National Institute Of Materials Physics - Romania

Up-conversion properties of Yb3+/Er3+ co-doped LiYF4 nanocrystals embedded in a sol-gel derived oxyfluoride glass-ceramic have been studied by comparison to the corresponding polycrystalline sample. Under 980 rim laser light pumping, both Yb3+/Er3+ co-doped LiYF4 samples show green ((H-2(11/2,) S-4(3/2)) I-4(15/2)) and red (F-4(9/2) I-4(15/2)) up-conversion luminescences explained by a two-photon processes. The tendency for the saturation of the red up-conversion luminescence band observed in the glass ceramic was ascribed to a back energy transfer process Er3+-Yb3+ and cross-relaxation processes caused by the high Yb3+ and Er3+ ions Concentrations in the nanocrystals. (C) 2015 Elsevier B.V. All rights reserved.

The ability of photoelectron spectro-microscopy with sub-micrometer lateral resolution to identify ferroelectric domains by analysis of surface band bendings is demonstrated on lead zirco-titanate PZT(1 1 1) thin films grown by pulsed laser deposition. Conventional synchrotron radiation X-ray photoelectron spectroscopy allowed one to derive the surface composition of the sample and evidenced shifts toward higher binding energy when the sample is subject to intense soft X-ray beam. A basic model is developed which supposes that photogenerated carriers reduce the depolarization field, yielding a lower torque applied to the ferroelectric polarization. As a consequence, the out-of-plane component of the polarization increases. Domain migration during irradiation with soft X-ray is inferred from the relative amplitude of the components with different binding energy. When the flux density of soft X-ray is on the order of 1011 photons/(s mu m(2)), metal Pb clusters are formed at the surface on areas with the out-of-plane component of the polarization pointing outwards only. (C) 2015 Elsevier B.V. All rights reserved.

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.

GeTe and GaSb thin films obtained by pulsed laser deposition were investigated by spectroscopic ellipsometry at controlled temperatures. The GeTe films were fully amorphous, while the GaSb films were partially crystalized in the as-deposited state. The Tauc-Lorentz model was employed to fit the experimental data. From the temperature study of the optical constants, it was observed the crystallization in the 150-160 degrees C range of GeTe amorphous films and between 230 and 240 degrees C of GaSb amorphous phase. A second transition in the resonance energy and the broadening parameter of the Lorentz oscillator was observed due to the crystallization of Sb after 250 degrees C. The temperatures of 85 degrees C and 130 degrees C are noticed as the start of the relaxation of the amorphous GeTe phase and as-deposited GaSb. The peaks of the imaginary part of the dielectric function red shifted after the phase change, while the variation with temperature of the crystalline phase follows the Varshni law. The electron-phonon coupling constants are 2.88 and 1.64 for c-GeTe and c-GaSb, respectively. An optical contrast up to 60% was obtained for GeTe films and a maximum value of 7.5% is revealed in the case GaSb, which is altered by the partial crystallinity of the as-deposited films. (C) 2015 AIP Publishing LLC.

The current-voltage (C-V) characteristic of epitaxial ferroelectric films is simulated assuming the presence of Schottky-type contacts at the two electrode interfaces. The model assumes that the overall capacitance of the metal-ferroelectric-metal (MFM) structure is composed of two parts: (i) one associated with the Schottky contacts, in which the ferroelectric polarization is saturated, the dielectric constant is independent on the voltage and only the linear response to the applied electric field is taken into account; (ii) one related to the ferroelectric volume, where the dielectric constant is voltage dependent through the hysteresis response of the ferroelectric polarization. Themost important result of the model is that it can simulate the experimentally observed thickness dependence of the dielectric constant without considering a so-called "dead layer" at the electrode interface. The model renders C-V characteristics in good qualitative agreement with the experimental ones in the case of an MFM structure based on epitaxial PZT films. The quantitative fit suggests that the behaviour of the ferroelectric polarization during the C-V measurement may be very different from its behaviour during the hysteresis measurement. This is explained by the fact that the two measurements have very different principles. It is also found that the dielectric constant of the ferroelectric volume has a different voltage dependence compared to the one derived from the hysteresis loop or from the experimental C-V characteristic. This is also related to the different measurement principles and to the fact that the measured capacitance of the MFM structure includes, besides the ferroelectric volume, the voltage dependent capacitance of the Schottky contacts. (C) 2015 Elsevier B.V. All rights reserved.

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.

Composites consisting of tantalum fiber/powder dispersed in a nanostructured W matrix have been consolidated by spark and pulse plasma sintering as well as by hot isostatic pressing. The microstructural observations revealed that the tungsten-tantalum fiber composites consolidated by hot isostatic pressing and pulse plasma sintering presented a continuous layer of Ta2O5 phase at the W/Ta interfaces, while the samples consolidated by spark plasma sintering evidenced a Ta+Ta2O5 eutectic mixture due to the higher temperature of this consolidation process. Similar results have been obtained for the tungsten-tantalum powder composites. A (W, Ta) solid solution was detected around the prior nanostructured W particles in tungsten-tantalum powder composites consolidated by spark and pulse plasma sintering. Higher densifications were obtained for composites consolidated by hot isostatic pressing and pulse plasma sintering. (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.