Palladium diffusion into bulk copper via the (100) surface.

Using low-energy electron microscopy, we measure the diffusion of Pd into bulk Cu at the Cu(100) surface. Interdiffusion is tracked by measuring the dissolution of the Cu(100)-c(2 × 2)-Pd surface alloy during annealing (T>240 °C). The activation barrier for Pd diffusion from the surface alloy into the bulk is determined to be (1.8 ± 0.6) eV. During annealing, we observe the growth of a new layer of Cu near step edges. Under this new Cu layer, dilute Pd remaining near the surface develops a layered structure similar to the Cu(3)Pd L 1(2) bulk alloy phase.

How Pb-overlayer islands move fast enough to self-assemble on Pb-Cu surface alloys.

Low-energy electron microscopy reveals that two-dimensional, approximately 50 000 atom, Pb-overlayer and vacancy islands both have diffusion coefficients of 25.6+/-0.8 nm2/sec at 400 degrees C on Pb-Cu surface alloys. This high mobility, key to self-assembly in this system, results from the fast transport of Pb atoms on the surface alloy and of Cu through the Pb overlayer. A high Pb vacancy concentration, predicted by ab initio calculations, facilitates the latter.

Origins of nanoscale heterogeneity in ultrathin films.

A key challenge in thin-film growth is controlling structure and composition at the atomic scale. We have used spatially resolved electron scattering to measure how the three-dimensional composition profile of an alloy film evolves with time at the nanometer length scale. We show that heterogeneity during the growth of Pd on Cu(001) arises naturally from a generic step-overgrowth mechanism relevant in many growth systems.

Reversible shape transition of pb islands on Cu(111).

Using low-energy electron microscopy, we have observed a reversible transition in the shape of Pb adatom and vacancy islands on Cu(111). With increasing temperature, circular islands become elongated in one direction. In previous work we have shown that surface stress domain patterns are observed in this system with a characteristic feature size which decreases with increasing temperature. We show that the island shape transition occurs when the ratio of the island size to this characteristic feature size reaches a particular value. The observed critical ratio matches the value expected from stress domains.

Thermal motion and energetics of self-assembled domain structures: Pb on Cu(111).

Low energy electron microscope measurements of the thermal motion of 50-200 nm diameter Pb islands on Cu(111) are used to establish the nature and determine the strength of interactions that give rise to self-assembly in this two-dimensional, two-phase system. The results show that self-assembled patterns arise from a temperature-independent surface stress difference of approximately 1.2 N/m between the two phases. With increasing Pb coverage, the domain patterns evolve in a manner consistent with models based on dipolar repulsions caused by elastic interactions due to a surface stress difference.

Nanostructures. Self-assembled domain patterns.

The ordered domain patterns that form spontaneously in a wide variety of chemical and physical systems as a result of competing interatomic interactions can be used as templates for fabricating nanostructures. Here we describe a new self-assembling domain pattern on a solid surface that involves two surface structures of lead on copper. The evolution of the system agrees with theoretical predictions, enabling us to probe the interatomic force parameters that are crucial to the process.