Davydov Splitting, Double-Resonance Raman Scattering, and Disorder-Induced Second-Order Processes in Chemical Vapor Deposited MoS2 Thin Films.

Atomically thin MoS2 hosts rich and distinct vibrational spectral features, which are prominent under selective excitation energies near the excitonic transitions. In this work, we have investigated the resonant Raman scattering of the MoS2 layers of different thicknesses, from monolayer to five-layer samples, measured near resonance with the A excitonic transition. We show that the near-resonance excitation (1.96 eV) resulted in a Davydov splitting of the out-of-plane A-like phonon mode (A1g) around 406 cm-1 caused by the weak interlayer interaction. The number of Davydov splitting components (N) equals the number of layers (NL) of the MoS2, suggesting that it can be used as a thickness indicator. The origin of various Davydov components is understood based on a simple nearest-interlayer interaction. We extend our investigation to identify some acoustic phonon modes associated with characteristic second-order double-resonance Raman and disorder-induced bands.