Layer-Dependent Variation in Raman Modes of MoS2

Raman spectroscopy is used widely to probe phonon properties and layer thickness in two-dimensional MoS2. However, relative peak heights of the out of plane (A1g) and in-plane (E2g^1) Raman modes can vary with layer numbers. The explanation for origin of this variation has remained elusive. In this work, we show through statistical analysis of hundreds of Raman spectra on hundreds of samples that the A1g/E2g^1 peak height ratio is ~1.4 for monolayer and <1.0 for bilayer MoS2. However, in tri- and four layered MoS2, the A1g/E2g^1 ratio can be both above or below 1.0, and gradually rises with thickness – approaching a saturation value of ~1.3. We find – based on measurements on different dielectrics and at multiple temperatures down to 8 K – that the peak height ratio is modulated by the A1g linewidth, which depends on both substrate-dependent electronic effects and thermally activated contributions associated with interlayer-related phonon scattering. In monolayer MoS2, the anomalous area ratio originates from suppression of the E2g^1 peak area and depends on excitation energy, suggesting the influence of its distinct electronic structure. Our results show that the variation of the A1g/E2g^1 peak height ratio arises from the coupling of linewidth and monolayer-specific Raman response.