Two-dimensional materials have some incredible properties that can offer entirely new technical possibilities – just think of graphene and its use in anything from sensor technology to solar cells. However, accurately measuring the extreme internal stresses and strains such materials undergo has been challenging to do, until now.
Researchers at the Vienna University of Technology (TU Vienna) have recently reported on new methods to measure such alterations – which can dramatically change the physical properties of the two-dimensional material – in Nature Communications.
The new technique allows researchers to visualize precisely – point for point – at a microscopic level, how the properties of materials, which are only a few atomic layers thick, might be altered by simple distortions.
If a material is compressed or strained, the distance between individual atoms changes, and this change can affect the electronic properties of the material, a phenomenon which has been exploited in semiconductor technology. For example, silicon crystals can be grown so that they are under permanent internal mechanical stress.
Two-dimensional materials consisting of an ultra-thin layer could offer greater potential, believes Professor Thomas Müller from the Photonics Institute in the Faculty of Electrical Engineering and Information Technology at TU Vienna: "A crystal can be stretched by perhaps one percent before it breaks. With 2D materials, deformation of ten or twenty percent is possible", he says.
The deformation and the type of mechanical stresses present within the material can have a direct effect on its electronic properties, which might be changed entirely, for example, its electrons ability 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 study. This might involve transmission electron microscopy to observe the surface, and then measuring the average distance between atoms and determining any compression or stretching from that, he says.
This process has now been made much more straightforward and accurate thanks to TU Vienna researchers. It employs an effect called frequency doubling, which is also known as the second harmonic generation. During the process, two photons with the same frequency interact with a nonlinear material and combine to form new photons with twice the energy, twice the frequency and half the wavelength of the initial photons.
"If you irradiate specific materials – in our case a layer of molybdenum disulfide – with a suitable laser beam, the material may reflect back light of a different color", explains Müller.
Two photons from the incoming laser beam – which is red - combine to form a single photon with double the energy and is then emitted from the materials as blue light. The intensity of the effect does depend on the internal symmetry of the material. Molybdenum disulfide has a honeycomb structure – much like graphene – which is slightly distorted when stretched or compressed. Although only a small distortion, the effect on the intensity of light reflected back from the material is dramatic.
Positioning a layer of molybdenum disulfide over a microstructure – much like putting a rubber blanket over a climbing frame – results in an intricate pattern of local distortions. Employing a laser to scan the material point for point allows for the creation of a detailed map of the 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 is a powerful new tool that can be used for local, targeted adjustment of material properties, says Müller: "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."
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