Strain tuning of interlayer phonons in bilayer MoS₂  

Strain engineering in atomically thin van der Waals (vdW) transition metal dichalcogenides (TMDs) greatly impacts their electronic, optical and mechanical properties. In-plane strain is expected to affect only the intralayer characteristics, however some previous work on epitaxial films suggests that it can also alter out-of-plane lattice parameters. In this work we provide direct experimental evidence that in-plane biaxial strain enhances interlayer interactions in bilayer vdW systems. Via low frequency Raman spectroscopy, we observe a significant hardening in the breathing modes of both 2H and 2R MoS₂ under tensile biaxial strain. First principal calculations assign this hardening to a reduced interlayer spacing and the resulting increase in the interlayer coupling strength. Our results highlight an anisotropic coupling between in-plane and out-of-plane degrees of freedom in vdW materials, expanding the understanding of strain effects and offering new pathways for designing nanoscale optomechanical devices.