Interlayer breathing and shear modes in few-trilayer MoS2 and WSe2

TL;DR

This study investigates low-frequency interlayer vibrational modes in few-layer MoS2 and WSe2, revealing their evolution with layer number and implications for mechanical and electronic properties.

Contribution

It uncovers and models interlayer breathing and shear modes in 2D TMDs, demonstrating their layer-independent force constants using Raman spectroscopy and first principles calculations.

Findings

Interlayer modes can be described by a simple linear chain model.

Force constants are consistent from bilayer to bulk materials.

Interlayer frictional properties are layer-independent.

Abstract

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently attracted tremendous interest as potential valleytronic and nano-electronic materials, in addition to being well-known as excellent lubricants in the bulk. The interlayer van der Waals (vdW) coupling and low frequency phonon modes, and how they evolve with the number of layers, are important for both the mechanical and electrical properties of 2D TMDs. Here we uncover the ultra-low frequency interlayer breathing and shear modes in few-layer MoS2 and WSe2, prototypical layered TMDs, using both Raman spectroscopy and first principles calculations. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model with only nearest-neighbour interactions. We show that the derived in-plane (shear) and out-of-plane (breathing) force constants from experiment remain the same from two-layer 2D crystals to the bulk materials, suggesting that the nanoscale interlayer frictional characteristics of these excellent lubricants should be independent of the number of layers.