Marco Castriota

Temperature dependence of lithium ion solvation in ethylene carbonate–LiClO4 solutions

The solvation mechanism of lithium ions in pure ethylene carbonate (EC) solutions has been studied in a wide concentration range by different techniques and for temperatures up to 100 °C. For low concentrations (R=[Li+]/[EC]⩽0.1) the solvation number of Li+ cations in the solution has been found to be ∼7, by using nuclear magnetic resonance techniques. This number decreases at higher concentration, and complexes Li+-EC with ∼3 solvent molecules per cation are formed when R=0.33. The temperature dependence of the solvation has been investigated for the more concentrated solutions by Raman spectroscopy. When the temperature increases, the relative intensity of the Raman bands upshifted by the lithium interaction also increases remarkably, indicating that Li+ ions form complexes with a higher solvation number. On the other hand, a higher degree of reassociation of Li+ cations with perchlorate anions is observed.

Temperature evolution of thermoreversible polymer gel electrolytes LiClO4/ethylene carbonate/poly(acrylonitrile)

Thermoreversible polymer gel electrolytes with ionic conductivities of about 10−3 S cm−1 at room temperature, were prepared from poly(acrylonitrile) (PAN)-ethylene carbonate (EC)-LiClO4 at different solvent/salt ratios. A temperature dependence was investigated for the electrical and mechanical properties, as well as for spectroscopic changes associate to the solvation mechanism of lithium ions in these gel. The measurements performed on this system concern ionic conductivity, study of self-diffusion coefficient by 1H–NMR, Raman spectroscopy and, finally, an accurate analysis of its mechanical proprieties by rheological tests. The gels present a strong-weak gel transition at about 70 °C, independent from salt concentration hence assignable only to the polymeric matrix. However, this transition is completely reversible on cooling, being this a necessary mechanical property for industrial applications. The Raman study has pointed out an increase, on heating, of the Li+ coordination number by EC molecules, a...

Characterization of rhenium oxide films and their application to liquid crystal cells

Rhenium trioxide exhibits high electronic conductivity, while its open cubic crystal structure allows an appreciable hydrogen intercalation, generating disordered solid phases, with protonic conductivity. Rhenium oxide thin films have been obtained by thermal evaporation of ReO3 powders on different substrates, maintained at different temperatures, and also by reactive magnetron sputtering of a Re metallic target. A comparative investigation has been carried out on these films, by using micro-Raman spectroscopy and x-ray diffraction. Two basic types of solid phases appear to grow in the films: a red metallic HxReO3 compound, with distorted perovskite structures, like in the bulk material, and ordered HReO4 crystals based on tetrahedral perrhenate ions. Because of its conduction properties, the electrical and electro-optical behaviors of ReO3 films deposited on standard indium tin oxide/glass substrate have been tested inside asymmetric nematic liquid crystal cells, showing an appreciable capability of rectification of their electro-optical response, in similar way to tungsten trioxide.Rhenium trioxide exhibits high electronic conductivity, while its open cubic crystal structure allows an appreciable hydrogen intercalation, generating disordered solid phases, with protonic conductivity. Rhenium oxide thin films have been obtained by thermal evaporation of ReO3 powders on different substrates, maintained at different temperatures, and also by reactive magnetron sputtering of a Re metallic target. A comparative investigation has been carried out on these films, by using micro-Raman spectroscopy and x-ray diffraction. Two basic types of solid phases appear to grow in the films: a red metallic HxReO3 compound, with distorted perovskite structures, like in the bulk material, and ordered HReO4 crystals based on tetrahedral perrhenate ions. Because of its conduction properties, the electrical and electro-optical behaviors of ReO3 films deposited on standard indium tin oxide/glass substrate have been tested inside asymmetric nematic liquid crystal cells, showing an appreciable capability of rec...

Changes of the electro-optic response of nematic liquid crystal cells due to inserted titania-vanadia films

We report a polarity-sensitive electro-optical response observed in nematic liquid crystal cells characterized by asymmetric insertion of thin films of titania-vanadia oxide with a Ti∕V atomic ratio of 1∕1, prepared by sol-gel synthesis on transparent indium tin oxide. The electro-optical effect is opposite to previously reported ones but is obtained using the same materials with a slightly different sol-gel route. The structural properties of titania-vanadia films are extensively studied with particular attention paid to the thermal history of the films. The measurements made of the films and on the nematic liquid crystal cell demonstrate that the crystalline structure of the film is responsible of the observed effect. Thermal annealing of the films results in formation of structures that dramatically change the electric properties of the films. A qualitative model is proposed to explain the surface charge distribution inside the nematic liquid crystal cell that results in the observed electro-optical ef...

Asymmetric nematic liquid crystal cells containing lead zirconium titanate (PZT) films

Interest in thin films of PZT (lead zirconium titanate PbZr0.47Ti0.53O3) has increased because of their potential applications. For instance, the insertion of PZT in asymmetric nematic liquid crystal cells induces a rectified electro-optical response similar to that observed in such cells because of the insertion of metal oxide layers with ionic conductivity. In the present work several PZT films have been investigated, obtained by different thermal treatments after sol-gel synthesis and spin coating deposition. The observed rectifying effect, due to the insertion of PZT films in asymmetric liquid crystal cells, has been attributed to the presence of an internal field made from the reorientation of nanodomains of PZT. The presence of such internal fields is demonstrated and an estimation of it is given. Moreover, asymmetric nematic liquid crystal cells made with PZT films show considerable improvements in both contrast and response time.

In situ polarized micro-Raman investigation of periodic structures realized in liquid-crystalline composite materials

In situ polarized micro-Raman Spectroscopy has been utilized to determine the liquid crystal configuration inside a periodic liquid crystalline composite structure made of polymer slices alternated to films of liquid crystal. Liquid crystal, Norland Optical Adhesive (NOA-61) monomer and its polymerized form have been investigated separately. The main Raman features, used as markers for the molecular orientation estimation, have been identified. In situ polarized Raman spectra indicate that the orientation of the liquid crystal director inside the structure is perpendicular to its polymeric slices. Results show the usefulness of in situ polarized micro-Raman spectroscopy to investigate liquid crystalline composite structures.

Morphological and electrical investigations of lead zirconium titanate thin films obtained by sol-gel synthesis on indium tin oxide electrodes

In recent years, interest in PbZr0.53Ti0.47O3 (PZT) films has been rapidly increasing due to their technological applications as ferroelectric materials. In the present work, PZT films are obtained by sol-gel synthesis and deposited by spin coating on a transparent conductor substrate, with a perspective application as rectifying layers in asymmetric nematic liquid crystal cells. An extensive investigation is carried out on the effects of different annealing temperatures, with regard to the film texture and to the electric polarization properties, by using low vacuum scanning electron microscopy, atomic force microscopy, and electrostatic force microscopy. It has been observed that PZT domains self-organize into flower-like dendritic structures with a “rosetta” shape at the early stage of crystallization, occurring for annealing temperatures higher than 600 °C; the dimensions of such structures increase versus the annealing temperature. The ferroelectric properties of the PZT films have been related to th...

Characterization of Tungsten Trioxide Thin Film Deposited by Spin Coating and the Effect on Their Insertion in Liquid Crystal Cells

ABSTRACT The asymmetric insertion of oxide layers having mixed conduction properties (ionic and electronic) in liquid crystal cells induce various kinds of electro-optical response of the liquid crystals, and this behaviour has been related to the structural differences of the inserted films. In this work, is reported a structural study of such oxide films deposited on indium tin oxide (ITO) covered glasses. To have further confirmations of the model and a better understanding of the basic mechanism underlying the rectification effect and the connections with the structural and electrical properties of the films, WO3 layers have been studied before and after thermal. Moreover the gelification via spin-coating, has been implemented and the optimization test of various relevant parameters have been performed. The chemical, structural and optical evolution has been extensively investigated as a function of the thermal annealing treatment, by performing vibrational spectroscopy analysis (micro-Raman and IR) impedance spectroscopy characterization and spectroscopic ellipsometry, before testing the films into the NLC cells.

Sputtering deposition and characterization of Ru-doped WO3 thin films for electrochromic applications

Mixed tungsten-ruthenium oxide thin films were prepared for the first time by dc magnetron co-sputtering technique and were studied by cyclic voltammetry, optical transmission measurements, Raman spectroscopy and the W L3 and Ru K edges X-ray absorption spectroscopy (XAS) in comparison with pure WO3 films. The Ru concentration was varied in the range from 0 to 28 at.%. XAS results suggest that the average local structure around both tungsten and ruthenium ions remains unchanged within experimental accuracy in all samples, moreover, for tungsten ions, it resembles that of pure WO3 films. However, the presence of the ruthenium ions affects the electrochemical and optical properties of the films. Our results suggest that mixed films are formed by tungsten trioxide grains surrounded by ruthenium oxide phase.

Asymmetric Response to Electric Field in Nematic Liquid Crystal Cells Containing Vanadium Oxide Thin Films Prepared by Sol-Gel Synthesis

ABSTRACT The asymmetric insertion of oxide layers having mixed conduction properties (ionic and electronic) in liquid crystal cells induces various kinds of electro-optical response of the liquid crystals, and this behaviour has been related to the structural differences of the inserted films. In this work, it is reported a structural study of such oxide films deposited on indium tin oxide (ITO) covered glasses. To have further confirmations of the model and a better understanding of the basic mechanism underlying the rectification effect and the connections with the structural and electrical properties of the films, Vanadium oxide layers have been studied before and after thermal treatment. Moreover the gelification via spin coating has been implemented and the optimisation tests of various relevant parameters have been performed. The chemical, structural and optical evolution has been extensively investigated as a function of the thermal annealing treatment, by performing vibrational spectroscopy analysis (micro-Raman and IR) and impedance spectroscopy characterization, before testing the films into the NLC cells.