Davydov splitting and polytypism in few-layer MoS ₂

Abstract The dependence of the interlayer interaction on the stacking order of MoS ₂ is investigated by Raman spectroscopy using three excitation energies of 2.81, 2.41, and 1.96 eV. The low-frequency Raman spectra show distinct correlation with the stacking type. The A-like intra-layer vibration mode at 405 cm ⁻¹ exhibit Davydov splitting due to interlayer interactions when the 1.96 eV excitation is used. The positions and the relative intensities of the Davydov-split peaks depend on the stacking type as well. By using the linear-chain model, the interlayer force constants are extracted and compared for 3-layer samples with different stacking types: the 3R-types have a larger force constant than the 2H-type. For the 2H-type, Davydov splitting is analyzed as a function of the number of layers. The force constants for the second-nearest-neighbor interaction are obtained and compared with other transition metal dichalcogenides.