National Institute Of Materials Physics - Romania

Ferroelectric materials in thin film form, largely used in a broad variety of high-tech applications, are characterized by performing electrical measurements on capacitor-like structures. A complete characterization should include hysteresis, capacitance, and current measurements performed at different bias voltages, frequencies and temperatures. The analysis of the experimental data should be made considering that the real test structure is metal-ferroelectric-metal, and taking care of the direct impact of the microstructure on the macroscopically measured quantities. Further on, the theoretical models developed to simulate the experimental results should be able to explain simultaneously the data obtained from different types of electrical measurements. The main type of electrical measurements performed on ferroelectric capacitors will be discussed in detail. A special attention will be given to some important problems such as: the electrode-ferroelectric interfaces; calculation of the dielectric constant; intrinsic-versus-extrinsic contributions to the value of the dielectric constant--fake hysteresis loops and the question "Is the presence of the hysteresis cycle solid evidence for the presence of ferroelectricity?"; non-conventional contributions to the polarization charge and their effect on the frequency dependence of the hysteresis loop; conduction mechanisms in different ferroelectric materials. Some "hot" topic will be also discussed, as for example the validity of the serial model in the case of ferroelectric multilayers, and the coexistence of ferroelectric and antiferroelectric behavior in some structures and multilayers. The presentation is supported by experimental material collected by the author in the last 7 years, especially during his extended stay at the Max Planck Institute from Halle, Germany. The author is very grateful to V. Stancu, I. Vrejoiu and K. Boldyreva for providing most part of the samples, and to M. Alexe, D. Hesse and U. Goesele for the useful discussions and funding during the stay in Halle. Part of the experimental work was made in the frame of the contract CEEX-44-DINAFER, financed by the Romanian Ministry of Education and Research.

Mossbauer spectroscopy studies of Fe-Co/MgO catalysts proved that a high dispersion degree of Fe may be achieved for catalysts containing Fe/Co in a ratio close to 1. During materials processing. the presence of hydrogen at high temperatures, even for short time, induced an increased particle size of the catalytic Fe. The evaluation of various iron species from their areas in the Mossbauer spectra revealed the dependence of the nanotube amount on (FeCo alloy + Fe2+) content in the catalyst, in good agreement with literature mechanisms. The Raman spectra show that carbon nanotubes synthesized from ethanol in argon flow on 4.5%Fe:4.5%Co/MgO are richer in low diameter (0.7-1.0nm) tubes than those obtained at low ethanol pressure, in agreement with the effects of hydrogen reported in literature. The close similarity with the Raman spectra of Aldrich double-walled carbon nanotubes and from literature, recommends the syntheses in argon flow as better conditions for double-walled carbon nanotube growth.

The paper discusses the possibilities for enhancement of laser emission parameters and power scaling of Nd-doped transparent ceramic materials. It is shown that by a correlated approach that accounts for the properties of the laser material, pumping conditions and details of laser design, the direct diode laser pumping of these lasers into the emitting level F-4(3/2) enables additional power scaling in the range of 50-60% compared with the traditional 800 nm pumping in F-4(5/2).(.) (C) 2008 Elsevier B.V. All rights reserved.

Confinement of 4-n-octyl-4'-cyanobiphenyl (8CB) to nanoporous molecular sieves with hexagonal structure of cylindrical pores (4.6 nm diameter) is studied. Thermogravimetric investigations have indicated that the pores are completely filled. Several surface species inside the pores and onto the external surface of the grains were demonstrated by differential thermal analysis and by in situ infrared spectroscopy. Arguments are given that bulk-like monomer and dimer species along with hydrogen bonded ones might coexist in the so-called surface layer, but their population varies drastically as function of the temperature. In addition, chemical changes of the confined liquid crystal are quite possible inside these nanopores, at temperatures lower than for the bulk. (c) 2008 Published by Elsevier B.V.

The infrared (800 nm range) radiation conversion to visible in Er3+ doped or (Nd, Yb, Tm) co-doped Sc2O3 transparent ceramics was investigated. The green upconversion (H-2(11/2) -> S-4(3/2)) -> I-4(15/2) emission in Er - 3 (at low Er content) under pulsed pumping into (I-4(15/2) -> I-4(19/2)) absorption lines shows a competition between ESA (excited state absorption) (I-4(15/2) -> I-4(19/2)) + (I-4(19/2) -> H-2(9/2)) and ETU (energy transfer upconversion) (I-4(11/2) -> I-4(15/2)) +(I-4(11/2) -> F-4(7/2)) two-step processes, function on the excitation wavelength. The upconversion transfer rates were estimated. The features of the blue (1)G(4) -> H-3(6) Tm3+ upconversion emission in (Tm, Nd, Yb):Sc2O3 ceramic, under pumping at 808.3 nm were also analyzed. At this wavelength both Nd3+ and Tm3+ present ground state absorption. The data suggest that the population of Tm3+ (1)G(4) emitting level is achieved by the energy transfer processes Nd(F-4(3/2) -> I-4(11/2)) + Tm(H-3(4) -> (1)G(4)), or the chain Nd(F-4(3/2) -> I-4(19/2)) + Yb(F-2(1/2) -> F-2(5/2)) followed by the Yb(F-2(5/2) -> F-2(7/2)) + Tm(H-3(4) -> (1)G(4)), function on Yb3+ content. (C) 2008 Elsevier B.V. All rights reserved.

We report on the fabrication of MgO nanorods and nanowires by a novel, all-self-assembled method, using Pulsed Laser Deposition. The seeds for the formation of these one-dimensional nanostructures consisted of self-assembled gold nanodots. Gold was used due to its well-known catalytic effect. Dimensions and surface density of the Au nano-dots, investigated by Atomic Force Microscopy, depend on the deposition conditions and the number of laser pulses. Morphology of the MgO nanorods and nanowires was investigated by SEM, and was found to depend strongly on the deposition conditions and on the architecture of the Au nanodots on the decorated substrates.

Artificial pinning centres produced by Gd2Ba4CuWOy (2411W) nanophase inclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO target containing 1 mol% 2411W have been investigated. For comparison, YBCO thin films have been grown under the same conditions using a pure (commercial) YBCO target. The resulting films were characterized by AC susceptibility (superconducting transition) and SQUID magnetometry (critical current density, J(c)), and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition). J(c) of the film grown from the composite target is practically the same as that measured for the reference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatures higher than 50 K, however, the presence of the nano-inclusions leads to increased J(c). The relative enhancement in J(c) increases with further increase in temperature. J(c) of the film containing nanophase inclusions is between two and three times higher than that of the reference sample at 77.3 K and 1.5 T. This can be explained by considering the dimensions of nanophase inclusions compared with the temperature-dependent coherence length. Susceptibility measurements reveal that, apart from a large and sharp transition at about 91 K, there is a secondary, smaller and broader transition between 88 and 84 K in the YBCO film containing nanophase inclusions. This can be explained by substitutions of compatible atoms between YBCO and the 2411W phase (Gd with Y), which might also create additional pinning centres of delta T-c-type.

Flux flow resistivity above critical temperature is analyzed based on the increased evidences that high temperature superconductors are intrinsically inhomogeneous, and local nonpercolating superconducting domains persist high above the critical temperature. It was found that above a certain field, the resistivity originated in the flow Of flux motion starts to decrease with increasing field, due to the predominance of the suppression of the superconducting droplets over the increase of vortex core density. (C) 2008 Elsevier B.V. All rights reserved.

We present a fast and simple method to prepare specimen,, for transmission electron microscopy Studies of oxide thin films deposited oil silicon Substrates. The method consists of scratching the film surface using I pointed diamond tip, in a special manner. Small and thin fragments are then detached front the film and its substrate. Depending oil the scratching direction, the fragments call be used for plan-view or cross-section imaging. High-resolution images call be also obtained from thin edges of the film fragments. The method is demonstrated in the Case Of HfO2 sol-gel films deposited on [100] Si wafer substrates.

On subjecting a bulk 2H-PbI2 crystal to vacuum annealing at 500 K followed by a sudden cooling at liquid nitrogen temperature stacking faults are generated that separate distinct layers of nanometric thickness in which different numbers of I-Pb-I atomic layers are bundled together. Such structures, containing two, three, four, five etc I-Pb-I atomic layers, behave as quantum wells of different widths. The signature of such a transformation is given by a shift towards higher energies of the fundamental absorption edge, which is experimentally revealed by specific anisotropies in the photoluminescence and Raman spectra. The quantum confining effect is made visible by specific variations of a wide extra-excitonic band (G) at 2.06 eV that originates in the radiative recombination of carriers (electrons and holes), trapped on the surface defects. The excitation spectrum of the G band, with p polarized exciting light, reveals a fine structure comprised of narrow bands at 2.75, 2.64, 2.59 and 2.56 eV, which are associated with the PbI2 quantum wells formed from two, three, four and five I-Pb-I atomic layers of 0.7 nm thickness. Regardless of the polarization state of the laser exciting light of 514.5 nm (2.41 eV), which is close to the band gap energy of PbI2 (2.52 eV), the Raman scattering on bulk as-grown PbI2 crystals has the character of a resonant process. For p polarized exciting light, the Raman scattering process on vacuum annealed PbI2 becomes non-resonant. This originates from the quantum well structures generated inside the crystal, whose band gap energies are higher than the energy of the exciting light.