Achieving Large, Tunable Strain in Monolayer Transition-Metal Dichalcogenides

TL;DR

This paper presents a simple polymer encapsulation method to apply controllable strain to monolayer TMDs, studying their lattice response via Raman spectroscopy and revealing strain-induced phonon and symmetry changes.

Contribution

Introduces a facile technique for applying tunable strain to monolayer TMDs and analyzes their lattice response through polarized Raman spectroscopy.

Findings

Strain causes mode-dependent redshifts in Raman spectra.

Degeneracy of in-plane E' modes splits under strain.

Large strain can alter polarization response of A' mode.

Abstract

We describe a facile technique based on polymer encapsulation to apply several percent controllable strains to monolayer and few-layer Transition Metal Dichalcogenides (TMDs). We use this technique to study the lattice response to strain via polarized Raman spectroscopy in monolayer WSe2 and WS2. The application of strain causes mode-dependent redshifts, with larger shift rates observed for in-plane modes. We observe a splitting of the degeneracy of the in-plane E' modes in both materials and measure the Gruneisen parameters. At large strain, we observe that the reduction of crystal symmetry can lead to a change in the polarization response of the A' mode in WS2. While both WSe2 and WS2 exhibit similar qualitative changes in the phonon structure with strain, we observe much larger changes in mode positions and intensities with strain in WS2. These differences can be explained simply by the degree of iconicity of the metal-chalcogen bond.