Imaging ofC70intramolecular structures with scanning tunneling microscopy

Adsorption of pure ${\mathrm{C}}_{70}$ and its mixture with ${\mathrm{C}}_{60}$ on the Cu(111)1×1 surface was investigated by using scanning tunneling microscopy. The pure ${\mathrm{C}}_{70}$ monolayer was found to form the 4×4-commensurate superlattice, similar to the case of pure ${\mathrm{C}}_{60}$, after a brief annealing at approximately 320 °C. The individual ${\mathrm{C}}_{70}$ molecules take the upright position with the long axis perpendicular to the surface. Introducing ${\mathrm{C}}_{60}$’s in the ${\mathrm{C}}_{70}$ layer allows some of the ${\mathrm{C}}_{70}$ molecules to take the lying-down position with the long axis parallel to the 〈1¯10〉 direction of the surface. By comparing with theoretical simulations, we propose that ${\mathrm{C}}_{70}$ molecules in the ${\mathrm{C}}_{70}$-${\mathrm{C}}_{60}$ mixture cease their internal rotation and are ratcheted with one of the hexagonal rings around the equator facing down the Cu(111) surface.