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arXiv · 1911.04451

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

Moshe G. Harats·Jan N. Kirchhof·Mengxiong Qiao·Kyrylo Greben·Kirill I. Bolotin
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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.

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Moshe G. Harats, Jan N. Kirchhof, Mengxiong Qiao, Kyrylo Greben, Kirill I. Bolotin. 2019-11-11. Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS$_2$. https://doi.org/10.1038/s41566-019-0581-5

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