Confined lattice dynamics of single and quadruple SnSe bilayers in [(SnSe)_1.04]_m[MoSe₂]_n ferecrystals

The Sn specific densities of phonon states in the SnSe subunits of [(SnSe)_1.04]_m[MoSe₂]_n ferecrystals with (m,n) = (1,1), (4,1) and in bulk SnSe were derived from nuclear inelastic scattering by the ¹¹⁹Sn Mössbauer resonance. Using different measurement configurations, phonons with polarization parallel and perpendicular to the ferecrystal plane were specifically probed. Vibrational properties and phonon spectral weight are found to strongly depend on the phonon polarization and layer count m. A highly peculiar feature of these ferecrystal densities of phonon states is the emergence of rather sharp high energy vibrational modes polarized perpendicular to the ferecrystal plane, which contrasts with usual findings in thin layered structures and nanostructured materials in general, and a depletion of modes with a gap appearing between acoustic and high energy modes. The spectral weight of these phonons depends on the overall SnSe content, m, but cannot be unambiguously attributed to SnSe-MoSe₂ interfaces. Considering the low energy part of lattice dynamics, ferecrystals exhibit rather low average phonon group velocities as compared to the speed of sound in the long wavelength limit. For the (1,1) ferecrystal, this effect is most pronounced for vibrations polarized in the ferecrystal plane. Thus, an experimental microscopic origin for the vibrational and bonding anisotropy in subunits of ferecrystals is provided.