Expanded Thermochromic Color Changes in VO 2 Thin Film Devices Using Structured Plasmonic Metal Layers
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Expanded Thermochromic Color Changes in VO2 Thin Film Devices Using Structured Plasmonic Metal Layers Yan Wang1, 2 and John F. Muth1 Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA 2 Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
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ABSTRACT We investigate metallic thin films on VO2 and show that the magnitude of the reflected color change in that visible portion of the spectrum as VO2 undergoes the insulating to metallic phase transition can be controlled by changing the type of metal, the thickness of the metal and by patterning the metal at the nano scale. We consider the role of surface plasmas in the metal film and show that in the near infrared, the magnitude of the reflectivity increase for metal coated VO2 films, but decrease for uncoated VO2 thin films. This is explained in the context of Fresnel equations and considering the large change in the imaginary part of the dielectric constant as the VO2 changes state from the insulating to metallic phase. INTRODUCTION Thermochromic material which is well known for the optical property of being able to change reversibly upon temperature has attracted much attention. Thermochromic thin films whose reflectance depends on temperature can be used as coating materials for smart windows or chromic glass [1]. Vanadium dioxide (VO2) has been one of the most interesting thermochromic materials because of its high infrared transmittance in insulating phase and high infrared reflectance in metallic phase. Thermochromics in infrared region have been observed from single VO2 films or VO2 films coating by oxides [2, 3]. VO2-based nanothermochromics has also been reported before [4, 5]. In this paper, we report an optical observation of the reflectivity change at the interface between VO2 and gold film. Three device structures were fabricated with Pulsed Laser Deposition (PLD) and e-beam deposition. The reflection and transmission spectra of these devices are compared at the phase transition region of the VO2 film. Reflectance of VO2 film decreases as we increase the temperature. However, we have seen opposite reflectance change after coating the VO2 film with a layer of gold as we change the temperature from low to high. This effect can be explained by dielectric constant change of the VO2 film during phase transition and the increasing in electron damping at the interface. EXPERIMENT Pulsed Laser Deposition (PLD) was used to deposit the VO2 films on an ITO covered glass substrate. A KrF excimer laser at a wavelength of 248 nm was focused on a homemade V2O5 target. The repetition rate and energy level of laser pulses were kept at 10 Hz and 200 mJ during deposition. The base pressure of the deposition chamber was pumped to 10-7 Torr. The
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depositions were performed in an ambient of 20 mTorr oxygen and the substrate temperature was maintained at 800 °C. After the deposition, the samples were cooled down at the deposition oxygen pressure to room temperature. Th
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