Researchers have long used scanning tunnelling microscopes (STMs) to move single atoms around on the surface of material with atomic-scale precision — allowing them to make nanometre-scale structures such as “quantum corrals”, which confine electrons to tiny regions on a surface. However, it has never actually been possible to measure the force required to move an individual atom, something that could improve our understanding of the structural and mechanical properties of materials.
Now, however, an international team of physicists has used a modified STM to measure the force needed to move a single cobalt atom on both platinum and copper surfaces (Science 309 1066). The breakthrough could help researchers build new nanoscale devices such as high-density magnetic memories.
An STM involves placing a tiny metal tip very near to a surface of interest and applying a voltage between the surface and tip. The tip is scanned with great precision across the surface and an image is generated by measuring the current of electrons that tunnel between tip and surface. The tip also exerts a force on the surface atoms and can be used to move individual atoms around on a surface.
Flexible tip
The strength of the tiny forces required to move an atom could, in principle, be measured by fitting an STM with a flexible tip that vibrates much like the prong of a tuning fork. When tip and atom are brought very close together, the force between the two would change the frequency of vibration — which is how some atomic force microscopes (AFMs) work. The problem is that a very stiff and stable tip is needed to move an individual atom with sufficient spatial accuracy, while a relatively floppy tip is needed to measure the force accurately.
Now, Markus Ternes and colleagues at IBM’s Almaden Research Center in California, the Swiss Federal Institute of Technology in Lausanne and the University of Regensburg in Germany hove found a way around this dilemma. To do this, the team modified a STM by mounting the tip on one prong of a quartz oscillator similar to that used to keep time in a wristwatch. The prong is about 40 times stiffer than a silicon-based cantilever used in an AFM.
Hopping atoms
The force need to move an atom from was measured by scanning the vibrating tip back and forth above a cobalt-occupied adsorption site and an adjacent empty site. At each successive pass, the tip was lowered by as little as 10 pm at a time until it hovered less than 100 pm from the cobalt atom. At first, the atom did not move, but as the tip got closer to the surface, the force exerted by the tip caused the atom to hop to the adjacent site. By looking at how the vibrational frequency of the tip changed during the hop, the team were able to determine the threshold force required to move the atom.
Using the modified STM, Ternes and his team found that it took about 210 pN (2.1 × 10–10 N) to move a cobalt atom 160 pm (1.6 × 10–10 m) between two adjacent adsorption sites on platininum. While this might not seem like much of a force, it is about 1014 times the force of gravity on a cobalt atom. A much lower force of about 17 pN was needed to move cobalt on copper.
Tiny oscillation
According to Ternes, the team’s success was down to their ability to limit the amplitude of the tip oscillation to about 25–30 pm, which is a fraction of the distance between adsorption sites. As well as allowing the team to move the atoms with great precision, this tiny amplitude allowed them to study the short-range chemical-bonding forces that affect how the cobalt moves from on site to another.
Ternes told physicsworld.com that the team plan to use the instrument to assemble atomic-scale magnetic structures on insulating surfaces – something that a standard STM cannot do because — unlike an AFM — it is unable to image and insulator. The tem believe that such structures could form the basis of very high-density data storage devices.
