A Study of the Optical Band Gap of Lithium Tungsten Trioxide Thin Films
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A STUDY OF THE OPTICAL BAND GAP OF LITHIUM TUNGSTEN TRIOXIDE THIN FILMS G. Berera, R.B. Goldner, F.O. Amtz, K.K. Wong, A. Ciaccia, M. Welch, T.E. Haas, L. Jauniskis Electro-Optics Technology Center, Tufts University, Medford MA 02155 ABSTRACT The insertion of lithium (lithiation) into tungsten trioxide results in the formation of the tungsten bronze LiWO3. Polycrystalline, rf sputter deposited thin films of Li1WO 3 were investigated for their application in Smart Window Devices. The optical band gap studies of these films revealed the narrowing of the intrinsic band gap as a consequence of lithium insertion. The results suggest that the rigid band model, which is generally adopted in interpreting the electronic structure of the tungsten bronzes may not be applicable in Li.WO. INTRODUCTION Thin films of tungsten trioxide (WO3) have been extensively researched for their potential applications in electrochromic devices [1,2]. Our attention was drawn to this material for it showed promise as the principal electrochromic layer in an electrochromic "Smart Window" device [3], useful for energy conservation purposes. When alkali metal atoms are incorporated into tungsten trioxide it results in the formation of tungsten bronzes and the optical state changes from a transparent to a reflective state. High reflectivities, approximately 80% in the near infrared (NIR) region, has been reported [4] for rf sputtered polycrystalline WO3 films when inserted with lithium atoms resulting in the formation of lithium tungsten bronze (Li3WO3). The electronic, optical and structural properties of the tungsten bronzes, mainly the sodium bronzes (NakWO3) have been investigated in depth. The band structure of the bronzes has been qualitatively explained in terms of a model developed by Goodenough [5]. In short, the conduction band in WO3 is derived from the three t2 (5d) tungsten orbitals and the valence band from the oxygen 2p atomic orbitals. The inserted alkali metal atom merely contributes one electron into the otherwise empty conduction band of WO3 suggesting a rigid band model for these bronzes. The band structure calculations of WO3 and Na•WO 3 [6,7] as well as the data on the optical constants of Na•WO 3 [8] is supportive of this model. However the validity of this theory has been questioned by several authors based on XPS studies [9,10]. In this paper we would like to report that our results from optical band gap studies of polycrystalline thin films of LiXWO 3 suggests the narrowing of the optical band gap as a function of lithium content [Li] and would like to show that the rigid band model may not be applicable in Li.WO 3. EXPERIMENTAL Tungsten trioxide films in the present study were prepared by an rf sputter deposition technique. The target was a 5-inch diameter partially reduced W0 3 - obtained from Cerac, Inc. The deposition parameters were the following: 200 Watts of power (13.5 MHz), an atmosphere of 10 mT of oxygen and a substrate temperature of approximately 430*C to 440 0C. The film thicknesses were 0.2 ± 0.02 jim for fi
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