Epitaxial stabilization and phase instability of VO2 polymorphs.

Shinbuhm Lee,Jong K Keum,Ho Nyung Lee,Ilia N Ivanov

doi:10.1038/srep19621

Abstract

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 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 VO2 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 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.

Highlights

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
By investigating the phase instability, phonon modes and transport behaviours, do we find distinctively contrasting physical properties of the VO2 polymorphs, but that the polymorphs can be on the verge of phase transitions when heated as low as ~400 °C
It has been shown that the VO2(A) and VO2(B) phases are metastable in bulk and undergo an irreversible phase change into VO2(R) upon heating10,12,16, resulting in a mixture of VO2 polymorphs

Summary

Results and Discussion

In order to selectively grow VO2 polymorphs, commercially-available perovskite-oxide substrates, including TbScO3 (TSO), SrTiO3 (STO), (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT), LaAlO3 (LAO) and YAlO3 (YAO), were used. As explained above, the VO2 polymorphs revealed a wide range of electronic ground states, i.e., metal [VO2(R)], semiconductor [VO2(B)] and insulator [VO2(A) and VO2(M1)], depending on their crystal structure This wide range of electronic ground states makes VO2 highly attractive over other transition metal dioxides, since most other binary oxides are either metal (CrO2: α -phase and β -phase) or insulator (TiO2: rutile, brookite and anatase). Since the VO2 polymorphs have a wide range of electronic ground states from metal [VO2(R)] and semiconductor [VO2(B)] to insulator [VO2(A) and VO2(M1)], our epitaxial thin films, which are known to be challenging to grow, will expedite our understanding of underlying physics and developing VO2 polymorphs-based electronic devices utilizing the wide selection of the electronic properties from a single composition

Methods

Author Contributions

Additional Information