Identifying the Structure of Two-Dimensional ACuO (A = Na, K, Cs) Film on Cu(111) with Atomic Resolution.

The deposition of alkali metals on oxide surfaces has garnered significant interest due to their critical role in enhancing various catalytic processes. However, the atomic-scale characterization of these structures remains elusive, owing to the complex and competing interactions among the oxygen, the alkali metals, and the metal atoms within the oxides. In this work, we grew alkali metals (Na, K, Cs) on the copper oxide films on the Cu(111) surface and found the formation of hexagonally ordered monolayer films. Via noncontact atomic force microscopy (nc-AFM), we could directly identify the positions of alkali metal cations and the chemical structures of CuO building block in the hexagonal superstructure. In combination with density functional theory (DFT) calculations and AFM simulations, we demonstrated that the alkali metal cations (Na, K, Cs) are chemically bonded with the oxygens in the copper oxides, forming an ACuO (A = Na, K, Cs) monolayer compound on the Cu(111) surface. Scanning tunneling spectroscopy (STS) measurement presents the increase of the density of states beyond zero bias (Fermi level, ) and onset of conduction band at 0.5 eV. In addition, the alkali metal modified copper oxide film shows a lower work function (∼3.5 eV), which is quantitively assessed through field emission resonance (FER) and further confirmed by measuring the contact potential difference and I(z) curves. These electronic properties of the ACuO ternary compound indicate the high chemical activity, which facilitates the adsorption of CO molecules with the oxygen binding with the alkali metal cations. These findings clarify the geometric and electronic structure of alkali metal modified copper oxide films and will contribute to unraveling its promoting reaction mechanism in heterogeneous catalysis at the molecular level.