Molybdenum oxide (MoO3) films were deposited on glass and silicon substrates held at temperature 473？K by RF magnetron sputtering of molybdenum target at various oxygen partial pressures in the range mbar. The deposited MoO3 films were characterized for their chemical composition, crystallographic structure, surface morphology, chemical binding configuration, and optical properties. The films formed at oxygen partial pressure of mbar were nearly stoichiometric and nanocrystalline MoO3 with crystallite size of 27？nm. The Fourier transform infrared spectrum of the films formed at mbar exhibited the characteristics vibrational bands of MoO3. The optical band gap of the films increased from 3.11 to 3.28？eV, and the refractive index increased from 2.04 to 2.16 with the increase of oxygen partial pressure from to mbar, respectively. The electrochromic performance of MoO3 films formed on ITO coated glass substrates was studied and achieved the optical modulation of about 13% with color efficiency of about 20 cm2/C. 1. Introduction Transition metal oxides constitute an interesting group of semiconducting materials because of their technological applications in various fields such as display devices, optical smart windows, electrochromic devices, and gas sensors [1, 2]. Among the transition metal oxides, molybdenum oxide (MoO3) exhibits interesting structural, chemical, and optical properties. MoO3 finds application as a cathode material in the development of high energy density solid state microbatteries [3, 4]. It is considered as a chromogenic material since it exhibits electro-, photo-, and gaso chromic (coloration) effects by virtue of which material is of potential for the development of electronic display devices [5]. MoO3 films in nanocrystalline form also find applications in sensors and lubricants [6]. It is also a promising candidate as a back contact layer for cadmium telluride solar cells in superstrate configuration because of its high work function, which possibly reduces the back contact barrier [7]. Various physical thin film deposition techniques such as thermal evaporation [8, 9], electron beam evaporation [10, 11], pulsed laser deposition [12, 13], and sputtering [14–18] and chemical methods such as electrodeposition [19], chemical vapour deposition [20], spray pyrolysis [21, 22], and sol-gel process [23–25] were employed for the growth of MoO3 films. Among these films deposition techniques, magnetron sputter deposition is an industrially practiced technique for the growth of oxide films. The physical properties of the sputter deposited MoO3
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