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Nanotechnology Letters

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Optical, electrical and thermochromic properties of VO2 thin films

Author(s): Professor Pandurang Ashrit

 Vanadium dioxide (VO2) thin films are being studied extensively due to their thermochromic properties i.e. mainly a reversible switching of their optical properties as a function of temperature. At room temperature they are semiconductors with a monoclinic structure and are transparent to infrared radiation. Beyond the transition temperature Tt, which is typically around 68oC in bulk form of VO2, the films exhibit a metallic behaviour with a tetragonal phase and become opaque to infrared wavelengths. Hence, the VO2 films undergo a drastic change in their optical, electrical and structural properties around the transition temperature. They also exhibit a hysteresis behaviour in the optical and electrical properties as a function of temperature around the Tt. This possibility to interactively control the physical properties of the VO2 thin films via thermal activation leads to many important applications, especially in the energy sector. These thermochromic changes are also important from the fundamental study point of view. In addition, the VO2 thin film nanostructure is found to play an important role in the thermochromic switching efficiency as well as in changing the transition temperature, making it possible to tailor their switching properties via the control of their nanostructure. In the present work, a systematic study of the thermochromic properties of sputter deposited and annealed VO2 thin films has been undertaken as a function of the film thickness. The effect of film thickness and nanostructure on the optical, electrical and thermochromic properties as well as on the hysteresis and Tt has been studied in details, The results are examined in the light of existing theories, where possible. The potential for tailoring the thermochromic properties for various applications will be discussed in the light of these results


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