Characterizations of mixed Bi/V oxide films, deposited via sol–gel route, used as electrodes in asymmetric liquid crystal cells

Abstract

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 BiCl 3 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 Bi 4V 6O 21 (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.