Field-induced cation migration in Cu oxide films by in situ scanning tunneling microscopy

We observed the formation of Cu metallic nanoscale structures of ∼20-nm diameter and ∼2-nm height on a Cu2O covered polycrystalline Cu film under an applied field using a scanning tunneling microscope tip in a high vacuum condition. We interpreted the results as the Cu cation transport through the copper oxide film towards the surface when a positive biased voltage (>1.5 V) was applied to the film to lower the activation energy of the cation migration. Scanning tunneling spectroscopy measurements showed that the field-induced nanostructures were pure metallic Cu with a characteristic broad peak near −0.45 eV. No structural change was observed when a negative bias was applied to the film.