National Institute Of Materials Physics - Romania

Publications

5781 articles found

2491. Spectroscopic ellipsometry investigations of Eu-doped oxy-fluoride glass and glass-ceramics

Authors: Galca, AC; Preda, N; Secu, CE; Luculescu, CR; Secu, M

Published: JUN 2012, OPTICAL MATERIALS, 34, 1496, DOI: 10.1016/j.optmat.2012.03.015

Oxyfluoride glass-ceramics in the system SiO2-Al2O3-CaF2-EuF3 containing Eu3+-doped CaF2 nanocrystals were produced by using controlled crystallization of melt-quenched glass. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 50 nm size. The Eu3+-dopant ions act as the nucleating agent necessary to initiate the crystallization process. The refractive index is higher in the glass ceramics than in the initial glass and varies as the annealing time increases. Two competitive processes are responsible for this behavior, the crystallization of the CaF2 phase and the decrease of the glass ceramic mass density. (C) 2012 Elsevier B.V. All rights reserved.

2492. Structure and Magnetic Properties of Nanosized Magnetite Obtained by Glass Recrystallization

Authors: Sandu, V; Nicolescu, MS; Kuncser, V; Popa, S; Pasuk, I; Ghica, C; Sandu, E

Published: JUN 2012, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 12, 5050, DOI: 10.1166/jnn.2012.4943

We present the preparation, structural and magnetic properties of nanosized magnetite obtained by the crystallization of a series of Fe-containing borosilicate glasses. Several compositions with the ratio Fe2O3/SiO2 spanning from 0.37 to 0.67 were investigated as a function of two nucleators Cr2O3 and P2O5, respectively, and modifiers and intermediates (Al2O3 and MgO). Mossbauer spectroscopy revealed the degree, the type and the location of disorder induced by a specific composition and nucleators. In addition to magnetite, it was also revealed the presence of large amounts of Fe-rich paramagnetic phases. The magnetic response is analysed in relation with the amount of Fe ions which remain dispersed in the glassy matrix as noninteracting (paramagnetic) ions. We discuss the role of the nucleators on the disorder in both tetrahedral and octahedral sites of the magnetite.

2493. Treatment of carrier scattering in quantum dots beyond the Boltzmann equation

Authors: Steinhoff, A; Gartner, P; Florian, M; Jahnke, F

Published: MAY 29 2012, PHYSICAL REVIEW B, 85, DOI: 10.1103/PhysRevB.85.205144

As quantum dots (QD) can confine a small number of carriers in localized states with discrete energies, it is questionable to neglect correlations between the carriers when describing their dynamics. We analyze the influence of carrier correlations in a single QD on Coulomb scattering processes, which are due to the contact with a quasicontinuum of wetting-layer (WL) states. Results obtained from a Boltzmann equation are compared with the fully correlated dynamics governed by a von Neumann-Lindblad equation. In a first step, we take into account correlations generated by the exact treatment of Pauli blocking due to the contributing QD carrier configurations. Subsequently, we include correlations generated by energy renormalizations due to Coulomb interaction between the QD carriers. It is shown that at low WL carrier densities, neither Pauli correlations nor Coulomb correlations can be safely neglected, if the dynamics of single-particle states in the QD are to be predicted qualitatively and quantitatively. In the high-density regime, both types of correlations play a lesser role and thus a description of carrier dynamics by a Boltzmann equation becomes reliable. Furthermore, the efficiency of WL-assisted scattering processes as well as scattering-induced dephasing rates depending on the WL carrier density are discussed.

2494. Tartarate precursors to CoxZn1-xGa2O4 spinel oxides

Authors: Mindru, I; Gingasu, D; Marinescu, G; Patron, L; Diamandescu, L; Feder, M; Calderon-Moreno, JM; Stanica, N

Published: MAY 15 2012, MATERIALS CHEMISTRY AND PHYSICS, 134, 483, DOI: 10.1016/j.matchemphys.2012.03.019

The CoxZn1-xGa2O4 spinels (x=0.25, 0.5, 0.75, 1) were synthesized by a wet chemistry method-the precursor route via tartarate decomposition. The complex precursors have been characterized by IR. UV-Vis, magnetic measurements and luminescence spectroscopy. XRD, SEM, IR. UV-Vis and luminescence spectroscopy were used for the structural and morphological investigation of CoxZn1-xGa2O4 spinels; magnetic susceptibilities of the spinel were also measured. The X-ray diffraction patterns confirmed the formation of gallate spinet phase. CoxZn1-xGa2O4. (C) 2012 Elsevier B.V. All rights reserved.

2495. Preparation of nanostructured ZnO nanorods in a hydrothermal-electrochemical process

Authors: Sima, M; Vasile, E; Sima, M

Published: MAY 1 2012, THIN SOLID FILMS, 520, 4636, DOI: 10.1016/j.tsf.2011.10.116

An array of ZnO nanorods, each nanorod being covered with a shell of porous ZnO was prepared in two steps by hydrothermal-electrochemical processes. The growth of ZnO nanorods was achieved in a zinc nitrate and hexamethylenetetramine aqueous solution on fluorine-doped tin oxide film. The porous ZnO shell was grown from a similar solution in the presence of eosin Y as nanostructuring agent. A dye-sensitized solar cell was assembled using as photoanode an eosin Y sensitized ZnO nanorod/shell layer. (C) 2011 Elsevier B.V. All rights reserved.

2496. Ferroelectric and dielectric multilayer heterostructures based on KTa0.65Nb0.35O3 and Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y grown by pulsed laser deposition and chemical solution deposition for high frequency tunable devices

Authors: Le Febvrier, A; Deputier, S; Bouquet, V; Demange, V; Ollivier, S; Galca, AC; Dragoi, C; Radu, R; Pintilie, L; Guilloux-Viry, M

Published: MAY 1 2012, THIN SOLID FILMS, 520, 4567, DOI: 10.1016/j.tsf.2011.10.142

Epitaxial growth of Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y (BZN) thin films was achieved on (100)(pc) LaAlO3 substrate by pulsed laser deposition (PLD) and by chemical solution deposition based on Pechini process. Effect of bismuth and zinc deficiency on the BZN thin films obtained by PLD was discussed, in relation with the starting target composition. Dielectric permittivity and bandgap values were determined from electrical and spectroscopic ellipsometry measurements performed on randomly oriented films grown on Pt/Si substrate. BZN thin films obtained by PLD exhibit, at 100 kHz, a dielectric constant of epsilon(r) = 203 and quite low dielectric losses of tan delta = 5 x 10(-2). Epitaxial ferroelectric - dielectric KTa0.65Nb0.35O3 (KTN) -Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y (KTN on BZN and BZN on KIN) bilayers were obtained by PLD on (100)(pc) LaAlO3 with the insertion of a suitable buffer layer of KNbO3 in the case of KTN on BZN. Such multilayer heterostructures with an epitaxial growth control of each layer are promising candidates for potential integration in microwave devices. (C) 2011 Elsevier B.V. All rights reserved.

2497. Interactions of exotic particles with ordinary matter

Authors: Lazanu, I; Lazanu, S

Published: MAY 1 2012, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 278, 77, DOI: 10.1016/j.nimb.2012.02.008

Weakly interacting massive particles (WIMPs) and strangelets are two classes of "exotic" particles not yet discovered, and in agreement with theoretical scenarios most probably produced in different early stages of evolution of the Universe. Some peculiarities of their energy loss in the electronic and nuclear interactions with ordinary matter are investigated. For the direct detection of WIMPs the signals produced by the stopping of recoils in matter are used for their identification. The influence of the orientation of the recoil in respect to crystal axes for crystalline silicon (as material for detectors) is analysed as average quantities: energy loss, and as transient thermal effects. For strangelets, the mechanisms of picking-up neutrons during their penetration into matter and the effects on electronic and nuclear stopping are considered. The clarification of the aspects related to the stopping of these hypothetical particles in matter will permit a better interpretation of some experimental results and could also contribute to the search for new techniques or materials for their detection, if they exist. (C) 2012 Elsevier B.V. All rights reserved.

2498. Investigations of the non-isothermal crystallization of Bi4Ge3O12 (2:3) glasses

Authors: Aldica, G; Polosan, S

Published: MAY 1 2012, JOURNAL OF NON-CRYSTALLINE SOLIDS, 358, 1227, DOI: 10.1016/j.jnoncrysol.2012.02.025

Amorphous Bi4Ge3O12 glass samples were produced by melt quenching procedure stating with Bi4Ge3O12 (BGO) powder, obtained by solid state reaction between oxides. The kinetics of non-isothermal crystallization of EGO nano-crystals has been investigated. Differential Thermal Analysis (DTA) can give the main parameters of crystallization with an exothermic peak from 813 K to 851 K depending on the heating rate, which was assigned to the crystallization of cubic BGO in the amorphous matrix and compared with the X-Ray Diffraction (XRD) patterns. The nano-crystal dimensions were calculated from the XRD patterns by using the Debye-Scherrer method and were compared with Transmission Electron Microscopy (TEM) images. It was shown that the Ozawa model is most suitable for describing the behavior of non-isothermal crystallization of EGO nano-crystals within the glass matrix. Experimental results suggest a disk-shape type growth mechanism for the Bi4Ge3O12 nano-crystallites. The Flynn-Wall-Ozawa method has shown that the average activated energy value is 385 +/- 14 kJ/mol which was computed within the same model and agrees very well with the activation energy of the crystallization. (C) 2012 Elsevier B.V. All rights reserved.

2499. Catalyst size limitation in vapor-liquid-solid ZnO nanowire growth using pulsed laser deposition

Authors: Marcu, A; Trupina, L; Zamani, R; Arbiol, J; Grigoriu, C; Morante, JR

Published: MAY 1 2012, THIN SOLID FILMS, 520, 4631, DOI: 10.1016/j.tsf.2011.10.126

Using a pulsed laser deposition system in a plane reflector configuration, we have grown single crystal ZnO nanowires using different gold catalyst quantities and different local particle fluxes. The number of ZnO nanowire over the square micrometer substrate surface proved to depend on both gold quantity and particle flux, in specific conditions. If for a "low particle flux" the nanowire density seems to strongly depend on the gold quantity, for a "high particle flux", the dependence is no longer observable. The results were interpreted in terms of the catalyst absorbed particle concentration dependence on the catalyst droplet size and incident particle flux. (C) 2011 Elsevier B.V. All rights reserved.

2500. Optical properties of amorphous-like indium zinc oxide and indium gallium zinc oxide thin films

Authors: Galca, AC; Socol, G; Craciun, V

Published: MAY 1 2012, THIN SOLID FILMS, 520, 4725, DOI: 10.1016/j.tsf.2011.10.194

The paper presents the optical properties of amorphous-like indium zinc oxide and indium gallium zinc oxide thin films with various In/(In + Zn) ratios obtained by Pulsed Laser Deposition. Thickness results obtained from simulations of X-ray Reflectivity and Spectroscopic Ellipsometry data were very similar. The dependence of density on stoichiometry resembles the corresponding dependence of the refractive index in the transparency range. A free carrier absorption was noted in the visible spectral range, leading to a weak absorbing thin transparent conductive oxide. On the other hand, the refractive index is smaller than those of based oxides (ZnO and In2O3), and counterbalance therefore the weak light absorption. (C) 2011 Elsevier B.V. All rights reserved.