Strain-shear coupling in bilayer MoS2

TL;DR

This paper reveals a coupling between in-plane strain and interlayer shear in bilayer MoS2, showing how strain affects phonon modes and enabling measurement of elastic constants via Raman spectroscopy.

Contribution

It uncovers the strain-shear coupling in layered materials and demonstrates how Raman spectroscopy can determine their mechanical constants.

Findings

Uniaxial strain causes anomalous splitting of interlayer shear phonon modes.

Strain induces shear mode hardening, contrary to intralayer phonon behavior.

Raman spectroscopy can measure almost all mechanical constants of layered materials.

Abstract

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intra- or inter-layer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstrating that Raman spectroscopy can determine almost all mechanical constants of layered materials.