Anomalous vibrational modes in few layer WTe₂ revealed by polarized Raman scattering and first-principles calculations

When layered transition-metal dichalcogenides (TMDs) are scaled down from a three- to a 2D geometry, electronic and structural transitions occur, leading to the emergence of properties not usually found in the bulk. Here, we report a systematic Raman study of exfoliated semi-metallic WTe₂ flakes with thickness ranging from few layers down to a single layer. A dramatic change in the Raman spectra occurs between the monolayer and few-layer WTe₂ as a vibrational mode centered at ~86.9 cm^-1 in the monolayer splits into two active modes at 82.9 and 89.6 cm^-1 in the bilayer. Davydov splitting of these two modes is found in the bilayer, as further evidenced by polarized Raman measurements. Strong angular dependence of Raman modes on the WTe₂ film thickness reflects that the existence of directional interlayer interaction, rather than isotropic van der Waals (vdw) coupling, is playing an essential role affecting the phonon modes, especially in anisotropic 2D WTe₂ material. Therefore, the strong evolution of Raman modes with thickness and polarization direction, can not only be a reliable fingerprint for the determination of the thickness and the crystallographic orientation, but can also be an ideal probe for such strong and directional interlayer interaction.