Publications

A hysteresis loop with three polarization states is obtained in the case of a symmetric epitaxial ferroelectric-interlayer-ferroelectric structure with bottom and top SrRuO3 electrodes. The ferroelectric layers are of PbZr0.2Ti0.8O3, while the interlayer is CoFe2O4. It is shown that the three polarization states can be separately accessed, suggesting that this type of structure can be used as building element for a three-state nonvolatile ferroelectric random-access memory (FERAM). The presence of the three-state memory effect is explained through a simple phenomenological model based on Landau-Ginzburg-Devonshire theory. The findings of this study can pave the way to multistate all-oxide FERAM devices, resulting in a 50% increase in the storage density compared to actual nonvolatile memories.

The positions of the low energy electron diffraction (LEED) spots from ferroelectric single crystal films depend on its polarization state, due to electric fields generated outside of the sample. Onemay derive the surface potential energy, yielding the depth where the mobile charge carriers compensating the depolarization field are located (delta). On ferroelectric Pb(Zr, Ti)O-3(001) samples, surface potential energies are between 6.7 and 10.6 eV, and d values are unusually low, in the range of 1.8 +/- 0.4 angstrom. When delta is introduced in the values of the band bending inside the ferroelectric, a considerably lower value of the dielectric constant and/or of the polarization near the surface than their bulk values is obtained, evidencing either that the intrinsic 'dielectric constant' of the material has this lower value or the existence of a 'dead layer' at the free surface of clean ferroelectric films. The inwards polarization of these films is explained in the framework of the present considerations by the formation of an electron sheet on the surface. Possible explanations are suggested for discrepancies between the values found for surface potential energies from LEED experiments and those derived from the transition between mirror electron microscopy and low energy electron microscopy.

Dielectric measurements of bismuth germanate oxides reveal significant changes in the amorphous phase, between 200 and 350 degrees C, where the crystallisation processes start below the glass transition temperature. The capacitive and loss measurements versus temperature and frequency suggest an incipient glass transition below 350 degrees C associated with an increase in the clusters, mainly those formed by GeO4 tetrahedra, responsible for the dipolar orientation effects. An increase in the capacitive parameter versus temperature at lower frequencies, especially over 250 degrees C has been associated with the increase in mobility of the clusters. The physical meaning of those processes has been associated with the formation of a highly viscous layer enriched in GeO4 which is formed during crystallisation. This layer acts as a diffusion barrier and hinders further crystal growth. A higher frequency is required in the crystallisation processes to compensate for the thermal disorder in the amorphous materials. The crystallisation process is identified by the decrease in the dielectric constant despite the increase in temperature.

Cobalt ferrite (CF) powder was synthesized by solid state reaction method at two different calcination temperatures (1120 K and 1320 K) then milled in two steps, gradually reducing the milling media size. The first milling step results in CF nanoparticles with crystallite size of 33 nm showing fairly high coercivity (3.7 kOe), > 5 times higher than the non-milled material (0.7 kOe). The high coercivity was correlated to the crystallite size close to the single-domain limit, and to the strain increase up to 2.1%. This value of strain is the highest ever reported in literature for the CF and brings to the highest figure of merit for permanent magnets, (BH)(max)= 2.16 MGOe. After the second milling step the powder displays particle size of 9 nm, release of strain (epsilon = 1.2%), coercivity reduction that approaches 250 Oe and decrease of the deblocking temperature from 421 K to 317 K. The large tunability obtained by multi-step milling allows to use CF in different applications. In particular, the milled CF powder characterized by high microstrain is a good candidate for the realization of rare-earth-free permanent magnets (at least on the basis of the (BH)(max) product). For the first time, a correlation between the spin-canting angle and the degree of inversion, the crystallite size and the microstrain is presented and discussed. [GRAPHICS] .

This contribution concerns the catalytic activity of functional layered double hydroxides (LDHs) for 1,4-addition of n-octanol to 2-propenonitrile aiming to investigate the influence of the organic interlayer anion nature on their physico-chemical and catalytic performances. Two series of functional LDHs, e.g. Mg2.5Al, and Zn2.5Al respectively, were synthesized by co-precipitation at pH 9.5 using a metal nitrates M2+ (NO3)(2)center dot 6H(2)O (M2+ = Mg, Zn), Al(NO3)(3)center dot 9H(2)O solution, an organic acid sodium salt (e.g. sodium dodecyl sulfate (NaDS), sodium laurate (NaL), or sodium stearate (NaS)) and NaOH solution for pH adjustment. XRD, DRIFTS and DR-UV-Vis-NIR characterizations indicated a better intercalation of DS in the interlayer region compared to L or S. The catalytic activity of these solids was related to both their organophilic character and basicity, Mg2.5Al-DS samples, which were more basic, showed a higher activity than Zn2.5Al-DS.

The crystallization mechanism of sol-gel-derived NaYF4:(Yb, Er) up-converting phosphors has been studied by differential scanning calorimetry analysis using both model-free and model fitting approaches. Structural and optical data have shown that the hexagonal NaYF4:(Yb, Er) phase crystallization process occurs at around 315 degrees C as a result of the thermal decomposition of the metal trifluoroacetates. As the annealing temperature increases, sphere-like microcrystals of about 1-2 mu m size (at 300 degrees C) break up into smaller ones (400-500 nm size) and finally collapse at higher temperatures (600 degrees C); the up-conversion luminescence signal intensity increases due to the crystallinity improvement and dehydration process. The crystallization process can be described as an autocatalytic-type reaction where the accompanying cubic NaYF4 phase played a catalytic role by reducing the energy barrier against the crystallization of the hexagonal NaYF4 phase, causing its fast self-accelerated crystallization. The energy resulting from the disintegration process of the initial NaYF4 microcrystals contributed to the growth and agglomeration processes and finally the collapse of the crystalline fragments with increasing temperature.

The exciton-phonon interaction, considered as a stimulated Raman scattering process, is studied in different semiconductor mixtures: PbI2/TiO2, PbI2/Si and CdS/Si. Raman spectra recorded at excitation wavelengths of 514.5 and 488 nm for PbI2 and CdS, respectively, reveal a strong enhancement of the Raman lines peaked at 97 and 305 cm(-1), evaluated by the ratio I-TK/I-300 (K) between the relative intensities of the spectra recorded in the temperature range of 88-300 K. It is found that PbI2 and CdS exhibit a decrease in the Raman intensity modes with decreasing temperature, while in TiO2 and Si an increase in the Raman lines intensities peaked at 138 and 520 cm(-1) is observed. This behavior can be explained by an energy transfer process from PbI2 or CdS towards TiO2 and Si. This explanation is supported by the schematic potential energy levels diagram obtained from the density of states, which is calculated using the density functional theory. According to this energy levels diagram, the electrons are expected to migrate directly from the conduction band (CB) energetic levels of the PbI2 and CdS towards the CB levels of TiO2 and Si.

The synthesis of Cu-0 nanoparticles on different supports and their activity in controlled living radical polymerization processes is presented. The type of support influences the final size of the copper nanoparticles as well as their adhesion to the support. These aspects have a direct influence on the characteristics of the polymers obtained. The best results were obtained for SiO2 particles, which afforded a good molecular weight distribution (Mw/Mn = 1.25). The activity, recovery and recycling of the catalyst was explored for ultrafast polymerization reaction of butyl acrylate. Further, the terminal bromine reactivity was used for the synthesis of a block poly(n butyl acrylate-block-styrene). The influence of ligand type on the control of the reaction was studied. Also, a straightforward polymerization procedure without any ligand afforded a polydispersity value of 1.38.

We propose a novel idea to improve the surface properties of carbon-based Pt-free electrocatalysts in Polymer Electrolyte Membranes (PEMs) and Alkaline Fuel Cells (AFCs). Our concept is based on the addition of oxygenophilic and hydrophobic ionic liquids (ILs) to form a thin passivating layer at the triple point between the electrocatalyst-electrolyte-gas interface where the Oxygen Reduction Reaction (ORR) takes place.

Graphene is widely used in nanotechnologies to amplify the photocatalytic activity of TiO2, but the development of TiO2/graphene composites imposes the assessment of their risk to human and environmental health. Therefore, reduced graphene oxide was decorated with two types of TiO2 particles co-doped with 1% iron and nitrogen, one of them being obtained by a simultaneous precipitation of Ti3+ and Fe3+ ions to achieve their uniform distribution, and the other one after a sequential precipitation of these two cations for a higher concentration of iron on the surface. Physico-chemical characterization, photocatalytic efficiency evaluation, antimicrobial analysis and biocompatibility assessment were performed for these TiO2-based composites. The best photocatalytic efficiency was found for the sample with iron atoms localized at the sample surface. A very good anti-inhibitory activity was obtained for both samples against biofilms of Gram-positive and Gram-negative strains. Exposure of human skin and lung fibroblasts to photocatalysts did not significantly affect cell viability, but analysis of oxidative stress showed increased levels of carbonyl groups and advanced oxidation protein products for both cell lines after 48 h of incubation. Our findings are of major importance by providing useful knowledge for future photocatalytic self-cleaning and biomedical applications of graphene-based materials.