Proton doping-induced chromism in VO₂ beyond the conventional metal-insulator transition approach

Chromism, the phenomenon of color change in response to external stimuli, encompasses various mechanisms, including electro-, photo- and thermo-chromism. These phenomena find applications in technologies like smart windows and displays. Transition metal oxides, such as VO₂, are known for chromic applications, generally the metal-insulator transition in VO₂ accompanied by a structural phase transition, making it a potential candidate for smart windows. This study demonstrates a different approach to chromism in VO₂ by proton doping thin films through the Pt catalyst effect. This method significantly alters the crystal structure and tunes the bandgap, resulting in a color change from brown to transparent with enhanced transmittance compared to the conventional metal-insulator transition chromism in VO₂. Hydrogen concentration in the thin film was investigated using elastic recoil detection analysis, revealing insights into the amount of protons. Successful proton doping induces a remarkable transition in crystal structure, generating lattice elastic energy at interstices, triggering micro-level changes. The controllability of this effect holds promise for applications in smart windows with instantaneous color changes and dynamic optical devices.