There are two types of vibrational modes in MoS2 (as in all layered structures): vibrational modes inside layers (intralayer) and modes resulting from the movements of complete layers (interlayer).
The intralayer modes are primary related to the chemical composition of a layer (or layers) and are their fingerprint; however these can be slightly influenced by the number of layers. The interlayer modes, due to the high mass of the layers, are observed at very low frequencies and their position depends on the number of layers. As a result, both types of modes can be used to determine the number of layers.
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