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We report two new first-order Raman modes in the spectra of few-layer MoS$_2$at 286~cm$^{-1}$ and 471~cm$^{-1}$ for excitation energies above 2.4~eV. Thesemodes appear only in few-layer MoS$_2$; therefore their absence provides aneasy and accurate method to identify single-layer MoS$_2$. We show that thesemodes are related to phonons that are not observed in the single layer due totheir symmetry. Each of these phonons leads to several nearly degeneratephonons in few-layer samples. The nearly degenerate phonons in few-layermaterials belong to two different symmetry representations, showing oppositebehavior under inversion or horizontal reflection. As a result, Raman activephonons exist in few-layer materials that have nearly the same frequency as thesymmetry forbidden phonon of the single layer. We provide here a generaltreatment of this effect for all few-layer two-dimensional crystal structureswith an inversion center or a mirror plane parallel to the layers. We show thatalways nearly degenerate phonon modes of different symmetry must occur and, asa result, similar pseudo-activation effects can be excepted.

Newly observed first-order resonant Raman modes in few-layer MoS$_2$