In nanoscience, the ultimate goal is to design better materials and devices by controlling the positions of the atoms, molecules, and molecular clusters on a substrate with exact precision. In a new study, researchers have developed a new method for controlling the movements and positions of clusters of gold atoms on an insulating substrate not only by using the tip of a microscope as is usually done, but also by using atom-sized defects in the substrate itself. The additional level of control offered by the defects could prove useful for constructing future nanodevices and nanomachines.
The researchers, Teemu Hynninen, et al., from institutes in Finland and France, have published their study on the manipulation of gold nanoclusters using defects in an NaCl surface in a recent issue of Scientific Reports.
In 1990, researchers first demonstrated that they could move single atoms by pushing them with the tip of a scanning tunneling microscope (STM). But while moving single atoms may be of great fundamental interest, it’s actually more practical to be able to move somewhat larger atomic clusters.
“For many applications—such as catalysis—clusters or molecules are more relevant than single atoms, so it makes sense to operate on larger units than just atoms,” Hynninen, at Aalto University and the Tampere University of Technology in Finland, told Phys.org. “Also, if you want to build something of considerable size (on the nanoscale) it’s easier if you can use bigger building blocks. Of course, you would never produce anything with nanomanipulation—it’s much too inefficient. Nanomanipulation is a technology with which one can design structures with absolute precision for further studies.”
Over the past several years, scientists have demonstrated how to move atomic clusters using a non-contact atomic force microscope (nc-AFM), which works due to a repulsive interaction that arises between the cluster and tip when they’re only a few angstroms apart. Generally, atomic clusters can be moved by a non-contact tip in two ways: by lowering the tip from directly above the cluster (which the researchers here call “kicking”), and by moving the tip toward the cluster from the side (which the researchers here call “sliding”).
Although kicking and sliding provide proven ways to move atomic clusters, both methods are limited by the tip’s scanning direction. That is, the direction of the cluster’s movement depends on the position of the tip.
In the new study, the scientists demonstrated that, by taking advantage of the natural defects in the substrate on which the atoms lie, they can move atomic clusters in a way that is not completely constrained by the tip’s position. As the researchers explain in their paper and in the YouTube video, an NaCl substrate can have vacancy defects due to missing Na ions and Cl ions. When the researchers deposited single neutral gold atoms onto the insulating NaCl substrate, they observed that the vacancies act as nucleation sites that allow the gold atoms and clusters to bond to the substrate.