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.

Electrical and electrooptical investigation of liquid crystals cells containing WO3 thin films

N. SCARAMUZZA;C. VERSACE;BARTOLINO, Roberto
2000

Abstract

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.
An interesting application of the fast ion transport properties of tungsten trioxide is presented, when it is inserted as an electrode in nematic liquid crystal ~NLC! cells. In a standard sandwichlike cell the nematic liquid crystal, confined between two transparent plane electrodes of purely electronic conductors @indium tin oxide ~ITO!#, undergoes a molecular reorientation under the action of an external electric field E. This electrically controlled birefringence ~electro-optical switching! is proportional to E2, thus polarity insensitive @L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials ~Springer-Verlag, New York, 1994!#. When a thin film of tungsten trioxide is deposited by magnetron sputtering onto one of the transparent ITO electrodes, and a NLC cell is assembled with such asymmetry, the electro-optical response becomes polarity sensitive @G. Strangi et al., Appl. Phys. Lett. 74, 534 ~1999!#. The analysis of this response suggests the occurrence of a reverse internal electric field, associated with the ionic diffusion process of protons always present in these sputtered WO3 films @E. Cazzanelli et al., Electrochim. Acta 44, 3101 ~1999!#. By using an opportune voltage waveform it is possible to evaluate such an internal field. Impedance and cyclic voltammetry measurements were carried out on these cells, comparing ‘‘as-deposited’’ and ‘‘annealed’’ tungsten trioxide electrodes. These studies confirm that an important ionic diffusion process is involved in the establishment of an internal electric field, which modifies the electro-optical response of the nematic liquid crystal cell.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11770/145865
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