National Institute Of Materials Physics - Romania

ZnO:Co thin films are prepared by electrochemical deposition at different overpotentials and investigated by scanning electron microscopy, optical spectroscopy, and cathodoluminescence, X-ray photoelectron spectroscopy and Kerr magnetometry (MOKE). An increase of structural disorder and crystallite reorientation are observed for cobalt-doped samples. MOKE magnetometry revealed room-temperature ferromagnetic behavior only for ZnO: Co deposited at -850 mV. These are the samples with the lower cobalt content and lowest structural perturbation of the ZnO matrix by cobalt. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A genuine GaAs surface was covered with a relatively thick layer (similar to nm) of native oxide pinning the surface Fermi level within the band gap of semiconductor. The method presented in this work is related to the sulphur passivation by treating n-GaAs in sulphide solutions (e.g. pure ammonium sulphide and sulphur monochloride) that combines both chemical and electronic passivation by reducing the surface state density. The presence of the covalent bonds As-S and Ga-S was putted into evidence by XPS analysis. It is presented the independence of XPS measurements on n-GaAs substrate orientation for (100) and (110) planes. At the surface of n-GaAs it is developed an adherent layer of sulphur compound as a result of chemical interaction of sulphur ions with n-GaAs surface.

A new analytical approximation of the diffraction peak size profile due to the lognormal distribution of spherical crystallites is derived. In comparison with the existing approximation constructed with the condition of conserving the volume-averaged column length of the exact profile [Popa & Balzar (2002). J. Appl. Cryst. 35, 338-346], the new approximation conserves both volume- and area-averaged column lengths. Moreover, it is a better approximation for large distribution dispersions. Like the old approximation, the present one is appropriate for implementation in the whole powder pattern fitting codes, Rietveld included.

Short time spark plasma sintering at 1200 degrees C for 10 min and under a uniaxial pressure of 95 MPa followed by annealing at 1100 degrees C for 5 h is proposed for synthesis of high density (99%) Ba(Ti0.87Sn0.13)O-3 ceramic. Starting powder of 450 nm was obtained by co-precipitation, while the average grain size in the sintered ceramic was of 580 nm. X-ray diffraction analysis of the as prepared Ba(Ti0.87Sn0.13)O-3 samples indicated the presence of single phase BaTi0.87Sn0.13O3 developed in the BaTiO3 cubic structure. Samples have high relative dielectric constant of 5680 and low dissipation factor (tan delta = 0.009) at Curie temperature of 57 degrees C and 1 kHz, good tunability and ferroelectric properties. Remarkable is that these results are from some points of view different than for the conventionally processed samples with similar stoichiometry. Some ideas in this regard are presented and this suggests that SPS can be useful for expanded possibilities in the control of the properties. (C) 2010 Elsevier B.V. All rights reserved.

Li-N-H and Li-Si-N-H composites have been synthesized by ball milling starting from lithium amide, lithium hydride and silicon powders. Hydrogen sorption behaviour was investigated for the Si-containing composites and a Li-N-H reference sample. The as-milled Si-containing sample showed the presence of the constituent powders but after few absorption-desorption cycles an important amount of Li(2)SiN(2) appeared in both de-hydrogenated and re-hydrogenated composites. This phase was present in higher amount in the de-hydrogenated sample than in the re-hydrogenated one. The presence of this phase was also confirmed by XPS measurements. Li-N-H and Li-Si-N-H composites in de-hydrogenated state also contained lithium imide. X-ray diffraction data indicated a reversible hydrogen generating reaction between lithium amide, lithium hydride and silicon to produce Li(2)SiN(2). lithium imide and hydrogen. The plateau pressure of Li-N-Si-H system is twice the one for Li-N-H at the same temperature (about 265 degrees C), as shown by pressure-composition isotherms in desorption mode. For the first time it has been observed a thermodynamic destabilization of Li-N-H by mixing it with Si. (C) 2010 Elsevier B.V. All rights reserved.

The etherification of biomass-based alcohols with various linear alpha-olefins under solvent-free conditions was followed in a space- and time-resolved manner on 9 mu m large H-Beta zeolite crystals by confocal fluorescence microscopy. This allowed us to visualize the interaction with the substrate and distribution of the coke products into the catalyst at the level of an individual zeolite crystal during the etherification process. The spectroscopic information obtained on the micrometer-scale zeolite was in line with the results obtained with bulk characterization techniques and further confirmed by the catalytic results obtained both for micrometer-scale and nanoscale zeolites. This allowed us to explain the influence of the substrate type (glycerol, glycols, and alkenes) and zeolite properties (Si/Al ratio and particle size) on the etherification activity. The etherification of the biomass-based alcohols takes place mainly on the external surface of the zeolite particles. The gradual blockage of the external surface of the zeolite results in a partial or total loss of etherification activity. The deactivation could be attributed to olefin oligomerization. The high conversions obtained in the etherification of 1,2-propylene glycol with long linear alkenes (up to 80%) and the pronounced deactivation of the zeolite observed in the etherification of glycerol with long linear alkenes (max. 20% conversion) were explained by the spectroscopic measurements and is due to differences in the adsorption, i.e., in the center of the zeolite particle for glycerol and on the external surface in the case of glycols.

Electron states at the SiO2/4H-SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of similar to 10(19) cm(-3) and 10(16) cm(-3), respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35-295 K. Three main categories of electron states are disclosed, true interface states (D-it), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of D-it and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstrated to be responsible for the parallel shifts and hysterezis effects occurring in the C-V characteristics. All three categories of electron states are reduced in concentration in the implanted samples. This holds particularly for NIToxfast with a peak at similar to 0.1 eV below the conduction band edge of 4H-SiC that is suppressed by at least two orders of magnitude relative to the nonimplanted samples. The decrease for D-it is also substantial (a factor of similar to 10) while the loss for NIToxslow is considerably smaller (only similar to 30%). The results provide firm evidence that NIToxfast and NIToxslow do not originate from the same kind of defect center. (C) 2010 American Institute of Physics. [doi:10.1063/1.3457906]

Unimodal porous system (denoted as SnTf-MCM-41) and bimodal porosity (denoted as SnTf-UVM-7) were prepared in a two-step synthesis. The triflic acid was incorporated into previously synthesized Sn-containing porous silicas. Chemical analysis was carried out by electron probe microanalysis (EPMA), textural properties were determined by transmission electron microscopy (TEM) and from nitrogen adsorption-desorption isotherms (77 K), while structural characterization by X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements. The structural characterization provided evidences on the direct interaction of triflate with incorporated tin species. The acidity of these catalysts was determined from the ammonia chemisorption measurements. The catalysts have been tested for transesterification of sunflower oil under both microwave and ultrasound activation. These experiments showed a direct reaction between the acidity of the catalysts and catalytic performances. No leaching of the triflate species or catalyst deactivation have been detected over several runs.

Holmium-doped BaTiO3 with composition Ba0.97Ho0.03TiO3 was prepared by sol-gel combustion method. A molar ratio of citrate/nitrate (CA/NO3- = 1.3) was used to prepare nanopowders of (Ba,Ho)TiO3. The structure and microstructure of (Ba,Ho)TiO3 powders and ceramics were investigated. The ceramics exhibit a dielectric constant of about 4400 and dielectric loss (tan delta = 0.267) at 10 Hz, and at the Curie temperature (T-c = 132 degrees C). The remanent polarization and the coercive field of Ba0.97Ho0.03TiO3 ceramics, at 1 kHz, were P-r = 6 mu C/cm(2) and E-C = 0.75 kV/cm. The dielectric and ferroelectric behavior of the holmium-doped BaTiO3 is influenced by the amphoteric character of Ho3+ ions.

The to study of the total oxide (SiO2+SiOx) thickness, SiO2 and SiOx (e.g. Si2O, SiO, Si2O3) thicknesses on Si(100) crystalline substrate with take-off angles ranging from 30 degrees to 80 degrees has been carried out by spectrometric method. The d(s) X-ray Photoelectron Spectroscopy (XPS) thicknesses were compared with d(EL) thicknesses obtained by fitting the Spectroscopic Ellipsometry (SE) spectra. A qualitatively good correlation is revealed. However, from these estimations of film thicknesses it results that ellipsometry analysis cannot be as accurate as in XPS evaluation. This is due to uncertainty of used optical constants as well due to very thin oxide films used in this work.