  Nanocontact properties of two-dimensional (2D) materials are closely
dependent on their unique nanomechanical systems, such as the number of atomic
layers and the supporting substrate. Here, we report a direct observation of
toplayer-dependent crystallographic orientation imaging of 2D materials with
the transverse shear microscopy (TSM). Three typical nanomechanical systems,
MoS2 on the amorphous SiO2/Si, graphene on the amorphous SiO2/Si, and MoS2 on
the crystallized Al2O3, have been investigated in detail. This experimental
observation reveals that puckering behaviour mainly occurs on the top layer of
2D materials, which is attributed to its direct contact adhesion with the AFM
tip. Furthermore, the result of crystallographic orientation imaging of
MoS2/SiO2/Si and MoS2/Al2O3 indicated that the underlying crystalline
substrates almost do not contribute to the puckering effect of 2D materials.
Our work directly revealed the top layer dependent puckering properties of 2D
material, and demonstrate the general applications of TSM in the bilayer 2D
systems.
