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Directly measuring the structural transition pathways of strain-engineered VO2 thin films.

Epitaxial films of vanadium dioxide (VO2) on rutile TiO2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO2 thin films deposited on isostructural MgF2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF2 substrate clearly reveals how elongation of V-V dimers accompanied by the shortening of V-O bonds triggers the intermediate M2 phase in the temperature range between 70-80 °C upon the heating-cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO2 thin films.