Titre de la dissertation: « Experimental and numerical study of the optical properties of thermochromic vanadium dioxide: influence of coupling with metal nanoparticles and nanostructuration ».

Promoteurs: Monsieur Michel Voué et Monsieur Stephanos Konstantinidis

Résumé de la dissertation: The aim of this PhD thesis is to develop a multicomponent platform that exploits the plasmonic resonance of metallic nanoparticles (NPs) and the Metal-Insulator Transition (MIT) of monoclinic vanadium dioxide (VO₂) to achieve a highly tunable optical response. In addition, VO₂ nanostructures such as tilted or straight nanocolumns, helices and zigzag, are synthesized by combining reactive magnetron sputtering with GLancing Angle Deposition (GLAD) to analyze the influence of the sample anisotropy on the optical response. VO₂ is popular because it is a thermochromic material that undergoes a phase transition at around 67°C, shifting from a semiconducting monoclinic to a metallic tetragonal state, which significantly alters its electrical and optical properties in the visible and infrared ranges. These unique properties make it an excellent candidate for applications in photonic devices, tunable optical filters and energy-saving windows.Initially, we successfully synthesized thermochromic VO₂ thin films using reactive magnetron sputtering. These films were analyzed using Raman spectroscopy and X-ray diffraction (XRD) to determine their chemical composition and crystalline phase. The thermochromic properties were highlighted through transmission and resistivity measurements as a function of temperature.We then conducted experimental research combining VO₂ films with gold nanoparticles (AuNPs) to achieve tunable plasmonic signals in response to temperature variations. We demonstrated the successful grafting of AuNPs onto the VO₂ film surface using (3-aminopropyl) trimethoxysilane (APTMS) linkers. We observed a noticeable shift in the plasmonic peak wavelength as a function of temperature for two distinct platforms: one with NPs positioned on top of the VO₂ film and another with NPs embedded within the film. The grafting of AuNPs onto VO₂ films resulted in a shift in the plasmonic peak wavelength with temperature, enhanced resistivity, transmission drops, and a reduction of the critical MIT temperature by 5°C. Embedding NPs within the VO₂ matrix further reduced the critical transition temperature to 35°C, making it promising for smart window applications. Concerning the plasmonic wavelength, COSMOL simulations allowed us to study and understand the influence of the NPs penetration inside a VO₂ matrix on the plasmonic modulation with temperature.Finally, we successfully synthesized thermochromic VO₂ nanostructures in various forms, including tilted nanocolumns, zigzags, and helices, utilizing a GLAD configuration. Ellipsometry analyses revealed significant anisotropy with sample rotation. Transmission analyses showed improved visible and infrared modulation compared to dense films of the same thickness, reinforcing their suitability for smart windows.This work paves the way for thin-film materials with high optical tunability and enhanced performance, potentially applicable in color displays, anti-counterfeiting, optoelectronic chips, and energy-saving smart windows.

La défense aura lieu à la salle Linus Pauling du Bâtiment Mendeleïev