Number of Layers of MOS2 Determined Using Raman Spectroscopy

A combined (low-frequency and fingerprint) Raman map of MoS2 layers.

A combined Raman map of MoS2 layers.

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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Number of Layers of MoS2 Determined Using Raman Spectroscopy
DescriptionThe two methods - Analysis of fingerprint modes (intralayer) and Analysis of low-frequency modes (interlayer) - give complementary results and allow the determination of the number of MoS2 layers. Method 2 (using low frequency modes) gives excellent contrast; however it does not show single layer regions (which is related to the nature of the modes, rising from interaction between at least two layers). Method 1 (using fingerprint modes) shows all the layers, but the contrast is poorer, particularly for higher numbers of layers. The best result can be obtained combining the two methods. All the measurements (low-frequency and fingerprint) were done using ultra-low frequency ULFTM filters which allow a high throughput measurement in a full Raman range, down to <10 cm-1.
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