Dynamical observations on the crack tip zone and stress corrosion of two-dimensional MoS2

Abstract

Whether and how fracture mechanics needs to be modified for small length scales and in systems of reduced dimensionality remains an open debate. Here, employing in situ transmission electron microscopy, atomic structures and dislocation dynamics in the crack tip zone of a propagating crack in two-dimensional (2D) monolayer MoS 2 membrane are observed, and atom-to-atom displacement mapping is obtained. The electron beam is used to initiate the crack; during in situ observation of crack propagation the electron beam effect is minimized. The observed high-frequency emission of dislocations is beyond previous understanding of the fracture of brittle MoS 2 . Strain analysis reveals dislocation emission to be closely associated with the crack propagation path in nanoscale. The critical crack tip plastic zone size of nearly perfect 2D MoS 2 is between 2 and 5 nm, although it can grow to 10 nm under corrosive conditions such as ultraviolet light exposure, showing enhanced dislocation activity via defect generation.

Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys

“…Indeed, defects severely reduce the critical strain of TMD crystals and cause the runaway propagation of cracks in a substantially smaller strain regime. Our conclusion closely agrees with a recently reported in-situ transmission electron microscopy study on monolayer MoS 2 , 26 where defects were found to serve as initial sites from which cracks propagate along Y-shaped junctions, similar to our observations in Fig. 3.…”

“…Given that the origin of the strain is composition conversion, we think that unsaturated bonds (at the edges) or defect sites (within the interior domains of the film) serve as local sites at which the incorporation of S atoms into the lattice of MoSe 2 is initiated. A similar contribution of defect sites in the formation of cracks and Y-shaped branching behavior has been recorded in TMD 26 films subjected to biaxial strain.…”

“…In addition, the Van der Waals interaction of monolayer crystals with the substrate provides a medium for the strain fields of individual cracks to mutually interact, defining a net propagation path between the Y-shaped junctions. Theoretical modeling based on molecular dynamics 27 and classical fracture mechanics 26 are effective for the qualitative prediction of the crack propagation path in monolayer MoS 2 crystals under biaxial strain. In these models, the strain field is assumed to extend to a critical radius of (r c ) from the tip of the propagating cracks.…”

Facile Doping in Two-Dimensional Transition-Metal Dichalcogenides by UV Light

“…Line patterns emerged with height ~ 0.8 nm and width ~ 50 nm, roughness was also increased to ~3 nm ( Figure 1f), which probably comes from the functionalization groups after oxidation of MoS 2 19-22 . Meanwhile, stress corrosion cracking (SCC) 29 was also initiated, with cracking depth 0.8 nm (1L thickness) and width ~ 50 nm.…”

“…While after UV irradiation under 65% humid level for 6 min, Figure 2a showed the topographic AFM image demonstrating the uniformity of the triangle shape 1L MoS 2 (thickness ~ 0.8 nm, roughness ~ 0.1 nm). Only several brighter lines caused by the SCC cracks due to initial oxidation and large strain mismatch between the flake and sapphire substrate 29,33 . It is also reasonable that the CVD grown samples were cracked under a lower humidity level than mechanical exfoliated samples with same exposure time, because the 6 CVD samples intrinsically possess more atomic defects or vacancies than the mechanical exfoliated samples.…”