Direct Observation of the Valence Band Edge by in Situ ECSTM-ECTS in p-Type Cu₂O Layers Prepared by Copper Anodization

Abstract

Polycrystalline Cu2O layers have been selectively grown by electrochemical anodization of polycrystalline Cu electrodes in an alkaline medium (pH 12.85). Uniform layers with thicknesses around 100 nm have been obtained. Using electrochemical impedance spectroscopy, it was concluded that the Cu2O films behave as a p-type semiconductor. The Mott−Schottky plot gives a value for the flat band potential of UFB = −255 mV vs silver/silver chloride electrode (SSC), an estimated carrier density NA = 6.1 × 1017 cm−3, and the space charge layer width was calculated to be WSCL = 9 nm at a band bending of 120 mV. The electronic structure of the Cu|Cu2O|electrolyte interface was for the first time probed by in situ electrochemical tunneling spectroscopy. The use of in situ electrochemical scanning tunneling microscopy allows us to directly observed the valence band edge and determine its position against the absolute energy scale to be EVB = −4.9 eV. Finally, we constructed a quantitative electronic diagram of the Cu|Cu2O|electrolyte interface, where the positions of the valence and conduction band edges are depicted, as well as the edge of the previously reported electronic subband.