Interlayer interaction and Davydov splitting in MoS₂ polytypes studied by Raman spectroscopy

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 [1]. 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 [2, 3, 4].

References
[1] J.-U. Lee et al., ACS NANO, 10, 1948 (2016).
[2] G. Froehlicher et al., Nano Lett., 15, 6481 (2015).
[3] K. Kim et al., ACS NANO, 10, 8113 (2016).
[4] W. Na et al., 2D Mater., 6, 015004 (2019).