Electrical and electro-optical investigations of liquid crystal cells containing TiO2–V2O5 thin films prepared by sol–gel synthesis

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 TiO2–V2O5 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 TiO2–V2O5 layer has been analyzed. The voltage thresholds of the Freedericksz transition are increased or decreased for anodic or the cathodic polarization, respectively, of the TiO2–V2O5 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 TiO2–V2O5 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.