A treasure chest for researchers

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Physicist Dr Nihit Saigal is a postdoctoral researcher in the research group of
Physicist Dr Nihit Saigal is a postdoctoral researcher in the research group of Ursula Wurstbauer. Using a micro-structure manipulation tool, developed at the Institute of Physics, for the rotation-adapted stacking of 2D crystal layers,he places an ultrathin layer of molybdenum disulphide onto a silicon dioxide underlay © University of Münster - Peter Leßmann

Physicists investigate 2D materials with very special properties

Postdoc Dr. Nihit Saigal, a member of Prof. Ursula Wurstbauer’s team at Münster University’s Institute of Physics, has got everything ready in the laboratory to produce an ultra-thin, two-dimensional material - a silver-coloured crystal of molybdenum disulphide, a viscoelastic polymer film... and sticking tape. He carefully places the crystal on the sticking tape, so that a little of the material remains stuck to it. He then presses these traces of the material several times onto the polymer film. In the process, the traces become thinner and thinner. He looks at the result under the microscope and searches for a spot where the molybdenum disulphide is extremely thin - i.e., as precisely as possible, as thin as a molecule layer.

These steps represent the beginning of a long process, in which the 2D layer is placed with a high degree of precision on a silicon underlay coated with silicon dioxide, where it will later be examined for its properties. Such 2D materials are certainly something else: extremely thin layers, one or two molecule layers thick, have fundamentally different properties than the same material in three dimensions. If layers consisting of different materials are combined, or if the individual layers are twisted against each other, the result is more new properties - it is possible, for example, to make insulators out of electrically conductive materials.

The best-known example of a 2D material is graphene. On 22 October 2004, the "Science" journal published an article showing for the first time how graphene - a carbon layer with the thickness of a single atom - can be produced, namely by pressing a piece of sticking tape onto a piece of graphite and then peeling it off. Graphite crystals consist of layers of carbon atoms in a honeycomb form, i.e. of countless layers of graphene. In their article, the researchers also described the properties of the new material, and even today these properties are still astonishing: "It is the thinnest material in the world, stronger than steel and more conductive than copper," says Ursula Wurstbauer, summing up the properties. "In addition, graphene is an excellent heat conductor, as well as being flexible, transparent and impermeable to liquids and gases." Physicists Andre Geim and Kostya Novoselov received the Nobel Prize for Physics for their discovery.

Since then, researchers have tracked down thousands of materials which have a similarly layered structure to graphene - a real treasure chest for research work. The team headed by Ursula Wurstbauer is just one of a series of groups at the Faculty of Physics that are working in this field. Among other things, in its work on a special class of metal compounds - the so-called transition metals dichalcogenides - the team is investigating a special case among layered 2D materials: the twist. Here, the researchers twist two or more layers of the material against each other at small angles, thus producing different properties in the material - regardless of the angle - which result from a superimposition of the molecule patterns.

"The twist is the key to producing a wide range of different properties," says Wurstbauer. "By making precise twists we control whether particles - for example, electrons - can move within the molecular lattice or not. In this way, the same material can, in extreme cases, have either superconductive or insulating properties. Or, with a certain twist - and only then - a material emits individual photons. This is of interest for possible applications in quantum technologies."

Dr. Emeline Nysten, a postdoc in Prof. Hubert Krenner’s working group, provides another example of the research being done. She is examining the impact that acoustic waves have on the electrical properties of two-layered 2D materials. The question behind this is: Can the surface acoustic waves control and programme the electric fields? "These effects may be of interest for producing non-volatile means of storage which can preserve information long-term just as a computer’s memory does," Nysten explains. "One advantage is that, unlike conventional magnetic means of storage, such ferroelectric storage can be further miniaturised and is more resilient to electromagnetic radiation."

Although the working groups at the Faculty of Physics are mostly undertaking basic research, there is an example of applied research in Ursula Wurstbauer’s team. A collaborative project with Prof. Rebecca Saive at the University of Twente is looking at improving the efficiency of solar cells. "The question," says Wurstbauer, "is how we can transfer the charge carriers from the solar cells into the power grid without any losses occurring. At the moment, it’s a bottleneck, and there is a high level of loss." What the researchers hope is that they can minimise these losses as a result of improved electrical

This article is from the university newspaper wissen