Layer-number dependent high-frequency vibration modes in few-layer transition metal dichalcogenides induced by interlayer couplings

Two-dimensional transition metal dichalcogenides（TMDs） have attracted extensive attention due to their many novel properties.The atoms within each layer in two-dimensional TMDs are joined together by covalent bonds,while van der Waals interactions combine the layers together.This makes its lattice dynamics layer-number dependent.The evolutions of ultralow frequency(< 50 cm-1) modes,such as shear and layer-breathing modes have been well-established.Here,we review the layer-number dependent high-frequency(> 50 cm-1) vibration modes in few-layer TMDs and demonstrate how the interlayer coupling leads to the splitting of high-frequency vibration modes,known as Davydov splitting.Such Davydov splitting can be well described by a van der Waals model,which directly links the splitting with the interlayer coupling.Our review expands the understanding on the effect of interlayer coupling on the high-frequency vibration modes in TMDs and other two-dimensional materials.