IBM scientists directly measure charge states of atoms

IBM scientists in collaboration with the University of Regensburg,Germany, and Utrecht University, Netherlands, have for the first timedemonstrated the ability to measure the charge state of individual atoms usingnoncontact atomic force microscopy.

Measuring with the precision of a single electron charge and nanometre lateralresolution, researchers succeeded in distinguishing neutral atoms frompositively or negatively charged ones. This represents a milestone in nanoscalescience and opens up new possibilities in the exploration of nanoscalestructures and devices at the ultimate atomic and molecular limits. Theseresults hold potential to impact a variety of fields such as molecularelectronics, catalysis or photovoltaics.

As reported in the June 12 issue of Science magazine, Leo Gross, FabianMohn and Gerhard Meyer of IBM's Zurich Research Laboratory in collaborationwith colleagues at the University of Regensburg and Utrecht University imagedand identified differently charged individual gold and silver atoms bymeasuring the tiny differences in the forces between the tip of an atomic forcemicroscope and a charged or uncharged atom located in close proximity below it.

To conduct these experiments, researchers used a combined scanningtunnelling microscope (STM) and atomic force microscope (AFM) operated invacuum at very low temperature (5 Kelvin) to achieve the high stabilitynecessary for these measurements.

The AFM in principle uses a sharp tip to measure the attractive forcesbetween 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 atuning fork, the other prong being fixed. The tuning fork, which is like thosefound in ordinary wristwatches, is actuated mechanically and oscillates withamplitudes as small as 0.02 nanometre, which is about one-tenth of an atom'sdiameter. As the AFM tip approaches the sample, the resonance frequency of thetuning fork is shifted due to the forces acting between sample and tip. Byscanning the tip over a surface and measuring the differences in the frequencyshift, a precise force map of the surface can be derived.

The extremely stable measurement conditions were crucial for sensing theminute differences in the force caused by the charge state switching of singleatoms. The difference between the force of a neutral gold atom and that of agold atom charged with an additional electron, for example, was found to beonly about 11 piconewton, measured at the minimum distance to the tip of abouthalf a nanometre above the atom. The measurement accuracy of these experimentsis better than 1 piconewton -- which is equal to the gravitational force thattwo 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 appliedbetween tip and sample, the scientists were able to distinguish positively fromnegatively charged single atoms.

This breakthrough is yet another crucial advance in the field ofatomic-scale science. In contrast to the STM, which can be used only onconducting materials, the AFM is independent of conductivity and can be usedfor investigating materials of all kinds, most importantly insulators. In thefield of molecular electronics, which aims at using molecules as functionalbuilding blocks for future computing devices, as well as for single-electrondevices, an insulating substrate is needed in order to avoid the leakage ofelectrons. This makes noncontact atomic force microscopy the investigationmethod of choice.

"The AFM with single-electron-charge sensitivity is a powerful toolto explore the charge transfer in molecule complexes, providing us with crucialinsights and new physics to what might one day lead to revolutionary computingdevices and concepts," explains Gerhard Meyer, who leads the STM andAFM-related research efforts at IBM's Zurich Research Laboratory. To study thecharge transfer in molecule complexes, scientists envision that, in futureexperiments, single atoms could be connected with molecules to formmetal-molecular networks. Using the tip for charging these atoms, scientistscould then inject electrons into the system and measure their distributiondirectly with the non-contact AFM (see figure 2).

IBM researcher Leo Gross points out other areas of impact beyondnanoscale computing: "The charge state and charge distribution arecritical in catalysis and photo conversion. Mapping the charge distribution onthe atomic scale might deliver insight into fundamental processes in thesefields."

This achievement follows a string of remarkable scientific advancesachieved by IBM scientists in recent years and represents a fundamental steptowards building computing elements at the molecular scale--computing elementsthat are expected to be vastly smaller, faster and more energy-efficient thantoday's processors and memory devices.

Using the qPlus AFM, a team at the IBM Almaden Research Centre was thefirst to measure in 2008 the force necessary to move an atom over a surface,paving the way for the present experiment. In 2007, Gerhard Meyer's team atIBM's Zurich Lab demonstrated a single-molecule switch that can operate flawlesslywithout disrupting the molecule's outer frame or shape. In 2004, the same groupcontrollably manipulated the charge state of individual atoms using an STM. Byinducing voltage pulses through the STM tip, they succeeded in charging anindividual atom on a thin insulating film with an additional electron.Importantly, the negatively charged atom remained stable until a voltage pulsewith the opposite bias was applied via the STM tip. This is the method used byscientists in the present experiments to charge the individual atoms.

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