through correlated studies using scanning electron microscopy (SEM), Raman spectroscopy and low-temperature photoluminescence (PL) we have demonstrated that the intercalated PbI2 with ammonia, poly (vinyl alcohol) and polyacrylamide are characterized by a distinctive signature in the Raman and photoluminescence spectra. After intercalation, the Raman spectrum of PbI2 reveals an orthorhombic structure, identical with that observed on the micrometric scale, of KPbI3 rod-like particles resulting from the reaction between Pb(NO3)(2) and KI, carried out in liquid media such as ethanol and acetonitrile. The rods and the intercalated PbI2 are characterized by a new and strong emission band at 2.23 eV (about 550 nm) that appears at 77 K under an excitation wavelength of about 340 nm. Modification of the Raman and PL spectra results from a compressing effect produced by the penetration between the PbI2 layers of different molecular species. The compression acting primarily on the iodine layers has the result of lowering the contribution of the 5p states of I- ions to the constitution of the electronic level situated at the top of valence band of PbI2, so that the photoluminescence of intercalated PbI2 acquires the characteristics of Pb2+ emission when it is dissolved in an alkali halide crystal.

754. Magneto-sensitive nickel nanowires fabricated by electrodeposition into multi- and single-ion track templates

Authors: Ohgai, T; Enculescu, I; Zet, C; Westerberg, L; Hjort, K; Spohr, R; Neumann, R

Published: OCT 2006, JOURNAL OF APPLIED ELECTROCHEMISTRY, 36, 1162, DOI: 10.1007/s10800-006-9200-5

Polycarbonate templates of ( 30 +/- 1) mu m thickness containing cylindrical etched-track nanochannels of ( 500 +/- 50) nm diameter were used for electrodeposition of Ni nanowires. Using 10(4) channels per cm(2), the most favourable deposition potential of -1.0 V was determined in a potentiostatic mode by varying the deposition potential with respect to an Ag/AgCl reference electrode over a range between -0.1 V and -1.5 V. The deposition efficiency at -1.0 V was estimated around 10%. The resulting single wires had a resistance around 200 W and showed an anisotropic magnetoresistance (AMR) effect of 1%, applicable to directionally sensitive magnetic field sensors.

755. Criteria for the occurrence of negative resonant magnetoresistance in magnetic tunnel junctions

Authors: Tolea, M; Aldea, A; Tolea, F

Published: SEP 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, R86, DOI: 10.1002/pssb.200642347

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.

756. Optical properties of low-dimensional PbI2 particles embedded in polyacrylamide matrix

Authors: Preda, N; Mihut, L; Baltog, I; Velula, T; Teodorescu, V

Published: JUN 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 913, DOI:

Small particles of Pbl(2) embedded in transparent polymer matrix have been studied by Raman scattering, UV-VIS absorption spectroscopy, low temperature photoluminescence (PL) and Scanning Electron Microscopy (SEM). The Pbl(2) crystallites were prepared by two different methods: i) by cooling to the room temperature a boiling saturated aqueous Pbl(2) solution containing polyacrylamide and ii) by chemical reaction of Kl and Pb(NO3)(2) in aqueous polyacrylamide solution. In both cases, beside the hexagonal platelets of Pbl(2) Were identified rods whose luminescence and Raman signature is quite different than of Pbl(2) platelets. Regardless the rods nature, as distinct KPbl(3) particles or lead iodide low-dimensional particles embedded in the polymer matrix, their photoluminescence is featured by an intense green emission (550 nm) appearing at low temperature at excitation wavelengths less then 350 nm.

757. Fractal structures of gold obtained by diffusion limited aggregation in alkali halide crystals

Authors: Enculescu, M; Enculescu, I; Topa, V

Published: JUN 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1233, DOI:

Nanostructures of gold embedded in potassium chloride matrices were obtained after the doped crystals growth, by electrolytical colouring, and by thermal treatment. When the TEM images of the nanostructures were analyzed using fractal geometry it means was proved that the self assembling of the metallic structures is governed by the diffusion limited aggregation (DLA) mechanism. The values of the fractal dimension, which were in all cases between 1.7 and 1.9, calculated using two methods, sandbox and box-counting, proved the DLA mechanism.

758. FexOy-SiO2 nanocomposites studied by Mossbauer spectroscopy

Authors: Predoi, D; Kuncser, V; Zaharescu, M; Jitianu, A; Crisan, M; Keune, W; Sahoo, B; Filoti, G; Raileanu, M

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 522, DOI:

FexOy-SiO2 nanocomposites were obtained via the sol-gel method, by using three different precursors for the silica matrix. The morphology and the iron phase composition is strongly influenced by the precursors. The presence of magnetically ordered goethite and fayalite is evidenced by low temperature Mossbauer spectroscopy. Subsequent annealing treatments performed on the initial samples have induced the formation of hematite nanoparticles in the silica matrix, mainly on the account of the goethite decomposition. The total amount and the size distribution of the hematite nanoparticles can be controlled via the initial precursors and the subsequent annealing conditions. The simultaneous analysis of the iron phase composition and of the local magnetic interactions in the analysed samples is performed.

759. Growth of ZnO micro and nanowires using the template method

Authors: Sima, M; Enculescu, I; Vasile, E

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 828, DOI:

The electrochemical deposition in ion track nanoporous membranes was employed for ZnO micro and nanowires preparation. A ZnNO3 bath was employed for the deposition, performed at -800 mV (vs. SCE) and at temperatures around 90 degrees C. Scanning and transmission electron microscopy were used to characterize the wires from a morphological point of view. Electron and X-ray diffraction were employed for the characterization of materials from the point of view of structure. The technique allowed the fabrication of nanowire arrays with good morphological and structural quality.

760. Mesoscopic Fano effect in Aharonov-Bohm rings with an embedded double dot

Authors: Tanatar, B; Moldoveanu, V; Tolea, M; Aldea, A

Published: 2006, LOW TEMPERATURE PHYSICS, PTS A AND B, 850, +, DOI:

We investigate theoretically in a tight-binding model the transport properties of the Aharonov-Bohm interferometer (ABI) with one dot embedded in each of its arms. For weak interdot coupling the model Hamiltonian describes the system considered in the experiments of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)]. The electronic transmittance of the interferometer is computed within the Landauer-Buttiker formalism while the coexistence of resonant and coherent transport is explicitly emphasized by using the Feschbach formula. The latter produces effective Hamiltonians whose spectral properties describe the tunneling processes through each dot. We reproduce numerically the stability charging diagrams reported in the experiments of Holleitner et al. When the magnetic flux is fixed and one dot is set to resonance the interferometer transmittance shows Fano, lineshapes as a function of the gate voltage applied to the other dot. Our model includes the effect of the magnetic field on the dot levels and explains the change of the asymmetric tail as the magnetic flux is varied. The transmittance assigned to the Fano, dips located in the almost crossing point of the charging diagrams shows Aharonov-Bohm oscillations.