Biaxial strain in atomically thin transition metal dichalcogenides

TL;DR

This study investigates how biaxial strain, applied via thermal expansion of polymer substrates, affects the optical properties and bandgap energies of monolayer transition metal dichalcogenides, revealing significant tunability.

Contribution

It demonstrates a controlled method to apply biaxial strain to TMD monolayers and measures their optical responses, providing new insights into strain-induced bandgap tuning.

Findings

Biaxial strain causes a redshift in bandgap energy, up to 94 meV/% in WS2.

Bandgap shifts follow the order WS2 > WSe2 > MoS2 > MoSe2.

Strain transfer modeled with finite element simulations aligns with experimental optical measurements.

Abstract

Strain engineering in single-layer semiconducting transition metal dichalcogenides aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper we study transition metal dichalcogenides monolayers deposited on polymeric substrates under the application of biaxial strain, both tensile and compressive. We can control the amount of biaxial strain applied by letting the substrate thermally expand or compress by changing the substrate temperature. After modelling the substrate-dependent strain transfer process with a finite elements simulation, we performed micro-differential spectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2, WS2, WSe2) under the application of biaxial strain and measured their optical properties. For tensile strain we observe a redshift of the bandgap that reaches a value as large as 94 meV/% in the case of single-layer WS2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order WS2 > WSe2 > MoS2 > MoSe2.