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Scanning Tunnelling Microscopy with Hydrogen

The selective use of a hydrogen molecule increases the resolution of conventional scanning tunnelling microscopes

A hydrogen molecule attached to the very top of the measuring tip of a scanning tunnelling microscope, in contact with the specimen being viewed, functions like a highly sensitive feeler. This is, in simplified terms, the principle behind "scanning tunnelling hydrogen microscopy" (STHM), which was developed by Jülich researchers. STHM helps scientists to generate even more detailed images on the nanometre scale and thanks to a relatively simple extension of existing technologies, allows them to look inside organic molecules for the first time. In the past, researchers had to make do with blurred cloud-like images, whereas now they can recognize molecular structures.

To gain such insights into the nanoworld, Jülich researchers make use of various scanning tunnelling microscopes. Their thin metal tips travel over the specimen surface like the needle of a record player and register the atomic irregularities and differences of approximately one nanometre (a billionth of a metre) with minuscule electric currents. The operating principle of these conventional scanning tunnelling microscopes (STMs) is rooted in the quantum mechanical tunnel effect. If a metallic tip is brought very close to a surface, the electrons can "tunnel" from the tip through the vacuum barrier into the specimen. If a current is applied during this process, a "tunnelling current" that is much easier to measure is created. This allows conclusions to be drawn about the structure of the specimen, because the current depends on the electronic structure of the specimen. This technique allows us, for example, to visualize semiconductor surfaces down to the atomic level. In contrast, the internal atomic structure of complex molecules cannot be recorded by conventional scanning tunnelling microscopy.

A molecule comprising two heavy hydrogen atoms, known as deuterium, can help out here, as the team of Jülich researchers headed by Prof. Stefan Tautz and Dr. Ruslan Temirov discovered. The deuterium on the measuring tip of the microscope functions like a sensor on the specimen surface. As it is very flexibly attached to the tip, it can follow the contours. This influences the currents flowing over the microscope tip. It is like adding an extra feeler to the highly sensitive tip, which increases the resolution of a scanning tunnelling microscope significantly.

One of the first molecules studied by the scientists was the compound perylene tetracarboxylic dianhydride (PTCDA). It consists of 26 carbon atoms, eight hydrogen atoms and six oxygen atoms, which form seven interconnected rings. Earlier scanning tunnelling micrographs with pure metal tips only visualized this molecule as a spot around a nanometre in size with no contours. The Jülich technique, much like an X-ray, allows us to image the internal honeycombed atomic structure, which is formed by the rings.

Normally, foreign substances are undesirable for measurements on the scanning tunnelling microscope. A strong vacuum therefore creates an absolutely clean, interference-free environment at – 260 °C. Luckily, tiny amounts attached to the specimen and the ice-cold tip are enough to produce a magnifying effect. Using computer-based calculations and in cooperation with the working group headed by Prof. Michael Rohlfing at Osnabrück University, the scientists developed a quantum mechanical theory on how this works. According to this theory, short-range "Pauli repulsion" is responsible for the effect. This is a quantum-physical force between the deuterium and the molecule, which modulates the conductivity and makes it possible to measure the fine structures in a highly sensitive manner.

What makes the discovery by the researchers at the Peter Grünberg Institute so spectacular is that the resulting technique can be easily coupled with commercial low-temperature scanning tunnelling microscopes and that the preparation of the specimens is predominantly based on standard techniques.
The Jülich STHM technique can be used to measure the atomic structure of flat molecules, which can be used as organic semiconductors or as part of fast and efficient future electronic components. Large three-dimensional biomolecules, such as proteins, could also be examined as soon as the techniques have been refined.