  The interlayer bonding in two dimensional materials is particularly important
because it is not only related to their physical and chemical stability but
also affects their mechanical, thermal, electronic, optical, and other
properties. To address this issue, we report the direct characterization of the
interlayer bonding in 2D SnSe using contact-resonance atomic force microscopy
in this study. Site specific CR spectroscopy and CR force spectroscopy
measurements are performed on both SnSe and its supporting SiO2 substrate
comparatively. Based on the cantilever and contact mechanic models, the contact
stiffness and vertical Young's modulus are evaluated in comparison with SiO2 as
a reference material. The interlayer bonding of SnSe is further analyzed in
combination with the semi-analytical model and density functional theory
calculations. The direct characterization of interlayer interactions using this
nondestructive methodology of CR AFM would facilitate a better understanding of
the physical and chemical properties of 2D layered materials, specifically for
interlayer intercalation and vertical heterostructures.
