3, Rice University, Houston, Texas, United States
2, Bruker Nano Surfaces, Minneapolis, Minnesota, United States
4, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
The search for new materials that could improve the existent scaling barriers of today’s electronics is in high demand1. Transition metal dichalcogenides (TMDC) are strong candidates on this matter due to their excellent electronic, mechanical and optical properties2. It was shown that mechanical strain can tune the electronic properties of TMDC3. Thus, stretching and bending methods are currently being used to deform these layered materials , utilizing for example atomic force microscopy4.
Another strategy to modify the behavior of these structures is by vertically stacking different combinations of TMDC. This is usually accomplished by methods like molecular beam epitaxy, mechanical transfer or chemical vapor deposition5. The production of devices made from these structures depends on the understanding of their electrical, optical and mechanical properties.
To shed light on deformation characteristics of vertical heterostructure MoS2/WS2, we performed fully atomistic molecular dynamics simulations using LAMMPS6. We considered monolayers of WS2 or MoS2 stacked on a monolayer MoS2. Similarities were found regarding the stress x strain in the structures at strains up to 1.5%. As the strain increases, the stress values shown drops at specific points, due to atomic rearrangements. Interestingly, this happens first for MoS2 bilayer, approximately at 1.6 % of strain. WS2 showed better resistance to tension and drop in stress was observed at about 2 % of strain. The main cause for this difference is due to interlayer interactions, stronger for WS2/MoS2 in comparison to MoS2/MoS2.
The sliding between these layered materials were also studied, applying a force on the top layer of the WS2/MoS2 layers to make it slide. The systems shown directional dependent plasticity that can be attributed to different levels of roughness at interface between the two layers.
We also analyzed the crack propagation for MoS2/MoS2 and MoS2/WS2. When the top layer is WS2, larger strain values are necessary to start altering the atomic arrangements of the structure prior to fracture. When the fracture happens, at first the crack propagates in a straight line and its edges have armchair configuration. This is due to the required force to cut the metal-chalcogenide layer along armchair-like edges, that is lower in comparison to zig-zag ones7. However, when the crack reaches the MoS2/WS2 interface, a deviation occurs and forms zig-zag edges. This shows that fracture patterns are highly chirality dependent in these heterostructures.
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