A novel combination of quantitative real-time microscopy measurements and atomistic modeling helps solve the problem of manipulating structures at the nanoscale.
The ability to work in the nanometer range offers the prospect of continuing the drive to miniaturization in electronics and designing novel devices with specific properties that rely on very small dimensions.
Two Sandia researchers used a tool called scanning tunneling microscopy (STM) to understand “self organization.” Thin films and solid surfaces are often observed to spontaneously “self-organize” into ordered patterns at nanometer to micron length scales. Such patterns are often too large to be manufactured by chemical synthesis and too small to be accessible to existing microtechnologies, based on photolithography.
Understanding the physics of nanoscale self-organized patterns is challenging, because it is difficult to manipulate them in any controlled way. This difficulty is compounded by the complexity of the involved nanometer-scale forces.
To surmount these difficulties, Sandia researchers Konrad Thürmer and Norm Bartelt combined quantitative real-time microscopy measurements and atomistic modeling to understand the physics needed to manipulate nanoscale structures.
The two first quantitatively analyzed the naturally occurring thermal vibrations in these structures using variable-temperature, atomic-resolution scanning tunneling microscopy. They chose a system simple enough to allow a precise determination of its atomic structure — a triangular array of vacancy islands, or holes, in a one-atom-thick monolayer silver film that develops spontaneously when a monolayer of silver grown on a ruthenium crystal is exposed to a tiny amount of sulfur.
They found that although the holes wander about their respective average positions, the arrays are extremely stable. Combining the real-time STM measurements with atomistic modeling, the researchers were able to determine the origin of this stabilizing force.
The thermal hole vibrations observed by STM were inconsistent with the conventional explanation for the surface ordering, which involved elastic distortions of the substrate. In a search for an alternative ordering mechanism, the researchers used atomically resolved STM images.
The hole-hole interactions originated from dislocations, which often formed to help accommodate mismatches between the lattice constants of the film and the substrate. The tendency of the dislocations to run along specific crystallographic directions connecting the holes, created stabilizing interactions, the researchers noted.
Since dislocations are extremely common in crystalline solids, scientists expect to discover other systems where self-organization is accomplished through similar mechanisms. The value of such ordered nanostructures could be greatly enhanced if scientists learn to tailor their properties.
In the case of the studied vacancy island lattice, the possibility of achieving this tailoring exists, because control of the holehole spacing can be obtained by adjusting the composition of the film.
For more information:
Konrad Thürmer, Ph.D., 925-294-4564, kthurme@sandia.gov
Norm Bartelt, Ph.D., 925-294-3061, bartelt@sandia.gov