Phase-field study on tip-force-induced metal–insulator transition and nucleation dynamics in VO2 thin films

Despite significant progress being made in research on the mechanism and control of vanadium dioxide (VO2) phase transition, localized metal-to-insulator transition (MIT) and monoclinic VO2 (M) nucleation evolution in rutile VO2 (R) thin films under the application of tip force remain unclear, particularly on how these processes depend on the film orientation and how the tip force selects M-phase variants. Here, based on phase-field simulations and theoretical analysis, the localized MIT and the domain nucleation evolution behavior in 001R and 110R oriented VO2 thin films under tip-force loading are revealed. Results show that under the conditions of identical film thickness and tip-force application, the 001R-oriented thin films are more prone to inducing localized MIT and typically form a quatrefoil morphology of domain nuclei mixed with M1/M2 phase variants. In contrast, MIT nuclei in 110R-oriented thin films have smaller sizes and typically form an olive-shaped along the 001R axis. Moreover, the occasional formation of antiphase boundaries in the same M-phase variant during MIT is found to significantly affect the morphology of domain nuclei. Our results reflect a strong impact of the film orientation on the tip-force-induced MIT of VO2 thin films, which is essentially due to the distinct eigenstrain characteristics of M-phase variants in the 001R and 110R coordinate systems, which modulate the degeneracy of M-phase variants and their selection during MIT. The work provides new insights into the tip-force-induced MIT and nucleation dynamics in VO2 thin films, and it is instructive for engineering of the VO2 phase transition and domain structure.