Layer dependent MoS2 Raman spectroscopy.
Like graphene, single-layer and few-layer molybdenum disulphide (moly) has distinctive signatures in its Raman spectrum. But first, let’s discuss the Raman spectrum of bulk MoS2.
The Raman spectrum of bulk MoS2 has two prominent peaks: an in-plane (E2g) mode located around 383cm-1 and an out-of-plane (A1g) mode which is located at 407cm-1. The in-plane mode corresponds to the sulphur atoms vibrating in one direction and the Molybdenum atom in the other, while the out-of-plane mode is a mode of just the sulphur atoms vibrating out-of-plane.
As MoS2 becomes single-layer these two modes evolve with thickness. The in-plane mode upshifts to 386cm-1 and the out-of-plane downshifts to 404cm-1. The difference of these two modes (~18cm-1) can be used as a reliable identification for monolayer MoS2.
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Phonons in single-layer MoS2 and WS2
. Molina-Sanchez et al, Physical Review B, 2011
. Theoretical paper discussing evolution of phonon dispersions, focus on the out-of-plane and in-plane mode. It discusses in detail the mechanisms of shifting of the two peaks.