CMC discovers flawed theory behind widely used materials

DNRF head of center Bo Brummerstedt Iversen is one of the authors of a new paper in Nature Materials which shows that parts of the theory behind van der Waals materials do not match reality. Therefore, the theory must be reconsidered before researchers can fully understand the materials, which may be of great importance in future energy research.    

The SPring-8 synchrotron facility in Japan, where the researchers conducted the experiments (Photo: Wikimedia Commons)

The theoretical modeling of so-called van der Waals materials needs to be reconsidered and improved. This is the conclusion of a newly published paper by the head of the Center for Materials Crystallography (CMC) at Aarhus University, Bo Brummerstedt Iversen, and colleagues. The paper was published in the recognized scientific journal Nature Materials, and the results of the research may be of great importance for future energy research.

“So far, there have been mostly theoretical calculations and few experiments in the field, but in our experiments, we have made very accurate measurements using one of the world’s most powerful X-ray facilities. The results show something different than the theory describes,” said Brummerstedt Iversen.

The materials are used in everything from pencils to batteries
Van der Waals materials are layered materials that are held together by the so-called van der Waals force, which denotes the weak interaction between the molecules in the layers of the materials. Therefore, “van der Waals materials” is a broad term, but the materials are particularly applicable in energy technologies, for example, in batteries.

One of the most commonly known van der Waals materials is graphite, which together with clay forms the so-called lead in pencils, but is also used to construct anodes in batteries. In the pencil, the weak interaction between graphite layers can be seen when the layers easily loosen from the lead as we write before transforming into letters on the paper. Graphite is also used to produce the wonder material graphene, a single-layer carbon atom material, far stronger than steel and an amazing conductor of heat and electricity.

In the newly published study, Brummerstedt Iversen and his colleagues worked with titanium disulfide, a generic van der Waals material. Therefore, the results can be applied as a basis for all materials that are held together by van der Waals forces.

“The result indicates that the interaction between the layers is stronger than previously thought. In the experiment, we see it because there are more electrons present between the layers than the theory says,” noted the CMC’s head of center, commenting on the measurements, which were conducted at the SPring-8 facility in Japan.

“Nothing useful comes out of working on a false basis”
In their paper Brummerstedt Iversen and his colleagues argue that when theory does not correctly predict the electron density in the materials, one cannot rely on other parts of the theory, which, for instance, describes materials’ ability to conduct and transport electrical energy. This is a vital feature in terms of understanding the potential of van der Waals materials in energy research.

“When the theory does not fit into our experiments, we do not have a correct model for van der Waals materials. It is problematic, because it is difficult to utilize these materials from a societal perspective if we do not fully understand the materials. This means that we must work towards matching experiments with theory. Nothing useful comes out of working on a false basis,” explained Brummerstedt Iversen.