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Posted: Dec 19, 2006
Single atom manipulation on a 3-D surface
(Nanowerk Spotlight) In recent years, the manipulation of single atoms and molecules has been a major advance in the application of the scanning tunneling microscope (STM). The main appeal of STM manipulation is the ability to access, control and modify the interactions between the tip and the adsorbate, a few angstroms apart. So far, however, atom manipulation using a STM or an AFM -tip has been restricted to flat surfaces. Manipulation of atoms on a rough terrain requires much more precise control at the atomic scale. Researchers now report extraction and manipulation of individual silver atoms on three dimensional silver nanoclusters. This is the first demonstration that individual atoms can be repeatedly pulled out from a silver cluster on a silver surface using STM tip. It is also the first atom manipulation work done on a 3-D surface. There are still very few research groups that have demonstrated single atom manipulation with atomic scale precision on flat surfaces. This remarkable achievement has an impact on the fundamental understanding of interactions between the matters. While it certainly is not
a commercial production technique, it does further the fundamental understanding of the interaction between atoms, and it is an atom production technique that can be used to extract the atoms for atomistic construction.
Professor Saw-Wai Hla at the Nanoscale & Quantum Phenomena Institute at Ohio University explains to Nanowerk how the atoms can be extracted from the cluster: "We first dip the STM-tip into the single crystal silver surface. This coats the tip with silver. Then by gently touching the tip to the surface at a flat surface area, some of the silver from the tip is transfer to the surface as a cluster."
After achieving a silver cluster on the surface, the researchers take a 3-D STM image of the cluster. Protruding parts of the cluster are chosen as the ideal target zones for atom extraction. To extract the atoms from the cluster, the tip is first approached very closely to the cluster (less than 0.6 angstrom or 0.06 nm distance).
"Just by approaching the tip to a very close proximity of the cluster results in loosening of the top-atom inside the cluster" says Hla. "When we move the tip laterally across the cluster surface, the loose atom follows the tip. Now we have just extracted one atom."
This procedure can be repeatedly performed as can be seen in this movie.
Atom extraction. (a) A three-dimensional STM image of a silver nanocluster deposited by tip-surface contact. The tip is brought close to the protruded part of the cluster and then moved laterally towards a destination on the Ag(111) surface. (b) An STM image acquired after this procedure shows a height reduction of the cluster protrusion and the extracted atom at the final surface destination. (c) The manipulation signal of this event reveals the atomistic details of the atom extraction. High peaks at the left side are caused by removal of the atom from the favorable adsorption sites on the cluster. The smooth single atom periodicity at the right side of the curve is due to the sliding mode manipulation of extracted atom on the flat terrace. (d) The drawings demonstrate the tip climbing up along the contour of top atom inside the cluster (left) and an abrupt decrease in tip height due to the removal of atom from the site (right). (e) A reverse process of (d) where the atom moves under the tip causes an abrupt increase in tip height followed by the tip moving a part of down slope of the atom. (Reprinted with permission from the American Physical Society)
The atomistic details of the atom extraction mechanism are explained by means of statistical analyses and theoretical modeling, which reveals that just by locating the STM-tip at required proximity of the nanocluster greatly reduces the extraction barrier facilitating repeated removal of the top atoms from the cluster.
Even though the atomistic dynamic of atom extraction can be understood from the already established knowledge of manipulation signals, the environment that the extracted atom faces during the process is clearly different from the atom manipulation on a flat surface. In particular, this atom extraction involves pulling out the top atom from a protruding part of a cluster, and then moving it along a rough terrain on a three dimensional cluster surface.
The energy required to extract the atom from the cluster is the energy barrier to move the atom from its original location to the next site within the cluster. In the absence of the tip, the energy barrier for the atom to diffuse over the step edge is 300 meV, which is much higher than the 35 meV barrier for a silver atom diffusion on a flat Ag(111) terrace. These barriers are altered by the presence of the tip as the latter drastically modifies the energy landscape of the system. The variation of the tip height has a dramatic effect on the potential energy of the cluster and of the extracted atom. It appears that the location of the tip in close proximity of the cluster is sufficient to extract the top-atom by overcoming the binding of the atoms within the cluster.
Hla sees this work as just the beginning: "We are continuing our investigations at atomic level interactions to get a deeper understanding on how atoms bind to form matters."
A paper on these findings, authored by Hla and collaborators at Kansas State University and the University of Central Florida, titled "Atom-by-atom extraction using scanning tunneling microscope tip-cluster interaction" will be published shortly in Physics Review Letters.