National Institute Of Materials Physics - Romania

Publications

5781 articles found

3431. 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.

3432. Capacitance-voltage characteristics of heterostructures with high leakage currents

Authors: Goldenblum, A; Stancu, V; Buda, M; Iordache, G; Pintilie, I; Negrila, C; Botila, T

Published: MAR 1 2008, JOURNAL OF APPLIED PHYSICS, 103, DOI: 10.1063/1.2844210

Leakage currents determine bumps in the capacitance-voltage characteristics of metal-oxide-semiconductor-type heterostructures with capacitance values larger than the insulator capacitance. A nearly parallel shift with frequency is also observed. These aspects are connected with the presence of a high density of interface states. We illustrate these effects for the case of nano-PbS/SiO2/Si heterostructures. The phenomena were simulated and we propose a method to quickly estimate the interface state density level of a heterostructure when leakage channels are present. (C) 2008 American Institute of Physics.

3433. Magnetite nanoparticles obtained from the system Fe(III)/Fe(II) - glycine

Authors: Gingasu, D; Mindru, I; Patron, L; Diamandescu, L; Feder, M

Published: MAR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 503, DOI:

Magnetite nanoparticles were synthesized from Fe3+/Fe2+ salts in the presence of glycine (2:1:8-2:1:3 ratio metal ions/glycine) using NH4OH 25% as precipitating agent. The parameters which influence the synthesis are discussed. The magnetite powders were characterized by XRD, IR spectra, magnetic measurements and Mossbauer spectra. The average diameter of crystallites is in the range 10 - 13 nm. The magnetization value is similar to 43 emu/g.

3434. 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.

3435. 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.

3436. Effect of additives on nickel nanowires electrochemical deposition

Authors: Matei, E; Enculescu, I; Enculescu, M; Neumann, R

Published: MAR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 511, DOI:

The paper presents the effect of additives on nickel nanowires preparation by employing a template approach. The nanostructures were obtained by electrochemical deposition in ion track nanoporous membranes. The main goal was to find a wetting agent which increases the pores filling efficiency. Due to the fact that the polycarbonate nanoporous membranes used as templates are strongly hydrophobic problems appear when filling the nanopores with the aqueous electrochemical baths. We found that polyvinylpyrrolidone (PVP) added in the bath improves membrane wetting, thus increasing the deposition efficiency up to 80%. Electrochemical polarization and chronoamperometry were employed for identifying and studying the processes which take place in the case of cathodic deposition of nickel nanowires. Scanning electron microscopy was employed for nanostructure morphology characterization.

3437. Coherent and incoherent transport through T-shaped double quantum dots

Authors: Moldoveanu, V; Tolea, M; Tanatar, B

Published: MAR 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 1104, DOI: 10.1016/j.physe.2007.08.012

We investigate the measurement induced dephasing of the Fano effect in the electronic transport through a double quantum dot mesoscopic interferometer coupled to a charge detector. The current and the differential conductance are computed within the Keldysh formalism, taking into account of the inelastic processes due to the dot-detector interaction. We show that the visibility of the Fano lineshape is reduced by applying a finite bias on the charge detector. (C) 2007 Elsevier B.V. All rights reserved.

3438. Study of iron oxide nanoparticles coated with dextrin obtained by coprecipitation

Authors: Vatasescu-Balcan, RA; Predoi, D; Ungureanu, F; Costache, M

Published: MAR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 696, DOI:

In this paper we present the preparation and biological tests of iron oxide nanoparticles coated with dextrin. Iron oxide was also prepared by co-precipitation. The samples were characterized by transmission electron microscopy (TEM) and 14 scanning electron microscopy (SEM). Biocompatibility test of the iron oxide nanoparticles coated with dextrin substrate has been done using primary osteoblast cell line. After osteoblast culture achievement, the cells were treated with trypsin 0.05% and splitted in 35/35 mm Petri dish. The influence in vitro was assessed using primary osteoblast cell line and various techniques to observe cell-particles interactions. The results of these analyses are presented in this work.

3439. 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.

3440. A study of patch antennas on ZST substrate

Authors: Avadanei, OG; Ioachim, A; Gasner, P; Pascariu, GN; Pascariu, V

Published: MAR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 644, DOI:

Compactness of microstrip antennas has made them very attractive for applications in communication systems. As the need for antenna miniaturization continues, a possible solution is offered by the utilization of high dielectric constant materials as substrate. The purpose of this paper is the study of patch antennas on high permittivity dielectric substrates (Zr-0.8 Sn-0.2)TiO4 - ZST. A comparative investigation of different types of patch and arrays was performed Two substrate thickness h(1)=1 mm and h(2)=2 mm at a frequency of f=1090 MHz was considered. The main advantage of ZST material as a dielectric substrate for patch antennas is the miniaturization of the antennas as well as a high stability with the temperature. For each case we presented the radiation pattern, the efficiency and the input impedance of the patch antennas. The patch antennas were designed using an improved cavity model (a further improved Richards cavity model). The radiation pattern for rectangular array with n = 4, 16, 32 are also presented. The numbers are imposed by the feeding system of the array, which must ensure the same phase for all the patches. A discussion on the suitability of patch antennas with high permittivity substrate is done.