  Transition metal dichalcogenides are 2D structures with remarkable
electronic, chemical, optical and mechanical properties. Monolayer and crystal
properties of these structures have been extensively investigated, but a
detailed understanding of the properties of their few-layer structures are
still missing. In this work we investigated the mechanical differences between
monolayer and multilayer WSe2 and MoSe2, through fully atomistic molecular
dynamics simulations (MD). It was observed that single layer WSe2/MoSe2
deposited on silicon substrates have larger friction coefficients than 2, 3 and
4 layered structures. For all considered cases it is always easier to peel off
and/or to fracture MoSe2 structures. These results suggest that the
interactions between first layer and substrate are stronger than interlayer
interactions themselves. Similar findings have been reported for other
nanomaterials and it has been speculated whether this is a universal-like
behavior for 2D layered materials. We have also analyzed fracture patterns. Our
results show that fracture is chirality dependent with crack propagation
preferentially perpendicular to W(Mo)-Se bonds and faster for zig-zag-like
defects.
