Nature of the Metal Insulator Transition in Ultrathin Epitaxial Vanadium Dioxide

We have combined hard X-ray photoelectron spectroscopy with angular dependent O K-edge and V L-edge X-ray absorption spectroscopy to study the electronic structure of metallic and insulating end point phases in 4.1 nm thick (14 units cells. along the c-axis of VO2) films on TiO2(001) substrates, each displaying an abrupt MIT centered at 300 K with width <20 K and a resistance change of Delta R/R > 10(3). The dimensions, quality of the films, and stoichiometry were confirmed by a combination of scanning transmission electron microscopy with electron energy loss spectroscopy, X-ray spectroscopy, and resistivity measurements. The measured end point phases agree with their bulk counterparts. This clearly shows that, apart from the strain induced change in transition temperature, the underlying mechanism of the MIT for technologically relevant dimensions must be the same as the bulk for this orientation.