It is well known that during the sputtering, the interaction between Ar plasma and WO3 bulk can highly distort and tilt the form works of the WO6 octahedra and thus results in an amorphous film. At the same time, a large number of oxygen vacancies and defects, which can reduce the tungsten ions from W6+ to W5+ or W4+, would be produced in the deposited films. This is responsible for the violet-red color and the amorphous XRD data of the films annealed below 300℃. The obvious crystallinity of the deposited films at 400℃ for 60 min implies that the defects and disordered structures in the films have been effectively activated by this temperature. Consequently, increasing the annealing time at 400℃ is obviously favorable to the crystallinity and the growth of the films.

 

 

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Several preparation techniques have been used to deposit tungsten oxide thin films, including sol-gel, hydrothermal technologies, chemical vapor deposition, thermal evaporation, pulsed laser deposition, and magnetron sputtering. In fact, reactive magnetron sputtering is one of the most versatile techniques to deposit oxide films, allowing the control of many properties of the films by changing the partial pressure of the reactive gas, the components with the different targets and even sputtering power. Many reports have been published explaining the mechanisms involved in thin film growth using magnetron sputtering, but very often these refer to tungsten oxide thin films obtained from a metallic tungsten target. In contrast, the reports on tungsten oxide thin films obtained from a WO3 target are seldom reported. In 2009, Acosta et al. reported the studies on the optical properties of tungsten oxide thin films by non-reactive sputtering of WO3 target and found that the argon pressure had a strong influence on the optical properties of the film. It has been predicted that a large amount of oxygen vacancies, defects and disordered structures in the deposited films, which are very critical in determining the crystalline, optical, photochemical and electric properties of tungsten oxide thin film, would be produced when the sputtering plasma interacts with WO3 bulk.

 

 

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As we all know, annealing, or thermal, treatment is one of the most effective ways to influence the structure and the properties of many films. To investigate the effects of the annealing temperatures on the structure and optical property of tungsten oxide thin films that were deposited by magnetron sputtering of WO3 bulk in a vacuum, the deposited films were annealed at 200℃ and 300℃ for 60 min and at 400℃ for 60 min and 180 min in air, respectively. We find that the annealing temperature of 400℃ can effectively influence the structure and optical property of the deposited films. The films annealed at 200℃ and 300℃ display violetred under the sunlight as well as the as-deposited ones. Nevertheless, the film annealed at 400℃ exhibits transparency and appeared to be blue colored.

 

 

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In the past few years, an increasing interest has been put on the tungsten oxide films due to their potential applicanons in smart windows, gas sensors, photocatalytic reactions, and optoelectronic devices, etc. The unique properties of the tungsten oxide films were usually determined by the oxygen defects and the valence states of tungsten ions. Besides, it is very important to improve the properties of tungsten oxide films and other oxide films by controlling their morphologies and crystalline phases, which mainly depend on the preparation technologies and thermal treating temperatures and atmospheres.

 

 

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