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

Abstract

We demonstrate that the shear elastic constant of misfit-layered dichalcogenide films ${[\mathrm{SnSe}]}_{n}{[{\mathrm{MoSe}}_{2}]}_{n}$ with $n=1,2,3$, synthesized by the modulated elemental reactants method, is ${c}_{44}\ensuremath{\approx}1$ GPa, an order of magnitude lower than ${c}_{44}$ 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 $\ensuremath{\approx}60$ nm followed by thermal annealing. We determine ${c}_{44}$ through measurements of the velocities of 700 nm wavelength surface acoustic waves propagating along the surface of Al/${[\mathrm{SnSe}]}_{n}{[{\mathrm{MoSe}}_{2}]}_{n}$/Si structures in combination with picosecond acoustics measurements of ${c}_{33}$ and calculations of the ${c}_{11}$, ${c}_{12}$, and ${c}_{13}$ elastic constants by density functional theory. We attribute the low value of ${c}_{44}$ to incommensurate interfaces between SnSe and ${\mathrm{MoSe}}_{2}$ layers and turbostratic disorder within the ${\mathrm{MoSe}}_{2}$ layers. We conclude that the ultralow shear modulus of disordered layered materials contributes significantly to their exceptionally low thermal conductivity.