Optical properties and electrochromic response of nanostructured molybdenum trioxide films
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e report on the electrochromic response of as-deposited and annealed nanostructured molybdenum trioxide films prepared with the glancing angle deposition (GLAD) technique. Morphology of the as-deposited films, obtained with an atomic force microscope (AFM), showed a typical grain size of 10 to 50 nm diameter. After annealing, the AFM images clearly showed the dominant presence of a layered structure, characteristic of the orthorhombic (a) phase of molybdenum trioxide, with typical grain dimensions of a few micrometers. The annealed samples showed pronounced coloration in the visible and near-infrared regions of the electromagnetic spectrum, while the as-deposited samples showed significant coloration only in the visible region.
I. INTRODUCTION
Molybdenum trioxide (MoO3) is one of the transition metal oxides known to exhibit the phenomenon of electrochromism, defined as the reversible coloration with the intercalation/deintercalation of electrons and ions.1 Such materials, among which are also tungsten trioxide (WO3), titanium dioxide (TiO2), and vanadium pentoxide (V2O5), are of interest for the fabrication of smart windows, variable reflectance mirrors, and displays. In addition to the pronounced electrochromism, molybdenum trioxide films show thermochromic and photochromic properties, defined as the reversible coloration with temperature change and photon absorption, respectively.1 The known crystalline phases of MoO3 include the a-phase, which is an orthorhombic phase with chains of corner sharing MoO6 octahedra, with van der Waal’s forces weakly binding together the layers of chains.1 Because of its layered structure, a-MoO3 has applications in ion storage, especially as cathode material in highenergy density secondary lithium batteries.2–4 There is a monoclinic phase (b-MoO3) with a framework structure as well.1 A hexagonal structure for bulk samples has also been reported in the literature.5 Previous work on MoO3 films includes studies of films prepared with techniques such as MOCVD,6 sputtering,7 thermal oxidation,8 and electron beam evaporation.9 These studies and others have indicated the importance of texture and nanostructure in determining the electrochromic properties of MoO3 films.6–13 Glancing angle deposition (GLAD) is a technique in which the dynamic control of azimuthal and tilt rotation of the substrate during deposition allows tailoring of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.43 J. Mater. Res., Vol. 26, No. 1, Jan 14, 2011
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nanostructure, morphology, and porosity of the resulting film.14 In this work, we use the GLAD technique with electron beam deposition to deposit nanostructured MoO3 films with varying degrees of porosity. In a previous work, we showed the enhancement of photochromism in nanostructured GLAD films.15 Here, we examine the effect of nanostructuring and annealing on the electrochromic properties of these films. We show that the nanostructuring introduced by the GLAD technique can
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