In this blog post, we briefly describe the molybdenum disulfide’s, MoS2, Raman spectrum.
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 the layer thickness 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). 
Don’t forget you can buy bulk MoS2 crystals at our store.
- Anomalous lattice vibrations of single- and few-layer MoS2. Lee et al, ACS Nano, 2011.Thickness-dependent Raman spectroscopy of MoS2 flakes
- 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.
- From bulk to Monolayer MoS2: Evolution of Raman scattering. Li et al, Advanced
Functional Materials, 2012.Multi-wavelength thickness-dependent Raman characterization of MoS2.
- Emerging photoluminescence in monolayer MoS2. Splendiani et al, Nano letters 2010Layer-dependence of photoluminescence from atomically thin MoS2.