National Institute Of Materials Physics - Romania

Publications

5781 articles found

2951. Investigation of microstructural properties of nitrogen doped ZnO thin films formed by magnetron sputtering on silicon substrate

Authors: Nicolescu, M; Anastasescu, M; Preda, S; Calderon-Moreno, JM; Osiceanu, P; Gartner, M; Teodorescu, VS; Maraloiu, AV; Kampylafka, V; Aperathitis, E; Modreanu, M

Published: MAY 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 1051, DOI:

Zinc oxide (ZnO) is a semiconductor with a wurtzite-type structure, useful for a variety of optical, optoelectronic and piezoelectric applications. We report here on the post deposition treatment (Rapid Thermal Annealing at 400 and 550 degrees C) impact on the microstructural properties of N-doped ZnO (ZnO:N) thin films grown on silicon substrate by r.f. magnetron sputtering of ZnN target. The ZnO:N films have been characterized by X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Scanning (SEM) and Transmission (TEM) Electron Microscopy coupled with Energy Dispersive X-ray (EDX) and Selected Area Electron Diffraction (SAED) respectively and Fourier Transform Infrared (FTIR) Spectroscopy. XRD confirms that ZnO:N films are polycrystalline in the as deposited state. SEM, TEM and XRD measurements revealed a polycrystalline film with preferentially oriented columnar crystals. AFM studies reveal a drastic change of ZnO surface morphology upon RTA and the existence of areas with different roughness in the sample thermally treated at 400 degrees C. FTIR, XRD and TEM results shows that disorder associated mainly with grain boundary defects decreases as the annealing temperature increases.

2952. Dielectric Ba(Ti1-x Sn (x) )O-3 (x=0.13) ceramics, sintered by spark plasma and conventional methods

Authors: Aldica, G; Cernea, M; Ganea, P

Published: MAY 2010, JOURNAL OF MATERIALS SCIENCE, 45, 2610, DOI: 10.1007/s10853-010-4234-9

A two-step sintering approach composed of spark-plasma-sintering (SPS) technique at 1000 A degrees C for 1 min and under a uniaxial pressure of 63 MPa followed by conventional sintering at 1400 A degrees C for 3 h is proposed for synthesis of dense Ba(Ti0.87Sn0.13)O-3 ceramics. Starting powders had grain size of about 90 nm and were obtained by co-precipitation. The SPS pellets consist of submicron (300-500 nm) grains. X-ray diffraction analysis of as-prepared Ba(Ti0.87Sn0.13)O-3 ceramic shows the occurrence of cubic and tetragonal phase coexistence for the pellets obtained after SPS processing and the presence of only tetragonal phase in the samples after the second (conventional) sintering. Grain uniformity in the final product is high, with average size of similar to 2 mu m. The apparent densities of the sintered pellets at temperature of 1400 A degrees C were similar to 92% of the theoretical value of Ba(Ti0.87Sn0.13)O-3. The ceramics exhibit a high relative dielectric constant of 6,550 and a dielectric loss (tan delta) = 0.078 at Curie temperature of 63 A degrees C and 10 Hz.

2953. Improvements of kinetic properties of LiBH4 by supporting on MSU-H type mesoporous silica

Authors: Comanescu, C; Guran, C; Palade, P

Published: MAY 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 708, DOI:

Lithium borohydride (LiBH4) is a promising material for hydrogen storage, featuring a high gravimetric storage density (above 13 wt. % in the first decomposition step). However, studies of re-hydrogenation of the decomposition products have proven less successful, leading us to the requirement of modified-LiBH4 materials with better absorption-desorption behavior. A Mo-doped mesoporous material was obtained starting from commercial MSU-H and ammonium molybdate. This mesoporous material (MSU-H-type functionalized with molybdenum salt) was used as support in order to impregnate LiBH4. Even though the thermodynamics of LiBH4 hydrogen absorption-desorption is not modified, the desorption kinetics of supported LiBH4 is improved compared to that of pure LiBH4. Moreover, after re-hydrogenation at 450 degrees C only 3.2 wt. % H-2 of storage capacity is recovered for LiBH4, while for supported LiBH4 we achieved almost 5.2 wt. % H-2.

2954. Porous Au-TiO2 aerogels nanoarchitectures for photodegradation processes

Authors: Peter, A; Baia, L; Baia, M; Indrea, E; Toderas, F; Danciu, V; Cosoveanu, V; Diamandescu, L

Published: MAY 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 1077, DOI:

Porous nanoarchitectures based on TiO2 aerogel and colloidal gold particles have been obtained by different synthesis routes: impregnation of TiO2 aerogel with Au colloid, impregnation of TiO2 gel with Au colloid and preparation of TiO2 gel in the presence of Au colloid. The as prepared composites have been thermal treated at 500 degrees C and their structural and morphological properties were investigated. The Raman spectra and XRD data demonstrated the anatase structure of the composite obtained by impregnation of TiO2 aerogel with Au colloid and the mixed anatase-brookite structure of those obtained by impregnation of TiO2 gel with Au colloid and by preparation of TiO2 gel in the presence of Au colloid, respectively. TEM analyses suggest that the Au particles on the TiO2 surface as individual particles, as well as clusters are present. The photocatalytic performances of the synthesized composites were tested in the salicylic acid photodecomposition process and it was found that they are strongly influenced by both anatase - brookite TiO2 structure and Au nanoparticles dispersion on the TiO2 surface.

2955. The study of the silicon oxide thickness on crystalline Si by X-ray photoelectron spectroscopy and spectroscopic ellipsometry

Authors: Cotirlan, C; Galca, AC; Ciobanu, CS; Logofatu, C

Published: MAY 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 1097, DOI:

The to study of the total oxide (SiO2+SiOx) thickness, SiO2 and SiOx (e.g. Si2O, SiO, Si2O3) thicknesses on Si(100) crystalline substrate with take-off angles ranging from 30 degrees to 80 degrees has been carried out by spectrometric method. The d(s) X-ray Photoelectron Spectroscopy (XPS) thicknesses were compared with d(EL) thicknesses obtained by fitting the Spectroscopic Ellipsometry (SE) spectra. A qualitatively good correlation is revealed. However, from these estimations of film thicknesses it results that ellipsometry analysis cannot be as accurate as in XPS evaluation. This is due to uncertainty of used optical constants as well due to very thin oxide films used in this work.

2956. Structural and electrochemical characterization of TiO2/Pt hybrid catalyst system for direct bio-ethanol fuel cell

Authors: Banu, A; Spataru, N; Teodorescu, VS; Maraloiu, AV; Voiculescu, I; Marcu, AM; Spataru, T

Published: MAY 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 1193, DOI:

This paper is focused on preparation and microstructure and electrochemical characterization of carbon-TiO2/Pt catalyst used as electrode in fuel cells applications. The platinum deposition was comparatively studied on graphitized carbon substrate and carbon substrate covered with titanium oxide. A strong influence on the platinum deposition, depending of the TiO2 presence, was found. The platinum nanoparticles about 5 nm in size are dispersed in aggregates of 50 to 100 nm on the catalyst surface. This dispersion is strongly affected by the titanium oxide and the substrate porosity. The Pt/TiO2/G and Pt/G electrodes were tested for their catalytic activity in the oxidation of ethanol. The prepared Pt/TiO2/G electrode show higher active surface area than for the Pt/G electrode, this increase in the surface area could be explained to be due to a spreading of wetting or the formation of new active sites at the Pt/oxide interface.

2957. Properties of highly electronegative plasmas produced in a multipolar magnetic-confined device with a transversal magnetic filter

Authors: Draghici, M; Stamate, E

Published: APR 21 2010, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 43, DOI: 10.1088/0022-3727/43/15/155205

Highly electronegative plasmas were produced in Ar/SF6 gas mixtures in a dc discharge with multipolar magnetic confinement and transversal magnetic filter. Langmuir probe and mass spectrometry were used for plasma diagnostics. Plasma potential drift, the influence of small or large area biased electrodes on plasma parameters, the formation of the negative ion sheath and etching rates by positive and negative ions have been investigated for different experimental conditions. When the electron temperature was reduced below 1 eV the density ratio of negative ion to electron exceeded 100 even for very low amounts of SF6 gas. The plasma potential drift could be controlled by proper wall conditioning. A large electrode biased positively had no effect on plasma potential for density ratios of negative ions to electrons larger than 50. For similar electronegativities or higher a negative ion sheath could be formed by applying a positive bias of a few hundred volts.

2958. Coulomb interaction and transient charging of excited states in open nanosystems

Authors: Moldoveanu, V; Manolescu, A; Tang, CS; Gudmundsson, V

Published: APR 15 2010, PHYSICAL REVIEW B, 81, DOI: 10.1103/PhysRevB.81.155442

We obtain and analyze the effect of electron-electron Coulomb interaction on the time-dependent current flowing through a mesoscopic system connected to biased semi-infinite leads. We assume the contact is gradually switched on in time and we calculate the time-dependent reduced density operator of the sample using the generalized master equation. The many-electron states (MES) of the isolated sample are derived with the exact-diagonalization method. The chemical potentials of the two leads create a bias window which determines which MES are relevant to the charging and discharging of the sample and to the currents, during the transient or steady states. We discuss the contribution of the MES with fixed number of electrons N and we find that in the transient regime there are excited states more active than the ground state even for N=1. This is a dynamical signature of the Coulomb-blockade phenomenon. We discuss numerical results for three sample models: short one-dimensional chain, two-dimensional (2D) lattice, and 2D parabolic quantum wire.

2959. 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

2960. Second-harmonic generation of 400nm blue-violet light in Gd1-xRxCa4O(BO3)(3) (R = Lu, Sc) crystals through noncritical phase-matching

Authors: Gheorghe, L; Loiseau, P; Aka, G

Published: APR 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 823, DOI:

The optical birefringence of Gd1-xRxCa4O(BO3)(3) (R = Lu or Sc) crystals can be controlled by changing the compositional parameter x. Two nonlinear crystals of Gd0.884Lu0.116Ca4O(BO3)(3) and Gd0.952Sc0.048Ca4O(BO3)(3) with large size and good quality have been grown by Czochralski method. According to our assumptions, the obtained results demonstrate that both crystals convert the near-infrared radiation of 800nm into blue-violet light (400nm) by type-I noncritical phase-matching (NCPM) second-harmonic generation (SHG) processes along Y axis.