National Institute Of Materials Physics - Romania

Powder mixtures of MgB2 and B4C with composition ((MgB2) + (B4C) x, x = 0.005, 0.01, 0.03) were consolidated by Spark Plasma Sintering at 1150 degrees C for 3 min. The average particle size of B4C raw powder was relatively high of 4 mm. Despite this, it is shown that processing processes are fast and, as in the case of the in-situ routes, for our ex-situ method carbon substitutes for the boron in the crystal lattice of MgB2. Specifics of microstructure are discussed based on electron microscopy observations. Carbon substitution and microstructure contribute to enhancement of the critical current density J(c) at high magnetic fields and of the irreversibility field H-irr. Samples are shown to be in the point pinning limit with some tendency toward the grain boundary pinning depending on B4C doping amount and temperature. An optimum composition is found for x = 0.01: for this sample, at 20 K, a J(c) of 100 A/cm(2) is obtained at 5.35 T. This value is higher than for the pristine MgB2 sample and for an optimum ex-situ nano-SiC-doped sample obtained for the same SPS processing conditions. (C) 2015 Elsevier B.V. All rights reserved.

High aspect ratio CuO nanowires are synthesized by a simple and scalable method, thermal oxidation in air. The structural, morphological, optical, and electrical properties of the semiconducting nanowires were studied. Au-Ti/CuO nanowire and Pt/CuO nanowire electrical contacts were investigated. A dominant Schottky mechanism was evidenced in the Au-Ti/CuO nanowire junction and an ohmic behavior was observed for the Pt/CuO nanowire junction. The Pt/CuO nanowire/Pt structure allows the measurements of the intrinsic transport properties of the single CuO nanowires. It was found that an activation mechanism describes the behavior at higher temperatures, while a nearest neighbor hopping transport mechanism is characteristic at low temperatures. This was also confirmed by four-probe resistivity measurements on the single CuO nanowires. By changing the metal/semiconductor interface, devices such as Schottky diodes and field effect transistors based on single CuO p-type nanowire semiconductor channel are obtained. These devices are suitable for being used in various electronic circuits where their size related properties can be exploited. (c) 2015 AIP Publishing LLC.

Ferroelectric/electric properties of PbZr0.52Ti0.48O3 (PZT) thin films grown by pulsed laser deposition (PLD) on two different substrates, Si (001) and SrTiO3 (STO) (001), were comparatively analyzed. The structural characterization has revealed the epitaxial relationship between the grown layers and the two types of substrates, with larger density of structural defects for the films deposited on Si (001) with buffer STO layer. The ferroelectric/electric properties are also different, with lower remnant polarization (about half of the value obtained on STO substrate), higher dielectric constant (about two times larger), and lower leakage current (about two orders of magnitude lower) for the PZT films deposited on Si (001) compared to those deposited on (001) STO substrates. Nevertheless, the results show that the use of a STO buffer layer on Si can be a solution to obtain good quality PZT capacitor structures without using expensive single-crystal oxide substrates. In this way, applications based on PZT capacitors (e.g. non-volatile memories, pyroelectric detectors, light switches, etc.) would be more easily integrated directly on Si wafers.

Hard carbon thin films were synthesized on Si (100) and quartz substrates by the Pulsed Laser Deposition (PLD) technique in vacuum or methane ambient to study their suitability for applications requiring high mechanical resistance. The deposited films' surface morphology was investigated by scanning electron microscopy, crystalline status by X-ray diffraction, packing and density by X-ray reflectivity, chemical bonding by Raman and X-ray photoelectron spectroscopy, adherence by pull-out measurements and mechanical properties by nanoindentation tests. Films synthesized in vacuum were a-C DLC type, while films synthesized in methane were categorized as a-C:H. The majority of PLD films consisted of two layers: one low density layer towards the surface and a higher density layer in contact with the substrate. The deposition gas pressure played a crucial role on films thickness, component layers thickness ratio, structure and mechanical properties. The films were smooth, amorphous and composed of a mixture of sp(3)-sp(2) carbon, with sp(3) content ranging between 50% and 90%. The thickness and density of the two constituent layers of a film directly determined its mechanical properties.

This work is focusing on generation, time evolution, and impact on the electrical performance of silicon diodes impaired by radiation induced active defects. n-type silicon diodes had been irradiated with electrons ranging from 1.5 MeV to 27 MeV. It is shown that the formation of small clusters starts already after irradiation with high fluence of 1.5 MeV electrons. An increase of the introduction rates of both point defects and small clusters with increasing energy is seen, showing saturation for electron energies above similar to 15 MeV. The changes in the leakage current at low irradiation fluence-values proved to be determined by the change in the configuration of the tri-vacancy (V-3). Similar to V-3, other cluster related defects are showing bistability indicating that they might be associated with larger vacancy clusters. The change of the space charge density with irradiation and with annealing time after irradiation is fully described by accounting for the radiation induced trapping centers. High resolution electron microscopy investigations correlated with the annealing experiments revealed changes in the spatial structure of the defects. Furthermore, it is shown that while the generation of point defects is well described by the classical Non Ionizing Energy Loss (NIEL), the formation of small defect clusters is better described by the "effective NIEL" using results from molecular dynamics simulations. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Bulk ceramics of Ga2S3 and rare-earth sulfides (EuS, Gd2S3, Er2S3) as well as combinations thereof have been prepared by spark plasma sintering (SPS). The disk-shaped ceramics were used as targets for pulsed laser deposition (PLD) experiments to obtain amorphous thin films. The properties of these new bulks and amorphous thin films have been investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), optical transmission spectroscopy, and atomic force microscopy (AFM). In order to test the photoexpansion effect in Gd2S3 and the possibility to create planar arrays of microlenses, the film was irradiated with femtosecond laser pulses at different powers. For low laser power pulses (up to 100mW power per pulse) a photoexpansion effect was observed, which leads to formation of hillocks with a height of 40-50 nm. EuS doped Gd2S3 thin film shows luminescence properties, which recommend them for optoelectronic applications.

Thin films of Sr2FeMoO6 (SFMO) were grown by pulsed laser deposition in non-optimized argon ambient pressures. The films were found to contain a high number of precipitates of foreign phases. The nature and microstructure of these phases were investigated in detail by high-resolution scanning transmission electron microscopy (STEM) and X-ray diffractometry (XRD). We found out that the dominant foreign phase embedded in the SFMO film matrix was SrMoO4 (SMO). Through STEM and XRD analysis, we determined that the SMO phase grows epitaxially with respect to the surrounding SFMO matrix and has a fairly good crystallinity. Although the SFMO films include many foreign precipitates, they still exhibit good conducting properties and moderate magnetization values. Tuning the growth of the SMO phase on top of SFMO films to obtain a natural tunnel barrier might pave the way for future applications of SFMO in spintronic devices.

A series of 3,4,5-tridodecyloxybenzyl pyridinium salts derived from 4-hydroxypyridine has been designed and prepared. The liquid crystalline properties of these compounds were investigated by polarized optical microscopy, differential scanning calorimetry and powder X-ray diffraction while their thermal stability was studied by thermogravimetric analysis. The N-3,4,5-tridodecyloxybenzyl-4-pyridone intermediate shows a monotropic columnar hexagonal mesophase ranging from 56 degrees C down to room temperature while the corresponding bromide dodecyl O-alkylated pyridinium salt shows one enantiotropic columnar mesophase and one additional monotropic columnar phase at lower temperatures. Replacing bromide ion (Br-) with other counterions (NO3-, BF4- and PF6-) resulted in mesophase suppression. These luminescent pyridinium salts show weak emission in dichloromethane solutions at room temperature and a pronounced red-shifted emission in solid state. Photoluminescent properties of the pyridinium salts do not depend significantly on the nature of counterion employed. (C) 2014 Elsevier B.V. All rights reserved.

The aim of this research is to prepare amorphous thin films of undoped gallium sulphide and doped with rare-earth sulphides, of rare-earth sulphides and to investigate their physical properties. We have prepared thin amorphous films of Ga2S3, EuS, Er2S3, Gd2S3, and Ga2S3 doped with rare-earth sulphides (Ga2S3:EuS, Ga2S3:Er2S3, Ga2S3:Gd2S3) by Pulsed laser Deposition (PLD). The corresponding targets for preparation of amorphous thin films were obtained by Spark Plasma Sintering (SPS) from commercially available powders of binary sulphides. The structural results for the undoped and doped Ga2S3 thin films indicate a packing of disordered layers similar to that of amorphous As2S3. Femtosecond laser irradiation of the Ga2S3 thin films shows a photoexpansion effect at low laser power (85-100 mW) and an ablation effect at higher laser power (above 105 mW). The threshold between low power and high power pulses is situated at higher value for Ga2S3 (100 mW) in comparison with the case of As2S3 thin films (20 mW). (C) 2014 Elsevier B.V. All rights reserved.