Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS$_2$

TL;DR

This study explores how non-uniform strain in monolayer WS$_2$ affects exciton and trion transport, revealing highly efficient exciton-to-trion conversion driven by strain-induced carrier redistribution, challenging previous assumptions about exciton funneling.

Contribution

It demonstrates that exciton funneling is negligible at room temperature, while strain-induced carrier redistribution enables near 100% exciton-to-trion conversion in WS$_2$, providing new insights for optoelectronic applications.

Findings

Exciton funneling is negligible at room temperature in strained WS$_2$.

Strain-induced carrier redistribution leads to nearly 100% exciton-to-trion conversion.

Optical spectroscopy and drift-diffusion modeling explain the observed phenomena.

Abstract

We investigate the transport of excitons and trions in monolayer semiconductor WS$_2$ subjected to controlled non-uniform mechanical strain. We actively control and tune the strain profiles with an AFM-based setup in which the monolayer is indented by an AFM tip. Optical spectroscopy is used to reveal the dynamics of the excited carriers. The non-uniform strain configuration locally changes the valence and conduction bands of WS$_2$, giving rise to effective forces attracting excitons and trions towards the point of maximum strain underneath the AFM tip. We observe large changes in the photoluminescence spectra of WS$_2$ under strain, which we interpret using a drift-diffusion model. We show that the transport of neutral excitons, a process that was previously thought to be efficient in non-uniformly strained 2D semiconductors and termed as "funneling", is negligible at room temperature in contrast to previous observations. Conversely, we discover that redistribution of free carriers under non-uniform strain profiles leads to highly efficient conversion of excitons to trions. Conversion efficiency reaches $\simeq 100\%$ even without electrical gating. Our results explain inconsistencies in previous experiments and pave the way towards new types of optoelectronic devices.