  Single- and few-layer MoS2 has recently gained attention as an interesting
new material system for opto-electronics. Here, we report on scanning Raman
measurements on few-layer MoS2 flakes prepared by exfoliation. We observe a
Raman mode corresponding to a rigid shearing oscillation of adjacent layers.
This mode appears at very low Raman shifts between 20 and 30 relative
wavenumbers. Its position strongly depends on the number of layers, which we
independently determine using AFM measurements and investigation of the other
characteristic Raman modes. Raman spectroscopy of the shear mode therefore is a
useful tool to determine the number of layers for few-layer MoS2 flakes.
