Room-Temperature Repositioning of Individual C(60) Molecules at Cu Steps: Operation of a Molecular Counting Device

C(60) molecules absorbed on a monoatomic Cu step have been reversibly repositioned at room temperature with the tip of a scanning tunneling microscope by performing controlled displacements along the step direction. We demonstrate the feasibility of building an abacus on the nanometer scale using single molecules as ``counters'', Cu monoatomic steps as ``rods'' that constrain the molecular motion to one dimension, and the scanning tunneling microscope as an ``actuator'' for counting operations.