Raman spectroscopy insights into the size-induced structural transformation in SnSe nanolayers

Raman spectroscopy is used to probe the structural changes in [SnSe]m[MoSe2]n ferecrystal thin films as a function of m, the number of bilayers of SnSe. In spite of the interleaved structure in the intergrowths, Raman spectra can be described as a superposition of spectra from the individual components, indicating that the interaction at the interface between the components is relatively weak. Analysis of room-temperature Raman spectra indicate that the MoSe2 layers separating the SnSe layers are nanocrystalline in all of the samples studied, with little change as the number of Se-Mo-Se trilayers (n) or SnSe bilayers (m) increases, reflecting the rotational disorder between adjacent trilayers. A thickness-dependent, continuous transition occurs in the SnSe layer as m is increased, from a pseudotetragonal structure when the layers are thin to a bulk-like orthorhombic SnSe structure when the SnSe layer thickness is increased. Polarization analysis of the Raman scattering from these materials allows the symmetry evolution of the SnSe layers through this transition to be determined.