How many atoms does the world make up

A scanning tunneling microscope with an add-on at the probe tip makes the internal molecular structure visible thanks to Pauli repulsion

Electron microscope shows molecular structure

Jülich - Looking closely at the outer shape of atoms and molecules, researchers have been able to do that for many years - thanks to a scanning tunneling microscope that scans the surface step by step. But Jülich researchers succeeded in making the internal structure of molecules visible. Instead of a blurred blob, their new images now reveal clear shapes of the atomic structure within organic molecules. The team explains how and why this method works in the journal "Physical Review Letters". With a small addition, commercially available scanning tunnel microscopes can also be converted for the inside view. In this way, proteins, but also organic semiconductor molecules, should be able to be examined more quickly and more precisely in the future.

"In order to increase the sensitivity for organic molecules, we have put a sensor and signal converter at the top," says Ruslan Temirov from the Institute for Bio- and Nanosystems (IBN) at Forschungszentrum Jülich. While the conventional, extremely fine metal tip of a scanning tunneling microscope scans the examined surface like a turntable needle - and perceives the ups and downs of only individual nanometers using the smallest electrical currents - Temirov's team attached this tip. The researchers hung a molecule made of two deuterium atoms on it so that it could move. Deuterium is a variant of hydrogen, also called heavy hydrogen. The loosely hanging deuterium molecule can follow the contours of the surface better than the fixed tip and also influences the flowing currents.

The team's success was particularly evident in the molecule of the compound perylenetetracarboxylic acid dianhydride (PTCDA), which is based on 26 carbon atoms. Together with eight hydrogen and six oxygen atoms, they form seven flat, connected rings around one nanometer (billionths of a millimeter) in size. Temirov and colleagues, on the other hand, made the honeycomb-like internal structure of the molecule visible with their new method. While the team was able to present the first images of the process almost two years ago, it has now provided the quantum mechanical operating principle behind it. The basis of the new measurement method is therefore the so-called short-range Pauli repulsion between the deuterium at the tip of the microscope and the molecule to be examined. This quantum physical force influences and modulates the conductivity of the tip and allows the fine structures to be measured with high sensitivity. The institute has applied for a patent on the method, which can be combined with commercial atomic force microscopes. Director Stefan Tautz explains: "The captivating simplicity of the method makes it so valuable for future research",

"The spatial dimensions inside molecules can now be determined in just a few minutes," says Tautz, "and the previous preparation of the samples is largely based on standard methods." After clarifying the principle of action, the researchers now want to try to calibrate the measured currents. This means that one could infer the type of atoms directly from the measured currents alone. Further refinements could also make it possible to measure large, non-flat biomolecules such as proteins. But even with the current status, the new method should be used quickly. For example, it helps to measure the charge distribution and internal structure of flat molecules - for example, when they are to be used as organic semiconductors or as part of faster and more efficient electronic components.