# Interface-induced collective phase transition in VO2-based bilayers studied by layer selective spectroscopy

## TL;DR

This study explores how the interface between two VO2 layers triggers a collective phase transition, using advanced spectroscopy techniques to observe changes in electronic and crystal structures.

## Contribution

The paper introduces layer-selective spectroscopy to reveal interface-induced phase transitions in VO2 bilayers, showing a collective behavior driven by interfacial energy balance.

## Key findings

- The monoclinic insulating VO2 layer transitions to a rutile metallic phase at the heterointerface.
- Temperature decreases cause the rutile metallic phase to revert to the monoclinic insulating phase.
- Phase transitions in the VO2 layer show in-plane phase separation of metallic and insulating regions.

## Abstract

We investigated the origin of collective electronic phase transitions induced at the heterointerface between monoclinic insulating VO2 and rutile metallic electron-doped VO2 layers using in situ soft X-ray photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) on nanoscale VO2/V0.99W0.01O2 (001)R bilayers. Thanks to the surface sensitivity of PES and XAS, we determined the changes in the electronic structure and V–V dimerization in each constituent layer separately. The layer selective observation of the electronic and crystal structures in the upper VO2 layer of the bilayer indicates that the monoclinic insulating phase VO2 layer undergoes a transition to the rutile metallic phase by forming the heterointerface. Detailed temperature-dependent measurements reveal that the rutile metallic phase VO2 undergoes a transition to the monoclinic insulating phase with a decrease in temperature, as in the case of a VO2 single-layer film. Furthermore, during the temperature-induced phase transition in the VO2 layer, the spectra are well described by an in-plane phase separation of the rutile metallic and monoclinic insulating phases. These results suggest that the interface-induced transition from the monoclinic insulating to the rutile metallic phase in the VO2 layer of bilayers occurs as a collective phase transition derived from the static energy balance between the interfacial energy and the bulk free energies of the constituent layers.

The online version contains supplementary material available at 10.1038/s41598-025-20752-w.