The self-organization of lead on silicon stands out for its remarkable efficiency and surprising new results suggest why. Most atoms sitting on surfaces like to go about their business by themselves. Alone they walk in random directions. Rarely do they move together, so when a billion atoms collectively decide to move 0.05 mm within 1 second below room temperature, it is exceptional. Researchers have found evidence of this â€˜superdiffusionâ€™ for lead on silicon using a technique known as low energy electron microscopy. The mass transport mechanism involves layers of lead atoms sliding across the surface all at once; the motion is believed to involve the correlated, instantaneous movements of single atoms. This superdiffusion mechanism is orders of magnitude faster than classical diffusion. Atomic and molecular diffusion on solid surfaces is critical to many physical and chemical phenomena including catalysis, surface supported nanoclusters and the formation of patterned structures. The discovery of this new mechanism provides important insight for designing other self-organizing systems.
K. L. Man, M. C. Tringides, M. M. T. Loy, and M. S. Altman, â€œSuperdiffusive Motion of the Pb Wetting Layer on the Si(111) Surfaceâ€, Physical Review Letters, 2013, 110, 036104. http://dx.doi.org/10.1103/PhysRevLett.110.036104.