National Institute Of Materials Physics - Romania

Publications

5714 articles found

2011. Pulsed laser deposition of nanocrystalline SiC films

Authors: Socol, G; Galca, AC; Craciun, D; Hanna, M; Taylor, CR; Lambers, E; Craciun, V

Published: JUL 1 2014, APPLIED SURFACE SCIENCE, 306, 69, DOI: 10.1016/j.apsusc.2014.01.201

Thin SiC films were grown on (1 0 0) Si substrates at temperatures from 400 to 1000 degrees C under various CH4 pressures by the pulsed laser deposition (PLD) technique using a KrF excimer laser. After deposition, films were in situ annealed at their deposition temperature under 500 mbar of CH4 for 1.0-1.5h. X-ray reflectivity investigations showed that films exhibited mass densities similar to SiC single crystal samples, while symmetrical and grazing incidence X-ray diffraction investigations found that films deposited at 800 degrees C or higher substrate temperatures were nanocrystalline. Modeling of spectroscopic ellipsometry measurements indicated that the refractive index values were similar to those reported for bulk SiC, while X-ray photoelectron spectroscopy investigations found that films contained in bulk a relatively low oxygen concentration of around 1.0 at.%. Nanoindentation results showed that the deposited SiC films were very hard, with hardness values above 40 GPa for films deposited at temperatures higher than 800 degrees C. (C) 2014 Elsevier B.V. All rights reserved.

2012. Effect of Silver Addition to Superconducting SmFeAsO1-x F (x)

Authors: Sandu, V; Aldica, G; Liu, ZY; Zhang, ZL; Suo, HL

Published: JUL 2014, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 27, 1641, DOI: 10.1007/s10948-014-2503-6

We investigate the effect of silver addition to polycrystalline Sm-1111 superconductor. We show that Ag improves the phase homogeneity of the samples drastically reducing the weight of the low temperature, hence, leading to less fluorinated phases. The intragranular critical current density is enhanced by adding a small amount of Ag in all pinning regimes but a higher content of Ag leads to a decrease of this quantity. A tentative explanation for this dependence is proposed.

2013. Electrochemical Studies on the Growth Process of the Zinc Oxide Films from Nitrate Solutions

Authors: Dumitru, CS; Sima, M; Cojocaru, A

Published: JUL 2014, REVISTA DE CHIMIE, 65, 839, DOI:

The cathodic deposition of zinc oxide film on Pt from an aqueous solution containing 0.05 M Zn(NO3)(2) was investigated using linear sweep voltammetry and electrochemical impedance spectroscopy. The film formation is an electroprecipitation process due to reduction of nitrate ion on the surface of working electrode in the presence of Zn2+ ions. A morphology of ZnO with hexagonal platelets was observed. The standard electromotive force of the cell reaction was calculated for both NO3/NO2 process and overall deposition process. Linear voltammograms and visual observations showed that polarizing the electrode at higher negative potentials than -1.1 V, the parallel reduction of zinc ion to zinc metal occurs together with reduction of nitrate ion to hydroxide ion; as a consequence, a partial amorphous state of ZnO film was recorded on SEM micrographs. Experimental impedance data and data calculated by fitting with an equivalent circuit showed that the ZnO formation involves an adsorption of complexed zinc ions on Pt electrode, before the charge transfer.

2014. Evidence of A-B site cooperation in the EuFeO3 perovskite from Eu-151 and Fe-57 Mossbauer spectroscopy, EXAFS, and toluene catalytic oxidation

Authors: Florea, M; Alifanti, M; Kuncser, V; Macovei, D; Apostol, N; Granger, P; Parvulescu, VI

Published: JUL 2014, JOURNAL OF CATALYSIS, 316, 140, DOI: 10.1016/j.jcat.2014.04.016

EuFeO3 perovskite was considered as a case study of the cooperative effects of the rare earth and transition metal elements in the total catalytic oxidation of aromatic hydrocarbons. For this purpose, a EuFeO3 perovskite was prepared using the citrate route. Extensive characterization was performed via ex situ and in situ methods. EXAFS revealed that oxygen vacancies are formed in the nearest neighborhood of Fe and next-nearest neighborhood of Eu. Combining Eu-151 and Fe-57 Mossbauer experiments also suggested local oxygen defects in the proximity of the Eu cations during the reaction, as well as cooperative electron delocalization effects between Eu and Fe. XPS has shown the +3 oxidation state of both Eu and Fe cations and the relatively strong ionicity of the Fe-O chemical bonds, as suggested by the weakened 2p(3/2) satellite in the Fe2p spectrum. A systematic change of this satellite with the A cation in AFeO(3) perovskites was also discussed. The thermal treatment of the catalyst at 623 K, in air or toluene, was accompanied by a partial reduction of Fe and a partial oxidation of Eu, respectively. The interaction between the two elements appeared to be influenced mainly by the temperature and only slightly by the sample environment during further treatments. Catalytic oxidation of toluene on this perovskite led only to the production of carbon dioxide and water with no side-product formation from partial oxidation reactions. All the characterization and catalytic results preferably agree with a Volkenstein mechanism. (C) 2014 Elsevier Inc. All rights reserved.

2015. Chalcogenide systems at the border of the glass-formation domain: A key for understanding the memory-switching phenomena

Authors: Popescu, M; Velea, A; Sava, F; Lorinczi, A

Published: JUL 2014, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 251, 1339, DOI: 10.1002/pssb.201350104

The compositions in the ternary chalcogenide systems from the demarcation region between the glass-formation domain (GFD) and the partially or fully crystalline formation domain seem to exhibit outstanding properties. We have shown in this paper that the compositions with the best memory-switching properties are situated at the border of the GFD in many systems. One can use this correlation to find new phase-change materials with better switching properties or to discover GFDs that were not observed yet. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2016. Electrical properties related to the structure of GeSi nanostructured films

Authors: Ciurea, ML; Stavarache, I; Lepadatu, AM; Pasuk, I; Teodorescu, VS

Published: JUL 2014, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 251, 1346, DOI: 10.1002/pssb.201350112

GeSi nanostructured films were obtained by cosputtering from two Ge and Si targets and subsequent annealing in furnace in N-2 for 5 h at 700, 800 and 900 degrees C, with the aim to show the correlation between electrical properties and crystalline structure of the films. The as-deposited films are amorphous, have a Ge: Si composition of 55: 45 and 185 nm thickness. The film structure was investigated by XRD and TEM, and the electrical behaviour was studied by measuring current-voltage (I-V) and current-temperature (I-T) characteristics and by discussing them in correlation with the structure. Different electrical behaviours of GeSi films corresponding to different structures have been evidenced. The 700 degrees C annealed GeSi films are formed of nanocrystals (7-15 nm) separated by amorphous regions (1-2 nm). These films present a superlinear I-V characteristic typical of high field-assisted tunnelling through potential barriers (amorphous regions) between nanocrystals. The I-T characteristic at low temperature follows a T-1/2 law showing a thermally activated tunnelling of carriers between neighboring GeSi nanocrystals. The films annealed at 800 and 900 degrees C have a like behavior, they are completely crystallized and present linear I-V and Arrhenius I-T dependences reflecting the polycrystalline behaviour. [GRAPHICS] . (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2017. Simulation of the structure of GeAs4Te7 chalcogenide materials during memory switching

Authors: Popescu, M; Sava, F; Velea, A; Lorinczi, A; Simandan, ID

Published: JUL 2014, CANADIAN JOURNAL OF PHYSICS, 92, 680, DOI: 10.1139/cjp-2013-0561

The complex chalcogenides with excellent memory switching properties are mainly situated close to the border of glass formation domain. The simulation of the structural changes occurring during the memory switching process of a ternary chalcogenide composition has been carried out. The transition of a high resistivity GeAs4Te7 amorphous cluster with 120 atoms to a low resistivity crystalline cluster was analyzed. The coordination of atoms changes from that corresponding to 8-N coordination rule (two for tellurium, three for arsenic, and four for germanium) in the amorphous phase to six (the same for all atoms) in metastable crystalline phase. Because of spatial constraints exercised by the amorphous matrix, the amorphous cluster cannot expand. In these circumstances Te atoms seem to be over-coordinated (up to sixfold-coordinated). During the switching process, the atoms are moving on distances up to 4.0 angstrom. The average displacement is of 2.36 angstrom.

2018. Micropatterned ZnO rod arrays prepared by Au-catalyzed electroless deposition

Authors: Florica, C; Preda, N; Enculescu, M; Enculescu, I

Published: JUL 2014, PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, 8, 652, DOI: 10.1002/pssr.201409089

Micropatterned ZnO was synthesized by an electroless deposition process using Au stripes as catalytic surfaces. The Au-patterned electrodes were prepared on SiO2/Si wafers using photolithography. The site-selective deposition of patterned ZnO hexagonal rod arrays is confirmed by scanning electron microscopy. The ZnO micropatterned surface revealed a conversion of wettability from hydrophilic to super-hydrophobic depending on the deposition reaction parameters. The electrical measurements carried out at room temperature before and after exposure to ammonia vapors of the patterned ZnO arrays show a resistance variation with exposure time. Highly reproducible, easy scalable and low-cost, photolithography and electroless deposition techniques could provide a facile approach to fabricate functionalized micropatterns, for a wide range of applications. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2019. Electronic structure and magnetism of new scandium-based full Heusler compounds: Sc(2)CoZ (Z = Si, Ge, Sn)

Authors: Birsan, A

Published: JUN 15 2014, JOURNAL OF ALLOYS AND COMPOUNDS, 598, 235, DOI: 10.1016/j.jallcom.2014.02.005

First principles FPLAPW calculations were performed in the framework of Density Functional Theory (DFT), to study the electronic structures and magnetic properties for the new full-Heusler compounds: Sc(2)CoZ (Z = Si, Ge, Sn). The investigated materials are stable against decomposition, in ferromagnetic configuration and crystallize in the inverse Heusler structures. The half-metallic properties as function of the variation of unit cell volumes are analysed regarding the fourth main group constituent elements. The electronic structure calculations for Sc2CoSi and Sc2CoSn show half-metallic characters, with indirect band gaps of 0.544 eV and 0.408 eV at optimised lattice parameters of 6.28 angstrom and 6.62 angstrom, respectively. For Sc2CoGe compound, the Fermi energy is not pinned inside the energy band gap from minority density of states, neither for unit cell contraction nor for enlargement. The calculated total magnetic moments are 1 mu B/f.u., for all compounds, in agreement with Slater-Pauling rule. (C) 2014 Elsevier B. V. All rights reserved.

2020. Dynamics and relaxation of sp biexcitons in disk-shaped quantum dots

Authors: Moldoveanu, V; Dinu, IV; Dragomir, R

Published: JUN 12 2014, PHYSICAL REVIEW B, 89, DOI: 10.1103/PhysRevB.89.245415

We study the effects of intraband relaxation processes on optical manipulation protocols for sp biexcitons hosted by CdTe disk-shaped quantum dots. The many-body states are calculated within the configuration interaction method starting from single-particle states given by the k.p theory. The time-dependent occupations of relevant many-body states are extracted from the von Neumann-Lindblad equation for the density operator. We mainly investigate the generation of sp biexcitons with two pulses of different polarizations sigma(+) and sigma(-). The fast hole relaxation processes prevent a high-fidelity controlled operation on sp biexcitons and lead to the occupation of some transient states which can be optically probed. More importantly, the many-body structure of the transient states consists of two holes on the s shell and antiparallel sp triplet states for electrons. Our simulations show that these triplet states are more stable against decoherence as they can only be damaged through slow electron relaxation. The configuration mixing due to correlation effects is also discussed.