Electrochromism of fluorinated and electron-bombarded tungsten oxide films

Reactive dc magnetron sputtering of W was performed in a plasma of Ar+O₂ with and without CF₄ addition and substrate bias. Structural studies by atomic force microscopy, X-ray diffraction, infrared reflectance spectroscopy, and Raman spectroscopy indicated that the electron e bombardment associated with a positive substrate bias led to grain growth and partial crystallization while maintaining a high density of W=O double bonds presumably on internal surfaces. Electrochemical measurements showed that the durability under extended Li⁺ intercalation/ deintercalation was excellent for e-bombarded oxide films and poor for oxyfluoride films. Spectrophotometric studies of the electrochromism yielded that the color/bleach dynamics was slow for the e-bombarded oxide but fast for the oxyfluoride. The range of optical modulation was large. Tandem films, with a thin protective layer of e-bombarded oxide covering a thicker oxyfluoride layer, were able to combine rapid dynamics with good durability.