At the last Graphene Week conference recently held in Athens, more than 700 international experts in graphene, a carbon-based sheet material with enormous potential, gathered. The congress was also attended by researcher Dr. Lucian Covaci (Arad-Romania, 1976) as the representative of the Condensed Matter Theory group led by Prof. François Peeters at the University of Antwerp, in Belgium.

Within the large European initiative Graphene Flagship, this group coordinates a project which, however, does not focus on graphene, but on other 2D materials of growing interest with a name that is difficult to pronounce: transition metal dichalcogenides (TMDs). Dr. Covaci details the characteristics and applications of these novel materials.

What exactly are the TMDs?

They are a family of two-dimensional semiconductor materials with the formula MX2, where M represents a transition metal –elements of the central part of the periodic table, such as molybdenum (Mo) or tungsten (W)– and X is a chalcogen –they are located in the column of the oxygen, such as sulfur (S), selenium (S) or tellurium (Te). Inserting a layer of atoms type M between two of type X we obtain the very thin, yet stable, monolayer of TMD. They have been known for half a century in its 3D version, but the graphene advances of the last decade have also triggered interest in these two-dimensional materials and their properties.

What are the advantages of TMDs with regard to graphene?

There are many benefits of the TMDs, and other two-dimensional materials like silicene, phosphorene or germanene, when compared to graphene, especially in electronics applications like field effect transistors. Within what is called bandgap energy, graphene is a zero-gap semiconductor, i.e. a semi-metal, and it can only be used as contact or gate. One requires a tunable gap in order to develop a transistor, and most TMDs are semiconductors with sizable bandgaps.

Are there any other differences?

Another shortcoming of graphene is the fact that spin-orbit interactions are very weak, and therefore a lot of interesting physics related to spintronics is missing. This is not the case in most of the other 2D materials, where the spin-orbit coupling is large, resulting in spin-split bands and the possibility of manipulating the spins of the electrons through electrical means. Nowadays ,there is a strong push towards combining TMDs with graphene, and this way inducing by proximity desired properties, like spin-orbit coupling.

And what is the European project you are working now?

It is called Trans2DTMD, a three-year project that began in 2016. This is a theoretical investigation of electronic transport in two-dimensional and 'functionalised' TMDs, a term that refers to that they are not pure, but have some element which changes their properties. The most interesting thing is that you can dope these materials by adding impurities (like introducing or varying some atom types) or changing its topology (at edges of nanoribbons and grain boundaries) to induce, for example, a metallic phase or magnetism.

An example of TMD: molybdenum disulfide (MoS2), which can be semiconducting (left) or metallic (right) depending on the arrangement of the atoms. / Trans2DTMD

And what are the main aims of Trans2DTMD?

On one hand, we develop novel numerical methods for electronic transport calculations in 2D materials. We use both commercial numerical codes and proprietary tools developed by us to investigate its structural, optical and electronic characteristics. On the other hand, we study the effects of defects and strains on the properties of metallic TMDs, as well as predict novel 2D materials with interesting properties.

What properties and applications have these TMDs?

Their topological properties are of utmost importance in electronic applications, for example, field effect transistors, photovoltaic devices and bio-sensors. TMDs are also relevant in spintronics, an emerging technology that exploits both the charge and spin of electrons, as well as in an even more novel one: the valleytronics, where energy "valleys" in the conduction band can trap and channel the electrons and act as a new degree of freedom.

What progress have you made so far?

To study these materials we are developing software based on the Pybinding code created at the University of Antwerp, which will also be an open source available to the entire scientific community. We have so far implemented efficient codes that can take advantage of state-of-the-art computational resources: computer clusters and video cards. This will allow for the investigation of generic systems where disorder effects on the electrical and optical conductivity of 2D materials.

What is this software for?

It will allow the simulation of very large systems (with billions of atoms) in order to capture effects that can then be observed in experimental samples. This is important since many of the theoretical tools fail to accurately study adatoms (individual atoms attached to a crystal) and defects that disperse the electricity. With this software, it will be possible to realize accurate spintronic and valleytronics simulations.

How many groups are involved in this project?

The Trans2DTMD project is coordinated by the Condensed Matter Theory group led by Prof. François Peeters at the University of Antwerp, with extensive experience in computational and theoretical tools to study TMDs. The second member of the consortium is the Theoretical Chemistry group led by Prof. Thomas Heine at the University of Leipzig (Germany). Coordination with Graphene Flagship and aspects related to graphene is done through the Technical University of Dresden (Germany). In addition, as associated partners, the University of Twente (Netherlands) specializes in spintronics; and the University of the Basque Country, provides calculations to analyze electron-electron interactions, an important aspect in 2D materials.

Have Trans2DTMD members already published any paper?

Together with other Graphene Flagship partners, the Leipzig University group has published in Nano Letters a paper about optical quality improvements in molybdenum diselenide (MoSe₂). They have also presented in 2D Materials calculations on potential and exotic properties of tantalum selenide (Ta₂Se₃), showing two Dirac points, rather than one as is the case of graphene. Within the effort to describe transport properties of two-dimensional materials, the University of Antwerp group has also submitted a study on the conductivity of twisted bilayer graphene for a wide range of angles of torsion and electric fields.