National Institute Of Materials Physics - Romania

Publications

830 articles found

411. Growth and structural characterization of orthorhombic and tetragonal SrCuO2 thin films

Authors: Mihailescu, CN; Pasuk, I; Athanasopoulos, GI; Luculescu, C; Socol, M; Saint-Martin, R; Revcolevschi, A; Giapintzakis, J

Published: AUG 1 2013, APPLIED SURFACE SCIENCE, 278, 135, DOI: 10.1016/j.apsusc.2012.12.029

Epitaxial SrCuO2 thin films were grown on (0 0 1) SrTiO3 substrates by pulsed laser deposition using a stoichiometric target. X-ray diffraction indicated that the SrCuO2 films undergo a structural phase transition as a function of the substrate temperature. Films deposited at temperatures below 600 degrees C exhibit a tetragonal phase with the c-axis oriented along the growth direction while films deposited at temperatures above 700 degrees C exhibit an orthorhombic phase with the b-axis oriented along the growth direction. Atomic force microscopy indicated that the as-grown film surfaces are rather smooth and the roughness increases with increasing substrate temperature. Energy dispersive X-ray spectroscopy in agreement with X-ray diffraction intensity ratio data revealed that all films are non-stoichiometric and contain Sr vacancies (Sr/Cu similar to 0.8). The influence of film-substrate lattice matching and substrate temperature on the structural phase transition is discussed. (C) 2012 Elsevier B.V. All rights reserved.

412. Structural and dielectric properties of Ba(X1/3Ta2/3)O-3 thin films grown by RF-PLD

Authors: Nedelcu, L; Scarisoreanu, ND; Chirila, C; Busuioc, C; Banciu, MG; Jinga, SI; Dinescu, M

Published: AUG 1 2013, APPLIED SURFACE SCIENCE, 278, 161, DOI: 10.1016/j.apsusc.2012.10.124

Ba(X1/3Ta2/3)O-3 (X = Mg, Zn) thin films were grown on commercial Pt-coated Si substrates by radiofrequency plasma beam assisted pulsed laser deposition (RF-PLD) method. Single phase Ba(Mg1/3Ta2/3)O-3 and Ba(Zn1/3Ta2/3)O-3 ceramic targets having an ordered hexagonal structure (with P (3) over bar m1 space group) were used for deposition. Structural, morphological and surface characterizations of the Ba(X1/3Ta2/3)O-3 (BXT) films were performed using X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray diffraction studies evidenced a cubic (disordered) perovskite structure, with unit cell typical for Pm3m space group. Scanning electron microscopy investigations showed that BXT films have a columnar microstructure, which is oriented perpendicular to the substrate. The temperature dependence of the dielectric permittivity of the films was recorded at 100 kHz. Unlike targets, the BXT films exhibit positive values of the temperature coefficient of the dielectric permittivity. BaMg1/3Ta2/3O3 and BaZn1/3Ta2/3O3 thin films with dielectric constant of about 22.5 and 25, respectively have been obtained. (C) 2012 Elsevier B.V. All rights reserved.

413. Photoluminescence decay time studies on ZnS in cubic and hexagonal phase and its mechanico-chemical interaction with polyaniline

Authors: Scocioreanu, M; Mihut, L; Baibarac, M; Baltog, I

Published: JUL 2013, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 250, 1431, DOI: 10.1002/pssb.201248234

We report new photoluminescence (PL) data generated by pulsed optical excitation on the composites achieved by a mechanico-chemical reaction between ZnS in the cubic (c) and wurtzite (w) phases and polyaniline-emeraldine base (PANI-EB). Under continuous and pulsed optical excitation, powders of the (w)ZnS and (w)ZnS/PANI-EB composites display PL with different spectral compositions. Contrary to expectations, the (c)ZnS and the (c)ZnS/PANI-EB composite displayed weak PL and decay time in the range of nanoseconds. This results from the great involvement of the surface states in nanometric powders which creates efficient channels for the non-radiative recombination of the carriers. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

414. On the properties of aluminium doped zinc oxide thin films deposited on plastic substrates from ceramic targets

Authors: Girtan, M; Vlad, A; Mallet, R; Bodea, MA; Pedarnig, JD; Stanculescu, A; Mardare, D; Leontie, L; Antohe, S

Published: JUN 1 2013, APPLIED SURFACE SCIENCE, 274, 313, DOI: 10.1016/j.apsusc.2013.03.046

We report on the deposition of Al doped ZnO (AZO) thin films on unheated polyethylene terephthalate (PET) substrates by pulsed laser deposition technique using a UV excimer laser and Al2O3:ZnO ceramic targets (1.5 and 2 wt% Al2O3). The deposited AZO films have been investigated by atomic force microscopy, scanning electron microscopy, X-ray diffraction, and optical spectrophotometry. Films present excellent optical and electrical properties (transmission in the visible range T > 85%; resistivity at room temperature rho = 1.3 x 10(-3) Omega cm) as electrodes for plastic solar cells. A good correlation was found between deposition conditions (laser fluence) and structural, morphological, optical and electrical properties. (C) 2013 Elsevier B.V. All rights reserved.

415. Possible mechanism of Ag photodiffusion in a-As2S3 thin films

Authors: Sava, F; Popescu, M; Lorinczi, A; Velea, A

Published: MAY 2013, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 250, 1003, DOI: 10.1002/pssb.201248517

Monitoring the silver photodiffusion in thin amorphous As2S3 film is addressed with a new experimental setup. A possible photo-diffusion mechanism of silver into the a-As2S3 thin film under green laser diode light (=532nm) irradiation is proposed. The proposed mechanism is based on a gradual filling of the structural voids existing in the network of the thin chalcogenide layer. This mechanism is supported by XRD measurements, optical absorption, and modeling data.

416. Disorder in order: silicon versus graphene

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A

Published: MAY 2013, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 250, 1010, DOI: 10.1002/pssb.201248516

The topological transition from order to disorder in crystalline silicon was investigated by a computer simulation procedure. The gradually introduction of topological WootenWinerWeaire defect states makes the crystal change in a more and more disordered assembly of atoms. The characterization of deformation energy around a single defect state is analyzed. The topological transition from graphene structure to an amorphous carbon layer, by introduction of a high number of StoneWales defect-type states was evidenced. The comparison of the disordered structure in tetrahedrally bonded semiconductors (silicon) and a two-dimensional network based on graphene structure was made.

417. Optics of microlenses created by irradiation of As2S3 amorphous chalcogenide films with femtosecond laser pulses

Authors: Popescu, M; Velea, A; Miclos, S; Savastru, D

Published: APR 1 2013, PHILOSOPHICAL MAGAZINE LETTERS, 93, 220, DOI: 10.1080/09500839.2012.760058

Arrays of microlenses have been created on the surface of thin amorphous films of As2S3 by localized photoexpansion induced by femtosecond laser pulses. The profile of a typical microlens has been mathematically described. Ray-tracing analysis has been performed to determine the focal points in the red and near-infrared spectrum of electromagnetic radiation. These lie at distances of 1.21m at =650nm and 1.33m at =1.5m above the top of the microlens. It is suggested that the microlenses could be used in two-dimensional optoelectronic circuits or in fibre optics.

418. ZnO films and nanorod/shell arrays electrodeposited on PET-ITO electrodes

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

Published: APR 2013, MATERIALS RESEARCH BULLETIN, 48, 1586, DOI: 10.1016/j.materresbull.2012.12.045

In this work, ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. ZnO films with high optical transmittance were prepared from a zinc nitrate solution using a pulsed current technique with a reduced pulse time (3 s). The X-ray diffraction pattern of ZnO film deposited on PET-ITO electrode showed that it has a polycrystalline structure with preferred orientations in the directions [0 0 2] and [1 03]. ZnO nanorods were synthesized on electrochemical seeded substrate in an aqueous solution containing zinc nitrate and hexamethylenetetramine. In order to increase the stability of PET-ITO electrode to electrochemical and chemical stresses during ZnO nanorods deposition the surface of the electrode was treated with a 17 wt% NH4F aqueous solution. Electrochemical stability of PET-ITO electrode was evaluated in a solution containing nitrate ions and hexamethylenetetramine. ZnO nanorod/shell arrays were fabricated using eosin Y as nanostructuring agent. Photoluminescence spectra of ZnO nanorod and ZnO nanorod/shell arrays prepared on the surface of PET-ITO electrode were discussed comparatively. By employing the 1.5 mu m-length ZnO nanorod/shell array covered with a Cu2O film a photovoltaic device was fabricated on the PET-ITO substrate. (C) 2012 Elsevier Ltd. All rights reserved.

419. Electrode polarization and interface effects in liquid crystal systems with mobile ions: development of a model of bipolar diffusion

Authors: Ganea, CP

Published: APR 2013, CENTRAL EUROPEAN JOURNAL OF PHYSICS, 11, 511, DOI: 10.2478/s11534-013-0204-6

The influence of mobile ions on the results of impedance spectroscopy (dielectric spectroscopy) measurements performed on a liquid crystal cell using the new mathematical model recently described was investigated. This mathematical model reformulates the fundamental equation system of continuity for mobile charge carriers and the Poisson equation using new variables. One makes the following assumptions: ions have different mobilities and diffusion coefficients, there is no generation-recombination process, the equilibrium carrier concentrations are uniform and equal each other, the electrodes are either completely blocking or blocked with adsorption-desorption processes. The final result is the analytical expression of the equivalent admittance for the system, allowing to have a clearer picture of the mobile ions and of the processes that occur at the electrode interface influencing the dielectric behavior.

420. Effect of tritiation on the superconducting properties of MgB2

Authors: Sandu, V; Aldica, G; Baibarac, M; Enachescu, M; Sandu, E; Chee, CY

Published: APR 2013, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 26, DOI: 10.1088/0953-2048/26/4/045014

We investigated the effect of tritium upload in the dense, polycrystalline MgB2 superconductor for 2 and 216 h. One basic property, the critical temperature T-c, slowly changes in time but is independent of the duration of tritium uptake. Immediately after tritiation, T-c shows a slight increase to 39.1 K but decreases to the critical temperature of the untritiated (pristine) sample, T-c = 38.8 K, after 370 days. The critical current density is strongly dependent on the tritium upload time, mainly at high magnetic fields. The underlying physics is discussed. This behaviour makes the use of MgB2 based conductors advantageous in tritium environments like fusion reactors.