Fe-doped Tungsten Oxide Thin Films

Tungsten oxide (WO3) is an electrochromic material which has been widely used as the basis for electrochromic-based devices such as displays and light modulating windows. Numerous methods are known for depositing tungsten oxide coatings onto a substrate. Vacuum deposition methods such as sputtering and evaporation are the most commonly used processes, although solution deposition methods are known as well. These solution-based methods generally involve one of three general types of tungsten oxide precursors: colloidal sols, alkoxide derivatives, and peroxy tungstic acid.

The gas sensing characteristics of Fe-doped (10%) tungsten oxide thin films of various thicknesses (100–500nm) prepared by electron beam evaporation. The performance of these films in sensing four gases (H₂, NH₃, NO₂ and N₂O) in the concentration range 2–10,000ppm at operating temperatures of 150–280°C has been investigated. The results are compared with the sensing performance of a pure WO₃ film of thickness 300nm produced by the same method. Doping of the tungsten oxide film with 10at.% Fe significantly increases the base conductance of the pure film but decreases the gas sensing response. The maximum response measured in this experiment, represented by the relative change in resistance when exposed to a gas, was ΔR/R=375. This was the response amplitude measured in the presence of 5ppm NO₂ at an operating temperature of 250°C using a 400 nm thick WO₃:Fe film. This value is slightly lower than the corresponding result obtained using the pure WO₃ film (ΔR/R=450). However it was noted that the WO₃:Fe sensor is highly selective to NO₂, exhibiting a much higher response to NO₂ compared to the other gases. The high performance of the sensors to NO₂ was attributed to the small grain size and high porosity of the films, which was obtained through e-beam evaporation and post-deposition heat treatment of the films at 300°C for 1h in air.

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