New theoretical studies of material properties as a function of thickness in chalcogenides

by Virginia Greco

A study carried out by researchers from the Institute for Theoretical Solid State Physics in Aachen (Germany) and from the ICN2, and recently published in Advanced Materials Communications, reveals that materials of two subfamilies of chalcogenides show different dependence of their properties on slab thickness. This reflects on ferroelectric characteristics, which are relevant to their applications in information technology.

After the discovery of the striking properties of graphene, a material made up of a single layer of carbon atoms, 2D and few-layer materials have been studied with increasing interest due to their possible applications in various fields. What makes them special is the fact that their properties differ from those of the corresponding bulk materials, which are composed by more layers of the same kind. These differences are strictly connected with the kind of bonds that keep the layers together.

In order to better understand their characteristics and predict the behaviour of possible novel 2D and few-layer materials, theoretical simulations and computational resources are largely employed. Among the recently discovered families of 2D compounds, group IV chalcogenides (which are composed of an element of the IV group of the elements table, as Ge or Sn, and a chalcogen, as S, Se or Te) are particularly interesting since they exhibit remarkable electronic properties, including in-plane ferroelectric polarization.

In a work developed in close collaboration by Dr Ider Ronneberger, from the Institute for Theoretical Solid State Physics in Aachen (Germany), and Dr Zeila Zanolli, from the ICN2 Theory and Simulation group, and recently published in Advanced Materials Communications, the properties of a few compounds, representative of two subfamilies of group IV chalcogenides, have been investigated as a function of the slab thickness, using thin-film computational models. In particular, the study focused on analyzing, on one side, the behaviour of thin films of two selenides, GeSe and SnSe, which are held together by covalent bonds in their bulk state, and, on the other, that of thin films of two tellurides, GeTe and SnTe, which show an unconventional form of bonding, called metavalent bonding (MVB).

The simulations showed that, in the case of the selenides, bulk properties are recovered increasing the thickness of the material to just a few layers, while the structure of the tellurides thin films exhibits pronounced deviations from the bulk counterparts, even for thicknesses exceeding 18 bilayers (a bilayer is the “unit” of layers used in this research because more suitable to this study). As a result, these two groups of materials also present different ferroelectric properties, which are relevant to their possible applications, in particular to information technology.

This study provides crucial information about the characteristics and the bondings of few-layer structures from different families of compounds and allows predicting the behaviour of other 2D materials. Such knowledge is key to developing tools to tune materials properties according to the desired application.

Reference article:

Ronneberger I., Zanolli Z., Wuttig M. and Mazzarello R., Changes of Structure and Bonding with Thickness in Chalcogenide Thin Films, Adv. Mater. 2020. DOI: 10.1002/adma.202001033