Just 17 piconewtons, or 60 trillionths of an ounce, is the force it takes to push a cobalt atom across a copper surface. This is one of the findings of a group of IBM scientists who have been testing a new kind of atomic force microscope (AFM), which has made the first measurements of the force required to move an individual atom.
It’s been nearly two decades since IBM’s Don Eigler and Erhard Schweizer showed how 35 xenon atoms could be positioned with atomic-scale precision to spell out the company’s logo, says Andreas Heinrich, a scientist at IBM’s Almaden Research Center, in San Jose, CA, and one of the researchers who made the device. “But after all this time, we still couldn’t answer the basic question of how much force it takes to move atoms,” he says.
The new microscope, developed at Almaden in collaboration with Franz Giessibl, a professor of physics at the Institute of Experimental and Applied Physics at the University of Regensburg, Germany, changes that. “From a scientific point of view, it’s interesting to understand how atoms and molecules interact,” says Heinrich. “But also we want to be able to build things.” To use individual atoms as building blocks, researchers need to know precisely how much force is required to lift them, pull them, and push them around.
“It provides very important input for theorists dealing with all kinds of atomic-scale problems, from nanostructures to friction,” says Udo Schwartz, a professor of mechanical engineering at Yale University. The new measurements, he says, give an indication of how stable nanostructures assembled from the bottom up would be. “Most importantly, by knowing the lateral force needed to move an individual atom over a surface, one can start to make models of how it changes by adding a second one, and the third one, and so on,” he says.
According to Oscar Custance of the Japanese National Institute for Materials Science, in Ibaraki, the IBM work is of fundamental importance, “as it provides scientists with an additional information channel about the nature of the chemical interactions.”
Scanning tunneling microscopes (STMs) can detect subatomic features of a surface by measuring tiny changes in currents flowing between the surface and a very sharp tip positioned near it. But although their resolution is extremely fine, STMs are unable to measure the forces involved in manipulating atoms.
In contrast, AFMs work by measuring the physical deflection of a tip on the end of a cantilever as it is brought close to a surface. The technique can be used to both manipulate atoms and measure some forces, but its resolution is lower, so it can’t gauge the subtle lateral forces involved in nudging atoms across a surface.