Enhanced Biaxial Compressive Strain Tuning of 2D semiconductors via Hot Dry Transfer on Polymer Substrates

TL;DR

This study demonstrates a novel hot-dry transfer method to induce large biaxial compressive strain in 2D WS$_2$ using polymer substrates, achieving unprecedented excitonic energy shifts at cryogenic temperatures.

Contribution

The paper introduces a pre-straining technique combined with hot-dry transfer to significantly enhance biaxial strain in 2D materials at cryogenic temperatures, surpassing previous strain levels.

Findings

Achieved up to ~0.5% biaxial compressive strain in WS$_2$

Observed a ~200 meV blueshift in exciton energies at 5 K

Strain transfer efficiency increases at cryogenic temperatures

Abstract

Strain engineering is an effective tool for tailoring the properties of two-dimensional (2D) materials, especially for tuning quantum phenomena. Among the limited methods available for strain engineering under cryogenic conditions, thermal mismatch with polymeric substrates provides a simple and affordable strategy to induce biaxial compressive strain upon cooling. In this work, we demonstrate the transfer of unprecedentedly large levels of uniform biaxial compressive strain to single-layer WS$_2$ by employing a pre-straining approach prior to cryogenic cooling. Using a hot-dry-transfer method, single-layer WS$_2$ samples were deposited onto thermally expanded polymeric substrates at 100 $^\circ$C. As the substrate cools to room temperature, it contracts, inducing biaxial compressive strain (up to ~0.5%) in the WS$_2$ layer. This pre-strain results in a measurable blueshift in excitonic energies compared to samples transferred at room temperature, which serve as control (not pre-strained) samples. Subsequent cooling of the pre-strained samples from room temperature down to 5 K leads to a remarkable total blueshift of ~200 meV in the exciton energies of single-layer WS$_2$. This energy shift surpasses previously reported values, indicating superior levels of biaxial compressive strain induced by the accumulated substrate contraction of ~1.7%. Moreover, our findings reveal a pronounced temperature dependence in strain transfer efficiency, with gauge factors approaching theoretical limits for ideal strain transfer at 5 K. We attribute this enhanced efficiency to the increased Young's modulus of the polymeric substrate at cryogenic temperatures.