Thursday, 1 March 2012

Scientists at IBM have discovered an innovative way to measure and visualize charge distribution at the molecular level. This is a major breakthrough for nanotechnologists, as it provides an avenue to better understanding single-molecule switching, and can help provide further insight into how bonds form between atoms and molecules.
Big Blue’s discovery is likely to help further research in everything from computer science to biology to solar energy.
It’s all in the technique
Measuring charges in an atom is nothing new; what’s new is capturing them as an image. To do this, the team of scientists used a special kind of atomic force microscopy called “Kelvin probe force microscopy” (KPFM).
KPFM is kind of like performing an X-ray, MRI, or ultrasonography. But where these examples provide complementary information about a person’s anatomy and health condition, KPFM helps researchers visualize and measure how molecules work.
“This work demonstrates an important new capability of being able to directly measure how [a] charge arranges itself within an individual molecule,” said Michael Crommie, Professor in the Department of Physics at the University of California, Berkeley. “Understanding this kind of charge distribution is critical for understanding how molecules work in different environments. I expect this technique to have an especially important future impact on the many areas where physics, chemistry, and biology intersect.”
 How KPFM works
KPFM requires that a very tiny scanning probe tip, charged with a small voltage, hover above a conductive molecule. The difference in electric potential between the tip and molecule, in turn, generates an electric field that can be both measured and visualized.
More specifically, with the application of voltage, the electric field becomes compensated. This allows the researchers to not only measure the molecule’s charge, but also visualize it. You see, some parts of the electric field display stronger signals than others; specifically, the regions that have the molecule’s electrons will display a greater signal for measurement when the charged tip hovers above it. Oppositely charged areas, on the other hand, yield a different charge because the direction of the electric field above them is reversed. These differences in charge can then be visualized with the display of light and dark areas on a micrograph (as seen below).
Big Blue’s big experiment
The sample material that the scientists used was the x-shaped molecule, naphthalocyanine. What’s unique about this material is that when you apply a voltage to it, the two hydrogen atoms in the center of the molecule switch places, and the electrons reshuffle to opposite parts of the “X” to re-apply themselves in order to keep everything cohesive. With KPFM, the team was able to observe this change in charge distribution.
The picture below captures the charge distribution in the X-shaped molecule, with the circles showing the way in which the electrical charge is being distributed within the molecule.
Quick side note: To achieve this sub-molecular resolution, a high degree of thermal and mechanical stability, combined with the atomic precision of the instrument itself, are all required over the course of the entire multi-day experiment. 
Potential moving forward
KPFM could be used to study charge separation and charge transport in charge-transfer complexes. As the name suggests, these are the areas that an electronic charge gets transferred, whether it’s where two or more molecules meet, or at the junctures connecting parts of one larger molecule. 
Having a better understanding of how CT complexes work in different materials could help advance the future of designing molecular-sized, super energy-efficient transistors for everything from sensors to smartphones to supercomputers.
“This technique provides another channel of information that will further our understanding of nanoscale physics. It will now be possible to investigate at the single-molecule level how charge is redistributed when individual chemical bonds are formed between atoms and molecules on surfaces,” explains Fabian Mohn of the Physics of Nanoscale Systems group at IBM Research Zurich. “This is essential as we seek to build atomic and molecular scale devices.”


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