Visualization of dark excitons in semiconductor monolayers for high-sensitivity strain sensing

TL;DR

This paper demonstrates how compressive strain in WS2 monolayers enhances the visibility of dark excitons through phonon scattering, enabling high-sensitivity strain sensing with a gauge factor over 10^4.

Contribution

It introduces a method to visualize dark excitons in TMD monolayers by applying strain, revealing their spectral properties and enabling ultra-sensitive strain detection.

Findings

Strain modifies band alignment and enhances dark exciton emission.

Dark exciton spectral features depend strongly on local strain environment.

Achieved strain sensing with a gauge factor exceeding 10^4.

Abstract

Transition metal dichalcogenides (TMDs) are layered materials that have a semiconducting phase with many advantageous optoelectronic properties, including tightly bound excitons and spin-valley locking. In Tungsten-based TMDs, spin and momentum forbidden transitions give rise to dark excitons that typically are optically inaccessible but represent the lowest excitonic states of the system. Dark excitons can deeply affect transport, dynamics and coherence of bright excitons, hampering device performance. Therefore, it is crucial to create conditions in which these excitonic states can be visualized and controlled. Here, we show that compressive strain in WS2 enables phonon scattering of photoexcited electrons between momentum valleys, enhancing the formation of dark intervalley excitons. We show that the emission and spectral properties of momentum-forbidden excitons are accessible and strongly depend on the local strain environment that modifies the band alignment. This mechanism is further exploited for strain sensing in two-dimensional semiconductors revealing a gauge factor exceeding 10^4.