Growing Ultrathin Cu2O Films on Highly Crystalline Cu(111): A Closer Inspection from Microscopy and Theory

Cu2O has been investigated for decades to understand the complex nature of oxidation and to utilize its high catalytic activity and intrinsic p-type character. However, the structures and intrinsic defects of Cu2O(111) surfaces have not been fully explored at the atomistic level, which is required to clarify some issues such as termination of Cu2O(111) surfaces. Here, our combined scanning tunneling microscopy (STM) and density functional theory (DFT) studies show that Cu2O(111) has a stoichiometric surface where the coordinately unsaturated Cu atoms appear with a hexagonal lattice. DFT simulations reflecting the orbital contributions of the STM tip present a good agreement with STM topography, unveiling the fine structures of Cu2O(111) surfaces that arise from coordinately saturated Cu atoms. Besides the possibility of kinetically formed oxygen vacancies reported in a previous work, two intrinsic defects identified in this work as a Cu vacancy (VCu) and Cu adatoms commonly exist on Cu2O(111) surfaces. Intriguingly, direct experimental evidence indicates that VCu plays the role of a hole provider in Cu2O. The topographic contrast of VCu is inverted by reversing the polarity of the sample bias, and VCu also exhibits strongly enhanced dI/dV spectrum at negative bias. These results imply that VCu is negatively charged due to its acceptor character. We expect that our observations will provide important information to establish an in-depth understanding of the fundamental properties of Cu2O.