Coffee beans spilled upon a table form no pattern. Instead, their distribution is dictated by the laws of chance. The same was generally believed true of atoms deposited upon a substrate, until now. Researchers at the Department of Energy�s Sandia National Laboratories report that they have imaged deposited atoms forming orderly, controllable 2-D nanopatterns.
The work, which was described in the August 30 issue of Nature, produced real-time video of atoms self-arranging themselves in the manner long predicted by a variety of theorists, but contrary to ordinary intuition. According to Sandia physicist Norm Bartelt, the ordered domain patterns that form spontaneously in a wide variety of chemical and physical systems as a result of competing interatomic interactions could serve as templates for fabricating nanostructures. It also may become possible to tailor characteristics for devices like photonic lattices.
In the Nature article, Bartelt and colleagues describe a new self-assembling domain pattern on a solid surface with two surface structures. One is a disordered lead/copper surface alloy produced by deposition of lead vapture onto copper. This is followed by deposition of a lead-overlayer phase. The scientists produced a sequence of low-energy electron-microscope images that show the overlayer growing over the surface alloy structure during lead deposition. Bartelt and colleagues report that the most striking feature of the sequence is the evolution of a pattern in the overlayer matrix from circular islands to stripes and then circular holes.
Observation of the real-time assembly process, along with control over physical factors that influence that process, may offer a means of finding out far more about the conditions under which atoms self-assemble than any theory could predict. As a result, this could help researchers understand how to influence that assembly into more desirable structures.
“There are many control knobs we can turn to create new patterns,” says Bartelt. Among them are temperature and material composition.
“The work – which to our knowledge is the first unambiguous observation of the expected sequence of domain patterns – can help us understand the new physics that manifests itself at these small length scales,” says Sandia project lead Gary Kellogg. “New materials with highly specialized properties necessary to meet defense and consumer needs can be fabricated only by tailoring the structure of the material on the nanometer scale. This work provides insight into how nature does this, and how humans can do the same.”
Sandia researchers were able to record real-time, real-space images using a LEEM that show exactly how the nanostructures are generated, self-assemble, and transform. “The close agreement between experiment and theory allows us to probe the key inter-atomic force parameters involved in the process,” says Kellogg.
Theorists long have believed that competing attractive and repulsive inter-atomic interactions can lead to the spontaneous formation of ordered patterns in widely varying chemical and physical systems. Potentially, such patterns could be used as templates for nanostructure fabrications. “There are precedents for people using these patterns for further growth of quantum dots," says Bartelt. “They can be the starting point of controllable patterns that extend into three dimensions.”
Though models have clearly predicted the possibility of controlling any pattern's geometry and order, depending on temperature and amount of secondary metal introduced, experimental verification of these models had remained elusive till now.
For self-assembled structures to become useful as nanostructure templates, the Sandia research team, which included Julie Last and Richard Plass in addition to Bartelt and Kellogg, reports that the two-dimensional self-assembled patterns must be both stable at room temperature and resistant to air exposure – conditions which are met with their lead/copper structure.