National Institute Of Materials Physics - Romania

Electrode interface is a key element in controlling the macroscopic electrical properties of the ferroelectric capacitors based on thin films. In the case of epitaxial ferroelectrics, the electrode interface is essential in controlling the leakage current and the polarization switching, two important elements in the read/write processes of nonvolatile memories. However, the relation between the polarization bound charges and the electronic properties of the electrode interfaces is not yet well understood. Here we show that polarization charges are controlling the height of the potential barriers at the electrode interfaces in the case of Pb(Zr,Ti)O-3 and BaTiO3 epitaxial films. The results suggest that the height is set to a value allowing rapid compensation of the depolarization field during the polarization switching, being almost independent of the metals used for electrodes. This general behavior open a new perspective in engineering interface properties and designing new devices based on epitaxial ferroelectrics.

Perovskite complex ceramic oxides, BaTiO3 doped with 0.5 mol% Nb2O5 and then nanocoated with SiO2 (abbreviated as BT-Nb-0.005/SiO2) was successful prepared using conventional sol-gel processing. Phase composition, particle morphology, structure, and electric properties of BT-Nb-0.005 core and BT-Nb-0.005/SiO2 core-shell were examined and compared, using X-ray diffraction, transmission electron microscopy and, dielectric and ferroelectric measurements. Core-shell composite with well-defined perovskite tetragonal phase of BaTiO3 was achieved. Furthermore, single crystalline BT-Nb-0.005/SiO2 core-nanoshell heterostructure with similar to 34 nm shell thick was prepared, which is a novelty in ferroelectrics field. The ferroelectric quality of BT-Nb-0.005 has suffered an alteration when the (BT-Nb-0.005)/SiO2 core-shell heterostructure was realized. One-dimensional BT-Nb-0.005/SiO2 core-shell heterostructure exhibits an improvement of dielectric losses and a decrease of dielectric constant, compared to uncoated BT-Nb-0.005. The (BT-Nb-0.005)/SiO2 core-shell material could be interesting for application in the composite capacitors. (C) 2013 Elsevier B.V. All rights reserved.

A novel series of Pd(II) and Pt(II) complexes based on cyclometallated imine ligands and N-benzoylthiourea (BTU) derivatives as auxiliary ligands has been prepared and their liquid crystalline properties as well as photophysical properties have been investigated. The crystal structure of one cyclometallated Pt(II) complex with N-(p-F-phenyl)-N'-benzoylthiourea as a co-ligand has been solved. The liquid crystalline properties have been investigated by a combination of DSC, POM and variable temperature powder X-ray diffraction. These new metallomesogens display either a monotropic SmC phase or both SmA and SmC phases, depending on the number of alkoxy groups attached to the imine ligand, alkyl chain length or the use of branched alkoxy terminal groups. We found that the introduction of branched alkoxy terminal groups lead to lower transition temperatures and stabilization of the SmC phase in both the Pd(II) and Pt(II) complexes. While the Pd(II) complexes display no emission, the Pt(II) complexes show good emission properties in solution, in the solid state and as a PMMA film at room temperature, and their investigation is reported. (C) 2013 Elsevier Ltd. All rights reserved.

We report the deposition by Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique of biomimetic nanocrystalline apatite coatings on titanium substrates, with potential application in tissue engineering. The targets were prepared from metastable, nanometric, poorly crystalline apatite powders, analogous to mineral bone, synthesized through a biomimetic approach by double decomposition process. For the deposition of thin films, a KrF* excimer laser source was used (lambda = 248 nm, tau(FWHM) <= 25 ns). The analyses revealed the existence, in synthesized powders, of labile non-apatitic mineral ions, associated with the formation of a hydrated layer at the surface of the nanocrystals. The thin film analyses showed that the structural and chemical nature of the nanocrystalline apatite was prevalently preserved. The perpetuation of the non-apatitic environments was also observed. The study indicated that MAPLE is a suitable technique for the congruent transfer of a delicate material, such as the biomimetic hydrated nanohydroxyapatite. (C) 2013 Elsevier B.V. All rights reserved.

In order to study specific phenomena at ferromagnetic/semiconducting interfaces, of potentially high interest in spintronics and information technology, structural aspects and magnetic properties of Fe thin films grown on Si(001) substrates by RF sputtering have been investigated using Fe-57 conversion electron Mossbauer spectroscopy (CEMS) and magneto-optic Keer effect (MOKE). Films of different thicknesses have been deposited either directly on crystalline Si substrates or on Cu buffer layers. An inherent Fe oxide layer is observed in all as prepared films, with a relative thickness decreasing drastically with the deposition time. The Cu buffer layer does not diminish either the interfacial diffusion or the oxidation process. An efficient method to prepare sharper oxygen- and silicon-free interfaces for an improved spin injection, via thermal treatment in hydrogen atmosphere, is proposed. Accordingly, the hydrogenation treatments are very efficient in the modification of the ferromagnetic film structure, phase composition, magnetic properties and interfacial mixing. (C) 2013 Elsevier B.V. All rights reserved.

Lead-free piezoelectric (Bi0.5Na0.5)(0.92)Ba0.08TiO3, doped with 5 mol %BiFeO3 (BNT-BT-BFO) nanowires was prepared from its corresponding precursor sol. A polycarbonate membrane with the thickness of 30 mu m and the pore diameter of 100 nm was used as template. Nanowires with average diameter of similar to 80 nm and different lengths of a few microns were prepared from BNT-BT-BFO precursor sol with similar to 0.03 M concentration. Ferroelectric and piezoelectric characterizations of BNT-BT-BFO nanowires were performed using AFM-PFM equipment. The HR-TEM micrograph of nanocrystals, SAED patterns, and XRD pattern indicated that BNT-BT-BFO nanowires crystallized on the lattice of rhombohedral (Na0.5Bi0.5TiO3) phase. The BNT-BT-BFO nanowires showed a significant piezoelectric response, suggesting a good piezoelectric behavior. The saturation magnetization of the nanowires, measured at room temperature and at 5 kOe magnetic field, was Ms = 0.061 emu/g. A saturated ferromagnetic hysteresis loop has been also obtained.

A new method to fabricate an Fe-PZT core-shell wire arrays developed in three steps is reported in this paper. This involves the electrochemical growth of an iron wire array by template method, deposition by spin coating of the PZT precursor prepared by sol-gel technique on the iron wires surface and annealing treatment to obtain Fe-PZT core-shell structures. The structure of the Fe-PZT core-shell wire array was characterized by scanning electron microscopy (SEM) and Raman scattering. Raman lines situated at 372 and 575cm(-1) indicate the formation of the FeO particles on the Fe wires surface as a result of the use of an aqueous solution for electrochemical synthesis. An upshift of Raman lines of Fe decorated with FeO wires was observed after formation of Fe-PZT core-shell structure. The annealing treatment of this structure involves a partial transformation of the FeO into Fe2O3 particles both with structure rhombohedral and cubic that were evidenced by Raman lines peaked at 243 and 497cm(-1), respectively. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Investigations on compact dielectric resonator antennas arc presented. Low-loss, high dielectric constant zirconium tin titanate (ZST) material was prepared by using solid state reaction method. The ZST resonators exhibit 36 dielectric relative permittivity, dielectric loss tangent 5x10(-4) and resonant frequency temperature coefficient between -4 to +4 ppm/degrees C. The developed resonators are very attractive for probe-fed resonator antenna for 2.66 GHz.

X-ray photoelectron spectroscopy (XPS) analyses of lead zirco-titanate Pb(Zr,Ti)O-3(001) single crystal thin layers as a function of the time spent between sample preparation by pulsed laser deposition and introduction in to a ultrahigh vacuum revealed the fact that freshly prepared samples showed a shift of the C 1s towards a higher binding energy, together with shifts of core levels originating from the substrate (particularly Ti 2p and O 1s) towards a lower binding energy. This behaviour is explained by considering that the molecules of contaminants (fatty acids, alcohols, esters) are adsorbed preferentially on areas exhibiting outwards polarization P(+). Thus, photoelectrons originating from contaminants will have larger binding energies because of the charge state of the P(+) areas, whereas the substrate XPS signals from these P(+) areas are attenuated by the contaminants, with the consequence of a prevalence of XPS substrate signals originating from the P(-) areas, shifted towards lower binding energies. Piezoresponse force microscopy confirmed the assumptions derived from XPS results and suggests the existence of an interplay between the adsorption of contaminants and the surface polarization state.

In this letter, we study the higher-order modes excited in a high-permittivity cylindrical dielectric resonator antenna (DRA), situated asymmetrically on top of a rectangular slot in the ground plane of a microstrip line. Besides the modes excited by a symmetric positioning of the DRA above the feeding slot, we identified five new modes: TEme(010), TEme(011), TEme(210), TEme(020), and TM212 respectively. The higher-order modes offer to the DRA antenna some interesting features: They have different radiation patterns for different operating modes; modes with close resonance frequency give wideband capabilities; and sometimes, when a mode has poor performance in one polarization, it is possible that another mode with close frequency can compensate it.