Better understanding of the concurrent structural and electronic transformations in VO₂ thin films is of utmost importance to enable fine-tuning of the metal-to-insulator transition for functional electronic devices. Despite this, detailed real space structure analysis of the VO₂ films, in particular near the film-substrate interface is lacking. Intermediate (transitional) structures present near the interface have also remained elusive due to the lack of atomically-resolved quantitative analysis. In this work, we resolve both V and O atomic columns and elucidate the phase coexistence of the strain-stabilized intermediate monoclinic (M2) phase nanolayer (~2nm) in epitaxial VO₂ films grown on TiO₂(001) substrate using pulsed laser deposition. We also unfold the crucial role of simultaneous imaging of the spatial configurations of the V and O atoms using atomic-resolution scanning transmission electron microscopy (STEM), in particular simultaneous recording high-angle annular dark field (HAADF) and integrated differential phase contrast (iDPC) images. The strength of iDPC we recently demonstrated by resolving for the first time hydrogen atoms at a metal – metal hydride interface [1] and in-situ imaging of reversible oxygen migration in a cross-section of a complex oxides electronic device [2]. Also the concept of ellipticity of the projected atomic columns during analysis of the images plays a key role in revealing different phases in VO₂. Our approach to analyze nanoscale phase coexistence in VO₂ sets a pathway to study and unravel subtle structural transitions in a wide range of correlated oxides, and thus offers substantial implications for the field of device electronics.

References

[1] S. de Graaf et al., Sci. Adv., 2020, 6 eaay4312.

[2] P. Nukala et al., Science 372, 2021, 630–635.