National Institute Of Materials Physics - Romania

Publications

830 articles found

621. Transport properties of electrodeposited ZnO nanowires

Authors: Tazlaoanu, C; Ion, L; Enculescu, I; Sima, M; Enculescu, M; Matei, E; Neumann, R; Bazavan, R; Bazavan, D; Antohe, S

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2507, DOI: 10.1016/j.physe.2007.07.013

ZnO nanowires were electrodeposited by using a template approach. Ion track polycarbonate foils (30 mu m thick) were used as templates. Current-voltage (I-V) characteristics were recorded in the temperature range 40-300 K. I-V characteristics show a symmetric, non-linear shape, at temperatures greater than 240 K and voltages greater than 15 V. At lower temperatures, linear characteristics were recorded for the voltage range used. The temperature dependence of the electrical resistance is activated, with activation energy of 0.42 eV at temperatures greater than 240 K. (C) 2007 Elsevier B.V. All rights reserved.

622. Single-beam pumped coherent anti-Stokes Raman scattering on carbon nanotubes thin films excited through surface plasmons

Authors: Baltog, I; Baibarac, M; Lefrant, S

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2385, DOI: 10.1016/j.physe.2007.07.022

In this paper, we propose an interpretation for an abnormal anti-Stokes Raman emission observed on nanometric thin films of different materials and in particular carbon nanotubes. We demonstrate that under a tight-focusing of the excitation light, a coherent anti-stokes Raman scattering (CARS) emission is produced, resulting from a wave mixing process between the incident laser light (omega(1)) and Stokes Raman light (omega(s)) generated by a surface enhanced Raman scattering (SERS) mechanism. Although the Stokes/anti-Stokes intensity ratio has been explained differently, we present here the results which corroborate the CARS emission. They can be summarized as follows: (i) a square relationship between the CARS signal intensity and the film thickness; (ii) a square relationship between the CARS signal intensity and the exciting laser intensity; (iii) a dependence of the CARS intensity on the numerical aperture (NA) of the microscope objective used for the detection of the anti-Stokes emission. Such effects are not specific to carbon nanotubes and have been observed with other materials accommodated in similar conditions on rough metallic surfaces acting as SERS supports. (c) 2007 Elsevier B.V. All rights reserved.

623. Vibrational and photoluminescence properties of composites based on zinc oxide and single-walled carbon nanotubes

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

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2564, DOI: 10.1016/j.physe.2007.09.034

Composites based on zinc oxide (ZnO) nanoparticles and single-walled carbon nanotubes (SWNTs) were investigated by photoluminescence (PL) and Raman scattering. The room temperature PL spectrum of ZnO at the excitation wavelength of 335 nm displays a narrow excitonic band, peaking at 368 nm (3.36eV), and a wide one, disclosing two components with maxima at about 444 nm (2.79eV) and 560 nm (2.21eV) which are associated to the local oxygen vacancies and oxygen excess, respectively. Regardless the preparation method of the ZnO/SWNTs compound (electrochemical, mechanicochemical and hydrothermal synthesis), PL spectra reveal an increased quenching tendency with the concentration of nanotubes in the composite mass. As distinct feature in the PL spectra of ZnO/SWNTs composites are two bands, one at 450 nm (2.75 nm) and another one attributed to the formation of a donor-acceptor complex at 680 nm (1.82eV). The role of the dispersion of carbon nanotubes in the ZnO/SWNTs composite is observed by Raman scattering with the excitation wavelength of 676 nm. (C) 2007 Elsevier B.V. All rights reserved.

624. Electrical properties of electrodeposited CdS nanowires

Authors: Ghenescu, M; Ion, L; Enculescu, I; Tazlaoanu, C; Antohe, VA; Sima, M; Enculescu, M; Matei, E; Neumann, R; Ghenescu, O; Covlea, V; Antohe, S

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2488, DOI: 10.1016/j.physe.2007.09.188

A template method was used to obtain cadmium sulfide (CdS) nanowires. Polymer ion tracks foils (30 mu m tick) were used as templates, after etching with solutions containing NaOH and methanol. CdS nanowires were electrochemically grown in the resulting pores. The nanowires were contacted by sputtering a gold layer on top of the membrane, and the electrical properties were recorded in the temperature range 40-300K. An activated electrical resistance was observed, with activation energy of 0.27eV at temperatures larger than 180 K. I-V characteristics show a symmetric, non-linear shape, in the voltage range used in this experiment. (C) 2007 Elsevier B.V. All rights reserved.

625. Luminescence and EPR study of ZnO : Mn : Cu nanowire array

Authors: Sima, M; Enculescu, I; Grecu, MN; Secu, M; Sima, M; Matei, E; Vasile, V

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2498, DOI: 10.1016/j.physe.2007.08.075

We report on photoluminescence studies on manganese and copper-doped ZnO nanowire array prepared by template method. The ZnO:Mn:Cu semiconductor grown using the electrochemical technique was characterized by electron paramagnetic resonance (EPR) and optical reflection measurements. Morphology and the structure were observed by SEM and X-ray diffraction. The photoluminescence of this system was correlated with manganese and copper concentration in the ZnO matrix. (C) 2007 Elsevier B.V. All rights reserved.

626. Investigation of emission properties of doped aromatic derivative organic semiconductor crystals

Authors: Stanculescu, A; Mihut, L; Stanculescu, F; Alexandru, H

Published: APR 2008, JOURNAL OF CRYSTAL GROWTH, 310, 1771, DOI: 10.1016/j.jcrysgro.2007.11.211

Fluorescence measurements have been made on pure and doped bulk, mechanically polished wafers of crystalline m-DNB and benzil obtained by cutting ingots grown by the Bridgman-Stockbarger method modified for organic compounds crystallization. By comparison with pure matrices, we have investigated the effect of an inorganic dopant (iodine, silver, sodium) and of an organic dopant (m-DNB, naphthalene) on the emission characteristics (position and shape) of these molecular crystals. A slight shift of the emission peaks through high energy and an intense emission peak situated around 2.35 eV correlated with the local trapping level attributed to structural defects, which are involved in radiative processes, have been evidenced in iodine-doped m-DNB. The emission peak of m-DNB-doped benzil situated in the high-energy range (2.97 eV) is associated with direct emission activity of m-DNB, suggesting that this is an active impurity in benzil molecular matrix. We have not observed in benzil any evidence of indirect action of the impurity molecules (atoms) associated with the traps represented by the structural defects that generate changes in the energy levels of the neighbouring molecules and are correlated with different growth conditions. We have not remarked any involvement of the studied inorganic metallic impurities and of some organic impurities, such as naphthalene, in the radiative recombination processes in benzil matrix. (c) 2007 Elsevier B.V. All rights reserved.

627. Thermoluminescence of ZnF2 : Mn

Authors: Jipa, S; Zaharescu, T; Kappel, W; Secu, M; Secu, CE; Lungulescu, M; Mantsch, A

Published: MAR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 557, DOI:

The thermoluminescence of undoped and Mn-doped ZnF2 Was investigated at the dopant concentration of 1.5 % mole Mn after gamma - ray irradiation at room temperature. The glow curve from undoped ZnF2 presents two peaks centered at 86 degrees C and 273 degrees C, while the ZnF2:Mn presents two peaks located at higher temperatures (325 degrees C and 365 degrees C), whose height ratio is modified with dose. The dose responses and fading process were also examined. The radiative thermostimulated recombination was found to be of the second kinetic order based on the geometric factor calculation. The trap parameters were obtained as activation energy and frequency factor.

628. On the optical thickness dependence of the electro-optical properties of an in-plane switching LC cell

Authors: Beica, T; Frunza, S; Zgura, I; Moldovan, R; Dinescu, A

Published: MAR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 563, DOI:

The optical transmission of a liquid crystal cell with in-plane switching (IPS) mode is studied as function of the thickness for different rubbing angles and anchoring energy. It was found that the light transmission has a maximum at the value of similar to 0.66 for the ratio of the optical path difference (introduced by the undisturbed cell) to X and that this value does not depend on lambda, in the case of a very strong anchoring energy and of a rubbing angle of 13 degrees. These experimental results agree very well with simulations using a simplified model. Moreover, the simulations have shown that the thickness corresponding to such a maximum optical path difference depends non-monotonously on the anchoring strength for five values of the rubbing angles (in the range 0 to 40 degrees). For a given anchoring energy this thickness decreases with increasing rubbing angle.

629. Nickel nanotubes prepared by electroless deposition in ion track templates

Authors: Enculescu, I; Sima, M; Enculescu, M; Matei, E; Molares, MET; Cornelius, T

Published: MAR 2008, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 2, 136, DOI:

Nickel nano- and micro- tubes were prepared by electroless deposition in ion track template membranes. By choosing the appropriate etching conditions membranes with cylindrical or conical pores were obtained allowing the preparation of cylindrical or conical tubules. The activation of the membranes was a two step process. The bath used for deposition was an acidic one. Typically for acidic bath deposition, by energy dispersive X ray analysis, a phosphorous content of up to 10 % was found in the deposit.

630. Photoluminescence properties of composites based on zinc oxyde and single-walled carbon nanotubes

Authors: Baibarac, M; Baltog, I; Husanu, M; Velula, T; Bucur, C; Mihut, L; Preda, N

Published: FEB 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 293, DOI:

Photoluminescence (PL) properties of the composites based on zinc oxide (ZnO) and single-walled carbon nanotubes (SWNTs) prepared by hydrothermal synthesis are studied in this paper. The emission and excitation spectra of nanometric ZnO powder are dramatically influenced by the adsorption of different molecules coming from the environment. Therefore, a special attention is given to reveal the influences of adsorption/de-sorption process on photoluminescence properties of zinc oxide nanoparticles. A quenching effect of the intrinsic PL is observed regularly when the ZnO is synthesized in the presence of SWNTs, i.e. when a ZnO/SWNTs composite is formed. A distinct feature of the ZnO/SWNTs composite, is an emission band with a maximum at 405-450 nm.