Number 152 (Story #2), November 19, 1993 by Phillip F. Schewe and Ben Stein THE DIFFUSION-CONTROLLED AGGREGATION of nanostructures on a substrate is directed by a hierarchy of energy barriers. For example, adsorbed atoms on a surface need more energy to climb from one terrace of atoms to another than to simply move across the a terrace surface. Moving along the edge of a terrace requires even more energy; departing from an island of terraces altogether requires more energy still. Klaus Kern at the Institute for Experimental Physics in Lausanne, Switzerland has studied the effect of atom mobility on the nucleation and growth of several types of atom clusters. He finds that in this world of atom archipelagos temperature (at least within a certain range) is destiny: different patterns of self-organization emerge depending on such environmental factors as temperature or the flux of new atoms onto the substrate. Horia Metiu of UC Santa Barbara compares this aggregation process to the activity at a construction site: the seemingly random motions of atoms, subject to a number of "building codes," do eventually arrive at a condition of geometrical order, whether in the form of one- dimensional strands or two-dimensional islands. (Holger Roder et al., Nature, 11 Nov. 1993.)