Direct Visualization of Nearly Free Electron States Formed by Superatom Molecular Orbitals in a Li@C₆₀ Monolayer

Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we directly determine the spatial and energetic distributions of superatom molecular orbitals (SAMOs) of an Li@C₆₀ monolayer adsorbed on a Cu(111) surface. Utilizing a weakly bonded [Li⁺@C₆₀] NTf₂^- (NTf₂^-: bis(trifluoromethanesulfonyl)imide) salt makes it possible to produce a Li@C₆₀ monolayer with high concentration of Li@C₆₀ molecules. Because of the very uniform adsorption geometry of Li@C₆₀ on Cu(111), the p-SAMO, populated above the upper hemisphere of the molecule, exhibits an isotropic and delocalized nature, with an energy that is significantly lower compared to that of C₆₀. The isotropic overlapping of p-SAMOs in the condensed monolayer of Li@C₆₀ results in a laterally homogeneous STM image contributing to the formation of a free-electron-like states. These findings make an important step toward further basic research and applicative utilization of Li@C₆₀ SAMOs.