June 18, 2009: Researchers from IBM and the U. of Regensburg (Germany) say they have demonstrated the ability to measure the charge state of a single atom, distinguishing between neutral/positive/negative ones, using noncontact atomic force microscopy (AFM) — an achievement they say opens up explorations into nanoscale structures and devices “at the ultimate atomic and molecular limits” for applications in molecular electronics, catalysis, and photovoltaics.

The work, reported in the June 12 issue of Science, imaged and identified differently charged gold and silver atoms by measuring the differences in forces between the AFM tip and charged/uncharged atoms located below it. The tool setup is a combined scanning tunneling microscope (STM) and atomic force microscope (AFM) operated in vacuum at very low temperature (5 Kelvin). The AFM incorporates a qPlus force sensor with a tip mounted on one prong of a tuning fork (the other prong is fixed), which actuates mechanically with ≤0.02nm oscillation amplitudes. As the AFM tip approaches the sample, the tuning fork’s resonance frequency shifts; scanning the tip over the surface and measuring the differences in frequency shift creates a force map of the surface; measuring the variation of force with voltage applied between tip and sample allowed them to distinguish between positively and negatively charged single atoms. The researchers found the difference in forces between a neutral gold atom and one with an additional electron was about 11piconewtons (accurate to <1piconewton), measured about half a nanometer above the atom.

Figure 1: Single atoms (orange) could be connected with molecules to form metal-molecular networks. Using the tip for charging these atoms, scientists could then inject electrons into the system and measure their distribution directly with the non-contact AFM. Understanding the charge distribution in molecules and molecular networks is a crucial step in the exploration of future computing elements on the nanoscale. (Source: IBM)

“The AFM with single-electron-charge sensitivity is a powerful tool to explore the charge transfer in molecule complexes, providing us with crucial insights and new physics to what might one day lead to revolutionary computing devices and concepts,” explains Gerhard Meyer, who leads the STM and AFM-related research efforts at IBM’s Zurich Research Laboratory. To study the charge transfer in molecule complexes, scientists envision that, in future experiments, single atoms could be connected with molecules to form metal-molecular networks (see Figure 1). Using the tip for charging these atoms, scientists could then inject electrons into the system and measure their distribution directly with the non-contact AFM (see Figure 2).

Figure 2: Model of the experimental setup (left). The gold atom is on a substrate covered with a very thin insulating film of sodium chloride, which also stabilizes the charged atom. The atomically-sharp AFM tip is brought into close proximity with the gold atom, up to a minimum distance of about 0.5nm. The tip, which is mounted on one prong of a tuning fork (not shown) oscillates with amplitudes as small as 0.02nm, about one-tenth of an atom’s diameter. Using this setup, the scientists were able to sense the minute differences in the force exerted by a neutral gold atom and a gold atom charged with one additional electron (right). (Source: IBM)