How Do 2D Materials Crack?

To understand how thin two dimensional materials crack, we must go beyond existing theory, scientists say.

AsianScientist (Feb. 13, 2017) – Scientists from the Center for Integrated Nanostructure Physics (CINAP) at the Institute for Basic Science (IBS) have observed the cracking of two dimensional molybdenum disulfide (MoS2) at the atomic level for the first time. Their findings, published in Nature Communications, could help design new 2D materials.

2D materials like MoS2 have emerged as an important asset for future electronic and photoelectric devices. However, the mechanical properties of 2D materials are expected to differ greatly from 3D materials. In particular, it is currently difficult to explain and predict the shape and seriousness of a crack based on existing theory.

Ductile materials like gold can withstand large strains before rupturing while brittle materials like glass only absorb relatively little energy before breaking suddenly, without elongation and deformation. At the nano-level, atoms move freer in ductile materials than in brittle materials; so in the presence of a pulling force (tensile stress) they can go out of position from the ordered crystal structure, or in technical terms, they dislocate. So far, this explanation (Griffith model) has been applied to cracking phenomena in bulk, but it lacks experimental data at the atomic or nano-scale.

In this study, IBS scientists observed how cracks propagate on 2D MoS2 after a pore was formed either spontaneously or with an electron beam.

“The most difficult point [of the experiments] was to use the electron beam to create the pore without generating other defects or breaking the sample,” explains Dr. Ly Thuc Hue, first author of this study. “So we had to be fast and use a minimum amount of energy.”

The atomic observations were done using real-time transmission electron microscopy. Surprisingly, even though MoS2 is a brittle material, the team saw atom dislocations 3-5 nm away from the front line of the crack, or crack tip. This observation cannot be explained with the Griffith model.

In order to create conditions that represent the natural environment, the sample was exposed to ultraviolet light. This caused the MoS2 to oxidize; atom dislocations occurred more rapidly and the stretched region expanded to 5-10 nm from the crack tip.

“The study shows that cracking in 2D materials is fundamentally different from cracking in 3D ductile and brittle materials. These results cannot be explained with the conventional material failure theory, and we suggest that a new theory is needed,” explained CINAP Professor Lee Young Hee.

The article can be found at: Ly et al. (2017) Dynamical Observations on the Crack Tip Zone and Stress Corrosion of Two-dimensional MoS2.


Source: Institute for Basic Science.
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