Phase transition driven discontinuity in thermodynamic size selection.

We show how an order-disorder phase transition in a two-dimensional system can discontinuously alter the shape and size of stress-stabilized self-assembled nanostructures. Low energy electron microscopy was used to study the dealloying of the Cu(111)-sqrt[3]×sqrt[3]-R30°-Bi surface alloy. The gradual expulsion of embedded bismuth from the alloy with increasing temperature induces a hard-hexagon-type order-disorder transition in the surface alloy. Our low energy electron microscopy results demonstrate how the loss of long-range order induces enormous changes in the domain patterns that the alloy forms with a Bi overlayer phase. We propose that the occurrence of phase transitions in one of the two surface phases that constitute a self-assembled domain pattern, provides a general, largely unexplored, mechanism that can be used to influence the morphological details of two-dimensional nanostructures.