Hopalong atom gets to work

Characterization

March 7, 2008

Schematic of an AFM tip measuring the force it takes to move a Co atom on a crystalline surface. The ability to measure the exact force required to move individual atoms is one of the keys to designing and constructing the small structures that will enable future nanotechnologies. (Courtesy of Markus Ternes, IBM.)

Atomic force microscopy (AFM) is increasingly finding a role in the assembly of components at the nanoscale by moving atoms from one site to another.

Previously, however, information on the force required to move an atom could only be obtained through theoretical calculations and simulations.

Scientists from IBM’s Almaden Research Center in collaboration with Franz J. Giessibl from the University of Regensburg in Germany have now measured these forces experimentally. They find the driving forces to be dependent on the nature of the adsorbate, the surface, and the lateral forces exerted by the tip on the atom [Ternes et al., Science (2008) 319, 1066].

“The main problem we had to overcome was to get the system so stable that we could perform the experiment with precision... and obtain a spatial resolution in the range of just a few picometers,” says Markus Ternes of IBM.

Measurements were obtained using a frequency-modulated AFM under ultrahigh vacuum. A stiff cantilever with a metal tip at its end is set into oscillation just above the surface and the driving forces are measured in the vertical and lateral directions.

Initially, the lateral force is zero but rises to 210 pN when the atom, in this case Co, is nudged and hops along a Pt(111) surface. This force is independent of the vertical component and decreases by a factor of ten when the substrate is replaced with Cu(111).

Although both surfaces are face-centered cubic crystals and the Co atom binds at threefold hollow sites, it is the chemical nature of the bond that plays the stronger role in keeping the atom firmly in place.

When the system is replaced with a CO molecule on Cu(111), the lateral force increases by an order of magnitude as the complexity of the component forces increases. For each of the systems studied, the measured energy barrier height is in accordance with density functional theory.

A clear idea of the forces involved in nanoscale manipulation can lead to valuable information for future information technology devices that could be built atom by atom.

“We need to figure out what to do once the current Si scaling fails and we all know that it’s going to fail at some point. We need to know what you can and can’t build, starting from the scale of single atoms,” comments Andreas J. Heinrich of IBM.

His colleague, Ternes adds, “If you want to progress to a larger scale, [you must] ask at the very beginning how much force you need to move something from A to B.”

Katerina Busuttil