National Institute Of Materials Physics - Romania

The leakage current in all oxide epitaxial (La,Sr)MnO3-ferroelectric-(La,Sr)MnO3 structures, where the ferroelectric layer is either BaTiO3 or Pb(Zr0.2Ti0.8)O-3, was analyzed on a broad range of temperatures and for different thicknesses of the ferroelectric layer. It was found that, although the structures are nominally symmetric, the current-voltage (I-V) characteristics are asymmetric. The leakage current depends strongly on the thicknesses of the ferroelectric layer, on temperature and on the polarity of the applied voltage. Simple conduction mechanisms such as space charge limited currents or thermionic emission cannot explain in the same time the voltage, temperature, and thickness dependence of the experimentally measured leakage currents. A combination between interface limited charge injection and bulk controlled drift-diffusion (through hopping in the case of BTO and through band mobility in the case of PZT) is qualitatively explaining the experimental I-V characteristics. (C) 2013 AIP Publishing LLC.

Metal-ferroelectric-metal structures based on epitaxial Pb(Zr0.2Ti0.8)O-3 thin films are prepared by pulsed laser deposition on single crystal SrTiO3 substrates ((001) orientation) with buffer SrRuO3 layer as bottom electrode. Pt, Cu, and SrRuO3 are used as top contacts. The current-voltage (I-V) measurements reveal a strong influence of the top electrode interface on the magnitude of the leakage current and the shape of the I-V characteristics. The lowest current values are obtained for top Cu and the highest for top Pt. Diode-like behavior is obtained for top Cu and Pt, but the forward and reverse biases are opposite in sign. Contrary to the case of BiFeO3 layers deposited on the same type of substrates, it was found that the diode-like behavior is not switchable with the polarization reversal although the polarization values are comparable. It is also shown that the metal-ferroelectric-metal (MFM) structure based on Pb(Zr,Ti)O-3 (PZT) can be simulated and modeled as a back-to-back connection of two Schottky diodes. The diode-like behavior of the MFM structure can be induced by a slight asymmetry of the potential barriers at the electrode interfaces behaving as Schottky contacts. The study ends with a critical discussion of the MFM structures based on PZT and BiFeO3 (BFO) layers. It is shown that the switchable diode-like behavior is not uniquely determined by the polarization reversal and is not a general characteristic for MFM structures. Such behavior may be present only if the polarization induced band-bending at the interface is generating an accumulation layer at the interface. This could be possible in BiFeO3 based MFM structures due to the lower band gap compared to Pb(Zr0.2Ti0.8)O-3 thin films. (C) 2013 AIP Publishing LLC.

The growth of gold layers on Pb(Zr,Ti)O-3 (PZT) deposited on SrTiO3 is investigated by X-ray photoelectron spectroscopy in the Au thickness range 2-100 angstrom. Two phases are identified, with compositions close to nominal PZT. The 'standard' phase is represented by all binding energies (Pb 4f, Ti 2p, Zr 3d, O 1s) sensibly equal to the nominal values for PZT, whereas the 'charged' phase exhibits all core levels are shifted by similar to 1 eV toward higher binding energies. By taking into account also scanning probe microscopy images together with recent photoemission results, the 'charged' phase belongs to P(+) regions of PZT, whereas the 'normal' phase corresponds to regions with no net ferroelectric polarization perpendicular to the surface. Au deposition proceeds in a band bending of Phi(PZT) - Phi(Au) similar to 0.4-0.5 eV for both phases, identified as similar shifts toward higher binding energies of all Pb, Ti, Zr, O core levels with Au deposition. The Au 4f core level exhibits also an unusually low binding energy component 1 eV below the 'nominal' Au 4f binding energy position (metal Au). This implies the existence of negatively charged gold, or electron transfer from PZT to Au, although the 'normal' PZT phase have a higher work function, as it is derived from the band bending. Most probably this charge transfer occurs toward Au nanoparticles, which have even higher ionization energies. High resolution transmission electron microscopy evidenced the formation of such isolated nanoparticles. (C) 2013 Elsevier B.V. All rights reserved.

Bi2O3 and Bi-metal powders were mixed with MgB2 powder. Starting compositions were (MgB2)(Bi2O3) x, x = 0.0025, 0.005, 0.015, and (MgB2)(Bi)(y), y = 0.01. Mixtures were processed by Spark Plasma Sintering (SPS) technique. As obtained composite samples show high density, above 94 % of the theoretical density. Samples with Bi2O3 for x = 0.0025 and 0.005 show higher critical current densities, J(c), at high fields and at 20 K than for the pristine sample. Their irreversibility field, H-irr, is also higher. On the other hand, sample added with Bi-metal has lower J(c) and H-irr at any temperature than for the pristine sample.

Lead-free piezoelectric (Bi0.5Na0.5)(0.92)Ba0.08TiO3, (abbreviated as BNT-BT0.08) wires were prepared using its corresponding precursor sol and, a polycarbonate template membrane. The polycarbonate membrane used has a thickness of 30 mu m and pore diameter of 300 nm. Uniform surface morphology of the wires with average diameter of 145 nm and a length of about 20 mu m was obtained when the concentration of the sol was 0.3 M. The ferroelectric and piezoelectric properties of an individual BNT-BT0.08 wire were investigated using the PFM technique. The hysteresis loops between the PFM phase and DC bias and, the characteristic butterfly loops of the PFM amplitude versus DC bias, indicated a ferroelectric and piezoelectric behavior of as-obtained BNT-BT0.08 wires.

The aim of this study was to obtain a novel hydroxyapatite-based material with high biocompatibility. The structural properties of the samples were well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS). The X-ray diffraction studies revealed the characteristic peaks of hydroxyapatite in each sample. Other phases or impurities were not observed. The scanning electron microscopy observations suggest that the doping components have no influence on the surface morphology of the samples, which reveals a homogeneous aspect of the synthesized particles for all samples. The presence of calcium (Ca), phosphor (P), oxygen (O) and silver (Ag) in the Ag:HAp is confirmed by energy dispersive X-ray (EDAX) and X-ray Photoelectron Spectroscopy analyses. Nanocrystalline silver doped HAp stimulated viability and potentiated the activation of murine macrophages. (C) 2012 Elsevier B.V. All rights reserved.

Over the last decade, field emission miniature x-ray tubes emerged as cutting-edge applications of nanotechnology, possessing massive potential for use in various important fields, including that of precision medical therapy. The article essentially presents a review of such new devices reported in the literature. Additional discussions on the necessity of stabilizing the electron beam that generates x-rays are also included, and a simple technique for minimizing the current fluctuations is described. It is also pointed out that further miniaturization of field emission x-ray sources may need new concepts in designing the tube in shapes acting as "self focusing" structures for the electron beams.

We report on the synthesis of novel ovine and bovine derived hydroxyapatite thin films on titanium substrates by pulsed laser deposition for a new generation of implants. The calcination treatment applied to produce the hydroxyapatite powders from ovine/bovine bones was intended to induce crystallization and to prohibit the transmission of diseases. The deposited films were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy. Pull-off adherence and profilometry measurements were also carried out. X-ray diffraction ascertained the polycrystalline hydroxyapatite nature of the powders and films. Fourier transform infrared spectroscopy evidenced the vibrational bands characteristic to a hydroxyapatite material slightly carbonated. The micrographs of the films showed a uniform distribution of spheroidal particulates with a mean diameter of similar to 2 mu m. Pull-off measurements demonstrated excellent bonding strength values between the hydroxyapatite films and the titanium substrates. Because of their physical-chemical properties and low cost fabrication from renewable resources, we think that these new coating materials could be considered as a prospective competitor to synthetic hydroxyapatite used for implantology applications. (C) 2012 Elsevier B.V. All rights reserved.

Polarized Raman scattering studies on stiff layered structure of surfactant intercalated with trilayer graphene were performed at different intensities and excitation wavelengths. The D and 2D Raman bands reach the highest and lowest intensity when the polarization of laser excitation light is oriented along and perpendicular on the edges. The 2D band discloses two lorentzian components, separated by similar to 40 cm(-1), which result from the action of interplanar forces, of Casimir nature. The value of similar to 40 cm(-1) is close to the energy value associated with E-2g interplanar layer shear mode evidenced so far only by neutron spectrometry. A new result regards the opposite variation of the intensities of D and 2D bands with the increase of the wavelength of the excitation light. This originates in the different origin of the D and 2D bands; the former is dependent on disorder including also the graphene edges while the latter, results from in a double resonant mechanism combined with a Casimir effect. One demonstrates that the magnitude of Casimir force, which activates interlayer vibration modes, depends on the carrier density on the graphene sheets which can be varied both by the intensity and the wavelength of the excitation laser light. (C) 2012 Elsevier Ltd. All rights reserved.

The high intensity and high repetition rate of the XFEL, the European X-ray Free-Electron Laser presently under construction in Hamburg, results in X-ray doses of up to 1 GGy in silicon sensors for 3 years of operation. Within the AGIPD Collaboration the Hamburg group has systematically studied X-ray-radiation damage using test structures and segmented sensors fabricated on high-ohmic n-type silicon. MOS Capacitors and Gate-Controlled Diodes from 4 vendors with different crystal orientations and different technological parameters, as well as strip sensors have been irradiated in the dose range between 10 kGy and 1 GGy. Current-Voltage, Capacitance/Conductance-Voltage and Thermal Dielectric Relaxation Current measurements were used to extract oxide-charge densities, interface-trap densities and surface-current densities as function of dose and annealing conditions. The results have been implemented into TCAD simulations, and the radiation performance of strip sensors and guard-ring structures simulated and compared to experimental results. Finally, with the help of detailed TCAD simulations, the layout and technological parameters of the AGIPD pixel sensor have been optimized. It is found that the optimization for sensors exposed to high X-ray doses is significantly different than for non-irradiated sensors, and that the specifications of the AGIPD sensor can be met. In 2012 sensors have been ordered, the first batch has been delivered recently, and first results on a comparison between simulations and measurements will be presented.