2D or not 2D: EPFL scientists revised a long-standing paradigm on the physics of layered materials

An international collaboration led by EPFL researchers has unveiled the unexpected nature of electron’s motion in a layered material. This discovery is disrupting current scientific beliefs, and can influence the design of future electronic devices based on 2D conductors.

We have unveiled the unexpected nature of electron’s motion in a layered material. Scientists have spent decades investigating layered crystals, as graphite, motivated by the emergence of surprising quantum properties such as superconductivity. They concluded that in these materials, electrons are confined within each individual atomic plane. Thanks to advanced microfabrication technologies, we have revised this paradigm in a highly debated layered materials hosting complex electronic phases such as superconductivity and insulating phases: 1T-TaS2. With large surprise, we observed that the current estimation that electrons move 1000 times better within the atomic planes is incorrect. In this material electrons have a surprising preference to jump between atomic planes, as a consequence of a unique self-organization of the atoms and creation of an orbital ordering. As layered materials are the building block for 2D-conductors this results will influence the design of future electronic devices based on 2D materials.

Due to their exceptional electronic conduction and optical properties, this class of 2D materials is expected to revolutionize the field of optoelectronic devices. This field of research started with the pioneering work on graphene, and was awarded the Physics Nobel prize in 2010. Since then it has grown thanks to the plentiful variety of accessible materials, the opportunity to easily fabricate electronic devices, and the ability to combine different layers into artificial materials with tailored properties.

Crystals of only one atom thickness are commonly obtained by starting with layered materials, where thousands of individual atomic planes are stacked together by weak van der Waals interactions. Scientists have spent many decades investigating layered crystals, motivated by the emergence of surprising quantum properties such as superconductivity, strong electronic correlations, and charge density waves. They concluded that in these materials, electrons are confined within each individual atomic plane, living in a two dimensional world and rarely adventuring along the third dimension.

The atomic structure of 1T-TaS2, where individual layers are stacked along the c-axis. At low temperature, the atoms change position, moving closer together to create cluster of 13 atoms with a characteristic Star of David shape. The central figure shows the atoms displacement with red arrows and highlight the cluster contours with a blue line. The graph on the right display the surprising conduction properties, that clearly shows that transport of electrical current is much easier along the out-of-plane direction, in particular as temperature is reduced. The inset displays the nanoscale pattern created by the 13-atom star cluster along each atomic plane. The individual stars form hexagonal domains of few nanometers in size (millionth of a millimeter).

Thanks to advanced microfabrication technologies—based on focused ion beams, and the precise characterization of the material using synchrotron radiation facilities—a team of EPFL scientists have challenged this long-standing belief. In their work, published on open access Nature partner journal 2D Materials and Applications (https://www.nature.com/articles/s41699-020-0145-z), they have investigated one of the most complex and debated layered transition metal dichalcogenide: 1T-TaS2. This compound fascinated scientists because it collects an extremely rich variety of electronic phases, and its properties are strongly influenced by the appearance of “stars” made by small clusters of 13 atoms that self-organize within the weakly bounded atomic planes.

On the left, the image of the microstructured sample created by researchers at EPFL, to measure with extreme precision the conduction properties of a layered single crystal of 1T-TaS2, coloured in purple. The arrows indicate the crystal’s orientation, with the atomic layer along the ab-plane.  The left image shows the result of the finite elements simulations to analyse the electric potential distribution, and validate the surprising results observed.

By measuring the flow of electrical current along both and perpendicular to the atomic planes, the EPFL team came to a surprising result. In certain conditions the electrons largely prefer to jump between different atomic planes, rather than moving within them. This result is not only stunning, but is also in stark contrast with what was previously published, which suggested electron’s motion is 1’000 times better along the planes. This great contrast of results comes from a new and extraordinary more precise experimental procedure developed in close collaboration between the Laboratory of Physics of Complex Matter (iPhys), and the Laboratory of Quantum Matter (MX). To further confirm the validity of their new results the team supplemented the work with finite element simulations to ensure that no room for doubt was left; that electrons do indeed prefer to move perpendicularly to the atomic planes.

The surprising nature of electrons motion in 1T-TaS2 is understood to originate from the characteristic orbital order within this material. The electrons follow the path defined by the overlap of each individual atomic orbital inside the crystalline lattice. For this specific compound, electrons move along the so called  orbitals which belong to the Ta atoms. These orbitals have a characteristic shape of two lobes extending in opposite directions. Inside the crystal structure of 1T-TaS2, the lobes extend perpendicular to the layers, helping electrons move more easily between then. This new evidence is of relevance for scientists, as it is a strong argument for revising how we understand the complex properties observed in this material, above all the sudden transformation into an enigmatic electrically insulating state at low temperatures. All previous theories that described this material’s electronic properties were based on the assumption that electrons live in a 2D world, and have little interaction with those in neighbouring layers. This new discovery is now supporting the emergence of new theories that carefully put into consideration the role of interaction between separated layers.

Edoardo Martino (left) and Konstantin Semeniuk (right) are the two EPFL scientists who led and coordinated the project.