Feeling the Force
A new kind of probe microscope can measure the force needed to push a single atom.
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.
The new device, reported in the current issue of the journal Science, combines both of these approaches. A sharp tip is placed at the end of a tiny horizontal quartz tuning fork, similar to the ones used to regulate wristwatches. “This tuning fork vibrates at its own resonant frequency,” says Markus Ternes, another member of the Almaden research team.
As the tip is brought into the proximity of an atom on a surface, forces acting between the two will alter the frequency at which the tuning fork vibrates. The new microscope measures changes in the piezoelectric currents generated by the vibrating quartz; like an STM, however, it also measures changes in a tunneling current flowing between the surface and the tip. The combination of techniques enables it to measure the forces exerted on individual atoms with unprecedented precision.
An atom hopping from one position to another on a surface is much like a sphere moving from one well in an egg carton to the next. By positioning the tip at various points around the atom, the IBM researchers take measurements of forces acting on it before it hops. It’s a bit like holding one magnet close enough to another to feel a pull but not close enough to actually move it.
With the atoms, however, the IBM researchers keep moving the tip closer, says Heinrich. “At some point, the force is too large, and that is when the atom hops,” he says. Although this happens too quickly to record, it is possible to calculate the force of the hop from measurements taken before it occurs.
The idea for the microscope’s tuning-fork arrangement originally came from Giessibl. But it was only the collaboration of the IBM researchers that yielded a device with electronics and materials sensitive enough to detect the lateral forces on individual atoms. One of the reasons the microscope is so sensitive compared with regular AFMs is that the quartz material it uses is roughly 40 times stiffer than the silicon used in most AFM cantilevers.
The probe has already led to some unexpected results. For example, for some reason, moving a cobalt atom across a platinum surface requires more than 12 times as much force as moving it across copper surface. Moving a carbon monoxide molecule across copper, on the other hand, requires 10 times as much force as moving a single cobalt atom. Until now, scientists were unaware of any such nuances: “Before this, we were flying blind,” says Heinrich.