Comparison between vanadium dioxide coatings on glass produced by sputtering, alkoxide and aqueous sol-gel methods

Thin films of vanadium dioxide (VO2) on glass substrates were produced by sputtering and two different sol-gel methods. The sputtering method used carefully controlled deposition rates from a vanadium metal target in an argon/oxygen atmosphere to ensure correct stoichiometry. The alkoxide sol was based on a vanadium triisopropoxy oxide precursor: the substrates were dip coated in a dry atmosphere and reduced by vacuum heat treatment. The aqueous sol was made by pouring molten vanadium pentoxide (V2O5) directly into distilled water. After dip coating the substrates were reduced by heat treatment in a low pressure carbon monoxide and carbon dioxide (CO/CO2) atmosphere. The electrical and optical properties of these films were then tested, and are presented here for comparison. All three methods produced films that switch well (as indicated by the conductivity measurements) with the aqueous technique giving the largest magnitude transition. The change in reflectance between the semiconductor and metallic states is similar for the alkoxide and sputtered samples, but smaller for the thicker film produced by the aqueous route. The aqueous sol-gel technique is easier and cheaper than the alkoxide method as expensive precursors and the need to coat in a dry atmosphere are eliminated, although a reducing gas mixture is necessary to reduce the films successfully. (C) 2002 Elsevier Science B.V. All rights reserved.