  The VO2 polymorphs, i.e., VO2(A), VO2(B), VO2(M1) and VO2(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 VO2 polymorphs are closely related to each other in a
thermodynamic framework. Here, we report epitaxial stabilization of the VO2
polymorphs to synthesize high quality single crystalline thin films and study
the phase stability of those 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 VO2 polymorphs, but that the polymorphs could be on the verge
of phase transitions when heated as low as ~400 oC. Our successful epitaxy of
both VO2(A) and VO2(B) phases, which are rarely studied due to the lack of
phase pure materials, will open the door to the fundamental studies of VO2
polymorphs for potential applications in advanced electronic and energy
devices.
