This shows the structure of 2D titanium disulfide and the forces between the different layers. Image: University of Tsukuba.
This shows the structure of 2D titanium disulfide and the forces between the different layers. Image: University of Tsukuba.

The discovery of graphene, with its high strength-to-weight ratio, flexibility, electrical conductivity and ability to form an impenetrable barrier, has led to an explosion of interest in 2D materials. Weak, long-range interactions between individual layers give 2D materials some of their most interesting behaviors, and so understanding the van der Waals interactions that hold these materials' layers together is crucial for further developing these materials. However, experimental support for theoretical modelling of van der Waals interactions has been wanting.

Now, an international team led by researchers at the University of Tsukuba in Japan and Aarhus University in Denmark has performed synchrotron X-ray diffraction experiments on titanium disulfide (TiS2) – a transition metal dichalogenide (TMD) material with a layered 2D structure – and compared the results with theoretical calculations. This benchmark work is reported in a paper in Nature Materials.

"The interaction between layers in van der Waals materials such as TiS2 has a significant bearing on their modification, processing and assembly," says co-author Eiji Nishibori from the University of Tsukuba. "By modelling experimental synchrotron data and comparing it with density functional theory calculations, we revealed surprising information about the nature of the electron sharing between layers in these materials."

TiS2 is an archetypal van der Waals material, with layers comprising sheets of titanium and sulfur atoms that interact through strong chemical bonds, where electrons are shared between atoms, resulting in a relatively fixed structure. Between these sheets, long-range van der Waals interactions attract the layers to one another, allowing them to build up to form solid materials.

These van der Waals interactions are known to be much weaker than those within the 2D sheets. However, using high-energy synchrotron X-ray radiation to precisely measure a single TiS2 crystal, the researchers were able to show that the interlayer interactions are in fact stronger than theory suggests, and involve significant electron sharing.

"This work provides a fundamental understanding of an exciting class of materials with numerous potential applications in technologies such as ion batteries, catalysis and superconductors," says lead author Hidetaka Kasai from the University of Tsukuba. "Our experiments are the first to reveal the true nature of the interactions that make 2D materials so interesting, and we hope they will underpin many future developments in this area."

The outstanding agreement of the synchrotron diffraction data with theoretical calculations in describing the intralayer Ti-S interactions supports the validity of these new-found differences for the long-range interactions across the interlayer gaps. The findings are expected to make major contributions to the fundamental understanding of weak chemical bonding in 2D layered materials in general, and to the development of TMD materials.

This story is adapted from material from the University of Tsukuba, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.