Optical Properties of Thermochromic VO2 Nanoparticles

Abstract

Thermochromic VO2 undergoes metal-insulator transition at temperatures relatively near room temperature. The transition affects its optical properties especially at the near infrared (NIR) wavelengths. Consequently, thin films of VO2 have been proposed for energy-saving applications such as window coatings. At high temperatures, metal VO2 has high reflectivity of NIR light which is absorbed or transmitted at low temperatures. However, if used as nanoparticles instead of thin films, the optical properties of the high-temperature metal VO2 are significantly different. Metal nanoparticles show strong surface plasmon resonance absorption which for VO2 is at NIR wavelengths changing the high reflectivity of NIR light in thin films to absorption in nanoparticles. We have studied the optical spectrum of VO2 nanoparticles using two different methods suitable for the calculation of optical properties of nanoparticles embedded in a dielectric coating layer. Effective medium theory (EMT), in this case in the form of Maxwell Garnett theory, can describe the absorption properties of nanoparticles but does not consider scattering of light. It can be taken into account by using the four-flux method with scattering and absorption efficiencies calculated with the Mie theory. The results from the two methods agree well at both low and high temperatures for small VO2 nanoparticles where the scattering contribution to the spectrum is small.