Salvatore Marino

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.

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.

Dielectric investigations on a bent-core liquid crystal

Dielectric measurements on a bent-core liquid crystal were carried out in the frequency range from 10 mHz to 100 kHz in planar aligned cells. Four relaxation ranges were detected during heating condition: two in a low frequency range of a few hertz probably due to conductivity and interface relaxation phenomena, another between 10 and 20 Hz, and another one in a range between 10 kHz and 100 kHz in smectic as in nematic and isotropic phases. The third relaxation response is no more visible during cooling conditions. Dielectric increments, distribution parameters, and relaxation frequencies have been evaluated at different temperatures by fitting data with Havriliak-Negami (H-N) relaxation function, which is an empirical modification of the Debye relaxation model. The presence of a relaxation response between 10 and 20 Hz and the relatively great values of the permittivity could suggest the presence of a ferroelectric response due to the presence of cybotactic clusters.

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.

Proton Presence and Motion in Rhenium-Oxide Films and Their Application to Liquid-Crystalline Cells

Disordered solid phases, containing appreciable amounts of hydrogen ions, are grown at the surface of rhenium oxide crystals, because of the high reactivity of this compound with ambient moisture. To investigate such phenomena, a comparative study is performed on ground powder and thermally evaporated or sputtered films using x-ray diffraction and micro-Raman spectroscopy. Two types of solid phases were found in the films: HxReO3 distorted perovskite structures, based on corner-sharing ReO6 octahedra as in the bulk crystals, and ordered HReO4 crystalline structures, based on tetrahedral perrhenate ions. The complex impedance measurements on ReO3 films support the hypothesis of mobile hydrogen ions in such defective films. Moreover, this relevant protonic conductivity allows the application of these films as active layers inserted into asymmetric nematic liquid-crystalline cells to rectify the electro-optical response of such devices, with performances quite similar to previously studied oxides such as WO3.

Electro-optical response of polymer-dispersed liquid crystal single layers of large nematic droplets oriented by rubbed teflon nanolayers

The surface orienting effect of rubbed teflon nanolayers on the morphology and electro-optical (EO) response of polymer-dispersed liquid crystal (PDLC) single layers of large nematic droplets was studied experimentally. In PDLC composites of the nematic liquid crystal (LC) E7 and NOA65 polymer, single droplets of LC with diameters as larger as 10 μm were confined in layers with a thickness of 10 μm, and the nematic director field was efficiently modified by nanostructuring teflon rubbing of the glass plates of the PDLC cell. For layered PDLCs arranged and oriented in this way, the modulated EO response by the dielectric oscillations of the nematic director exhibits a selective amplitude-frequency modulation controllable by both temperature and voltage applied, and is simply related to the LC droplet size. That may be of practical interest for PDLC-based modulators operating in the infrasound frequency range.

Dielectric Characterisation of an Orthoconic Antiferroelectric Liquid Crystal Mixture

Dielectric properties of the orthoconic smectic liquid crystalline mixture W-129, which presents both antiferroelectric and ferroelectric smectic C phases, have been studied in the frequency range from 10 mHz to 100 kHz in planar aligned cells. Some important relaxation modes were detected in SmCa* and SmC* phases. Dielectric increments, distribution parameters and relaxation frequencies of these modes have been evaluated at different temperatures. From the study of dielectric spectra a great variety of relaxation responses emerges that could indicate the presence of different SmCa* subphases.