IBM scientists directly measure charge states of atoms

Tony Austin
Monday, 15 June 2009 07:38

Opinion and Analysis

Scientists at  IBM's Zurich Research Laboratory, in collaboration with two European universities, have demonstrated the ability to measure the charge state of individual atoms using noncontact atomic force microscopy.

You have to be impressed with IBM's fundamental scientific research projects. Early this year I reported how they had demonstrated ultra-high resolution MRI microscopy.

IBM has been a pioneer in nanoscience and nanotechnology ever since the development of the scanning tunneling microscope (STM) in 1981 by IBM Fellows Gerd Binnig and Heinrich Rohrer at IBM’s Zurich Research Laboratory. For this invention, which made it possible to image individual atoms and later on to manipulate them, Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986.

The atomic force microscope (AFM), an offspring of the STM, was developed by Binnig in 1986. The STM is widely regarded as the instrument that opened the door to the nanoworld.

Now -- in collaboration with the University of Regensburg, Germany, and Utrecht University, Netherlands -- IBM researchers, measuring with the precision of a single electron charge and nanometer lateral resolution, have succeeded in distinguishing neutral atoms from positively or negatively charged ones.

They regard this as a milestone in nanoscale science which opens up new possibilities in the exploration of nanoscale structures and devices at the ultimate atomic and molecular limits. These results hold potential to impact a variety of fields such as molecular electronics, catalysis or photovoltaics.

As reported in the June 12 issue of Science magazine, Leo Gross, Fabian Mohn and Gerhard Meyer of IBM’s Zurich Research Laboratory in collaboration with colleagues at the University of Regensburg and Utrecht University imaged and identified differently charged individual gold and silver atoms by measuring the tiny differences in the forces between the tip of an atomic force microscope and a charged or uncharged atom located in close proximity below it.

To conduct these experiments, researchers used a combined STM and AFM operated in vacuum at very low temperature (5 Kelvin) to achieve the high stability necessary for these measurements.

The AFM in principle uses a sharp tip to measure the attractive forces between the tip and the atoms on a substrate. In the setup of the present work, the AFM uses a qPlus force sensor consisting of a tip mounted on one prong of a tuning fork, the other prong being fixed.

The tuning fork, which is like those found in ordinary wristwatches, is actuated mechanically and oscillates with amplitudes as small as 0.02 nanometer—which is about one-tenth of an atom’s diameter. As the AFM tip approaches the sample, the resonance frequency of the tuning fork is shifted due to the forces acting between sample and tip. By scanning the tip over a surface and measuring the differences in the frequency shift, a precise force map of the surface can be derived.

The extremely stable measurement conditions were crucial for sensing the minute differences in the force caused by the charge state switching of single atoms.

The difference between the force of a neutral gold atom and that of a gold atom charged with an additional electron, for example, was found to be only about 11 piconewton, measured at the minimum distance to the tip of about half a nanometer above the atom.

The measurement accuracy of these experiments is better than 1 piconewton—which is equal to the gravitational force that two adults exert on each other over a distance of more than half a kilometre. Moreover, by measuring the variation of the force with the voltage applied between tip and sample, the scientists were able to distinguish positively from negatively charged single atoms.

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