Thermochromic vanadium dioxide thin films from electric field assisted aerosol assisted chemical vapour deposition Michael E.A. Warwick a,b , Ian Ridley c , Russell Binions d, ⁎ a Department of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street, London WC1H 0AJ, United Kingdom b UCL Energy Institute, Central House, 14 Upper Woburn Place, London, WC1H 0NN, United Kingdom c School of Property, Construction and Project Management, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia d School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom abstract article info Available online 24 June 2013 Keywords: Chemical vapour deposition Thermochromic Electric fields Vanadium dioxide Thermochromic vanadium dioxide thin films were deposited via aerosol assisted chemical vapour deposition from a precursor solution of vanadyl acetylacetonate in ethanol at 525 °C on to fluorine doped tin oxide coat- ed glass substrates. A potential difference was applied between the top plate and substrate during the depo- sition to generate an electric field with a positive bias applied to the substrate. The films produced were analysed and characterised by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectros- copy, Raman spectroscopy, contact angle and variable temperature UV/Visible spectroscopy. It was found that the presence of an electric field during deposition could lead to a marked change in the microstructure and functional properties of the deposited films; specifically the wetting and thermochromic properties. An increase in field strength lead to a reduction in crystallite and agglomerate size compared to films grown without the presence of an electric field. © 2013 Elsevier B.V. All rights reserved. 1. Introduction In recent years there has been much interest in thin films of vanadi- um (IV) oxide (VO 2 ) for use in intelligent glazing systems. [1,2] The in- terest stems from the inherent thermochromic transition which VO 2 under goes causing a transition between semi conductor and metallic properties, occurring at 68 ºC for pure single crystals [3]. The transition is due to a structural change from the low temperature monoclinic phase (VO 2 M) to the high temperature rutile phase (VO 2 R). [4] This change results in a significant change in optical and electrical proper- ties. The low temperature VO 2 M phase is semiconducting and trans- mits a wide range of solar radiation. In contrast the high temperature VO 2 R is metallic and far more reflective to solar radiation especially that in the infrared region. This transition and accompanying properties makes VO 2 an inter- esting candidate for variable temperature heat mirror there are still some problems which need to be addressed before it could be consid- ered viable. The main concern for this technology is the high temper- ature at which the transition occurs and how this could be reduced [5]. The ideal temperature for the transition is thought to be between 20 and 30 ºC although this will vary depending on the environment in which it is going to be used [5–7]. Previous investigations into the use of dopants have shown some success in reducing the transition temperature. The dopants that have been found to effectively lower the transition temperature are high valance metal ions. The most effective of these ions is tungsten, which has been the subject of significant investigation showing that dopant levels ~2 at% can drop the transition temperature ~25 ºC [8]. This does however lead to a reduction in the variation of the infrared optical properties associated with the transition. It has also been seen that a reduction in film thickness also leads to a reduction in the transition temperature [6]. This decrease in transi- tion temperature is thought to occur as a result of an increase of strain within the film [9]. There has also been work done that shows a cor- relation between deposited particle size reduction and a reduction in transition temperature [10]. It is the reduction of particle size that this paper will report on. It has been previously reported that applied electric fields during CVD reactions can cause a decrease in particulate size as well as some variation in orientation and growth preference [11–14]. In this paper we report on the use of electric fields in the growth of VO 2 thin films with a continual positive bias applied to the substrate. 2. Experimental section Reactions were carried out in a quartz cold walled reactor set up as shown in Fig. 1. A 0.1 M solution of [VO(acac) 2 ] (Aldrich 99.99%) was made up by dissolving 0.384 g in 15 ml of ethanol. The solutions were stirred for 20 min prior to use to ensure the [VO(acac) 2 ] had dissolved Surface & Coatings Technology 230 (2013) 163–167 ⁎ Corresponding author. Tel.: +44 2078825305. E-mail addresses: michael.warwick.09@ucl.ac.uk (M.E.A. Warwick), r.binions@qmul.ac.uk (R. Binions). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.06.077 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat