Abstract:We present in-situ transmission electron microscopy of crack propagation in a freestanding monolayer MoS and molecular dynamic analysis of the underlying mechanisms. Chemical vapor deposited monolayer MoS was transferred from sapphire substrate using interfacial etching for defect and contamination minimization. Atomic resolution imaging shows crack tip atoms sustaining 14.5% strain before bond breaking, while the stress field decays at unprecedented rate of 2.15 GPa Å. Crack propagation is seen mostly in the … Show more
“…During the transgranular propagation, stress-concentration is highly localized at Mo atoms of the crack tip (Figure c); however, the stress on S atoms becomes remarkably relieved (Figure S6c). This reveals that the Mo atoms play a crucial role in load bearing, while S atoms are very important in sustaining large stress at the sharp crack tip, in agreement with recent MD calculations . No evident lattice reconstruction in both tip regions is detected.…”
Section: Resultssupporting
confidence: 89%
“…There have also been a number of pioneering studies examining the mechanical properties of defect-free MoS 2, both experimentally and theoretically. ,− However, very few attempts have been made to assess the effect of topological defects on the mechanical properties of MoS 2 crystals. − Using computer simulations, it was revealed that point defects, including V 1S , V 2S , V Mo , V MoS 2 , and V MoS 3 , degrade the in-plane tensile properties of single-crystal MoS 2 because of stress-concentration at the defect locations. , Very recently, in situ experiments and molecular dynamics (MD) simulations showed that the crack propagating behavior in crystalline domain of CVD-grown monolayer MoS 2 was governed by the concentration of point defects ahead of the crack tip zone and exposed environments. , For example, crack deflections occurred in sparse vacancy defects of MoS 2 crystals, while, as a result of migration of vacancies in the strain fields into networks, a transition from brittle to ductile in fracture mechanism was discovered as the density of vacancy defects exceeds a critical value …”
Pristine monocrystalline molybdenum disulfide (MoS) possesses high mechanical strength comparable to that of stainless steel. Large-area chemical-vapor-deposited monolayer MoS tends to be polycrystalline with intrinsic grain boundaries (GBs). Topological defects and grain size skillfully alter its physical properties in a variety of materials; however, the polycrystallinity and its role played in the mechanical performance of the emerging single-layer MoS remain largely unknown. Here, using large-scale atomistic simulations, GB structures and mechanical characteristics of realistic single-layered polycrystalline MoS of varying grain size prepared by confinement-quenched method are investigated. Depending on misorientation angle, structural energetics of polar-GBs in polycrystals favor diverse dislocation cores, consistent with experimental observations. Polycrystals exhibit grain-size-dependent thermally induced global out-of-plane deformation, although defective GBs in MoS show planar structures that are in contrast to the graphene. Tensile tests show that presence of cohesive GBs pronouncedly deteriorates the in-plane mechanical properties of MoS. Both stiffness and strength follow an inverse pseudo Hall-Petch relation to grain size, which is shown to be governed by the weakest link mechanism. Under uniaxial tension, transgranular crack propagates with small deflection, whereas upon biaxial stretching, the crack grows in a kinked manner with large deflection. These findings shed new light in GB-based engineering and control of mechanical properties of MoS crystals toward real-world applications in flexible electronics and nanoelectromechanical systems.
“…During the transgranular propagation, stress-concentration is highly localized at Mo atoms of the crack tip (Figure c); however, the stress on S atoms becomes remarkably relieved (Figure S6c). This reveals that the Mo atoms play a crucial role in load bearing, while S atoms are very important in sustaining large stress at the sharp crack tip, in agreement with recent MD calculations . No evident lattice reconstruction in both tip regions is detected.…”
Section: Resultssupporting
confidence: 89%
“…There have also been a number of pioneering studies examining the mechanical properties of defect-free MoS 2, both experimentally and theoretically. ,− However, very few attempts have been made to assess the effect of topological defects on the mechanical properties of MoS 2 crystals. − Using computer simulations, it was revealed that point defects, including V 1S , V 2S , V Mo , V MoS 2 , and V MoS 3 , degrade the in-plane tensile properties of single-crystal MoS 2 because of stress-concentration at the defect locations. , Very recently, in situ experiments and molecular dynamics (MD) simulations showed that the crack propagating behavior in crystalline domain of CVD-grown monolayer MoS 2 was governed by the concentration of point defects ahead of the crack tip zone and exposed environments. , For example, crack deflections occurred in sparse vacancy defects of MoS 2 crystals, while, as a result of migration of vacancies in the strain fields into networks, a transition from brittle to ductile in fracture mechanism was discovered as the density of vacancy defects exceeds a critical value …”
Pristine monocrystalline molybdenum disulfide (MoS) possesses high mechanical strength comparable to that of stainless steel. Large-area chemical-vapor-deposited monolayer MoS tends to be polycrystalline with intrinsic grain boundaries (GBs). Topological defects and grain size skillfully alter its physical properties in a variety of materials; however, the polycrystallinity and its role played in the mechanical performance of the emerging single-layer MoS remain largely unknown. Here, using large-scale atomistic simulations, GB structures and mechanical characteristics of realistic single-layered polycrystalline MoS of varying grain size prepared by confinement-quenched method are investigated. Depending on misorientation angle, structural energetics of polar-GBs in polycrystals favor diverse dislocation cores, consistent with experimental observations. Polycrystals exhibit grain-size-dependent thermally induced global out-of-plane deformation, although defective GBs in MoS show planar structures that are in contrast to the graphene. Tensile tests show that presence of cohesive GBs pronouncedly deteriorates the in-plane mechanical properties of MoS. Both stiffness and strength follow an inverse pseudo Hall-Petch relation to grain size, which is shown to be governed by the weakest link mechanism. Under uniaxial tension, transgranular crack propagates with small deflection, whereas upon biaxial stretching, the crack grows in a kinked manner with large deflection. These findings shed new light in GB-based engineering and control of mechanical properties of MoS crystals toward real-world applications in flexible electronics and nanoelectromechanical systems.
“…This shows that under uniaxial tension loading, the crack tends to propagate along the zigzag edge, implying that the zigzag's edge energy is lower than the armchair's edge energy. In a recently published study, Islam et al 27 explained the crack mechanism that preferentially propagates along the zigzag edge, which is consistent with the previous experimental study 43 for MoS 2 material. Figure 2 The atomic shear strain evolution and deformation behavior of monolayer MoS 2 membrane under tension ( a ) along the x (armchair) direction at 1 K, ( b ) along the y (zigzag) direction at 1 K. …”
For practical application, determining the thermal and mechanical characterization of nanoporous two-dimensional MoS2 membranes is critical. To understand the influences of the temperature and porosity on the mechanical properties of single-layer MoS2 membrane, uniaxial and biaxial tensions were conducted using molecular dynamics simulations. It was found that Young’s modulus, ultimate strength, and fracture strain reduce with the temperature increases. At the same time, porosity effects were found to cause a decrease in the ultimate strength, fracture strain, and Young’s modulus of MoS2 membranes. Because the pore exists, the most considerable stresses will be concentrated around the pore site throughout uniaxial and biaxial tensile tests, increasing the possibility of fracture compared to tensing the pristine membrane. Moreover, this article investigates the impacts of temperature, porosity, and length size on the thermal conductivity of MoS2 membrane using the non-equilibrium molecular dynamics (NEMD) method. The results show that the thermal conductivity of the MoS2 membrane is strongly dependent on the temperature, porosity, and length size. Specifically, the thermal conductivity decreases as the temperature increases, and the thermal conductivity reduces as the porosity density increases. Interestingly, the thermal and mechanical properties of the pristine MoS2 membrane are similar in armchair and zigzag directions.
“…Our findings can be qualitatively understood in the framework of several theoretical works, , where molecular dynamics simulations show that crack propagation in MoS 2 monolayers is governed by sulfur vacancy concentrations ahead of the crack tip. For pristine MoS 2 , the crack propagates through a preferential direction with the least energy release; sparse vacancies create crack deflection and therefore strain energy.…”
In
two-dimensional crystals, fractures propagate easily, thus restricting
their mechanical reliability. This work demonstrates that controlled
defect creation constitutes an effective approach to avoid catastrophic
failure in MoS2 monolayers. A systematic study of fracture
mechanics in MoS2 monolayers as a function of the density
of atomic vacancies, created by ion irradiation, is reported. Pristine
and irradiated materials were studied by atomic force microscopy,
high-resolution scanning transmission electron microscopy, and Raman
spectroscopy. By inducing ruptures through nanoindentations, we determine
the strength and length of the propagated cracks within MoS2 atom-thick membranes as a function of the density and type of the
atomic vacancies. We find that a 0.15% atomic vacancy induces a decrease
of 40% in strength with respect to that of pristine samples. In contrast,
while tear holes in pristine 2D membranes span several microns, they
are restricted to a few nanometers in the presence of atomic and nanometer-sized
vacancies, thus increasing the material’s fracture toughness.
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