Davydov splitting and polytypism in few-layer MoS2

The dependence of the interlayer interaction on the stacking order of MoS2 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.