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Direct Observations of Nanoscale Self-Assembly and Pattern Formation Solid Surfaces
by R. Plass, N. C. Bartelt, and G. L. Kellogg








Motivation—Competing short-range attractive and long-range repulsive interactions can lead to the spontaneous formation of ordered domain patterns in widely varying physical and chemical systems.  The potential use of such patterns as templates for the fabrication of nanostructures has fostered considerable interest in the physics underlying self-assembly.  Theoretical models and computer simulations have predicted the possibility of controlling pattern geometry, long-range order, and feature size by varying the coverage and temperatrue, but, until now, experimental verification of these models has remained elusive. 

AccomplishmentWe have discovered that two phases of Pb atoms on the (111) crystal surface of copper, self-assemble into ordered, nanoscale domain patterns.  Real-time, real-space images recorded using the low energy electron microscope (LEEM) show exactly how the structures are generated, how they self-organize, and how the phases transform. Stable domains, with sizes ranging from 10-100 nanometers, result from a competition between elastic interactions, which favor small domains, and ordinary coarsening processes, which favor large domains.  The pattern type, feature size, and degree of long-range order vary controllably with surface composition and temperature.  Moreover, the continuous evolution of the domain structures from circular islands to stripes to "inverted" islands with increasing lead coverage (shown in Fig. 1 below) agrees quantitatively with previous theoretical descriptions based on competing long- and short-range interactions. Such models have been invoked many times to explain observations of stripe and droplet patterns in a wide range of systems from molecular films on liquid surfaces to two-dimensional magnetic systems, but specific predictions of the models have not, until now, been verified.  Our investigation of the lead/copper system thus provides the first experimental demonstration of the predictive capabilities of the theory and allows us to explore the interactions underlying self-assembly in a well-defined physical system.  From our observations and these models, we have determined the stress difference between the two phases and the free energy of domain boundary formation for the two-phase system.   These results define the parameters required for self assembly on solid surfaces and allow us to predict other systems for which the process should occur.  We also find that the structures, formed at elevated temperatures in ultrahigh vacuum, are stable in air at room temperature suggesting applications as templates for nanostructure fabrication. 

SignificanceThe spontaneous formation of two-dimensional domain structures with controllable shapes and dimensions is an intriguing phenomenon with significant potential applications in patterning nanotechnologies.  Our observations show unambiguously that self-assembled lead structures on the close-packed surface of copper encompass the complete range of phases expected when long- and short-range forces compete.  The system is simple enough to characterize the composition and structure of the component phases in atomic detail and yet displays the same cooperative behavior found in much more complicated magnetic and chemical systems. Examination of the lead/copper system thus allows a rigorous test of existing theories, refinements to the theories, and a quantitative determination of the key force parameters involved in self-assembly processes.

 

Fig. 1  LEEM images of lead/copper surface phases at 400°C showing pattern progression with increasing amount of lead  (2.3 m field of view)
Publications: 
R. Plass, J. A. Last, N. C. Bartelt and G. L. Kellogg, Nature 412, 875 (2001).
R. Plass, N. C. Bartelt, and G. L. Kellogg, J. Phys.: Cond. Matter 14, 4227 (2002).
R. van Gastel, R. Plass, N. C. Bartelt, and G. L. Kellogg. Phys. Rev. Lett. 91, 055503 (2003).
Work supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering
Last modified December 29, 2003
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