Using a scanning transmission electron microscope it is possible, by directly imaging single atoms, to investigate the motion and distribution of heavy (high-Z) atoms deposited on the surfaces of thin low-Z films. In this study, the heavy atoms U, Pt, Au, In, Cd, Ag, and Pd were deposited on 15-Å-thick amorphous carbon substrates. It is found that adatom motion is strongly dependent upon local substrate structure and adatom species. Diffusion coefficients and activation energies for single atoms, as well as dimers, trimers, etc., have been obtained by directly measuring the motion of the atoms. Pair-density and nearest-neighbor distributions were measured and used to derive the interatomic potential and the rate of adatom nucleation. It is found that the interatomic potential is very weak and long ranged
