Effects of ion beam bombardment on electrochromic tungsten oxide films studied by X-ray photoelectron spectroscopy and Rutherford back-scattering

The effect of ion bombardment on thermally evaporated and magnetron sputter ed tungsten oxide films were investigated using X-ray photoelectron spectro scopy (XPS). Results show that irrespective of the porosity and crystallini ty of the film samples formed with different techniques and conditions, ion bombardment induced preferential sputtering of oxygen, resulting in a decr ease of oxygen/tungsten (O/W) ratio with increasing sputtering time. Sample s experienced electrochromic switching cycles also show the same effect, ex cept that a higher O/W ratio is detected because the tungsten oxide film re acts with the LiClO4-propylene carbonate electrolyte. Angle-resolved XPS ex periments further confirm preferential sputtering of oxygen, suggesting tha t ion beam sputtering used in XPS for pre-cleaning and depth profile analys is of tungsten oxide must be used with caution. Rutherford back-scattering gives more reliable composition data of tungsten oxide, since it does not i nvolve any sputtering process. (C) 2000 Elsevier Science S.A. All rights re served.