Graphene stands out among the two-dimensional solids with its enormous electrical conductivity, but it is also very tensile. This extreme tensile strength is a result of the honeycomb-shaped arrangement of atoms in the material. The removal of some atoms from the material, including the associated bonds, should intuitively lead to a reduction in this tensile strength, but scientific studies have shown both a small reduction and a significant increase, as reported by the University of Vienna.
Graphene completely isolated for investigations
Researchers led by group leader Jani Kotakoski at the University of Vienna have now been able to clarify these contradictions with new measurements. The experiments took place in airless ultra-clean chambers, which were also connected to each other by airless metal tubes. This allowed the samples to pass from one device to the other without ever coming into contact with the ambient air. "This unique system, which we developed at the University of Vienna, allows us to study 2D materials without interference," explains Kotakoski. Wael Joudi, first author of the study, adds: "This is the first time that we have succeeded in permanently isolating the graphene from the ambient air and the foreign particles it contains during this type of experiment. Otherwise, these would be deposited on the surface within a short time and affect both the experiment and the measurement."
In fact, the focus on meticulous cleanliness of the material surface led to the discovery of the so-called accordion effect with regard to the mechanical tensile strength of graphene: Even the removal of just two neighboring atoms causes a certain curvature of the originally flat material. Together, several such curvatures result in a corrugation of the graphene: "You can imagine it like an accordion. When pulled apart, these waves are flattened, which requires much less force than stretching the flat material, which ultimately makes it more elastic," reports Joudi. Rika Saskia Windisch and Florian Libisch, both theoretical physicists at the Vienna University of Technology, have been able to confirm both the wave formation and the resulting lower tensile strength of the material with simulations.
Contradictory properties generated by disruptive factors
The experiments also showed that foreign particles on the surface of the material not only suppress this effect, but actually have the opposite effect. Specifically, this makes the material appear more tensile, which also explains the contradictions in the past. "This shows the importance of the measurement environment when dealing with 2D materials. The results open up a way to regulate the tensile strength of graphene and thus pave the way for potential applications," concludes Joudi. These could include, for example, the integration of electronics into clothing (wearable electronics).