Atoms and Electrons Watched at Work

Kieler layer crystals are used worldwide as a basis for research into the nanocosmos

Tantalum disulfide
Scientists all over the world use samples of tantalum disulfide from Kiel for their experiments. © Siekmann, University of Kiel
tantalum disulfide
Its simple structure at the atomic level makes tantalum disulfide an ideal material for many physical experiments. © Siekmann, Uni Kiel

The properties of materials are determined by their atomic structure. If atoms and electrons change their arrangement, the characteristics of a material also change. The complex processes involved are being investigated by a working group of the Christian-Albrechts-Universität zu Kiel (CAU) using a unique material: tantalum disulfide. The Kieler Group specializes in the production of this layered crystal, and its samples are used by research teams worldwide as a basis for experiments on dynamic processes in materials. Together with the Universities of Göttingen, Duisburg-Essen and Aarhus, three studies based on tantalum disulphide from Kiel have recently been published, showing previously unknown phenomena in the movement of electrons and atoms. For example, the results could provide information in the long term, how these movements can be controlled and thus properties of materials could be changed in a targeted way. They appeared in the journals “Nature Physics “ ,” Physical Review Letters “and” Physical Review B “.

A Drosophila of material physics

It is a material as unique as it is simple, with which scientists want to find out more about the diverse behavior of electrons and atoms. “Tantalum disulfide is a nanocosm of its own, incredibly rich in quantum physical phenomena,” enthuses Kai Roßnagel, professor of solid state research with synchrotron radiation at the CAU and senior scientist at the German Electron Synchrotron (DESY). “This offers us almost inexhaustible possibilities of investigation.” If it is cooled or exposed to flashes of light, for example, its atoms and electrons rearrange. The material is thus from the conductor to the insulator or vice versa.

Tantalum disulfide
A kind of “crystal sandwich”: Between two layers of sulfur atoms (red) lies a layer of the metallic tantalum (gray). © Matthias Kallänen

With the help of the special layered crystal, Roßnagel and his colleagues want to better understand how such special material properties arise. On the other hand, they want to find out how and how quickly properties can be changed. Here, they benefit from the simple layered structure of tantalum disulfide. It makes it easier to interpret quantum physical phenomena. “Tantalum disulfide is therefore ideal as a reference material for solid state physics. Insights gained here can also be transferred to other materials, “says Roßnagel. “For material physics, it’s sort of what the Drosophila fly is for biology.”

Layered Crystals from Kiel: International Trademark in Nanoproduction

Tantalum disulfide is a kind of “crystal sandwich”: between two layers of sulfur atoms is a layer of the metallic tantalum. Together, they are just half a nanometer thick. Several of the three-ply stacks on top of each other eventually form the layered crystal. Roßnagel’s team has more than 35 years of experience in the production and analysis of the sought after research material. The high-purity chemical sources tantalum and sulfur are placed in a quartz ampoule and then their ends are heated to different degrees in a special oven. In six to eight weeks, the multi-layered crystals grow in the ampoule.

Tantalum disulfide
Prof. Dr. med. Kai Roßnagel (left), Tim Riedel and Kerstin Hanff the crystals with the special properties. © Siekmann, University of Kiel

In recent years, samples of Kiel tantalum disulfide have become a trademark in international nanotechnology research. Due to their quality, research groups worldwide source the crystals for their experiments from Kiel. Through the discussion of their results with the experts of the CAU, numerous research cooperations and joint publications on the samples from Kiel have emerged. Recently, three studies have been published in a short time. They all deal with the question of how electrons and atoms behave in tantalum disulfide and thus influence the function of the material.

New studies show previously unknown motion phenomena at the atomic level

In cooperation with the University of Aarhus, the scientists investigated how electrons move within a tantalum disulfide crystal. In low-temperature spectroscopy experiments, they found that their mobility between the layers was significantly higher than along the layers. “The fact that the individual layers, as it were, ‘talk to each other’ and electricity flows more easily vertically between the layers, has surprised us”, ordersSolid-state and surface physicists Roßnagel the results.

The Kiel team, together with colleagues from the universities of Duisburg-Essen, Hamburg and the Swiss city of Friborg, examined why the electrons are so immobile at low temperatures within a layer of tantalum disulfide. They put on the outside free electrons on a layer of tantalum disulfide. Since an electron is already bound to all the atoms and electrons with negative electrons repel each other, the electrons “hopped” from atom to atom. In their experiments, the scientists were able to measure how long an electron needed for this. “For the first time, we were able to demonstrate explicitly why the current flow is suppressed at this point: The electrons are virtually in the way of their own.”

In a study with the University of Göttingen, the scientists finally investigated the process of ultrafast structural transformations in tantalum disulfide. Thanks to a new method from Göttingen, the atomic rearrangement could be recorded in super-slow motion: the abruptly driven out of their regular formation atoms found their position in the new structure only gradually over initially isolated ordered areas that were slowly growing larger and growing together.

Long-term ultrafast transistors conceivable

Tantalum disulfide is still a material from basic research, but in principle new electronic components are conceivable. “After all, tantalum disulfide is a switch. One day, this would make an ultrafast transistor possible, “says Roßnagel. In the future he wants to observe the processes in the layered crystal with the high-performance X-ray laser European XFEL in real time. “With each new measurement method, we make new discoveries in the material. Here, together with the electrons, we will watch the atoms live at work. “As a spokesman for an international consortium, Roßnagel is currently initiating the construction of an experiment for ultrafast spectroscopy in the northern German city of Schenefeld. For 2019 the first attempts are planned. They should help to decisively expand the understanding of the nano-cosmos.

Source : Christian-Albrechts-Universität zu Kiel (CAU)