Abstract
We experimentally investigate the semiconductor-to-metal transition (SMT) in vanadium dioxide thin films using an infrared thermographic technique. During the semiconductor to metal phase change process, VO2 optical properties dynamically change and infrared emission undergoes a hysteresis loop due to differences between heating and cooling stages. The shape of the hysteresis loop was accurately monitored under different dynamic heating/cooling rates. In order to quantify and understand the effects of different rates, we used a numerical modelling approach in which a VO2 thin layer was modeled as metamaterial. The main experimental findings are interpreted assuming that both the rate of formation and shape of metallic inclusions are tuned with the heating/cooling rate. The structural transition from monoclinic to tetragonal phases is the main mechanism for controlling the global properties of the phase transition. However, our experimental results reveal that the dynamics of the heating/cooling process can become a useful parameter for further tuning options and lays out a macroscopic optical sensing scheme for the microscopic phase change dynamics of VO2. Our study sheds light on phase-transition dynamics and their effect on the infrared emission spectra of VO2 thin films, therefore enabling the heating/cooling rate to be an additional parameter to control infrared emission characteristics of thermal emitters. The hysteresis loop represents the phase coexistence region, thus being of fundamental importance for several applications, such as the operation of radiative thermal logic elements based on phase transition materials. For such applications, the phase transition region is shifted for heating and cooling processes. We also show that, depending on the way the phase change elements are heated, the temperature operation range will be slightly modified.
Highlights
Thermochromic materials offer a drastic phase transition around their critical temperature (Tc), where significant optical, electrical and magnetic changes take place
In this work we investigate, both theoretically and experimentally, the dynamic tuning of infrared emission from a single VO2 thin film on sapphire substrate in the 3.3-5.1 μm wavelength range [20,21]
Heating rate emerges as a promising parameter contributing to the resulting spectral features such as emittance or reflectance could play important role in future device designs based on phase-change materials such as VO2 thin films
Summary
Thermochromic materials offer a drastic phase transition around their critical temperature (Tc), where significant optical, electrical and magnetic changes take place. In this work we investigate, both theoretically and experimentally, the dynamic tuning of infrared emission from a single VO2 thin film on sapphire substrate in the 3.3-5.1 μm wavelength range [20,21]. Within this wavelength range, we employed a highly emissive object placed below the sample to show different emission features arising at different heating/cooling rates. Heating rate emerges as a promising parameter contributing to the resulting spectral features such as emittance or reflectance could play important role in future device designs based on phase-change materials such as VO2 thin films. The agreement between our experimental findings and numerical simulations reveals that the process of metallization can be modeled as a binary system where inclusion aspect ratio is modulated by the heating rate
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