Emergence of Two-Dimensional F-Induced Kagome Lattice on the Cu(111) Surface

We report the first observation of well-ordered fluorine monolayers (MLs) on the Cu(111) surface. Fluorine atoms were adsorbed on the copper surface through the controlled decomposition of fluorinated fullerene (C60F18) molecules, which were previously deposited on the substrate at submonolayer coverage. The desorption of fluorine atoms from the C60F18 fullerene molecules and their adsorption on the Cu surfaces were evidenced by X-ray photoelectron spectroscopy experiments. Analysis of the core-level shifts indicates a strong F–Cu interaction, but the formation of copper fluoride (CuF2) can be ruled out. Two types of ordered fluorine structures were observed in scanning tunneling microscopy measurements. We found a kagome lattice with 5 × 5 cell with a fluorine coverage of 0.48 ML and a distorted, kagome-like structure with a 7 × 7 cell with a fluorine coverage of 0.49 ML, where some of the 6 rings of the ideal kagome lattice become 5 or 7 rings. The two structures coexist at temperatures up to 200 °C. Density functional theory calculations show that the observed structures are stable and that the F atoms occupy face-centered cubic (fcc)-hollow (the fcc-hollow site is situated in the gap between three atoms in the upper layer with no atom in the layer below), hexagonal close-packed (hcp)-hollow site (which is also located in the gap between three atoms, but with the atom in the layer below), and bridge sites. The energy difference between the two structures is very small (11 meV/F atom), which may explain their coexistence at room temperature. Simpler structures of comparable coverage, in particular a honeycomb lattice with a 2 × 2 cell and a coverage of 0.5 ML, are found to be less stable. The present results provide new insight into the fluorine–copper interfacial interaction, which is of great importance for technological applications such as fluorine ion shuttle batteries.