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Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2

Nature Photonics volume 14, pages 324–329 (2020)Cite this article

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Abstract

In recent years, there has been ongoing effort in achieving efficient transport of excitons in monolayer transition metal dichalcogenides subjected to highly non-uniform strain. Here we investigate the transport of excitons and trions in monolayer semiconductor WS2 subjected to controlled non-uniform mechanical strain. An atomic force microscope (AFM)-based setup is applied to actively control and tune the strain profiles by indenting the monolayer with 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 WS2, 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 WS2 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 two-dimensional semiconductors and termed as funnelling, 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 up to about 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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Fig. 1: Experimental setup.
Fig. 2: Photoluminescence spectra of samples.
Fig. 3: Comparison between the experimentally measured photoluminescence spectra and the model predictions.
Fig. 4: Spatial dependency of properties of sample A.
Fig. 5: Spatial maps of the ratio between the total trion emission and the exciton emission for sample A.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Change history

  • 27 March 2020

    In the HTML version of this Article, Fig. 5 appeared with the caption for Fig. 4 and vice versa; this has now been amended. The PDF version is correct.

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Acknowledgements

We thank R. Netz for fruitful discussions. We also thank K. Höflich for technical support. This work was supported by the European Research Council Starting grant 639739, and DFG CRC/TRR 227.

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Authors and Affiliations

  1. Department of Physics, Freie Universität Berlin, Berlin, Germany

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

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  1. Moshe G. Harats
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  2. Jan N. Kirchhof
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Contributions

M.G.H. and K.I.B. planned and designed the experiment and wrote the paper. M.G.H. and J.N.K. performed the nanoindentation experiments. M.G.H., J.N.K. and M.Q. fabricated the samples. The experiments, data analysis and the theoretical model were done by M.G.H. K.G. contributed to the theoretical model.

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Correspondence to Kirill I. Bolotin.

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Supplementary Figs. 1–12 and discussion.

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Harats, M.G., Kirchhof, J.N., Qiao, M. et al. Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2. Nat. Photonics 14, 324–329 (2020). https://doi.org/10.1038/s41566-019-0581-5

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