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

Abstract

Abstract We investigated the origin of collective electronic phase transitions induced at the heterointerface between monoclinic insulating VO 2 and rutile metallic electron-doped VO 2 layers using in situ soft X-ray photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) on nanoscale VO 2 /V 0.99 W 0.01 O 2 (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 VO 2 layer of the bilayer indicates that the monoclinic insulating phase VO 2 layer undergoes a transition to the rutile metallic phase by forming the heterointerface. Detailed temperature-dependent measurements reveal that the rutile metallic phase VO 2 undergoes a transition to the monoclinic insulating phase with a decrease in temperature, as in the case of a VO 2 single-layer film. Furthermore, during the temperature-induced phase transition in the VO 2 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 VO 2 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.