This shows how a 2D material such as molybdenum disulphide can be stretched and distorted. Image: TU Wien.
This shows how a 2D material such as molybdenum disulphide can be stretched and distorted. Image: TU Wien.

Two-dimensional (2D) materials such as graphene, which consist of only one or a few atomic layers, have proved to be a very promising aspect of materials science over recent years. They demonstrate remarkable properties that open up completely new technical possibilities, from sensor technology to solar cells.

However, there is one important phenomenon that could not be measured accurately up until now: the extreme internal stresses and strains that 2D materials may be subjected to, which can often drastically alter the material's physical properties. Researchers at TU Wien in Austria have now successfully measured these distortions in 2D materials at a microscopic level, making it possible to observe precisely (point-for-point) how the properties of a 2D material may be altered as a result of a simple distortion. This new measurement method is reported in a paper in Nature Communications.

When a material is stretched or compressed, the distance between its individual atoms changes, and this distance has an influence on the electronic properties of the material. This phenomenon has been used in semiconductor technology for years: silicon crystals, for example, can be grown so that they are permanently under internal mechanical stress.

However, 2D materials offer much greater potential. "A crystal can be stretched by perhaps 1% before it breaks. With 2D materials, deformation of 10% to 20% is possible," says Thomas Müller in the Photonics Institute (Faculty of Electrical Engineering and Information Technology) at TU Wien. Depending on the deformation and mechanical stresses present within the material, its electronic properties can completely change, such as the ability of electrons to absorb incoming light.

"Up until now, if you wanted to measure stresses present in this type of material you had to rely on extremely complicated measurement methods," explains Lukas Mennel, lead author of the paper. One such method involves observing the surface of the 2D material with a transmission electron microscope, measuring the average distance between the atoms and then deducing any stretching or compression from that measurement. The TU Wien researchers have now made this process much simpler and more accurate.

They have done this by taking advantage of a remarkable effect called frequency doubling. "If you irradiate specific materials – in our case a layer of molybdenum disulphide – with a suitable laser beam, the material may reflect back light of a different color," explains Müller. Two photons in the incoming laser beam can combine to form one photon with double the energy, which is emitted from the material.

However, the intensity of this effect depends on the internal symmetry of the material. Usually, molybdenum disulphide has a honeycomb-like structure, i.e. hexagonal symmetry. If the material is stretched or compressed, this symmetry is slightly distorted – and this small distortion has a dramatic effect on the intensity of the light reflected back from the material.

If a layer of molybdenum disulphide is placed over a microstructure, the result is a complex pattern of local distortions. The laser can now be used to scan the material point-for-point and thus obtain a detailed map of these stretches and compressions. "In doing so, not only can we measure the severity of these deformations, but we can also see the exact direction in which they run," explains Mennel.

This imaging method can now be used for the local, targeted adjustment of material properties. "For example, custom material deformations in solar cells could ensure that free charge carriers are diffused away in the right direction as quickly as possible", says Müller. This research on 2D materials means that a powerful new tool is now available.

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