Raman spectrum of molybdenum disulfide (MoS2)

The Raman spectrum of MoS2 consists of 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.

Raman spectrum of MoS2 showing its evolution with layer thickness

As the layer thickness reduces to becomes single-layer these two modes evolve with thickness, a clear evolution in MoS2 Raman spectrum. 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. 

Additionally, the photoluminescence of MoS2 changes with the number of layers. This means that while bulk MoS2 is an indirect semiconductor (1.3eV), monolayer MoS2 is a direct gap (~1.8eV). [4]

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Molybdenum disulfide (MoS2) Crystal

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References

1. Anomalous lattice vibrations of single- and few-layer MoS2. Lee et al, ACS Nano, 2011.Thickness-dependent Raman spectroscopy of MoS2 flakes

2. 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.

3. From bulk to Monolayer MoS2: Evolution of Raman scattering. Li et al, Advanced Functional Materials, 2012.Multi-wavelength thickness-dependent Raman characterization of MoS2.

4. Emerging photoluminescence in monolayer MoS2. Splendiani et al, Nano letters 2010Layer-dependence of photoluminescence from atomically thin MoS2.