While it may not make much of an anniversary present, a gold chain built atom by atom by UC Irvine physicist Wilson Ho offers an answer to one of the basic questions of nanotechnology—how small can you go?

In the first study of its kind, Ho and his colleagues have discovered the molecular phase when a cluster of atoms develops into a solid structure, a finding that can have a significant impact in the future development of metal structures built at the molecular scale. The study—which appears on the Science Express website, a service of Science magazine—also suggests a limit on the tiniest size that electrically conductive molecules can be constructed, and it presents a new method for researchers to build and examine these structures.

"This research answers fundamental questions on how solids form from an assembly of single atoms," said Ho, the Donald Bren Professor of Physics & Astronomy and Chemistry. "It allows us for the first time to see matter form in its smallest unit, and it can have important implications for the construction of metallic nanostructures that can be used in catalysis, electronic circuits and data storage."

Ho, working with fellow UCI researchers Niklas Nilius and T. Mitch Wallis, employed a scanning tunneling microscope to build a chain of gold atoms in order to measure how electron states change as more atoms are added to the chain. Starting with a single atom and adding one at a time, the researchers succeeded in measuring the electrical conductivity in these states as the atoms shared electrons, and these measurements varied dramatically as atoms were added to the chain. The scanning tunneling microscope enabled the researchers not only to manipulate individual atoms but also to capture images of the chain and measure its properties. As a result, the researchers were able to obtain a clear connection between the geometry of the fabricated nanostructure and its electronic properties.

As the researchers added the fifth and sixth atoms, the chain began to exhibit the collective properties of a bulk structure, which is when atoms in a molecule lose their individual characteristics and assume those of the overall structure. It is at this point when a metallic molecule becomes conductive and can be used as an electrical conduit.

Ultimately, the gold chain reached 20 atoms long, although in principle there is no limit to how long a chain can be built. In measurements taken as the chain grew from six atoms to 20, the states for the electrons showed only small variations and had practically converged to show properties typical of solids with a larger number of atoms. Ho said that the consistency of these latter measurements further support the concept that a functional gold structure can be built with as few as six atoms.

"What these experiments provide is a new way to study the electronic properties of materials at a nanoscale," Ho said. "We have been able to build a gold bulk structure with six atoms, but in a larger scale, we are starting to answer the question of how many atoms are needed to build a material that has potential utility.

"While it is not practical to mass produce these chains as one-dimensional conductors, catalysts or data storage devices, these studies provide a scientific basis for future nanotechnology," he added. "The results from this research contribute to our understanding of the behavior of matter as a function of its size."

In further research, Ho and his colleagues are studying the electronic properties both of gold atoms constructed in a two-dimensional array and of a chain of silver atoms. Ho used gold in this study because of its unique electronic properties that can be readily observed and controlled through the use of a scanning tunneling microscope. By extending the present study to include other types of atoms, it would be possible to understand a wide range of materials such as alloys, magnets and catalysts at the nanoscale.

Nilius is a UCI postdoctoral researcher, and Wallis is a graduate student on leave-of-absence from Cornell University. The National Science Foundation funded the research.