National Institute Of Materials Physics - Romania

Publications

860 articles found

691. Optical spectroscopy and thermoluminescence of electrolytically colored KCl:Tl+ crystals

Authors: Polosan, S; Enculescu, M; Secu, M; Apostol, E

Published: NOV 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 3011, DOI:

The paper presents the spectroscopic properties of electrolytic colored KCl:Tl and KCI:Tl+Ca crystals and the assignments of the new defects which are formed during electron injection process. The new formed defects Tl-0 and Tl-0+Ca2+ with the metal in the anionic sites, presents very high photoluminescence properties in the near infrared region. Together with those centers, the F-centers, their aggregates and the holes centers are formed during electrolytic coloring process. Their properties were studied by optical absorption (OA), photoluminescence (PL) and thermoluminescence (TL) measurements and compared with those obtained by ionizing irradiation in uncolored KCl:Tl crystals. The results suggest a more efficient Tl+-> Tl-0 process in the case of Ca2+ codoped samples due to the prolonged time of electrons injection, low temperature and high electric field.

692. Preparation and Properties of Cobalt Doped ZnO Nanowires

Authors: Enculescu, I; Matei, E; Sima, M; Neumann, R; Granville, S; Ansermet, JP

Published: NOV 2008, IEEE TRANSACTIONS ON MAGNETICS, 44, 2680, DOI: 10.1109/TMAG.2008.2003242

The template method was used for preparing Zn1-xCoxO nanowires with x ranging from 0.01 to 0.05. Thus, electrochemical deposition was employed for filling up the pores of polycarbonate ion track nanoporous membranes with the desired material. The method allows a good control over the morphology and composition of the deposited nanowires, as evidenced by scanning electron microscopy (SEM) and energy dispersive X ray analysis (EDX). Measurements of the magnetic properties showed a paramagnetic behavior of the nanowire arrays for the whole set of temperatures and Co concentrations.

693. Luminescence tuning of Si/SiO2 nanoparticles in aqueous solutions

Authors: Marcu, A; Sima, C; Grigoriu, C; Enculescu, I; Iliescu, B

Published: NOV 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 3134, DOI:

We investigated the possibility of tuning luminescent spectra for a given Si/SiO2 nanoparticle size distribution in aqueous solution. The luminescence spectra of Si/SiO2 Suspensions in aqueous solutions of different particle/cluster concentrations were studied. The different Si/SiO2 nanoparticle/cluster concentrations were produced by both centrifugation and dilution. Depending on the centrifugal force we found a reversible shift up to 100 nm toward UV, regardless of the excitation wavelength. The shift could be reverse by sonication of the sample. A-scenario based on particle and cluster size variation is also discussed.

694. Structural and morphological properties of NiCu magnetic thin films

Authors: Bazavan, D; Bazavan, R; Enculescu, I; Matei, E; Ion, L; Antohe, S

Published: NOV 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 3057, DOI:

NiCu thin films for magnetic applications have been produced by using an electrochemical method. Their structural and morphological properties have been investigated by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM and EDX) and are discussed in correlation with the growth conditions.

695. Phase-change electrical memory elements and devices

Authors: Popescu, M; Lorinczi, A; Sava, F; Velea, A; Matei, E; Socol, G; Mihailescu, IN

Published: OCT 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2621, DOI:

Electrical memory elements based on Ge-Sb-Te pure and doped by Sn-Se have been obtained by pulsed laser deposition on special substrates covered by gold as well as on common glass. A set of electrical memory elements in a 4x4 matrix structure on the glass substrate has been produced. Devices with 3 and 10 memory elements have been constructed and tested for their memory properties. The special features of the voltage-current characteristics have been revealed.

696. Structural and hydrogen absorption/desorption properties of YNi(4-x)Al(x)Mg compounds (with 0 <= x <= 1.5)

Authors: Stan, C; Andronescu, E; Predoi, D; Bobet, JL

Published: AUG 11 2008, JOURNAL OF ALLOYS AND COMPOUNDS, 461, 234, DOI: 10.1016/j.jallcom.2007.07.064

The compounds YNi(4-x)Al(x)Mg with 0 <= x <= 1.5, crystallizing in the C15 structure type (MgCu(4)Sn-type), have been elaborated by mechanical milling and a subsequent heat treatment at 650 degrees C for 1 h. It was possible to synthesize the compounds YNi(4-x)Al(x)Mg with x up to approximately 1.15 and the lattice parameters was in good agreement with those already reported. All the samples, excepting the YNi(4)Mg compound, absorbed and desorbed hydrogen reversibly and no structural change is reported after the hydrogenation. A direct relationship between the lattice parameters and the equilibrium plateau pressure was found. (C) 2007 Elsevier B.V. All rights reserved.

697. Calcium phosphate ceramics for biomedical applications

Authors: Predoi, D; Vatasescu-Balcan, RA; Pasuk, I; Trusca, R; Costache, M

Published: AUG 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2155, DOI:

Calcium phosphate compounds have been studied for biomedical applications due to chemical and structural similarity to the mineral phase of bone and tooth. The composition, physico-chemical properties, crystal size and morphology of synthetic apatite are extremely sensitive to preparative conditions and sometimes it resulted into non-stoichiometric calcium deficient hydroxyapatite (HAp) powders. The present paper describes the synthesis of calcium phosphate ceramics powders via a sol-gel method, The powders were sintered at 600 and 800 degrees C. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used for characterization and evaluation of the phase composition, morphology and particle size of samples, The functional group of the coatings were analyzed using Fourier transform infrared (FTIR). The XRD analysis revealed a well crystallized HA structure at both temperatures. At 800 degrees C a small amount of CaO (about 0.4%) was detected. The mean crystallite size, determined from the breadth of the diffraction lines, increases from 30 nm at 600 degrees C to about 110 nm at 800 degrees C. FTIR spectra showed the presence of various PO43- and OH- groups present in the powders. Osteoblast cells were used to determine cell proliferation, viability and citotoxicity after interaction with the prepared bioceramics, by MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, a tetrazole) assay. We note that the viability of HAp-600 starts to decrease after 12h, while the viability of HAp-800 decreases after 6h.

698. Optical properties of CaF2 : Eu3+ nanoctystals embedded in transparent oxyfluoride glass ceramic

Authors: Polosan, S; Secu, CE

Published: AUG 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2137, DOI:

Transparent oxyfluoride glass ceramic in the system SiO2-Al2O3-CaF2-EuF2 containing CaF2:Eu3+ nanocrystals were produced by annealing of the initial glasses slightly above the CaF2 phase crystallisation peak. The relative good transparency of these materials was due to the small 65 nm, nucleated CaF2 nanocrystallites. X-ray diffraction data have shown an increase of the cell parameters for CaF2:Eu3+ compared with CaF2 which was attributed to the substitution of Ca2+ ions with Eu3+ ions in the CaF2 nanoparticles. Optical investigations using UV-VIS-NIR absorption and luminescence spectroscopy have shown an increase of the splitting of the Eu3+-ion associated luminescences and small shifts of the Eu3+ ion characteristic IR absorption peaks. These effects have been attributed to the Eu3+ environment evolving from the glass to the crystalline-like one.

699. Transport through quantum dots with magnetic impurities

Authors: Tolea, M; Aldea, A; Bulka, BR

Published: JUL 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 1758, DOI:

We analyze the electronic transport through a quantum dot that contains a magnetic impurity. The coherent transport of electrons is governed by the quantum confinement inside the dot, but is also influenced by the exchange interaction with the impurity. The interplay between the two gives raise to the singlet-triplet splitting of the energy levels available for the tunneling electron. In this paper, we focus on the charge fluctuations and, more precisely, the height of the conductance peaks. We show that the conductance peaks corresponding to the triplet levels are three times higher than those corresponding to singlet levels, if electronic correlations are neglected (for non-interacting dots, when an exact solution can be obtained). Next, we consider the Coulomb repulsion and the many-body correlations. In this case, the singlet/triplet peak height ratio has a complex behavior. Usually the highest peak corresponds to the state that is lowest in energy (ground state), regardless if it is singlet or triplet. In the end, we get an insight on the Kondo regime for such a system, and show the formation of three Kondo peaks. We use the equation of motion method with appropriate decoupling.

700. Resonant Raman scattering and absorption spectroscopy studies on individual carbon nanotubes in surfactant solutions

Authors: Husanu, M; Baibarac, M; Preda, N; Baltog, I

Published: JUL 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 1726, DOI:

Carbon Nanotubes are exotic low-dimensional systems, and due to their remarkable properties, are promising components for technological applications. The electronic and vibrational signatures raising from their existence as individual entities or associated in bundles, is still a controversed subject. A careful analysis of the G (similar to 1600 cm(-1)) and RBM (100-200 cm(-1)) band features as: profile, peak position and intensity in the Raman spectra may be a valuable indication about their dispersion state. The UV-VIS-NIR absorption spectrum comes to support this assumption. Observing the absorption spectra associated to resonant excitation of semiconducting and metallic nanotubes, may reveal complementary informations concerning the individualization procedure efficiency.