Ultralow shear modulus of incommensurate [SnSe]n[MoSe2]n layers synthesized by the method of modulated elemental reactants

We demonstrate that the shear elastic constant of misfit-layered dichalcogenide films [SnSe]n[MoSe2]n with n=1,2,3, synthesized by the modulated elemental reactants method, is c44≈1 GPa, an order of magnitude lower than c44 of typical layered crystals that have weak interlayer van der Waals bonding. The films are synthesized by alternating deposition of the elements to a total thickness of ≈60 nm followed by thermal annealing. We determine c44 through measurements of the velocities of 700 nm wavelength surface acoustic waves propagating along the surface of Al/[SnSe]n[MoSe2]n/Si structures in combination with picosecond acoustics measurements of c33 and calculations of the c11, c12, and c13 elastic constants by density functional theory. We attribute the low value of c44 to incommensurate interfaces between SnSe and MoSe2 layers and turbostratic disorder within the MoSe2 layers. We conclude that the ultralow shear modulus of disordered layered materials contributes significantly to their exceptionally low thermal conductivity.