The tetragonal-like to rutile structural phase transition in epitaxial VO₂/TiO₂ (001) thick films

A controllable metal–insulator transition (MIT) of VO₂ has been highly desired due to its huge potential applications in memory storage, smart windows or optical switching devices. Recently, interfacial strain engineering has been recognized as an effective approach to tuning the MIT of epitaxial VO₂ films. However, the strain-involved structural evolution during the MIT process is still not clear, which prevents comprehensively understanding and utilizing interfacial strain engineering in VO₂ films. In this work, we have systematically studied the epitaxial VO₂ thick films grown on TiO₂ (001) single crystal substrate and the structural transition at the boundary of MIT region. By using in situ temperature-dependent high-resolution x-ray diffractions, a tetragonal-like ('T-like') to 'rutile' structural phase transition is identified during the MIT process. The room-temperature crystal phase of epitaxial VO₂/TiO₂(001) thick film is clarified to be tetragonal-like, neither strained-rutile phase nor monoclinic phase. The calculated atomic structure of this T-like phase VO₂ resembles that of the M1 phase VO₂, which has been verified by their similar Raman spectra. More, the crystal lattices of the coexisted phases in the MIT region were revealed in detail. The current findings will not only show some clues on the MIT mechanism study from the structural point of view, but also favor the interface engineering assisted VO₂-based devices and applications in the future. (paper)