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This work is aimed at a deeper understanding of the polarity-sensitive electro-optical response observed in a liquid crystal (LC) cell with asymmetric insertion of thin films of TiO[SUB2]-V[SUB2]O[SUB5] having a Ti/V atomic ratio of 1/1, prepared by sol-gel synthesis on a transparent indium tin oxide substrate. After preliminary structural characterization of the films, the electro-optical response of the liquid crystal cells containing a TiO[SUB2]-V[SUB2]O[SUB5] layer has been analyzed. The voltage thresholds of the Freedericksz transition are increased or decreased for anodic or the cathodic polarization, respectively, of the TiO[SUB2]-V[SUB2]O[SUB5] film. In such a way a polarity-sensitive electro-optical response is generated that has the same frequency as the field applied. Impedance and cyclic voltammetry measurements were performed on liquid crystal cells having TiO[SUB2]-V[SUB2]O[SUB5] films inserted as the electrode, for as-deposited films as well as for films annealed at 400 °C. The LC cell containing thermally annealed Ti/V 1/1 film showed a rectified square wave response instead of the usual impulsive one, quadratic versus electric field. On the contrary, the LC cell containing as-deposited Ti/V 1/1 film exhibits more complex electro-optical behavior with a weak asymmetric response. All the measurements suggest that charge redistribution of the ions, always present in the films, occurs during the intercalation-deintercalation processes induced by the voltage applied, and this is responsible for changes of the effective electric field that act on the liquid crystal layer. [ABSTRACT FROM AUTHOR]

Titania and titania/vanadia thin films were prepared via solgel process, starting from alcoholic solution of Ti (OBu) 4 for pure titania films and from Ti (OBu) 4 /VO(OPr i ) 3 for mixed oxide films. The main requirements of these materials for applications in electrochromic devices were investigated, i.e. high charge capacity and electrochemical stability. Microstructural and electrochemical characterizations were carried out both on gels and films leading to the following results: titania films showed a high electro-chemical stability associated with a good charge capacity, whereas mixed oxide systems displayed a high stability again but a lower intercalation/deintercalation charges. The first electro-optic measurements were also performed for two films with different Ti:V atomic ratio, to check the interaction with liquid crystals in modified nematic liquid crystal cells: the cell containing films poorer in vanadia give some rectification effect, but they have a very complex dependence on the applied potential and very slow relaxation times; on the contrary films richer in vanadia exhibits very good response in the liquid crystal cells, compared with another mixed conductivity oxides. [ABSTRACT FROM AUTHOR]

In the search of new materials for electrodes in electrochromic devices and in asymmetric liquid crystal cells, some modified vanadium oxide thin films has been investigated. The new gels have been generated by the addition of 10% of bismuth (atomic ratio) to the precursors mixture. The sol–gel route started from metal alkoxides, vanadium oxo iso-propoxide and BiCl3 dissolved in ethanol (99%), to give the atomic ratio Bi/V=1:10.The crystallization temperature of such bulk gels has been found by DTA analysis to occur at 320 °C, while only seeds of a crystal phase are found after annealing the dip-coated films at 400 °C, identified as Bi4V6O21 (PDF card no. 33-222). Microscopic visual investigation and micro-Raman analysis were performed on the deposited films, “as grown” and “400 °C annealed”. Finally, the films have been tested as electrodes in asymmetric nematic liquid crystal (NLC) cells, to investigate the rectification effect. The electro-optic measurements were performed in modified nematic liquid crystal cells for both “as-deposited” and annealed films. The voltage dependence of the electro-optical response and the current flow have been analyzed at the same time in the voltage range −10/+10 V. A model based on the electrical interaction between both the indium tin oxide (ITO)-Bi/V oxide films and Bi/V oxide films–liquid crystal interfaces is proposed, to explain the electric and electro-optic asymmetric responses of the cells. [Copyright &y& Elsevier]