National Institute Of Materials Physics - Romania

Publications

5714 articles found

3261. Non-covalent functionalization of carbon nanotubes: Experimental evidence for isolated and bundled tubes

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

Published: OCT 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 41, 69, DOI: 10.1016/j.physe.2008.06.001

Single-walled carbon nanotubes (SWNTs) may be used in technological applications as individual entities instead of bundled tubes. The Raman signature of these two forms is still a controversial subject. The profile, peak position and relative intensity of the Raman band associated with the tangential vibration modes (G band similar to 1600cm(-1)) and radial vibration modes (RBM similar to 100-200cm(-1)) are the main parameters which can reveal the dispersion state of nanotubes. Resonant Raman scattering and absorption spectroscopy on SWNTs furnish complementary information concerning the efficiency of the nanotube dispersion in individual entities. (C) 2008 Elsevier B.V. All rights reserved.

3262. Single-beam pumped coherent anti-Stokes Raman scattering on carbon nanotubes

Authors: Lefrant, S; Buisson, JP; Mevellec, JY; Baltog, I; Baibarac, M

Published: OCT 2008, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 245, 2224, DOI: 10.1002/pssb.200879567

Giant optical fields locally generated in nanoscale metal structures through the local excitation of surface plasmons underlie a variety of nonlinear optical processes. Using silver and gold rough metallic films, reproducible enhancements of the Raman signal on various materials including carbon nano-tubes have been demonstrated. In addition, an abnormal anti-Stokes Raman emission, reminiscent of Coherent anti-Stokes Raman Scattering (CARS), is accurately observed on nano-metric thin films. We demonstrate that under a tight focusing of the excitation light, a CARS-type emission, resulting from a wave mixing process between the incident laser light and Stokes Raman light generated by a SERS mechanism, is produced. The results which corroborate the CARS emission are the following: a square relation between the CARS signal intensity and the film thickness; a square relationship between the CARS signal intensity and the exciting laser intensity; a dependence of the CARS intensity on the numerical aperture (NA) of the microscope objective used for the detection of the anti-Stokes signal; a polarization ratio in the anti-Stokes side always greater in comparison with that measured for an usual Stokes Raman signal. We demonstrate also that this mechanism may contribute to a clear identification of the state of the observed nanotubes, i.e. in isolated or bundled forms, even when tubes are in powders. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3263. Substituted cobalt ferrites for sensors applications

Authors: Caltun, O; Dumitru, I; Feder, M; Lupu, N; Chiriac, H

Published: OCT 2008, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 320, E873, DOI: 10.1016/j.jmmm.2008.04.067

The aim of this work was to study the influence of the sintering temperature and of the substitution level on the Curie temperature and on the magnetostriction coefficients for four series of cobalt ferrites sintered by conventional ceramic method. The microstructure of the samples was characterized using X-ray diffractometry and scanning surface electron microscopy. Vibrating sample magnetometry and strain gauge measurements were used to characterize the magnetic and magnetostriction properties. The study proves that a small amount of manganese substitution for iron decrease drastically the Curie temperature but in the same time decrease the specific magnetization and magnetostriction coefficient. Despite this drawback, the manganese substituted cobalt ferrites present the largest strain derivative. The maximum of the magnetostriction coefficient is also influenced by the stoichiometry of the cobalt ferrites. The magnetostriction coefficient of cobalt ferrite can be enhanced by substitution of small fraction of iron or cobalt by silicon or manganese, this implying the modification of the strain derivative as well as magnetostriction at lower magnetic fields. The maximum magnetostriction can be tuned by substitution levels and sintering parameters. (C) 2008 Elsevier B.V. All rights reserved.

3264. 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.

3265. Dual behavior of gold nanoparticles, as generators and scavengers for free radicals

Authors: Ionita, P; Spafiu, F; Ghica, C

Published: OCT 2008, JOURNAL OF MATERIALS SCIENCE, 43, 6574, DOI: 10.1007/s10853-008-2987-1

3266. Superconducting MgB2 ceramics and tapes prepared from mechanically milled powders

Authors: Badica, P; Kondo, T; Togano, K; Aldica, G

Published: OCT 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2756, DOI:

Bulks and tapes of MgB2 with 5 wt.% SiC were prepared from Mg, B and SiC. Mixing of the raw powders was performed in Ar atmosphere by high energy milling using a planetary mill. Number of balls (10 or 20), size (10 mm or 7 mm, respectively), rotation speed (200 rpm or 400 rpm) and milling time (up to 3 h) were varied. Powders were compared through XRD and SEM. From the as-prepared powders, bulk and tape samples were prepared and investigated by magnetic (SQUID) and transport (four-probe method) measurements. Results suggest that through variation of the milling conditions such as rotation speed, size and number of balls it is possible to control the value of the critical current density, J(c) and the slope of the J(c)(B) curve, For different heat treatment conditions milling with 400 rpm and using 20 balls of 7 mm always resulted in lower J(c)(B) than for the tapes or bulks produced from hand-milled mixture. Use of 10 balls of 10 mm keeping all the other conditions constant changed the slope of the J(c)(B) curve possibly improving J(c) above a crossover at 4 T. Mild milling with a rotation speed of 200 rpm (20 balls x 7 mm) resulted in enhancement of J(c) in the entire investigated region B <= 14 T, but the slope was steeper than for the best sample produced from the hand-milled mixture and a crossover is likely to occur for B > 14 T leading to lower J(c)-values. The best samples had at 4.2 K J(c) = 3.6 x 10(-2) A center dot cm(-2) at 14 T and J(c) = 5 x 10(3) A center dot cm(-2) at 10 T.

3267. Electrical characterization of thiols self-assembled layers on GaP (111) structures

Authors: Ghita, RV; Lazarescu, V; Logofatu, C; Negrila, CC; Lazarescu, MF

Published: OCT 2008, MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING, 11, 397, DOI: 10.1016/j.mssp.2008.11.012

The experimental studies on III-V semiconductor compounds surface passivation phenomena are mainly dedicated to solve some technological problems as those regarding the ways to keep the chemical stability of native oxides on surfaces. Self-assembled monolayers (SAMs) provide a simple way to produce relatively ordered structures at a molecular scale, which seems to be capable to protect the clean surface against the evolution of oxidation process. In this respect, thin films of SAMs of aliphatic thiol (dodencanthiol-CH3(CH2)(11)SH) and aromatic thiol (4, 4' tiobisbenzenthiol-S (C6H4SH)(2) have been deposited on the surface of Gap (111) samples. The electrical properties measurements of some structures based on GaP compound was performed. There were recorded current-voltage (I-V) characteristics for complex structures AuGeNi/R-SH/GaP and AuGeNi/Ar-SH/GaP in darkness and also exposed to a Xe lamp. In dark and in "reverse bias" way, the I-V characteristics present the feature of a Zenner diode for GaP/Ar-SH and a gradual increase of current for GaP/R-SH. In dark and "in forward bias" way, the current increases as for a normal diode for both GaP/Ar-SH and GaP/R-SH structures. The complex structures (e.g.: In/AuGeNi/R-SH/GaP/R-SH/AuGeNi/In) are less sensitive to light. The SEM analysis performed on a GaP/R-SH surface shows a continuous packed up layer while GaP/Ar-SH looks like an inhomogeneous deposition of layers with different thickness regions. The diodes' ideality factors determined from I-V characteristics are unusually high (n >> 2) as a possible result of inhomogeneous Schottky contacts or due to ageing effects, in the field of degradation. (C) 2009 Elsevier Ltd. All rights reserved.

3268. Precipitation of super-fine calcium carbonate by controlled double jet method: solid phase characterization and estimation of kinetic parameters

Authors: Mateescu, CD; Isopescu, R; Branzoi, F; Mihai, M; Chilibon, I

Published: OCT 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2674, DOI:

Super - fine calcium carbonate was prepared by double jet precipitation method. The values of the estimated kinetics parameters show that nucleation is the dominant mechanism in all experimental runs.

3269. ERRORS IN EVALUATION OF THE KINETIC PARAMETERS IN TEMPERATURE PROGRAMMED REDUCTION

Authors: Munteanu, G; Miclea, C; Segal, E

Published: OCT 2008, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 94, 321, DOI: 10.1007/s10973-008-8998-y

The possibility to determine the kinetic parameters for temperature programmed reduction of Cu/Fe3O4 using only one TPR profile is analyzed. The same data are analyzed both by Friedman's iso-conversional method and another one previously derived and published by the authors. One shows that taking into account the experimental restrictions of Monti and Baiker, the Friedman's method, although gives values of the activation energy smaller than the real values, indicates a very similar dependence of these on the reduction degree. On the basis of some synthetic data one shows that the errors are very large when these recommendations are neglected, being possible to determine a false dependence on the degree of reduction.

3270. Investigation on the nitrogen doping of multilayered, porous TiO2 thin films

Authors: Gartner, M; Osiceanu, P; Anastasescu, M; Stoica, T; Stoica, TF; Trapalis, C; Glannakopoulou, T; Todorova, N; Lagoyannis, A

Published: SEP 30 2008, THIN SOLID FILMS, 516, 8189, DOI: 10.1016/j.tsf.2008.04.027

N-doping is often used to improve the photocatalytic properties of TiO2, films in order to achieve visible light response. In this work, we study the effect of annealing treatment (temperature and atmosphere) on the structural and optical properties of undoped and N-doped TiO2, films by means of X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE) and UV-VIS optical transmission spectroscopy. Porous five-layer TiO2, films were deposited by sol-gel method on quartz substrate and thermally treated in oxygen or NH3 flow at 500 and 600 degrees C. Significant doping effect was achieved after annealing at 600 degrees C in NH3 and a shift in optical bandgap value down to visible range (2.69 eV) was observed. (C) 2008 Elsevier B.V. All rights reserved.