National Institute Of Materials Physics - Romania

The compensation of the depolarization field in ferroelectric layers requires the presence of a suitable amount of charges able to follow any variation of the ferroelectric polarization. These can be free carriers or charged defects located in the ferroelectric material or free carriers coming from the electrodes. Here we show that a self-doping phenomenon occurs in epitaxial, tetragonal ferroelectric films of Pb(Zr0.2Ti0.8)O-3, consisting in generation of point defects (vacancies) acting as donors/acceptors. These are introducing free carriers that partly compensate the depolarization field occurring in the film. It is found that the concentration of the free carriers introduced by selfdoping increases with decreasing the thickness of the ferroelectric layer, reaching values of the order of 10(26) m(-3) for 10 nm thick films. One the other hand, microscopic investigations show that, for thicknesses higher than 50 nm, the 2O/(Ti+Zr+Pb) atomic ratio increases with the thickness of the layers. These results suggest that the ratio between the oxygen and cation vacancies varies with the thickness of the layer in such a way that the net free carrier density is sufficient to efficiently compensate the depolarization field and to preserve the outward direction of the polarization.

The sites of incorporation of Cu2+ impurity ions in Bi12GeO20 single crystals co-doped with copper and vanadium have been investigated by electron paramagnetic resonance (EPR). While the X-band EPR spectra consist of a simple broad (Delta B similar to 50 mT) line with anisotropic lineshape, the W-band EPR spectra exhibit well resolved, strongly anisotropic lines, due to transitions within the 3d(9)-D-2 ground manifold of the Cu2+ ions. The most intense group of lines, attributed to the dominant Cu2+(I) center, displays a characteristic four components hyperfine structure for magnetic field orientations close to a < 110 > direction. The g and A tensor main axes are very close to one of the 12 possible sets of orthogonal < 1-10 >, < 00-1 > and < 110 > crystal directions. Several less intense lines, with unresolved hyperfine structure and similar symmetry properties, mostly overlapped by the Cu2+(I) spectrum, were attributed to Cu-2 (+)(II) centers. The two paramagnetic centers are identified as substitutional Cu2+ ions at Bi3+ sites with low C-1 symmetry, very likely resulting from different configurations of neighboring charge compensating defects. (C) 2015 Elsevier Inc. All rights reserved.

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.

In this paper, we apply results of analysis of the Raman spectrum from an amorphous carbon film modified with iron (a - C : Fe) which leads to construction of clusters containing fragments of graphene and a layer of iron atoms. Such clusters may be candidates for the role of microwave radiation absorbers. For the Raman experiments we performed deposition of a thin film of a -C : Fe. Details of the film deposition process, together with the corresponding Raman experiments are presented in this paper. Comparison with a literature model was performed for the intensity ratio of the maxima for specific Raman bands, using the letter D to represent " disorder" and the letter G to represent "graphite". Comparison gives evidence for the existence of nanosize fragments of graphene embedded in an amorphous matrix of the a - C : Fe film. The diameter of the fragments is predicted to attain a value of about 1.2 nm. Moreover, in a comparison with experimental data for defective graphite, the position of the maximum of the D band for a- C : Fe appeared to be red shifted. This could support the proposition that damping of Raman-active oscillations occurs by an electron gas in fragments of graphene after introduction of iron, e. g. after intercalation. On the basis of these estimates, we modeled a symmetrical polycyclic aromatic hydrocarbon having a similar size and converted it into fragment of a graphene plane by removal of hydrogen atoms occupying the edge states. To preserve symmetry, we placed atoms of iron on top of the fragment, situating them exactly above the centers of hexagons at a certain distance and placed another fragment of graphene on the top of the iron atoms, symmetrically. For a distance of 2.52 angstrom between the centers of the hexagon and the iron atom, distortions of carbon-carbon valence bonds and angles were found to be minimal, as shown through optimization of the system's geometry using the Avogadro molecular editor. This result supports an hypothesis of graphene fragments during the growth of an amorphous carbon film modified by simultaneous addition of a dopant metal such as iron. This example may illustrate the stability of a two-dimensional electron gas confined between the fragments.

Nano-composite magnets with L1(0) structure derived from binary FePt alloys and prepared as melt-spun ribbons are of current interest due to their higher operating temperature and the ability to be cast as a two-phase magnet with exchange spring magnetic properties, as both soft and hard magnetic phase may emerge from the same metastable precursor, i.e. the disordered cubic A1 phase. The present paper studies the effect of Mn addition on the thermal stability and phase structure, on the abundance of the hard magnetic phase and relative proportion of the soft ones, on the microstructure of the alloy as a function of temperature and on the overall magnetic properties. The interplay of the various magnetic sublattices in the ordering of the L1(0) phases as a consequence of introducing antiferromagnetically coupled Mn atoms in the alloy composition is discussed and interpreted in terms of microstructural changes induced by this addition as revealed by high resolution transmission electron microscopy and Xray diffraction. The temperature evolution of the phase composition and structural parameters is monitored using synchrotron radiation powder diffraction, while the compositional aspects are investigated using proton-induced X-ray emission and energy dispersive X-ray spectroscopy. Magnetic measurements reveal the magnetic parameters of interest (coercivity, remanence, Curie temperature, saturation magnetization), as well as the exchange-coupled two-phase nature of these magnets and provide information that hints at possible spin reorientation transitions in the Mn-containing planes of the L1(0) superlattice. (C) 2015 Elsevier Ltd. All rights reserved.

Dense samples (relative density > 93%) of bulk MgB2 with GeO2 additions were obtained by Spark Plasma Sintering. The critical current density J(c) of the added samples is improved at high magnetic fields when compared to the pristine sample. The optimum composition is for MgB2(GeO2)(0.005). For this sample, a J(c)(20 K) = 10(2) A/cm(2) is obtained at 5.1 T versus 3.9 T for the pristine sample. Ge substitution in the crystal lattice of MgB2 can be considered negligible, and T-c,T-onset and T-c,T-midpoint from magnetization measurements scatter within 0.2 and 0.6 K, respectively. TEM investigations show some specific details at nano scale: the tendency to form secondary phases (25-100 nm) with sphere-like or irregular shapes is observed and discussed. Samples are composites and the residual strain of MgB2 is constant for pristine and GeO2-added samples. Therefore, pinning enhancement leading to improvement of Jc for the GeO2 added samples is purely a 'microstructure' effect due to the presence of secondary phases. The point pinning is determined to be the predominant mechanism. Addition of a higher amount of GeO2 is shifting the pinning mechanism toward a grain boundary pinning. (C) 2015 Elsevier Masson SAS. All rights reserved.

The present study reveals the synthesis of metallic particles at atmospheric pressure using a radiofrequency low-temperature argon plasma jet. Copper bulk material of the powered electrode acted as solid precursor in the process and the metallic particles were obtained directly in gaseous environment. The particles were collected onto Si (100)-oriented substrates downstream of the atmospheric plasma jet and were ex situ characterized via optical microscopy, scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. We obtained copper particles, reddish brown in color, either of nano-or micro-metric sizes. Additionally, the plasma species and gas temperature were determined by OES. In conclusion, it was demonstrated that conversion of bulk metals into particles is possible at atmospheric pressure using a radiofrequency plasma jet and the development of several applications is recommended.

The recent observation of a fundamental direct bandgap for GeSn group W alloys and the demonstration of low temperature lasing provide new perspectives on the fabrication of Si photonic circuits. This work addresses the progress in GeSn alloy epitaxy aiming at room temperature GeSn lasing. Chemical vapor deposition of direct bandgap GeSn alloys with a high Gamma- to L-valley energy separation and large thicknesses for efficient optical mode confinement is presented and discussed. Up to 1 mu m thick GeSn layers with Sn contents up to 14 at. % were grown on thick relaxed Ge buffers, using Ge2H6 and SnCl4 precursors. Strong strain relaxation (up to 81%) at 12.5 at. % Sn concentration, translating into an increased separation between Gamma- and L-valleys of about 60 meV, have been obtained without crystalline structure degradation, as revealed by Rutherford backscattering spectroscopy/ion channeling and transmission electron microscopy. Room temperature reflectance and photoluminescence measurements were performed to probe the optical properties of these alloys. The emission/absorption limit of GeSn alloys can be extended up to 3.5 mu m (0.35 eV), making those alloys ideal candidates for optoelectronics in the mid-infrared region. Theoretical net gain calculations indicate that large room temperature laser gains should be reachable even without additional doping.

We present the synthesis of tungsten nanoparticles using a gas aggregation cluster source based on a magnetron sputtering discharge. The nanoparticles have sizes between 70 and 100 nm and a dendritic morphology, with branches emerging from the center and evolving in a flower-like pattern. Structural investigations revealed the presence of alpha-W and residual beta-W crystalline phases. Post deposition oxidation of the nanoparticles is also investigated.

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.