Structure Identification of CO Monolayer on Ag(111) Using Atomic Force Microscopy

Local structure analysis in physically adsorbed small molecule systems on metal surfaces remains challenging. The structural models of monolayers formed by weakly adsorbed CO molecules on Ag(111) surfaces have long been controversial. In this study, the structure of the CO monolayer is determined through high-resolution atomic force microscopy (AFM) observations at 4.5 K. Contrary to a previously proposed model based on scanning tunneling microscopy experiments [Phys. Rev. B 71, 153405 (2005)], it is found that the CO monolayer adopts a close-packed structure. Additionally, a superstructure associated with higher-order commensurate between the $\sqrt{31}\times \sqrt{31}$ lattice of Ag(111) and the 4 × 4 lattice of CO is identified. A structural model, involving the tilt of the CO molecular axis, is proposed based on AFM observations and density functional theory (DFT) calculations. Thermal fluctuations of the CO molecules are also observed, and the energy barrier derived from the hopping rate aligns with estimates from DFT calculations. These results indicate that AFM is powerful for atomic-level analysis of physisorption systems.