Publications

The strong correlation between morphology and charge storage properties of HfO2/Ge/HfO2/Si trilayer structures was evidenced. The morphology of structures deposited by magnetron sputtering and electron beam evaporation was tailored by rapid thermal annealing and investigated by transmission electron microscopy, Raman and X-ray photoelectron spectroscopies. The best hysteresis loops (capacitance-voltage characteristics) were obtained for trilayers with high density Ge nanocrystals located in the position of as-deposited Ge layer. The decrease of Ge nanocrystals density narrows the memory window, by spreading Ge atoms into HfO2 matrix (sputtered trilayers), or by Ge atoms expulsion toward HfO2 nanocrystals surface (evaporated trilayers). (C) 2015 Elsevier Ltd. All rights reserved.

FePt-based alloys are currently under scrutiny for their possible use as materials for perpendicular magnetic recording. Another possible application is in the field of permanent magnets without rare-earths, magnets that may operate at higher temperatures than the classic Nd-Fe-B magnets. Within this study, FeCoPt alloys prepared by rapid solidification from the melt are structurally and magnetically characterized. In the as-cast FeCoPt ribbons, a three-phase structure comprising well-ordered CoFePt and CoPt L1(0) phases embedded in a disordered fcc FePt matrix was evidenced by XRD, HREM and SAED. Extended transmission electron microscopy analysis demonstrates the incipient formation of ordered L1(0) phases. X-ray diffraction was used to characterize the phase structure and to obtain the structural parameters of interest for L1(0) ordering. In the as-cast state, the co-existence of hard magnetic CoFePt and CoPt L1(0) tetragonal phases with the soft fcc FePt phase is obtained within a refined microstructure made of alternatively disposed grains (grain sizes from 1 to 7 nm). Following a thermal treatment of 1 h at 670 degrees C, the soft magnetic fcc matrix phase transforms to tetragonal L1(0) phases (disorder-order transition). The resulting CoPt and CoFePt L1(0) phases have grains of around 5-20 nm in size. In the as-cast state, magnetic measurements show a quite large remanence (0.75 T), close to the value of the parent L1(0) FePt phase. Coercive fields of about 200 kA/m at 5 K were obtained, comparable with those reported for some FePt-based bulk alloys. Upon annealing both remanence and coercivity are increased and values of up to 254 kA/m at 300 K are obtained. The polycrystalline structure of the annealed FeCoPt samples, as well as the formation of multiple c-axis domains in different CoPt and CoFePt regions (which leads to a reduction of the magneto-crystalline anisotropy) may account for the observed coercive fields that are lower than in the case of very thin FeCoPt films. A Curie temperature of about 820 K (close to 550 degrees C) is reported for the Fe35Co15Pt50 alloy which opens wide possibilities for the use of such magnets in high operating temperature industrial applications. The present results indicate that ternary FeCoPt alloys hold a great potential as a novel class of rare earth free exchange-spring coupled nanocomposite magnets. (C) 2015 Elsevier B.V. All rights reserved.

The possibility to intercalate noble gas atoms below epitaxial graphene monolayers coupled with the instability at high temperature of graphene on the surface of certain metals has been exploited to produce Ar-filled graphene nanosized blisters evenly distributed on the bare Ni(111) surface. We have followed in real time the self-assembling of the nanoblisters during the thermal annealing of the Gr/Ni(111) interface loaded with Ar and characterized their morphology and structure at the atomic scale. The nanoblisters contain Ar aggregates compressed at high pressure arranged below the graphene monolayer skin that is decoupled from the Ni substrate and sealed only at the periphery through stable C-Ni bonds. Their in-plane truncated triangular shapes are driven by the crystallographic directions of the Ni surface. The nonuniform strain revealed along the blister profile is explained by the inhomogeneous expansion of the flexible graphene lattice that adjusts to envelop the Ar atom stacks.

The Peierls transition of polycrystalline CuGeO3 samples was investigated by thermal and magnetic investigations. We find that spin disorder induced by chain breaking and grain boundaries has similar effects to doping but without suppressing the spin-Peierls transition temperature or the spin-Peierls gap.

Aluminum doped zinc oxide (AZO) thin films were obtained by pulsed laser deposition on (001) SrTiO3 (STO) on a range of substrate temperatures during ablation between 300 degrees C and 600 degrees C. A hexagonal system lying on a cubic one should be difficult to be obtained in epitaxial form. The geometrical selection of the AZO growth on (001) STO is not giving a unique preferential orientation. Two orientations, c-axis (along [001]) and 110, have been observed experimentally with different ratios at different substrate temperature. Discussions are made with respect to the temperature dependence of lattice mismatch between the two cases and the cubic surface of the substrate, and to the substrate surface morphology and terminating atomic layer composition. The 110 AZO is the main phase at deposition temperature of 550 degrees C, while for other substrate temperatures the 001 is the preferential orientation. The conductive character of 110 AZO thin film have been inferred from both ellipsometry spectra and current-voltage measurements. Excepting the samples deposited at 300 degrees C, the lowest resistivity is recorded for the samples with 110 AZO as the main phase. (C) 2015 Elsevier B.V. All rights reserved.

Aluminophosphate glasses from the Li2O-BaO-Al2O3-P2O5 system with the addition of nonmagnetic and paramagnetic rare earth ions, were prepared using a wet nonconventional method to process the raw materials, followed by a melting-quenching procedure. The glasses obtained were characterized with respect to their magnetic and magneto-optical properties using superconducting quantum interference device magnetometry and spectroscopic ellipsometry. The assumption of a linear dependence of the Verdet constant on the magnetic susceptibility, with a proportionality constant dependent on the type of vitreous matrix and doping ion, is critically discussed. The diamagnetic and paramagnetic contributions to the Faraday rotation were separately analyzed and specific designs for optimal active and passive elements are proposed.

This work reports on the unprecedented magnetron sputtering deposition of submicrometric hollow cones of bioactive glass at low temperature in the absence of any template or catalyst. The influence of sputtering conditions on the formation and development of bioglass cones was studied. It was shown that larger populations of well-developed cones could be achieved by increasing the argon sputtering pressure. A mechanism describing the growth of bioglass hollow cones is presented, offering the links for process control and reproducibility of the cone features. The composition, structure, and morphology of the as-synthesized hollow cones were investigated by energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), grazing incidence geometry X-ray diffraction (GIXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)-selected area electron diffraction (SAED). The in vitro biological performance, assessed by degradation tests (ISO 10993-14) and cytocompatibility assays (ISO 10993-5) in endothelial cell cultures, was excellent. This allied with resorbability and the unique morphological features make the submicrometer hollow cones interesting candidate material devices for focal transitory permeabilization of the blood brain barrier in the treatment of carcinoma and neurodegenerative disorders.

A series of flexibly linked bis(pyridinium) salts with various counterions (Br-, PF6-, BF4- and OTf-) was designed and prepared starting from corresponding N-alkylated 4-pyridones precursors with mesogenic 3,4,5-tris(alkyloxy)benzyl moieties (alkyl=dodecyl or tetradecyl). These salts were investigated for their liquid crystalline properties by a combination of differential scanning calorimetry, polarising optical microscopy and temperature-dependent powder X-ray diffraction (XRD). Their thermal stability was checked by thermogravimetric analysis. All bis(pyridinium) salts, except the triflate salt with shorter terminal carbon chain, display an enantiotropic liquid crystalline behaviour with a hexagonal columnar (Col(h)) phase assigned on the basis of its characteristic texture and XRD studies. It was found that these luminescent bis(pyridinium) salts show weak emission in dichloromethane solutions at room temperature and a pronounced red-shifted emission in solid state. The emission properties of these bis(pyridinium) salts do not depend significantly on the nature of counterion employed.

Gallium antimonide (GaSb) is the basis of the most photovoltaic and thermophotovoltaic (TPV) systems and its innovative technological aspects based on modern ultra-high vacuum techniques are in trend for device achievement. The real surface of GaSb is modified by technological processes that can conduce to problems related to the reproducible control of its surface properties. The GaSb surface is reactive in atmosphere due to oxygen presence and exhibits a native oxide layer. The evolution of native oxides during the ion sputtering, chemical etching and thermal annealing processes for preparing the surface is presented in detailed way. Ratios of surface constituents are obtained by Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS). Moreover, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Atomic Force Microscopy (AFM) and Low-Energy Electron Diffraction (LEED) are used for characterization. The surface stoichiometry is changed using a specific etchant (e.g. citric acid) at different etching time and is analyzed by ARXPS, SEM, EDS and AFM methods. The experimental results provide useful information regarding surface native oxides characteristics on n-GaSb(100) to be taken into account for development of low resistance contacts for TPV devices based on GaSb alloy. (C) 2015 Elsevier B.V. All rights reserved.

A model for current voltage characteristics of a thin film metal-ferroelectric-metal structure is constructed by combining the electrostatics of a polarized ferroelectric film with the balanced flow of charge through its interfaces. Using a set of fitting parameters, good agreement with several sets of experimental data is obtained for different system temperatures. The influence of model parameters on the current-voltage characteristic is discussed. Best fit values of some of these parameters correlate well with ab initio calculations in the literature, supporting the idea of low dielectric permittivity of the interface transition layers in the ferroelectric.