Spectroscopic analysis of tungsten oxide thin films
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Chintalapalle V. Ramana and Satya K. Gullapalli Department of Mechanical Engineering, University of Texas at El Paso, Texas 79968 (Received 2 March 2010; accepted 14 July 2010)
We present a detailed study of the morphology and composition of tungsten oxide (WO3) thin films, grown by radio frequency magnetron reactive sputtering at substrate temperatures varied from room temperature (RT) to 500 C, using infrared (IR) absorption, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). This work includes valuable new far-IR results about structural changes in microcrystalline WO3. Both IR absorption and Raman techniques reveal an amorphous sample grown at RT and initial crystallization into monoclinic structures for samples grown at temperatures between 100 and 300 C. The Raman spectra of the samples grown at high temperatures indicate, apart from the monoclinic structure, a strain effect, with a distribution revealed by confocal Raman mapping. XPS indicates that the film surface maintains the stoichiometry WOx, with a value of x slightly greater than 3 at RT due to oxygen contamination, which decreases with increasing temperature. I. INTRODUCTION
Tungsten oxide (WO3) has been extensively investigated, especially in the form of thin films, where controlling material characteristics such as crystallinity, grain size, nano- and micro-porosity, stoichiometry, and surface reactivity can be used to enhance its photochromic,1–3 electrochromic,3–6 thermochromic,7,8 and gasochromic9,10 properties. Important reasons for the impressive amount of research dedicated to this metal oxide are its unique properties, which consist of relatively high melting point, diverse, but thermodynamically stable structures with variations in temperature and pressure growth conditions, and low-cost manufacturing procedures. In addition, the high compatibility of WO3 with microelectronic processing and ease of integration into portable devices with the characteristics of low power consumption and online operation, have resulted in its extensive use in nanotechnology applications. It is a primary candidate for use in photocatalysts, anti-dazzle mirrors, smart windows, and gas sensing devices.10–14 In order to achieve stable, selective, and reliable devices, accurate preparation of the functional material is crucial; many factors must be taken into account to warrant homogeneous WO3 grain characteristics such as shape and size, distribution, porosity, and surface conditions. This is a consequence of the fact that, despite the simplicity of the principle and use of metal oxides for a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0294 J. Mater. Res., Vol. 25, No. 12, Dec 2010
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industrial purposes, the actual mechanism is quite complex and not yet fully understood in a way that allows total control in a confinement regime. Thus, fundamental investigations of WO3 at the nano- and micro-scale, where structural defects and perturbation of t
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