National Institute Of Materials Physics - Romania

Interfaces formed by gold and copper on single crystal layers of (0 0 1) PbZr0.2Ti0.8O3 (PZT) produced by pulsed laser deposition and exhibiting outwards polarization are analyzed by X-ray photoelectron spectroscopy. The stoichiometry of the layers reproduces reasonably that of the PZT target. The band bending occurring at the interface between PZT and the metals is investigated by analyzing the core level shifts as function on the metal deposition. It is found that for Au/PZT(0 0 1) the gold layer is not continuous and the observed band bendings can be attributed to a Schottky mechanism, whereas for Cu/PZT(0 0 1) the copper layer is continuous; in this latter case, the observed band bendings towards higher energies (lower binding energies) can be attributed to a concomitant bending due to the Schottky effect together with the disappearance of the initial bending due to the outwards polarization of the samples. Metal Pb is observed to segregate only in the case of Cu/PZT(0 0 1), therefore the surface self-reduction might also be connected to the presence of a metal with lower work function, which for larger coverage forms a continuous metal layer, able to provide electrons to the surface. High resolution transmission electron spectroscopy yielded the disappearance of the tetragonal distortion in the case of Cu/PZT(0 0 1), in line with the assumption of disappearance of the polarization-induced band bending. (C) 2015 Elsevier B.V. All rights reserved.

Ultraviolet laser irradiation of films composed of graphene oxide (GO) and GO-magnetite (Fe3O4) nanoparticles deposited on polydimethylsiloxane substrates is carried out. The irradiations are performed in vacuum and ammonia-rich gas environments. Electron and scanning probe microscopies reveal a rippling process in GO sheets as the accumulation of laser pulses proceeds, being the effect more pronounced with the increase of laser fluence. X-ray photoelectron spectroscopy analyses point to laser-induced chemical reaction pathways in GO completely different depending on the environment and the presence or absence of Fe3O4 nanoparticles. It is demonstrated that GO-based films with diverse type of oxygen- and nitrogen-containing chemical groups can be obtained by means of laser irradiation processes. The sheet resistance of these materials is also correlated to their structure and composition. (C) 2015 Elsevier Ltd. All rights reserved.

The influence of SiO2 layer in a core-shell BNT-BTCe@SiO2 heterostructure on the electrical properties of the BNT-BTCe core ceramic was investigated. The core-shell composite was prepared by coating BNT-BTCe powder with silica layer by a chemical process. BNT-BTCe grain-core was a solid solution of (Bi0.5Na0.5)TiO3 (BNT) and Ba0.95Ce0.05TiO3 (BTCe) with 92: 8 molar ratio. The thickness of the amorphous silica shell was about 128 nm and the size of the grain core was about 250 nm. The structure, microstructure, dielectric, ferroelectric and piezoelectric properties of BNT-BTCe@SiO2 core-shell powder and sintered ceramic were investigated and compared with those of the BNT-BTCe base material. We found that the values of permittivity (epsilon(r)) and dielectric losses of core-shell ceramic are lower and higher, respectively, than those of BNT-BTCe ceramic indicating that the SiO2 does not improve the dielectric properties of BNT-BTCe ceramic. For the BNT-BTCe@SiO2 composite, the silica layer leads to the increase of diffuseness degree of the phase transition and to a better ferroelectric relaxor. The sintered BNT-BTCe@SiO2 heterostructure shows higher remnant polarization P-r, higher coercive field E-c and higher piezoelectric constants (k(t), d(33), g(33), c(33)(E) and S-11(E)) than BNT-BTCe ceramic. The results of our study suggest that BNT-BTCe@SiO2 core-shell material could be interesting for applications in the ferroelectric and piezoelectric domains.

The time-dependent transport through single-molecule magnets (5MM) coupled to magnetic or non-magnetic electrodes is studied in the framework of the generalized Master equation (GME) method. We calculate the transient currents which develop when the molecule is smoothly coupled to the source and drain electrodes. The signature of the electrically induced magnetic switching on these transient currents is investigated. Our simulations show that the magnetic switching of the molecular spin can be read indirectly from the transient currents if one lead is magnetic and it is much faster if the leads have opposite spin polarizations. We identify effects of the transverse anisotropy on the dynamics of molecular states. (C) 2015 Elsevier Ltd. All rights reserved.

W-Si-C composites were fabricated by active filler controlled pyrolysis of W powder (high tungsten content) and W-SiC powder mixtures (low tungsten content), infiltrated by a preceramic polymer and heat treated at temperatures from 1600 to 2000 degrees C. Material with high volume fraction of W in initial powder-polymer mixture, formed a composite material composed of W, W2C and W5Si3 with closed porosity in a single polymer infiltration and pyrolysis (PIP) cycle. After heat treatment at 1700 degrees C the material exhibited flexural strength above 350 MPa, hardness of 7.8 GPa and indentation modulus of 250 GPa. Room temperature thermal conductivity of the composite was rather low, 23 Wm(-1) K-1, however, thermal conductivity increased with increasing temperature achieving 35 Wm(-1) K-1 at 1000 degrees C. The effect of W as active filler in W-SiC powder mixtures with low volume fraction of tungsten was negligible. Therefore, six polymer infiltration and pyrolysis cycles were used to achieve significant densification with 15% porosity. The material fabricated at 1800 degrees C was composed of SiC, WC and WSi2 and exhibited flexural strength of 400 MPa and room temperature thermal conductivity of 100 W m(-1) K-1, which decreased to 32 W m(-1) K-1 at 1000 degrees C. (C) 2015 Elsevier B.V. All rights reserved.

Bioactive glasses are currently considered the suitable candidates to stir the quest for a new generation of osseous implants with superior biological/functional performance. In congruence with this vision, this contribution aims to introduce a reliable technological recipe for coating fairly complex 3D-shaped implants (e.g. dental screws) with uniform and mechanical resistant bioactive glass films by the radio-frequency magnetron sputtering method. The mechanical reliability of the bioactive glass films applied to real Ti dental implant fixtures has been evaluated by a procedure comprised of "cold" implantation in pig mandibular bone from a dead animal, followed by immediate tension-free extraction tests. The effects of the complex mechanical strains occurring during implantation were analysed by scanning electron microscopy coupled with electron dispersive spectroscopy. Extensive biocompatibility assays (MTS, immunofluorescence, Western blot) revealed that the bioactive glass films stimulated strong cellular adhesion and proliferation of human dental pulp stem cells, without promoting their differentiation. The ability of the implant coatings to conserve a healthy stem cell pool is promising to further endorse the fabrication of new osseointegration implant designs with extended lifetime. (C) 2015 Elsevier Ltd. All rights reserved.

A comprehensive study of optical transitions in direct-bandgap Ge0.875Sn0.125 group IV alloys via photo-luminescence measurements as a function of temperature, compressive strain and excitation power is performed. The analysis of the integrated emission intensities reveals a strain-dependent indirect-to-direct bandgap transition, in good agreement with band structure calculations based on the 8-band k.p and deformation potential methods. We have observed and quantified Gamma valley-heavy hole and Gamma valley-light hole transitions at low pumping power and low temperatures in order to verify the splitting of the valence band due to strain. We will demonstrate that the intensity evolution of these transitions supports the conclusion about the fundamental direct bandgap in compressively strained GeSn alloys. The presented investigation, thus, demonstrates that direct-bandgap group IV alloys can be directly grown on Ge-buffered Si(001) substrates despite their residual compressive strain.

Electric and pyroelectric properties of AIN layers deposited on Si substrates with different resistivities were investigated. The dielectric constant was found to be around 12, while the conductance determined from dc current measurements was found to be in the 10(-9) to 10(-19) S range. The pyroelectric measurements were performed in voltage mode using two types of IR sources: a laser diode with 800 nm wavelength and a black body at 700 degrees C. A peculiar behavior was observed for the signal recorded when the laser diode was used as IR source. It was found that the Si substrate is introducing a signal component, due to the photogenerated carriers, which is adding to the pyroelectric signal generated by the AIN layer. This component is strongly dependent on the resistivity of the Si substrate. For strongly doped Si (Si++) the signal generated into the substrate represents only 10% of the recorded pyroelectric voltage. For electronic grade Si the signal generated into the substrate is about 100 times larger than the pyroelectric signal generated in the AIN layer. This effect can be used as an optical amplification of the pyroelectric signal. The frequency dependence observed for the pyroelectric signal recorded when the black body is used as IR source is typical for a pyroelectric detector. A value as large as 12.4 degrees C m(-2) K-1 was obtained for the pyroelectric coefficient using for estimation the constant signal at low modulation frequencies of the IR beam. However, the value of the pyroelectric coefficient is strongly affected by the electrical conductance of the AIN layer. As the conductance is frequency dependent it results that the value of the pyroelectric coefficient is frequency dependent, the value from above being valid only for very small frequencies of the temperature variation. It was also found that the electric and pyroelectric properties are dependent on the crystalline quality of the AIN layer. (C) 2015 Elsevier B.V. All rights reserved.

Electrical properties of ferroelectric capacitors based on PbZr0.52Ti0.48O3 thin films grown by pulsed laser deposition on silicon substrate with SrTiO3 buffer layer grown by molecular beam epitaxy were studied. A SrRuO3 layer was deposited as bottom electrode also by pulse laser deposition and Pt, Ir, Ru, SrRuO3 were used as top contacts. Electrical characterization comprised hysteresis and capacitance-voltage measurements in the temperature range from 150 K to 400 K. It was found that the macroscopic electrical properties are affected by the electrode interface, by the choice of the top electrode. However, even for metals with very different work functions (e.g. Pt and SrRuO3) the properties of the top and bottom electrode interfaces remain fairly symmetric suggesting a strong influence from the bound polarization charges located near the interface. (C) 2015 Elsevier B.V. All rights reserved.