Optical properties and surface morphology of evaporated (WO 3 ) 1 x –(Fe 2 O 3 ) x thin films A.Z. Moshfegh a, * , R. Azimirad b , O. Akhavan a a Department of Physics, Sharif University of Technology, P.O. Box 11365-9161, Tehran, Iran b Department of Physics, Tehran University, P.O. Box 19395-3667, Tehran, Iran Received 27 April 2004; accepted in revised form 11 February 2005 Available online 21 April 2005 Abstract Thin films of (WO 3 ) 1 x – (Fe 2 O 3 ) x composition were deposited by thermal evaporation on glass substrates and then all samples were annealed at 400 -C in air. Optical properties such as transmittance, reflectance, and optical bandgap energy of the ‘‘as deposited’’ and the annealed films were studied using ultraviolet –visible spectrophotometry. It was shown that the annealing process did not substantially change the optical transmittance and reflectance of all the films except the films having x = 0.75. By increasing Fe 2 O 3 content in the films from x =0 to x =0.75, optical bandgap energy decreased from 3.4 to about 1.3 eV and from 3.1 to 2.1 eV for the ‘‘as deposited’’ and the annealed samples, respectively. Moreover, using X-ray photoelectron spectroscopy, we have shown that the ‘‘as deposited’’ (WO 3 ) 0.25 – (Fe 2 O 3 ) 0.75 films contained Fe 3 O 4 , and it converted to Fe 2 O 3 after the annealing process. Using atomic force microscopy, it was found that the grain size and surface roughness of the annealed films weregreater than the ‘‘as deposited’’ ones. In addition, for the annealed films with intermediate x ’s, we have observed a smoother surface with smaller grain size as compared to films with the other x ’s. D 2005 Elsevier B.V. All rights reserved. PACS: 78.20Ci; 78.66.Li; 68.37.Ps Keywords: Tungsten oxide; Iron oxide; Optical properties; Surface morphology 1. Introduction Tungsten oxide (WO 3 ) thin film has been extensively studied during past years due to its potential technological applications in selective catalysts [1,2], electrochromic devices [3–9], gas sensors [10,11], gaschromic [12] and nanowires [13]. WO 3 thin films can be prepared by various deposition techniques such as thermal evaporation [14], magnetron sputtering [11], solution spray [7], pulsed laser ablation [15], chemical vapor deposition [16] and sol–gel coating [17]. The optical properties of metal oxides are of great concern for practical applications including photo-catalysts and electrochromic devices. The study of optical absorption, particularly the absorption edge, has proved to be very useful for elucidation of the electronic structure of solids. It is possible to determine whether the optically induced transition is direct or indirect and allowed or forbidden by analyzing the optical absorption spectra. Moreover, the surface structure and surface morphology of the metal oxides are also important for different applications. In fact, the electrochromic devices are made of amorphous oxides [4], while crystalline phase plays a major role in catalysts and sensors [18]. This is because the minor change in their chemical composition and crystalline structure could modify different properties of the metal oxides. In several works, efforts have been made to improve surface and optical properties of WO 3 by mixing it with other metal oxides such as: TiO 2 [3,4], MoO 3 [6,19,20], and V 2 O 5 [2–4]. Another metal oxide that could possibly modify the properties of WO 3 is Fe 2 O 3 . This is because pure Fe 2 O 3 has excellent properties similar to WO 3 [4,21 – 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.02.019 * Corresponding author. Fax: +98 21 601 2983. E-mail address: moshfegh@sharif.edu (A.Z. Moshfegh). Thin Solid Films 484 (2005) 124 – 131 www.elsevier.com/locate/tsf