National Institute Of Materials Physics - Romania

Publications

873 articles found

801. Magnetic interactions and spin structure in composite Fe/Nd2Fe14B systems

Authors: Kuncser, V; Valeanu, M; Lifei, F; Predoi, D; Palade, P; Schinteie, G; Crisan, O; Filoti, G

Published: MAY 2004, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 272, 799, DOI: 10.1016/j.jmmm.2003.12.1279

Local magnetic interactions and spin configurations in alpha-Fe/Nd2Fe14B composite exchange-spring magnets have been investigated by Mossbauer spectroscopy and the magnetic behaviour was analysed by magnetometry. A new method for the evaluation of the coupling strength between soft and hard magnetic phases is proposed. (C) 2003 Elsevier B.V. All rights reserved.

802. On the optical properties of micro- and nanometric size PbI2 particles

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

Published: APR 7 2004, JOURNAL OF PHYSICS-CONDENSED MATTER, 16, 2356, DOI: 10.1088/0953-8984/16/13/014

Scanning electron microscopy, Raman scattering, UV-VIS absorption spectroscopy and low temperature photoluminescence (PL) were used to examine small particles produced by the chemical reaction between Pb(NO3)(2) and KI in different liquid media: water, methanol, ethanol and acetonitrile. By stoichiometric changes in the synthesis reaction, platelets of PbI2 and rods probably of KPbI3 are produced. Regardless of shape and size, these particles exhibit almost the same PL, which consists of two intense bands centred around the 2.5 (E band) and 2.0 eV (G band), in turn similar to that of a crystalline slide or a micrometric powder, both prepared from a PbI2 single-crystal grown from the melt. Crystalline PbI2 platelets exhibit an E band with two components, at 2.49 (E-F band) and 2.47 eV (E-T band), originating in the recombination of the free and trapped excitons produced by inter-band irradiation. A close relation between the enhancement of the E-T and G band reveals that they are linked to the surface defects. For the rod-like particles, the PL spectrum is somewhat similar to that of a Pb2+ ion introduced into an alkaline halide lattice, which as for any ns(2) ion displays two emission bands, A(T) and A(X), whose correspondents are E and G bands.

803. Electrodeposition of CdTe nanorods in ion track membranes

Authors: Sima, M; Enculescu, I; Trautmann, C; Neumann, R

Published: MAR 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 125, DOI:

CdTe nanowires were electrodeposited in ion track membranes with pore diameters in the range 100-2000 nm. Acidic and basic baths were tested for obtaining materials with good stoichiometry. The cyclic voltammograms were compared for the case of deposition on a carbon rotating disc and on the porous membrane substrates, the differences being attributed to the additional resistance induced by the pores. For larger pores a tendency of growing hollow structures was observed.

804. Spin magnetization of a strongly correlated electron gas confined in a two-dimensional finite lattice

Authors: Nita, M; Dinu, V; Aldea, A; Tanatar, B

Published: FEB 2004, PHYSICAL REVIEW B, 69, DOI: 10.1103/PhysRevB.69.073106

The influence of disorder and interaction effects on the ground state polarization of the two-dimensional correlated electron gas is studied by numerical investigations of the unrestricted Hartree-Fock approach. With the model of Anderson disorder a continuous increase of the spin magnetization until the fully polarized regime is obtained. The ferromagnetic ground state is found to be favorable when the electron number is lowered and the interaction and disorder parameters are suitably chosen.

805. Structural and magnetic properties of iron species/SiO2 nanocomposites obtained by sol-gel methods

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

Published: 2004, SECOND SEEHEIM CONFERENCE ON MAGNETISM, PROCEEDINGS, 3510, DOI:

The phase composition, local structure and magnetic interactions were investigated in FexOx-SiO2 nanocomposites obtained by modern sol-gel methods. The very sensitive method of Mossbauer spectroscopy has evidenced the strong influence of the silica matrix precursor on the iron phase composition in the initial composites as well as on the size distribution of the hematite nanoparticles obtained by suitable thermal annealing treatments.

806. C-60 aggregates in pyrrolidine and N-methyl-2-pyrrolidinone evidenced by surface-enhanced Raman scaterring spectra

Authors: Baibarac, M; Preda, N; Mihut, L; Velula, T; Baltog, I; Mevellec, JY; Lefrant, S

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 417, 587, DOI: 10.1080/15421400490478434

At room temperature, soluted C-60 in pyrrolidine and N-methyl-2-pyrrolidinone associates slowly in stable aggregates. The process originates in the charge transfer from the solvent molecules as electron donors towards fullerene. In the aggregate phase the intermolecular interactions lead to a large reduction in parent I-h C-60 symmetry such that a modified phonon spectrum is observed. Using the surface-enhanced Raman scattering we demonstrate that the Raman range below 800 cm(-1) is most diagnostic for aggregate assignment. In particular, the strong decrease of the radial breathing mode and appearance of new bands at 110 and 93 cm(-1) due to the interball interactions must be noticed.

807. Photoluminescence studies on [C60] fullerene solutions in N-methyl-2-pyrrolidinone and pyrrolidine

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

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 415, 21, DOI: 10.1080/15421400490478209

Photoluminescence (PL) spectra of C-60 solutions in N-methyl-2-pyrrolidinone (NMP) show a drastic increase of the PL intensity if the solutions are stored longer periods of time. Two distinct spectral components, one in the red (1.92-2.00 eV) and another in the green (2.18-2.3 eV) were identified in the whole PL emission. Their weights depend on C-60 concentration and on storage time. The two emissions come from two successive processes running with different kinetics. Charge transfer, leading to (C-60, NMP) loose molecular complexes and a slow aggregation process of these entities seems to originate the red and green emission, respectively. Similar PL properties are found when pyrrolidine (P) is used as solvent.

808. Electrochemical deposition of PBSe1-xTex nanorod arrays using ion track etched membranes as template

Authors: Sima, M; Enculescu, I; Visan, T; Spohr, R; Trautmann, C

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 418, 755, DOI: 10.1080/15421400490478885

Lead chalcogenides (PbS, PbSe, PbTe) are narrow band gap semiconductors which have been studied in the field of IR detection and thermoelectric devices. The template method is a general approach for synthesizing nanomaterials within the pores of membranes. The membranes employed contain cylindrical pores with monodisperse diameters, and corresponding cylindrical nanostructures are obtained. The aim of the present study was to prepare PbSe1-xTex nanorod arrays using electrodeposition. The process was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The resulted PbSe1-xTex nanostructures were characterized using scanning electron microscopy (SEM) as well as energy dispersive X-ray analysis (EDAX).

809. Emission properties of T-a-centres in KCl

Authors: Topa, V; Tsuboi, T; Polosan, S

Published: AUG-DEC 2004, RADIATION MEASUREMENTS, 38, 734, DOI: 10.1016/j.radmeas.2004.01.021

Electrolytic colouration at about 600degreesC and 350 V/cm, for KCl crystal containing Pb2+ and for KCl:Pb2+ crystals co-doped with Li+, Na+ and Rb+ has been undertaken. Several absorption bands were observed in both doped and co-doped crystals in the visible-UV region. Excitation into these bands gives rise to the same 0.86 eV emission band except for the Li+-co-doped crystal which gives rise to a 0.80 eV emission band. These absorption bands are due to the same T-a-centre related to Pb-. The observed infrared emission intensity of the crystal with F-centres is higher than of without F-centres. (C) 2004 Elsevier Ltd. All rights reserved.

810. Electronic transport in nanosystems

Authors: Wulf, U; Racec, ER; Racec, PN; Aldea, A

Published: DEC 15 2003, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 23, 681, DOI: 10.1016/j.msec.2003.09.080

We demonstrate the application of the Landauer-Buttiker formalism (LBF) to MIS (metal-insulator-semiconductor)-type quantum nanostructures and analyze their static and dynamical response properties. For the time-dependent problem, we employ the random phase approximation (RPA) and calculate the irreducible polarization from the one-particle scattering wave functions in the Hartree approximation. For their calculation, we employ the R-matrix approach which we have developed over the past years. We find in the low frequency limit a reasonable agreement with the static Hartree calculations and experimental capacitance data. (C) 2003 Published by Elsevier B.V.