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.