Hydrated Transition Metal Oxide Films for Reversible Switching in Near IR Region
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ABSTRACT Electro-optical properties of thin films of hydrated transition metal oxides, WOy3 2(H 2 0) and WOy3 1/3(H 2 0), have been investigated in situ using photoelectrochemical methods. These films can reversibly change their light absorption in the near IR region due to the double charge injection upon the applied electric field in electrolyte media. Taking into account the high stability and fast switching time, these hydrated tungsten oxide films can be utilized for efficient light modulation in the near infrared region of the spectrum.
INTRODUCTION During the past two decades, electrochromic phenomena in transition metal oxides, such as W0 3, MoO 3 , TiO 2 , have been investigated extensively. The unique effects of electrically induced reversible coloration and charge storage in thin transition metal oxide films have diverse applications which range from high-contrast information displays with a wide view angle, automobile rearview mirrors, which automatically control the reflectance to "smart" windows allowing regulation of solar energy passing through them [1-21. A coloration into the deep blue color is achieved by cathodic polarization of the film in an electrolyte medium. The hydrated oxide WOy2(H20) and WO 3yl/3(H2 0) films can not be used as materials for optical switching in visible region of the spectrum because their induced absorption bands are located in the wavelength range from 0.6 to 2.2 gim and peaked at 1.2 -1.3 gim. Nevertheless, the modulated transmission of these films in the near infrared region can be easily monitored using the applied potential and they can be used for switching applications. Reversible reactions of electron and proton injection/ejection cause a strong and fast variation in the film transmittance. The reaction proceeds according to the general mechanism: W0 3 "n(H20) + xH+ + xe"
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