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 TiO 2-V 2O 5 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 2-V 2O 5 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 2-V 2O 5 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 2-V 2O 5 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. © 2002 American Institute of Physics.