"X-ray view" into the molecule

Researchers are developing a novel method for observing atomic structures

Research Center J lich
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It is part of everyday life for nanotechnologists to examine individual atoms with their microscopes. So far, however, it has been difficult to observe atomic structures that were inside organic molecules. In the journal "Physical Review Letters" Jülich researchers now present a novel method that allows an "X-ray view" into the molecule. It could make it easier to analyze organic semiconductors and proteins.

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For a look into the nanoworld, the researchers used a scanning tunneling microscope. Its thin metal tip moves like the needle of a turntable over the sample surface and registers the atomic bumps and differences of about one nanometer by means of very small electrical currents - that is one billionth of a millimeter. But even if the tip of the microscope is only one atom wide, so far could not be seen in the interior of molecules.

Molecule of two deuterium atoms

"In order to increase the sensitivity for organic molecules, we have put a sensor and signal transducer at the top, " explains Ruslan Temirov from Forschungszentrum Jülich. The two functions fulfill a small molecule of two deuterium atoms, also called heavy hydrogen. As it hangs very movably at the top, it can follow the contours and affect the currents that flow over the microscope tip.

PTCDA made the beginning

As one of the first molecules, Temirov and his colleagues investigated the compound perylenetetracarboxylic dianhydride (PTCDA). It consists of 26 carbon, eight hydrogen and six oxygen atoms, which form seven contiguous rings. In previous photographs, it is only shown as a roughly one nanometer large contourless spot. However, the Jülich scanning tunneling microscope shows - as on an x-ray - now the inner, honeycomb-like structure formed by the rings. display

"The impressive simplicity of the method makes it so valuable for future research, " says Professor Stefan Tautz from the Jülich Institute for Bio- and Nanosystems. The method is now patent pending and can be easily coupled with commercial scanning tunneling microscopes.

Calibration of the measured currents

"The spatial dimensions inside molecules can now be determined in just a few minutes, " continues Tautz, "and the previous preparation of the samples is largely based on standard procedures." In the next step, the Jülich scientists want to calibrate the measured current intensities. If this succeeds, it could be deduced directly from the measured current intensities on the type of atoms.

Quantum-mechanical effect explained

After she had already published the first pictures with the new method in 2008, the research group around Tautz and Temirov could now explain the quantum mechanical principle of deuterium at the microscope tip. A computer-aided calculation of researchers led by Professor Michael Rohlfing at the University of Osnabr ck also helped. The so-called short-range Pauli repulsion, a quantum-physical force between deuterium and molecule, modulates the conductivity and makes it possible to measure the fine structures very sensitively.

The J Verfahrenlicher method can be used to measure the structure and charge distribution of flat molecules, which could be used as organic semiconductors or as part of future, fast and efficient electronic devices. But large three-dimensional biomolecules like proteins could also be investigated, according to the researchers - if the methods are refined.

(idw - Forschungszentrum J lich, 20.08.2010 - DLO)