From Bulk to Monolayer MoS2: Evolution of Raman Scattering

Abstract

Molybdenum disulfide (MoS₂) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of $ E_{2{\rm g}}^1 $ and $ A_{{\rm 1g}}^{} $ peaks vary monotonously with the layer number of ultrathin MoS₂ flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons are found to become weaker when the layer number of MoS₂ decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS₂. The asymmetric Raman peak at 454 cm⁻¹, which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS₂, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode ($ A_{{\rm 2u}}^{} $ ). Our findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS₂ is scaled down from three‐ to two‐dimensional geometry.