Control of biaxial strain in single-layer Molybdenite using local thermal expansion of the substrate

TL;DR

This paper presents a novel method to controllably apply biaxial tensile strain to single-layer MoS2 using local thermal expansion of a silicone substrate, enabling tunable bandgap engineering observable via optical spectroscopy.

Contribution

It introduces a laser-heating technique exploiting thermal expansion mismatch to induce and control biaxial strain in monolayer MoS2, which was previously challenging to achieve.

Findings

Biaxial strain up to 0.2% applied to MoS2.

Redshift in photoluminescence correlates with strain levels.

Band gap modulation of 105 meV per percentage of biaxial tensile strain.

Abstract

Single-layer MoS2 is a direct-gap semiconductor whose electronic band structure strongly depends on the strain applied to its crystal lattice. While uniaxial strain can be easily applied in a controlled way, e.g., by bending of a flexible substrate with the atomically thin MoS2 layer on top, experimental realization of biaxial strain is more challenging. Here, we exploit the large mismatch between the thermal expansion coefficients of MoS2 and a silicone-based substrate to apply a controllable biaxial tensile strain by heating the substrate with a focused laser. The effect of this biaxial strain is directly observable in optical spectroscopy as a redshift of the MoS2 photoluminescence. We also demonstrate the potential of this method to engineer more complex strain patterns by employing highly absorptive features on the substrate to achieve non-uniform heat profiles. By comparison of the observed redshift to strain-dependent band structure calculations, we estimate the biaxial strain applied by the silicone-based substrate to be up to 0.2 percent, corresponding to a band gap modulation of 105 meV per percentage of biaxial tensile strain.