Shear anisotropy-driven crystallographic orientation imaging in flexible hexagonal two-dimensional atomic crystals

Hexagonal two-dimensional (2D) atomic crystals commonly reveal intrinsically isotropic elastic properties, but stretching or bending deformation can lead to their mechanical symmetry breaking. So far, little work has been done on strain-induced in-plane anisotropic shear behaviors of such 2D atomic crystals. Here, in theory, we predict the appearance of in-plane shear stiffness anisotropy under uniaxial normal tension strain in monolayer molybdenum disulfide. We verify experimentally such a shear characteristic based on friction-driven stretch deformation during a contact scan by employing transverse shear microscopy, and we demonstrate the visualization of anisotropic shear deformation as a function of crystallographic orientation. The present work provides deep insights into flexibility governed interactions among friction, deformation, and in-plane elastic characteristics in 2D atomic crystals.