Optical anisotropy of nanostructured vanadium dioxide thermochromic thin films synthesized by reactive magnetron sputtering combined with glancing angle deposition

Abstract

In this study, we explore the optical and thermochromic properties of monoclinic vanadium dioxide (VO₂) nanostructures, which undergo a reversible phase transition from an insulating to a metallic state at around 68 °$C$. This phase transition is crucial for applications such as photonic devices, tunable optical filters, and energy-efficient windows. While the performance of VO₂ can be optimized by tailoring its nanostructure and film morphology, to the best of our knowledge, no prior work in the literature has successfully synthesized VO₂ nanostructures with well-defined morphology and high VO₂ purity using the Glancing Angle Deposition (GLAD) technique.

In this work, by combining reactive magnetron sputtering of a vanadium target in an Argon-Oxygen atmosphere with GLancing Angle Deposition (GLAD), we synthesized thin films of VO₂, followed by post-deposition annealing in an oxygen-rich environment. Through GLAD we elaborate anisotropic nanostructures, including tilted and straight columns morphologies. Optical characterizations techniques, such as ellipsometric measurements and grazing incidence X-ray diffraction (GIXRD), were employed to evaluate the crystalline phase and dielectric functions of the films in both their metallic and insulating states. For the tilted nanocolumns, azimuthal Mueller matrix measurements reveal pronounced anisotropic effects. Optical transmission studies show that nanostructured films, particularly those with pillar morphologies, display superior thermochromic performance, with increased transmission, enhanced infrared modulation, and broader hysteresis compared to dense films. The influence of nanostructure porosity on the optical response is also confirmed through simulations using both COMSOL and the Berreman matrix methods, which demonstrate strong agreement in reflectivity predictions. Our work represents a significant advancement in the synthesis of well-defined VO₂ nanostructures, opening new pathways for optimizing the material properties for advanced optical and thermochromic applications.