Anisotropic Diffusion Barrier of Benzene on Cu(111) Measured by Single-Molecule Manipulation

Anisotropic surface diffusion of atoms and molecules originates from the underlying crystallographic symmetry and the presence of step edges. However, accurate experimental determination of diffusion barriers remains challenging due to intrinsic limitations of existing techniques. Here, we employ atomic force microscopy-based single-molecule manipulation combined with lateral force spectroscopy to investigate the diffusion of benzene on Cu(111). By quantifying the threshold forces required to induce controlled molecular motion, we resolve pronounced anisotropy in diffusion barriers on atomically flat terraces, identifying the close-packed direction ([110]) as the energetically preferred pathway. Near step edges, we find that lateral displacement along the lower step edge requires significantly higher forces than other in-plane directions, which we attribute to steric constraints imposed by the step geometry. The use of a Cl-terminated tip suppresses short-range chemical interactions with the adsorbate, minimizing perturbation of the potential energy landscape and enabling reliable extraction of diffusion barriers. These results provide experimentally constrained insights into anisotropic molecular diffusion on crystalline surfaces and establish improved benchmarks for molecular dynamics simulations and thin-film growth modeling.