From bulk to monolayer MoS ₂: Evolution of Raman scattering

Molybdenum disulfide (MoS ₂) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of E _2g ¹ and A _1g peaks vary monotonously with the layer number of ultrathin MoS ₂ 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 MoS ₂ decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS ₂. The asymmetric Raman peak at 454 cm ^-1, which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS ₂, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode (A _2u. Our findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS ₂ is scaled down from three- to two-dimensional geometry.