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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.
Two-dimensional transition metal dichalcogenides (TMDs) have been extensively addressed due to their many novel properties. The atoms within each layer in two-dimensional TMDs were joined together by covalent bonds, while van der Waals interactions combine the layers together. This makes its own 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. Suu Davydov splitting can be 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.