In this article we review the WS2 Raman spectrum. Its Raman spectrum is quite similar to it’s dichalcogenide neighbour material, molybdenum.
The Raman spectrum of bulk WS2 has two prominent peaks: an in-plane (E2g) mode located around 350cm-1 and an out-of-plane (A1g) mode which is located at 420cm-1. The in-plane mode corresponds to the sulphur atoms vibrating in one direction and the tungsten atom in the other, while the out-of-plane mode is a mode of just the sulphur atoms vibrating out-of-plane.
As WS2 becomes single-layer these two modes evolve with thickness. The in-plane Raman mode upshifts, although very slowly (less than 1 cm-1), and the out-of-plane mode downshifts to 417cm-1. This means that monolayer WS2 can be reliably identified by the shift of the A1g mode.
Perhaps a more direct way of monitoring the transition to monolayer, in the case of WS2, is the observation of its photoluminescence. While bulk does not show any photoluminescence, as a result of its indirect band gap nature, monolayer shows strong and sharp photoluminesence which is a signature of a transition to monolayer state.
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 Lattice dynamics in mono- and few-layer sheets of WS2 and WSe2 Zhao et al. Nanoscale, 2013
 Identification of individual and few layers of WS2 using Raman Spectroscopy Berkdemir et al. Scientific Reports, 2013
 Electrical and optical characterization of atomically thin WS2 Georgiou et al. Dalton Transaction, 2014