Wet‐Etching Induced Abnormal Phase Transition in Highly Strained VO 2 /TiO 2 (001) Epitaxial Film

The metal–insulator transition (MIT) behavior in vanadium dioxide (VO 2 ) epitaxial film is known to be dramatically affected by interfacial stress due to lattice mismatching. For the VO 2 /TiO 2 (001) system, there exists a considerable strain in ultra‐thin VO 2 thin film, which shows a lower T c value close to room temperature. As the VO 2 epitaxial film grows thicker layer‐by‐layer along the “bottom‐up” route, the strain will be gradually relaxed and T c will increase as well, until the MIT behavior becomes the same as that of bulk material with a T c of about 68 °C. Whereas, in this study, we find that the VO 2 /TiO 2 (001) film thinned by “top‐down” wet‐etching shows an abnormal variation in MIT, which accompanies the potential relaxation of film strain with thinning. It is observed that even when the strained VO 2 film is etched up to several nanometers, the MIT persists, and T c will increase up to that of bulk material, showing the trend to a stress‐free ultra‐thin VO 2 film. The current findings demonstrate a facial chemical‐etching way to change interfacial strain and modulate the phase transition behavior of ultrathinVO 2 films, which can also be applied to other strained oxide films.