Shape and decay of two- and three-dimensional islands on Au(1 1 0)

We observe an extraordinary equilibrium shape, almond-like, of two-dimensional adatom and vacancy islands on the Au(1 1 0)-surface: each island contains two smoothly curved steps, joined at two sharp corners. Two different types of corners can be found: corners where two steps seem to cross and corners where two pairs of steps reach the corner from the same side and end there. We refer to these as crossing and termination sites. We show that combinations of these sites allow the formation of step network structures that accommodate any height variation at low energy cost. These network of steps provide the means to connect the perfectly oriented parts of the Au(1 1 0)-surface, which are at different heights. The precise equilibrium, almond island shape is completely determined by two different step energies. These energies also determine the equilibrium angle, under which the four steps end at a termination site. By measuring this angle at different temperatures we determine the two step energies. Finally, we concentrate on the spontaneous decay of the almond-shaped islands at elevated temperatures. We observe that the disappearance of the top level of a mound is accompanied by an unusual splitting of the underlying terrace into two separate parts. An explanation for this behaviour is found in the structure of the step networks. A quantitative analysis of the decay of islands shows that individual terrace levels on Au(1 1 0) cannot adopt their shapes independently from each other, even when their edges are separated by large distances; they always represent part of a three-dimensional object.