Tungsten doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride Russell Binions, Clara Piccirillo, Ivan P. Parkin ⁎ Department of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street, London, WC1H 0AJ, United Kingdom Available online 23 March 2007 Abstract The atmospheric pressure chemical vapour deposition reaction of vanadyl acetylacetonate and tungsten hexachloride led to the production of thin films of tungsten doped monoclinic vanadium dioxide on glass substrates. Scanning electron microscopy indicated that the films had a columnar island growth morphology. Transmission and reflectance measurements elucidated a significant change in properties in the IR portion of the spectrum either side of the metal to semiconductor transition. Variable temperature transmission studies show that the metal to semiconductor transition was lowered by doping tungsten into the films and that this effect was dependent on the amount of tungsten doping. © 2007 Elsevier B.V. All rights reserved. Keywords: APCVD; Vanadium dioxide; Thermochromic; Thin films 1. Introduction Thin films of vanadium (IV) oxide have received much interest in recent years due to the potential application as an intelligent window coating [1,2] and other advanced applica- tions such as data storage [3,4] or infrared modulators [5]. These technologies are based on the thermochromic metal to semiconductor transition which occurs in the pure material at 68 °C. There is an associated structural adjustment from the low temperature monoclinic phase (VO 2 M) to the higher tempera- ture rutile phase (VO 2 R) [6]. In accordance with this structural change there are significant changes with the electrical conductivity and optical properties. The rutile material is metallic and reflects a wide range of solar radiation, whereas the monoclinic is a semiconductor and is not nearly as reflective. For VO 2 to be effective as an intelligent window coating it is desirable to lower the transition temperature from 68 °C to nearer room temperature. Doping studies have shown that the transition temperature can be altered by the incorporation of metal ions into the VO 2 lattice [7,8]. It was found that the most effective metal ion was tungsten [9]. Tungsten doped VO 2 films have been prepared by a variety of methods including sol–gel [10], sputtering [11], and CVD methodologies [6,12–23]. CVD routes to the production of doped VO 2 films are generally considered more attractive because of the compatibility of CVD processes with high volume glass manufacture and the physical properties of CVD produced films which are usually adherent and long lasting. Various studies have been conducted on the growth of tungsten doped thin films using vanadium tetrachloride or vanadium oxychloride as a vanadium precursor and tungsten hexachloride or tungsten ethoxide as a precursor to tungsten doping. In all cases water was used as an oxygen precursor [9,24]. In the current study vanadyl acetylace- tonate was utilised as the vanadium precursor and tungsten hexachloride as the tungsten doping precursor. Tungsten hexachloride was preferred to tungsten ethoxide because previous studies have highlighted the poorer mass transport properties of tungsten ethoxide [22]. Vanadyl acetylacetonate has received some attention in the production of VO 2 films [19,20] but little attention in doping studies. 2. Experimental A 98% nitrogen, 2% oxygen mixture was obtained from the British Oxygen Company (BOC) and used as supplied in the Surface & Coatings Technology 201 (2007) 9369 – 9372 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. E-mail address: i.p.parkin@ucl.ac.uk (I.P. Parkin). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.03.026