Splitting of monolayer out-of-planeA1′Raman mode in few-layerWS2

We present Raman measurements of mono- and few-layer ${\mathrm{WS}}_{2}$. We study the monolayer ${A}_{1}^{\ensuremath{'}}$ mode around $420{\mathrm{cm}}^{\ensuremath{-}1}$ and its evolution with the number of layers. We show that with increasing layer number there are an increasing number of possible vibrational patterns for the out-of-plane Raman mode: in $N$-layer ${\mathrm{WS}}_{2}$ there are $N$ $\mathrm{\ensuremath{\Gamma}}$-point phonons evolving from the ${A}_{1}^{\ensuremath{'}}$ monolayer mode. For an excitation energy close to resonance with the $A$ excitonic transition energy, we were able to observe all of these $N$ components, irrespective of their Raman activity. Density functional theory calculations support the experimental findings and make it possible to attribute the modes to their respective symmetries. The findings described here are of general importance for all other phonon modes in ${\mathrm{WS}}_{2}$ and other layered transition-metal dichalcogenide systems in the few-layer regime.