National Institute Of Materials Physics - Romania

Publications

873 articles found

741. Transport through a multiply connected interacting mesoscopic system using the Keldysh formalism

Authors: Dinu, IV; Tolea, M; Aldea, A

Published: SEP 2007, PHYSICAL REVIEW B, 76, DOI: 10.1103/PhysRevB.76.113302

We apply the Keldysh formalism in order to derive a current formula easy to use for a system with many sites, one of which is interacting. The main technical challenge is to deal with the lesser Green's function. It turns out that, in the case of the left-right symmetry, the knowledge of the lesser Green's function is not necessary and an exact current formula can be expressed in terms of retarded Green's functions only. The application is done for a triangular interferometer which gives a good account of the Fano-Kondo effect. It is found that the interference effects, in the context of Kondo correlations, give rise to a point in the parameter space where the conductance is temperature independent. We include a comparison with the results from Ng's ansatz [Phys. Rev. Lett. 70, 3635 (1993)], which are less accurate, but can be used also in the absence of the above mentioned symmetry.

742. Influence of geometrical properties on light emission of ZnO nanowires

Authors: Enculescu, I; Sima, M; Enculescu, M; Enache, M; Vasile, V; Neumann, R

Published: SEP 2007, OPTICAL MATERIALS, 30, 75, DOI: 10.1016/j.optmat.2007.01.002

The present paper deals with the preparation and study of optical properties of ZnO nanowires. Nanoporous ion track polycarbonate foils were used as templates for ZnO nanowires growth by electrochemical deposition. The growth was performed using an aqueous electrochemical bath at a temperature of 70 degrees C. SEM and TEM were employed to study the morphological properties of the nanowires. Electron diffraction was employed for structural characterization. Reflection and photoluminescence spectroscopy were used for optical characterization. A comparison between the properties of electrodeposited thin films and nanowires of ZnO was made from the point of view of photoluminescence properties. (c) 2007 Elsevier B.V. All rights reserved.

743. Electrolytic reduction of Tl+ ions in KCl crystals

Authors: Polosan, S; Tsubi, T; Apostol, E; Topa, V

Published: SEP 2007, OPTICAL MATERIALS, 30, 97, DOI: 10.1016/j.optmat.2006.11.006

Electrolytic coloration has been made for KCl crystals doped with Tl+ ions and doped with Tl+ and Ca2+ ions. Intensity decrease of the Tl+ absorption bands is observed in the two colored crystals, while several new absorption and emission bands are observed. They are attributed to Tl- ions which were converted from Tl+ ions by electron injection during electrolytic coloration. Different absorption and emission bands are observed for KCI:TI and KCI:T1 + Ca crystals. Sharp absorption band appears at 285 nm in the colored KCl:Tl and at 400 and 503 nm in the colored KCLTl + Ca. Infrared emission bands are observed at 1470, 1600 and 2185 nm in colored KCI:Tl, while at 1630 and 1900 nm in colored KCl:Tl + Ca. It is suggested that the absorption and emission bands in KCl:Tl + Ca are due to Tl- ions perturbed by Ca2+ or vacancy at the nearest neighbor. (c) 2006 Elsevier B.V. All rights reserved.

744. Asymmetric localization in disordered Landau bands

Authors: Nita, M; Aldea, A; Zittartz, J

Published: JUN 6 2007, JOURNAL OF PHYSICS-CONDENSED MATTER, 19, DOI: 10.1088/0953-8984/19/22/226217

We show that, due to band mixing, the eigenstate localization within the disordered Landau bands gets an asymmetric structure: the degree of localization increases in the lower part of the band and decreases in the upper one. The calculation is performed for a two-dimensional lattice with the Anderson disorder potential and we prove that this effect is related to the upper shift of the extended states within the band and is enhanced by the disorder strength. The asymmetric localization and the energy shift disappear when the interband coupling is switched off.

745. Iron oxide in a silica matrix prepared by the sol-gel method

Authors: Predoi, D; Crisan, O; Jitianu, A; Valsangiacom, MC; Raileanu, M; Crisan, M; Zaharescu, M

Published: JUN 4 2007, THIN SOLID FILMS, 515, 6323, DOI: 10.1016/j.tsf.2006.11.148

FexOy-SiO2 nanocomposites were prepared by sol-gel method by using two SiO2 sources and Fe(SO4)(2)center dot 7H(2)O as raw materials. The amorphous gels were thermally treated up to 1000 degrees C. The initial gel and the thermally treated samples were characterized by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) and infrared spectroscopy. The presence of hematite was confirmed by the obtained Mossbauer spectra which showed the characteristic sextet. The total amount and the size distributions of the hematite nanoparticles can be controlled via the initial precursors and subsequent by annealing conditions. (C) 2006 Elsevier B.V. All rights reserved.

746. Thermal studies of magnetic spinel iron oxide in solution

Authors: Predoi, D; Valsangiacom, CM

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1799, DOI:

Small particles of iron - based magnetic spinel oxide are technologically interesting because of their magnetic properties. The synthesis from solution, generally achieved by coprecipitation, offers some advantages: simple and rapid preparation, control of particle size and composition, various possibilities to modify the particle surface state allowing to make homogeneous and stable dispersion in liquid or solid media. This work reports the thermal studies of magnetic iron spinel oxide nanoparticles. For this study, the Fe2+-Fe3+ mixture (Fe2+/Fe3+=0.5) was introduced in an alkaline solutions of which the pH was kept constant. The influence of the particles sizes on the Y -> aFe(2)O(3) transformation is analyzed.

747. IR and thermal studies of iron oxide nanoparticles in a bioceramic matrix

Authors: Birsan, C; Predoi, D; Andronescu, E

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1824, DOI:

Magnetic iron oxide nanoparticles have a great potential in various applications in biomedical research and technology. Iron oxide particles such as magnetite (Fe3O4) is by far the most commonly employed magnetic materials for biomedical applications. Bioceramic composites were obtained by combining the magnetite with two biocompatible components (hydroxyapatite and bioglass). Their thermal behaviour has been studied by thermal analysis (DTA and TG). The interaction of iron oxide with the bioceramic matrix has been investigated using IR spectroscopy.

748. Time-resolved luminescence spectroscopy of Eu2+ in BaFCl : Eu2+ X-ray storage phosphor

Authors: Secu, M; Secu, CE; Vasile, V; Predoi, D; Gazdaru, DM

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1802, DOI:

Photoluminescence (PL) spectra recorded at room temperature (RT) in polycrystalline BaFCI:Eu2+ X-ray storage phosphor have shown the europium associated luminescences, a broad luminescence band at 385nm (3.22eV) with a 3.3 +/- 0.1 mu s PL lifetime accompanied by a luminescence line at about 368nm (3.37eV). The energy difference Delta EB-L =226cm(-1) between the two excited levels from which the europium associated band and line luminescences took place was computed. An additional broad luminescence band at about 425nm (2.92eV) with 0.7 +/- 0.1 mu s PL lifetime has been attributed to the europium luminescence perturbed by an impurity. Time resolved spectroscopy measurements have shown that no other impurity luminescences (like oxygen-vacancy centres) can be observed at RT except the europium associated ones.

749. Characterisation of magnetic iron oxide in hydroxyapatite

Authors: Birsan, M; Predoi, D; Birsan, C; Secu, CE; Andronescu, F

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1832, DOI:

In order to be used for medical applications, a material must exhibit many specific characteristics, of which the most fundamental is related to biocompatibility. Lately, the iron oxide nanoparticles are preferred for biomedical applications because they present a superparamagnetic behaviour at room temperature. The preparation method and conditions of iron oxide nanoparticles represents some of the most important challenges that will determine the particle size and shape, the size distribution, the surface chemistry of the particles and consequently their magnetic properties. The samples were characterized by X-ray diffractions (XRD), transmission electron microscopy (TEM) and infrared spectroscopy.

750. The colloidal route of the sol-gel process - an alternative to produce Fe3O4-SiO2 nanocomposites

Authors: Raileanu, M; Crisan, M; Ballo, A; Cosma, C; Petre, J; Stan, C; Predoi, D; Valsangiacom, CM; Tolea, F; Secu, C

Published: MAY 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1402, DOI:

The iron oxide-based nanomaterials have a great importance because of their impact on a wide number of industries. They have many properties and applications in domains as: environmental protection, biomedical, catalysis, information displays and electronics. In order to avoid the tendency of nanoparticles to aggregate, they are often included in sol-gel derived silica matrices, thus being also ensured a homogeneous dispersion of the ultra-fine metal oxide particles in the host matrix. The reasons of the attempt are both an economic and a non-pollutant one, tacking into account the fact that it is well known that aqueous silica sol is cheaper and less toxic than TEOS. The Fe3O4 was introduced in the reaction mixture as aqueous suspension. A final iron content related to SiO2 of 3% wt. was chosen for the prepared nanocomposite. Thermal analysis, IR spectroscopy, XRD and TEM methods have been used for the structural characterisation,. Some experiments of water depollution (from As) have been proceeded using the prepared Fe3O4-SiO2 nanocomposite.