Atomically resolved phase coexistence in VO2 thin films

TL;DR

This study uses atomic-resolution electron microscopy to directly observe phase coexistence and intermediate monoclinic phases in VO2 thin films, revealing detailed atomic structures including oxygen columns.

Contribution

It provides the first detailed real-space atomic structure analysis of VO2 thin films, including oxygen atoms, and uncovers strain-stabilized intermediate phases.

Findings

Discovery of nanometer-thick M2 phase layers in VO2 films.

Direct imaging of both V and O atomic columns.

Identification of strain-stabilized intermediate phases.

Abstract

Concurrent structural and electronic transformations in VO2 thin films are of twofold importance: enabling fine-tuning of the emergent electrical properties in functional devices, yet creating an intricate interfacial domain structure of transitional phases. Despite the importance of understanding the structure of VO2 thin films, a detailed real space atomic structure analysis in which also the oxygen atomic columns are resolved is lacking. Moreover, intermediate atomic structures have remained elusive due to the lack of robust atomically resolved quantitative analysis. Here, we directly resolve both V and O atomic columns and discover the presence of the strain-stabilized intermediate monoclinic (M2) phase nanolayers (less than 2 nm thick) in epitaxially grown VO2 films on a TiO2 (001) substrate, where the dominant part of VO2 undergoes a transition from the tetragonal (rutile) phase to the monoclinic M1 phase. We unfold the crucial role of imaging the spatial configurations of the oxygen anions (in addition to V cations) utilizing atomic-resolution electron microscopy. Our approach sets a direct pathway to unravel the structural transitions in wide range of correlated oxides, offering substantial implications for e.g. optoelectronics and ferroelectrics.