Fracturing of Polycrystalline MoS₂ Nanofilms

The possibility of tailoring the critical strain of two-dimensional (2D) materials will be crucial for the fabrication of flexible and stretchable devices. While crystalline MoS2 monolayer shows tensile strength comparable to that of steel, a large concentration of defects and grain boundaries in polycrystalline MoS2 significantly degrades its mechanical properties. In this paper, the fracture in polycrystalline MoS2 films with an average grain size below 10 nm is studied at the micro- and nanoscale using electron microscopy. Two samples with different thicknesses and grain orientations horizontal and vertical to the sample plane are measured. The critical uniaxial strain is determined to be approximately 5% and independent of the sample morphology. However, electron beam irradiation is found to enhance the interaction between MoS2 and polydimethylsiloxane (PDMS) substrates, leading to an increased critical strain that can exceed 10%. This enhancement of strain resistance was used to fabricate a mechanically robust array of MoS2 lines 1 mm in length. Finally, nanoscale crack propagation studied by transmission electron microscopy showed that cracks propagate along the grain boundaries as well as through the grains, preferentially along van der Waals planes. These results provide insight into the fracture of polycrystalline 2D materials and a method to enhance the critical strain.