TITLE: Gear-like rolling motion at the nanometer scale

SPEAKER: Dr. Michael R. Falvo

Using an atomic force microscope (AFM) equipped with an advanced user interface designed for manipulation (the nanoManipulator), we have performed sophisticated manipulation studies of nanometer scale objects. We will present results of AFM/LFM manipulation studies of carbon nanotubes on graphite (HOPG). In this system, the contact zone consists of two facing graphene lattices. We find the friction is highly anisotropic depending on the degree of commensurability, and that there are certain orientations where the CNT "locks-in" to atomic registry with the substrate. The lateral force required to move the tube in this "locked-in" (commensurate) state is up to 10 times larger than the out-of-registry state. We also observe that rolling motion occurs only in this lock-in state. We have never observed rolling on any other substrate. We have also performed atomistic computer simulations identifying the energy barriers for sliding and rolling, elucidating atomic-scale features of slip-roll motion and explaining the details of the lateral force data in terms of the intrinsic faceting of multiwall carbon nanotubes. Our experiments and simulations show that interlocking of the atomic lattices in the contact region of two bodies can determine whether the body slides or rolls. In essence, the atomic lattice can act like a gear mechanism.