| The majority of these lead/silicon islands are seven layers high. |
That's why researchers at the DOE's Ames Laboratory are studying the processes that control how custom-made nanoscale materials grow. Exact control of layer thickness and atomic uniformity of thin films and nanostructures is considered the Holy Grail in nanotechnology, says Michael Tringides, an Ames physicist.
Deposited metal atoms usually stack up in islands of widely varying heights. But lead atoms grown on silicon at about -126°F seem to be "intelligent" and choose only one height "related to their electronic structure," explains Tringides. "Keeping electrons confined in small metal islands requires them to occupy sharp energy levels as dictated by the laws of quantum mechanics." Confinement implies that total electron energy depends strongly on nanostructure size or shape. The relationship is called Quantum Size Effects, or QSE. "A consequence is that certain film thicknesses are more stable than others," says Tringides.
Tringides explains that although QSE is the driving force for height uniformity, the islands need the right temperature and surface coverage to form. At higher temperatures, the islands evolve into multiheight mounds, which limits their potential for room-temperature applications. But researchers discovered oxygen can restrict the upward motion of the lead atoms. This extends stability to higher temperatures and makes the resulting structures more amenable to practical applications.