instrumentals
AFM tells elements apart Forces measured between single atoms and an AFM tip reveal the chemical identities of the atoms.
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ÓSCAR CUSTANCE
he atomic force microscope (AFM) has expanded its résumé—not only does it produce pretty images of surfaces with nanoscale resolution but it can now also chemically recognize atoms. Óscar Custance and colleagues at Osaka University (Japan), Universidad Autónoma de Madrid, the Japan Science and Technology Agency, and the Academy of Sciences of the Czech Republic have pushed the AFM’s capabilities so that it can identify tin, lead, and silicon atoms on a surface (Nature 2007, 446, 64 – 67). “One of the holy grails of probe microscopy is the ability to identify atoms and molecules,” says Craig Prater of Veeco. Although similar studies have been done in the past with scanning tunneling microscopy, Prater says the problem has been that the method is limited to conducting substrates. The virtue of an AFM is that it can interact with a broader range of samples, including insulators. “To my knowledge, this is the first demonstration of identification of individual atoms using an AFM,” he says. Custance and colleagues operated their microscope in a mode known as force modulation. A cantilever is oscillated at its resonant frequency while its amplitude is held constant. As the tip at the end of the cantilever is brought closer to a surface, “we measure a shift in the resonant frequency of the cantilever, which changes when a force is applied to the end of the cantilever,” explains Custance. The method is sensitive enough to pick up forces that are as small as a few piconewtons. The investigators brought the silicon tip close to the surface of an alloy of tin, lead, and silicon. “To get atomic resolution, we occasionally touch the tip to the surface to modify the apex and make it really sharp,” says Custance. “The apex should terminate in a single atom.”
(a) Schematic of operating an AFM cantilever in dynamic mode so that (b) chemical bonding between the outermost-tip atom and a surface atom (indicated by the green stick) gives rise to atomic contrast.
The single atom at the apex was scanned over the surface. As the atom began to form covalent bonds with individual atoms on the surface, Custance and colleagues picked up changes in the short-range forces as shifts in the cantilever’s resonant frequency relative to its frequency when no force is applied. But “the problem is we cannot control which atom we have at the very end of the tip. It could be silicon, it could be lead, it could be tin,” says Custance. So, the investigators sidestepped the potential problem of deducing the nature of the tip—instead of looking at the absolute values of forces between the tip and surface atoms, they analyzed ratios. They calculated the ratio of the maximum attractive short-range forces be-
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tween two curves taken with the same tip. The ratio, called the relative interaction ratio, stays nearly constant for a given element. “The method is independent of the tip,” says Custance. “The ratio cancels out the chemical contribution from the atom at the end of the tip.” The investigators went on to identify atoms of tin, silicon, and lead—which are topographically indistinguishable— on the flat alloy surface. They systematically recorded the total tip–surface interaction forces for each atom, and when they calculated the ratios, the values fell into three distinct groups. Franz Giessibl at the University of Regensburg (Germany) says it’s noteworthy that the investigators did their experiments at room temperature. To date, single-atom work by probe microscopy “was done at low temperature, where everything is quiet and stays put,” he explains. “It requires much thinking into how to build your instrument to make it less susceptible to thermal drift problems.” Over the years, he says, the investigators have diligently minimized the noise level in their homebuilt AFM, to the point that they could carry out such precise experiments at room temperature. The investigators are excited about combining chemical identification with a method they developed for maneuvering single atoms into complex structures with an AFM (Nat. Mater. 2005, 4, 156–159). “We will be able to dope semiconductors with different materials and create arrays on surfaces to change not only the arrangement of atoms but also the composition of the arrays, to increase the performance of the device,” says Custance. They will also attempt to carry out chemical reactions on a surface by precisely manipulating different atoms and identifying both the reactants and products. a —Rajendrani Mukhopadhyay © 2007 AMERICAN CHEMICAL SOCIETY
