National Institute Of Materials Physics - Romania

Publications

860 articles found

671. Polypyrrole - porous silicon nanocomposites

Authors: Popa, C; Turcu, R; Craciunescu, I; Nan, A; Ciurea, ML; Stavarache, I; Iancu, V

Published: SEP 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2324, DOI:

In this work we report the synthesis and characterization of the polypyrrole - porous silicon nanocomposites. Our main goal is to control the growth of conducting polypyrrole into the alveolar pores of porous silicon, in order to get a functionalized nanocomposite with the required properties for application as electrode in microfluidic devices. Porous silicon was electrochemically etched from (100) p-type silicon wafers. Different silicon resistivities (from 1.8 to 11 Omega cm) and preparation conditions (different electrolyte concentrations and current densities) were used. The electrodeposition of polypyrrole into porous silicon was carried out galvanostatically, at a constant current density 2.5 mA/cm(2) from a solution containing acetonitrile, the monomer, pyrrole and p-toluensulfonic acid as electrolyte. The polymerization time was varied in order to investigate the growing process of polypyrrole into the pores. The morphology and of the as-prepared nanocomposites was investigated by SEM. The existence of PPY into porous silicon is evidenced by the EDX spectra of the nanocomposite. The electrical characterization evidences the relative contributions of the polypyrrole and the substrate resistivity, respectively.

672. Hybrid metal (gold)-inorganic (silica) nanoparticles: Synthesis, characterization, and spin-labeling

Authors: Ionita, P; Ghica, C; Caproiu, MT; Ionita, G

Published: SEP 2008, JOURNAL OF INORGANIC AND ORGANOMETALLIC POLYMERS AND MATERIALS, 18, 419, DOI: 10.1007/s10904-008-9210-3

Water-soluble gold nanoparticles, capped with captopril, have been synthesized and characterized. Their average size is 2.3 nm, with a spherical shape. These gold nanoparticles can be easily labeled with stable free radicals (4-amino-tempo) by a coupling reaction performed in the presence of 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ). Both synthesized and spin-labeled gold nanoparticles can be incorporated into much bigger (100 nm) silica nanoparticles using the Stober method, thus forming hybrid metal (gold)-inorganic (silica) nanoparticles. These hybrid silica nanoparticles (containing or not spin-labeled gold nanoparticles) can be easily spin-labeled with another stable free radical (4-isocyanato-tempo), leading to the formation of a double spin-labeled material. In this way, some stable free radicals are attached on the gold surface while others are attached on the silica surface. Three types of EPR spectra were recorded and discussed for the hybrid gold-silica nanoparticles: (1) where the spin labels are attached to the embedded gold nanoparticles, (2) where the spin labels are attached to the silica nanoparticles, and (3) in the case of the double spin labeled material where both gold and silica nanoparticles are spin-labeled. Influence of different solvents on the EPR spectra is also discussed.

673. Fe-catalyzed carbon nanotubes growth on fluidized powders by remote radiofrequency plasma beam

Authors: Vizireanu, S; Stoica, D; Birjega, R; Ghica, C; Teodorescu, V; Nistor, L; Ganea, R; Dinescu, G

Published: AUG 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2060, DOI:

The paper reports the application of a new method combining the Plasma Assisted Chemical Vapour Deposition with the Fluidized Bed Reactor principles for the production of carbon nanotubes on powdered catalysts. An argon plasma beam injected with acetylene in presence of hydrogen was used as fluidized environment for targeting, smashing and mixing the catalytic powder located on the bottom of a cylindrical quartz reactor. The described technique generated plate-like or fibrous like carbon nanostructures on fine Fe particles and carbon nanotubes with 5-10 nm diameters on Fe supported ZSM-5 zeolite.

674. Investigations of conduction mechanism in Cr2O3 gas sensing thick films by ac impedance spectroscopy and work function changes measurements

Authors: Pokhrel, S; Simion, CE; Quemener, V; Barsan, N; Weimar, U

Published: JUL 28 2008, SENSORS AND ACTUATORS B-CHEMICAL, 133, 83, DOI: 10.1016/j.snb.2008.01.054

This paper investigates the conduction mechanism in Cr2O3 gas sensing thick films. A citrate combustion method was used for the preparation of the metal oxide and ethanol vapour as a test gas. The manner in which surface reactions induced electrical changes are affecting the sensor signals inputs was explored by simultaneous dc and work function changes (Kelvin probe method). The identification of the contributions to conduction of the different sensing layer elements was made possible by ac impedance spectroscopy measurements. A conduction model, which qualitatively explains the experimental findings, was elaborated on the basis of the acquired experimental data and the information provided in literature. The model validity should apply, besides Cr2O3, to all p-type metal oxides used as gas sensitive materials. (C) 2008 Elsevier B.V. All rights reserved.

675. Analytical expression for the quantum dot contribution to the quasistatic capacitance for conduction band characterization

Authors: Buda, M; Iordache, G; Mokkapati, S; Fu, L; Jolley, G; Tan, HH; Jagadish, C; Buda, M

Published: JUL 15 2008, JOURNAL OF APPLIED PHYSICS, 104, DOI: 10.1063/1.2959681

This paper demonstrates an analytical expression for the quasistatic capacitance of a quantum dot layer embedded in a junction, where the reverse bias is used to discharge the initially occupied energy levels. This analysis can be used to determine the position and the Gaussian homogeneous broadening of the energy levels in the conduction band, and is applied for an InGaAs/GaAs quantum dot structure grown by metal organic chemical vapor deposition. It is shown that the Gaussian broadening of the conduction band levels is significantly larger than the broadening of the interband photoluminescence (PL) transitions involving both conduction and hole states. The analysis also reveals a contribution from the wetting layer both in PL and modeled C-V profiles which is much stronger than in typical molecular beam epitaxy grown dots. The presence of a built-in local field oriented from the apex of the dot toward its base, contrary to the direction expected for a strained dot with uniform composition (negative dipole), is also derived from fitting of the C- V experimental data. (c) 2008 American Institute of Physics.

676. Electron and hole trapping in irradiated PbBr2 : Tl single crystals

Authors: Stefan, M; Nistor, SV; Darabont, A; Neamtu, C; Goovaerts, E

Published: JUN 2008, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 266, 2761, DOI: 10.1016/j.nimb.2008.03.179

Several hole trapped centres, namely Tl2+, self-trapped hole and A-type centres, were observed by ESR in PbBr2:Tl single crystals after X-ray irradiation at 77 K. The corresponding spectra are visible up to 200 K for all three defect centres. The STH centres are observed at higher temperatures than in the undoped PbBr2 single crystals, probably due to the stabilising effect of the Tl+ impurities. The ESR parameters of the Tl2+ centre point to stronger covalency effects in PbBr2 than in the isostructural PbCl2 crystals. All hole trapped centres started decaying around 160 K, probably due to recombination with nonparamagnetic electron trapped centres. Besides a small concentration of self-trapped electron centres, no other electron trapped centres were observed. (c) 2008 Elsevier B.V. All rights reserved.

677. Irradiation defects in superhard cubic boron nitride single crystals

Authors: Nistor, SV; Ghica, D; Stefan, M; Nistor, LC; Goovaerts, E; Taniguchi, T

Published: JUN 2008, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 266, 2787, DOI: 10.1016/j.nimb.2008.03.117

Single crystallites of superhard, semiconducting, n-type, amber colored and p-type, blue colored, Be-doped cubic boron nitride have been irradiated either with UV (350 nm) light or with an intense beam of accelerated (1 MeV) electrons. The examination of the irradiated samples at low temperatures by high frequency W (95 GHz)-band electron spin resonance reveals several new, radiation-induced, isotropic paramagnetic centers. The UV irradiation of both types of crystals yields centers involving very likely protons. In the amber c-BN crystals the irradiation with I MeV electrons results in the formation of vacancy associated paramagnetic defects and quasi-free electrons in colloidal particles. (C) 2008 Elsevier B.V. All rights reserved.

678. Doped sol-gel TiO2 films for biological applications

Authors: Gartner, M; Trapalis, C; Todorova, N; Giannakopoulou, T; Dobrescu, G; Anastasescu, M; Osiceanu, P; Ghita, A; Enache, M; Dumitru, L; Stoica, T; Zaharescu, M; Bae, JY; Suh, SH

Published: MAY 20 2008, BULLETIN OF THE KOREAN CHEMICAL SOCIETY, 29, 1042, DOI:

Mono and multilayer TiO2(Fe, PEG(600)) films were deposited by the dip-coating on SiO2/glass substrate using sol-gel method. In an attempt to improve the antibacterial properties of doped TiO2 films, the influence of the iron oxides and polyethilenglycol (PEG(600)) on the morphological, optical, surface chemical composition and biological properties of nanostructured layers was studied. Complementary measurements were performed including Spectroscopic Ellipsometry (SE), Scanning Electron Microscopy (SEM) coupled with the fractal analysis, X-Ray Photoelectron Spectroscopy (XPS) and antibacterial tests. It was found that different concentrations of Fe and PEG(600) added to coating solution strongly influence the porosity and morphology at nanometric scale related to fractal behaviour and the elemental and chemical states of the surfaces as well. The thermal treatment under oxidative atmosphere leads to films densification and oxides phase stabilization. The antibacterial activity of coatings against Escherichia Coli bacteria was examined by specific antibacterial tests.

679. Investigation of metal-oxide semiconductor field-effect transistor-like Si/SiO2/(nano)crystalline PbS heterostructures

Authors: Stancu, V; Buda, M; Pintilie, L; Pintilie, I; Botila, T; Iordache, G

Published: APR 30 2008, THIN SOLID FILMS, 516, 4306, DOI: 10.1016/j.tsf.2007.11.116

The influence of the gate voltage on photoconductivity in P-Si/SiO2/(nano)crystalline PbS, Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET)-like structures is investigated. The effect on the photo-signal in the wavelength range 500-1180 nm is opposite compared to the effect in the wavelength range 1600-2500 nm: a negative gate voltage increases the signal in the short wavelength range and decreases it for the long wavelength range. The opposite is valid for a positive gate bias. An on-off ratio better than 85 times is obtained for the spectral range 500-1180 nm, corresponding to the absorption in p-Si and better than 3.5 times for the spectral range 1600-2500 nm corresponding to the absorption in the (nano)crystal line PbS region, respectively. (c) 2007 Elsevier B.V. All rights reserved.

680. 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.