Epitaxial stabilization and phase instability of VO 2 polymorphs

The VO 2 polymorphs, i.e., VO 2 (A), VO 2 (B), VO 2 (M1) and VO 2 (R), have a wide spectrum of functionalities useful for many potential applications in information and energy technologies. However, synthesis of phase pure materials, especially in thin film forms, has been a challenging task due to the fact that the VO 2 polymorphs are closely related to each other in a thermodynamic framework. Here, we report epitaxial stabilization of the VO 2 polymorphs to synthesize high quality single crystalline thin films and study the phase stability of these metastable materials. We selectively deposit all the phases on various perovskite substrates with different crystallographic orientations. By investigating the phase instability, phonon modes and transport behaviours, not only do we find distinctively contrasting physical properties of the VO 2 polymorphs, but that the polymorphs can be on the verge of phase transitions when heated as low as ∼400 °C. Our successful epitaxy of both VO 2 (A) and VO 2 (B) phases, which are rarely studied due to the lack of phase pure materials, will open the door to the fundamental studies of VO 2 polymorphs for potential applications in advanced electronic and energy devices.