The monolayers of two-dimensional (2D) crystals define a new class of atomically thin elastic materials, exhibiting simultaneously exceptional elastic stiffness [1–3] and extremely small bending rigidity.[4, 5] The discrete nature in the thickness of 2D-crystal membranes is expected to introduce new nanomechanical phenomena in the monolayer regime when classical plate idealizations fail.[6] Here, we report a novel type of rippling instability in substrate-supported monolayer molybdenum disulphide (MoS2) induced by centrosymmetric bilayer epitaxy. Interlayer coupling originating from 2H bilayer growth creates 3-fold biaxial strain in the triangular monolayer, which is stretched along the altitude directions and sheared along the bilayer edges. The resulting quasi-periodical textures of parallel one-dimensional nanoripple arrays (1D NRAs) show surprisingly robust resistance to the perturbation of substrate roughness, and disobey the classical elasticity theory. The results elucidate that nanomechanical behavior is not specific to sp2-bonded graphene, but a genuine feature for 2D-crystal monolayers when interlayer coupling is absent. The formation of 1D NRAs significantly changes the nanotribological properties and the energy band structures of monolayer MoS2, which may pave a new way in strain and friction engineering in monolayer transitional metal dichalcogenides.

The monolayers of two-dimensional (2D) crystals define a new class of atomically thin elastic materials, exhibiting simultaneously exceptional elastic stiffness [1–3] and extremely small bending rigidity.[4, 5] The discrete nature in the thickness of 2D-crystal membranes is expected to introduce new nanomechanical phenomena in the monolayer regime when classical plate idealizations fail.[6] Here, we report a novel type of rippling instability in substrate-supported monolayer molybdenum disulphide (MoS2) induced by centrosymmetric bilayer epitaxy. Interlayer coupling originating from 2H bilayer growth creates 3-fold biaxial strain in the triangular monolayer, which is stretched along the altitude directions and sheared along the bilayer edges. The resulting quasi-periodical textures of parallel one-dimensional nanoripple arrays (1D NRAs) show surprisingly robust resistance to the perturbation of substrate roughness, and disobey the classical elasticity theory. The results elucidate that nanomechanical behavior is not specific to sp2-bonded graphene, but a genuine feature for 2D-crystal monolayers when interlayer coupling is absent. The formation of 1D NRAs significantly changes the nanotribological properties and the energy band structures of monolayer MoS2, which may pave a new way in strain and friction engineering in monolayer transitional metal dichalcogenides.