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Boffins ‘see' charge within a single molecule

Applications include molecular-scale computing devices
Mon Feb 27 2012, 16:06
Kelvin probe force microscopy

BOFFINS HAVE for the first time managed to "see" an image of the charge distribution within a single molecule.

The breakthrough was made by Fabian Mohn, Leo Gross, Nikolaj Moll and Gerhard Meyer of IBM Research in Zurich who directly imaged the charge distribution within a single naphthalocyanine molecule using a special kind of atomic force microscopy called Kelvin probe force microscopy at low temperatures and in ultrahigh vacuum.

According to IBM, the ability to "see" for the first time how charge is distributed in a molecule could end up having a significant impact on nano-electronics. The company predicts that its impact might even be comparable to what the invention of MRI scanners has done for healthcare.

The technology builds on existing techniques including scanning tunnelling microscopy (STM),which can be used for imaging electron orbitals of a molecule, and atomic force microscopy (AFM), which can be used for resolving its molecular structure. kpfm-measurement-scheme

To measure the charge distribution, IBM scientists used an offspring of AFM called Kelvin probe force microscopy (KPFM). When a scanning probe tip is placed above a conductive sample, an electric field is generated due to the different electrical potentials of the tip and the sample.

The researchers explained that, with KPFM, this potential difference can be measured by applying a voltage such that the electric field is compensated. Therefore, KPFM does not measure the electric charge in the molecule directly, but rather the electric field generated by this charge. The field is stronger above areas of the molecule that are charged, leading to a greater KPFM signal. Furthermore, oppositely charged areas yield a different contrast because the direction of the electric field is reversed. This leads to the light and dark areas in the micrograph (or red and blue areas in coloured ones).

The achievement will enable fundamental scientific insights into single-molecule switching and bond formation between atoms and molecules, according to the researchers.

Furthermore, it introduces the possibility of imaging the charge distribution within functional molecular structures, which apparently holds great promise for future applications such as solar photoconversion, energy storage, or molecular scale computing devices. It added that practical applications for the technology include solar photoconversion, energy storage, or molecular scale computing devices.

Michael Crommie, professor of Condensed Matter Physics at the University of Berkeley said, "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." µ



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