Structure and shape of epitaxial metal clusters

The structure of small gold particles produced by evaporation on alkali-halide surfaces is studied using ultra-high resolution electron microscopy. During growth two main types of particles are observed; five-fold (icosahedral and decahedral) and regular fcc structures. Weak beam and ultra-high resolution projected potential images are obtained. The different shapes of the particles are thus disclosed. For fcc triangular, rhombic, pyramidal, and irregular shapes are found. Evidence towards the transition from icosahedron to fcc cubo-octahedron occurring possibly through an intermediate spherical state is presented. Ultra-high resolution projected potential images of icosahedrons show that the interatomic distance along a radial direction does not change substantially. Particle distortions predicted by energy calculations based on a pair-wise Lennard-Jones potential are not observed. Also, no dislocations were found in the tetrahedra interface, for particle sizes below ∼300 Å.