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Scanning tunneling microscopy is used to study low angle grain boundaries at the surface of nearly planar copper nanocrystalline (111) films. The presence of grain boundaries and their emergence at the film surface create valleys composed of dissociated edge dislocations and ridges where partial dislocations have recombined. Geometric analysis and simulations confirm valleys and ridges are created by an out-of-plane grain rotation driven by reduction of grain boundary energy. These results indicate that in general it is impossible to form flat twodimensional nanoscale films of copper and other metals exhibiting small stacking fault energies and/or large elastic anisotropy. Main Text:Nanocrystalline metal materials are widely used as electrical contacts and interconnects in ultralarge-scale integrated circuits (1). Technologically important properties of these materials (2-4) are strongly influenced by the presence and density of surfaces, grain boundaries (GBs) and dislocations within them; each studied extensively (5-7). Using transmission electron microscopy (TEM), GBs with high-symmetry tilt axes have been intensively studied at the atomic scale (8), where TEM provides a plan view of atomic columns along the tilt axis of the GB. At the granular length scale, the formation, rotation and coalescence of sub-grains during annealing have also been studied by TEM (7,9). Here, we present the first investigation of nanocrystalline metal films from the multi-grain scale down to the atomic scale using scanning tunneling microscopy (STM). Uniquely, STM can map the local three-dimensional topography of GB intersections at surfaces with picometer precision and is insensitive to the degree of tilt axis misalignment from high symmetry directions that typically hampers TEM analysis of GB structure (10, 11). We identify shifts in the GB tilt axis away from that of the original low angle GB (LAGB) in nanocrystalline copper films. We show this phenomenon is accompanied by GB energy minimization and results in the unavoidable introduction of ridges and valleys into the film.

We study the interaction of single Fe atoms on Cu͑111͒ and Ag͑111͒ substrates with low-temperature scanning tunneling microscopy and kinetic Monte Carlo simulations. In Fe/Cu͑111͒, a self-assembled hexagonal quasisuperlattice with perturbation of around 20% dimers/clusters is obtained. In Fe/Ag͑111͒, however, a disorderlike structure is found even though long-range interactions among atoms are observed. In combination with kinetic Monte Carlo simulations, possible mechanisms of the superstructure formation are discussed. We find that two parameters, i.e., the ratio of adatom interaction energy ͑the depth of the first energy minimum͒ to diffusion barrier and the square of the repulsive ring radius versus the superstructure lattice constant, play important roles for superstructure formation.