Adsorption and motion of C(60) molecules on the Pb-covered Si(111) surface

Adsorption and diffusion of C(60) molecules at a monolayer Pb-covered Si(111) surface is studied with a scanning tunneling microscope (STM). At room temperature, C(60) molecules stably adsorb with two configurations: one with a hexagonal ring and the other with a pentagonal ring bonded to the Si(111) substrate. We can manipulate the molecules out of the stable states with a STM tip by decreasing the tunneling gap. The molecules would then randomly diffuse at the surface and momentarily stop at some metastable states. Eventually, they settle down at a stable state and become immobile again. Through monitoring the change in the intramolecular features during the entire motion processes, we believe that random thermal rolling may occur during the diffusion process of C(60) molecules at the surface. A possible schematic potential diagram for the diffusion of this spherelike molecule at the Pb-covered surface is proposed.