National Institute Of Materials Physics - Romania

Publications

860 articles found

691. Electrical characteristics of nano-PbS/SiO2/Si heterostructures obtained by chemical bath method

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

Published: DEC 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3802, DOI:

The PbS/SiO2/Si structures show a clockwise hysteresis of capacitance-voltage (C-V) characteristics. The loop width increases with temperature and sweep time. The conductance-voltage (G-V) characteristics also present a hysteresis effect with two bumps. Both characteristics show a strong shift with frequency. We show that these effects are determined by the positive mobile ions injected in the oxide region during the chemical bath deposition of PbS and by the high concentration Of Si/SiO2 interface states. The corresponding C-V and G-V characteristics are simulated and the main peculiarities of the experimental results are well reproduced by the modeled curves.

692. Paramagnetic silica-coated gold nanoparticles

Authors: Ghica, C; Ionita, P

Published: DEC 2007, JOURNAL OF MATERIALS SCIENCE, 42, 10064, DOI: 10.1007/s10853-007-1980-4

Water soluble gold nanoparticles, obtained by the reduction of the gold (III) chloride with sodium borohydride in the presence of citric acid or thioctic acid, were covered with a paramagnetic silica layer using the Stober method, yielding a hybrid metallic-inorganic nanomaterial (gold nanoparticles, with an average size of 5 nm, embedded into silica nanoparticles, with an average size of 100 nm). The paramagnetic silica layer was formed by copolymerization of a paramagnetic silica precursor (derived from 3-aminopropyltrimethoxysilane) with tetramethyorthosilicate. The paramagnetic silica precursor was obtained by coupling 3-aminopropyltrimethoxysilane with 3-carboxy-proxyl free radical. TEM pictures show that each silica nanoparticle of about 100 nm in size embedded about 10 gold nanoparticles. These hybrid nanoparticles are quite stable and exhibit the expected paramagnetic characteristics, as seen by electron paramagnetic resonance. The accessibility of methanol through the silica layer was also studied. Depending on the capping ligands of the gold nanoparticles (citric or thioctic acid), different silica networks are formed, as seen by the mobility of the spin-label inside the silica layer. The EPR spectra showed that the paramagnetic silica layer is very robust and the mobility of the spin-probe inside the silica layer is very little affected by methanol. However, if spin-labeled thioctic acid protected gold nanoparticles were used in the material synthesis, the mobility of the spins attached to the gold surface is quite high in the presence of methanol, while the spins embedded into the silica layer remains immobilized.

693. Intermediate-valence behavior of YbCu5-xAlx around quantum critical point measured by resonant inelastic x-ray scattering at Yb L-3 absorption edge

Authors: Yamamoto, K; Yamaoka, H; Tsujii, N; Vlaicu, AM; Oohashi, H; Sakakura, S; Tochio, T; Ito, Y; Chainani, A; Shin, S

Published: DEC 2007, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76, DOI: 10.1143/JPSJ.76.124705

An x-ray spectroscopy study was performed for YbCU5-xAlx (x = 0.0-2.0) using high-resolution partial-fluorescence yield (PFY) and resonant inelastic x-ray scattering (RlXS) at the Yb L-3 absorption edge. From a careful analysis of the PFY and RlXS spectra, the mean-valence of Yb was estimated and is found to increase quite rapidly for compositions between 0 <= x <= 1.0, indicating strong intermediate-valence behavior. The mean valence continues to increase gradually with x and becomes nearly trivalent for x = 2.0. Our results, however, reveal that valence fluctuation occurs even at x = 1.5, for which non-Fermi liquid quantum critical behavior is known. The valence evolution cannot be explained solely in terms of chemical pressure effects. The study suggests the role of valence-coupled spin fluctuations for the quantum critical behavior.

694. (Te/SnSe2)(3) multilayers deposited by pulsed laser deposition. Structure and gas sensing properties

Authors: Lorinczi, A; Sava, F; Tomescu, A; Simion, C; Socol, G; Mihailescu, IN; Popescu, M

Published: NOV 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3492, DOI:

Multilayers made of alternative layers of composition SnSe2 and tellurium (three pairs) have been prepared by pulsed laser deposition (PLD). The complex films have been investigated by X-ray diffraction. After a special annealing in oxygen fluence at various temperatures, up to 620 degrees C, the films were investigated both by X-ray diffraction and for gas sensing. Gas sensing properties were tested for water vapours, carbon monoxide (CO) and methane (CH4). After annealing at high temperature, the structure of the multilayer samples show oxide phases like TeO2, SnO2 and a mixed oxide ternary phase SnTe3O8. The annealed multilayers show good sensing properties to water vapours and CO gas.

695. Ge dots embedded in silicon dioxide using sol-gel deposition

Authors: Stoica, TF; Gartner, M; Teodorescu, VS; Stoica, T

Published: OCT 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3274, DOI:

A versatile method, that of sol-gel deposition was used to produce Si1-xGexO2 films on Si substrates. Samples after annealing at different temperatures are investigated by different techniques including high-resolution transmission electron microscopy and spectral ellipsometry. It was revealed that, by annealing at high temperatures, Ge accumulates in small clusters (5-10nm) inside of the oxide layer. By ellipsometric investigations, valuable data about the temperature evolution of the composition and optical constants were obtained.

696. Trapping phenomena in silicon-based nanocrystalline semiconductors

Authors: Ciurea, ML; Iancu, V; Mitroi, MR

Published: OCT 2007, SOLID-STATE ELECTRONICS, 51, 1337, DOI: 10.1016/j.sse.2007.07.002

In this paper, the trapping phenomena in silicon-based nanocrystalline semiconductors are studied. We propose a general and complete model for optical charging spectroscopy measurements, which takes into account the trapping-detrapping-retrapping processes and all the types of discharge currents that can occur in a nanocrystalline system. The model was applied to the measurements performed on multi-quantum well structures, (nc-Si/CaF2)(50), as well as on nanocrystalline porous silicon. This way, the trap parameters that are not directly measurable were determined. (C) 2007 Elsevier Ltd. All rights reserved.

697. Thickness dependence of crystallization process for hydroxyapatite thin films

Authors: Mercioniu, I; Ciuca, S; Pasuk, I; Slav, A; Morosanu, C; Bercu, M

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2538, DOI:

Hydroxyapatite (HAP) thin films (0.2 pm, 0.3 pm and 1.2 mu m thickness) were grown by RF magnetron sputtering in argon atmosphere onto silicon substrates at high deposition rates (0.6 mu m/h). Crystalline HAP films were obtained using a low temperature (100 degrees C) followed by post-deposition annealing at 300 degrees C, 450 degrees C, 500 degrees C and 550 degrees C in environmental air for 1 hour. An important influence of the films thickness upon the crystallization degree was noticed at intermediate annealing temperatures, as obtained from XRD measurements. For low and high temperatures similar values were obtained with a better crystallization degree for the thinner films. FTIR absorption led to the same conclusion considering the shape of stretching and bending PO4 lines. This suggests that the crystallization process has a diffusion component besides usual thermal activation process.

698. Phototransport and photoluminescence in nanocrystalline porous silicon

Authors: Iancu, V; Ciurea, ML; Stavarache, I; Teodorescu, VS

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2643, DOI:

The paper presents the phototransport and photoluminescence investigations on nanocrystalline porous silicon. Spectral dependence curves of the phototransport and photoluminescence were taken at room temperature. A typical phototransport curve presents several maxima and shoulders, while the photoluminescence curves present only one maximum. The temperature dependence of the phototransport was measured in the 85 - 250 K range for different wavelengths. All these curves present only one activation energy on the whole temperature interval. The experimental results are interpreted taking into account a quantum confinement model. Previous microstructure investigations proved that the studied nanocrystalline porous silicon is formed by a nanowire network. Then, the electron Hamiltonian can be split into the sum of a 1D longitudinal Bloch part and a 2D transversal part, the nanowire wall acting like a potential well. Thus, a number of discrete energy levels will appear into the band gap. Both phototransport and photoluminescence maxima are due to the transitions between these levels, corresponding to a mean nanowire diameter of 3.2 nm, in good agreement with the previous microstructure and electrical transport investigations. On the contrary, the temperature dependence of the phototransport is determined by the surface states located on the internal surface and/or the interface between nanocrystallites.

699. Percolation phenomena in Si-SiO2 nanocomposite films

Authors: Stavarache, I; Ciurea, ML

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2647, DOI:

Current-voltage characteristics of samples containing silicon nanodots embedded in an amorphous silicon dioxide matrix were investigated. A percolation mechanism was evidenced by the dependence on the concentration of the initial differential conductance and by the appearance of several thresholds in the I-V characteristics.

700. Nanocrystalline Er : YAG thin films prepared by pulsed laser deposition: An electron microscopy study

Authors: Stanoi, D; Popescu, A; Ghica, C; Socol, G; Axente, E; Ristoscu, C; Mihailescu, IN; Stefan, A; Georgescu, S

Published: JUL 31 2007, APPLIED SURFACE SCIENCE, 253, 8272, DOI: 10.1016/j.apsusc.2007.02.113

We report the synthesis of well crystallized stoichiometric Er:YAG thin films by pulsed laser deposition from Er:YAG targets in low pressure oxygen, followed by a post-deposition treatment at 1400 degrees C in environmental air for 19 h. The structure and morphology of the films were examined by transmission electron microscopy and selected area electron diffraction. We demonstrated that the subsequent application of pulsed laser deposition and heat treatment resulted in the formation of well crystallized single phase cubic Er:YAG nanostructured coatings. (c) 2007 Elsevier B.V. All rights reserved.