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We present Raman measurements of mono- and few-layer WS2. We study themonolayer A'1 mode around 420 cm(-1) and its evolution with the number oflayers. We show that with increasing layer number there is an increasing numberof possible vibrational patterns for the out-of-plane Raman mode: in N-layerWS2 there are N Gamma-point phonons evolving from the A'1 monolayer mode. Foran excitation energy close to resonance with the excitonic transition energy wewere able to observe all of these N components, irrespective of their Ramanactivity. Density functional theory calculations support the experimentalfindings and make it possible to attribute the modes to their respectivesymmetries. The findings described here are of general importance for all otherphonon modes in WS2 and other layered transition metal dichalcogenide systemsin the few layer regime.

Splitting of the monolayer out-of-plane A'1 Raman mode in few-layer WS2