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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|Titolo:||Electrical and electro-optical investigations of liquid crystal cells containing WO3 thin films|
|Data di pubblicazione:||2000|
|Appare nelle tipologie:||1.1 Articolo in rivista|