National Institute Of Materials Physics - Romania

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839 articles found

531. Kinetical study of the nonstoichiometric vapour growth process in the system ZnSe:I-2

Authors: Stanculescu, A

Published: AUG 2010, CRYSTAL RESEARCH AND TECHNOLOGY, 45, 799, DOI: 10.1002/crat.201000059

We have studied the growth of ZnSe crystals by chemical transport in a closed system in nonstoichiometric conditions, and we have deduced that the interface kinetics is the phenomenon limiting the growth process. The effect on the growth process of the deviation from stoichiometry of the II-VI compound was investigated using a mathematical model that involves indirect data computed from directly obtained experimental values. The experimental crystallization rate was compared with the maximum value of the transport flux calculated using the Arizumi-Nishinaga model. The influence of the stoichiometry of the source material and of the variations in the growth parameters (supercooling, geometrical dimension, specific loading of the ampoule and iodine concentration) on the ZnSe crystal growth process has also been studied. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

532. Investigations of the non-isothermal crystallization of CaF2 nanoparticles in Sm-doped oxy-fluoride glasses

Authors: Aldica, G; Secu, M

Published: JUL 15 2010, JOURNAL OF NON-CRYSTALLINE SOLIDS, 356, 1636, DOI: 10.1016/j.jnoncrysol.2010.06.017

Oxyfluoride glass samples in the system SiO2-Al2O3-CaF2-SmF3 were produced by using the melt-quenched technique and the kinetics of non-isothermal crystallization of CaF2 nano-crystals has been investigated. Differential thermal analysis (DTA) has shown an exothermic peak at about 700 degrees C which was assigned to the CaF2 nano-crystals precipitation within the glass matrix as was identified by X-ray diffraction (XRD) analysis of the annealed glass samples. It was shown the Ozawa model is most suitable for describing the behaviour of non-isothermal crystallization of CaF2 nano-crystals within the glass matrix. The mean value of the Avrami exponent was found 1.16 which indicates the diffusion-controlled growth process of nano-particles in the glass system with zero nucleation rate as the single operative mechanism; the activation energy of the crystallization is E-d = 379 +/- 25 kJ/mol (3.93 +/- 0.26 eV). The activation energy of the nano-crystals growth was computed within the same model and agrees very well with the activation energy of the crystallization. (C) 2010 Elsevier B.V. All rights reserved.

533. Dynamics of cyanophenyl alkylbenzoate molecules in the bulk and in a surface layer adsorbed onto aerosil. Variation of the lengths of the alkyl chain

Authors: Frunza, S; Schonhals, A; Frunza, L; Beica, T; Zgura, I; Ganea, P; Stoenescu, D

Published: JUN 16 2010, CHEMICAL PHYSICS, 372, 60, DOI: 10.1016/j.chemphys.2010.04.031

The molecular mobility of 4-butyl- and 4-pentyl-4'-cyanophenyl benzoate (CP4B, CP5B) and their composites prepared from aerosil A380 was investigated by broadband dielectric spectroscopy in a large temperature range. Thermogravimetric and infrared investigations were additionally performed. High silica density (larger than 7 g aerosil/1 g of liquid crystal) was selected to observe a thin layer adsorbed on the surface of the silica particles. The data were compared with those of the member of the series with six carbon atoms in the alkyl tail. Bulk CP4B and CP5B show the dielectric behaviour expected for liquid crystals. For the composites one relaxation process is observed at frequencies much lower than those for the corresponding bulk, which was assigned to the dynamics of the molecules in a surface layer. The temperature dependence of the relaxation rates (and of the dielectric strength) shows a crossover behaviour with two distinguished regimes. At higher temperatures the data obey the Vogel-Fulcher-Tammann law, whereas an Arrhenius law is observed at lower temperature, in a close similarity to the behaviour of a constrained dynamic glass transition. The estimated Vogel and crossover temperature is independent on the tail length, while the activation energy for the low temperature branch increases weakly with increasing the alkyl tail. (C) 2010 Elsevier B. V. All rights reserved.

534. Photocatalytic activity of pulsed laser deposited TiO2 thin films in N-2, O-2 and CH4

Authors: Socol, G; Gnatyuk, Y; Stefan, N; Smirnova, N; Djokic, V; Sutan, C; Malinovschi, V; Stanculescu, A; Korduban, O; Mihailescu, IN

Published: JUN 1 2010, THIN SOLID FILMS, 518, 4653, DOI: 10.1016/j.tsf.2009.12.051

We report on pulsed laser deposition of TiO2 films on glass substrates in oxygen, methane, nitrogen and mixture of oxygen and nitrogen atmosphere. The nitrogen incorporation into TiO2 lattice was successfully achieved, as demonstrated by optical absorption and XPS measurements. The absorption edge of the N-doped TiO2 films was red-shifted up to similar to 480 nm from 360 nm in case of undoped ones. The photocatalytic activity of TiO2 films was investigated during toxic Cr(VI) ions photoreduction to Cr(III) state in aqueous media under irradiation with visible and UV light. Under visible light irradiation, TiO2 films deposited in nitrogen atmosphere showed the highest photocatalytic activity, whereas by UV light exposure the best results were obtained for the TiO2 structures deposited in pure methane and oxygen atmosphere. (C) 2009 Elsevier B.V. All rights reserved.

535. Optical properties of zinc oxide thin films doped with aluminum and lithium

Authors: Galca, AC; Secu, M; Vlad, A; Pedarnig, JD

Published: JUN 1 2010, THIN SOLID FILMS, 518, 4606, DOI: 10.1016/j.tsf.2009.12.041

Aluminum doped Zinc Oxide (AZO) and Lithium doped Zinc Oxide (LZO) thin films are obtained by Pulsed Laser Deposition (PLD) method. These films are characterized by using Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD) and Photoluminescence (PL). By modeling the ellipsometry spectra we get the dielectric functions, the optical band gap E(g), and the electrical properties. Our results show the influence of the processing parameters on the optical and structural properties of doped ZnO thin films. The post-annealing treatment applied to AZO thin films, changes strongly the optical properties, by lowering the resistivity and red-shifting the band gap. (C) 2009 Elsevier B.V. All rights reserved.

536. Engineered beam shaping effect in anisotropic photonic crystals

Authors: Rasoga, O; Dragoman, D

Published: APR 10 2010, APPLIED OPTICS, 49, 2167, DOI: 10.1364/AO.49.002161

We demonstrate that an anisotropic photonic crystal can modify the shape of a highly convergent incident optical beam. The beam shape engineering is relatively easy, and the photonic crystal is less alignment demanding than beam shapers that incorporate several optical systems. The shape of the output beam can be controlled by an appropriate choice of the angular divergence of the beam, the number of periods and the birefringence values and layer widths of the constituent materials. (C) 2010 Optical Society of America

537. Involvement of cyan and ester groups in surface interactions of aerosil-cyanophenyl alkyl benzoate systems with high silica density: Infrared investigations

Authors: Frunza, L; Frunza, S; Zgura, I; Beica, T; Gheorghe, N; Ganea, P; Stoenescu, D; Dinescu, A; Schonhals, A

Published: APR 2010, SPECTROCHIMICA ACTA PART A-MOLECULAR AND BIOMOLECULAR SPECTROSCOPY, 75, 1235, DOI: 10.1016/j.saa.2009.12.029

Composites prepared from aerosil A380 and liquid crystals (LCs) of 4-n-alkyl-4'-cyanophenyl benzoate type, with four to six carbon atoms in the alkyl chain were investigated by infrared spectroscopy. Their high silica content (of 2-7 g aerosil/1 g of LC) was given by thermogravimetric investigations and allows the observation of a rather thin LC layer on the silica particles. Several surface species onto the external surface of the grains were demonstrated. Arguments are given that monomer and dimer species are present in the bulk cyanophenyl benzoate materials while bulk-like species along with hydrogen-bonded ones coexist in the so-called surface layer of the composites. The main interaction of LC molecules with the aerosil surface is by hydrogen bonding taking place with the involvement of the cyan group. There is a contribution of ester carbonyl group to these surface interactions but this cannot be well quantified. (C) 2009 Elsevier B.V. All rights reserved.

538. MAPLE deposited thin monomer films of maleimidic derivatives for photonics

Authors: Stanculescu, A; Albu, AM; Socol, G; Stanculescu, F; Socol, M; Preda, N; Rasoga, O; Girtan, M; Iulian, I

Published: MAR 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 739, DOI:

This paper presents a study of some thin monomer films deposited by MAPLE. These monomers have been synthesised from maleic anhydride and aniline derivatives and are characterized by a molecular structure adequate for the generation of optical non-linear phenomena. UV-VIS, FTIR, Raman and Photoluminescence spectroscopy have been used to investigate the influence of the experimental conditions on the properties of the films. X-Ray Diffraction has evidenced a certain degree of disorder in the thin films, which is correlated with the randomly orientated molecules and AFM images have revealed different roughness for different monomer layers and different substrates. Second harmonic measurements have shown a strong signal emitted by the thin film prepared from the monomer with one [-NH-NH-] donor and two [-NO(2)] acceptor groups. This thin film is characterized by good transparency and low photoluminescence emission at excitation with the wavelength of the second harmonic (lambda-400 nm).

539. Specificity of defects induced in silicon by RF-plasma hydrogenation

Authors: Ghica, C; Nistor, LC; Stefan, M; Ghica, D; Mironov, B; Vizireanu, S; Moldovan, A; Dinescu, M

Published: MAR 2010, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 98, 785, DOI: 10.1007/s00339-009-5527-1

Silicon wafers have been submitted to hydrogen RF-plasma treatment in various experimental conditions. Hydrogen RF-plasma treatment induced two kinds of effects on Si wafers, depending on the treatment conditions: surface corrugation and formation of structural defects below the free surface. Atomic force microscopy (AFM) investigations showed that the surface roughness significantly increased with the treatment duration, leading to the formation of pyramidal humps on the surface. The structural defects resulting after the plasma treatments were investigated by conventional and high-resolution transmission electron microscopy (CTEM and HRTEM) techniques. The specificity of the induced extended defects due to hydrogen decoration was emphasized. Three types of extended defects were identified and characterized: planar defects in the {111} and {100} planes and nanometric voids. Point defects related to the hydrogenation process were investigated by electron paramagnetic resonance (EPR) in correlation with the electron microscopy results.

540. Structure and properties of silver doped SnSe2 and Ge2Sb2Te5 thin films prepared by pulsed laser deposition

Authors: Popescu, M; Velea, A; Sava, F; Lorinczi, A; Tomescu, A; Simion, C; Matei, E; Socol, G; Mihailescu, IN; Andonie, A; Stamatin, I

Published: MAR 2010, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 207, 520, DOI: 10.1002/pssa.200982900

Amorphous thin films of SnSe2 and Ge2Sb2Te5 doped by different amount of silver (0.1, 0.2, 0.5 and 1 Ag atoms per formula unit) have been prepared by pulsed laser deposition (PLD) starting from solid polycrystalline targets. The films were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The good gas sensing properties for CO, as well as the sensitivity for CH4 and NO of the Ag; doped SnSe2 films have been demonstrated in the composition SnSe2Ag0.2. The structural effect of silver introduced in Ge2Sb2Te5 matrix has been investigated. The freshly deposited thin films doped by various amount of Ag develop three phases: an amorphous one, and two crystalline phases consisting of a major fcc cubic phase of AgSbTe2 and a minor cubic phase of composition Ag2Te. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim