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Two sets of WO3 thin films were prepared by rf sputtering from tungsten oxide and tungsten metal targets, respectively. Lithium insertion studies were carried out in these two types of films. It has been found that the films sputtered from the metal targets showed an irreversible uptake of lithium without any coloration. The aim of the present work is to study this specific behavior of such films in comparison to those deposited from tungsten oxide targets. These films have been studied by thermogravimetry, UV-Vis spectroscopy, electron probe, ellipsometry, and ATR-FTIR spectroscopy. The stoichiometry of both types of films is found to be identical. The ATR-FTIR spectra show a difference in the W-O stretching vibration pattern of the two types of films, clearly indicating structural differences in the as- deposited WO3 films. However, both types of films show the presence of little or no water in the as-deposited state. Upon the insertion of lithium it is found that the films deposited from the oxide target show the formation of 'bronze' right from the very early stage of insertion as indicated by the blue coloration. This formation is reflected in the ATR-FTIR spectra by the increasing slope of the absorption in the mid-infrared range. Contrary to this behavior, the films deposited from the metal target do not show any coloration in the initial stages of lithium insertion. The infrared spectra corresponding to this behavior show little change in the mid-infrared range. However, a widening of the W-O stretching band is observed in these spectra reflecting a structural reorganization in the films. With continued lithium insertion, however, the behavior in the two types of films becomes similar. Hence, in the initial stages of coloration the quantity of lithium participating in the electrochromic coloration of the films deposited from metal target seems to be lower than in the other types of film. The optical constants of these films have also been determined using reflection- transmission ellipsometry showing important differences in the films. Hence, the electrochromic behavior of sputtered tungsten oxide films depends strongly on the type of deposition.
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