Discovery of nanoscale reduced surfaces and interfaces in VO2 thin films as a unique case of prewetting

VO2 thin films grown on SiOx/Si substrates have been characterized at the sub-nanometer level by Cs-corrected scanning transmission electron microscopy along with electron energy loss spectroscopy. Reduced transitional regions of 2–3 nm thick were found at both the surface and the interface, where the vanadium valence progressively changes from +4 to +2. The formation of these nanometer-thick surficial and interfacial layers can be interpreted as a unique case of prewetting, and it explains the degradation of metal-to-insulator transition properties in VO2 thin films.