National Institute Of Materials Physics - Romania

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5781 articles found

4161. Photoconductivity and optical properties in composites of poly(paraphenylene vinylene) and single-walled carbon nanotubes

Authors: Mulazzi, E; Perego, R; Aarab, H; Mihut, L; Lefrant, S; Faulques, E; Wery, J

Published: OCT 2004, PHYSICAL REVIEW B, 70, DOI: 10.1103/PhysRevB.70.155206

We present photoconductivity and Raman scattering data obtained from composite films of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes at different weight concentrations from 0% to 64%. It is found that the introduction of nanotubes in the PPV precursor polymer solution, heated at 300degreesC to perform conversion into PPV, yields drastic modifications in both the structural features of the composite components and in the electronic properties of the composites. The PPV polymer matrix becomes more disordered due to the introduction of nanotubes which induce a shortening of the polymer conjugated segments as shown by Raman scattering spectra. In addition, these spectra yield information about the evolution from small bundles to thick bundles of single-walled nanotubes as function of their concentration x. Photoconductivity data show that the percolation regime begins at x=2%, indicating that a migration network for the photogenerated charges is established above this threshold. By using a model based on distributions of PPV conjugated lengths and their changes as function of x, we calculate the Raman scattering band shapes and their relative intensities. The theoretical results lead to a comprehensive interpretation of experimental data.

4162. The electrodeposition of semiconductor nanowires with thermoelectric properties using "template" method

Authors: Sima, M; Enculescu, I; Visan, T

Published: OCT 2004, REVISTA DE CHIMIE, 55, 746, DOI:

A binary telluride (CdTe) as well as a ternary telluride (PbSeTe) were electrodeposited in the pores of a track-etched polycarbonate membrane, resulting nanowires and nanotubes. Either acid or alkaline solutions were used for preparing CdTe compound, whereas the ternary PbSe1-xTex compound was deposited from an acid solution, only. The investigation of electrode mechanisms was performed by cyclic voltammetry. The nanostructures were visualized using scanning electron microscopy (SEM).

4163. Copper tubes prepared by electroless deposition in ion track templates

Authors: Bercu, B; Enculescu, I; Spohr, R

Published: OCT 2004, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 225, 502, DOI: 10.1016/j.nimb.2004.06.011

Polycarbonate porous membranes were prepared by etching the ion tracks contained in foils irradiated in the fluence range of 1 ion/sample to 10(8) ions/cm(2). The membranes were used as templates for the preparation of tubes employing auto-catalytic copper deposition. A three-step process was used: pre-activation, activation and copper deposition. The deposition bath employed formaldehyde as reducing agent and tartrate as complexing agent. Copper nucleation was studied in order to optimize the activation steps thus allowing the preparation of thin tubes. The external diameters of the tubes ranged from 300 to 2000 nm. The inner diameters varied as a function of the time-of-exposure to the electroless bath. The length of the tubes was 30 mum, corresponding to the thickness of the foils. (C) 2004 Elsevier B.V. All rights reserved.

4164. Nonmonotonic temperature dependence of the experimentally determined vortex-creep activation energy in disordered high-temperature superconductors

Authors: Miu, L; Popa, S; Noji, T; Koike, Y; Miu, D; Diaz, S; Chouteau, G

Published: OCT 2004, PHYSICAL REVIEW B, 70, DOI: 10.1103/PhysRevB.70.134523

The temperature T variation of the normalized vortex-creep activation energy U* determined in standard magnetization relaxation experiments for Pb2Sr2Y0.53Ca0.47Cu3O8+delta single crystals with random point disorder exhibits a maximum, which moves to lower-T values by increasing the external magnetic field oriented parallel to the c axis. The nonmonotonic U*(T) dependence is related to the change of the vortex pinning barriers involved in the creep process across the order-disorder transition in the vortex system (accompanied by the occurrence of the second magnetization peak), in a dynamic scenario. The decrease of U* with decreasing T in the low-T region is caused by the shift of the current density J range probed in standard magnetization measurements toward the critical current density, and the significant U*(J) variation in the elastic-creep domain. The dynamic approach is confirmed by the behavior of highly disordered top-seeded melt-grown YBa2Cu3O7-delta crystals at low T, for which no second magnetization peak appears, and U* does not depend on T.

4165. Second harmonic generation and impedance spectroscopy at n-GaAs(100) electrodes

Authors: Lazarescu, V; Lazarescu, MF; Santos, E; Schmickler, W

Published: SEP 30 2004, ELECTROCHIMICA ACTA, 49, 4238, DOI: 10.1016/j.electacta.2004.04.017

Second harmonic generation from n-GaAs(100) electrodes in sulfuric acid solution has been investigated at an incident wavelength of 1064 nm over a potential range where Faradaic processes are excluded. In the p-in/p-out polarization configuration, the rotational anisotropy in the nonlinear response was found to depend both on the doping level and the applied potential. The effects of the applied potential on the isotropic amplitude of the SHG signal turned out to be closely related to the population of surface states as revealed by impedance spectroscopy, indicating that these states could play a significant role in second harmonic generation. (C) 2004 Elsevier Ltd. All rights reserved.

4166. Pyroelectric current spectroscopy: example of application on Nb doped Pb(Zr0.92Ti0.08)O-3 ceramics for infrared detection

Authors: Pintilie, L; Pereira, M; Gomes, MJM; Boerasu, I

Published: SEP 21 2004, SENSORS AND ACTUATORS A-PHYSICAL, 115, 190, DOI: 10.1016/j.sna.2004.01.020

Pyroelectric current spectroscopy (PCS) was used to investigate the homogeneity of the Nb doped lead zirconate-titanate Pb(Zr-0.92 Ti-0.08)O-3, (PZT) ceramics intended to be used in the infrared (IR) pyroelectric detectors manufacturing. The non-homogeneous composition in the ceramic volume is reflected by the occurrence of several peaks in the PCS spectra, suggesting the presence of PZT phases with different Zr/Ti ratio, thus having different transition temperatures. The non-homogeneity was confirmed by compositional analysis performed using an Energy Dispersive X-ray Spectrometer (EDAX) attached to a Scanning Electron Microscope (SEM). PCS can be a useful tool in optimizing the calcination and sintering parameters (temperature and time) during ceramic preparation. (C) 2004 Elsevier B.V. All rights reserved.

4167. Optical and electronic properties of polyvinyl alcohol doped with pairs of mixed valence metal ions

Authors: Bulinski, M; Kuncser, V; Plapcianu, C; Krautwald, S; Franke, H; Rotaru, P; Filoti, G

Published: SEP 7 2004, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 37, 2441, DOI: 10.1088/0022-3727/37/17/015

The electronic mechanisms induced by the UV exposure of thin films of polyvinyl alcohol doped with pairs of mixed valence metal ions were studied in relation to their optical behaviour by Mossbauer spectroscopy and optical absorption. The results obtained definitely point to the role of each element from the pair in the electronic mechanism involved, with influence on the optical properties regarding applications in real-time holography and integrated optics.

4168. Application of elevated magnetic fields during growth of BiSrCaCuO superconducting whiskers and studies of growth defects for better understanding of the growth mechanism

Authors: Badica, P; Togano, K; Awaji, S; Watanabe, K; Iyo, A; Kumakura, H

Published: SEP 1 2004, JOURNAL OF CRYSTAL GROWTH, 269, 534, DOI: 10.1016/j.jcrysgro.2004.05.115

Superconducting whiskers of Bi-2212 have been grown by an airtight crucible method in elevated magnetic fields, up to 10 T. Growth behavior with and without application of a magnetic field, as well as the SEM detailed study of the growth defects of the whiskers allow to emphasize some aspects of the growth mechanism. The conclusion is that whiskers are growing mainly through continuous crystallization at the base-end and from the so-called 'micro-crucible' formed by the glassy or powdered substrates. The supply of cations by a vapor transfer is very probable, but is taking place to the micro-crucible reaction site rather than to the droplet, as in the classic vapor-liquid-solid (VLS) growth mechanism. Therefore, whiskers grow by a modified VLS-microcrucible mechanism (MVLS-MC). Application of magnetic fields during growth has proven to be a powerful tool to investigate and influence some processes. Observation of stoichiometric Bi(Pb)-2212, CuO and Ca2CuO3 whiskers is also reported. (C) 2004 Elsevier B.V. All rights reserved.

4169. Microwave synthesis and characterization of Co-ferrite nanoparticles

Authors: Bensebaa, F; Zavaliche, F; L'Ecuyer, P; Cochrane, RW; Veres, T

Published: SEP 1 2004, JOURNAL OF COLLOID AND INTERFACE SCIENCE, 277, 110, DOI: 10.1016/j.jcis.2004.04.016

Stable CoFe(2)O(4) nanoparticles have been obtained by co-precipitation using a microwave heating system. Transmission electron microscopy images analysis shows an agglomeration of particles with an average size of about 5 nm, and X-ray diffraction reveals the presence of a pure ferrite nanocrystalline phase. X-ray photoelectron spectroscopy and thermal gravimetric analysis show the presence of organic matter in the range of about 16 wt%. The magnetic response in DC fields is typical for an assembly of single-domain particles. The measured saturation magnetization is slightly larger than the bulk value, probably due to the presence of small amounts of Co and Fe. AC magnetization data indicate the presence of magnetic interactions between the nanoparticles. Crown Copyright (C) 2004 Published by Elsevier Inc. All rights reserved.

4170. Influence of deep levels on space charge density at different temperatures in gamma-irradiated silicon

Authors: Menichelli, D; Scaringella, M; Miglio, S; Bruzzi, M; Li, Z; Fretwurst, E; Pintilie, I

Published: SEP 1 2004, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 530, 145, DOI: 10.1016/j.nima.2004.05.062

In this work, it is shown that the analysis of thermally stimulated currents (TSC) and current transients (CT) at constant temperature can be a suitable tool to study the influence of deep levels on space charge density N(T) in irradiated silicon diodes. In particular, the occurrence of space charge sign inversion (SCSI) can be related to signal discontinuities in TSC and CT measurements. This approach has been adopted in this work to study devices made of standard Float Zone (FZ) and Diffusion Oxygenated Float Zone silicon, irradiated by gamma-rays up to a dose of 300 Mrad. Our study shows that all the samples are inverted at 50 K after a low-temperature excitation. Several space charge sign inversions, from positive to negative and vice versa, have been observed between cryogenic and room temperature, and have been related to carriers emission from dominating deep traps. Only standard FZ silicon remains inverted at room temperature after a dose of 300 Mrad. (C) 2004 Elsevier B.V. All rights reserved.